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84 1079

This is to certify that the

thesis entitled
Influence of Ventilation Rate on Potato
Quality Out of Storage

presented by

Todd D. Forbush

has been accepted towards fulfillment
of the requirements for

_Mfl_$_terLS__degree in A9 - Enq -

 

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Date W

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

 

 

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Michigan State

University J

 

 

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PLACE IN RETURN BOX to move this checkout from your record.
TO AVOID FINES return on or before duo due.

DATE DUE DATE DUE DATE DUE

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MSU I. An Mmdlvo Action/Emil Opponunlty In!
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Influence of Ventilation Rate on

Potato Quality Out of Storage

BY
Todd David Forbush

A THESIS

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

MASTER OF SCIENCE
Department of Agricultural Engineering

1989

9030715

ABSTRACT

INFLUENCE OF VENTILATION RATE ON

POTATO QUALITY OUT OF STORAGE

BY

Todd D. Forbush

A potato storage research facility was constructed and
instrumented to study the effects of varying the ventilation
rate through the pile on the quality of stored potatoes.

The facility consisted of three storage bins measuring
2.4 m square by 4.9 m deep. The ventilation systems were
controlled using a FANCOM 1056 environmental control
computer.

The three ventilation rates studied were 56 m3/tonne-
hr, 112 m3/tonne-hr, and 168 m3/tonne-hr. The
study was conducted for three storage seasons 1986 - 1989.

A variable speed fan capable of delivering from 0 - 112
m3/tonne-hr was substituted for the 168 m3/tonne-hr rate for
the second and third storage seasons.

The results indicate that average weight loss within
the bins was not a function of airflow rate. However,
weight loss did vary vertically within the potato bins
ventilated at the recommended ventilation rate for Michigan.

The potatoes stored in the research bins attained an

 

ac

tC

Todd D. Forbush

acceptable process color following storage durations of 115
to 142 days.

A finite difference temperature generation-weight loss
simulation model (Lerew, 1978) was used to evaluate
different management strategies for removing moisture from
the surface of potato tubers. The simulation studies
indicated that ventilating with high airflow rates (112
m3/tonne-hr) and low inlet relative humidities (65%) will
result in acceptable moisture removal with minimal weight
loss.

A madel was developed to predict the time required for
pile temperature changes to be made. The model was used to
study the effect of airflow rate and inlet conditions on the
time required to cool the potato pile. The results of this
study indicated that when cooling a potato pile in 0.5 °C
increments, the cooling is primarily controlled by latent heat
energy transfer.

It was concluded that the current ventilation
recommendations are acceptable given the state of potato
storage monitoring in Michigan. Advances in control and
monitoring systems may lead to a reduction in ventilation
requirements based on information collected during each

storage season.

Dedicated To

My wife Kristen

ii

Brook
major
and t
expre
have

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commi

their

Agric
sPeci
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even

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Willi
many

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to my
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to CO

ACKNOWLEDGEMENTS

I would like to express my appreciation to Dr. Roger
Brook for taking on the responsibilities of serving as my
major professor following the death of a very good friend
and teacher, Dr. Burton Cargill. The opinions and ideas
expressed by these two men, however different they were,
have given me the ability to approach challenges with a new
perspective. .

Many thanks also to the members of the guidance
committee, Dr. John Gerrish and Dr. Arthur Cameron, for
their contributions to this work.

Thanks also to the faculty and staff of the
Agricultural Engineering department. Four friends deserve
special recognition for their support throughout this
project. These include; Dennis Welch, who often sacrificed
even his personal vehicle to assure that the spuds were
harvested, Dirk Maier, for his many suggestions and
willingness to lend a helping hand, Keith Tinsey, who shared
many hours on the road in "spud van" in deep discussion, and
John Mills for his assistance in completing this work.

I would also like to express my greatest appreciation
to my wife Kristen for the support given me, and for putting
up with all the hours spent working on this and other
projects.

Last, but not least, to my parents Marshall and Margret
Forbush for their support and for giving me the perseverance

to complete this work.

iii

 

List

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Table of Contents

LiSt Of Tables 0 O O O O O O O O O O O O O I 0

List of Figures . . . . . . . . . . . . . . .

Nomenclature . . . . . . . . . . . . . . . . .

1.

1.1
1.2

Introduction . . . . . . . . . . . . . . .

The Potato in Storage . . . . . .
Potato Storage Ventilation . . .

2. ObjECtives O O O O O O O O O O O O O O O O

3. Literature Review . . . . . . . . . . . . .

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Managing the Potato Storage .
Equalization and Drying Phase
Wound Healing Phase . . . .
Pre-Conditioning Phase . . .
Cooling Phase . . . . . . .
Holding Phase . . . . . . .
Reconditioning Phase . . . .
Weight Loss During Storage .
Potato Quality Requirements
Potato Storage Ventilation
Recommendations . . . . . . . .
Potato Storage Ventilation
Modeling . . . . . . . . . . . . .

4. Methods and Materials . . . . . . . . . .

4.1

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Storage Research Facility
Construction . . . . . . .
Ventilation System Design
Research Potato Samples .
Environmental Control . .
Sensor Placement . . . . .
Ventilation Fan Control . .
Fresh Air and Relative Humidity
Control . . . . . . . . . . . . .
Computer Influences and Setpoints
Data Logging . . . . . . . . . .
Storage Management Procedures
Suberization . . . . . . . .
Pre-Conditioning . . . .
Cool-Down . . . . . . .
Holding . . . . . . . .
Evaluation Procedures .
Potato Quality . . . . .
n

Environmental Control A aly 1

iv

4.7 Computer Modeling and Simulation . .
4.7.1 Condensation Control Simulation . .
4.7.2 Temperature Change Simulation . . .
5. Results and Discussion . . . . . . . . . . .
5.1 Weight Loss Results . . . . . . . .
5.2 Weight Loss Discussion . . . . . . .
5.2.1 Differences Between Bins . . . . . .
5.2.2 Weight Loss Within Each Bin . . . .
5.3 Pre-Harvest Internal Sugar Analysis
Results . . . . . . . . . . . . . .
5.4 Pre-Harvest Internal Sugar Analysis
Discussion . . . . . . . . . . . . .
5.5 Process Quality Results . . . . . .
5.6 Process Quality Discussion . . . . .
5.7 Environmental Control Results . . .
5.8 Environmental Control Discussion . .
5.9 Ventilation Fan Operation Results .
5.10 Ventilation Fan Operation Discussion
5.11 Temperature Change Analysis Results
5.12 Temperature Change Discussion . . .
5.13 Temperature Change Model . . . . . .
5.14 Temperature Change Modeling
Discussion . . . . . . . . . . .
5.15 Finite Difference Control Model
Results . . . . . . . . .
5.16 Finite Difference Control Model
Discussion . . . . . . . . . . . .
5.17 Simulation of a Condensation Problem
5.18 Discussion of a Condensation Problem

6. Summary and Conclusions . . . . .
7. Suggestions for Future Research .

List of References . . . . . . . . .

 

 

 

Tabl
1.1

1.2

3.1

5.4

5.5

5.5

5.7

5~8

5.6

5.7 '

5.8

List of Tables

Utilization of the 1983 Potato Crop . . .

Distribution of Annual Potato Production
by Harvest Season in the United States . .

Ventilation Rate Recommendations for
Different Potato Production Regions
throughout the World . . . . . . . . . . .

Influences and Setpoints used for the
FANCOM 1056 Computer for the Control of
a Potato Storage Environment . . . . . . .

List of Parameters Stored on the Personal
Computer by the FANCOM Control System . .

Weight Loss Percent of the Potatoes
Stored in the Simulated Storage Facility
for the 1986-87 Storage Season . . . . .

Weight Loss Percent of the Potatoes
Stored in the Simulated Storage Facility
for the 1987-88 Storage Season . . . . .

Weight Loss Percent of the Potatoes
Stored in the Simulated Storage Facility
for the 1988-89 Storage Season . . . . .

Internal Sugar Content of Potato Tubers
Before Harvest 1987-88 . . . . . . . . . .

Internal Sugar Content of Potato Tubers
Before Harvest 1988-89 . . . . . . . . . .

Frito-Lay Cook Test Results for the
Potatoes Stored in the Simulated Storage
Research Facility for the 1986-87

Storage Season . . . . . . . . . . . . . .

Frito-Lay Cook Test Results for the
Potatoes Stored in the Simulated Storage
Research Facility for the 1987-88

Storage Season . . . . . . . . . . . . . .

Frito-Lay Cook Test Results for the
Potatoes Stored in the Simulated Storage
Research Facility for the 1988-89

Storage Season . . . . . . . . . . . . . .

vi

16

29

30

44

44

45

49

49

53

54

55

 

 

 

 

 

5.9

5.10

5.11

5.12

5.13

5.14

5.15

5.15

5.17

5'18

5.9

5.10

5.11

5.13

5.17

Process Color of Potatoes Taken form the
Potato Pile Surface within the Simulated
Storages throughout the 1988-89

Storage Season . . . . . . . . . . . . . . 57

Percent Internal Glucose of Potatoes Taken
from the Potato Pile Surface within the
Simulated Storages throughout the 1988-89
Storage Season . . . . . . . . . . . . . . 57

Sucrose Rating of Potatoes Taken from the
Potato Pile Surface within the Simulated
Storages throughout the 1988-89 Storage

Season . . . . . . . . . . . . . . . . . . 58

Average and Standard Deviation of the
Differences Between the Desired and Measured
Average Pile Temperature for the Simulated
Storage Research Facility for Three

Storage Seasons . . . . . . . . . . . . . 67

Average and Standard Deviation of the
Differences Between the Desired and Measured
Inlet Relative Humidity for the Simulated
Storage Research Facility for Three

Storage Seasons . . . . . . . . . . . . . 68

Fan Time Operation for the Simulated
Storages, 1987-88 Storage Season . . . . . 70

Fan Time Operation for the Simulated
Storages, 1988-89 Storage Season . . . . . 71

Average and Standard Deviation of Time,
Relative Humidity, and Temperature
Differential for 0.5 0C Temperature Change
during the 1988-89 Storage Season . . . . 77

Time Required for a Temperaturs Change

from 12 °C to 11.5 °C in a 4 m Column of
Potatoes with Inlet Conditions of 11.0 °C,

90 % RH, a 0.25 °C Temperature Gradient

within the Potato, and Various

Airflow Rates . . . . . . . . . . . . . . 79

Predicted Weight Loss and Time Required

to Remove Condensation from the Surface

of Potato Tubers Utilizing Various Air

Inlet Conditions and Ventilation Rates . . 91

vii

 

 

 

Figu1
4.1

4.3

4.4

5.1

5.2

5.3

5.4

5.5

List of Figures

Figure Page
4.1 Overview of the Simulated Storage

FaCility O O O O O O O O O O O O O O O O 19

4.2 Detail of Ventilation System for the
Simulated Storage Bins . . . . . . . . . 20

4.3 Detail of Sensor Placement within the
Simulated Storage Research Bins . . . . 25

4.4 Electrical Analogy used to Model the
Heat and Mass Transfer System Involved.
while Cooling Potatoes . . . . . . . . . 41

5.1 Sucrose Rating and Percent Glucose
Readings of Tubers throughout the 1987
Growing Season . . . . . . . . . . . . . 51

5.2 Sucrose Rating and Percent Glucose
Readings of Tubers throughout the 1988
Growing Season . . . . . . . . . . . . . 52

5.3 Sucrose Rating, Percent Glucose, Chip
Color, and Average Temperature for
Simulated Storage Bin #1 during the
1988-89 Storage Season . . . . . . . . . 59

5.4 Sucrose Rating, Percent Glucose, Chip
Color, and Average Temperature for
Simulated Storage Bin #2 during the
1988-89 Storage Season . . . . . . . . . 60

5.5 Sucrose Rating, Percent Glucose, Chip
Color, and Average Temperature for
Simulated Storage Bin #3 during the
1988- 89 Storage Season . . . . . . . . . 61

5.6 Predicted Time Required to Complete a
0. 5 oC Temperature Change in a Four Cubic
Meter Column of Potatoes, with a 0. 5 °C
Internal Temperature Gradient, Inlet
Relative Humidity of 90 % and Various
Airflow Rates . . . . . . . . . . . . . 80

5.7 Predicted Time Required to Complete a
0. 5 °C Temperature Change in a Four Cubic
Meter Column 0 Potatoes with a an Airflow
Rate of 56.1 m /tonne-hr, an Inlet Relative
Humidity of 90 % and Various Internal
Temperature Gradients . . . . . . . . . 81

viii

 

 

 

 

5.8

5.9

5.10

5.11

5012

5.8

5.9

5.10

5.12

Predicted Time Required to Complete a

0.5 °C Temperature Change in a Four Cubic
Meter Column of Potatoes, with a 0.5 °C
Internal Tempegature Gradient, an Airflow
Rate of 56.1 m /tonne-hr, and Various

Inlet Relative Humidity Levels . . . . . 81

Comparison of the Simulated and Real

Average Pile Temperatures during a Steady
State Storage Period for the 1988-89 Storage
Season at an Airflow Rate of 56.1 (Top) and
112.2 m /tonne-hr (bottom) . . . . . . . 86

Comparison of the Simulated and Real

Average Pile Temperatures during a
Temperature Change Period for the 1988-89
Storage Season at an Airflow Rate of

56.1 (Tap) and

112.2 m /tonne-hr (bottom) . . . . . . . 87

Comparison of the Simulated and Real

Pile Temperature at a Distance of 1.22 m

from the Storage Floor during a Temperature
Period for the 1988-89 Storage Season at an
Airflow Rate of 56.1 (Top) and

112.2 m3/tonne-hr (bottom) . . . . . . . 88

Water Film Thickness on the Tuber Surfaces
Simulated within a Four Meter Deep Pile with
Two Meters of 21 °C Temperature Potatoes on
Top of Two Meters of 10 °C Temperature
Potatogs at a ventilation Rate of

56.1 m /tonne-hr

(Top : Complete Recirculation)

(Bottom : Inlet Conditions,

15.5 °c, so 3 RH) . . . . . . . . . . . 92

ix

1. Introduction

Potatoes were one of the first crops planted as the
pioneers moved into the state of Michigan. Potato
cultivation has grown from small scale production to its
current status as an industry. In 1984, a total of 688,200
tonnes of potatoes were harvested from 23,085 ha of Michigan
soil for an average yield of 29.8 tonnes/ha. This placed the
state ninth in production among potato producing states in
the United States. The production area for the United
States was 541,364 ha in 1984, with an average yield of 30.4
tonnes/ha for a total production of 16.44 million tonnes.

Potato utilization is outlined in Table 1.1.

 

Table 1.1 Utilization of the 1983 United States Potato

Crop.
Percent by Volume Use

82.5 % Human Consumption
(32.6 % Fresh )
(49.9 % Processed)

0.9 % Starch, Flour

1.5 % Animal Feed

7.6 % Seed

7.4 % Shrinkage & Loss

* Taken from National Potato Council's Potato Statistical
Yearbook, 1983-84 (Anonymous, 1984).

 

Potatoes are harvested during all four seasons in the
United States, with the majority of the annual crop
harvested in the fall. The production distribution among

these harvest periods is given in Table 1.2.

 

 

 

Table 1

* Take
Year

 

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Table 1.2 Distribution of Annual Potato Harvest by
Season in the United States.

Percent of Production Harvest Season
0.7 % Winter
5.9 % Spring
5.8 % Summer
87.6 % Fall

* Taken from National Potato Council's Potato Statistical
Yearbook, 1983-84 (Anonymous, 1984).

 

Table 1.2 indicates the importance of the storage of
potatoes in the United States to maintain a uniform supply
of potatoes for use throughout the year. It is estimated
that 8.73 million tonnes or 53 % of the United States potato
crop is held in storages across 15 states from October
through the month of December, with 2.29 million tonnes or
13.9 % of the Crop remaining in storage through May 1
(Anonymous, 1984). The necessity of maintaining high
quality potatoes through a storage period of nine months is
evident.

1.1 The Potato in Storage

The goal of potato storage is to maintain a near
harvest-quality potato throughout the storage season. The
potato storage is not a hospital; there can be no recovery
for poor quality potatoes in going into the storage.

The quality of potatoes from storage is directly
proportional to the quality of the potato going into the
storage. Factors such as potato maturity, bruising,

disease, stress during the growing season, and foreign

material content in the storage (dirt, vines, stones, etc.),
all play a major role in determining the potato quality out
of storage.

The definition of "quality" is based on the market for
which the potato is destined. For table stock potatoes, it
is critical that the potato maintain a smooth unscarred
surface and a high percentage of its original internal
moisture. These quality indices can be controlled by
maintaining a storage environment that limits the water loss
of the potato due to evaporation and respiration. For
potato processors, a low reducing sugar content is necessary
for acceptable process color. The reducing sugar content of
the potato is a function of the variety, growing conditions,
maturity at harvest, the storage environment and other
factors which are not clearly understood. To avoid the
build up of reducing sugars, a storage environment must be
maintained that will inhibit the low temperature conversion
of starch to sugar (Rastovski et. a1., 1987), while
enhancing the progression of tuber respiration to remove
existing reducing sugars.

Adequate environmental control is important to maintain
an acceptable level of potato quality throughout storage.
Ventilation air is the means of controlling the potato
storage environment. The adequacy of the environmental
control is directly dependent on the performance of the

ventilation system.

1.2 Potato Storage Ventilation Systems

Systems used for the ventilation of potato storages
have evolved regionally throughout the world since the
introduction of through-the-pile ventilation. At the
International Potato Storage Symposium held at Michigan
State University in 1985, ventilation recommendations ranged
from 22 m3/hr-tonne for the Eastern Oregon and Idaho regions
of the United States (Waelti, 1989) to 150 m3/hr-tonne for
several Western European countries including The Netherlands
(Hesen et. a1. , 1989), Hungary (Horvath, 1989), and West
Germany (Leppack, 1989). The reasons for these variations
include local climate, market requirements, and condition of
potatoes going into the storage. Hesen (1989) states that
'the shorter the duration of favorable outside air
temperature, the higher the ventilation rate required'. He
argued that the high ventilation rates 'make it possible to
keep potatoes at the mean minimum temperature of (the)
outside ambient (temperature)'. But he also stated that 'in
modern storages, lower ventilation rates could be used, but
the storage managers are reluctant to change .. '. Horvath
(1989) agreed with Hesen stating that 'just after
harvesting, highly humid cool air, suitable for the uniform
drying and cooling of potatoes, is available during limited
periods .... therefore the ventilation systems are designed
with the highest reasonable ventilation performance.‘

In response to the many questions that were raised

during International Symposium, this study has evolved as an

attempt to justify or verify the current recommendations and

practices in ventilation system design in Michigan.

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2.0 Objectives

The goal of this study is to determine the effect of
different rates of through-the-pile ventilation on potato
quality from potato storages in Michigan, justify the
current ventilation recommendations for Michigan, and
determine a probable cause for the differing ventilation
recommendations found elsewhere. The following set of
individual objectives were established to attain the goal of
this study:

1) Construct an on-farm storage research facility which
would acourately control and monitor the potato storage
environment with different air flow rates.

2) Modify an existing potato storage temperature and
weight loss model (Lerew, 1978) and compare its predicted
results with the experimental results.

3) Evaluate management strategies for removing moisture
from and cooling potatoes stored in bulk piles with the

simulation models.

3. Literature Review
3.1 Managing The Potato Storage Environment
The storage environment is the key to the success of
potato storage as it directly affects the market quality of
stored potatoes. Three aspects of the storage environment
that are important to the quality of potatoes are the
storage temperature, ventilation air relative humidity, and
oxygen/carbon dioxide levels within the storage. The
management of the potato storage depends on the quality
of the crop that will be placed into the storage bin. The
following sections outline storage management for mature
potatoes grown and harvested under ideal conditions, these
procedures are based on conversations with potato growers in
Michigan. Three terms require definition:
1) Mature potatoes - Potatoes with good skin set harvested
from potato plants with dead vines.
2) Ideal growing conditions - No excessive stress on
the tubers, ie. moisture,
nutrient, heat, or mechanical.
3) Ideal harvest conditions - soil temperatures between
10 and 18 °C with moisture content

below field capacity.

3.1.1 Equalization and Drying Phase
The pulp temperature of potatoes harvested under ideal
conditions should be between 10 and 18 °C. The
tuber pulp temperature often ranges 5 Co or more throughout

a bin of freshly harvested potatoes due to time of harvest

ax

and soil temperatures. The equalization and drying phase
takes place immediately after placing the potatoes in
storage to allow pile temperature to equalize. The
equalization and drying phase is also required to dry any
surface moisture from the potatoes. Traditional management
schemes would suggest that the main ventilation fan should
run continuously during this phase. The average potato pile
temperature should equalize within 2 0C of the potato pulp
temperature upon entry into storage. The completion of the
equalization and drying phase is indicated by a dry pile of
potatoes with a uniform pile temperature. The equalization
and drying phase may take up to one week or may not be
required at all if the potatoes are harvested from uniform

soil under ideal harvest conditions.

3.1.2 Wound Healing Phase

The wound healing phase is initiated upon the
completion of the equalization phase. The purpose of wound
healing is to allow the tubers to heal wounds from
harvesting and handling that occurred prior to storage.
Pisarczyk (1982) reported that suberization or wound healing
is vitally important to the success of extended storage.
The potato pile temperature is generally allowed to seek its
own level in the range of 15 to 18 °C as long as
temperatures of 21 °C are not exceeded (Schaper et. a1.
1989; Cargill et. a1. 1989). The relative humidity should
be in the 95 to 100% range, with some producers advocating

that condensation must form on the surfaces within the

 

90

SU

potato storage, or the potato bin must ”sweat" for good
suberization.

Varns et. a1. (1986) reported that the process of
allowing the potatoes to "go through a sweat" was conducive
to high carbon dioxide build up within the potato pile.
Carbon dioxide levels in excess of 6% were measured in a
commercial potato storage facility in North Dakota. It has
also been noted that high levels of carbon dioxide, above 1%
for some varieties (Cameron, 1988), inhibit the formation of
the wax or suberin layer in potatoes. Wax (suberin) layers
prohibit the entry of pathogens which will damage and limit
the storage potential of potatoes.

Fan operation recommendations vary during suberization
from continuous ventilation at low rates (Sparks et. a1.,
1968) to fan operation in the evenings only to assure no C02
build up occurs (Schaper et. a1., 1989). Cargill et. a1.
(1989) recommended that ventilation fans should be run at
least eight hours a day in two four hour intervals to
maintain acceptable temperatures and reduce the
concentration of carbon dioxide generated by potato
respiration. The common industry practice in Michigan is to
run ventilation fans continuously at a rate of
56 m3/tonne-hr to minimize carbon dioxide accumulation,

while maintaining a uniform pile temperature.

10

3.1.3 Pre-Conditioning Phase

The pre-conditioning phase of storage has been
commercially adapted due mostly to the unpredictable nature
of reconditioning process varieties that have been stored at
temperatures in the 8 to 10 C range following suberization.
Schaper et. a1. (1989) indicated that inadequacies exist in
reconditioning with the statement that "certain lots of
potatoes do not recondition to the desired color, while
those that do will not stay in the desired condition for an
indefinite period of time.“

The concept of pre-conditioning was defined by Singh
(1974) as the holding of potatoes at 18 °C for six weeks
prior to long term storage at low temperatures. The purpose
of pre-conditioning is to eliminate pools of reducing sugars
prior to low temperature storage. The current industry
practice for pre-conditioning is to maintain the storage
environment similar to the suberization phase. The main
difference being that the potato pile temperature is
actively controlled around the 16 °C level. At this
temperature, the respiration rate of potatoes is sufficient
to respire or "burn” away existing reducing and non-reducing
sugar accumulated within tubers.

The duration of the pre-conditioning phase is dependent
on the process color of the potatoes, as determined by sugar
content. Potatoes harvested in an immature state may take

months to condition, and may never process adequately. Pre-

CC

at

In

11

conditioning is complete when desirable process color is

attained.

3.1.4 Cooling Phase

The rate of cooling is dictated by the ambient
conditions if refrigeration is not available. Time periods
when cooling air is available generally exist only in the
night in many potato producing regions; a time when manual
control is difficult (Hesen et. a1., 1989; Thornton, 1989).
For this reason, it is critical to have a control system
that is capable of introducing cool fresh air into the
storage any time that it is available.

The nature of the cooling process is dependent on the
desired market for the potato. A table stock or seed potato
producer is interested in reducing weight loss and soft rot,
which may dictate a quick cooling phase to the temperature
range of 5 °C. Process potato producers must be concerned
with the reducing sugar content of the potatoes, which may
be controlled by slower cooling practices. A maximum
cooling rate of 3 °C per week for a processing potato
storage is recommended by Cargill et. a1. (1989) to reduce
temperature stress and avoid reducing sugar build up. Other
process storage recommendations (Tolsma, 1988) suggest
cooling at a rate as fast as possible to 10 °C, then slow

cooling if desired below this temperature.

12

3.1.5 Holding Phase

Once the potatoes are cooled to the proper holding
temperature, the amount of ventilation is reduced to a
minimum level capable of maintain a uniform pile temperature
within 0.5 0C (Cargill et. a1., 1989) of the desired level.
A fine level of temperature control is necessary to maintain
the required process color as storage managers lower the
average pile temperature near the critical level for the
variety being stored. A uniform pile temperature also helps
maintain a uniform product throughout the storage season,

which increases the value of the crop at the market place.

3.1.6 Reconditioning Phase

Reconditioning is the practice of warming the
potatoes from the holding temperature to obtain desirable
process color and reduce the handling damage that occurs
while unloading the storage. During reconditioning, the
potatoes are slowly warmed by either allowing generated heat
to remain within the storage, or through the use of heaters.
Ventilation air with a low relative humidity is generally
recommended to reduce the possibility of moisture condensing
on the cool tubers as warming occurs (Schaper et. a1.,
1989). The unpredictable nature of the reconditioning of

process potatoes was previously stated.

13

3.2 Weight Loss During Storage

The potato industry suffers large losses each year due
to weight loss throughout the storage season. It is
estimated that shrinkage and loss figures for the potato
crop in the United States has ranged from 6 to 10% annually
over the past decade (Anonymous, 1984).

Yeager (1977) studied the effect of potato pile
pressure and storage environment relative humidity on weight
loss of potatoes stored for a six month period. The work
was done with small lots of potatoes stored in boxes
measuring 0.03 m3. A spring system was developed which
placed pressure on the potatoes similar to the pressure
experienced within a pile of potatoes. Yeager concluded
that the relative humidity of the storage environment was
the most important parameter studied. He also reported that
pile pressure may affect weight loss.

Villa (1973) developed a semi-theoretical mathematical
model for predicting moisture losses from horticultural
products in storage. The model included an estimate of the
skin permeability and surface area of potatoes (cv. Monona).
Villa concluded that there was a correlation between the
velocity of the ventilation air and the moisture lost, but
the effect was slight; doubling the airflow rate resulted in
an increase in predicted moisture loss of only 15%.

A comprehensive study of weight loss and respiration
rates of small lots of potato tubers was conducted by Hunter

(1985). The conclusion of Hunter's work was that the weight

lo:
on
de
pe
be

14

loss is a function of potato variety, preharvest treatment,
curing regimen, storage temperature and vapor pressure
deficit. Hunter also concluded that if ventilation is
performed with high relative humidity air, weight loss may

be correlated to tuber respiration rate.

3.3 Potato Quality Requirements

The definition of the potato quality from storage is
determined by the end user of the product. Gould (1988)
defined a series of quality parameters which are of
importance to the processing industry. These include the
shape of the tubers, external defects such as bruises or
sprouts present, specific gravity of the tubers, and the
process color as evaluated with color charts or an Agtron
colorimeter.

The color of the finished product is affected by the
concentration of reducing sugars (glucose and fructose)
present in the tuber upon processing. The potato tuber is a
living organism which has a solids content primarily made up
of starches (Rostovski, 1987). Starch within the potato is
converted to sucrose, a twelve carbon disaccharide which is
broken down enzymatically into the reducing sugars glucose
and fructose. These reducing sugars are then consumed in
the respiration process of the tuber throughout the storage
season for the generation of energy.

The size of the free sugar pools within the tuber is
determined by the cultivar, maturity at harvest, and

temperature of storage. Sowokinos (1987) proposed a

 

Ch
th

ut

15

Chemical Maturity Monitoring (CMM) technique to determine
the chemical maturity of the tubers upon harvest. CMM
utilizes sugar analysis methodology to evaluate the tuber
flesh and determine the potential of the tuber to obtain
acceptable sugar levels after harvest. Sowokinos found that
if potatoes (cv. Norchip) had a Sucrose Rating below 1.0,
and a glucose percentage below 0.035 %, then the potatoes
could survive long term storage and still produce potato
chips with acceptable process color upon removal from long

term storage.

3.4 Potato Storage Ventilation Recommendations

An international meeting of potato storage researchers
took place at Michigan State University in 1985 to discuss
the practices and recommendations employed for the storage
of potatoes throughout the world. Table 3.1 lists the
ventilation rate recommendations for the different potato
production regions represented.

The rationale given for the recommendations in Table
3.1 centered around the drying and cooling process which

take place during the initial phases of storage.

Tab

16

 

Table 3.1. Ventilation Rate Recommendations for Different
Potato Production Regions throughout the World.

 

Ventilation Production Reference
Rate Region
m3/tonne-hr
72-108 Great Britain Statham (1989)
100-150 West Germany Leppack (1989)
64- 92 Belgium Hesen (1989)
100-150 The Netherlands Hesen (1989)
120-150 Hungary Horvath (1989)
69- 83 Poland Karwowski (1989)
50-100 Denmark,Norway
Finland Rasmussen (1989)
24- 72 Japan Wada (1989)
United States
19- 32 Northwest Thornton (1989)
26- 56 Red River Valley Schaper (1989)
38- 56 Michigan Cargill (1989)
22- 50 Northeast Hunter (1989)

 

3.5 Potato Storage Ventilation Modeling

The evolution of the digital computer has advanced the
area of mathematical modeling and simulation of physical
events. Prior to the computer age, the determination of
cause-and-effect relationships was limited to time consuming
experimental procedures. In the field of agriculture, where
experimentation is restricted to a seasonal exercise, the
practices of mathematical modeling and simulation are
especially valuable tools. Models allow researchers to
evaluate theories throughout the year, thus minimizing the
amount of experimentation required.

Misner (1973) developed a simulation routine to model

the cooling of potatoes. The model used the set of

l7

differential equations for simultaneous heat and mass
transfer developed by Bakker-Arkema et. a1. (1967). A
finite difference approach to computing the potato and
ventilation air parameters during cooling was applied. An
attempt was made to determine the optimum ventilation rate
based on a cost analysis which included the cost of the fan,
the cost of fan operation, and economic losses due to
product moisture loss. Misner concluded that an
economically feasible range for the ventilation rate would
be from 69 to 97 m3/tonne-hr, based on a bulk density of
1085 kg/m3 of potatoes, and a pile depth of 4 m.

Lerew (1978) investigated different ventilation
management practices and control procedures utilizing much
the same approach as Misner. Lerew that control of the
ventilation system designed to ventilate only when excessive
temperature build-up had occurred, resulted in a lower
weight loss than did other ventilation control strategies.

Ofoli (1984) approached the problem of optimizing
potato storage ventilation rates by attempting to minimize
entropy production during ventilation. The conclusion of
-this work was that the dominant parameter in entropy
production in the ventilation of potato storages is the
velocity of airflow over the tubers. The optimum
ventilation rate with respect to entropy production arrived
at by Ofoli was in the range of 35 to 40 m3/tonne-hr, based
on a pile depth of 4 m and a bulk density of 1085 kg/m3.

4.0 Methods and Materials
4.1 Storage Research Facility Construction

The criteria proposed for the storage research facility
was to allow the study of potato storage utilizing various
ventilation rates under simulated commercial storage
conditions. Three bins were constructed each
measuring 2.4 K 2.4 X 5.5 m high. This allowed medium scale
research on 16 to 18 tonnes of potatoes, which is the
approximate yield from 0.4 ha of land. The bins utilized
panel construction methods to facilitate the complete
removal from the commercial bin in which they were located
after completion of the storage season. The Simulated
Storage research facility was located within a commercial
storage bin at Sandyland Farms, Howard City, MI. Figure 4.1
gives an overview of the Simulated Storage research
facility.

4.2 Ventilation System Design

The ventilation systems of the simulated storages were
designed with independent fans, intakes, and distribution
duct systems. Half-round ducts were used to distribute air
under the pile, as is done in commercial potato storages.
Proportionally controlled louvers were used to mix fresh air
with recirculated air for temperature control. Each system
was supplied with a humidifier to maintain adequate inlet
relative humidity. Figure 4.2 illustrates the ventilation

systems of the simulated storage research bins.

18

19

 

 

 

 

30

 

 

 

 

 

 

 

k 6 0
Bin ll
Bin l4
(Mill Bin) Bill '2
Bin 3
,— Ibin Fm
L D Pain Air Plen- Cmtrol lhl

 

 

Figure 4.1 Overview of the Simulated Storage Facility.

HainFms

Fresh Air Fm Attic

20

 

 

\f
___I

$3

 

 

 

l l Sililltad Sta-up
Fm
—©Q—
Bm1meh1
Halt lhnd [but till hull
WmUhuhnflmt

 

 

 

 

 

 

W1

:\R"M““V% Pmn Lmr Ennis:

onnnmuam

 

Figure 4.2 Detail of Ventilation System for the Simulated
Storage Bins.

selec
for t
desig
(1986
maint
The s
apprc

throu

Peta
Atla
deve
Rota
rand
the

Cont

k9)

21

Ventilation rates of 56, 112 and 168 m3/hr-tonne were
selected as 1 x, 2 x, and 3 X the recommend ventilation rate
for the state of Michigan. The ventilation systems were
designed using the criteria outlined by Cargill et. a1.
(1986). The mean air velocities within the system were
maintained below 2.5 m/s with a slot velocity of 4.7 m/s.
The slot velocity of all three ventilation systems were
approximately equal to assure similarity in the flow of air
through the pile.

The ventilation system delivering 168 m3/hr-tonne was
modified after the first storage season by the substitution
of a variable-speed-fan. This system was designed to
deliver ventilation air at a rate that varied from 0 - 112
m3/hr/tonne, depending on the required ventilation demanded
by the control system. The slot was designed to deliver air

at 4.7 m/s at a ventilation rate of 56.1 m3/tonne-hr.

4.3 Research Potato Samples

The Simulated Storage facility was first filled with
potatoes in 1986 with commercially grown potatoes (cv.
Atlantic). Atlantic is a round white variety that was
developed as a high specific gravity variety suited for the
potato chip industry. Ten tubers were collected from a
randomly selected potato hill within each of five regions in
the field each week beginning prior to vine kill and
continuing through harvest. Four sample bags (approx. 11

kg) were placed with the potato pile at each of three levels

22

1.2, 2.4, and 3.6 m from the floor within the simulated
storages.

In 1987, sampling tubes constructed of 20 cm diameter
PVC pipe were inserted through the side wall of each
Simulated Storage at levels of 0.9, 1.8, and 2.7 m from
the floor. These tubes extended 0.6 m into the pile, and
allowed for the collection of samples to evaluate potato
conditions within the potato pile throughout the storage
season.

4.4 Environmental Control

The ventilation system was controlled using a FANCOM
1056 environmental control computer developed in The
Netherlands by FANCOM B.V. (commercially available at
Techmark Inc., Lansing, MI).1 The 1056 is a micro-processor
based controller capable of controlling the ventilation rate
(variable and fixed speed fan), the fresh and recirculation
air volume, and the heating, cooling, and humidifying
devices. The Fancom 1056 uses a decentralized control
theory, which required that each storage bin be controlled
independently. The advantage of decentralized control is
the reduced risk of loss in the event of controller failure.

The Fancom controller uses a multiple input, single
output control procedure to operate the fan, louvers,
heater, and humidifier. This control allows the user to

specify the "influence" different storage parameters have on

 

1The mention of commercial products is for clarification and
does not constitute endorsement of such products by
Michigan State University or The United States Government.

23

the control of the ventilation devices. For example, the
air circulation is determined by the pile temperature
differential and the amount of fresh air required. The
amount of fresh air required is determined by the pile
temperature, main air temperature, outside air temperature,
and the C02 content of the air in the storage. Thus, the
fan operation is influenced either directly or indirectly by
five environmental parameters measured by the computer. The
computer determines the amount of air circulation required
for each of the influences, then controls the fan to supply
the largest requirement for air circulation. Adjustments
are made with other ventilation equipment to accommodate
further requirements.

The performance of the 1056 was evaluated after the
1986-87 storage season, and program modifications were made
by the programmers at FANCOM B.V. The following changes
were made for the 1987-88 storage season: 1) an increase in
pile temperature sensor resolution from 0.5 °C to 0.1 °C;
2) additional aspirated psychrometers were placed above the
potato pile to measure the air conditions; 3) three
additional gas sampling tubes were placed within the potato
pile; and 4) air volume sensors were added to determine the
amount of fresh and recirculation air used. The air volume
sensors consisted of 30 cm diameter fans mounted in the
fresh and recirculation air inlets. The fan rotated at a
speed preportional to the volume of air that was drawn

through it. A hot wire anemometer (Solomat model 556C) was

us
wh

of

24

used to measure the velocity of the air through the Opening,
which was converted to a volume reading for the calibration
of the air volume sensors.
4.4.1 Sensor Placement

The control computer allowed sampling of 16 analog and
8 digital inputs, while utilizing l6 relay and 2 analog
outputs for control purposes. Information on the condition
of the air within the potato pile was collected from four
pile temperature sensors and four sampling tubes (1987
modification) for carbon dioxide and oxygen analysis. Both
the temperature sensors and the gas sampling tubes were
placed at 0.9, 1.8, 2.7, and 3.6 m from the floor of the
storage in the center of each simulated storage. Wet bulb
and dry bulb air temperatures were collected before the air
entered the potato pile, and as the air exited the pile
(1987 modification). The computer used this information to
calculate and report the absolute and relative humidity of
the air.. Temperature sensors were also placed at the fresh
air intake and in the mixing chamber to report air
conditions. Figure 4.3 is a detailed illustration of the
sensor placement in each simulated storage.

The temperatures were measured using Nickel-100 sensors
calibrated to have a resolution of either 0.5 or 0.1 °C
depending on the temperature measured. Aspirated

psychrometers using Nickel-100 temperature sensors were used

Figu

25

Serum Place-mt within 31:118th Potato Stung:

 

 

o7 740

C)

’ASN

 

IL

C2) C2) CD (:3

CD (:3 (:3 (:3

(:3 (:3 (:3 (:3

 

 

 

 

 

 

 

 

o-mth-pm

CIJhflMIhquflLifl!

<3-MrhMIM-ulu
o-ammuaquug

 

Figure 4.3 Detail of Sensor Placement within the Simulated

Storage Research Bins.

26

for humidity measurement. A software multiplexing system
within the control computer routed the gas samples from the
potato pile through the analysis equipment. The carbon
dioxide analysis was performed with a Siemens model ZFPCS
infrared gas analyzer. The oxygen sensing unit was a

Phillips model PW9612 analyzer.

4.4.2 Ventilation Fan Control

As previously stated the operation of the main
ventilation fan may be influenced by up to five different
environmental factors. The factors that influenced fan
control in this research project were the pile temperature
differential and fresh air requirement. The air circulation
was adjusted by the computer when the largest differential
between any two pile temperature sensors exceeded 0.5 °C to
assure that a uniform pile temperature was maintained. The
amount of ventilation due to a pile temperature differential
in excess of 0.5 °C was based on the "influence" of a pile
temperature differential on air circulation. An example of

the required air circulation calculation is given below:

Highest Pile Temperature 11.5 °C
Lowest Pile Temperature 10.0 0C
Allowable Difference 0.5 °C
Pile Differential Influence 100.0 %

Air Circulation = ((11.5 - 10 - 0.5) * 1.0) * 10 = 10 %
The computer adjusts the circulation to 10 % of maximum if a

variable speed is in use, or turns on a single speed fan.

27

4.4.3 Fresh Air and Relative Humidity Control

The amount of fresh and recirculation air was
proportionally controlled by the FANCOM computer through a
single Honeywell model M945B1057 actuator motor in each
storage. In the 1986-87 storage season a potentiometer
feedback was used to report the actual louver position to
the computer. This feedback system was replaced with an air
volume measurement system in the 1987-88 storage season to
assure that the correct mixture of fresh and recirculation
air was used.

Fresh air and a heater were used by the computer to
maintain the main air plenum temperature within a specified
range. If the temperature of the plenum varied from this
range, the ventilation fan was turned on and the fresh air
louver was adjusted by sending a low voltage pulse to the
actuator motor for a set pulse time. After a pulse repeat
time had elapsed, the temperature requirements were re-
evaluated, and additional control commands were made by the
computer. The minimum pulse and pulse repeat times used for
this research were 0.2 and 16 seconds respectively. The
pulse time was automatically adjusted above the minimum
pulse time by the computer based on the differential between
the desired and measured air volume position.

A relative humidity measurement of the air as it
entered the potato pile was used for relative humidity
control. A wheel type humidifier capable of applying three

gallons of water in 24 hours was used to increase the

relativ
the but
the bu}
this r

the 19

influ
Table
adViI

for

dead
10$

frw

28

relative humidity of the inlet air to the desired range. If
the humidity was below the desired level, a relay energized
the humidifier. The setpoint for the relative humidity for
this research was 92% for the 1986-87 season, and 95% for

the 1987-88 and 1988-89 seasons.

4.4.4 Computer Influences and Setpoints
The 1056 is a flexible control computer. The
influences and setpoints used in this research are listed in
Table 4.1. This information was determined through the
advice of control experts from Fancom B.V., and is intended

for use with the 1056 only.

4.4.5 Data Logging

The 1056 control computers were networked with a
dedicated Zenith personal computer for data
logging and remote operation. Information was collected
from the storage computers at the desired sampling rate, 10
- 30 minute intervals for this study, and stored on the PC's
hard drive for future reference. A partial list of the data
values stored on the PC is given in Table 4.2.

A phone modem and a commercially available
communication software package (Carbon.Copy, Meridian
Technology) was used to access the PC remotely. The
software and modem allowed a remote PC to view the measured

storage conditions and make control parameter changes.

Table

Code

HHHHI—a

29

 

Table 4.1 Influences and Setpoints used for the FANCOM 1056
Computer for the Control of a Potato Storage
Environment.

 

Code Number Description Setting
100 Desired Room Temperature 10 - 15 C
103 Heat Up Time 0.1 min
104 Cool Down Time 0.1 min
105 Range Room Temp. Control 1.1 C
107 - Influence of Pile Temp. 2.0 C/C
on Room Temp.

114 Maximum Fresh Air (60% of

total airflow)

115 Influence of Pile Temp. 0.0 %
on Fresh Air

116 Influence of Room Temp. 0.3 %/C
on Fresh Air

117 Influence of Relative Hum. 0.0 %/%
on Fresh Air

118 Influence of Carbon Dioxide 0.0 %/%
on Fresh Air

119 Minimum Circulation 0.0 m3/s

120 Maximum Circulation (vary by bin)

123 Influence Differential Pile 1.0 %/C
Temp. on Circulation

124 Allowable Pile Differential 0.5 C

135 Delay of Room Temp. Influence 0.2 min
on Fresh Air

137 Graphics Sample Time 15.0 min

166 Zero If Air Volume Measured 0

167 Influence of Low Outside Temp. 2 %/C
on Fresh Air Volume

168 Influence of High Outside Temp. 2 %/C
on Fresh Air Volume

171 ' One If RH controlled by l
a humidifier

175 Minimum Pulse Fresh Air 0.2 sec

176 Pulse Repeat Time Fresh Air 0.2 min

177 Diff. Switch Fresh Air 0.0

178 Minimum Pulse Circulation 0.5 sec

179 Pulse Repeat Time Circulation 0.1 min

 

Table 4

Vatit
1986
reco
SPEC
PrOc
infc
mana

ObsE

30

 

Table 4.2 List of Parameters Stored on the Personal
Computer by the FANCOM Control System.

 

l) Bin Number
2) Date
3) Time
4) Main Plenum Temperature
5) Outside Temperature
6) Relative and Absolute Humidity (Above Pile,
and Inlet)
7) Carbon Dioxide (Four levels in Pile and
Outside)
8) Pile Temperatures 1 - 4
9) Measured Fresh and Recirculation Air Volumes
10) Lowest Measured Pile Sensor .
11) Highest Measurengile Sensor
12) Desired Main Plenum Temperature
13) Desired Pile Temperature
14) Desired Relative Humidity

A complete listing of the values stored is given in the
FANCOM user manual for the 1056 (Fancom, 1986, 1987).

 

4.5 Storage Management Procedures

The condition of the potatoes taken from the field
varied for each storage season studied. In September of
1986, Michigan experienced one of the worst floods in
recorded history, while the drought of 1988 caused many to
speculate on the storability of the potato crop. The
procedure used for storage management was based on
information gathered from commercial potato storage
managers. The management decisions were based on

observations and are explained below.

31

4.5.1 Suberization

The potatoes were harvested in October with an average
pulp temperature of 10 0C over the three years of the study.
Upon placement into storage, the potatoes were subjected to
a period of continuous ventilation with total air
recirculation to allow the uniform build up of temperatures
into the 15 °C range for suberization. The ventilation was
continued with the controllers set to maintain the
temperature of 15 oC and a relative humidity setpoint of 92
- 95% throughout the 2 to 3 week suberization period. The
controllers were switched into the automatic mode for fan

control after suberization was complete.

4.5.2 Pre-Conditioning

The pre-conditioning phase was initiated after the
completion of the suberization phase. The storage
environment was controlled by the computers throughout the
remainder of the storage season. The environment was
controlled in the 15 °C temperature range with a relative
humidity setpoint of 92 - 95%. The purpose of this phase of
storage is to allow the potatoes to "burn" or respire off
excess internal sugar pools. Potato samples were tested
once a week for process color and sugar content during the
pre-conditioning phase. As the sugar content was reduced,
the process color improved. The pre-conditioning phase was
terminated when the potatoes maintained a desirable process
color and sugar content for two weeks in a row. This phase

of storage took from 3 to 8 weeks to complete.

32

4.5.3 Cool Down

The potatoes were slowly cooled from the pre-
conditioning temperature of 15 oC toward the desired
temperature of 10 °C. The rate of cooling was 0.5 0C per
week in the 1986-87 and 1987-88 storage seasons, while a
cooling rate of 1 °C per week was used for the 1988-89
season. At the slower cooling rate, the potatoes were
marketed during the cool down phase in late February at
temperatures in the 11.5 to 12 oC range. In the 1988-89
season, the potatoes were cooled to 10 °C by the first week
of January, then placed in a holding phase. Potato samples
were analyzed during cooling to assess the process color and

sugar content.

4.5.4 Holding

The holding phase was the final phase for the 1988-89
season. The environment was maintained at the desired
conditions with the fans running only when required by
temperature changes within the potato pile. The relative
humidity was controlled at the 95% level. Samples were
taken for process color and tuber sugar content throughout
the holding phase. The potatoes were marketed from storage

at a temperature of 10 °C in early March, 1989.

4.6 Evaluation Procedures
Several evaluation procedures were used to quantify the
physiological response of the potatoes during the storage

season. These evaluations were also used to help make

33

management decisions on the operation of the storage and
equipment. The 1056 environmental control computers were
evaluated for temperature and relative humidity control
efficiency.

4.6.1 Potato Quality

Potato quality was monitored from the initiation of
tubers in the field throughout the storage season. The
potatoes stored in this study were of the cultivar Atlantic,
which is a high gravity potato developed specifically for
the potato chip industry. The most critical quality factor
is the color of the potato chip. Other important factors
include the percent solids and internal and external defects
such as bruises, sprouts or discoloration. The processor
retains the right to reject loads which have an unacceptable
appearance, placing the burden of delivering an acceptable
product to the market on the storage manager. For this
reason, a chemical sprout inhibitor (CIPC) was applied after
suberization and the internal sugar content and process
color were monitored throughout all phases of potato
development.

The procedure for internal sugar content analysis was
outlined by Sowokinos (1988). A Yellow Springs Instrument
(YSI model 27) industrial analyzer was used to determine the
content of internal reducing (glucose and fructose) and non-
reducing (sucrose) sugars. A sample of 10 tubers were
randomly selected, and a center core of each was obtained by

block peeling. These cores were mixed together, and 200

34

grams of potato tissue selected. An extract taken by
juicerating the tissue was diluted and analyzed for glucose
content.‘ This extract was then subjected to invertase
enzymatic action which breaks the existing sucrose molecules
into six carbon sugars (glucose and fructose). The extract
was again tested for glucose content, and the percent glucose
and sucrose rating calculated.

Weight loss analysis was performed using the twelve
11.4 kg sample bags placed at three levels within the
storage. The bags were weighed upon placement into and
removal from storage. The results were calculated in terms
of weight loss as a percentage of the original weight.

Original Wt. - Final Wt.
Wt. Loss % = -------------------------- (4.1)
Original Wt.

This information was compiled for all levels within
each potato pile. A two way analysis of variance
(ventilation rate x pile depth) with four replications was
performed to determine the weight loss variation between
different levels under the same ventilation rate, and
between different ventilation rates.

4.6.2 Environmental Control Analysis

The computerized environmental control and monitoring
system generated 5 megabytes of data to be analyzed for each
storage season. A series of Pascal computer programs were
developed to evaluate the controller performance and storage

environment.

35

One set of programs was designed to evaluate the
performance of the controller by calculating the difference
between measured and desired temperature and relative
humidity values. The programs calculated the mean and
standard deviation of the difference. Time periods when the
controller was making temperature changes were evaluated
separately. The temperature change information was not used
to determine the uniformity of temperature and relative
humidity control. '

Step changes of -0.5 °C were performed throughout the
storage season to lower the temperature of the potato pile.
Temperature change time periods were separated from the
information base and analyzed individually. The information
consisted of the temperature at four points in the potato
pile, ventilation rate, and ventilation air temperature and
humidity levels. The temperature change characteristics in
terms of the time required for the temperature change and
the average temperature and humidity driving potentials
throughout the temperature change were calculated from this
data. The information was then subjected to a statistical
analysis using the students t-test with 10 temperature
change time periods to determine the temperature change
characteristics at each ventilation rate and between
ventilation rates.

As previously stated, the main ventilation fan was
activated to correct out of range pile temperatures and non-

uniform pile temperatures. Another set of programs extracted

36

information on the duration and frequency of ventilation.
The fan operation was separated into two categories based on
the reason for ventilation. If the fan operated to cool the
potato pile, the fan operation period was attributed to
temperature control. Otherwise, the fan operation was

attributed to the control of potato pile uniformity.

4.7 Computer Modeling and Simulation

An existing computer model (Lerew, 1978) was modified
to include a ventilation control routine similar to that
used in the Fancom 1056 control computer. A heat generation
rate was used which was similar to measured in the simulated
storages. The computer model used finite difference
techniques to estimate vertical temperature profiles and
weight loss characteristics of bulk stored potatoes. The
modified simulation model was verified against the data

collected from the control computers.

4.7.1 Condensation Control Simulation

Burton and Wigginton (1970) found that a film
thickness-time product of 7.5 x 10'5 h-m at 10 0C will
result in the initiate tuber decay. Hylmo et. a1. (1976)
stated that free water on the surface of tubers helps the
growth of fungi and bacteria, which will cause losses.

Condensation on the tuber surfaces may occur at the
interface between successive days of harvest while filling a

potato storage bin. At this interface, potatoes that were

37

harvested one day (pulp temperature 21 oC) could come into
direct contact with potatoes harvested the next day (pulp
temperature 10 0C). A diagonal region, determined by the
angle of repose, corresponding to the face of the pile is
affected by this temperature differential. Condensation may
occur on the cooler potatoes, and must be controlled with
minimum loss. .

The computer simulation model developed by Lerew (1978)
and modified for this research was used to evaluate
ventilation strategies to remove this condensation. The
condensation problem studied was that of having a 4 m deep
pile of potatoes with 2 m of 21 °C pulp temperature potatoes
in the lower portion of the pile, and 2 m of 10 °C potatoes
in the upper half of the pile.

The ventilation strategies studied were as follows:

1) No Ventilation
2) Recirculation with 56 m3/tonne-hr ventilation
3) Fixed Inlet Conditions of:

a) 15.6 c, 65 % as, with 28 m3/tonne-hr ventilation
b) 15.6 C, 65 RH, with 56 m3/tonne-hr ventilation
c) 15.6 C, 65 RH, with 112 m3/tonne-hr ventilation
RH, with 28 m3/tonne-hr ventilation

e) 15.6 c, 85 as, with 56 m3/tonne-hr ventilation

dPWWWfl

f) 15.6 C, 85 RH, with 112 m3/tonne-hr ventilation
These conditions are representative of the typical fall
weather conditions in Michigan.

The water film thickness on the tuber surface was

38

calculated by taking the total amount of moisture condensed,
based on psychrometric data. This amount of water was

then distributed evenly over the surface area of the
potatoes where condensation occurred. The temperature rise
of the pile, or heat generation was calculated using a
temperature-dependent respiration equation obtained from

Misener and Shove (1976):
o = 6.99 * T - 17.7 ‘ (4.2)

The equation is valid for a tuber temperature range of 4.5
to 21.0 °C, and was obtained by linear regression. Weight
loss was calculated based on skin permeability and air
velocity (Villa and Bakker-Arkema, 1974). It was assumed in
this work that the velocity of the air has an effect on
convective moisture transfer until a critical ventilation
rate is obtained (Burton, 1963). Above the critical
ventilation rates of 42 and 12 m3/tonne-hr for freshly
harvested and well suberized unsprouted potatoes
respectively, the permeability of the tuber skin limits the

amount of moisture loss.

4.7.2 Temperature Change Simulation
The characteristic response to the step temperature changes
were observed throughout the storage experiment. A simple
model (See Figure 4) was developed to obtain an
approximation of the potato pile's response to the step

cooling process. Energy transfer was modeled using heat and

39

 

 

B Moss

 

'————“vo«\/\/ ’\/\Av\/_——_”
R Conduction R Convection
C Potatoes -4

—
—F

 

 

 

Figure 4.4 Electrical Analogy used to Model the Heat and
Mass Transfer System Involved while Cooling
Potatoes.

mass transfer theories.

The model assumed parallel paths of heat and mass

transfer from the potato into the ventilation air stream.

The heat transfer path included the series combination of

the resistance to energy flow from conduction and

convection.

The ventilation air temperature was assumed to be
constant and cooler then the center temperature of the
potato. The thermal conductivity (Kc) was taken from Lerew

(1978) as 2450 J/hr-m-OC. The convective heat transfer

40

coefficient was calculated using equation 4.3 as described

in Lerew (1978):

n = v*e*952.1*ae‘°~41 (4.3)
rho*V*e

Re = --------- (4.4)
mu*a

Equation 4.3 is valid for Reynolds numbers above 50,
which was the case for all ventilation rates studied with
this model. Equation 4.5 and 4.6 were used to estimate

energy transfer through conduction and convection,

respectively:
goond. = Kc“1""('1'internal ‘ Tsurface) (4-5)
Hoonv. = h"‘A’W'I'surface ' Tair) (4-5)

The temperature at the center of the potato was assumed
constant. The potato surface temperature was calculated by
varying the conduction driving potential.

The Biot modulus, a measure of the ratio of convective
to conductive heat transfer, was calculated using equation
4.7 at a ventilation rate of 56 m3/tonne-hr with air
conditions of 10 °C and 95% relative humidity. A Biot
modulus > 0.1 indicates that the conduction of heat through
the product is significant as compared to the convection of

heat from the surface of the product (Holman, 1976):

Biot = ------- (4.7)

41

The convective heat transfer coefficient calculated as
previously stated was 21834 J/hr-m2-°C with a thermal
conductivity of 2450 J/hr-m-°C, and a tuber diameter of
0.045 m. The resultant Biot number of 0.4 indicates that
the conduction of heat through the potato has a significant
influence on the cooling process. The calculation at the
minimum ventilation rate used in this study resulted in the
lowest Biot modulus. At higher ventilation rates, the
convective heat transfer coefficient increases, thus
increasing the Biot number. The mass transfer process
was modeled as the resistance to moisture flow through the
tuber skin in series with moisture convection from the tuber
surface. Due to the high internal moisture content of the
potato, the vapor pressure at the surface of the potato was
calculated as the saturated vapor pressure at the potato
surface temperature. The surface temperature of the potato
was calculated as stated previously.

The equation used to predict mass transfer rate was
derived by Lerew (1978) and is stated by equations 4.8 -
4.10. Equation 4.9 is derived using the Colburn J factor
relationship between the convective heat and moisture
transfer coefficients at 10 oC. The relationship of the
skin permeability is the product of the membrane thickness
and the resistance to moisture flow. Villa and Bakker

(1974) found that this was a function of the Vapor Pressure

42
Deficit, which is expressed by equation 4.10:

. (bl*hd / T(abs))*b2
M = -------------------- *gamma*a*(VPD) (4.8)
Rde1*hd + b2

hd 8.96 * 10'4 * n (4.9)

Rdel 4.94 * 10'4 + 3.31 * 10‘5 * vpo (4.10)

The amount of energy removed while cooling a square
meter column of potatoes, 4 meters deep, 0.5 °C was calculated

using equation 4.11:
Energy TraUSfer = M*Cp*(Torgina1‘ Tfinal) (4.11)

The model was then used to demonstrate the effects of
varying the temperature gradient within the potato, the
inlet relative humidity and the air flow rate on the time
required for energy transfer determined by eqn. 4.11. The
mass lost during the cooling process was investigated by
multiplying the moisture loss rate by the time required for

the energy transfer.

5. Results and Discussion

5.1 Weight Loss Results

The weight loss (expressed in percentage of original

weight) for the 1986-87, 1987-88 and 1988-89 storage seasons

of the Simulated Storage Research facility are presented in

Tables 5.1, 5.2 and 5.3, respectively.

The information is

reported by level within the potato pile, and as an average

for each simulated storage.

 

Table 5.1 Weight Loss Percent of the Potatoes Stored in the

Simulated Storage Facility for the 1986-87

Storage Season.

 

 

Bin #1 Bin #2 Bin #3

V ntilation Rate
m /tonne-hr 56 112 168

Weight Loss Percent (Std. Deviation)
Above Floor
1.22 m* 18 (3.2) 15 (2.9) 15 (1.7)
2.44 m 10 (1.3) 12 (2.0) 15 (1.1)
3.66 m 11 (1.9) 11 (1.7) 15 (2.9)
Average 13 (4.4) 13 (2.6) 15 (2.8)

 

*Each level represents four samples.

 

43

44

 

Table 5.2 Weight Loss Percent of the Potatoes Stored in the
Simulated Storage Facility for the 1987-88
Storage Season.

 

Bin #1 Bin #2 Bin #3

t

V ntilation Rate
m /tonne-hr 56 112 52

 

Weight Loss Percent (Std. Deviation)
Above Floor

1.22 m ** 4.4 (1.4) 7.8 (0.9) 9.2 (0.9)
2.44 m 5.3 (0.9) 7.9 (0.9) 7.9 (1.1)
3.66 m 5.2 (3.5) 8.0 (1.1) 8.0 (0.6)
Average 5.3 (2.3) 7.9 (1.0) 8.3 (1.3)

 

Average of variable rate for entire storage season.

** Each level represents four samples.

 

 

Table 5.3 Weight Loss Percent of the Potatoes Stored in the
Simulated Storage Facility for the 1988-89
Storage Season.

 

Bin #1 Bin #2 Bin #3
V ntilation Rate
m /tonne-hr 56 112 37*

 

Weight Loss Percent (Std. Deviation)
Above Floor

**

1.22 m 8.8 (0.5) 10.0 (0.5) 9.3 (1.0)
2.44 m 7.0 (1.1) 7.9 (0.6) 5.6 (0.7)
3.66 m 6.1 (1.1) 6.2 (0.4) 5.6 (1.2)
Average 7.3 (1.5) 9.2 (1.3) 6.8 (2.0)

 

Average of variable rate for entire storage season.

** Each level represents four samples.

 

45

A statistical analysis was performed using a two way

analysis of variance (ventilation rate x storage depth)

using four replications to determine differences in weight

loss between the storage bins and- between levels within

each storage (levels 1, 2 and 3 relate to samples stored at

1.22, 2.44 and 3.66 m from the floor respectively). The

significant differences (95% confidence level) are presented

below:

1)

2)

3)

4)

5)

5)

7)

3)

9)

bin #2 and bin #3 for the 1986-87 storage season.

bin #1 and bin #2 for the 1987-88 storage season.

bin #1 and bin #3 for the 1987-88 storage season.

1.22 and 2.44 m from the floor in bin #1 for the 1986-87
and 1988-89 storage seasons.

1.22 and 3.66 m from the floor in bin #1 for the 1986-87
and 1988-89 storage seasons.

1.22 and 2.44 m from the floor in bin #2 for the 1988-89
storage season.

1.22 and 3.66 m from the floor in bin #2 for the 1988-89
storage season.

2.44 and 3.66 m from the floor in bin #2 for the 1988-89
storage season.

1.22 and 2.44 in bin #3 for the 1988-89 storage

season .

5.2 Weight Loss Discussion
5.2.1 Differences Between Bins

In the 1986-87 storage season, a weight loss difference

of 2.0 % occurred between bin #2 and bin #3. Weight loss

46

differences of 2.6% and 3.0 % were recorded between bin #1
and bins #2 and #3 respectively for the 1987-88 season. No
significant differences were recorded between bins for the
1988-89 storage season.

The bins of the simulated storage facility differ only
in air flow rates as listed in Tables 5.1 - 5.3. Burton
(1963) found that above the critical ventilation rate of
12 m3/tonne-hr for well suberized tubers, moisture loss
was not a function of air velocity (ventilation rate), due
to limitation of moisture transfer through the tuber skin.
Villa (1973) modeled this phenomenon as a function of skin
permeability and vapor pressure deficit and obtained
correlations between weight loss of potatoes (cv. Monona)
and intersticial air velocity. He concluded that the
environmental air velocity has little effect on moisture
losses from Monona potato tubers. Consistent significant
differences did not exist between simulated storages
throughout the three years of experimental data collected.
The results obtained in this study indicate an agreement
with the information collected by Burton (1963), and Villa

(1973).

5.2.2 Weight Loss Within Each Bin
The driving potential for moisture transfer from the
tuber surface into the ventilation air is the vapor pressure
deficit. This is determined by the temperature and humidity

of the ventilation air and tuber surface temperature. As

47

the air passes through the potato pile, the amount of water
in the air is increased as moisture is taken up from the
potatoes. The point at which the ventilation air becomes
saturated moves up in the column of potatoes as the
ventilation rate is increased. Grahs et. a1. (1978) defined
a critical ventilation rate at which the ventilation air
would become saturated at the top of the pile. At an inlet
relative humidity of 95 %, the critical ventilation rate was
calculated by Grahs as 72 m3/tonne-hr.

The weight loss uniformity from the top to the bottom
of a potato pile should increase as the ventilation rate
increased above the critical ventilation level, due to a
more uniform vapor pressure deficit throughout the pile.
This was noted in the weight loss results from Bin #3 in the
1986-87 storage season, and Bin #2 throughout the three
storage seasons. The potatoes stored in these bins
consistently had a lower standard deviation in weight loss
within the bin then did the storage bins ventilated at
lower rates in the same season.

Ventilation at a rate lower than the critical
level would lead to greater vapor pressure deficits in the
lower regions of the potato pile than in the regions where
the ventilation air is saturated. This hypothesis would
suggest that the moisture lost in the lower regions of the
potato pile ventilated below the critical rate should be
greater then moisture lost in the upper regions of the pile.

This was the case in bin #1 for the 1986-87 and 1988-89

48

storage seasons. Increased weight loss in the lower regions
of the pile was also noted in bins #2 and #3 for the 1988-89
storage season. The inlet air relative humidity was
controlled to 92 % for the 1986-87 storage season, and 95 %

for the 1987-88 and 1988-89 storage seasons.

5.3 Pre-Harvest Internal Sugar Analysis Results
Tuber sugar analyses were performed throughout all
stages of tuber development for the 1987-88 and 1988-89
seasons, and this information is presented in Tables 5.4 and
5.5 respectively. Figures 5.1 and 5.2 present this data in

graphical format.

 

Table 5.4 Internal Sugar Content of Potato Tubers
Before Harvest 1987-88.

 

Date or Location
Pre-Harvest Samples

Glucose (%) Sucrose Rating

8-15-87 0.04 0.52
8-25-87 0.05 0.22
8-29-87 0.05 0.15
9-08-87* 0.05 0.45
9-16-87 0.04 0.40
9-18-87 0.05 0.39

10-02-87 0.05 0.47

10-07-87 0.06 0.79

*Chemical Vine Kill Applied.

 

49

 

Table 5.5 Internal Sugar Content of Potato Tubers
Throughout Development 1988-89.

 

Date or Location Glucose (%) Sucrose Rating

Pre-Harvest Samples
8-02-88 0.086 0.21
9-06-88 0.108 0.39
9-12-88* 0.086 0.52
9-20-88 0.043 0.52
10-04-88 0.065 0.90
10-11-88 0.065 0.54

*cnemical Vine Kill Applied.

 

5.4 Pre-Harvest Sugar Content Discussion

The data presented in Table 5.4 for the 1987 growing
season indicated a constant to decreasing sugar content
(both sucrose rating and percent glucose) until vine
killing. The data presented in Table 5.5 for the 1988
growing season, indicate that a gradual decrease in glucose
content occurred throughout the growing season, while the
sucrose rating remained relatively constant.

In both the 1987-88 and 1988-89 growing season, the
potatoes were harvested with the sucrose ratings less than
1.0 which is acceptable by criteria defined by Sowokinos
(1988). The percent glucose readings, however, were greater
than the 0.035 % level which is defined as the acceptable

range for this value in Norchip potatoes.

50

 

1.2

. Samples taken during tuber growth
1.0-

Sucrose Rating
0
0'!
l

'l a sucrose
I I I rI I I I17 I I r

1 8 15 22 29 5 12 19 26 3 10 17

AUGUST SEPTEMBER OCTOBER

 

 

 

0.14

 

Samples taken during tuber growth

Glucose %

 

 

e Glucose
0000 IIIIII I IIIIIIIII “171113—71 IIIIII [flimliiim'Iiii

, m1 ...... In" mm
1 8 15 22 29 5 12 19 26 3 10 17
AUGUST SEPTEMBER OCTOBER

Temporal time 1987

 

Figure 5.1 Sucrose Rating and Percent Glucose Readings of
Tubers throughout the 1987 Growing Season.

Sucrose Rating

Glucose Z

51

 

 

 

 

 

 

 

 

L2
2 Samples taken during tuber growth
1.0-l
d C
4
(18-
(16-
j e ‘0 .
0.4" Q
J
.l
(12-'
a sucrose
0.0 YYYYYY I YYYYYY I WWWWW I IIIIII I YYYYYY I IIIIII I IIIIII I IIIIII I IIIIII I IIIIII I’ IIIIII I
18152229512192631017
AUGUST SEPTEMBER OCTOBER
0J4-
O 12 1 Samples taken during tuber growth
‘ o
(110-
.. .
(108-
‘ o 0
(106-
(104- '
(102-
0 GI
,0.00 .. q...,. ......,......1.......... .3533”.-.
1815222951219 31017
AUGUST SEPTEMBER OCTOBER ‘
Temporal time 1988

Figure 5.2 Sucrose Rating and Percent Glucose Readings of

Tubers throughout the 1988 Growing Season.

52

5.5 Process Quality Results

The potatoes stored in the simulated storage facility
were marketed through Frito-Lay Inc. each year. Results of
the standard Frito-Lay cook test are presented in Tables
5.6, 5.7 and 5.8 for the three storage seasons. Results are
presented by vertical location only for the 1987—88 and
1988-89 seasons. Upon removal from storage in the 1987-88
and 1988-89 season, tuber sugar analyses were performed

and are presented in Tables 5.7 and 5.8.

 

Table 5.6 Frito-Lay Cook Test Results for the Potatoes
Stored in the Simulated Storage Research Facility
for the 1986-87 Storage Season.

 

 

Bin #1 Bin #2 Bin #3
Ventilation
R te
m /tonne-hr 56 112 168
Color* 61 57 56
Appearance** 14 25 12

 

*Agtron Scale calibrated at 0 (M-OO) and 90 (M-SO) on red
mode.

**Frito-Lay Quality criteria, 0
15

excellent,
not acceptable.

 

 

Table 5.7' Frito-Lay Cook Test Results for the Potatoes
Stored in the Simulated Storage Research Facility
for the 1987-88 Storage Season.

 

 

 

Bin #1 Bin #2 Bin #3

Ventilation
R te *
m /tonne-hr 56 112 52
1.22 9 Level
Co1or** *** 52 53 63
Appearance 0 3 2
Percent

Glucose 0.010 0.010 0.022
Sucrose

Rating 0.08 0.15 0.56
2.44 9 Level
Color 63 63 60
Appearance 0 0 0
Percent

Glucose 0.006 0.006 0.027
Sucrose

Rating 0.06 0.10 0.61
3.66 m Level
Color 63 62 63
Appearance 0 1 0
Percent

Glucose 0.016 0.006 0.027
Sucrose

Rating 0.08 0.09 0.41
*Average of variable rate for entire storage season.

**Agtron Scale calibrated at 0 (M-OO) and 90 (M-90) on red

mode.

***Frito-Lay Quality criteria, 0 = excellent,
= not acceptable.

15

 

 

Table 5.8 Frito-Lay Cook Test Results for the Potatoes
Stored in the Simulated Storage Research Facility

for the 1988-89 Storage Season.

 

 

 

Bin #1 Bin #2 Bin #3

Ventilation
R te *
m /tonne-hr 56 112 37
1.22 3 Level
Color** 67 62 65
Appearance 0 8 0
Percent

Glucose 0.028 0.023 0.032
Sucrose

Rating 0.736 1.10 0.490
2.44 3 Level
Color 61 68 64
Appearance 2 3 3
Percent

Glucose 0.011 0.023 0.017
Sucrose

Rating 0.45 0.90 0.409
3.66 5 Level
Color 61 63 58
Appearance 0 0 1
Percent

Glucose 0.039 0.028 0.032
Sucrose

Rating 0.695 0.653 0.695
*Average of variable rate for entire storage season.

**Agtron Scale calibrated at 0 (M-00) and 90 (M-90) on red

made.
*

**Frito-Lay Quality criteria, 0

= excellent,
15 = not acceptable.

 

55

The following observations are noted from the data presented
in Tables 5.6, 5.7 and 5.8:

1) A11 potatoes stored in the simulated facility during

the 1987-88 and 1988-89 maintained acceptable process
quality throughout the storage season, and were acceptable
for market.

2) The potatoes stored in Bin #2 during the 1986-87 season
were rejected by the processor, due to defects.

3) The potatoes stored in Bin #3 during the 1986-87 season
were accepted as marginal quality product due to defects.

4) Samples tested throughout all three years of this

study attained an acceptable process color reading (above 55
Agtron) upon removal from storage.

5) All samples tested for tuber sugar content upon

removal from storage had acceptable levels of percent
glucose and sucrose rating.

The tuber sugar content and process color of the
potatoes were monitored throughout the 1988-89 storage
season by the Frito-Lay field lab in Sidney, MI. Samples
were taken from the surface of the potato pile of each
simulated storage bin on approximately two week intervals.
These readings, presented in Tables 5.9, 5.10, and 5.11 and
Figures 5.3, 5.4, and 5.5, were used as management tools to
determine the state of the potato pile, and the
environmental requirements to maintain potato quality

throughout the storage season.

56

 

Table 5.9 Process Color of Potatoes Taken from the
Potato Pile Surface within the Simulated Storages
throughout the 1988-89 Storage Season.

 

 

Bin #1 Bin #2 Bin #3

Ventilation

Rate *

m /tonne-hr 56 112 52
11-04-88 60** 62** 62**
11-18-88 60** 6o** 60**
12-02-88 62 66 60
12-16-88 61 61 61
01-04-89 63 62 64
01-27-89 66 67 64
02-06-88 61 63 61
02-16-89 65 62 63

 

* Average of variable rate for entire storage season.

**

Dark internal coloration noted.

 

 

Table 5.10 Percent Internal Glucose of Potatoes Taken from
the Potato Pile Surface within the Simulated
Storages throughout the 1988-89 Storage Season.

 

 

Bin #1 Bin #2 Bin #3
Ventilation
R te *
m /tonne-hr 56 112 37
11-04-88 0.022 0.024 0.019
11-18-88 0.024 0.015 0.022
12-02-88 0.009 0.026 0.032
'12-16-88 0.032 0.030 0.017
01-04-89 0.011 0.015 0.013
01-27-89 0.024 0.026 0.015
02-06-89 0.022 0.032 0.013
02-16-89 0.015 0.054 0.024

 

* Average of variable rate for entire storage season.

 

S7

 

Table 5.11. Sucrose Rating of Potatoes Taken from the
Potato Pile Surface within the Simulated
Storages throughout the 1988-89 Storage Season.

 

 

Bin #1 Bin #2 Bin #3

Ventilation

R te *

m /tonne-hr 56 112 37
11-04-88 0.86 0.82 0.74
11-18-88 0.49 0.45 0.41
12-02-88 0.45 0.57 0.33
12-16-88 0.49 0.49 0.57
01-04-89 0.37 0.33 0.33
01-27-89 0.41 0.45 0.49
02-06-89 0.57 0.49 0.57
02-16-89 0.70 1.10 0.49

 

* Average of variable rate for entire storage season.

58

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61

The following points are suggested by this data:

1) The sucrose rating, which is an indication of the

amount of sucrose within the tuber, was near the critical
value of 1.0 upon placement into storage in both the 1987
and 1988 growing seasons. A decrease in the sucrose rating
occurred from November 4, to November 18, 1988.

2) The sucrose rating generally decreased through January
4, 1989, then began an increase throughout the remainder of
the storage season.

3) As shown in Figures 5.3 - 5.5, the average pile
temperature was gradually lowered to 10 C on January 4, 1989
where it was maintained until the potatoes were marketed.

4) The percent glucose content was in the acceptable range
(% glc < 0.035) throughout the storage season until February
6, 1989, at which time this reading for Bin #2 had increased
0.054%. However the process color had not yet been affected

by this increase in glucose content.

5.6 Process Quality Discussion

The potatoes stored in the simulated storage facility
were utilized for potato chip production in all three
seasons studied. Quality evaluations were performed by the
processor, to determine the acceptability of the potatoes to
the process industry.

In the 1986-87 storage season, bin #2 was rejected at
the processor due to poor quality. The process color of
this lot of potatoes was acceptable. However the defects,

such as pressure bruise and internal discoloration were not

62

acceptable. During the same storage season, the potatoes
stored in bins #1 and #3 were accepted with marginal
quality. The Frito-Lay appearance numbers for the simulated
storage research bins were all above 10 on a scale where
0 is excellent and 15 is not acceptable.

I hypothesize that the reason for this poor quality was
a combination of circumstances which took place both before
and during storage. In September 1986, Michigan received
14 inches of rain in a 14 hour period, thus the potatoes
were harvested from saturated soil. Free water was evident
on the surface of most of the potatoes used to fill the
simulated storage facility. The potato condition going into
storage required that fresh air be heated and passed through
the pile to remove this surface moisture. However, in the
1986-87 storage season, the design of the simulated storages
was such that outgoing exhaust air, which needs to flow at
the same rate as incoming fresh air, was required to pass
through a pressure louver into the commercial storage bin.
The ventilation system in the simulated storage facility was
not capable of creating the pressure differential across the
louver to allow it to open when the commercial bin was in a
ventilation cycle. Therefore, no fresh air was available
during the initial storage phase to allow drying of the
potatoes. The controllers had required the fresh air
louvers to open 100% in an attempt to draw fresh air at
different times throughout the storage season, with no fresh

air response. As the commercial bin exhaust fan turned on,

63

or the main ventilation fan was shut off, the fresh air was
drawn in at high rates and the pile was overcooled. Large
temperature fluctuations occurred throughout the storage
season, and poor control was the result as previously noted.
The average difference between the desired and measured
average pile temperature ranged from 1.8 to 2.2 °C, with a
standard deviation ranging from 1.4 to 1.8 °C. These
fluctuations coupled with the condition of the potatoes
coming into the storage are assumed to be the cause of the
poor quality of the 1986-87 stored potatoes.

The ventilation system of the simulated storage
research facility was modified for the 1987-88 and 1988-89
storage seasons. A duct, which allowed exhaust air to pass
through the commercial bin to a work area in the storage
facility was installed. Exhaust fans were also installed in
each simulated storage bin and activated whenever fresh air
was required. To determine whether the ventilation rate was
what the computer requested, air flow sensors were installed
in the fresh and recirculation air inlets. The fresh-air
flow was used as the feedback for the louver control
(instead of the position of the louvers as was done in the
1986-87 storage season). These modifications provided the
required temperature and air movement to maintain potato
quality throughout the storage season.

In the 1987-88 and 1988-89 storage season, the potatoes
were marketed from the simulated storage facility at high

levels of quality following 115 to 142 days of storage. A11

64

potato lots attained a process color of 60 or higher on an
Agtron scale calibrated at 0 (M-00) and 90 (M-90). The
appearance numbers for the 1987-88 season ranged from 0 to
3, while during the 1988-89 season this number ranged from 0
to 8.

The information presented in Tables 5.9 - 5.11
represents the tuber sugar content and process color of
potatoes taken from the surface of the potato pile for
analysis throughout the storage season. This data is also
presented per bin along with average pile temperature in
Figures 5.3 - 5.5.

The process color of the potatoes in the storages
ranged from 60 to 62 at the beginning of the 1988-89
storage season. Internal discoloration, however, was noted
in the stem end of the tubers. The sucrose rating was also
near the critical level of 1.0, ranging from 0.74 to 0.86.
glucose levels were below the maximum allowable level of
0.035 %, ranging from 0.019 to 0.022 %. A pre-conditioning
management scheme, as defined by Sowokinos (1988),
was utilized to prepare the potatoes for long term storage.
The pile temperature of the simulated storage bins was
maintained at the 15.6 °C level through December 2, 1988
when the process color ranged from 60 to 66, and the sucrose
rating had dropped to a range from 0.33 to 0.57. The
glucose levels remained relatively stable throughout this
period of storage, and ranged from 0.009 to 0.032 % on
December 2, 1988.

65

The desired average pile temperature of the simulated
storage bins was lowered to 10 °C at a rate of 1 °C per week
beginning on December 3, 1988. This process lasted until
January 10, 1989. Process Color and internal sugar level
readings were taken on January 4, 1989, at an average potato
pile temperature of 10.6 °C. The process color ranged from
62 to 64, the sucrose rating was the lowest of the 1988-89
season ranging from 0.33 to 0.37, and the glucose percentage
was at an acceptable level ranging from 0.011 to 0.015 %.

The holding temperature of 10 °C is low by industry
standards for the Atlantic variety of potatoes. This
variety is known by commercial growers as only a short
term storage variety which would not attain acceptable
process color at storage temperatures below 12.8 °C. An
increase in sucrose rating occurred from January 4, 1989
until the potatoes were marketed on March 2 or 3, 1989. The
process color of the potatoes had not yet been affected by
the low storage temperature. Sowokinos (1988) suggests that
the sucrose rating is a forecasting tool to determine the
processability and storability of potatoes. The theory is
based on the fact that the twelve carbon, non-reducing
disaccharide, Sucrose is hydrolyzed into two six carbon
reducing sugar molecules by the enzyme invertase. These
reducing sugars are subjected to non-enzymatic browning
through the Millard reaction upon processing at high
temperatures (190 to 200 °C). Thus, it is reducing sugar

content, as measured by the percent glucose reading, that

66

determines the process color of potatoes processed for
potato chips and fries. Sowokinos argued that the presence
of excess sucrose during the storage season (sucrose rating
above 1.0) will result in an accumulation of glucose, thus
decreasing the processability of potatoes from storage.
Based on this information, it is hypothesized that the
process color of the potatoes would have begun to decrease
if the potatoes were held in storage until April or May.
Further work is required to determine a temperature
management strategy that will result in maintaining
acceptable process color from the Atlantic variety
throughout long term storage.
5.7 Environmental Control Results
The results of the temperature and relative humidity

control evaluation are presented in Tables 5.12 and 5.13.

 

Table 5.12 Average and Standard Deviation of the Difference
Between the Desired and Measured Average Pile
Temperature for the Simulated Storage Research
Facility for Three Storage Seasons.

 

 

Bin #1 Bin #2 Bin #3
Ventilation
R te
m /tonne-hr 56 112 variable
1986-87 1.8 (1.4) 2.2 (1.8) 2.1 (1.6)
1987-88 0.2 (0.1) 0.2 (0.3) 0.2 (0.2)
1988-89 0.2 (0.4) 0.2 (0.1) 0.2 (0.2)

 

67

 

Table 5.13 Average and Standard Deviation of the Difference
Between the Desired and Measured Inlet Relative
Humidity for the Simulated Storage Research
Facility for Three Storage Seasons.

 

 

Bin #1 Bin #2 Bin #3
Ventilation
Rate
m /tonne-hr . 56 112 variable
1986-87 4 (2) 3 (4) 4 (2)
1987-88 4 (1) 5 (3) 4 (3)
1988-89 9 (4) 4 (3) 6 (5)

 

5.8 Environmental Control Discussion

The control of the simulated storage environment was
performed by a FANCOM 1056 environment control computer.
The two storage parameters that were controlled were the
potato pile temperature, and the ventilation air relative
humidity. The computer was instructed to operate a
humidifier placed at the air inlet to the potato pile when
the relative humidity was below the setpoint of 92 % during
the 1986-87 storage season, and 95 % during the 1987-88 and
1988-89 storage seasons. The average difference between
measured and desired relative humidity ranged from 3 to 9 %
over the three storage seasons studied, with a standard
deviation ranging from 1 to 5%. The inaccuracies of the
sensor could account for variations of up to 4 % at a
temperature of 10 oC and a relative humidity of 98%.

The temperature of the potato pile was controlled using

fresh air for cooling, and small resistance heaters for

68

heating. Heating was not required throughout most of the
storage time studied due to the heat generation of the
potato pile. The temperature variation noted in the average
potato pile for the 1986-87 storage season was due to the
failure of the ventilation system during the process of
drawing fresh air for cooling as explained previously. The
problem may have been compounded by the 0.5 °C accuracy of
the sensors used. In the following two storage seasons,

0.1 °C sensors were used, and the average difference

between the desired and measured average pile temperature

was 0.2 0C.

5.9 Ventilation Fan Operation Results
The ventilation fan operation for the 1987-88 and 1988-
89 seasons is outlined in Tables 5.14 and 5.15. The
1986-87 data were not analyzed due to the lack of

ventilation measurement.

69

 

Table 5.14. Fan Time Operation for the Simulated Storages,
1987-88 Storage Season.

 

Bin #1 Bin #2 Bin #3
Time, Days 85 134 142
Ai flow ' *
(m /tonne/hr) 56 112 52
Fan Time **
Hours 267 1381 2352
% 13 43 69
Number of
Cycles:
Pile Temp.
Diff. 46 92 126
Fresh Air
Required 68 146 238
Cycle Time hr.
Pile Temp.
Diff. 2.4 6.4 12.0
Fresh Air
Required 2.3 5.4 3.5
Cycle Reason, %
Pile Temp. ‘
Diff. 40 43 64
Fresh Air
Required 60 57 36

 

 

Variable airflow, value is average airflow for season
Ventilation experiment ended on December 29, 1987 due to
potato quality considerations.

 

 

Table 5.15. Fan Time Operation Analysis for the Simulated

Storages, 1988-89 Storage Season.

 

Time, Days
Ai flow
(m /tonne/hr)
Fan Time
Hours
%
Number of
Cycles:
Pile Temp.
Diff.
Fresh Air
Required
Cycle Time hr.
Pile Temp.
Diff.
Fresh Air
Required
Cycle Reason,
Pile Temp.
Diff.
Fresh Air
Required

Bin #1

136
56

2483

76

267
244

4.3

46
54

Bin #2

136
112

2260
69

263

215

1.8

8.3

21

79

137
37

2205
67

432
344

47
53

Bin #3

*

 

* O O O 0
Variable airflow, value 18 average airflow for season

 

71

Observations based on the information presented in
Tables 5.14 and 5.15 are listed below:
1) The fan time increased for all bins from the 1987-88
to the 1988-89 season.
2) The ventilation rate of the variable speed fan averaged
less than 56 m3/tonne/hr for both storage seasons studied.
3) Proportionately more ventilation time was required in
bin #2 to bring in fresh air then to correct pile temperature
differentials for both seasons studied.
4) Fans cycled an average of 1, 1.8, and 2.6 times per day
in storage for the 1987-88 season, for bins #l, #2, and #3
respectively, while 3.8, 3.5, and 5.7 cycles per day was the

average for the 1988-89 storage season.

5.10 Ventilation Fan Operation Discussion

The main ventilation fan was used to move the tempered
air through the potato pile. Each fan was turned on any
time fresh air was required, or when the temperature
differential between two pile sensors exceeded 1.0 °C in the
1986-87 storage season (or 0.5 °C in the 1987-88 and 1988-89
seasons).

The volume of air used for controlling the temperature
within the simulated storages was measured using air volume
sensors during the 1987-88 and 1988-89 season. The data
from the simulated storages on the amount of ventilation
used indicated that the 1988-89 storage season required more

ventilation then the 1987-88 season. The increase in

72

ventilation time may be attributed to the longer period of
high temperature required for the pre-conditioning phase of
storage. In the 1988-89 season, the pile temperature was
maintained at the 16 °C level for the first 8 weeks of
storage, while this temperature was only maintained for 4
weeks in the 1987-88 season. During periods of high
temperature storage, the metabolism of the tuber is higher
than it is at lower temperatures. This was also evident in
the lower weight loss figures for the 1987-88 storage season
as compared with the 1988-89 season. Another difference
between the potatoes stored in the 1987-88 and 1988-89
seasons was the drought conditions which the developing
tubers survived during the 1988 growing season.

The on time of the main ventilation fan ranged from 2.4
to 12.0 hours for pile temperature differential correction,
and from 2.3 to 5.4 hours for drawing fresh air during the
1987-88 season. The same information collected from the
1988-89 season revealed that a range of 1.8 to 4.3 hours per
ventilation event was required for pile temperature
differential corrections while the ventilation time required
to draw fresh air ranged from 3.3 to 8.3 hours/cycle. The
main difference between the 1987-88 and 1988-89 storage
seasons was the average number of ventilation events per day
in storage. During the 1987-88 season, the bins averaged
1.0, 1.8, and 2.6 ventilation events per day, while 3.8,
3.5, and 5.7 events per day were required to control the

temperature of the potatoes in the simulated storages.

73

The practice of commercial potato storage ventilation in
Michigan varies considerably from one operator to another.
One storage manager may ventilate 15 minutes per hour during
the holding period of storage, while another may advocate
four cycles of 2 hours each day. The reason for this
variation stems mostly from the experience of each storage
manager in storing the potatoes produced on their own land.
The conditions under which the potatoes are produced has an
effect on the respiration of the tubers, which will change
the ventilation management procedures required for each
potato crop. The control computer applied the basic theory
for storage ventilation: the removal of respiration by-
products.

Data on bin #1 (56 m3/tonne-hr) collected during the
initial months of the 1987-88 storage season indicated that
ventilation fan operation was required only 13 % of the
first 85 days in storage. Periods of 10 days occurred when
no ventilation was required. During these periods without
ventilation, natural convection caused an accumulation of
moisture in the upper 1 m of the potato pile. On December
29, 1987, the automatic ventilation of the experiment was
terminated, and manual operation of ventilation fans was
used to maintain product quality at an acceptable level.
During this storage period, the potato pile was not
generating heat at a standard rate, and pile temperature
uniformity was within the desired range of 0.5 0C. The

potatoes metabolic rate seemed to have slowed to a level

74

which required little ventilation to maintain a desirable
environment. This behavior was not noted during the 1986-87
or the 1988-89 storage seasons, and was also not the case in
bins #2 or #3 during the 1987-88 storage season. The
storage temperature during this period of low metabolism was
16 0C, which normally supports high metabolic rates in
potatoes.

The data indicated that a lower percentage of
ventilation time was required to eliminate pile temperature
differentials in bin #2 (112 m3/tonne-hr) than bin #1 (56
mh3/tonne-hr) or bin #3 (variable) throughout both storage
seasons studied . This suggests that higher ventilation
rates may be more effective at the preventing prolonged pile
temperature differentials. An explanation for this may be
the distance from the air inlet to the point at which the
air reaches thermal equilibrium with the surface of the
tuber. At higher ventilation rates thermal equilibrium is
attained further from the air inlet, if at all. If this is
the case, tubers at different levels in the storage are
subjected to more uniform ventilation air conditions. This
argument, as well as the data from the simulated storages
would suggest that a higher ventilation rate would be better
suited to the correction of temperature differentials.

Bin #3 was equipped with a variable speed fan that was
operated by the controller for the 1987-88 and 1988-89
storage seasons. The ventilation rate was determined as

previously discussed, and was proportional to the problem

 

 

75

measured within the potato pile. The fan was capable of
delivering from 0 - 112 m3/tonne-hr. The average
ventilation rate used was 54 and 37 m3/tonne-hr for the
1987-88 and 1988-89 storage seasons respectively. During
the 1987-88 storage season, the fan ran more hours than
either bins #l or #2. Less time was spent on ventilation of
bin #3 for the 1988-89 storage season than either bins #1 or
#2.

The influences within the control computer were changed
between the 1987-88 and 1988-89 storage seasons in bin #3 to
more effectively control the variable speed fan. The
ventilation rate required for the correction of pile
temperature differentials was increased by a factor of two.
The data indicated that the average ventilation time for pile
temperature differential correction was 12.0 hours during
the 1987-88 season. The ventilation time required to
correct pile uniformity problems during the 1988-89 season
was 2.4 hours. The reduction in the ventilation time for
pile uniformity correction in bin #3 for the 1988-89 season
would indicate that the ventilation rate used for the 1988-
89 storage season was more effective at obtaining a uniform
pile temperatures than the rate used for the 1987-88 storage
season.

5.11 Temperature Change Analysis Results

Over the period of the 1988-89 storage season, ten

0.5 °C temperature changes were performed per bin to bring

the average pile temperature from 15 °C to 10 ° C. The data

 

76

recorded during these temperature change periods is reported

in Table 5.16.

 

Table 5.16. Average and Standard Deviation of Time,
Relative Humidity and Temperature Differential
for 0.5 °C Temperature Change during the 1988-
89 Storage Season.

 

Bin Number Time Relative Humidity Temperature
Hours Percent Diff. (C)
1 5.6 (1.7) 84 (2) 0.6 (0.3)
2 11.2 (0.8) 91 (3) 0.6 (0.3)
3 5.6 (2.3) 89 (l) 0.7 (0.6)

 

A statistical analysis performed on the data in Table
5.16 yielded the following significant (95% level)
differences:
1) The temperature change took less time for both bins #1
and #3 then for bin #2.
2) The relative humidity of the inlet air during the
temperature changes was less for bin #1 then bin #2 and

bin #3.

5.12 Temperature Change Discussion
The response of the potato pile to this step
temperature change was monitored, and the results were
previously presented. The variable speed fan in bin #3
was set up to operate in a low airflow made during cooling

periods, and its operation was similar to that of the fan in

77

bin #1. Temperature changes took less time in bins #1 and
#3 (95 % significance level) then in bin #2.

The cooling of potatoes which consist of approximately
80 % water, occurs by simultaneous heat and mass transfer
processes. An analysis of the Biot number, as calculated
previously, yields information which suggests that both the
conduction of heat through the potatoes and the convection
of heat from the surface of the potatoes must be considered.
Mass transfer is limited by the permeability of the tuber
skin and the vapor pressure deficit (Villa, 1973). The
condition of the inlet air determines the ratio of the
amount of energy which can be transferred through the heat
and mass pathways. If cooling is conducted with high
humidity air, the heat transfer path carries the bulk of the
energy which is removed. Low humidity cooling air increases
the vapor pressure deficit, thus increasing the amount of
energy passed throughout the mass transfer pathway.

The relative humidity of the inlet air was
significantly higher (95 % level) for the 1988-89 storage
season in bin #2 (91 %) than the relative humidity of the
cooling air used in bin #1 (84 %). There were no
statistically significant difference between bins on the
average temperature driving potential. A mean separation of
0.13 °c existed between bin #2 (0.62 °C) and bin :3 (0.75
°C). The increased amdunt of time required to cool the
potatoes in bin #2 over bin #1 may be attributed in part to

the difference in inlet relative humidity. However time

78

differences of the magnitude found experimentally are only

partially explained by the observed differences.

5.13 Temperature Change Model

The model developed which simulated energy transfer as
parallel heat and mass transfer was used to help define the
role of each form of energy transfer in the cooling of a
potato pile. This model calculated the time required for a
temperature change of 0.5 °C in a column of potatoes 1 meter
square by 4 meters deep. Table 5.17 presents a series of
times to remove the energy required for this temperature

change.

 

Table 5.17. Time Required for a Temperature Change from 12
°c to 11.5 °c in a 4 m Column of Potatoes with
Inlet Conditions of 11.0 C, 90 % RH, a 0.25 °C
Temperature Gradient within the Potato, and
Various Airflow Rates.

 

AiS Flow Rate Time to Cool
m /tonne-hr hours
28 7.5
56 5.4
84 4.5
112 4.0
140 3.7
168 3.5

 

 

 

79

The model was then used to determine the effect of
varying the inlet conditions and air flow rates on the time
required for the temperature change of 0.5 0C previously
defined. Figures 5.6, 5.7 and 5.8 present the results of
simulation runs with different combinations of ventilation
rate, temperature gradient within the potato pile and inlet

relative humidity levels.

 

 

 

l\

 

Time ( hours)

 

 

 

 

 

 

 

 

28 168
Ventilation Rate ( m3 / tonne-hr)

Figure 5L6 Predicted Time Required to Complete a 0.5 0C
Temperature Change in a Four Cubic Meter Column
of Potatoes, with a 0.5 °C Internal Temperature
Gradient, Inlet Relative Humidity of 90 % and
Various Airflow Rates.

80

 

 

 

Time ( hours)

 

Figure 5.7

Time (hours)

Figure 5.8

 

 

 

 

 

 

 

 

 

 

 

 

 

0 1.0

Internal Temperature Gradient (0 C)

Predicted Time Required to Complete a 0.5 °C
Temperature Change in a Four Cubic Meter Column
0 Potatoes with an Airflow Rate of 56

m /tonne-hr, an Inlet Relative Humidity of 90 %,
and Various Internal Temperature Gradients.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

80 .
Relative Humidity (percent) 100

Predicted Time Required to Complete a 0.5 °C
Temperature Change in a Four Cubic Meter Column
of Potatoes with a 0.5 °C Internal Temperature
Gradient, an Airflow Rate of 56 m /tonne-hr,
and Various Inlet Relative Humidity Levels.

81

The model was also used to determine the amount of
mass transferred during the cooling process studied. The
inlet relative humidity, temperature gradient within the
potato, and the airflow rate were varied over the ranges
shown in Figures 5.6 through 5.8. This set of model runs
resulted in a common volume of mass transferred (over all

conditions studied) of 2.2 kg.

5.14 Temperature Change Modeling Discussion

A simple temperature change model was developed to
determine the effects of the varying airflow rates and
inlet air conditions on the time required to cool a 4 :03
column of potatoes from 12 to 11.5 0C. The model predicted
a cooling time of 5.4 hours to make this temperature
correction using a ventilation rate of 56 m3/tonne-hr, an
inlet temperature of 11.0 °C, an inlet relative humidity of
90 %, and an internal temperature gradient within the
7.6 cm diameter potato of 0.25 °C. This prediction
compared favorably with the measured 5.6 hour average time
required to make a 0.5 °C temperature change in bin #1 of
the simulated storage facility.

The model predicted a time of 4.0 hours for the
previously described temperature change using a 112
m3/tonne-hr ventilation rate, which did not compare well
with the experimentally determined time of 11.2 hours. The
experiment seems to have been influenced by some factor

that is not currently explainable. It is interesting to

82

note that the model predicts that a 29% reduction in the
time needed to remove the energy from the potato column,
from 5.4 to 4.0 hours, requires a 100 % increase in

airflow. This indicates that the cooling process as
simulated by this model at the described inlet conditions is
primarily controlled by factors which are not affected by
the ventilation rate.

The model predicts that an increase of inlet relative
humidity from 80 to 95 % will result in an increase in
cooling time from 2.8 to 7.0 hours at a ventilation rate of
56.1 m3/tonne-hr, an inlet temperature 1.0 °C below the
average potato temperature and an internal temperature
gradient of 0.5 °C. Increasing the inlet air relative
humidity decreases the vapor pressure deficit, which is the
driving potential for the moisture transfer process. This
large increase in cooling time would indicate that the
moisture transfer pathway is carrying a majority of the
energy removed during the cooling process studied.

Varying the temperature gradient within the potato
tuber during the cooling process shifts the temperature
driving potential from 100 % convective potential to 100 %
conductive potential. The model predicts a time increase
from 5 hours to 8 hours to accomplish the temperature change
as the 1.0 °C temperature differential is shifted from 100 %
convection (tuber surface to air temperature) to 100 %
conduction (tuber center to tuber surface temperature).

This would indicate that resistance to heat transfer through

83

a conduction pathway is greater than the resistance posed by
the convection pathway at a ventilation rate of 56
m3/tonne-hr and an inlet relative humidity of 90 %. As the
ventilation rate was increased, the time to cool by
convection would decrease, while the time to cool by
conduction would remain constant.

Throughout a cooling cycle, the temperature gradient
within the potato will begin at 0 oC, and proceed to a
steady state value determined by the temperature of the
inlet air and the conductive heat transfer coefficient of
the potato. As the internal temperature gradient is
increased, an increasing portion of the energy transferred
during the cooling process must pass through the heat
conduction pathway.

The amount of mass transferred during the model
studies discussed above was constant at 2.2 kg per cooling
cycle. The mass flow rate varied one order of magnitude
over the relative humidity range studied. However, the time
varied one order of magnitude inversely to the mass flow
rate. Thus the overall mass transferred was not changed.
This simulation model indicates that the mass flow rate
varies inversely as the time required for the energy
transfer to cool the column of potatoes. This would
substantiate the previous discussion that the mass flow
rate, as determined by the vapor pressure deficit is a
critical parameter in determining the energy transfer and

the cooling rate of potatoes.

84

5.15 Finite Difference Control Model Results

The modified finite difference temperature and weight
loss simulation was compared with data collected from the
storages. Figure 5.9 presents plots of the measured and
predicted values for the average pile temperature during a
steady state control period. The experimental data was
taken from a time period in January 1989.

The simulation was compared to a temperature
change period during January of the 1988-89 storage season.
The simulation run is presented in Figure 5.10 for the 56
m3/tonne-hr ventilation rate, and Figure 5.11 for the 112

m3/tonne-hr ventilation rate.

15

85

 

14—

12

Temperature (deg C)
:7
N

 

 

a—a real-ave
a—a aim-ave

 

 

15

 

I t ‘ I l ‘ I ' I I I

10 20 30 4O 50 60 7O

 

14-

13-

Temperature (deg C)

12

 

W'

IF. unkwwe
ova dukwwl
I ' I ' I ' I ' '

 

 

 

10 20 30 4o ' 50 60 70
Time (hours)

Figure 5.9 Comparison of the Simulated and Real Average
Pile Temperatures during a Steady State Storage

Period for the 1988-89 Storage Season

tan

Airflow Rate of 56.1 (Top) and 112.2 m /tonne-hr
(bottom).

86

 

 

 

 

 

 

 

 

 

 

 

 

 

 

15
A
L)
01
Q)
'0
v
Q)
L
3
4d
0
33
CL 13-
E
Q)
*—
s—a real-ave
12 a—a sim-ave
l T
0 5 1'0 1'5 20 25 30
15
G
0‘ .
14-
.8 Hi
V I
m r*
L
3
4.1
O
b
a. 13-
E
0
*—
s—a real-ave
a—a sim-ove
12 I I I I I
0 5 1O 15 20 25 30

Time (hours)

Figure 5.10 Comparison of the Simulated and Real Average
Pile Temperatures during a Temperature Change
Period for the 1988-89 Storage Season t an

Airflow Rate of 56.1 (Top) and 112.2 m /tonne-hr
(bottom).

87

 

 

 

 

 

 

 

 

 

 

 

15
A
L)
8‘ 144
3
Q)
L.
:1
4.1
O
6
CL 13J
E
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Figure 5.11 Comparison of the Simulated and Real Pile
Temperature at a Distance of 1.22 m from the
Storage Floor during a Temperature Change

Period for the 1988-89 Storage Season t an
Airflow Rate of 56.1 (Top) and 112.2 m /tonne-hr
(bottom).

88

5.16 Finite Difference Control Model Discussion

The finite difference temperature and weight loss
prediction model developed by Lerew (1978) was modified by
the addition of a temperature and humidity control algorithm
similar to the FANCOM 1056 environmental control computer.
The heat generation equation was also modified to match the
heat generation measured from the potatoes used for this
experiment. The predicted vs. measured potato temperatures
were compared with data collected during the 1988-89 storage
season. The steady state simulation results indicated that
the temperature simulation of the potato storage responded
similarly to the temperature history of the experimental
potato storage studied.

A comparison was made between the control simulation
and the experimental data for a temperature change period of
the simulated storage facility. The simulation predicted
average pile temperature was changed 0.5 0C in the same time
that the actual average pile temperature was changed.
However a temperature overshoot of 0.6 °C was noted in the
simulation (Figures 5.10 and 5.11). This indicates that the
simple control routine used to approximate the FANCOM 1056
controller was not closing the fresh air louver quickly
enough.

The FANCOM control program utilizes a minimum pulse and
pulse repeat time to control the fresh air louver for

cooling. The controller pulse and pulse repeat times are

89

varied above the minimum value based on the louver position
adjustment required. This feature, which allows increased
the controller to adjust the air inlets quickly if large
louver position changes are required, was not included in the
simulation model written to evaluate the controller.

erhaps this difference is the cause of the temperature

overshoot noted in the simulation of the potato storage.

5.17 Simulation of a Condensation Problem

The modified finite difference simulation was used to
study a problem of potential condensation on the surface of
potato tubers. The simulation study was performed on a
4 m deep column of potatoes with 2 m of warm
potatoes (21 °C) placed on top of 2 m of cool potatoes (10
°C). The simulation model predicts the volume of moisture
in the ventilation air. If the air comes into contact with a
cool surface, a calculation is made on the amount of
moisture deposited on the cool surface. A typical plot of
predicted water film thickness on the cool potato surfaces
versus the position in the column is presented in Figure
5.12. This data was generated by simulation runs with
ventilation modeled as complete recirculation, and with
fixed inlet conditions. The amount of time required to
remove the condensation from the tuber surface using
different air flow rates and inlet conditions is presented

in Table 5.18.

90

 

Table 5.18 Predicted Weight Loss and Time Required to
Remove Condensation from the Surface of Potato
Tubers Utilizing Various Air Inlet Conditions
and Ventilation Rates.

 

Ventilation Inlet Air Time Weight Lost

Rate Conditions
Temp. RH

m3/tonne-hr . (°C) (%) hrs. Percent

56 Recirculation ----------

28 15.5 65 100 0.95

56 15.5 65 45 0.80

112 15.5 65 20 0.35

28 15.5 80 160 0.95

56 15.5 80 95 0.92

112 15.5 80 40 0.55

 

The simulation of this condensation problem yielded the
following predictions.
1) In the complete recirculation mode, the condensation was
not removed from the surface of the tubers within the 250
hour simulation.
2) The simulation predicted that high air flow rates would
remove surface condensation in less time with less weight
loss.

5.18 Discussion of a Condensation Problem

The modified temperature and weight loss simulation was
used to study different ventilation system management
strategies to alleviate condensation from the surface of
potato tubers. The problem studied was one which occurs
during the filling of a potato storage bin in cool weather.

As the grower harvests potatoes throughout the day, the

91

 

0.100
. o—o 10 hour

H 100 hour
H 190 hour
0-050 x—x 280 hour

 

0.060-

 

 

Water Film Thickness, nanometers

 

 

 

 

0.000- ,
0 1 2 3 4
File Height. Meters

OJOO
g H 10 hour
.66 4 H 30 hour
E H 80 hour
0 0.080- H 90 hour
6
c _ 5
6 0.0604 ‘ °
0
" l
C
‘6
:3 0£MO« . . 2 =
)—
g 4
II 0.020-
L.
0
a 1
o
3 0.000

0 1 2 3 4

 

Pile Height. Meters

Figure 5.12 Water Film Thickness on the Tuber Surfaces
Simulated within a Four Meter Deep Pile with
Two Meters of 21°C Temperature Potatoes on Tap
of Two Meters of 10°C Tem mgerature Potatoes at
a Ventilation Rate of 56 m /tonne-hr.
(Top : Complete Recirculation)o
(Bottom : Inlet Conditions, 15. 5 °C, 80% RH)

92

tuber pulp temperature increases as solar radiation heats up
the soil. At the interface between batches of potatoes
harvested on successive days in a storage bin, cool potatoes
harvested in the morning at a pulp temperature of in the
range of 10 0C are placed in direct contact with potatoes
harvested the previous day with a pulp temperature in the
range of 21 °C. The potential exists for condensation on
the surface of the cooler potatoes piled on top of the
warmer potatoes along the angled face of the pile. The
effectiveness of different ventilation conditions could be
measured by the simulation; The simulation could also be
extended to the problem of removing surface moisture from
potatoes coming from wet soil.

The simulation of this problem included varying the
ventilation rate and inlet air relative humidity to
determine the most effective means of removing the
condensation from the surface of the tubers, with a minimum
weight loss.

The simulation model predicted that using the
ventilation system in the complete recirculation made during
this phase of storage, which is a common practice, will
result in the transfer of moisture from the upper level of
the pile to the lower levels of the potato pile. Thus the
moisture is distributed over the surface of all potatoes in
the pile. The moisture was not removed from the tuber
surfaces over the 250 hour simulation time.

Further simulation work indicated that if the inlet

93

relative humidity were set at 65%, the time required to
remove the condensation from the pile was reduced as
compared to setting the relative humidity at 80%. The inlet
air was set at the average temperature between the two
potato pulp temperatures. Changing the inlet relative
humidity changes the water holding capacity of the
ventilation air. Increasing water holding capacity of
the ventilation air, which was set at the average of the in
potato pulp temperature used in the simulation, would $
account for the reduced time required to remove the
condensation from the surface of the tubers.
Simulations were also performed at different airflow
levels. The results indicated that as the airflow rate was
increased, the time required to remove the condensation was
proportionally reduced. The total volume of moisture that
must be removed is constant for all simulations. Thus if
the ventilation air conditions are fixed, the time required
to remove the condensation should vary inversely with the

volume of air passed over the surface of the potatoes.

6. Summary and Conclusions

Three years of research were conducted on the storage
of potatoes with ventilation systems designed with different
ventilation rates. The following conclusions were drawn in.
an attempt to quantify the differences in storage
performance based on ventilation rate.
1) Average weight loss within the simulated storage
research bins was not a function of ventilation rate.
2) Weight loss was more uniform from the top to the bottom
of the potato pile at high ventilation rates.
3) The pre-conditioning management strategy outlined by
Sowokinos (1987) produced high quality potatoes (cv.
Atlantic) for storage at 10 °C for 135 days.
4) The environmental control of the simulated storage
research bins was effectively carried out using the FANCOM
1056 environmental control computer. A controller was
developed specifically for the potato storage industry
by FANCOM B.V. utilizing information gathered from this
study.
5) The simulation runs performed to determine an adequate
management strategy for removing moisture from the
surface of potato tubers indicated that high ventilation
rates (112 m3/tonne-hr) performed better then low rates in
terms of time required for moisture removal and estimated

weight loss during this time.

94

9S

6) The result of calculations with the model developed
indicates that the time required for a temperature change of
the magnitude studied was primairly controlled by latent
heat transfer, and was not adversely affected by changes in
ventilation rate or internal temperature gradients within
the potatoes.

The goal of this research was to determine the effect
of different rates of through-the-pile ventilation for
potato storage in Michigan, justify current ventilation
recommendations, and determine a probable cause of the
differing ventilation recommendations found throughout the
world. The previously stated conclusions of this research
indicate that the current ventilation recommendation in
Michigan is higher than required for the holding phase of
storage. The ventilation of a potato storage, however, is
also dependent on the condition of the potatoes going into
the storage. Storage ventilation rate recommendations are
primarily affected by the condition of the potatoes at
harvest time. In West Germany and The Netherlands where the
requirement to dry potatoes coming from the field is
commonly cited (Hesen, 1989; Leppack, 1989), recommended
ventilation rates are in the range of 100 to 150 m3/tonne-
hr. In Michigan, where "fall harvest conditions vary"
(Cargill, 1989) ventilation rate recommendations are in the

range of 38 to 56 m3/tonne-hr. In the Pacific Northwest,

96

where harvest conditions are "dry and warm" (Waelti, 1989),
ventilation rates in the range of 22-32 m3/tonne-hr are
recommended.

The current ventilation recommendations in Michigan are
acceptable for the state of potato storage monitoring that
exists. Each storage season presents a different problem
to be controlled by the ventilation system. The lack of
monitoring systems within potato storage bins to determine
if condensation is occurring, or if potato pile temperatures
are out of range presents the need for excessive ventilation
to assure that potatoes are not lost. As monitoring systems
are employed, the ventilation rate may be varied depending
on measured ventilation needs to conserve energy and reduce

storage costs.

 

7. Suggestions for Future Research

Differences exist between potato storage ventilation
recommendations throughout the world. Previous discussion
indicated some possible causes for these differences. An
attempt was made to justify the current recommendations in
the state of Michigan. The next logical step in research is
to monitor the economic costs, in terms of energy
consumption, which are required by these different
ventilation strategies throughout the storage season. The
result of a study of this nature would indicate the economic
consequences of storage ventilation. A result of this
study might be that using variable rate ventilation system
designed to meet both the requirements of conditioning the
product after harvest, and maintaining this quality once
attained, would be a desirable alternative to using one
ventilation rate. A monitoring and control system would be
required to determine the needs of the potato pile

throughout the storage season.

97

List of References

ANONYMOUS. 1984. Potato Statistical Yearbook, National
Potato Council, Denver, Colorado. USA.

Bakker-Arkema, F. W., Bickert, W. G. and Patterson, R. J.
1967. Simultaneous Heat and Mass Transfer During the
Cooling of a Deep Bed of Biological Products Under Varying
Inlet Air Conditions. J. of Agr. Eng. Res. 12(4): 297-307.

Burton, W. G., 1963. The Basic Principles of Potato Storage
as Practiced in Great Britain. Europ. Pot. J. 6:77-92.

Burton, W. G. and Wigginton, M. J. 1970. The Effect of a Film
of Water Upon the Oxygen Status of a Potato Tuber. Pot.
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Cameron, A. and Hammerschmidt, R. 1988. Michigan Potato
Research Report, Michigan State University, E. Lansing,
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Cargill, B. F., Price, R. C. and Forbush, T. D. 1989.
Requirements and Recommendations for Potato Storage in the
Midwest USA. In: Potato Storage Technology and Practice,
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FANCOM. 1987. Unpublished Operators Manual 1056
Environmental Control Computer, Fancom B.V. 5980 AC
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FANCOM. 1986. Unpublished Operators Manual 1056
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Gould, W. A. 1988, Testing for Quality. In: Chipping Potato
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Graphs, L., Hylmo, B., Johansson, A., and Wikberg, C. 1978.
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Hesen, J. C. and Rostovski, A. 1989. Potato Storage
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Holman, J. P. 1976. Heat Transfer. McGraw-Hill Book Co.,
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Horvath, S. 1989. Special Problems of Potato Storage in

Hungary. In: Potato Storage Technology and Practice,
ASAE, St. Joseph, Michigan, USA. pp 83-102.

98

99

Hunter, J. H. 1985. Heat of Respiration and Weight Loss in
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Hunter, J. H. and Belyea, S. 1989. Requirements and
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Hylmo, B., Persson, T., and Wikberg, C. 1976. Bulk Storing
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Karwowski, T. 1989. Potato Storage in Poland. In: Potato
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Leppack, E. 1989. Conditions and Recommendations for Potato
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Michigan, USA. pp 39-64.

Lerew, L. E. 1978. Development of a Temperature-Weight Loss
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Misner, G. C. 1973. Simulated Cooling of Potatoes. Ph.D.
thesis. University of Illinois. University Microfilms, Ann
Arbor, Michigan.

Misner, G. C. and Shove, G. C. 1976. Simulated Cooling of
Potatoes. Trans. ASAE. 19:967-969.

Ofoli, R. Y. 1984. Entropy Production by Biological
Products in Storage. Ph.D. thesis. Michigan State
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Pisarczyk, J. M. 1982. Field Harvest Damage Affects Potato
Tuber Respiration and Sugar Content. Amer. Pot. J. 59:205-
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Rasmussen, R. 1989. Potato Storage in Scandinavia. In:
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Rastovski, A. and vanEs A. 1987. Storage of Potatoes.
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Schaper, L. A. and Preston, D. A. 1989. Requirements and
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100

Singh, R. P., Heldman, D. R., Cargill, B. F. and Bedford,
C. L. 1974. Factors Influencing Reconditioning of Potatoes
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Sowokinos, J. R. and Preston, D. A. 1988. Maintenance of
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Sparks, W. C., Smith, V. T., Garner, J. G. 1968.
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pp 9-38.

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Arbor, Michigan.

Villa, L. G. and Bakker-Arkema, F. W. 1974. Moisture Losses
for Potatoes during Storage. ASAE Tech. Paper No. 74-6510.

Wada, T. 1989. The Potato Industry in Japan. In: Potato
Storage Technology and Practice, ASAE, St. Joseph,
Michigan, USA. pp 173-188.

Waelti, H. 1989. Potato Storage and Ventilation in the
Pacific Northwest. In: Potato Storage Technology and
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Joseph, Michigan.

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