TRANSFER OF LISTERIA MONOCYTOGENES DURING CUTTING, SLICING, DICING,
AND SUBSEQUENT STORAGE OF CANTALOUPE AND HONEYDEW MELONS
By
Rocky D. Patil

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Food Science – Master of Science
2017

i

ABSTRACT
TRANSFER OF LISTERIA MONOCYTOGENES DURING CUTTING, SLICING,
DICING, AND SUBSEQUENT STORAGE OF CANTALOUPE AND HONEYDEW
MELONS
By
Rocky D. Patil
The 2011 multistate listeriosis outbreak associated with whole cantaloupe has heightened
concerns regarding commercial handling and preparation practices for melons. In response, two
separate studies were conducted to: (1) assess the transfer of Listeria monocytogenes during
cutting, slicing, dicing in melons and (2) measure growth during subsequent storage of
cantaloupes. Initially, cantaloupe and honeydew melon rinds containing L. monocytogenes
populations (J22F, J29H, and M3) of 6.1 and 4.4 log CFU/cm2, stored at 4 and 30oC were cut
with a sterile cork borer through the blossom scar, stem scar, and circumference regions of the
rind to evaluate the L. monocytogenes transfer to the edible melon flesh. L. monocytogenes
transfer ranged from 1.2 to 4.0 and 0.2 to 1.8 log CFU/cm2 for cantaloupe and honeydew.
Overall, no significant difference in L. monocytogenes transfer was seen at different depths or
product temperatures (P > 0.05). L. monocytogenes transfer from one inoculated melon to
subsequent uninoculated melons was evaluated after mechanical slicing (Vollrath Redco 401N)
of the intact fruit and dicing (Nemco 55650 dicer) of the flesh. Diced cantaloupe samples were
stored (4, 7, and 10oC for seven days) and microbiologically analyzed for growth and survival of
L. monocytogenes. During slicing, L. monocytogenes was transferred to uninoculated cantaloupe
and honeydew melons were statistically similar (P > 0.05), ranging from 0.1 to 1.5 and 0.1 to 0.5
log CFU/g, respectively. During the storage of diced cantaloupes, L. monocytogenes generation
time was 0.74 days for 10oC, with minimal growth observed at 4 and 7oC.

ii

ACKNOWLEDGEMENTS

I would like to sincerely thank my advisor, Dr. Elliot Ryser, for allowing me to join his
food safety laboratory at Michigan State University. I am forever thankful for the opportunity to
gain more experience in food science and learn more about this field from my advisor. Without
Dr. Ryser’s guidance in my project, I would not be able to complete my work, which I am
grateful in gaining a better understanding of designing lab experiments. In addition, I would like
to thank my committee members: Dr. Bradley Marks, Dr. John Linz, and Dr. Eva Almenar for
their support and providing insight and constructive criticism in the design of my experiments.
In addition, I would like to thank my fellow lab mates: Hamoud, Lin, Victor, Andy,
Harry, Chelsea, and Gordon for their support and willingness to help in assisting and answering
questions from course work to lab work. Overall, my time in the Ryser laboratory was made
enjoyable with my fellow coworkers. Finally, I would like to thank my research assistants, Jake
and Sarah, in helping me during my experiments, without them I would have had a difficult time
completing these projects.
Finally, I would like to thank my parents for all their unconditional love and support
during my time at Michigan State University. I know that with their trust and belief in me that I
will eventually achieve my goals. I am thankful for all that they have done for me.

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TABLE OF CONTENTS

LIST OF TABLES ........................................................................................................................ vi
LIST OF FIGURES .................................................................................................................... viii
Chapter 1: Introduction ...................................................................................................................1
Chapter 2: Literature Review ..........................................................................................................5
2.1 Foodborne Pathogens ..................................................................................................................... 6
2.2 Listeria monocytogenes .................................................................................................................. 6
2.3 Foodborne Listeriosis ..................................................................................................................... 7
2.4 Human Listeriosis & Treatment .................................................................................................... 9
2.5 Fresh-cut Produce Contamination Issues ................................................................................... 10
2.6 Outbreaks Associated with Melons ............................................................................................ 12
2.7 Listeria spp. in Produce ............................................................................................................... 17
2.8 Cantaloupe Rind Structure ........................................................................................................... 18
2.9 Sources of Contamination during Cutting ................................................................................. 19
2.10 Commercial Slicing and Dicing of Fresh-Cut Produce ......................................................... 20
2.11 Contamination of Melons .......................................................................................................... 21
2.11.1 Preharvest Contamination of Melons ............................................................................... 21
2.11.2 Postharvest Contamination of Melons ............................................................................. 22
2.12 Industry Practices to Minimize Contamination during Melon Processing .......................... 24
2.13 Temperature ................................................................................................................................. 29
2.14 Application of Sanitizers to Reduce Contamination ............................................................... 30
2.14.1 Application of Sanitizers on Surface of Melons ............................................................. 30
2.14.2 Inactivation Treatments for Surface of Cantaloupes ...................................................... 32
2.15 Food Safety Regulation to Minimize Cantaloupe Contamination ....................................... 32
Chapter 3: Extent of Listeria monocytogenes Transfer during Cutting of Cantaloupe and
Honeydew Melon ..........................................................................................................................34
3.1 Materials and Methods ................................................................................................................. 35
3.1.1 Experimental Design ............................................................................................................. 35
3.1.2 Produce ................................................................................................................................... 35
3.1.3 Bacterial Strains & Inoculation ........................................................................................... 35
3.1.4 Cantaloupe and Honeydew Melon Coring ......................................................................... 36
3.1.5 Comparison of Coring Tools ................................................................................................ 37
3.1.6 Microbiological Analysis ..................................................................................................... 38
3.1.7 Surface Population Calculations ......................................................................................... 38
3.1.8 Statistical Analysis ................................................................................................................ 39
3.2 Results ............................................................................................................................................ 40
3.2.1 L. monocytogenes populations on rind surfaces of cantaloupe and honeydew melon . 40
3.2.2 L. monocytogenes transfer from rind to cantaloupe and honeydew melon flesh .......... 41
3.2.3 Impact of melon region and temperature on L. monocytogenes transfer ....................... 41
3.2.4 Comparison of coring tools .................................................................................................. 47
iv

3.3 Discussion ...................................................................................................................................... 48
Chapter 4: Transfer of Listeria monocytogenes during Mechanical Slicing and Dicing and
Growth during Subsequent Storage of Cantaloupe and Honeydew Melon ..................................53
4.1 Materials And Methods ................................................................................................................ 54
4.1.1 Experimental Design ............................................................................................................. 54
4.1.2 Produce ................................................................................................................................... 54
4.1.3 Bacterial Strains ..................................................................................................................... 54
4.1.4 Cantaloupe and Honeydew Melon Inoculation for Slicing .............................................. 55
4.1.5 Cantaloupe Inoculation for Dicing ...................................................................................... 55
4.1.6 Cantaloupe and Honeydew Melon Slicing ......................................................................... 55
4.1.7 Cantaloupe Dicing and Storage ........................................................................................... 56
4.1.8 L. monocytogenes Growth on Diced Cantaloupe .............................................................. 57
4.1.9 Microbiological Analysis ..................................................................................................... 58
4.1.10 Surface Population Calculations ....................................................................................... 58
4.1.11 Statistical Analysis .............................................................................................................. 59
4.2 Results ............................................................................................................................................ 59
4.2.1 L. monocytogenes transfer during melon slicing ............................................................... 59
4.2.2 L. monocytogenes transfer from inoculated melons to subsequent uninoculated melons
during dicing .................................................................................................................................... 60
4.2.3 L. monocytogenes growth in ten diced cantaloupe halves during storage ..................... 62
4.2.4 L. monocytogenes growth on inoculated diced cantaloupe pieces .................................. 64
4.3 Discussion ...................................................................................................................................... 65
Chapter 5: Conclusions and Future Recommendations ................................................................69
APPENDICES ..............................................................................................................................72
Appendix A – Coring of Cantaloupe and Honeydew Melon ......................................................... 73
Appendix B – Slicing and Dicing of Cantaloupe and Honeydew Melon ..................................... 79
REFERENCES .............................................................................................................................86

v

LIST OF TABLES

Table 2.1: Foodborne illness outbreaks associated with honeydew melons adapted from Danyluk
et al. (2014). ..…………………………………………………………………………..………. 15
Table 2.2: Foodborne illness outbreaks associated with cantaloupes adapted from Danyluk et al.
(2014). ..………………………………………………………………………………………... 16
Table 3: Raw data for L. monocytogenes populations on surface of cantaloupe and honeydew
melon at the stem scar, blossom scar, and rind dip-inoculated (log CFU/cm2) and stored for 24 h
at 4oC. .………………………...………………………………………………………………... 73
Table 4: Raw data for L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the stem scar. ...….... 73
Table 5: Raw data for L. monocytogenes population transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the blossom scar. ..... 74
Table 6: Raw data for L. monocytogenes population transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the rind circumference
area. …………………………………………………………………..………………………… 74
Table 7: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar. …..…...... 75
Table 8: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar. …….. 75
Table 9: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference are.
………………………………………………………………………………………………….. 76
Table 10: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar. ....…....… 76
Table 11: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar. …...... 77
Table 12: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference
area. .............................................................................................................................................. 77

vi

Table 13: Raw data for transfer of L. monocytogenes populations from the cantaloupe rind dipinoculated and stored at 4oC to the interior melon flesh by using an apple corer and cork borer to
compare bacterial transfer. ……………………………………………………………………... 78
Table 14: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe to
uninoculated melon halves during mechanical slicing. Inoculation level at (5.2 Âą 0.4 log
CFU/cm2). ……………………………………………………………………………………… 79
Table 15: Raw data for L. monocytogenes populations transferred from inoculated honeydew
melon to uninoculated melon halves during mechanical slicing. Inoculation level at (4.2 Âą 0.2
log CFU/cm2). …………………………………………………………………………...……... 80
Table 16: Raw data for L. monocytogenes populations transferred from cantaloupe halves during
mechanical dicing. Inoculation level at (5.5 ± 0.1 log CFU/cm2). ..…………………………... 81
Table 17: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated cantaloupe halves during mechanical dicing. Inoculation level at (3.3 Âą
0.2 log CFU/cm2). ……………………………………………………………………………… 81
Table 18: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 4oC. ……...…………… 82
Table 19: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 7oC. .....…………..…… 83
Table 20: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 10oC. …….…………… 84
Table 21: Raw data for L. monocytogenes population growth on cantaloupe dices during
subsequent storage of seven days at 4oC, 7oC, and 10oC. ………………………...…………..... 85

vii

LIST OF FIGURES

Figure 2.1: Flow diagram of melon processing from field to consumer for retail and foodservice
establishment using field pack and shed pack methods adapted directly from the Food and Drug
Administration (FDA, 2009). .……………………………………...…………………………... 24
Figure 3.1: The coring locations at the A) circumference of the rind B) blossom scar and C)
stem scar region. ………………..…………………………………………………………….... 37
Figure 3.2: “Farberware Classic Apple Corer.” Amazon. n.p., n.d. Web. 24 Oct. 2016. ….…... 38
Figure 3.3: Mean (ÂąSE) L. monocytogenes populations on surface of cantaloupe and honeydew
melon at the stem scar, blossom scar, and rind dip-inoculated and stored for 24 hours at 4oC.
Different letters above the bars from Tukey-Kramer HSD comparisons indicate statistical
significance (P < 0.05) between surface population means. …………………………………… 41
Figure 3.4: Mean (ÂąSE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the stem scar. ........... 43
Figure 3.5: Mean (±SE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the blossom scar. ….. 44
Figure 3.6: Mean (ÂąSE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the rind circumference
area. ……………………………………………………………………………………...……... 44
Figure 3.7: Mean (±SE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar. ..........….. 44
Figure 3.8: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar. ........... 45
Figure 3.9: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference
area. .............................................................................................................................................. 45
Figure 3.10: Mean (±SE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from the stem scar. ……... 46
Figure 3.11: Mean (±SE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from the blossom scar. …. 46
Figure 3.12: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated stored at 4 and 30oC for 24 h to the cantaloupe flesh from the rind circumference area.
………...…………………………………………………………………………………….…... 47

viii

Figure 3.13: Mean (ÂąSE) transfer of L. monocytogenes populations from the cantaloupe rind to
the interior melon flesh by using an apple corer and cork borer to compare bacterial transfer. .. 48
Figure 4.1: “Vollrath 401N Redco Lettuce King I ¾” Cut Vegetable Shredder and Slicer – 10
Blades.” Webstaurant Store. n.p., n.d. Web. 10 Aug. 2016. …………………………………… 56
Figure 4.2: “Nemco 55650 Easy Lettuce Kutter (Cutter)” Webstaurant Store. n.p., n.d. Web. 10
Aug. 2016. ……………………………………………………………………………………… 57
Figure 4.3: Mean (Âą) SE L. monocytogenes populations on cantaloupe halves that were sliced.
Inoculation level at (5.2 ± 0.4 log CFU/cm2). ......……………………………………………… 60
Figure 4.4: Mean (Âą) SE L. monocytogenes populations on honeydew melon halves that were
sliced. Inoculation level at (4.2 ± 0.2 log CFU/cm2). …….…………………………………… 60
Figure 4.5: Mean (ÂąSE) L. monocytogenes populations on cantaloupe halves that were diced.
Inoculation level at (5.5 ± 0.1 log CFU/cm2). ………………………...………………………... 61
Figure 4.6: Mean (Âą) SE L. monocytogenes populations on cantaloupe halves that were diced.
Inoculation level at (3.3 ± 0.2 log CFU/cm2). …………..……………………………………… 62
Figure 4.7: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 4oC. ………………………………………………………………..………………...... 63
Figure 4.8: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 7oC. …………………………………………………………………….……………... 64
Figure 4.9: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 10oC. .…………….………………………………………………………………….... 64
Figure 4.10: Mean (Âą SE) L. monocytogenes population growth on cantaloupe dices during
subsequent storage of seven days and stored at 4oC, 7oC, and 10oC. ………………………….. 65

ix

Chapter 1: Introduction

1

Recently, people have been consuming more fresh produce in their diet due to its year
round availability and health benefits (Pollack, 2001). This increased consumption of raw fruits
and vegetables is not without risk. Due to common preharvest and postharvest production
practices, fresh fruits and vegetables carry an inherent risk for pathogen contamination. Fresh
produce contributed to approximately 46% of all foodborne outbreaks between 1998 to 2008
(Scallan et al., 2011). Melons, especially cantaloupes, have been a source of foodborne
outbreaks due to their netted surfaces that have an increased risk of becoming contaminated from
the environment and challenges in washing such surfaces during handling and processing.
Since 1994, there has been a drastic increase in the number of cantaloupe outbreaks
associated with Salmonella contamination (Walsh et al., 2013). Cantaloupe surfaces are
susceptible to microbial contamination due to their netted porous rind which allows for greater
attachment of pathogens (Fan et al., 2006). Historically, Listeria monocytogenes has not been
linked to cantaloupe outbreaks until 2011 when the “Rocky Ford” outbreak was documented in
Colorado. This multistate outbreak caused 147 illnesses leading to 33 fatalities and 1
miscarriage (McCollum et al., 2013). The FDA concluded that contamination most likely
occurred during the cooling step allowing for the growth and survival of L. monocytogenes on
cantaloupes (FDA, 2012). The infectious dose of L. monocytogenes in healthy individuals has
been estimated to be 107 CFU/g of food (Farber et al., 1996). In this outbreak, L. monocytogenes
was most likely transferred from the rind to the edible flesh during postharvest cutting, slicing,
dicing, or peeling.
These cantaloupe outbreaks have negatively affected the melon industry. Cantaloupe
prices dropped 33.6% due to the adverse national media exposure in 2011 (Bottemiller, 2011).
The outbreak from the “Rocky Ford” region of Colorado impacted the entire melon market even

2

though it only comprised 2% of the cantaloupe market. Overall, this outbreak heightened fears
from consumers regarding the safety of fresh produce, questions were raised concerning how
cantaloupes were processed and handled during preharvest conditions. These cantaloupe
outbreaks have led to the development and implementation of melon processing guidelines to
prevent microbial contamination of melon surfaces (FDA, 2005).
Cantaloupe and honeydew melons can be contaminated at any stage during preharvest
and postharvest processing. During processing (e.g. cutting, slicing, and dicing), pathogens can
be translocated from the rind inward to the edible flesh. In the food industry, there are no
standardized practices for the cutting, slicing, and dicing of melons as companies use either
manual or automated processes (Wang and Ryser, 2014). Problems can occur when
contaminated produce is sliced or diced, transferring pathogens to subsequent slices or dices
from the potentially contaminated blade or pusher of the equipment. Previous research has
assessed the transfer of pathogens during slicing of delicatessen meats, tomatoes, and onions
(Scollon et al., 2013; Vorst et al., 2006; Wang and Ryser, 2013). Additional studies have
quantified the transfer of L. monocytogenes during mechanical dicing of celery and determined
the effects of temperature on the growth of pathogens during storage (Kaminski et al., 2014). To
my knowledge, no research has been conducted on the transfer of L. monocytogenes from
contaminated to uncontaminated melons during mechanical slicing and dicing and subsequent
growth in the melon flesh at normally encountered storage temperatures. Also, the bacterial
transfer from the rind to the edible flesh on different rind regions and product temperatures was
evaluated. Due to the recent multistate outbreak with L. monocytogenes in cantaloupe, the lack
of research currently available on the transfer of L. monocytogenes in mechanical processing,
handling and storage of cantaloupe, the impact of safety concerns to the produce industry and

3

public health agencies, and the increasing consumption of fresh produce in the public, there is a
great need for this research.
Overall, knowledge gained from these studies will aid in the development and application
of strategies to minimize the transfer of pathogens during the slicing, dicing, storage, and
transportation of fresh-cut produce. Furthermore, the information gained from these experiments
will help industry target interventions strategies that will aid in reducing pathogen crosscontamination during commercial processing, handling, and transportation of fresh-cut produce.
The objectives of this study were:
1. To assess the transfer of L. monocytogenes during cutting of cantaloupe and honeydew
melon, impacted by three different regions of the rind (i.e., blossom scar, stem scar, and
circumference of the rind) and product temperature.
2. To quantify L. monocytogenes transfer from one inoculated cantaloupe or honeydew
melon to subsequent uninoculated melons during mechanical slicing and dicing.
3. To assess the growth of pathogens under commonly used storage conditions for diced
cantaloupe.

4

Chapter 2: Literature Review

5

2.1 Foodborne Pathogens
Foodborne pathogens pose a hazard to the public as millions worldwide are affected by
gastrointestinal diseases yearly resulting in a loss of work productivity and increased medical
expenses. Also, foodborne pathogens are a greater risk to the youth, pregnant women, elderly,
and immunocompromised individuals. Foodborne illnesses are still poorly understood and even
its long term effects are relatively unknown even with its regularity in the population. The
occurrence of foodborne illness annually in the U.S.A. is approximately 46 million incidents
resulting in 250,000 hospitalizations and 3,000 deaths (Scallan et al., 2011). Recently, new
foodborne pathogens have emerged from animal reservoirs that have unintentionally affected
humans as the pathogen was identified as the source of the outbreak. Furthermore, genetic
evolution in microorganisms along with changes in agricultural and manufacturing practices
have led to the emergence of new foodborne pathogens. The emergence of new public health
threats, complications in new global supply chain systems, and large multi-state foodborne
outbreaks have led to the development of public health surveillance programs like FoodNet,
PulseNet, National Antimicrobial Resistance Monitoring System (NARMS), and National
Outbreak Reporting System (NORS) which track human infections caused by specific pathogens.
2.2 Listeria monocytogenes
Listeria monocytogenes is a rod shaped, gram positive facultative anaerobic bacterium
which grows between -1.5 to 45oC (Petran and Zottola, 1989). The typical size of L.
monocytogenes is approximately 0.4 Âľm by 1 to 1.5 Âľm in length. L. monocytogenes is a unique
pathogen which is widely found in nature due to its ability to survive at refrigeration
temperatures and grow in high salt environments. Like many other foodborne pathogens, L.
monocytogenes has a flagella which increases its infectivity during gastrointestinal cell invasion

6

in humans (Bigot et al., 2005). The regulation of the flagella is temperature-dependent as it is
fully functional at temperatures between 20 to 25oC, but at 37oC, the motility genes are down
regulated leading to decreased flagellin production inhibiting its movement (Peel et al., 1988).
The genus Listeria consists of ten different species consisting of L. monocytogenes, L. ivanovii,
L. innocua, L. welshimeri, L. seeligeri, L. grayi, L. fleischmannii, L. marthii, L. rocourtiae, and
L. weihenstephanensis. (den Bakker et al., 2013; Volokhov et al., 2002). The newly identified
species of Listeria (L. rocourtiae, L. marthii, L. weihenstephanensis, and L. fleischmannii) are
typically isolated from environmental samples at low occurrences. L. monocytogenes is virulent
in humans and animals while L. ivanovii and L. seeligeri are only virulent in animals.
2.3 Foodborne Listeriosis
Foodborne illnesses have become a significant health concern leading to approximately
48 million illnesses and 3,000 deaths annually in the U.S.A. (CDC, 2011). Even if L.
monocytogenes infection occurs in low frequency compared to other pathogens, it is widely
found in food processing facilities and has the ability to survive in refrigerated temperatures
making it difficult to control and making it one of the most hazardous concerns to our food
supply. Annually in the United States, L. monocytogenes is responsible for approximately 2,500
listeriosis cases leading to a hospitalization rate of 91% and fatality rate of 20% (Mead et al.,
1999). L. monocytogenes has been the causative agent in several foodborne outbreaks including
the deadliest outbreak linked to cheese in 1985 (Linnan et al., 1988). Furthermore, several food
products like poultry, meat, produce, and dairy products have been vehicles of transmission
during L. monocytogenes outbreaks.

7

Human listeriosis is caused by the ingestion of foods containing L. monocytogenes with
higher infection rates in immunocompromised individuals. Typically, human listeriosis is rare
but if acquired, the fatality rate is approximately 20% (Low and Donachie, 1997). The infectious
dose for L. monocytogenes varies widely depending on the immune status, the amount of
contaminated food consumed and virulence of the particular L. monocytogenes strain (VasquezBoland et al., 2001). Contamination levels greater than 107 L. monocytogenes cells per gram of
food are most often associated with listeriosis (Farber et al., 1996). These scientific findings
prompted the European Food Safety Authority to establish a maximum threshold of 100 CFU/g
for foods in which L. monocytogenes is unable to grow (EFSA, 2013). Overall, the safety limit
established by this food regulatory agency holds food processors accountable to implement and
establish good manufacturing practices (GMP) to prevent cross-contamination of L.
monocytogenes during food preparation and handling.
Recently, consumption of produce has resulted in the most illnesses among food products
with 25,222 cases and second highest number of outbreaks with (696 cases) from 2001 to 2010
(DeWaal and Glassman, 2013). Fresh fruits and vegetables can become contaminated at any
stage during preharvest or postharvest. Furthermore, the increased availability of fruits and
vegetables in the market and higher consumption rates of minimally processed produce in the
population has led to increased occurrence of foodborne outbreaks (Buck et al., 2003). Listeria
spp. has been detected and isolated from a wide range of products including bean sprouts,
cabbage, chicory, cucumber, eggplant, lettuce, mushrooms, potatoes, radish, salad vegetables,
and tomatoes (Buck et al., 2003). The increased number of outbreaks associated with fresh fruits
and vegetables has heightened concerns about the safety of fresh produce resulting in the

8

development of food safety strategies to prevent contamination throughout the entire farm-tofork continuum.
2.4 Human Listeriosis & Treatment
Listeria infections are primarily associated with immunocompromised and pregnant
individuals who acquire it through the consumption of contaminated food.
Immunocompromised and elderly individuals have a greater likelihood of acquiring Listeria
infections as their immune systems have a larger proportion of helper T-cells that are unable to
protect against foodborne pathogens (Swaminathan and Gerner-Smidt, 2007). Pregnant women
who are infected with Listeria can transmit the disease to their fetus through the bloodstream
causing fetal distress which could be fatal (Swaminathan and Gerner-Smidt, 2007). The
FAO/WHO Listeria Risk Assessment Group that determined the elderly and fetuses were 2.6 and
14 times more likely to acquire invasive listeriosis compared to healthy individuals, respectively
(Buchanan et al., 2004). Overall, invasive listeriosis has a greater likelihood of infecting
individuals with predisposed risk factors.
The severity of foodborne illnesses primarily depends on the contamination level, the
amount of food consumed and the health of the individual. Listeriosis can be a self-limiting
diarrheal illness for which no antibiotic treatment is necessary, but individuals who contract
invasive listeriosis are treated with ampicillin, penicillin, and amoxicillin (Temple and Nahata,
2000). Recently, L. monocytogenes has become resistant to antimicrobials like penicillin and
tetracycline indicating that future resistance could occur to commonly administered treatments
used for Listeria infections (Walsh et al., 2001). Prazak tested Listeria strains from water and
environmental samples for antibiotic resistance and determined that approximately 95% of the

9

samples were resistant to two or more antibiotics (Prazak et al., 2002). These findings suggest
that Listeria is acquiring antibiotic resistance reducing the effectiveness of current treatments.
2.5 Fresh-cut Produce Contamination Issues
Fresh produce is grown in non-sterile environments allowing pathogens to be present on
the produce and is ultimately being identified as the source of foodborne outbreaks. L.
monocytogenes has been recovered from minimally processed fruits and vegetables and survive
and even grow under refrigeration conditions. The microbiological contamination of fresh-cut
produce begins at food-processing facilities with increased efforts having been made to prevent
L. monocytogenes contamination before it occurs due to the implementation of the Food Safety
Modernization Act of 2010 (FDA, 2010).
Fresh-cut fruits and vegetables can be contaminated by pathogens during processing,
packaging, or storage (Nguyen-the and Carlin, 1994). The growth of pathogens on fresh-cut
produce depends on the level of microbial contamination during processing, the properties of the
pathogen and the fresh produce. Fresh produce is susceptible to contamination during handling
from unhygienic workers or improperly cleaned equipment. For example, contamination from
handlers primarily depends on handling time and nature of the produce. Processors need to focus
on personal hygiene of workers as excessive handling of produce can lead to potential crosscontamination and transmission of pathogens (Costa, 2015). In 1991, an outbreak caused by
Salmonella Poona infection on precut cantaloupes resulted in 400 illnesses suggesting improved
hygienic practices need to be adopted to prevent contamination at this level (CDC, 1991). Freshcut processors need to require workers in contact with produce to wear gloves and use hand dips
before entering processing facilities. Overall, the implementation of GMPs and sanitation
programs are important to curb poor worker hygiene.

10

During processing, equipment surfaces can be contaminated by inaccessible sites that
promote bacterial growth and attachment on the surface of fresh-cut produce. The slicing and
cutting of fresh produce results in the accumulation of liquid losses from the produce allowing
for growth of microorganisms and potential cross-contamination of pathogens on the equipment
surface. These slicing, dicing, and cutting processes can potentially lead to a seven-fold increase
in the population of pathogens in fresh-cut produce (Brackett, 1996). These findings
demonstrate that significant contamination occurs during minimal processing of fresh produce.
The application of sanitizer treatments and washing of fresh-cut produce is an important
step to reduce the microbial load. Thorough washing of fresh produce is a necessary
preventative measure to reduce pathogens as there are not any thermal treatments for raw foods.
Water is used in fresh-cut processing during washing, cooling, and rinsing. Previous studies
have documented that chlorinated wash water used in washing leafy greens has contained 103
bacteria/ml while the product contained 106 CFU/g (Nguyen-the and Prunier, 1989). This
finding has shown that contamination of produce can occur during washing and potentially
resulting in the transfer of pathogens during this process. If wash water is not properly sanitized,
it can be the source of contamination to any produce during processing. Furthermore, increasing
organic material in the wash water reduces the efficacy of the sanitizers in the wash water
indicating the importance of changing wash water to prevent potential contamination (Davidson
and Ryser, 2014).
Recently, the effects of packaging and storage temperatures have been evaluated on
fresh-cut fruits and vegetables. Fresh-cut produce is held at refrigerated temperatures to prevent
the growth of pathogens during storage. Temperature is known to alter the respiration rate of
produce in packages, changing the gaseous composition of the environment, and affecting the

11

growth rate of pathogens (Nguyen-the and Carlin, 1994). Storage of fresh-cut produce under
refrigerated temperatures inhibits the growth of pathogens especially psychotropic organisms.
Temperature abuse of fresh-cut produce leads to the proliferation of microorganisms as there is
significant decrease in the lag and generation times (Gil and Selma, 2006).
2.6 Outbreaks Associated with Melons
Fresh-cut melons have been increasingly implicated in foodborne outbreaks.
Cantaloupes and honeydew melons can be microbiologically contaminated during the preharvest,
processing, distribution, and preparation stages. Since 1994, there has been a significant increase
in foodborne outbreaks associated with cantaloupes primarily due to increased consumption of
raw produce in diets and its year round availability (Bowen et al., 2006). Melon outbreaks have
been predominantly linked to microbiological contamination from Salmonella species
accounting for 56% of the outbreaks (Walsh et al., 2013).
Recently, studies have been conducted to evaluate Salmonella and Listeria contamination
in cantaloupe production facilities in Mexico and United States. A survey of cantaloupes from
eight cantaloupe farms and packaging sheds determined that 1.8% of the samples were positive
for Salmonella (Castillo et al., 2004). In another survey for cantaloupe contamination in
California, there were no melons contaminated with Salmonella in the fields from 1999 to 2001
(Suslow, 2004). During 2013, environmental samples were collected from production field sites
in Arizona to detect for the presence of L. monocytogenes from air, soil, cantaloupe, water, and
rhizosphere biomass (Kumar et al., 2015). An evaluation determined S. enterica and L.
monocytogenes was primarily absent on cantaloupes and honeydew melons after pre-shipment
from production sites in Arizona and California from 2011 to 2014 (Suslow et al., 2015). The
FDA has imposed strict regulations on imported melons to the United States as these melons

12

have been linked to several foodborne outbreaks. The CDC determined that three consecutive
multistate outbreaks linked to imported cantaloupes from Mexican farms resulted in 155 cases
and two fatalities from 2000 to 2002 (CDC, 2002). The FDA concluded the source of those
outbreaks was primarily due to unsanitary conditions in the packaging facilities in Mexico. The
FDA conducted a survey comparing domestic and imported melons to assess the levels of
surface contamination on the melons (FDA, 1999). Overall, higher levels of contamination were
seen for imported as compared to domestic melons as pathogens were detected on 5.3% to 2.4%,
respectively. These findings led to the implementation of the “Melon Safety Plan” which called
for the use of chlorinated water to wash melons and any surfaces contacting melons during
transportation.
Melons especially cantaloupes have been the source of foodborne outbreaks associated
with Salmonella infections. Cantaloupes are inclined to surface contamination, because they
grow on the ground allowing pathogens to attach to the rind potentially leading to the
internalization and formation of biofilms (Annous et al., 2005; Fan et al., 2006). A study
determining the prevalence of Salmonella in produce in South Texas found that only cantaloupes
tested positive for Salmonella indicating its rind was more susceptible to surface contamination
compared to other fruits and vegetables (Duffy et al., 2005). The first melon outbreak associated
with Salmonella was recorded in 1955 when 17 cases were linked to the consumption of sliced
watermelon from a local retailer (Gaylor et al., 1955). In 1990, a multistate outbreak associated
with Salmonella Chester led to 245 cases including two deaths in 30 states (Ries et al., 1990).
This outbreak was determined to be caused by the transfer of pathogens from the rind to the
edible flesh during cutting as the melons were not washed prior to consumption. In 1991,
another multistate outbreak caused by Salmonella Poona was linked to contaminated cantaloupes

13

from Texas. This outbreak led to 400 cases in 23 states and two provinces in Canada (CDC,
1991). These foodborne outbreaks associated with melons have been predominately caused by
unsanitary washing of whole melons and temperature abuse to cut melons leading to the
proliferation of pathogens in the edible flesh.
L. monocytogenes was not implicated in any melon-associated cases of illness until the
2011 “Rocky Ford” cantaloupe outbreak in Colorado. This outbreak resulted in 147 illnesses
leading to 33 deaths and 1 miscarriage affecting consumers in 28 states (McCollum et al., 2013).
The genomic characterization of this L. monocytogenes strain consisted of 1/2a and 1/2b serotype
which shown unique traits associated with the 1/2a (Laksanalamai et al., 2012). The majority of
listeriosis cases were observed in immunocompromised individuals above the age of 60 (CDC,
2011). Based on FDA investigations, possible factors contributing to this outbreak included
improper facility design and packing facility equipment, and postharvest processing during the
cooling step causing the proliferation and survival of L. monocytogenes in cantaloupes (FDA,
2012).
Identification of potential sources of contamination during melon processing can be
helpful in future risk assessments to prevent the growth and survival of L. monocytogenes.
Adherence to strict food safety regulations and learning from past outbreaks can help the food
industry to prevent future outbreaks. Melon outbreaks occur more frequently during the summer
months while imported melons are the source of melon contamination during December to April
(Walsh et al., 2013). Multistate foodborne outbreaks have been more frequently linked with
cantaloupes compared to honeydew melons as shown in Table 2.1 and 2.2 (Danyluk et al., 2014).
Melon outbreaks have increased in recent years in both domestic and imported melons

14

suggesting increased food safety programs and implementation of strategies to prevent
contamination during the entire farm-to-fork continuum.
Year

Location

Pathogen

Location of
Consumption

Cases
(Deaths)

Food
Vehicle

1998

US (IA)

Norovirus

Restaurant

41(0)

2001

US (CO)

Norovirus

Restaurant

100(1)

2002

US (DC)

Staphylococcus
aureus

Not reported

8(0)

2003

US
(multistate)
US (CO)
US

68(2)

Honeydew,
strawberries*
Honeydew,
pineapple*
Honeydew,
cheese
(pasteurized),
potato (fried)*
Honeydew

39(0)
11(0)

Honeydew*
Honeydew*

Salmonella
Grocery store, hospital,
Newport
nursing home, restaurant
2003
Shigella sonnei
Hotel restaurant
2007
Salmonella
Private home, restaurant
Litchfield
*Denotes a suspected food vehicle which was epidemiologically linked

Table 2.1: Foodborne illness outbreaks associated with honeydew melons adapted from Danyluk
et al. (2014).

15

Year

Location

Pathogen

Location of
Consumption

Cases
(Deaths)

Food
Vehicle

1990

US (multistate)

Salmonella Chester

245(2)

Cantaloupe*

1991

Salmonella Poona

400(0)

Cantaloupe*

1997

US (IL, MI), and
Canada
US (CA)

24(0)

Cantaloupe

1997
1998

US (OR)
Canada (ON)

9(0)
22(0)

Cantaloupe*
Cantaloupe*

2000

US (multistate)

E. coli O157:H7
Salmonella
Oranienburg
Salmonella Poona

Restaurant salad
bars
Grocery stores,
restaurants
Private home,
grocery store,
restaurant
Restaurant
Supermarket

47(0)

Cantaloupe*

2000

US (MN)

Norovirus

Nursing home,
home care,
private home,
restaurant,
school
Workplace

33(0)

2001

US (OR)

Salmonella spp.

2(0)

2001
2001

US (multistate)
US (MN)

Salmonella Poona
Norovirus

Nursing home,
home care,
restaurant
Private home
Workplace

Cantaloupe,
sandwich
(turkey)*
Cantaloupe*

50(2)
42(0)

2001

US (WA)

Not reported

Restaurant

4(0)

2002

US (multistate),
Canada

Salmonella Poona

58(0)

2004
2005

US (Not reported)
US (UT)

E. coli O157:H7
Salmonella spp.

Nursing home,
home care,
private home
Not reported
Private home

6(0)
126(0)

2007

US (CA)

Private home

11(0)

2008

US (CO)

Salmonella
Litchfield
Salmonella Newport

Private home

5(0)

2008
2008

US (multistate)
US (multistate),
Canada
US (CA)
US (multistate)
US (multistate)
US (multistate)

Salmonella Saphra

Salmonella Javiana
Not reported
Salmonella
Hospital, private
Litchfield
home
2008
Norovirus
Restaurant
2011
Salmonella Panama
Private home
2011
L. monocytogenes
Grocery retailer
2012
Salmonella
Not reported: all
Typhimurium and
source from
Salmonella Newport
same farm
*Denotes a suspected food vehicle which was epidemiologically linked

Cantaloupe*
Cantaloupe,
pineapple*
Cantaloupe,
pineapple*
Cantaloupe*

Cantaloupe*
Cantaloupe,
chicken, corned
beef*
Cantaloupe*

10(0)
51(0)

Cantaloupe,
hamburger
meat*
Cantaloupe
Cantaloupe*

23(0)
20(0)
147(33)
261(3)

Cantaloupe*
Cantaloupe*
Cantaloupe
Cantaloupe

Table 2.2: Foodborne illness outbreaks associated with cantaloupes adapted from Danyluk et al.
(2014).

16

2.7 Listeria spp. in Produce
Listeriosis outbreaks are typically associated with dairy and meat products, but recently a
few were triggered by the consumption of fresh produce like cantaloupe and celery (CDC, 2011;
Gaul et al., 2013). L. monocytogenes has been detected and isolated in agricultural fields, soil,
water, fruits and vegetables, suggesting that contamination can occur during preharvest. After
2008, all six listeriosis outbreaks have been associated with produce grown in the United States
suggesting the possible source of contamination was the result of structural defects or inadequate
cleaning and handling of produce (Jackson et al., 2015). The majority of the outbreaks
associated with cantaloupes have been linked to Salmonella rather than Listeria. A few studies
have investigated the properties of Listeria to determine why it is a less frequent cause of
foodborne outbreaks. Ukuku demonstrated that the attachment ability of Listeria to the surface
of whole cantaloupes was less compared to Escherichia and Salmonella (Ukuku and Fett, 2002).
Furthermore, transfer studies from the rind to the edible melon flesh of cantaloupe indicated that
lower populations of Listeria were transferred compared with Salmonella (Ukuku et al., 2012).
Listeria outbreaks involving produce occur relatively sporadically, but minimal processing of
produce increases the products susceptibility to potential surface contamination.
Recently, outbreaks associated with fresh-cut produce have been steadily increasing with
increased availability of produce and consumption by consumers. Microbial contamination of
produce can occur at any stage during the farm-to-fork continuum, and it is difficult to
completely remove Listeria once contamination has occurred. L. monocytogenes is commonly
associated with fruits and vegetables due to its contact with the soil which could potentially
harbor any pathogenic microorganisms. Chemical disinfectants are only marginally effective in
reducing microbial contamination on the surface of fresh produce (Rees et al., 2013). Some

17

pathogenic organisms can penetrate and internalize in the plant tissue of produce which can
protect them from the application of sanitizers. L. monocytogenes can enter plant tissue by
though natural opening like the stomata or be taken up by the roots through water movement. If
Listeria enters the internal tissue, it would most likely occur during postharvest.
There are several risk factors associated with potential Listeria contamination in produce
fields emphasizing the importance of developing and implementing GAPs to prevent surface
contamination of minimally processed fruits and vegetables. The potential risk factors
associated with produce fields are difficult to pinpoint as there are many environmental factors
like climate, land interaction, and topographical variations (Strawn et al., 2013). A greater
understanding of the possible environmental risk factors can aid in changing field practices to
reduce contamination risks. One research group identified potential field practices that can
increase the likelihood of Listeria contamination which included manure application, presence of
wildlife, worker activity, quality of irrigation water, soil, and buffer zone presence (Strawn et al.,
2013). Furthermore, the timing of these environmental factors significantly impacted the
presence of Listeria contamination in produce fields. These findings can help farmers to
evaluate their current farming practices and implement preventative measures to minimize
contamination associated with harvesting.
2.8 Cantaloupe Rind Structure
The cantaloupe rind protects the interior flesh from contamination. However, unlike the
smooth surface of honeydew melon, the netted surface of cantaloupe allows pathogens to attach
to the rind, form biofilms on its surface and internalize into the edible flesh (Annous et al.,
2005). For example, the cantaloupe surface is netted allowing for more bacterial attachment sites
compared with the smooth surface of honeydew melons. The cantaloupe rind prevents the

18

effective removal of pathogens due to its netted surface (Annous et al., 2005). Furthermore, the
ability of pathogens to attach to the netted rind leads to higher transfer of pathogens by cutting
into the edible flesh, suggesting the importance of effective removal of pathogens from the
cantaloupe surface (Ukuku and Sapers, 2001). The contamination levels on honeydew melons
are lower compared with cantaloupe leading to reduced microbial transfer into the edible flesh
during cutting (Ukuku and Sapers, 2005). The netted rind of the cantaloupe indicates the
disruption of the cuticle leads to higher levels of pathogen internalization into the edible flesh in
the presence of contaminated water during cantaloupe processing. Overall, cantaloupe’s netted
rind allows for higher levels of bacterial attachment compared with the non-netted structure of
honeydew melon leading to different pathogen transfer rates to the edible flesh during
processing.
2.9 Sources of Contamination during Cutting
Cantaloupes are susceptible to higher levels of bacterial contamination due to their netted
surface. Contamination can readily occur during cutting when pathogens are transferred from
the rind by the knife to the edible melon flesh (Beuchat, 1996). Even after the application of
sanitizers to the melon rind, bacterial transfer can still occur by cutting but at lower levels
compared without prior sanitization. A study conducted by Selma demonstrated that fresh-cut
cantaloupe pieces were still positive for E. coli when the rind contained 4.3 to 8.3 log CFU/g
before cutting. However, E. coli was not detected in the fresh-cut cantaloupe when the rind
contained 3.3 log CFU/g (Selma et al., 2008). The potential transfer of pathogens to the edible
flesh is the result of contamination that has occurred during production as there are no further
microbial treatments to reduce bacterial transfer introduced by cutting. Previous bacterial
transfer studies involving cantaloupe and honeydew melons indicated that approximately 3.3 and

19

2.0 log/CFU/g of aerobic mesophilic bacteria were transferred to fresh-cut pieces during cutting
(Ukuku and Fett, 2004). Recently, a quantitative microbial risk assessment (QMRA) was created
for L. monocytogenes which concluded that the risk of consuming fresh-cut cantaloupes was tenfold higher compared to whole cantaloupes (Wang et al., 2015). Furthermore, the manner of
cantaloupe processing can affect the transfer of pathogens as it was determined that peeling the
rind before cutting resulted in less contamination instead of first cutting then peeling (Castillo,
2009). Presently, there is limited data on the transfer of pathogens during peeling, slicing, and
dicing of melons during processing.
2.10 Commercial Slicing and Dicing of Fresh-Cut Produce
Foodborne outbreaks have been attributed to the commercial processing of fresh-cut
produce as bacterial transfer can occur during processing. Slicing and dicing of fruits and
vegetables can lead to the transfer of L. monocytogenes from the surface of contaminated
produce to subsequent uncontaminated produce during processing. Commercial slicing and
dicing can lead to potential cross-contamination of pathogens during processing due to
contamination of blades and pushers of the equipment. Currently in the food industry, there are
no standardized practices for slicing or dicing fruits and vegetables as companies employ manual
cutting with knives or semi-manual practices with hand-operated slicers to make fresh-cut
produce (Wang and Ryser, 2014).
Previous studies have indicated that inoculated fresh produce can contaminate subsequent
sliced/diced produce due to contamination of the slicer’s blades and pusher resulting in extended
bacterial transfer during processing. One study comparing the transfer of Salmonella during
manual slicing and electric slicing of tomatoes showed that Salmonella transfer was greater with
the manual slicer as it decreased 2.7 and 4.4 log CFU/tomato, respectively (Wang and Ryser,

20

2013). Another study assessing transfer of L. monocytogenes during mechanical slicing of
onions indicated that bacterial transfer was quantifiable up to the 20th onion with 0.3 log CFU/g
when one onion was initially inoculated to contain 6.1 log CFU/g (Scollon et al., 2013). A study
on the dicing of celery showed that L. monocytogenes easily spread from inoculated to
previously uninoculated celery during processing (Kaminski et al., 2014). These studies clearly
show that bacterial transfer can occur from contaminated fresh-cut produce to subsequent
uncontaminated fresh-cut produce during slicing and dicing and reinforce the importance of
applying sanitizers to slicers and dicers during processing to minimize the transfer of pathogens
to fresh-cut produce.
2.11 Contamination of Melons
2.11.1 Preharvest Contamination of Melons
The factors responsible for contamination of melons are due to indirect and direct contact
with fecal material in the preharvest environment. Potential indirect routes of fecal
contamination result from irrigation water, airborne particles from animal production, and field
processing (Ukuku et al., 2005). Irrigation water has a high risk of potentially contaminating
fruits and vegetables. The CDC indicated that contamination of cantaloupe rinds occurred from
contact with contaminated water during the production and handling (CDC, 2002). The survival
of L. monocytogenes in soil depends on several factors including soil type, presence of decaying
vegetation, and climate conditions (Sauders and Wiedmann, 2007). Furthermore, direct fecal
contamination can occur by wild animals like reptiles, birds, and rodents defecating in the fields
and on the produce, respectively (Geldreich and Bordner, 1971). The external rind of cantaloupe
has a porous, netlike structure allowing for the attachment of microorganisms making it difficult
to remove during the cleaning process with sanitizers or antimicrobial agents (Park and Beuchat,

21

1999). Consequently, watermelon and honeydew melon are less likely to microbial attachment
due to their smoother surfaces compared to cantaloupe. Also, poor hygiene of field workers
during harvesting can lead to the contamination of melons. The outbreaks associated with
melons during the preharvest stage have led to the formation and implementation of Good
Agricultural Practices (GAP) which reduces the risk of contamination.
2.11.2 Postharvest Contamination of Melons
Melons can be contaminated during several stages of postharvest processing like
handling, packaging, transportation, and distribution of the product as depicted in Figure 2.1
(FDA, 2009). Typically, cantaloupes and other melon varieties are field packaged and shipped
directly to the market without prior washing. Field packaging of cantaloupes reduces the risk of
contamination by limiting the transfer of pathogens to other cartons by not introducing water. It
is critical that field packaging equipment and facility packinghouses are sanitary to prevent the
microbial load from increasing during processing (Castillo et al., 2004). Furthermore, FDA
strongly recommends that packers clean, sanitize, and remove pests from packing equipment
before operations begin.
If melons are shed-packaged, the melon dump operations and cooling methods can be
potential sources of contamination. In packinghouse operations, melons are transferred from
bins either by dry or water dump operations possibly leading to cross-contamination between
melons or from the equipment surface. The purpose of washing the melons during the rinsing
process is to remove soil or any organic matter instead of removing pathogens from the
cantaloupe surface. Some facilities have water systems that recycle water which increase the
risk of potential contamination of produce. After the melons are removed from the dump
operations, the melons are cooled by either forced-air cooling or chilled water drench which

22

could result in approximately a two to three-fold reduction of pathogens on the surface of the
melon (Annous et al., 2004). Furthermore, melons should be cooled promptly after harvesting to
minimize any pathogen growth on the surface of the melons. During the distribution and
transportation of melons, processers need to follow Good Manufacturing Practices (GMPs) to
minimize the cross-contamination and growth of pathogens. Fresh-cut melons should be stored
at refrigeration temperatures between 0 to 5oC to prevent the growth of pathogenic
microorganisms (FDA, 2009).
Several outbreaks associated with melons have occurred at food service establishments
due to contamination during cutting and peeling. Previous studies have determined that
microbial contamination from the rind can be transferred by a knife while cutting into the edible
portion of the melon posing a hazard to consumers (Ukuku and Fett, 2002). The FDA suggests
that melons with external damage or decay be disposed as there is an increased risk of
contamination from foodborne pathogens in the edible flesh (FDA, 2009). Furthermore, retail
employees should wash their hands and not directly touch cut melons to prevent crosscontamination. All equipment that comes in contact with the melon should be thoroughly
washed and sanitized prior to use. Overall, these guidelines are helpful in assessing risks
regarding transfer of pathogens while cutting.

23

Figure 2.1: Flow diagram of melon processing from field to consumer for retail and foodservice
establishment using field pack and shed pack methods adapted directly from the Food and Drug
Administration (FDA, 2009).
2.12 Industry Practices to Minimize Contamination during Melon Processing
Governmental agencies and food industries understand the importance of maintaining the
safety of melon processing during each stage in the farm-to-fork continuum. It is essential that
Good Manufacturing Practices (GMPs), Standard Operating Procedures (SOPs), and Sanitation
Standard Operating Procedures (SSOPs) be strictly followed during melon production,

24

processing, packaging, distribution and retail display because each stage can introduce microbial
contamination. Some major sources of microbial contamination occur primarily due to
temperature abuse during storage and infected food processors. Preventative measures should be
implemented to prevent contamination of melons; because once contaminated during production
pathogens may be difficult to remove at each successive step.
Melons are primarily grown in non-sterile environments that can allow pathogens to
attach to the rind. Cantaloupes and honeydew melons are in contact with soil, irrigation water,
and animal feces. The FDA recommends melon producers conduct environmental assessments
and evaluate field practices involved with melon production to determine any possible sources of
contamination. The agency suggests producers determine the presence of wildlife, flooding
possibilities, and land history as these are environmental factors that could introduce pathogens
during initial production (FDA, 2009). For example, the geographical features of the melon
production site can introduce potential contamination due to water runoff from other fields,
waste hazards, or production facilities. Climate conditions of excess rainfall or prevailing winds
could transport pathogens from the soil to the cantaloupe rind. Also, humidity can increase the
activity of animals around the production sites and this climate favors the growth of pathogens.
Furthermore, melon production sites should be properly maintained to minimize these risk
factors.
The establishment of proper hygienic policies for workers to follow during melon
production and processing is essential to minimize the transfer of pathogens. Companies need to
have GMPs and SOPs for worker health and hygienic standards. It is vital that workers who
have direct contact with cantaloupes either in the production and processing stage maintain a
high level of personal hygiene. These workers should refrain from actions that could

25

contaminate cantaloupes like sneezing or spitting in the production or handling sites. Any
worker showing symptoms of foodborne illness should be relieved of their duties. Also, sanitary
facilities should be located away from the production sites and readily available to workers to
prevent any human contamination in the fields.
The characteristics of the surface of the melon rind have played a significant role in
pathogens adhering and growing on the rind. Cantaloupes have been the source of the most
foodborne outbreaks in melons due to their netted rinds (Harris et al., 2003). Melons with netted
surfaces are more susceptible to microbial contamination as it is difficult to remove pathogens
from the surfaces compared with smoother surfaces (Ukuku and Fett, 2002). The FDA strongly
suggests to minimize risk factors associated with melon production and processing, because once
surface contamination occurs it is difficult to remove pathogens on melons with netted rinds.
Also, the stem scar can be a source of contamination as it may allow for the infiltration of
pathogens into the edible melon flesh (Richards and Beuchat, 2004). Recently, a study by
Macarisin et al. (2015) used dye to mimic L. monocytogenes during hydro-cooling showed
pathogen infiltration occurred at the stem scar resulting in the contamination of the hypodermal
mesocarp and calix of the cantaloupe. Melon processors should focus on implementing and
adhering to postharvest practices that minimize contamination especially on the stem scar.
In postharvest operations, sanitation programs have to be properly enforced during field packing
and in the packaging facilities to minimize surface contamination of melons. FDA recommends
melon processors to determine if the cleaning processes of removing pathogens from the melons
are potentially a source of contamination (FDA, 2009). It is essential that all field packaging
equipment be properly cleaned especially any surfaces in contact with the melons to prevent any

26

cross-contamination. Also, field packing equipment needs to be protected from any potential
pest infestations to minimize pathogen exposure.
A possible source of cross-contamination during melon processing occurs during the
melon dump operation when melons are transferred from field containers to dry or water dump
operations. In this operation, contamination can occur between produce, water, and equipment
surfaces (Akins et al., 2005). FDA recommends that packinghouses find alternative methods for
transporting melons from the containers to dump units to reduce the transfer of pathogens.
Furthermore, processing equipment that is in contact with melons should be regularly cleaned
and sanitized to reduce contact contamination. If melons are transferred to wet dump units, it is
critical that the water is monitored for disinfectant levels to ensure that the water is not
contaminated from pathogens from the surface of the melon rind as it could recontaminate
incoming melons. Also, the time that melons are in the wet dump units should be limited to
prevent any cross-contamination. If the melons are hot from field heat when dumped into cold
wet dump tanks, the large temperature differential created can lead to infiltration of pathogens.
Therefore, pre-cooling of the cantaloupes or warming of the water is necessary before dump tank
washing.
After melon dump operations, melons have to be cooled to minimize microbial growth on
the rind. Melons are typically cooled by flume immersion, a chilled water drench, or forced-air
cooling (Park and Beuchat, 1999). For forced-air cooling, the FDA strongly recommends that
the equipment be regularly sanitized to prevent cross-contamination. If the melons are cooled by
submersion in cold water, processors need to decrease the contact time to reduce potential
infiltration by pathogens into the edible flesh due to the increased temperature gradient. Any
delays in cooling could allow for the multiplication of pathogens on the surface of the rind

27

(Behrsing et al., 2003). To prevent this issue, operators need to enforce regulations that ensure
proper cooling to reduce moisture as this could lead to the proliferation of pathogen growth on
melons.
During fresh-cut operations, the edible melon flesh can be contaminated during cutting,
slicing, dicing, or peeling from surface microbial contamination. Whole melons with any visible
damage to the rind should not be used for fresh-cut produce as they have a higher risk of
transmitting pathogens to the edible melon flesh. Also, thorough washing of melons prior to
cutting or peeling operations can reduce microbial surface contamination by 2 to 3 logs (Park
and Beuchat, 1999). Other methods to remove potential pathogens from the surface include
scrubbing the crevices with sanitizer or applying hot water or steam to reduce pathogens on the
rind. The removal of pathogens from the surface primarily depends on the sanitizers and contact
time with the melons, but excessive contact time can lead to the infiltration of pathogens through
the stem scar or rind into the edible melon flesh. During fresh-cut processing, knives should be
regularly sanitized to reduce the likelihood of pathogens being transferred to the melon flesh.
Refrigerated storage of fresh-cut and whole melons is necessary to maintain quality and prevent
the growth of pathogens during distribution. FDA recommends that fresh-cut melons be held at
0 to 5oC to prevent growth of pathogens (Del Rosario and Beuchat, 1995). Other whole melon
varieties are stored at different temperatures to maintain optimal quality as cantaloupes and
honeydew melons are stored from 2.2 to 5oC and 7 to 10oC, respectively.
Recently, foodborne outbreaks have been caused by user handling in retail and
foodservice operations by accidental contamination to edible melon flesh during cutting or rind
removal. Furthermore, a survey indicated that approximately 6% of the population does not
wash their produce and 35% do not wash melons before consumption (Li-Cohen and Bruhn,

28

2002). These findings suggest that more educational programs need to emphasize safe handling
and preparation of fresh produce before consumption to prevent possible foodborne outbreaks.
The FDA recommends the following guidelines during the preparation of fresh-cut melons to
minimize microbial contamination. It is recommended that before cutting melons that
individuals wash their hands thoroughly to prevent transfer of human pathogens. Melons that
have any visible damage or decay to the rind should not be used to prepare fresh-cut melons.
The FDA Food Code recommends that the outer surface of melons be washed under cool water
before being cut or added to other ingredients in retail establishments (FDA, 2009). Fresh-cut
melons should be immediately refrigerated between 0 to 5oC and should be consumed within
seven days.
2.13 Temperature
Temperature abuse during storage of fresh-cut melons can increase microbial growth and
amplify the risk to consumers. The FDA recommends that fresh-cut melons be stored between 0
to 5oC to prevent the proliferation of foodborne pathogens (FDA, 2009). Previous studies have
indicated that higher levels of contamination on the rind can result in increased pathogen transfer
allowing melon flesh to be contaminated and grow during refrigeration if temperature abuse
occurs during storage (Golden et al., 1993). In a study conducted by Ukuku and Fett (2002)
assessing L. monocytogenes growth after the direct application of sanitizers like chlorine or
hydrogen peroxide, L. monocytogenes growth was inhibited at 4oC until 15 days of storage while
populations reached up to 4.9 logs at 8 and 20oC after 15 days, respectively. In another
experiment, the growth of pathogens in fresh-cut melons increased significantly between 4 to 6 h
of incubation at 25oC reaching 6.8 logs, but no significant change was observed on fresh-cut
cantaloupes at 5oC (Delrosario and Beuchat, 1995). In a study assessing the effects of storage

29

temperature of fresh-cut melons inoculated with Salmonella populations remained unchanged
and decreased 1 log at 5oC during 12-day storage for cantaloupe and honeydew melon,
respectively (Ukuku and Sapers, 2007). Also, statistically greater growth of Salmonella was
seen at 10 and 22oC compared to 5oC during 12 days of storage. During storage of fresh-cut
cantaloupe and honeydew melon, Salmonella populations increased from 2.0 to 3.6 log and 1.9
to 3.0 log during storage at 10oC, respectively (Ukuku and Sapers, 2007). Overall, it is critical to
store fresh-cut melons at temperatures between 0 to 5oC to minimize pathogen and reduce the
risk of infection to consumers.
Delayed storage time can also lead to increased microbial growth. In one study,
Salmonella populations increased 0.3 log and 1.3 log when the fresh-cut melons were held at
22oC for 3 and 5 hours before storage at 5oC, respectively (Ukuku and Sapers, 2007). The study
indicated that holding fresh-cut melons at room temperature for an extended period of time leads
to proliferation of pathogens, and suggests that fresh-cut melons should be stored immediately at
5oC to prevent further microbial contamination. Recently, a QMRA conducted on L.
monocytogenes on fresh-cut cantaloupes and determined temperature at retail and household
storage after cutting was the most significant factor in the risk assessment (Wang et al., 2015).
Overall, these scientific studies have led to guidelines being adopted in the U.S.A. to minimize
proliferation of foodborne pathogens by storing fresh-cut produce below 5oC. (FDA, 2009).
2.14 Application of Sanitizers to Reduce Contamination
2.14.1 Application of Sanitizers on Surface of Melons
In the food industry, melons are initially washed with water before sanitizers are applied
to the surface of the melons. The cantaloupe netting poses some difficulties in the removal of
pathogens from the rind even after the application of chemical sanitizers. Sanitizer efficacy

30

varies in the removal of pathogens from the rind of cantaloupes. Recently, a study determined
spraying octenidine hydrochloride on cantaloupes at preharvest resulted in a 2 log CFU/cm2
reduction of Listeria (Keelara et al., 2015). A study by Craigshead et al. (2015) assessed the
efficacy of Bacillus subtilis UD 1022 on cantaloupe rinds as there was a reduction of 2 to 3 log at
22oC which are possibly encountered in temperature abuse conditions during storage and
transport. For example, a study conducted by Rodgers indicated that pathogen reduction on
cantaloupe rind was approximately 6 log CFU/g when dipped in a solution consisting of chlorine
dioxide, ozone, chlorinated trisodium phosphate, and peroxyacetic acid (Rodgers et al., 2004).
There have been conflicting results from other studies suggesting that bacteria removal from
cantaloupe rind is relatively difficult. For example, Ukuku and Fett (2004) observed a reduction
of 2.5 log when cantaloupes were washed in 2.5% or 5.0% hydrogen peroxide. Another study by
Alvarado-Casillas determined that there was a reduction of up to 2.9 log when cantaloupes were
dipped in hypochlorite at a concentration of 1000 mg/L (Alvarado-Casillas et al., 2007).
Researchers have determined that immersing cantaloupes in hot water at 90oC for 60 seconds can
effectively reduce pathogens up to 4 logs while maintaining the integrity of the rind (Ukuku and
Fett, 2004). A study conducted by Ukuku et al. (2004) determined the presence of natural
microflora on the cantaloupe rind inhibits the growth and survival of L. monocytogenes
suggesting that possible recontamination can occur if natural microflora is removed by the
application of sanitizers. Recently, a nisin-based sanitizer was effective in the reduction of L.
monocytogenes transfer from the cantaloupe rind positive only by enrichment (Ukuku et al.,
2015). Sanitizers reduce the potential contamination on the surface of melons, reducing the
potential pathogens transferred from the rind to the edible melon flesh ultimately reducing a
consumer’s risk for illness.

31

2.14.2 Inactivation Treatments for Surface of Cantaloupes
Recently, new inactivation treatments were evaluated to determine if there was
significant reduction of L. monocytogenes on the cantaloupe surface while still maintaining the
quality of the melon. A study performed by Mahmoud evaluated the effects of X-ray treatments
on L. monocytogenes and quality of the whole cantaloupe. It was determined 5 log reduction of
L. monocytogenes occurred when 2.0 kGY X-ray was exposed on the cantaloupe rind while color
and firmness of the melons were maintained during storage at 22oC for 20 days (Mahmoud,
2012). The use of alginate coatings with 2.0 % cinnamon bark oil and 0.5% soybean oil on the
surface of cantaloupes resulted in the reduction of L. monocytogenes to 1.3 log CFU/cm2 while
also preventing any color change or softening of the melon during 15-day storage (Zhang et al.,
2015). Kaminski performed surface pasteurization of cantaloupes by using a heated wash at
65oC for 45 seconds or heated wash followed by an application of peroxyacetic acid spray that
resulted in a reduction in L. innocua of 2.3 to 4.3 log CFU/cm2 (Kaminski et al., 2015).
Mukhopadhyay et al. (2015) applied high pressure processing of 500 MPa at 8oC to achieve a 6
log reduction of L. monocytogenes in cantaloupe puree which had minimal impact to the quality.
Overall, these inactivation treatments have demonstrated significant reduction of pathogens on
the rind and edible flesh while maintaining the quality of the cantaloupe.
2.15 Food Safety Regulation to Minimize Cantaloupe Contamination
Regulatory agencies like the Food and Drug Administration (FDA) impose strict
legislation on the importation and distribution of cantaloupes to the ensure safety of imported
products. These agencies realize that food safety programs are necessary to implement
preventative strategies to minimize the risks associated with melon processing. Furthermore,
regulatory agencies understand that strict enforcement of GMPs is essential to eliminate potential

32

microbial contamination on melons; because once contaminated removal of these pathogens with
sanitizers can no longer be assured. The FDA considers fresh-cut cantaloupe to be a potentially
hazardous food due to its ability to support bacterial growth because of its high water activity
and mild acidity (Bhagwat, 2006). This finding suggests that the contamination of cantaloupes
can occur at any stage of postharvest melon processing.
The Salmonella outbreaks traced to cantaloupe that included two deaths in California
(Green et al., 2005) led the FDA to ban the importation of cantaloupes from Mexico during 2000
to 2002. These contamination events have led to economic repercussions against Mexican
cantaloupe exporters as the production of Mexican cantaloupes declined by 92% and only made
up 3% of all U.S. cantaloupe imports (SAIP-SAGARPA, 2002). The cantaloupe import ban
from Mexico has improved food safety standards, but not all growers are restricted by the
legislation as growers are judged by their past safety performance (Anonymous, 2005). Overall,
the majority of outbreaks associated with cantaloupes are due to cross-contamination and direct
contact of pathogens. The documentation and implementation of regulatory actions has instilled
the practice of GMPs in melon processing to prevent microbial contamination and future
outbreaks.

33

CHAPTER 3: Extent of Listeria monocytogenes Transfer during Cutting of Cantaloupe and
Honeydew Melon

34

3.1 Materials and Methods
3.1.1 Experimental Design
One cantaloupe or honeydew melon was inoculated with a 3-strain avirulent cocktail of
L. monocytogenes (J22F, J29H, and M3), stored at 4 or 30oC for 24 h mimicking temperatures
encountered in processing facilities or field heat, respectively. After inoculation, melons were
cut through the stem scar, blossom scar, and rind midpoint (i.e., between the stem and blossom
scar) with a sterile, stainless steel cork borer to obtain five 25 mm length and 18 mm diameter
core samples in each region. All core samples were aseptically cut into five 5 mm-long pieces,
yielding five separate depth sections (0-5 mm, 6-10 mm, 11-15 mm, 16-20 mm, and 21-25 mm).
Each depth section was sampled using composite sampling from the 5 core samples taken from
each separate rind location. All experiments were performed in three replicates.
3.1.2 Produce
Cantaloupes (5- inch diameter) (Cucumis melo var. cantalupensis) and honeydew melons
(6 – 7 inch diameter) (Cucumis melo var. inodorus) free of any visible bruising or defects were
purchased from a local retailer, stored in 4oC refrigerator and used within 2 days of purchase.
Melons were washed under running tap water to remove any debris before any experimentation
proceeded.
3.1.3 Bacterial Strains & Inoculation
Three avirulent strains of Listeria monocytogenes (J22F, J29H, and M3) were obtained
from the laboratory of Dr. Sophia Katharian (North Carolina State University, Raleigh, NC).
J22F is a transposon mutant of H7550 and purine biosynthesis (purB) that are attenuated for
systemic infection. J29H is a non-hemolytic, transposon mutant of H7550 and hly gene. M3 is a
non-hemolytic, transposon mutant consisting of TN916 transposon in the hly gene inhibiting

35

Listeriolysin O (LLO) secretion. Stock cultures were maintained at -80oC in trypticase soy broth
containing 0.6% (w/v) yeast extract (TSBYE; Becton and Dickinson, Sparks, MD) and 10%
(v/v) glycerol (Sigma Chemical Co., St. Louis, MO). The stock cultures of each strain were
streaked onto trypticase soy agar containing 0.6% (w/v) yeast extract (TSAYE; Becton and
Dickinson) and incubated at 37oC for 24 h to prepare working cultures. Single colonies from the
TSAYE plates were subjected to two successively transfers in 9 ml of TSBYE at 37oC for 24 h.
The cultures were combined in equal volumes to obtain a 3-strain avirulent L. monocytogenes
cocktail containing 107 and 109 CFU/ml for cantaloupe inoculation and 109 CFU/ml for
honeydew melon inoculation, respectively. Cantaloupe and honeydew melons were dipinoculated in 4.5 L L. monocytogenes cocktail. Cantaloupe inoculation levels reached
approximately ~6 log CFU/cm2 and ~4 log CFU/cm2. Honeydew melon inoculation levels
reached near ~5 log CFU/cm2. Intact inoculated melons were air-dried for 1 h in a biological
safety cabinet and stored at 4 and 30oC for 24 h to simulate processing plant and field
temperatures. Prior to experimentation, rind concentrations of L. monocytogenes were confirmed
by direct plating.
3.1.4 Cantaloupe and Honeydew Melon Coring
An 18-mm diameter sterile, stainless steel cork borer was used to obtain five 25 mm
length core samples from each of the blossom scar, stem scar, and circumference regions of the
rind depicted in Figure 3.1. After aseptically cutting each core sample into five 5 mm-long
pieces (0-5 mm, 6-10 mm, 11-15 mm, 16-20 mm, and 21-25 mm), each of the five pieces from
the same depth and regions were composited in a sterile Whirl-Pak bag (Nasco, Fort Atkinson,
WI) and assessed for numbers of L. monocytogenes within 2 h.

36

Figure 3.1: The coring locations at the A) circumference of the rind B) blossom scar and C)
stem scar region.
3.1.5 Comparison of Coring Tools
Experiments were performed on cantaloupe alone using the 18-mm diameter sterile
stainless steel cork borer and 18-mm diameter sterile stainless steel apple borer. The apple borer
had a serrated blade section, corer head which was 30-mm in length and 18-mm in diameter and
is pictured in Figure 3.2. The cork borer and apple corer were used to obtain five 30 mm-long
core samples from the circumference of the rind to determine any potential differences in transfer
between the two coring tools at 4oC.

37

Figure 3.2: “Farberware Classic Apple Corer.” Amazon. n.p., n.d. Web. 24 Oct. 2016.
3.1.6 Microbiological Analysis
The five cantaloupe or honeydew melon pieces weighing ~ 25 g were diluted 1:2 in
sterile phosphate-buffered solution (PBS), homogenized in a stomacher (Stomacher 400
Circulator, Seward, Worthington, UK) at 260 rpm for 1 min, serially diluted in PBS and then
plated with or without membrane filtration using 0.45-Âľm-pore-size membrane filters (Millipore
Corp., Billerica, MA) on Listeria differential agar base, trypticase soy agar containing 0.6%
yeast extract, 0.1% esculin, and 0.05% ferric ammonium citrate (FAC), or Listeria
differential/selective agar base, Modified Oxford Agar (MOX, Neogen Corporation, Lansing,
MI) consisting of moxalactam and colistin sulfate as selective agents to obtain black Listeria
colonies from esculin hydrolysis which were counted after 24 to 48 h of incubation at 37o C. Any
samples negative for Listeria by direct plating were subsequently enriched for 48 h and then
streaked on Modified Oxford Agar (MOX).
3.1.7 Surface Population Calculations
The surface population of L. monocytogenes on cantaloupe and honeydew rinds were
determined by Eq. (1):

38

N=

CxD
nxA

where N is the Colony Forming Units per square centimeter (CFU/cm2), C is the number of
colonies plated, D is the dilution factor, n is the number of surface rind samples (n = 5), and A is
the rind surface area sampled (A = r2) in square centimeters.
The population of L. monocytogenes on the side surface area of the cantaloupe and
honeydew melon flesh cores were determined by Eq. (2):

N=

CxD
nxS

where N is the Colony Forming Units per square centimeter (CFU/cm2), C is the number of
colonies plated, D is the dilution factor, n is the number of core melon flesh samples (n = 5), and
S is the surface area of each corer section, side surface area, (SA = 2rh) of melon flesh in
square centimeters.
3.1.8 Statistical Analysis
All experiments were performed in triplicate and reported as log CFU/cm2. Results were
analyzed using a two-way mixed design analysis of variance (ANOVA) in SAS 9.4 (SAS
Institute Inc., Cary NC) were the temperature was designated as between factor and depth as
within factor. The Tukey’s test was used to determine significant differences in L.
monocytogenes transfer between the five core depths, three rind regions, and two temperatures
during simulated cutting with a cork borer. The Tukey’s test was used to determine if significant
differences in coring methodologies between the cork borer and apple corer were present.
Different letters above the bars from Tukey’s test comparisons indicate statistical significance (P
< 0.05) between surface population means.

39

3.2 Results
3.2.1 L. monocytogenes populations on rind surfaces of cantaloupe and honeydew melon
Higher populations of L. monocytogenes were attached to the surface of dip-inoculated
cantaloupe compared to honeydew melon (P < 0.05) (Figure 3.3). L. monocytogenes populations
on the cantaloupe rind were not significantly different at any localized regions of the stem scar,
blossom scar, or circumference of rind (P > 0.05). For honeydew melon, significantly lower L.
monocytogenes were seen on the rind circumference compared to the blossom and stem scar
regions (P < 0.05). A two-way ANOVA examined the effect of melon type and localized
regions of the rind, blossom scar, and stem scar on the attachment of L. monocytogenes on the
rind surface in Figure 3.3. There was a significant interaction between the melon type and
localized region on surface populations of L. monocytogenes on the rind surface (P < 0.05). The
analysis of the fixed effects indicated there were significantly greater attachment of L.
monocytogenes on cantaloupe compared to honeydew melon (P < 0.05). There were significant
differences in the attachment of L. monocytogenes in the localized regions of the rind surface (P
< 0.05).

40

8.0

A

Populations (log CFU/cm2)

AB
ABC

6.0

BC

C
D

4.0

2.0

0.0
Cantaloupe
Stem Scar

Honeydew
Stem Scar

Cantaloupe
Blossum Scar

Honeydew
Blossum Scar

Cantaloupe
Rind

Honeydew Rind

Figure 3.3: Mean (ÂąSE) L. monocytogenes populations on surface of cantaloupe and honeydew
melon at the stem scar, blossom scar, and rind dip-inoculated and stored for 24 h at 4oC.
Different letters above the bars from Tukey-Kramer HSD comparisons indicate statistical
significance (P < 0.05) between surface population means.
3.2.2 L. monocytogenes transfer from rind to cantaloupe and honeydew melon flesh
After surface inoculation, the difference in L. monocytogenes populations on the rind
affected the numbers of L. monocytogenes to the edible flesh during cutting. There was ~3
log/cm2 reduction in L. monocytogenes populations transferred from the rind to the 5 mm melon
flesh depth (P < 0.05) (Figure 3.4 – 3.12). By assessing the factor of depth, these results indicate
there is a significant difference of populations of L. monocytogenes transferred due to cutting
depth after the depth zero, rind, populations (P < 0.05). However, core samples taken from the
same region, similar L. monocytogenes populations were seen at the five different depth
populations after the rind to 5 mm population decreases were observed (P > 0.05).
3.2.3 Impact of melon region and temperature on L. monocytogenes transfer
For honeydew melon, greater L. monocytogenes transfer occurred for core samples taken
from the stem and blossom scar regions compared to the circumference of rind (P < 0.05)
(Figure 3.4 – 3.6). A two-way ANOVA was performed to determine the effect of temperature
41

and depth on the transfer of L. monocytogenes by cutting. There was no statistically significant
interaction between temperature and depth for cutting in both cantaloupe and honeydew melons
within the depths of the rind, blossom scar, and stem scar (P > 0.05). An analysis of the fixed
effects at the stem scar of the cantaloupe showed there was a significant difference of the
bacterial transfer due to temperature and depth (P < 0.05). However, the effects at the rind
circumference and blossom scar indicated that there was a statistically significant difference in
transfer of L. monocytogenes due to depth (P < 0.05). There was no significant difference due to
storage temperature at either 4 or 30oC (P > 0.05).
However, these same differences were not seen for cantaloupe (Figure 3.7 - 3.9). All
core samples yielded statistically similar Listeria populations for cantaloupe cut at 4 or 30oC (P >
0.05). By comparing the cutting of cantaloupe at 4 and 30oC by two-way analysis, there was no
significant difference in L. monocytogenes populations transferred by temperature (P > 0.05).
There was statistical difference in the transfer of L. monocytogenes due to depth in the stem scar
region (P < 0.05); however greater L. monocytogenes transferred by cutting in the blossom scar
and circumference of the rind at 4oC compared with 30oC (P < 0.05). There was also a
significant difference in the transfer of L. monocytogenes due to depth (P < 0.05).
There was a reduction in enumerated cells from the surface inoculated L. monocytogenes
populations to the edible melon flesh in all the transfer studies (Figure 3.4 – 3.12) performed at 4
and 30oC. The initial surface contamination ranged from 5.2 to 7.0 log CFU/cm2 in cantaloupe
(P > 0.05) and 3.4 to 5.2 log CFU/cm2 in honeydew melon (P < 0.05). The level of L.
monocytogenes transfer from rind surface to 5 mm depth varied from 2.8 to 3.6 log CFU/cm2 in
cantaloupe (P < 0.05) and 1.1 to 1.8 log CFU/cm2 in honeydew melon (P < 0.05) at 4oC. The
level of L. monocytogenes transfer from the rind surface to 5 mm depth varied from 1.9 to 4 log

42

CFU/cm2 in cantaloupe (P < 0.05) and 0.7 to 1.8 log CFU/cm2 in honeydew melon (P < 0.05)
stored at 30oC. There was no statistical significant difference observed in the transfer
populations compared with the storage temperature conditions of either the cantaloupe or
honeydew melons (P > 0.05).
Transfer of L. monocytogenes between the various depths varied without any statistical
significance observed between the depths (P > 0.05). L. monocytogenes was detected at the
depths of 5 mm to 25 mm from 1.2 to 4.0 log CFU/cm2 in cantaloupe and 0.2 to 2.1 log CFU/cm2
in honeydew melon at 30oC (P > 0.05). L. monocytogenes was detected at the depths of 5 mm to
20 mm from 2.2 to 3.6 log CFU/cm2 in cantaloupe and 0.3 to 1.8 log CFU/cm2 in honeydew

Populations (log CFU/cm2)

melon stored at 4oC (P > 0.05).

6

A

A

5
4
3

BC BC

2

B
BC BC

BC

BC

C

1
0
-1

Surface

5 mm

10 mm

15 mm

20 mm

Depth (mm)

Figure 3.4: Mean (ÂąSE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the stem scar.

43

Populations (log CFU/cm2)

6

A

5

A

4
3

B

2

B

B

B

B

B

B

B
B

B

1
0
Surface

5 mm

10 mm

15 mm

20 mm

25 mm

Depth (mm)

Figure 3.5: Mean (ÂąSE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the blossom scar.

Populations (log CFU/cm2)

5

A

4

A

3

B

B

2

B

B

B

B

B

B
B

B

1
0
Surface

5 mm

10 mm

-1

15 mm

20 mm

25 mm

Depth (mm)

Populations (log CFU/cm2)

Figure 3.6: Mean (ÂąSE) L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the rind circumference
area.

8
7
6
5
4
3
2
1
0

B

B
B

B

B

B

Surface

5 mm

10 mm

15 mm

B

B

20 mm

B

B

25 mm

Depth (mm)

Figure 3.7: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar.

44

Populations (log CFU/cm2)

7

A

AB

6

BC

5

BC

4

C
C

C

3

C

C
C

2
1
0
Surface

5 mm

10 mm

15 mm

20 mm

Depth (mm)

Populations (log CFU/cm2)

Figure 3.8: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar.

7
6

A
A

5
4

B

3

B

B

B
B

B

10 mm

15 mm

2

B

B

B

B

1
0
Surface

5 mm

20 mm

25 mm

Depth (mm)

Figure 3.9: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference
area.

45

Populations (log CFU/cm2)

7
6
5
4

B
B

3

B

2

B

B

B

B

10 mm

15 mm

1

B

B

B

20 mm

25 mm

0
Surface

5 mm

Depth (mm)

Populations (log CFU/cm2)

Figure 3.10: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from the stem scar.

7
6
5
4
3
2
1
0

A
A
B

Surface

B

5 mm

B

B

10 mm

B

B

15 mm

B

B

20 mm

Depth (mm)

Figure 3.11: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from the blossom scar.

46

Populations (log CFU/cm2)

6
5
4

A
A
B

3

B

2

B

B

B

B

B

B

B

B

1
0
Surface

5 mm

10 mm

15 mm

20 mm

25 mm

Depth (mm)

Figure 3.12: Mean (ÂąSE) L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from the rind circumference
area.
3.2.4 Comparison of coring tools
Overall, statistically similar L. monocytogenes populations were seen at the six different
core depth samples for the two coring implements (P > 0.05) (Figure 3.13). Subsequent transfer
of L. monocytogenes varied from 3.0 to 3.9 log CFU/cm2 using the cork borer and 3.1 to 4.1 log
CFU/cm2 using the apple corer in the cantaloupe (P > 0.05). A two-way ANOVA was
performed to determine if there was any effect of the different coring tools and transfer of L.
monocytogenes populations by depth. There was no significant interaction between the effect of
the coring method and depth (P > 0.05). The analysis of the fixed effects showed that there was
no difference in transfer of L. monocytogenes due to the use of different coring tools (P > 0.05).

47

7

A
Populations (log CFU/cm2)

6

Apple Corer

A

Cork Borer

5

B
4

BC

BCD
BCD

BCD
CD

CD

D

3

CD

D
D

D

25 mm

30 mm

2
1
0
Surface

5 mm

10 mm

15 mm

20 mm

Depth (mm)

Figure 3.13: Mean (ÂąSE) transfer of L. monocytogenes populations from the cantaloupe rind to
the interior melon flesh by using an apple corer and cork borer to compare bacterial transfer.
3.3 Discussion
Previous studies mimicking procedures used in commercial and home preparation of
fresh-cut melons have shown that bacteria can readily transfer from the rind to the edible melon
flesh during cutting, with the extent of contamination on the rind determining the numbers of
organisms transferred (Beuchat, 1996; Ukuku and Fett, 2004; Selma et al., 2008). Other studies
have evaluated the ability of chemical sanitizer to reduce microbial contaminants on the rind,
thereby reducing transfer during cutting (Ukuku and Fett, 2002; Ukuku et al. 2005, 2015).
One of the objectives of this study was to evaluate transfer of L. monocytogenes from
inoculated cantaloupe and honeydew melon into the edible flesh at 5 mm increments using a
cork borer. Cantaloupe and honeydew melons were dip-inoculated with L. monocytogenes to
simulate surface contamination possibly occurring during dump tank washing, cooling, and
subsequent storage (Parnell et al., 2005). These potential sources of melon rind contamination

48

emphasize the need to follow recently revised GAPs to minimize both preharvest and postharvest
contamination (FDA, 2011). Furthermore, insanitary kitchen operations can lead to potential
cross-contamination as pathogens can be transferred from the rind to edible flesh by improper
cleaning and sanitizing of melons before preparation. Cantaloupe rind is more susceptible to
surface contamination due to its porous and netted nature compared with the honeydew melon’s
smoother surface (Annous et al., 2005). Previous studies have demonstrated that cantaloupe rind
can support increased pathogen growth compared to honeydew melon with total coliform counts
being 1.4 log higher on cantaloupe (Cabrera 2003, unpublished data). The extent of L.
monocytogenes can be transferred observed in this study was based on initial populations on the
rind at the time of cutting with less transfer seen at the lower inoculation level.
Structural differences between the rind of cantaloupe and honeydew melon largely dictate
the extent of surface contamination. Previous bacterial attachment studies have shown less
bacteria attachment to the rind of honeydew melon compared to cantaloupe (Ukuku et al., 2005).
According to Barak et al. (2003), netted cantaloupe rind provided greater opportunities for S.
Poona attachment. Furthermore, Suslow (2004) has shown that infiltration of pathogens can
occur through the stem scar and ground spot area as pathogens were present 5 mm below the
melon rind. In our study, L. monocytogenes attachment to the cantaloupe rind was not
statistically different at either the stem car, blossom scar, or circumference of the rind (P > 0.05).
Statistically similar transfer of L. monocytogenes during coring was observed. For honeydew
melon, initial L. monocytogenes populations were 1 log CFU/cm2 higher on the stem and
blossom scar regions compared with the smoother portions of the rind which led to higher
numbers of L. monocytogenes in melon flesh from the stem and blossom scar regions. This

49

research supports that the inherent differences in melon structure can truly impact the microbial
safety and risks present with different melon types.
As a result of worker mishandling and poor hygienic practices, cantaloupe and honeydew
melons can be readily contaminated during the postharvest stage of production. Previous studies
have suggested as few as 150 bacteria per cm2 are needed on the cantaloupe rind for
contamination to occur while cutting (Suslow and Cantwell, 2001). During cutting of cantaloupe
and honeydew melon, the initial level of surface contamination directly impacted the number of
L. monocytogenes transferred to the edible flesh during coring. When contaminated produce is
cut with a knife, bacteria are first transferred from the produce surface to the knife and then to
the cut flesh with the number of bacteria transferred decreasing as cutting continues (Zilelidou et
al., 2015). A previous study performed by Vorst et al. (2006) indicated that knives inoculated
with L. monocytogenes at 5 and 3 log CFU per blade were able to contaminate 20 and 5 slices of
deli meat during slicing, respectively. Despite obvious surface differences between cantaloupe
and honeydew melon, an initial 3 log CFU/cm2 decline in L. monocytogenes populations was
seen from the rind to the first 5 mm section of melon flesh with no significant difference seen
thereafter. This suggests that all edible flesh is susceptible to contamination from the rind
surface during cutting. Hence, L. monocytogenes can be readily transferred from the rind to the
interior edible flesh of cantaloupe and honeydew melon during cutting.
As part of our study, the effect of cantaloupes and honeydew melon holding temperature
prior to preparation on transfer of L. monocytogenes during coring was also evaluated. In this
study, the melons were dip-inoculated, held at 4 or 30oC for 24 h and then enumeration prior to
mechanical preparation. Recently, a study performed by Nyarko et al. (2016) shows no
difference in L. monocytogenes populations based on cantaloupe cultivars over 15-day storage

50

period at different temperatures. In this same study, L. monocytogenes populations decreased on
the rind sections and increased on the stem scar region during 15 days of storage while there was
no difference in L. monocytogenes populations after 1 day of storage. This study contributes to
the field of food safety and supports enumeration of surface L. monocytogenes populations
would be by impacted significantly by 1 day of storage at various temperatures. Nyarko et al.
(2016) indicated that a difference in L. monocytogenes surface populations between the two
cantaloupe cultivars tested was not statistically different. Our study examines two different
melon types (cantaloupe and honeydew) and observed a statistically significant difference in the
L. monocytogenes populations enumerated from the surfaces of the stem scar, blossom scar, and
rind regions (P < 0.05). Findings from this study demonstrate that no statistical difference in
bacterial transfer was seen with temperature prior to coring at 4 or 30oC in the honeydew melon
rind, blossom scar, and stem scar (P > 0.05). However, this study shows there was greater
bacterial transfer observed in 4oC compared with 30oC in the cantaloupe rind and blossom scar
(P < 0.05). These findings and those by Nyarko et al. (2016) suggest that further studies are
needed to investigate differences in survival of L. monocytogenes on the surfaces of other
commonly consumed melon types.
Biofilm formation and bacterial attachment of L. monocytogenes is impact by knife blade
materials present in a production facility. This can ultimately lead to an impact on transfer rate
of L. monocytogenes during cutting and processing of fresh produce (Rodriguez et al., 2007).
The force and angle of the knife while cutting could also impact the number of bacteria
transferred as previously demonstrated during slicing of delicatessen meats (Vorst et al., 2006).
In this study, a stainless steel cork borer and apple corer were used to simulate cutting since both
yielded core samples for L. monocytogenes quantification at 5 mm increments. Using the cork

51

borer, core samples were retained inside the cork borer which were removed by pushing core
samples with sterile forceps. In contrast, cores obtained using the apple corer could be easily
removed without the risk of further bacterial transfer. The two-way analysis indicate comparison
of the cork and apple borers does not differ in the L. monocytogenes transfer in the depth within
the cantaloupe flesh while cutting. Overall, no significant difference in L. monocytogenes was
seen between the cork and apple borers (P > 0.05). This suggests that use of either the cork or
apple borer methodologies can be used to illicit statistically similar transfer results in subsequent
experiments.
In conclusion, this study demonstrates that L. monocytogenes can readily transfer from
the melon surface into the interior melon flesh during cutting in commercial, restaurant and
home settings. This study uses a cork borer to simulate cutting processes that allow for the
quantification of L. monocytogenes from the rind to specific depths in the edible melon flesh.
Appropriate chemical sanitizers and washing methods must be used to minimize surface
contamination, which in turn may reduce the likelihood of bacteria being transferred to the edible
flesh during cutting. The quantitative findings from this research should be helpful in current
risk assessments and help determine best practices for fresh-cut preparation cantaloupe and
melons.

52

CHAPTER 4: Transfer of Listeria monocytogenes during Mechanical Slicing and Dicing
and Growth during Subsequent Storage of Cantaloupe and Honeydew Melon

53

4.1 Materials and Methods
4.1.1 Experimental Design
Two cantaloupe or honeydew melon halves inoculated with a 3-strain avirulent cocktail
of Listeria monocytogenes were sliced followed by subsequent slicing of eight uninoculated
melon halves of the same type to assess bacterial transfer during preparation. Two cantaloupe
halves without its rind were inoculated with a 3-strain avirulent cocktail of L. monocytogenes
followed by subsequent dicing of eight uninoculated cantaloupe halves were stored for 7 days at
4, 7, or 10oC to evaluate growth of L. monocytogenes.
4.1.2 Produce
Cantaloupes (5-inch diameter) (Cucumis melo var. cantalupensis) and honeydew melons
(6 – 7-inch diameter) (Cucumis melo var. inodorus) were selected free of any visible bruising or
rind defects and purchased from a local retailer and stored for no longer than 2 days at 4oC prior
to experimentation. Before surface inoculation, all melons were washed under cool running tap
water to remove any debris.
4.1.3 Bacterial Strains
Three avirulent strains of L. monocytogenes (J22F, J29H, and M3) were obtained from
the laboratory of Dr. Sophia Katharian (North Carolina State University, Raleigh, NC). J22F is a
transposon mutant of H7550 and purine biosynthesis (purB) that are attenuated for systemic
infection. J29H is a non-hemolytic, transposon mutant of H7550 and hly gene. M3 is a nonhemolytic, transposon mutant consisting of TN916 transposon in the hly gene inhibiting
Listeriolysin O (LLO) secretion. Stock cultures were maintained at -80oC in Trypticase soy
broth containing 0.6% (w/v) yeast extract (TSBYE; Becton and Dickinson, Sparks, MD) and
10% (v/v) glycerol (Sigma Chemical Co., St. Louis, MO). The stock cultures of each strain were

54

streaked onto Trypticase soy agar containing 0.6% (w/v) yeast extract (TSAYE; Becton and
Dickinson) and incubated at 37oC for 24 h to obtain working cultures. Singles colonies from the
TSAYE plates were subjected to two successive transfers in 9 ml of TSBYE at 37oC for 24 h.
Thereafter, the cultures were combined in equal volumes to obtain one 3-strain avirulent L.
monocytogenes cocktail.
4.1.4 Cantaloupe and Honeydew Melon Inoculation for Slicing
Retail cantaloupe and honeydew melons were washed then dip-inoculated in a 3-strain
avirulent cocktail of L. monocytogenes containing ~109 CFU/ml for 10 min to contain ~5 log
CFU/cm2 and ~4 log CFU/cm2 for slicing, air-dried for 1 h and then stored at 4oC for 24 h. Prior
to slicing, the whole melons were cut in half starting at the stem end after which the seeds were
removed aseptically with a knife from the cavity of melons. The initial L. monocytogenes
populations on the melon were confirmed by direct plating as detailed below.
4.1.5 Cantaloupe Inoculation for Dicing
Retail cantaloupe was washed then followed by removal of the cantaloupe rind with a
sterile knife. The whole rind-free melon was dip-inoculated in a 3-strain avirulent cocktail for 30
s to contain ~3 log CFU/cm2 and immediately diced. Prior to dicing, the whole melons were cut
in half starting at the stem end after which the seeds were removed from the cavity of melons
without rind. The initial L. monocytogenes populations on the melon were confirmed by direct
plating as detailed below.
4.1.6 Cantaloupe and Honeydew Melon Slicing
Two cavity-cleaned inoculated melon halves were manually sliced with the rind on using
an aluminum manual slicer with bladed contact area of 233 cm2 (Vollrath Redco 401N, Vollrath
Company, Sheboygan, WI) to obtain nine 0.75 inch slices followed by eight cavity-cleaned

55

uninoculated melons halves (Figure 4.1). After slicing, alternate melon slices (~150 to 250
g/slice) had its rind aseptically removed by a sterile knife before being transferred into a sterile
Whirl-Pak bag (Nasco, Fort Atkinson, WI) for the enumeration of L. monocytogenes within 2
hours.

Figure 4.1: “Vollrath 401N Redco Lettuce King I ¾” Cut Vegetable Shredder and Slicer – 10
Blades.” Webstaurant Store. n.p., n.d. Web. 10 Aug. 2016.
4.1.7 Cantaloupe Dicing and Storage
Two cavity-cleaned, inoculated rind-free melon halves were manually diced using an
aluminum manual dicer with bladed contact area of 498 cm2 (Nemco 55650 dicer, Nemco Inc.,
Hicksville, OH) to obtain 1 inch cubes followed by eight cavity-cleaned uninoculated rind-free
melon halves (Figure 4.2). After dicing, 100 g of diced cantaloupe from each melon half were
weighed into sterile Whirl-Pak bags (Nasco, Fort Atkinson, WI) to quantify L. monocytogenes.
Additionally, 100 g of diced cantaloupe from each half was equally divided into three sterile

56

PLA trays (Nature Works, Minnetonka, MN) and stored at 4, 7, or 10oC for 3 and 7 days to
evaluate L. monocytogenes growth during storage.

Figure 4.2: “Nemco 55650 Easy Lettuce Kutter (Cutter)” Webstaurant Store. n.p., n.d. Web. 10
Aug. 2016.
4.1.8 L. monocytogenes Growth on Diced Cantaloupe
One whole cantaloupe had its rind aseptically removed by cutting with a knife, after
which the melon flesh was inoculated and mechanically diced using the Nemco dicer.
Inoculated cantaloupe pieces (100 g) were placed in sterile PLA, trays which were then stored at
4, 7, or 10oC for 7 days and were tested daily for number of L. monocytogenes. Since little if any
growth was seen at 4 or 7oC, generation times were only calculated for cantaloupe samples stored
at 10oC using the following Eq(1):
G=

t
3.3 log 𝑏/𝐵

57

where t is time in days, b is the L. monocytogenes population at time 1, and B is the L.
monocytogenes population at time 2.
4.1.9 Microbiological Analysis
One hundred-gram samples of cantaloupe or honeydew melon pieces were added to
sterile Whirl-Pak filter bags, diluted 1:1 in sterile UVM, and homogenized in a stomacher
(Stomacher 400 Circulator, Seward, Worthington, UK) at 260 rpm for 2 minutes. The sample
homogenates were serially diluted in phosphate-buffered solution (PBS) and surface-plated on
Listeria differential agar base, Trypticase soy agar containing 0.6% yeast extract, 0.1% esculin,
and 0.05% ferric ammonium citrate or Listeria differential/selective agar base, Modified Oxford
Agar (Neogen Corporation, Lansing, MI) consisting of moxalactam and colistin sulfate as
selective agents or processed with membrane filtration using 0.45-Âľm-pore-size membrane filters
(Millipore Corp., Billerica, MA) and plated Modified Oxford agar to obtain black L.
monocytogenes colonies from esculin hydrolysis which were counted after 24 to 48 h of
incubation at 37o C. Any samples negative for L. monocytogenes by direct plating were
subsequently enriched for 48 h and then streaked on Modified Oxford Agar to determine
presence or absence of L. monocytogenes.
4.1.10 Surface Population Calculations
The surface population of L. monocytogenes for honeydew melon with rind and
cantaloupe with or without rind were determined by Eq. (2):

N=

CxD
nxA

where N is the Colony Forming Units per square centimeter (CFU/cm2), C is the number of
colonies plated, D is the dilution factor, n is the number of surface samples with or without rind

58

(n = 4), and A is the surface area (A = length x width, 5 x 5 cm) of the rind or melon flesh in
square centimeters.
4.1.11 Statistical Analysis
The direct plate counts for L. monocytogenes from triplicate experiments were converted
into log CFU/g for each. Sliced or diced melon half was subjected to one-way analysis of
variance by using SAS 9.4 (SAS Institute Inc., Cary NC). The Tukey’s test was used to
determine significant differences in L. monocytogenes transfer between sliced cantaloupe and
honeydew melon halves subsequent uninoculated melon halves were sliced. The same
comparison test was used to determine if significant differences also existed in L. monocytogenes
transfer during dicing and growth on diced cantaloupes during storage. The letters within the bar
graphs indicate statistical significance from the Tukey’s test comparison (P < 0.05) between the
sliced or diced halve population means.
4.2 Results
4.2.1 L. monocytogenes transfer during melon slicing
Cantaloupes and honeydew melons inoculated with a 3-strain avirulent L. monocytogenes
contained 5.2 Âą 0.4 and 4.2 Âą 0.2 log CFU/g on the rind, respectively, after 1 h of air-drying
followed by 24 h storage at 4oC. Thereafter, manually slicing two cavity-cleaned inoculated
melon halves resulted in 2.2 Âą 0.4 log and 1.7 Âą 0.6 log CFU/g in the edible flesh of cantaloupes
and honeydew melons after slicing (Figure 4.3 & 4.4). When two inoculated cantaloupe halves
were sliced followed by 8 uninoculated halves, L. monocytogenes transfer was not significantly
different (P > 0.05) for halves 3 through 9 (0.2 - 1.5 log CFU/g) while only halve 10 was
significantly different compared to halve 3 (P < 0.05). When honeydew melon was sliced, L.
monocytogenes transfer was not statistically different for halves 3 through 10 (0.1 – 0.5 log

59

CFU/g). All uninoculated cantaloupe and honeydew melon halves yielded L. monocytogenes by
either direct plating or enrichment.

Populations (log CFU/g)

3

A

A

2

BC

BC

1

C

0
1
-1

2

3

4

5

6

7

8

9

10

Inoculated
Melon Halves

Figure 4.3: Mean (Âą) SE L. monocytogenes populations on cantaloupe halves that were sliced.
Inoculation level at (5.2 Âą 0.4 log CFU/cm2).

Populations (log CFU/g)

3

A
AB

2
1

BC

C

0
1
-1

2

3

4

5

6

7

8

9

10

Inoculated
Melon Halves

Figure 4.4: Mean (Âą) SE L. monocytogenes populations on honeydew melon halves that were
sliced. Inoculation level at (4.2 Âą 0.2 log CFU/cm2).
4.2.2 L. monocytogenes transfer from inoculated melons to subsequent uninoculated melons
during dicing
Dip-inoculation of rind-free cantaloupes with 3-strain avirulent L. monocytogenes
cocktail containing of L. monocytogenes yielded populations of 5.5 Âą 0.1 log and 3.3 Âą 0.2 log

60

CFU/cm2 on the surface. Manual dicing of the two cavity-cleaned inoculated cantaloupe halves
led to L. monocytogenes populations of 4.8 Âą 0.3 log CFU/g and 3.3 Âą 0.4 log CFU/g in the
edible melon flesh (Figure 4.5 & 4.6). Significantly greater L. monocytogenes transfer (P < 0.05)
from the inoculated to uninoculated melon halves was seen for 3rd and 4th halves (2.6 and 3.7 log
CFU/g) compared to halves 5 through 10 (0.7 – 1.6 log CFU/g) with L. monocytogenes again
found in all samples by either direct plating or enrichment (Figure 4.5).
6

A
Populations (log CFU/g)

5

A
AB

4

AC

3

C

C
C

2

C
C

C

1
0
1

2

Inoculated

3

4

5

6

7

8

9

10

Melon Halves

Figure 4.5: Mean (ÂąSE) L. monocytogenes populations on cantaloupe halves that were diced.
Inoculation level at (5.5 Âą 0.1 log CFU/cm2).

61

Populations (log CFU/g)

4

A

A
AB

3

B

2

B

B

B

B

1

0
1

2

Inoculated

3

4

5

6

7

8

9

10

Melon Halves

Figure 4.6: Mean (Âą) SE L. monocytogenes populations on cantaloupe halves that were diced.
Inoculation level at (3.3 Âą 0.2 log CFU/cm2).
4.2.3 L. monocytogenes growth in ten diced cantaloupe halves during storage
Diced cantaloupe pieces stored at 4 and 7oC for 7 days resulted in the average increased
growth of L. monocytogenes population by 0.5 and 3.3 log CFU/g, respectively (Figure 4.7 &
4.8). During storage of diced cantaloupe at 4oC, there was no significant difference in the
growth of L. monocytogenes during the 7-day storage (P > 0.05) except for halve 6 which had
more growth at 7 days compared with 3 days (P < 0.05) (Figure 4.7). When compared to diced
cantaloupe stored at 7 and 10oC, there were no diced cantaloupe halves with more growth during
the storage at 4oC, respectively (Figure 4.7 – 4.9). There was significant growth observed with
L. monocytogenes growth at 7 and 10oC (P < 0.05) with increases of 3.3 and 5.4 log CFU/g on
the uninoculated cantaloupe dices compared at 4oC. L. monocytogenes population after growth
in uninoculated diced cantaloupe pieces ranged from 4.8 to 5.9 log CFU/g in melon halves from
3 to 10 at 10oC (Figure 4.9). At 7oC storage, there was significant difference in the growth of L.
monocytogenes populations transferred to the uninoculated cantaloupe halves during dicing (P <
0.05). For example, all the cantaloupe dices from halves 3 to 10 had significant growth at 7 days

62

compared with 0 day (P < 0.05). In cantaloupe dices (halves 3, 4, 8-10), growth at 7 days was
significantly greater than at 0 and 3 days (P < 0.05) (Figure 4.8). For halves 5 and 6, there was
significant difference in growth between 0, 3, and 7 days (P < 0.05). At 10oC storage, there was
a significant difference in growth of L. monocytogenes populations transferred to uninoculated
cantaloupe halves 3 and 4 because there was more significantly more growth at 3 and 7 days
compared to 0 day (P < 0.05) (Figure 4.9). For cantaloupe halves 5 to 10, there was a significant
difference in growth between all the storage day durations (P < 0.05).

Populations (log CFU/g)

9
8
7
6
5
4

A A

A

A
A A
A

3

A A

A

A
A
A

2
1

A

A

A

A

A

A

AB B

A A A

A

A
A

A A A

9

10

0
1

2

Inoculated

3

4

5

6

7

8

Cantaloupe Halves
Day 0

Day 3

Day 7

Figure 4.7: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 4oC.

63

Populations (log CFU/g)

9
8

A

A

7

A
AB

6

AB

A

5
B

4

B

B

3

A

AB
B

B
B

2

A

A

A

B

B B

B

A

A

B

C

C

B

B

B
B

1
0
1

2

3

4

5

Inoculated

6

7

8

9

10

Cantaloupe Halves
Day 0

Day 3

Day 7

Figure 4.8: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 7oC.
10

A

A

Populations (log CFU/g)

9

A

A

8

A

A

A

7
6
B

A

A

A
A

A

B

5
4

A

A

B

B

B

B

B

B
B

B

3
2

C

C

C

C

C

C

1
0
1

2

Inoculated

3

4

5

6

7

8

9

10

Cantaloupe Halves
Day 0

Day 3

Day 7

Figure 4.9: Mean (Âą SE) L. monocytogenes populations on cantaloupe halves that were diced and
stored at 10oC.
4.2.4 L. monocytogenes growth on inoculated diced cantaloupe pieces
A whole cantaloupe without the rind was dip-inoculated in a 3-strain avirulent cocktail of
L. monocytogenes of ~5 log CFU/ml to contain 3.8 Âą 0.1 log CFU/cm2 on the melon surface
which was immediately followed by mechanical dicing. During the seven day storage at 4, 7,

64

and 10oC, L. monocytogenes counts reached maximum levels of 4.7 Âą 0.1, 6.2 Âą 0.1, and 9.2 Âą
0.1 log CFU/g, respectively (Figure 4.10). L. monocytogenes populations had minimal growth
during the seven day storage at 4oC as there was an increase of ~1 log CFU/g. At 7 and 10oC
storage, there was an increase of 2.4 and 5.4 log CFU/g during the seven day storage. During the
storage of diced cantaloupes, the L. monocytogenes generation time was calculated to be 0.74
days at 10oC as it exhibited significant growth.

Populations (log CFU/g)

4o C

7o C

10o C

10
9
8
7
6
5
4
3
2
1
0
0

1

2

3

4

5

6

7

Time (Days)

Figure 4.10: Mean (Âą SE) L. monocytogenes population growth on cantaloupe dices during
subsequent storage of seven days and stored at 4oC, 7oC, and 10oC.
4.3 Discussion
This study assessed the transfer of L. monocytogenes from inoculated to uninoculated
melon halves during mechanical slicing and dicing. The results from these studies demonstrate
that bacterial transfer readily occurs during mechanical slicing and dicing similar to crosscontamination observed in previous experiments with onions (Scollon et al., 2013; Scollon and
Ryser, 2014), tomatoes (Wang and Ryser, 2013), and celery (Kaminski et al., 2014). Cantaloupe
and honeydew melons were dip-inoculated in the slicing experiment to simulate surface
contamination from soil or irrigation water, whereas in the dicing study, rind-free cantaloupe

65

halves were dip-inoculated to simulate contamination during rind removal (Chimbombi et al.,
2013). Chimbombi et al. (2013) demonstrated in cantaloupe flesh that Salmonella growth
occurred between 10 to 30 h at 23oC and no growth was observed up to 5 h after inoculation.
This study supports the use of dip inoculation procedures for cantaloupe used in the present
study with sampling procedures immediately, within 10 minutes, following inoculation.
However, unusually high inoculation levels were needed to obtain quantitative data that could be
modeled for future risk assessments.
Surveys have shown Salmonella to be present on field-grown cantaloupe in Mexico
(Bhagwat, 2006) indicating that proper implementation of Good Agricultural Practices (GAP) is
necessary to minimize contamination of melons during processing. Cantaloupes and honeydew
melon rinds have opposing textural compositions with cantaloupe rind far more inclined to
pathogen contamination compared with honeydew melon due to its netted surface (Barak et al.,
2003).
Previous studies have demonstrated the importance of applying sanitizer treatments to
reduce surface contaminants on fresh produce since any remaining pathogens can be transferred
to previously uncontaminated product during subsequent processing. In one study evaluating
Escherichia coli O157:H7 transfer during pilot-scale production of fresh-cut lettuce (Buchholz et
al., 2012) showed that pathogen spread to previously uncontaminated product was relatively
random especially when the initial contamination level was below 3 log CFU/cm2. In slicing
experiment involving onions, Scollon et al. (2013) determined that transfer of L. monocytogenes
in onions throughout subsequent slicing regardless of the initial inoculation level. In our study,
the starting population of L. monocytogenes determined the extent of subsequent transfer to
uninoculated melon halves.

66

In the dicing experiments, the initial inoculation level also impacted the extent of L.
monocytogenes transfer to subsequently diced product. These findings are similar to those seen
for celery with a 2 log decrease in L. monocytogenes observed after dicing 8 batches of
previously uncontaminated product (Kaminski et al., 2014). In this study, L. monocytogenes
growth was also assessed in the diced cantaloupe halves during subsequent storage at 4, 7, and
10oC. After seven days of storage, L. monocytogenes populations were significantly greater in
diced cantaloupe stored at 10oC compared to 4 and 7oC (P < 0.05) as these findings were similar
to the growth of L. monocytogenes on fresh-cut cantaloupe at temperature abuse conditions
performed by Huang et al. (2015) and data could be used to model growth kinetics of L.
monocytogenes transfer at different storage temperatures (Fang et al., 2013; Yoon et al., 2014).
Nyarko et al. (2016) presented similar finding at 4oC with an artificially created cantaloupe rind
juice suggesting L. monocytogenes was able to grow ~3 log CFU/ml, after 7 days of storage
when adequate nutrients and moisture were available at lower temperatures. Our study noticed
modest growth after 4oC. The difference could have been caused by Nyarko et al. (2016) having
prepared artificially macerated rind juice, which would have damaged cellular tissues of
cantaloupe making nutrients readily available for microbial growth, liquid growth medium. In
our study microbial growth was determined over 7 days on diced intact pieces of cantaloupe in
which nutrients may not have been as readily available, station growth medium. These findings
indicate the proper temperature storage of fresh-cut cantaloupe is necessary to prevent growth of
pathogens during storage.
The results obtained during dicing and subsequent storage of cantaloupe are similar to
those observed in several previous studies indicating that temperature is the primary factor in the
growth of L. monocytogenes. During 12 days of storage, Ukuku (2007) reported no significant

67

growth of Salmonella on cubes of cantaloupe at 5oC while an increase of 3.6 logs was observed
at 10oC. The growth of diced cantaloupes at 4oC in our study was similar to the growth model
predicted by Danyluk et al. (2014), which noted 1 log growth over 6 days of storage. Another
fresh-cut cantaloupe study by Ukuku and Fett (2002) showed that even after exposing
cantaloupes inoculated with Salmonella at 1000 ppm chlorine-based sanitizer or 5% H2O2 for 2
min, the pathogen was not eliminated during cantaloupe storage at 4 or 8oC. Furthermore,
Lamikanra et al. (2000) compared the biochemical changes of cantaloupe during storage at 4 and
20oC, respectively. The study indicated that prolonged storage at 20oC resulted in increased
acidity and lactic acid production and loss of 17% of soluble solids leading to increased
microbial growth. These findings emphasize that storage of fresh-cut cantaloupe pieces at
temperatures less than 5oC can inhibit the growth of most pathogens, while higher temperatures
that may be encountered during retail display can lead to amplification of microorganisms
overtime.
In conclusion, this is the first study to show that L. monocytogenes can be transferred
from an inoculated melon halve with or without its rind to uninoculated melon halves during
subsequent mechanical slicing and dicing. Furthermore, when stored under temperature abuse
conditions, L. monocytogenes can reach potentially hazardous levels in diced cantaloupe. These
results reinforce proper washing and use of chemical sanitizers or other treatments such as hot
water to decrease microbial populations on the rind before slicing or dicing with thorough
washing of honeydew melons also recommended. Sliced and diced melon remains a perishable
product that needs to be properly refrigerated and consumed in a timely manner to minimize the
risk of foodborne illness.

68

Chapter 5: Conclusions and Future Recommendations

69

The increased availability and health benefits offered by the consumption of fresh fruits
and vegetables are not without its risks as half of all foodborne outbreaks are associated with
fresh produce. Melons, especially cantaloupes, have been a major source of outbreaks due to
their netted surface that increases the risk of contamination during preharvest and challenges
with cleaning during postharvest processing. The application of sanitizers to the rind surface is
necessary to reduce the transfer of L. monocytogenes into the edible melon flesh during
commercial processing.
The first objective of this study was to quantify the transfer of L. monocytogenes from the
rind into the edible melon flesh by cutting the rind in three different areas and storing the melon
at different product temperatures. The results demonstrated L. monocytogenes can be readily
transferred from the rind to interior flesh during preparation. L. monocytogenes transfer ranged
from 1.2 to 4.0 and 0.2 to 1.8 log CFU/cm2 for cantaloupe and melon rind when rinds were
inoculated with L. monocytogenes populations of 6.1 and 4.4 log CFU/cm2, respectively.
Overall, no significant difference in L. monocytogenes transfer was seen at different depths, or
product temperatures (P > 0.05). Therefore, these findings emphasize thorough washing of the
rind with a sanitizer is important to minimize translocation of L. monocytogenes to the interior
melon flesh.
The second study quantified the transfer of L. monocytogenes from one inoculated
cantaloupe or honeydew melon to an uninoculated melon during mechanical slicing and dicing
followed by storage at 4, 7, 10oC for seven days. L. monocytogenes populations in subsequently
uninoculated cantaloupe and honeydew melons were similar (P > 0.05). The transfer of L.
monocytogenes reached a maximum of 1.5 and 0.5 log CFU/g in cantaloupe and honeydew
melons, respectively. During the storage of diced cantaloupes, the L. monocytogenes generation

70

time was calculated to be 0.74 days at 10oC. This study demonstrates that L. monocytogenes
transfer was similar between cantaloupe and honeydew melons while slicing. These findings
reinforce the importance of refrigeration of diced cantaloupes in an effort to prevent the growth
of L. monocytogenes during storage and transport. Overall, these findings will be helpful in
current risk assessments associated with commercial processing, storage and transportation of
melons.
Future work includes conducting experiments to gain a better understanding of L.
monocytogenes transfer during industry specific commercial melon processing. In the cutting
experiments, L. monocytogenes transfer due to the maturity and ripeness of the melons or the
application force of the core borer should be assessed. L. monocytogenes transfer comparing
mechanical and non-mechanical peeling from inoculated melons to subsequent uninoculated
melons would add value to the field of food microbiology. In the dicing experiment, the melon
flesh was directly inoculated to obtain consistent L. monocytogenes populations in an effort to
determine the transfer to uninoculated melons during dicing. Furthermore, the growth of L.
monocytogenes during extended storage at different temperatures was assessed. The addition of
humidity and modified atmosphere parameters could also add value and should be investigated
in future studies. These experiments demonstrate that L. monocytogenes transfer can occur
during processing and that sanitizer application to the melon rind and equipment is necessary to
mitigate contamination, recontamination and cross-contamination during commercial melon
processing.

71

APPENDICES

72

Appendix A
Coring of Cantaloupe and Honeydew Melon

Cantaloupe Stem Scar
Honeydew Stem Scar
Cantaloupe Blossom Scar
Honeydew Blossom Scar
Cantaloupe Rind
Honeydew Rind

Replicate 1
6.6
5.1
6.6
4.5
6.1
2.8

Replicate 2
6.5
5.3
6.2
5.0
5.9
4.1

Replicate 3
6.8
5.3
5.8
5.4
5.8
3.8

Table 3: Raw data for L. monocytogenes populations on surface of cantaloupe and honeydew
melon at the stem scar, blossom scar, and rind dip-inoculated (log CFU/cm2) and stored for 24 h
at 4oC.
Temperature
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm

Replicate 1
5.1
1.4
1.6
1.3
< 1.0 CFU/g*

Replicate 2
5.3
1.7
1.2
1.3
1.6

Replicate 3
5.3
2.2
1.4
1.3
1.1

30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm

5.6
2.2
1.5
1.8
1.9

5.1
1.6
1.5
1.5
1.8

4.7
1.6
1.6
1.5
2.6

*LOD/2 = < 1.0 CFU/g

Table 4: Raw data for L. monocytogenes populations transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the stem scar.

73

Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
4.5
1.2
0.8
0.9
0.6
0.5

Replicate 2
5.0
1.8
2.0
1.6
1.7
1.2

Replicate 3
5.4
2.2
1.5
1.1
1.3
0.9

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

4.1
1.6
1.4
1.2
1.6
1.6

3.8
1.7
1.5
1.6
1.4
1.5

4.0
1.9
2.4
1.9
2.3
1.0

Table 5: Raw data for L. monocytogenes population transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the blossom scar.
Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
2.8
0.4
0.2
0.2
< 1.0 CFU/g*
< 1.0 CFU/g*

Replicate 2
4.1
1.8
1.2
1.3
1.1
1.1

Replicate 3
3.8
1.1
1.0
0.5
0.5
0.5

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

3.9
1.1
1.0
0.4
0.4
0.7

3.0
< 1.0 CFU/g*
< 1.0 CFU/g*
< 1.0 CFU/g*
< 1.0 CFU/g*
0.2

3.2
1.1
0.5
0.6
< 1.0 CFU/g*
0.6

*LOD/2 = < 1.0 CFU/g

Table 6: Raw data for L. monocytogenes population transferred from the honeydew rind dipinoculated and stored at 4 and 30oC for 24 h to the honeydew flesh from the rind circumference
area.

74

Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
6.6
3.2
2.8
3.1
3.0
1.7

Replicate 2
6.5
3.0
3.0
2.8
2.5
3.6

Replicate 3
6.8
4.5
3.0
5.0
2.9
2.5

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

6.6
4.5
3.6
3.2
3.2
2.8

7.6
2.7
2.2
2.0
2.2
1.7

6.9
4.7
3.7
3.9
3.6
3.6

Table 7: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar.
Temperature
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm

Replicate 1
6.6
4.4
2.9
3.1
2.9

Replicate 2
6.2
3.1
2.4
5.4
4.4

Replicate 3
5.8
2.8
2.6
2.3
2.1

30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm

5.5
2.5
1.7
1.7
1.2

5.5
2.7
2.2
1.7
1.2

5.9
3.7
3.4
3.0
1.7

Table 8: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar.

75

Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
6.1
2.8
2.4
2.2
2.5
2.4

Replicate 2
5.9
2.8
3.3
3.2
1.9
2.0

Replicate 3
5.8
2.8
2.5
2.3
2.2
2.1

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

5.0
1.0
0.9
1.0
0.5
0.5

5.3
1.6
1.2
1.4
1.2
1.4

5.3
2.9
2.4
2.4
1.9
1.9

Table 9: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference
area.
Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
4.4
1.3
1.4
1.0
1.0
1.0

Replicate 2
4.2
1.0
1.2
1.2
1.0
1.2

Replicate 3
4.4
1.9
1.3
1.2
1.3
0.9

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

5.6
3.5
2.8
2.4
2.5
0.8

4.0
1.2
0.5
1.2
1.2
1.2

5.2
2.2
1.3
1.0
1.0
1.2

Table 10: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from stem scar.

76

Temperature
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm

Replicate 1
4.3
1.7
1.2
1.5
1.2

Replicate 2
4.1
1.5
0.9
0.5
0.9

Replicate 3
4.4
1.5
1.5
1.2
0.5

30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm

5.1
2.3
1.7
1.3
1.2

4.0
1.2
0.5
1.2
1.2

5.9
1.2
0.5
0.2
1.2

Table 11: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from blossom scar.
Temperature
4
4
4
4
4
4

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm

Replicate 1
4.1
1.8
1.7
1.4
1.0
1.2

Replicate 2
3.9
2.0
2.0
1.3
1.3
1.6

Replicate 3
4.2
1.8
1.2
1.4
1.2
0.9

30
30
30
30
30
30

Surface
5 mm
10 mm
15 mm
20 mm
25 mm

5.2
2.5
1.5
1.7
1.5
1.2

5.7
2.2
1.3
1.0
1.0
0.5

5.3
0.9
0.2
0.9
0.2
0.2

Table 12: Raw data for L. monocytogenes populations transferred from the cantaloupe rind dipinoculated and stored at 4 and 30oC for 24 h to the cantaloupe flesh from rind circumference
area.

77

Method
Apple Corker
Apple Corker
Apple Corker
Apple Corker
Apple Corker
Apple Corker
Apple Corker

Depth
Surface
5 mm
10 mm
15 mm
20 mm
25 mm
30 mm

Replicate 1
5.2
4.1
3.8
3.8
3.5
3.4
3.2

Replicate 2
5.5
4.2
3.1
3.4
3.2
3.2
3.5

Replicate 3
5.5
4.1
3.2
3.1
2.9
2.9
2.5

Cork Borer
Cork Borer
Cork Borer
Cork Borer
Cork Borer
Cork Borer
Cork Borer

Surface
5 mm
10 mm
15 mm
20 mm
25 mm
30 mm

5.5
3.8
3.6
3.3
3.1
3.1
3.0

6.3
3.8
3.1
3.1
2.8
2.9
3.0

5.5
4.0
3.3
3.2
3.1
2.9
3.1

Table 13: Raw data for transfer of L. monocytogenes populations from the cantaloupe rind dipinoculated and stored at 4oC to the interior melon flesh by using an apple corer and cork borer to
compare bacterial transfer.

78

Appendix B
Slicing and Dicing of Cantaloupe and Honeydew Melon

Inoculated Cantaloupe Halve 1a
Inoculated Cantaloupe Halve 1b
Inoculated Cantaloupe Halve 2a
Inoculated Cantaloupe Halve 2b
Uninoculated Cantaloupe Halve 3a
Uninoculated Cantaloupe Halve 3b
Uninoculated Cantaloupe Halve 4a
Uninoculated Cantaloupe Halve 4b
Uninoculated Cantaloupe Halve 5a
Uninoculated Cantaloupe Halve 5b
Uninoculated Cantaloupe Halve 6a
Uninoculated Cantaloupe Halve 6b
Uninoculated Cantaloupe Halve 7a
Uninoculated Cantaloupe Halve 7b
Uninoculated Cantaloupe Halve 8a
Uninoculated Cantaloupe Halve 8b
Uninoculated Cantaloupe Halve 9a
Uninoculated Cantaloupe Halve 9b
Uninoculated Cantaloupe Halve 10a
Uninoculated Cantaloupe Halve 10b

Replicate 1
1.4
1.4
2.6
1.8
1.3
1.2
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g

Replicate 2
2.7
2.6
2.7
2.6
2.1
1.9
1.3
1.3
1.1
1.2
0.7
0.6
0.4
0.4
1.7
1.3
0.6
0.8
< 0.25 CFU/g
< 0.25 CFU/g

Replicate 3
2.2
1.6
2.4
2.5
1.3
1.2
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
0.9
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
0.2
< 0.25 CFU/g
< 0.25 CFU/g

*LOD/2 = < 0.25 CFU/g

Table 14: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe to
uninoculated melon halves during mechanical slicing. Inoculation level at (5.2 Âą 0.4 log
CFU/cm2).

79

Inoculated Honeydew Halve 1a
Inoculated Honeydew Halve 1b
Inoculated Honeydew Halve 2a
Inoculated Honeydew Halve 2b
Uninoculated Honeydew Halve 3a
Uninoculated Honeydew Halve 3b
Uninoculated Honeydew Halve 4a
Uninoculated Honeydew Halve 4b
Uninoculated Honeydew Halve 5a
Uninoculated Honeydew Halve 5b
Uninoculated Honeydew Halve 6a
Uninoculated Honeydew Halve 6b
Uninoculated Honeydew Halve 7a
Uninoculated Honeydew Halve 7b
Uninoculated Honeydew Halve 8a
Uninoculated Honeydew Halve 8b
Uninoculated Honeydew Halve 9a
Uninoculated Honeydew Halve 9b
Uninoculated Honeydew Halve 10a
Uninoculated Honeydew Halve 10b

Replicate 1
2.7
2.5
1.5
1.5
0.6
0.8
0.4
0.3
0.6
0.3
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
0.2
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g

Replicate 2
1.8
2.5
0.8
1.1
0.5
0.2
0.3
< 0.25 CFU/g
0.8
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
0.2
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g

Replicate 3
1.5
1.2
2.3
1.6
0.9
< 0.25 CFU/g
1.5
< 0.25 CFU/g
1.0
0.4
1.1
0.2
0.6
< 0.25 CFU/g
0.4
< 0.25 CFU/g
< 0.25 CFU/g
< 0.25 CFU/g
0.4
0.3

*LOD/2 = < 0.25 CFU/g

Table 15: Raw data for L. monocytogenes populations transferred from inoculated honeydew
melon to uninoculated melon halves during mechanical slicing. Inoculation level at (4.2 Âą 0.2
log CFU/cm2).

80

Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 2
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 10

Replicate 1
4.9
5.0
4.1
2.7
1.7
1.1
1.9
1.5
1.6
1.8

Replicate 2
4.4
4.6
2.9
2.0
0.7
0.5
0.3
0.5
0.3
1.1

Replicate 3
5.0
5.0
3.7
2.6
2.3
0.6
0.5
0.7
2.2
0.6

Table 16: Raw data for L. monocytogenes populations transferred from cantaloupe halves during
mechanical dicing. Inoculation level at (5.5 Âą 0.1 log CFU/cm2).

Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 2
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 10

Replicate 1
3.4
3.0
1.3
1.0
1.2
1.2
0.7
0.7
1.1
0.7

Replicate 2
2.8
3.6
1.7
1.0
0.7
1.0
2.0
2.0
2.4
1.2

Replicate 3
3.8
3.2
2.9
3.0
0.7
0.1
< 0.5 CFU/g
< 0.5 CFU/g
1.0
0.1

*LOD/2 = < 0.5 CFU/g

Table 17: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated cantaloupe halves during mechanical dicing. Inoculation level at (3.3 Âą
0.2 log CFU/cm2).

81

Day
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7

Cantaloupe Halves
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10

Replicate 1
3.4
3.8
4.1
3.0
3.9
3.8
1.3
1.4
2.1
1.0
1.0
1.0
1.2
1.0
0.6
1.2
1.8
1.7
0.7
2.3
2.6
0.7
1.8
2.0
1.1
1.7
2.6
0.7
1.2
1.1

Replicate 2
2.8
2.8
2.7
3.6
2.7
2.1
1.7
0.8
1.6
1.0
2.4
1.8
0.7
1.7
1.7
1.0
< 0.5 CFU/g
2.1
2.0
2.6
2.2
2.0
0.9
1.8
2.4
0.1
1.0
1.2
1.0
1.5

Replicate 3
3.8
3.7
4.0
3.2
3.4
4.2
2.9
2.5
2.4
3.0
0.8
1.1
0.7
0.7
1.7
0.1
< 0.5 CFU/g
2.2
< 0.5 CFU/g
< 0.5 CFU/g
1.9
< 0.5 CFU/g
0.3
1.8
1.0
< 0.5 CFU/g
1.5
0.1
0.6
1.2

*LOD/2 = < 0.5 CFU/g

Table 18: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 4oC.

82

Day
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7

Cantaloupe Halves
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10

Replicate 1
3.4
5.1
6.3
3.0
4.7
5.5
1.3
2.5
4.2
1.0
2.2
4.4
1.2
3.6
3.6
1.2
2.6
5.1
0.7
4.0
4.5
0.7
2.8
4.7
1.1
2.8
4.2
0.7
2.7
2.9

Replicate 2
2.8
3.6
5.6
3.6
3.6
6.1
1.7
2.4
6.5
1.0
0.9
5.5
0.7
2.1
5.0
1.0
2.3
5.2
2.0
1.2
5.0
2.0
0.1
5.2
2.4
1.9
5.7
1.2
0.3
5.6

Replicate 3
3.8
5.8
7.5
3.2
5.8
7.7
2.9
3.9
5.3
3.0
2.9
4.6
0.7
3.0
4.0
0.1
2.5
4.2
< 0.5 CFU/g
1.1
3.3
< 0.5 CFU/g
1.8
3.7
1.0
2.2
3.0
0.1
1.5
2.9

*LOD/2 = < 0.5 CFU/g

Table 19: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 7oC.

83

Day
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7
0
3
7

Cantaloupe Halves
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 1
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Inoculated Cantaloupe Halve 2
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 3
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 4
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 5
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 6
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 7
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 8
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 9
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10
Uninoculated Cantaloupe Halve 10

Replicate 1
3.4
7.7
8.0
3.0
7.8
7.8
1.3
6.1
7.2
1.0
5.3
7.2
1.2
4.2
4.9
1.2
5.1
5.5
0.7
5.1
6.7
0.7
4.6
6.4
1.1
3.9
6.2
0.7
3.9
5.1

Replicate 2
2.8
6.6
7.9
3.6
6.4
7.7
1.7
5.3
6.9
1.0
4.8
6.1
0.7
4.8
7.5
1.0
3.9
6.9
2.0
4.4
7.2
2.0
4.5
7.3
2.4
4.1
7.1
1.2
3.7
5.8

Replicate 3
3.8
7.7
9.3
3.2
7.9
9.0
2.9
6.6
8.1
3.0
5.1
6.6
0.7
5.0
7.4
0.1
4.3
7.4
< 0.5 CFU/g
4.4
6.6
< 0.5 CFU/g
4.2
6.6
1.0
3.0
6.5
0.1
2.9
5.4

*LOD/2 = < 0.5 CFU/g

Table 20: Raw data for L. monocytogenes populations transferred from inoculated cantaloupe
halves to uninoculated halves during mechanical dicing and stored at 10oC.

84

Temperature
4
4
4
4
4
4
4
4
7
7
7
7
7
7
7
7
10
10
10
10
10
10
10
10

Days
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7

Replicate 1
3.9
3.9
4.3
4.5
4.7
4.7
4.4
4.7
3.9
4.8
4.6
5.0
5.6
6.0
5.9
6.3
3.9
6.1
7.9
8.4
9.0
9.0
9.0
9.3

Replicate 2
3.9
4.1
4.2
4.4
4.6
4.8
4.5
4.7
3.9
4.7
4.8
4.7
5.5
6.0
6.2
6.1
3.9
6.1
7.9
8.5
8.9
9.0
9.2
9.1

Replicate 3
3.8
3.6
4.3
4.5
4.5
4.7
4.7
4.6
3.8
4.7
4.6
4.7
5.4
5.9
6.2
6.2
3.8
6.3
7.9
8.4
8.8
9.1
9.1
9.1

Table 21: Raw data for L. monocytogenes population growth on cantaloupe dices during
subsequent storage of seven days at 4oC, 7oC, and 10oC.

85

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