_::_____:_E__:_::___22:33:: __ mmm.

. .0
I ..|.

Pawn .

I. . n
Li.
I
4.. 5'
I

nag
«I:
«an
? u

 

LIBRARY '

Michigan Stave
University

 

 

T;- 0‘20

ABSTRACT

IMPROVING THE KEEPING QUALITY OF CUT HEAD LETTUCE

By

Thomas Richard Krahn

Keeping quality of cut (salad prepared) head lettuce
was affected by temperature, modified atmospheres and
chemical treatments.

Temperature control was the most effective means of
improving keeping quality with a 0°C temperature being pref-
erable; a range 0° to 2.200 was acceptable, however, storage
life decreased proportionally as temperature increased.
Chemical treatments tested, including sodium bisulfite,
calcium chloride, calcium oxalate and calcium nitrate, were
of little or no value in extending storage life. Modified

atmospheres with 2 to 3% 0 provided substantial benefit at

2
normal atmospheric pressure, however, when these conditions
were created by hypobaric methods, keeping quality of cut
head lettuce was substantially reduced.

Storage in a dry untreated condition, in an impermeable
heat sealed film, resulted in the most desirable product

after two weeks storage, especially if a cold water rinse

was used before product consumption.

Thomas Richard Krahn

Sliced radishes provided a worthwhile indicator of
browning or dehydration, when evaluating a handling system.
Chemical treatments provided a degree of benefit in improving
keeping quality of sliced radishes, while modified atmos—

pheres were not as important as for out head lettuce.

IMPROVING THE KEEPING QUALITY OF CUT HEAD LETTUCE

By

Thomas Richard Krahn

A THESIS
Submitted to

Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Horticulture

1973

ACKNOWLEDGEMENTS

The author would like to express his appreciation to
all those who assisted in making this research possible.
Specifically I would like to thank the Michigan State Uni—
versity School of Packaging for their cooperation in provid—
ing the plastic films tested. The Michigan State University
food processing center provided the cut head lettuce and I
thank them for saving me many hours of hand labor.

Special appreciation is extended to Dr. Robert C.
Herner for his continuous guidance in conducting the research
and in the preparation of this manuscript. Valuable assis—
tance was also received from Dr. D. H. Dewey and Dr. G.
McBride.

Financial aid in the form of a year's educational leave
was received from the Alberta Department of Agriculture and
is gratefully acknowledged.

Last but not least, I thank my wife, Lila and our two

children Carla and Kevin who helped make it all possible.

ii

TO THE EXAMINING COMMITTEE

The body of this thesis has been prepared in a publica-

tion format and will be submitted for publication to the

Journal of the American Society for Horticultural Science.

iii

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES
ABSTRACT
MATERIALS AND METHODS
Temperature effects on respiration
Modified atmospheres
Handling methods
RESULTS
Temperature effects on respiration
Modified atmospheres
Handling methods
DISCUSSION
LITERATURE CITED . . . . . . .
APPENDIX
REDUCED PRESSURE STORAGE

SLICED RADISHES AS AN INDICATOR

iv

Page

vi

NCDCI) CD C‘s-1:42" U.) l—’

WNNNF-‘H
I\)\O\OU'I\O

Table

10.

LIST OF TABLES

Effect of temperature on the respiration rate
of cut head lettuce . . . . .

Effect of selected controlled atmospheres and
temperature on keeping quality of cut head
lettuce after 2 weeks storage . . . .

Effect of reduced pressure storage on keeping
quality of cut head lettuce after 1 week of
storage . . . . . . . . . . . .

The influence of plastic films on keeping
quality of cut head lettuce after 2 weeks
storage . . . . . . . .

Treatment effects on keeping quality of cut
head lettuce after 2 weeks storage in com-
mercial polyethylene bags .

The effect of chemicals on keeping quality of
cut head lettuce after 2 weeks storage in
commercial polyethylene bags

Effect of temperature and treatment on the
keeping quality of cut head lettuce stored
in commercial polyethylene bags for 2 weeks

Effect of reduced pressure storage on the
keeping quality of cut vegetables after 1
week storage at 2 temperatures . .

The effect of selected controlled atmospheres
on the keeping quality of sliced radishes
after 2 weeks storage . . .

Effect of chemical treatments on the keeping
quality of sliced radishes stored for 2
weeks in commercial polyethylene bags

Page

10

ll

13

l6

l8

23

31

3A

35

LIST OF FIGURES

Figure

l. The effect of plastic film and temperature
on the modification of O levels in
packages of cut head letéuce, stored for
two weeks

2. The effect of treatment on the keeping
quality of cut head lettuce stored in com-
mercial polyethylene bags for 2 weeks at 0°
and 5°C . . . . . . . . . . .

3. Reduced pressure storage test equipment .

vi

Page

1“

17
3O

Abstract. Keeping quality of cut head lettuce (salad
prepared) was affected by temperature, modified atmospheres
and chemical treatments.

Temperature control was the most effective means of
improving keeping quality with a 0°C temperature being pref-
erable; a range 0° to 2.2°C was acceptable, however, storage
life decreased proportionally as temperature increased.
Chemical treatments tested, including sodium bisulfite,
calcium chloride, calcium oxalate and calcium nitrate were
of little or no value in extending storage life. Modified
atmospheres with 2 to 3% 02 provided substantial benefit at
normal atmospheric pressure, however, when these conditions
were created by hypobaric methods, keeping quality of cut

head lettuce was substantially reduced.

 

The use of fresh cut head lettuce (lettuce prepared in
a manner suitable for use in tossed salads), and other pre-
pared foods is a rapidly expanding segment of today's food
industry, benefiting from the ever increasing consumption of
food outside the home. Restaurants and other serving insti-
tutions are facing a problem of how to efficiently prepare
these various food items; therefore, centralized preparation,
storage, transportation and handling of cut head lettuce may
take on additional significance. Long distance transporta-

tion of fresh cut head lettuce offers advantages in shipping

no waste, and reducing labor requirements at the serving
destination. However, quality maintenance over an extended
period presents new and varied problems for produce handlers.

The limited research conducted on the handling of pre—
pared and cut vegetable products for salads and relishes has
mostly concerned coleslaw and root vegetables, with rela-
tively little attention given to the handling and storage of
cut lettuce.

The shelf life of coleslaw was extended by proper
handling, with the main benefit accruing from low tempera-
tures (6,11,12,15,2U). The limiting factor in the storage
of pre—peeled turnips and rutabagas was discoloration and
shriveling (2,3); chemical dips being of little benefit.
Peeled, sliced and packaged parsnips stored best at low
temperatures and the prevention of discoloration was aided
by dipping in chemicals, such as sodium acid sulfite and
sorbic acid (13). Shelf life of pre-peeled French fry
potatoes was extended by treatment with the antibiotics
oxytetracycline and chlorotetracycline (5,7). Antibiotics
also improved the keeping quality of cut fresh vegetables
including cucumbers, chicory, escarole, lettuce, radish and
red cabbage (A). Discoloration of cut cabbage was reduced
by rinsing with cold water, followed by spin drying (8).
Fresh packaged kale stored best at 0°C and 90 to 95% rela-
tive humidity (11). The shelf life of prepackaged and
shelled lima beans was extended mainly by hydrocooling while

chemical treatments, including antibiotics, were of little

or no value (1A). Apple slices stored at l.l°C benefited
from sulfite treatments which were needed to preserve the
light color of apple slices stored for several weeks (19,
20).

Head lettuce responds favorably to controlled atmos-
phere conditions for transit and storage, provided C0

2

levels are not above 1 to 2% (25) and 0 levels do not reach

2
zero (16).

Reduced pressure storage has received wide attention
in recent research, and has been found to be of benefit for
various fruit, vegetable and flower crops (26,27).

An investigation was conducted to determine the effect
of various treatments on the keeping quality of cut head
lettuce. Our studies included the evaluation of temperature
effects on metabolic activity, the influence of modified
atmospheres, both controlled atmospheres and hypobaric
methods, and the modification of handling techniques to more

adequately meet the needs of cut head lettuce users and han-

dlers.

Materials and Methods

Freshly cut (salad prepared) head lettuce was obtained
from the Michigan State University food processing center,
or hand out as required. All experiments were conducted
during 1972, at temperatures of 0° and 5°C (unless otherwise
indicated). Evaluations generally were made following two

weeks of storage.

All evaluations were made on a subjective basis and
ratings established on a scale of l to 9. Ratings were
based on the degree of browning or discoloration, texture,
flavor and general appearance. The following descriptions

should be used when interpreting our data.

Rating Description

9 fresh condition, the ideal

8 no browning or discoloration, excel-
lent general condition

7 slight browning, good overall condition

6Z slight-some browning, good general
appearance

5 some-moderate browning, fair general
appearance

A moderate-severe browning, poor general
appearance

3 severe browning, poor general condition

2 very poor general condition, some evi-
dence of decay

1 complete loss of texture, totally
unacceptable

ZMinimum rating at which quality is acceptable for use
in food Operations.

Temperature effects on respiration. Five chambers con-

 

taining various quantities of cut head lettuce ranging from
280 to A10 grams were connected to an air supply with a flow
rate of approximately 1 ml/min flowing through the chambers.
CO2 evolution was monitored by daily extraction of a 1 ml
sample of air, and analyzing it by gas chromatography.

Modified atmospheres. a) Controlled atmospheres. A

 

 

wide range of concentrations and combinations of CO2 and 02

were investigated and related to their suitability for
preserving freshness of cut head lettuce. These were: 100%
nitrogen (0% C02—0% 02), 20% 002-2% 02, 10% CO2-2% O2, 5%
CO2-2% O2, 10% C02—5% O2, 5% C02-5% 0
air (0% CO

5% co —10% o and

2’ 2 2’
2—21% 02). After placing two replicates of cut

head lettuce in each chamber, approximately 6 cubic feet in
volume, the atmosphere conditions were established by flush-
ing each chamber with pure nitrogen (N2) until the desired

level of O was reached; then pure CO was added to estab-

2 2

lish the desired level of CO2. Daily monitoring of atmos-

phere composition was conducted by removing a representative
1 ml air sample and analyzing it by gas chromatography for
CO2 and 02. Atmosphere modifications were made as required

by flushing the chambers with CO or N

2 2'

b) Reduced pressure storage. Samples of cut head lettuce

 

were placed in mesh bags or in perforated plastic bags before
placing in vacuum chambers (8 liter desiccators), at 8.8°

and 0°C; chambers were evacuated to 1/10 and 1/5 atmosphere
with a vacuum pump. Each chamber was continuously supplied
with fresh humidified air. A control was established by
using a sealed non-perforated polyethylene bag and maintained
in atmospheric pressure and 0°C. Evaluations were made after
one week.

c) Plastic films. Cut head lettuce is normally transported

 

and stored in a plastic film, therefore, we investigated the
effect of varying the type of film on the keeping quality of

lettuce. Four films, 2 mil polypropylene, 2 mil

polyvinylchloride (PVC), 2 mil experimental polyethylene
and a commercially manufactured 2 mil polyethylene bag were
evaluated. In all cases the bags were made by heat sealing
a continuous sheet of the film on three sides, except the
commercial polyethylene bag which was only heat sealed to
close it. Two experiments were conducted.

Cut head lettuce was stored in heat sealed bags made
of each film. An analysis of the internal atmosphere of
each bag was made before opening by removing two representa-
tive 1 ml air samples. One air sample was analyzed for CO2
and O2 and the other for ethylene by gas chromatography.

The levels present suggested the inclusion of an ethylene
absorbent in the second experiment.

The same films as used in the first experiment were
used; 2 replicates of each film were placed at each tempera-
ture, one containing approximately 180 grams of Purafil (an
CgHu absorber available from H. E. Burroughs and Assoc. Inc.
Chamblee, Georgia). Each was monitored daily to determine
the rate of modification of CO2, 02 and C2Hu levels inside
the package. A small patch of rubberized self sealing con-
tact cement was applied to each package, through which

syringes were inserted for atmosphere sampling.

Handling methods. Several experiments were conducted

 

to determine the effect of handling methods, such as chemi-
cal treatments, method of chemical application and tempera-

ture,on keeping quality.

a) Chemicals. Following preliminary investigations, a trial

 

replicated A times was conducted comparing the following
treatments: 1) 0.2% sodium bisulfite (NaHSO3) solution dip,
2) cold water dip and 3) dry, untreated. Produce was stored

in heat sealed 2 mil polyethylene bags; final CO and O

2

levels were checked and related to quality ratings.

2

We investigated the feasibility of using various cal-
cium salt solutions including a saturated solution of
calcium oxalate, a 1% solution of calcium chloride, and a
1% solution of calcium nitrate, as a replacement for sodium
bisulfite. Lettuce was stored in 2 mil polyethylene heat
sealed bags and quality ratings related to final CO2 and 02

levels.

b) Generating sulfur dioxide. Several different procedures

 

for generating sulfur dioxide (802) within commercial poly—
ethylene bags, were evaluated, with the objective of replac-
ing the standard solution dip. Quantities of dry sodium
bisulfite ranging from 0.1 to 0.6% of total weight, were
placed in paper envelopes or plastic vials and inserted in
each package.

c) Temperature. The effect of temperature in relation to

 

length of storage and keeping quality was determined by the
evaluation of 3 treatments; 1) dry, untreated, 2) cold H20,
and 3) 0.2% NaHSOB; at 0° and 5°C, every 2 days over a two
week period. A similar experiment was conducted at 0°, 2.2°

and 3.3°C.

Results

Temperature effects on respiration. Carbon dioxide pro-

 

duction was calculated as milligrams of CO per kilogram hour

2
(Table 1); data is the average of 5 replicates, and reflects
the influence of temperature on cut head lettuce respiration.
Respiration rates at 5°C were approximately A0% higher than

at 0°C.

Modified atmospheres. a) Controlled atmospheres. Modi—

 

 

fication of normal atmospheres, by changing CO and/or 0

2 2
levels had a definite effect on the keeping quality of cut
head lettuce. Browning and discoloration was substantially
reduced in comparison with storage in regular atmosphere con-
ditions. Texture and flavor were also maintained at a high
level. Controlled atmospheres in combination with low tem-
perature resulted in the best quality after two weeks of

storage (Table 2).

b) Reduced pressure storage. Table 3, summarizes the

 

results of our experiments with cut head lettuce using
reduced pressure storage. The quality ratings indicate
that reduced pressure storage was not suited to the storage
of cut head lettuce. The reduction in quality was due to
severe browning, and texture loss.

0) Plastic films. The type of plastic film used in handling

 

cut head lettuce may well have an effect on the length of
time such lettuce can be kept. Our results also indicate
that keeping quality is related to the levels of CO2 and 02.

These levels are related to the permeability of the film and

Table 1. Effect of temperature on the respiration rate of
cut head lettuce.

 

 

Time of determination CO production (mg/Kg. hr.)

 

2

Days 0°C 5°C
1 A.6 8.2

2 7.2 7.9
5.8 7.9

A 5.7 7.6

5 5.6 7.8

6 5.4 7.2
7 5.2 7.0

 

10

Table 2. Effect of selected controlled atmospheres and
temperature on keeping quality of cut head let—

tuce after 2 weeks storage.

 

 

Temperature % COAtmosphgrSZ Rating % wt. loss
5°C 0 0 (100% N2) 7 i .3 .9
20 2 7 i .7
10 2 5 i .7 .0
5 2 6 i .9 .0
10 5 .5 i .A .3
5 5 A i .6 .A
5 10 5 i- .A .5
0 21 (Air) A i- .A .5
0°C 0 0 (100% N2) 7 i .0 .3
20 2 6 i .2
10 2 7 i .A .A
5 2 6 i .3 .3
10 5 7 i .3 .3
5 5 .5 i .2 .3
5 10 A i .A .2
0 21 (Air) 5 i .A .3

 

11

Table 3. Effect of reduced pressure storage on keeping
quality of cut head lettuce after 1 week of

 

 

 

storage.
Temperature Atmosphere Treatment Rating % wt. change
pressure
8.8°C 76 mm Hg. mesh bag 1 ——1
76 mm Hg. plastic bag 1 -—
0°C 76 mm Hg. mesh bag 2 —A.0
152 mm Hg. mesh bag 6 +.8
152 mm Hg. plastic bag 5 —.6
760 mm Hg. control 8 +.3

 

Lettuce under these conditions was completely broken down
and not weighed.

12

the degree of efficiency in sealing the film (10,21). We

again point out the importance of low 0 levels and the

2
effect of temperature (Table A). Highest quality was
attained at 0°C and where the 02 was lowest and CO2 was
highest.

Ethylene concentrations of .02 to .6 ppm were found in
the internal atmospheres of various packages. The use of an
ethylene absorber reduced the ethylene levels to .006 to .01
ppm, however, there was no apparent correlation between the
concentrations of ethylene found and the keeping quality of
cut head lettuce during storage over a 2 week period.

The rate at which 02 levels approach 2 to 3%, the
desired level, is dependent on the film's permeability. The
most rapid decline in 02 concentration occurred during the
first 2 days of storage and generally leveled off after A or
5 days (Fig. 1). When produce was stored at higher tempera-
tures, such as 5°C, considerable benefits in improved quality
occurred from proper sealing of films and consequent increase
in the modification of atmospheres. We attributed the dif-
ference in keeping quality between the two types of poly—
ethylene used, to the fact that the commercially manufactured
bag was only heat sealed on one side, while the experimental
polyethylene was heat sealed on all sides. A correlation
existed with quality ratings and internal atmosphere (Table

A).

Handling methods. The standard procedure in commercial

 

handling of cut head lettuce, has been to dip the product in

13

Table A. The influence of plastic films on keeping quality
of cut head lettuce after 2 weeks storage.

 

 

 

Temperature Plastic film Rating ngnal %02
5°C comm. polyethylene 2 i .7 1.0 17.0
exp. polyethylene 3 i .9 6.0 8.6
polyvinylchloride 5 t .A 9.0 8.A

polypropylene 6 i .A 10.0 A.8

0°C comm. polyethylene 5 i .3 2.0 18.8
exp. polyethylene 5 i .A 3.0 13.2
polyvinylchloride 6 i .6 7.0 10.8

polypropylene 7 i .A 9.0 11 2

 

$ oxygon

as oxygen

1A

    

.C‘..

-‘O-
’0- o- .
.Q

“----."‘ §

\

§~.

 

 

 

#
2 4 6 . 10 ‘2 14
Joys

.-... comm poly

 

 

 

mono poly
.....poly|nop
21 —P"
9““
‘S .-..o“‘ .---'----.
13 F ‘s I‘ w"""”
15 '
12 -
9 L
6 L
3 b o .c
o ‘ ‘ w
2 4 O . 1O '1 '4
Joy:

The effect of plastic film and temperature on the
modification of 0 levels in packages of cut head
lettuce, stored for two weeks.

15

a 0.2% solution of sodium bisulfite before bagging. This
procedure generally delays the browning of the cut product,
but results in considerable water soaking especially after
several days of storage. Flavor and texture may be affected,
and a rather objectionable odor is often detected.

a) Chemicals. Table 5, compares the standard treatment, with

 

a cold H2O treatment and a dry out control at temperatures of
0° and 5°C. Our results indicate that the present method may
not be the most practical or beneficial. Severe water soak-

ing occurred in the cold H O and sodium bisulfite treatments

2
partially because sufficient water could not be removed. As
shown in Table 5 and Fig. 2 the best results were obtained
at 0° and 5°C when out head lettuce was held in a dry
untreated condition.

Table 6 shows the effect of several other chemicals
which have received some use in other areas of freshness
preservation. Our results indicate an excessive degree of

browning and general discoloration with these chemicals.

b) Generating sulfur dioxide. An attempt was made to

 

generate SO2 inside packages by the insertion of various
quantities of dry sodium bisulfite. When this chemical was
placed in paper envelopes, lettuce immediately surrounding
the envelopes became 'slimy'; as structure was completely
destroyed. In cases when considerable quantities (A and 8
grams) were used, the entire bag of lettuce became slimy
and when similar quantities were placed in plastic vials

with perforated tops, no benefit beyond the dry control was

16

Table 5. Treatment effects on keeping quality of cut head
lettuce after 2 weeks storage in commercial poly—
ethylene bags.

 

 

 

Temperature Treatment Rating Cg:nal %O2
5°C 0.2% NaHSO3 5.0 i .3 11.9 3.0
cold H20 .7 i .8 8.2 A.9

dry untreated 5.3 i .5 5.3 8.8
0°C 0.2% NaHSO3 5.5 i .2 7.0 6.2
cold H2O A.9 i .9 5.1 8.2
dry untreated 6.3 i .3 A.0 12 O

 

l7

 

 

 

5°C
9:-
. 3
¢ .=.
2 6’ =
o _
h
3
7:.
U 3-
2 4 6 a 10 12 14
days
l'clry
0 °C i sodium bisulfite
9b Ewaier
a
c,
2:. 6r-
3
I
E
0' 3t

 

2 4 6 8 10 12 14
days
Fig. 2. The effect of treatment on the keeping quality of

cut head lettuce stored in commercial polyethylene
bags for 2 weeks at 0° and 5°C.

18

Table 6. The effect of chemicals on keeping quality of cut

head lettuce after 2 weeks storage in commercial

polyethylene bags.

 

 

 

Temperature Chemical Rating Cg:nal g
5°C sat. calcium oxalate 3 A.9 13.8
1% calcium chloride 3 5.3 10.6
1% calcium nitrate 3 A.5 5.3
0.2% sodium bisulfite 5 11.9 3.0
0°C sat. calcium oxalate 3 .2 1A.2
1% calcium chloride 3 2.0 18.0
1% calcium nitrate A 3.3 9.9
0.2% sodium bisulfite 5 7.0 6.2

 

19

achieved. In most cases when sliminess was detected, color
of the product was normal but texture was not.

0) Temperature. The most important factor in holding cut

 

head lettuce appears to be proper temperature control as
shown in Tables A and 5. A range of 0° to 2.2°C was accept—
able, however, as storage temperature increased storage life

was decreased proportionally.

Discussion

The respiration rate of cut head lettuce is an important
factor in determining the proper handling techniques for such
products. We demonstrated a clear correlation between tem-
perature and respiration rate and this in part explains the
beneficial effect of temperatures near 0°C in all of our
experiments (Table l). The value of low temperatures for
horticultural crop storage in reducing respiration rates is
well documented (17,23). The respiration rates of injured
produce is normally higher than for regular produce; an
increase of 20 to 25% in respiration rate for cut kale leaves
was noted when comparing to whole kale leaves (11). Tempera-
ture control is thus of greater significance for out prod-
ucts.

The use of modified atmospheres for the storage and
transportation of head lettuce has been established (6,22,
25), as well as for cabbage (l) and other horticultural
products (12,18). We also found this technique to be of
use for the storage of cut head lettuce; higher levels of

C02 and lower levels of 02 than those recommended for head

20

lettuce provided the best results (Table 2). We did not
experience problems with CO2 injury, except some slight

damage occurring at 20%. This is contrary to evidence
reported for CO2 injury to head lettuce at concentrations

as low as 1% (16,25). This difference is attributed to the
less compact nature of cut head lettuce as compared to heads
of lettuce. It should be emphasized that the maximum benefit
of controlled atmosphere results when it is combined with
temperatures of 0°C. These ideal conditions may be achieved
in transit, as has been the case for other produce items (16),
however, at the point of destination, the serving institution,
it may not be practical to expect modified atmosphere capa-
bilities. At this point we would suggest that adequate
refrigeration facilities are the most critical.

For this reason we spent considerable time evaluating
various types of plastic films, films that when properly
sealed have the capability of creating their own modified
atmosphere condition (10,21). The permeability of films
will affect the rate of O2 depletion and CO2 build up (9).
Respiration rate is also influenced by temperature (11,12)
and internal atmosphere (10). Our evidence indicates that
the film polypropylene is considerably more effective in
creating these conditions, than the other films tested
(Table A, Fig. 1). We would again emphasize the value of
proper sealing. The difference between the two poly-
ethylene films must be attributed to the method of bag

manufacture. Handlers of cut head lettuce probably could

21

improve their operations by purchasing only tOp quality
plastic bags.

Atmosphere modification by reduced pressures, generally
created unfavorable conditions for cut head lettuce storage
(Table 3). Storage life was less than one week, and quality
was considerably below that of the controls. The technique
appears to be of no value for this type of storage.

Ethylene was found not to be a factor in the browning
of cut head lettuce, although measurable quantities were
produced by the injured tissue over two weeks of storage.

Antibiotics and chemical dips have provided considerable
benefit in improving storage and shelf life of coleslaw, pre—
peeled French fry potatoes and other vegetable products (3,
A,7,2A). We evaluated several chemicals for effectiveness
in improving keeping quality of cut head lettuce. Because
of the present concern over food additives and food purity
we did not test any antibiotics, but instead concentrated
our efforts in develOping a handling method superior to the

current practice of using a 0.2% NaHSO dip. As previously

3
mentioned, the inability to remove excess water from dipped
lettuce before bagging results in excessive water soaking of
the product, and a resultant effect on texture and general
appearance. We also found this to be the case with other
chemical or water dips. Tables 5 and 6 reflect the level of
quality achieved after two weeks of storage. Sodium bi-

sulfite does prevent the majority of browning while the

other chemicals, calcium oxalate, calcium chloride, calcium

22

nitrate and water do not; in fact these latter treatments
seem to enhance the process.

Our evidence thus suggests that the best method of
handling cut head lettuce is to simply cut and bag the
product without additional processing. There is some evi-
dence of dehydration after 10 to 1A days of storage when
this method is used, however, this can be corrected by a
cold water rinse immediately before use. The rinse also
serves to restore freshness of the product.

The effect of temperature control cannot be replaced
by any other technique; this supports data published on the
storage of commodities such as Brussel sprouts (18), head
lettuce (10), cabbage (l) and products like coleslaw (12)
and sliced kale (11). Temperature maintenance in the range
of 0°—2.2°C is critical, with temperatures at the bottom of
that range preferred. Storage life will be cut sharply as
temperatures range above 2.2°C and this decline cannot be
significantly affected by chemical treatment or modified
atmosphere.

We have further evidence that cut head lettuce should
not be stored above 2.2°C if it is to be kept for more than
A days, the range of 0°-2.2°C will give satisfactory results
up to 8 days (Table 7) and as shown by Fig. 2, at 0°C
storage life may be as long as 10 days.

The best handling practice therefore is very tempera-
ture dependent. Our evidence strongly suggests that a

combination of the following factors, listed in their order

23

Table 7. Effect of temperature and treatment on the keep—
ing quality of cut head lettuce stored in com—

mercial polyethylene bags for 2 weeks.

 

 

Treatment

Days stored

Temperature

Rating

 

Dry

Sodium bisulfite

Dry

Sodium bisulfite

Dry

Sodium bisulfite

A

l2

l2

3.3°C

2.2°
00
3-3°
2.2°
00
33°
2.2°
00
33°
2.2°
00
33°
2.2°
00
33°
2.2°

00

EPQWSNNONNO

WWWU'l-C-‘UQU'I

 

2A

of importance will achieve the most favorable results when
handling cut head lettuce:
l) a 0°C temperature
2) a dry cut product
3) a properly sealed permeable package such as the
plastic polypropylene
A) the use of a 5 minute cold water rinse before con-

sumption to restore freshness.

(l)

(2)

(3)

(A)

(5)

(6)

(7)

Literature Cited

Adair, C. N. 1971. Influence of controlled-atmosphere
storage conditions on cabbage postharvest decay fungi.

Plant Dis. Reporter 55(10):86A-868.

 

Alexander, B. and F. J. Francis. 1963. Packaging and

storage of prepeeled turnips. Proc. Amer. Soc. Hort.

 

s91. 8A 513—518.

and . 196A. Packaging and

 

 

storage of prepeeled rutabaga. Proc. Amer. Soc. Hort.

 

 

 

Sci. 85:A57-A6A.
Carroll, V. J., R. A. Benedict and C. L. Wrenshall.

1957. Delaying vegetable spoilage with antibiotics.

Food Tech. 11:A90-A93.

 

Ceponis, M. J. and B. A. Friedman. 1957. Temperature
and shelf-life studies with pre-peeled potatoes. Amer.
Pot. J. 3A(l2):333—3Al.

Francis, F. J. 195A. Discoloration and quality main—

tenance in coleslaw. Proc. Amer. Soc. Hort. Sci. 75:

 

 

AA9—A55.

, B. L. Amla and A. Kiratsous. 1959.

 

Control of exudation in pre-peeled French—fry potatoes

with antibiotics. Food Tech. l3:A85-A88.

 

25

(8)

(9)

(10)

(ll)

(12)

(13)

(1A)

(15)

(16)

(17)

26

Greidanus, P. 1969. Prepackaging of vegetables.

Annual Report Sprenger Institut, pp. 71-72.

 

Hardenburg, R. E. 195A. How to ventilate packaged

produce. Pre-Pack-Age 7(6):1A-l7.

 

1971. Effect of in—package environ-

 

ment on keeping quality of fruits and vegetables.
HortSci. 6(3):198—201.
Hruschka, H. W. 1971. Storage and shelf life of

packaged kale. U. S. D. A. Marketinquesearch Report

 

 

923:7—10.
Hudson, D. E. and T. L. Senn. 1961. The influence of
temperature and modified atmosphere on the shelf life

of coleslaw. A. S. A. W. 58th Ann. Proc., p. 189.

 

 

Kaufman, J. 1961. Parsnips. Produce Marketing A(5):

 

25-26.

and M. J. Ceponis. 1962. Extended shelf

 

life for green lima beans. Produce Marketing 5(7):32-

 

33.

and J. M. Lutz. 195A. Lengthening the

 

shelf life of packaged coleslaw. Pre—Pack—Age 8(1):

 

23-26.

Lipton, W. J. 1968. Low 02 atmosphere benefits and

dangers. United Fresh Fruit and Vegetable Assoc. 1968

 

Yearbook, pp. 99—103.
Lutz, J. M. and R. E. Hardenburg. 1968. The commer-
cial storage of fruits, vegetables, and florist and

nursery stocks. Agric. Handbook 66, U. S. D. A.

 

(18)

(19)

(20)

(21)

(22)

(23)

(2A)

(25)

27

Lyons, J. M. and L. Rappaport. 1959. Effect of con-
trolled atmosphere on storage quality of brussel

sprouts. Proc. Amer. Soc. Hort. Sci. 81:32A-331.

 

 

Ponting, J. D., R. Jackson and G. Watters. 1971.
Refrigerated apple slices: effects of pH, sulfites
and calcium on texture. J. of Food Sci. 36:3A9-350.

2 and . 1972.

 

 

 

Refrigerated apple slices: preservative effects of

ascorbic acid, calcium and sulfites. J. of Food Sci.

 

37:A3A—A36.
Rogers, H. 1963. Films for produce packaging.

Produce Marketing 6(10).

 

Singh, B., D. J. Wang, D. K. Salunkhe and A. R. Rahman.
1972. Controlled atmosphere storage of lettuce 2.
effects on biochemical composition of leaves. J. of

Food Sgi. 37:52—55.

, C. C. Yang, D. K. Salunkhe and A. R. Rahman.

 

1972. Controlled atmosphere storage of lettuce 1.
effects on quality and the respiration rate of lettuce
heads. J. of Food Sci. 37:A8-51.

Smith, W. L., Jr. and R. E. Hardenburg. 195A. Anti—
biotic and other chemical dips reduce discoloration of

packaged coleslaw. Phytopathology AA:389-390.

 

Stewart, J. K. and M. Uota. 1971. C02 injury and

market quality of lettuce held in controlled atmos—

pheres. Proc. Amer. Soc. Hort. Sci. 96:27—30.

 

 

All ll l| lllllll III III. I 'llllll.

28

(26) Tolle, W. E. 1969. Hypobaric storage of mature—green
tomatoes. Marketing Research Report #8A2, U. S. D. A.
(27) Wu, M. T. and D. K. Salunkhe. 1972. Subatmospheric

pressure storage of fruits and vegetables. Utah Sci.

33(l)=29-3l-

APPENDIX

REDUCED PRESSURE STORAGE

In addition to testing reduced pressure storage on cut
head lettuce, its effect on endive, cabbage (coleslaw),
carrot sticks, celery sticks, and sliced radishes was

evaluated.

Materials and Methods

Cut products as listed were obtained from the Michigan
State University food processing center and placed in vacuum
chambers at two temperatures, 8.8° and 0°C. Samples were
contained in mesh bags and in perforated plastic bags;
chambers were evacuated to 1/10 and 1/5 atmosphere. A con-
trol was established by placing each vegetable in a sealed
non—perforated plastic bag and holding at 0°C. Products were
stored for 1 week before evaluating. Each evacuated chamber
was humidified by bubbling air through flasks of water (Fig.
3).

Results and Discussion

Results of our experiments are summarized in Table 8.
These data indicate that no benefit was achieved by the use

of reduced pressure at either temperature. In all cases of

29

 

gownsowmou
Eudow>

 

 

 

 

 

 

 

Amman
:Hscneflasm

opmaswon a: o >

 

 

o>am> Mao psgm

hmpoa BOHH

new Among
(\b
Ax

 

 

 

 

nlflnul

Reduced pressure storage test equipment.

3.

F1

31

Table 8. Effect of reduced pressure storage on the keeping
quality of cut vegetables after 1 week storage at
2 temperatures.

 

 

 

Product Temfiiga- IEESREII Treatment Rating cgagge
Celery 8.8°C 76 mesh bag 6 - 7
plastic bag 5 +.6

1A2 mesh bag 7 -3.A

0°C 76 mesh bag 6 -2.A

1A2 plastic bag 6 +.7

760 control 7 — 5

Carrots 8.8°C 76 mesh bag 5 -8.0
76 plastic bag 5 - 5

1A2 mesh bag 5 -6.8

0°C 76 mesh bag 5 - 5

1A2 plastic bag 7 +.5

760 control 6 -l.0

Radishes 8.8°C 76 mesh bag 3 -l.5
76 plastic bag 1 - A

1A2 mesh bag 2 -5.2

0°C 76 mesh bag A —6.0

1A2 plastic bag A +.8

760 control 7 -l.3

Cabbage 8.8°C 76 mesh bag 2 - 7
76 plastic bag 3 +.2

0°C 1A2 mesh bag 6 1.A

1A2 plastic bag 3 +.l

760 control 8 +.5

Endive 8.8°C 76 mesh bag 3 - 7
76 plastic bag 6 -8.7

0°C 1A2 mesh bag 6 -5.8

1A2 plastic bag 6 -1.3

760 control 8 -3.5

 

32

higher temperature, the leafy vegetables showed damage; almost
complete discoloration of the coleslaw, severe dehydration and
discoloration of endive. Radishes generally showed a con-
siderable degree of discoloration, while carrots and celery
tended to become dehydrated and in some cases texture was
affected.

Keeping quality was not improved by increasing humidi—
fication in each chamber; this was done by using an additional
water flask through which the air was passed before entering
the chamber.

The technique of reduced pressure storage appears to be

of no benefit for the storage of fresh cut vegetables.

SLICED RADISHES AS AN INDICATOR

From our reduced pressure storage data, we concluded
that sliced radishes would be a good indicator of the degree
of browning of produce, for a particular handling system.
Therefore, in combination with our cut head lettuce experi-
ments described previously, sliced radishes were used in

specific instances.

Materials and Methods

Modified atmospheres. a) Controlled atmospheres.

 

 

Sliced radishes were stored in controlled atmosphere chambers,
established at 5% 002-2% 02, 10% C02—2% 02, 5% cog—5% 02,

5% CO -10% 02, 5% 002—15% 02 and 0% co -21% 0 (Air).

2 2 2

33

Chambers were located at temperatures of 5° and 0°C and
radishes were evaluated after two weeks of storage. Atmos-
pheres were established and chambers monitored in the same
manner previously indicated.

Chemicals. An investigation of the effect of various

 

chemical dips on the keeping quality of sliced radishes was
conducted. Chemicals used were sodium bisulfite, calcium
oxalate, calcium chloride, calcium nitrate and water. After
treatment radishes were placed in heat sealed commercial

polyethylene bags and stored for two weeks at 5° and 0°C.

Results and Discussion

Modified atmospheres. a) Controlled atmospheres.

 

 

Controlled atmosphere conditions which were of value in pro-
moting the keeping quality of cut head lettuce (high CO2,
low 02 levels), do not appear to have the same effect for
sliced radishes (Table 9).

As is indicated by the rating column, a certain amount
of variability exists. The ratings are generally an indica—
tion of darkening of the radish slices, however, they should
be interpreted in the same manner as those described for the
cut head lettuce evaluation. The weight loss column also
indicates a moderate amount of dehydration, and this con—
tributes to a lowering of quality.

Chemicals. From our data (Table 10), we can see that

 

sliced radishes respond to chemical treatment somewhat better

than out head lettuce does, and further suggests that

3A

Table 9. The effect of selected controlled atmospheres on
the keeping quality of sliced radishes after 2
weeks storage.

 

 

Atmosphere

 

Temperature % CO2 % 02 Rating % wt. loss
5°C 5 2 2 2.1
10 2 A 1.9
5 5 A 2
5 10 7 1.7
5 15 6 2.3
O 21 (Air) 6 2.7
0°C 5 2 A 2.2
10 2 5 2.7
5 5 5 2.8
5 10 5 2.8
5 15 5 3.1
0 21 (Air) 3 3.1

 

35

Table 10. Effect of chemical treatments on the keeping

quality of sliced radishes stored for 2 weeks
in commercial polyethylene bags.

 

 

 

Temperature Treatment Rating Cg:nal g2
5°C 0.2% sodium bisulfite 5 11.3 A.0
sat. sol. calcium oxalate 6 A.9 13.8
1% sodium chloride 6 5.3 10.6
1% sodium nitrate 6 A.5 5.3
water 5 ”-9 2-3
0°C 0.2% sodium bisulfite 6 6.A 9.1
sat. sol. calcium oxalate 6 3.2 1A.2
1% sodium chloride 5 2.0 18.0
1% sodium nitrate 6 3.3 9.7
water 5 1.6 12.1

 

36

controlled atmospheres are of little value, since the best

ratings reflect much higher 0 levels than we found best for

2
cut head lettuce. Dehydration does not seem to be a problem
when storing in plastic bags, and radishes drain easier than
out head lettuce, therefore, water soaking does not become a

problem and may account for the increased benefit of chemical

dips for sliced radishes over their effect on cut head lettuce.

 

”71711111171177?“MIEIFMEIIMQIMWWS