• TABLE OF CONTENTS
HIDE
 Front Cover
 The authors
 Introduction
 Cooling requirements
 Cooling methods
 Management guidelines
 Conclusion
 Reference
 Figures 5A-10B
 Back Cover






Group Title: Circular - Florida Extension Service - 941
Title: Cooling Florida sweet corn
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00014497/00001
 Material Information
Title: Cooling Florida sweet corn
Series Title: Circular Florida Extension Service
Physical Description: 21 p. : ill. ; 28 cm.
Language: English
Creator: Talbot, Michael T ( Michael Thomas ), 1948-
Sargent, Steven A ( Steven Alonzo )
Brecht, Jeffrey Karl
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla.
Publication Date: [1991]
 Subjects
Subject: Sweet corn -- Florida   ( lcsh )
Vegetables -- Cooling   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 11)
Statement of Responsibility: Michael T. Talbot, Steven A. Sargent and Jeffrey K. Brecht.
General Note: "September 1991."
Funding: Circular (Florida Cooperative Extension Service) ;
 Record Information
Bibliographic ID: UF00014497
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 001665297
oclc - 24645563
notis - AHX7077

Table of Contents
    Front Cover
        Front Cover
    The authors
        Unnumbered ( 2 )
    Introduction
        Page 1
    Cooling requirements
        Page 1
    Cooling methods
        Page 2
        Page 3
        Page 4
    Management guidelines
        Page 5
        Page 6
        Page 7
        Page 8
    Conclusion
        Page 9
        Page 10
    Reference
        Page 11
    Figures 5A-10B
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Back Cover
        Back Cover
Full Text





September 1991


Cooling Florida Sweet Corn

Michael T. Talbot, Steven A.,Sargent, and Jeffrey K. Brecht
*i


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Florida Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, Dean


Circular 941


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*Michael T. Talbot is Assistant Professor, Department of Agricultural Engineering; Steven A. Sargent is Assistant Professor and
Jeffrey K. Brecht is Associate Professor, Department of Vegetable Crops; IFAS, University of Florida, Gainesville FL 32611









Introduction
The sugar content of standard sweet corn culti-
vars, which largely determines quality, decreases
rapidly at normal temperatures [1]1. Loss of
tenderness and sweetness are not acceptable to
consumers. New supersweet cultivars with twice
the sugar content of standard cultivars lose their
sweetness more slowly during marketing and have
improved consumer satisfaction. However, proper
temperature management is important even with
the supersweet corn varieties [2, 3]. Proper tem-
perature management of sweet corn begins with
precooling (rapid removal of field heat) from field
temperatures which can be above 30C (860F).
Rapid removal of field heat is critical to retard
deterioration of sweet corn. The recommendation
for maximum quality retention of sweet corn is
precooling to near OC (32oF) within 1 hour of
harvest and maintaining at OoC (32oF) throughout
the marketing channels [5]. Commercial sweet
corn operations in Florida rarely achieve this ideal
cooling criterion due to various factors, including
volume of corn handled, cooling and handling
equipment availability and capability, economics,
energy, and market conditions.
Florida leads the nation in sweet corn production
with an average annual value of $69.7 million for
the last 5 years [4]. Sweet corn yields in Florida
averaged 219 crates per acre with 11.4 million
crates packed from 51.9 thousand acres, with a
season average f.o.b. of $6.13 per crate over the last
5 years.
Sweet corn growers and packers in Florida are
aware of the value of their crop and of the quality
demands of consumers. They are using good
temperature management but are interested in
additional improvements. Most sweet corn in
Florida is hydrocooled or vacuum cooled in wooden
crates. However, a recent commercial effort was
undertaken to utilize slush icing (or package icing)
for precooling sweet corn in wax-impregnated
fiberboard cartons. Room cooling of sweet corn is
too slow to be an acceptable precooling method, and
refrigerated trucks cannot remove field heat during
transit due to inadequate refrigeration capacity.
This publication presents cooling requirements,
cooling methods, quality parameters, and manage-
ment guidelines for maintaining the quality of
Florida sweet corn during handling and shipping
operations. Studies conducted at various commer-
cial sweet corn precooling operations from Home-
stead to Alachua over several growing seasons are
'Numbers in brackets refer to cited references.


discussed. The results of sweet corn quality evalu-
ation judged from a consumer perspective are also
presented. Initial quality evaluations were com-
pared to the quality after precooling and
postharvest handling to simulate the sweet corn
condition observed by the consumer. Recommenda-
tions are presented for packinghouse operators
concerning improvements possible in system
performance, such as increasing residence time
within a hydrocooler to achieve better cooling or
lowering the cold room temperature to prevent
warming of vacuum-cooled corn.

Cooling requirements
Understanding of the cooling requirements of
horticultural commodities requires an adequate
knowledge of their biological responses. Fresh
horticultural crops are living organisms, carrying
on many biological processes essential to the
maintenance of life. They must remain alive and
healthy until processed or consumed. Energy that
is needed for these life processes comes from the
food reserves that accumulated while the commodi-
ties were still attached to the plant [6].
Respiration is the process by which the food
reserves are converted to energy. Through a
complex sequence of steps, stored food reserves
(sugars and starches) are converted to organic acids
and subsequently to simple carbon compounds.
Oxygen from the surrounding air is used in the
process while carbon dioxide is released. Some of
the energy is used to maintain the life processes
while excess energy is released in the form of heat,
called "vital heat." This heat must be considered in
the temperature management program.
The respiration rate varies with commodity, in
addition to cultivar, maturity or stage of ripeness,
injuries, temperature, and other stress related
factors. Sweet corn has a high respiration rate, 40
mg CO/kg-h (8,900 Btu per ton per day) at OOC
(32oF) [5]. The major determinant of respiration
activity is the product temperature. Since the final
result of respiration activity is product deteriora-
tion and senescence, achieving as low a respiration
rate as possible is desirable. For each 10C (18F)
temperature increase respiration activity increases
by a factor of 2 to 4 [5]. For example, the respira-
tion of sweet corn at 10C (500F) is 112 mg CO2/kg-h
(24,600 Btu per ton per day), almost three times
greater than at 0C (32oF). Therefore, sweet corn
must be rapidly precooled to slow its metabolism
(physiological deterioration) in order to provide
maximum quality and storage life for shipping and
handling operations.









Introduction
The sugar content of standard sweet corn culti-
vars, which largely determines quality, decreases
rapidly at normal temperatures [1]1. Loss of
tenderness and sweetness are not acceptable to
consumers. New supersweet cultivars with twice
the sugar content of standard cultivars lose their
sweetness more slowly during marketing and have
improved consumer satisfaction. However, proper
temperature management is important even with
the supersweet corn varieties [2, 3]. Proper tem-
perature management of sweet corn begins with
precooling (rapid removal of field heat) from field
temperatures which can be above 30C (860F).
Rapid removal of field heat is critical to retard
deterioration of sweet corn. The recommendation
for maximum quality retention of sweet corn is
precooling to near OC (32oF) within 1 hour of
harvest and maintaining at OoC (32oF) throughout
the marketing channels [5]. Commercial sweet
corn operations in Florida rarely achieve this ideal
cooling criterion due to various factors, including
volume of corn handled, cooling and handling
equipment availability and capability, economics,
energy, and market conditions.
Florida leads the nation in sweet corn production
with an average annual value of $69.7 million for
the last 5 years [4]. Sweet corn yields in Florida
averaged 219 crates per acre with 11.4 million
crates packed from 51.9 thousand acres, with a
season average f.o.b. of $6.13 per crate over the last
5 years.
Sweet corn growers and packers in Florida are
aware of the value of their crop and of the quality
demands of consumers. They are using good
temperature management but are interested in
additional improvements. Most sweet corn in
Florida is hydrocooled or vacuum cooled in wooden
crates. However, a recent commercial effort was
undertaken to utilize slush icing (or package icing)
for precooling sweet corn in wax-impregnated
fiberboard cartons. Room cooling of sweet corn is
too slow to be an acceptable precooling method, and
refrigerated trucks cannot remove field heat during
transit due to inadequate refrigeration capacity.
This publication presents cooling requirements,
cooling methods, quality parameters, and manage-
ment guidelines for maintaining the quality of
Florida sweet corn during handling and shipping
operations. Studies conducted at various commer-
cial sweet corn precooling operations from Home-
stead to Alachua over several growing seasons are
'Numbers in brackets refer to cited references.


discussed. The results of sweet corn quality evalu-
ation judged from a consumer perspective are also
presented. Initial quality evaluations were com-
pared to the quality after precooling and
postharvest handling to simulate the sweet corn
condition observed by the consumer. Recommenda-
tions are presented for packinghouse operators
concerning improvements possible in system
performance, such as increasing residence time
within a hydrocooler to achieve better cooling or
lowering the cold room temperature to prevent
warming of vacuum-cooled corn.

Cooling requirements
Understanding of the cooling requirements of
horticultural commodities requires an adequate
knowledge of their biological responses. Fresh
horticultural crops are living organisms, carrying
on many biological processes essential to the
maintenance of life. They must remain alive and
healthy until processed or consumed. Energy that
is needed for these life processes comes from the
food reserves that accumulated while the commodi-
ties were still attached to the plant [6].
Respiration is the process by which the food
reserves are converted to energy. Through a
complex sequence of steps, stored food reserves
(sugars and starches) are converted to organic acids
and subsequently to simple carbon compounds.
Oxygen from the surrounding air is used in the
process while carbon dioxide is released. Some of
the energy is used to maintain the life processes
while excess energy is released in the form of heat,
called "vital heat." This heat must be considered in
the temperature management program.
The respiration rate varies with commodity, in
addition to cultivar, maturity or stage of ripeness,
injuries, temperature, and other stress related
factors. Sweet corn has a high respiration rate, 40
mg CO/kg-h (8,900 Btu per ton per day) at OOC
(32oF) [5]. The major determinant of respiration
activity is the product temperature. Since the final
result of respiration activity is product deteriora-
tion and senescence, achieving as low a respiration
rate as possible is desirable. For each 10C (18F)
temperature increase respiration activity increases
by a factor of 2 to 4 [5]. For example, the respira-
tion of sweet corn at 10C (500F) is 112 mg CO2/kg-h
(24,600 Btu per ton per day), almost three times
greater than at 0C (32oF). Therefore, sweet corn
must be rapidly precooled to slow its metabolism
(physiological deterioration) in order to provide
maximum quality and storage life for shipping and
handling operations.








Sweet corn is not a chilling-sensitive crop (crops
which must be stored at temperatures generally
above 10C (500F) to prevent physiological damage).
Therefore, it can be safely cooled to a temperature
of OOC (320F). The recommendation listed in the
introduction indicated the requirement of precool-
ing to near 0C (32F) within 1 hour of harvest and
maintaining at o0C (320F) throughout the market-
ing channels. The required rate of cooling during
precooling can be expressed in terms of the half-
cooling time or the 7/8-cooling time. These values
remain constant for the particular set of precooling
conditions from which determined. The half-
cooling time is the time required to remove one half
of the temperature difference between the initial
pulp temperature and the cooling medium tempera-
ture. For commercial precooling, it is recommended
[8] that 7/8 of the difference between the pulp
temperature and the cooling medium temperature
(7/8-cooling time) be removed prior to storage and
transport. Under ideal circumstances the 7/8-
cooling time is equal to about three times the
amount of the half-cooling time.
For example, if corn is harvested at 30C (860F)
and cooled in a hydrocooler with a water tempera-
ture of OC (320F), the half-cooling time would be
the time required to remove 15oC (270F)2 or for the
corn to be cooled to 150C (59F)3. For the same
situation, the 7/8-cooling time would be the time
required to remove 260C (470F)4 or for the corn to be
cooled to 4C (390F)6. By developing a precooling
schedule [8], the 7/8 cooling time could be estab-
lished. Therefore, after precooling for a time period
equal to the 7/8-cooling time or when the pulp
temperature reached 4C (390F), the corn would be
removed from the precooler and moved to cold
storage for additional cooling to 0C (320F).

Cooling methods
The selection of a particular precooling method is
determined by several factors including: the rate of
cooling required, compatibility of the method with
the commodities to be cooled, subsequent storage
and shipping conditions, and equipment and
operating costs.
During precooling, the sensible heat (or field
heat) from the product is transferred to the ambi-
ent cooling medium. The rate of heat transfer, or
cooling rate, is critical for the efficient removal of


field heat and is dependent upon two factors:cooling
medium temperature, and contact between cooling
medium and product. In addition, the product must
remain in the precooler for sufficient time to
remove the heat (7/8-cooling time) to achieve
maximum cool. This is particularly important
during busy periods when it may be tempting to
"push" product through the precooler. A correctly-
sized precooler should have sufficient capacity so as
to provide adequate residence time for precooling,
while at the same time not slowing subsequent
packing and/or handling operations. The cooling
medium (air, water, crushed ice) must be main-
tained at a constant temperature throughout the
cooling period. If the refrigeration system is under-
sized for the amount of product requiring precool-
ing, the temperature of the medium will increase
over time. The cooling medium must also have
intimate contact with the surfaces of the sweet
corn. Inappropriately designed containers can
markedly reduce flow of the cooling medium.
The cooling rate is not only dependent upon
cooling-medium temperature, and contact with the
commodity; it is also dependent on the cooling
method employed.

Vacuum cooling
Vacuum cooling (Figure 1) is the most rapid
method of precooling sweet corn, although it is
most efficient for commodities with a high surface-
to-volume ratio such as leafy crops. This method is
based on the principle that the boiling point of
water lowers as atmospheric pressure is reduced.
Cooling is achieved on a large scale by reducing the
atmospheric pressure inside a large, air-tight,
strongly constructed steel vacuum chamber (tube)


2 [30 0]* 1/2 = 15
3 [30 15] = 15
4 [30 01 7/8 = 26
5 [30 26] = 4


([86 32] 1/2 = 27)
([86 27] = 59)
([86 32] 7/8 = 47)
([86 47] = 39)


Figure 1. vacuum cooler cnamOers ana panetizea creates or
sweet corn.









containing the sweet corn packed in containers and
palletized. The pressure in the chamber is reduced,
which reduces the pressure of water vapor in the
chamber. When the water vapor pressure in the
chamber is reduced below that in the product's
intercellular spaces, water will evaporate from the
product resulting in cooling. The water on the
product surface boils at the desired precooling
temperature, usually near OC (32oF) wet bulb
temperature when the chamber pressure reaches
4.6 mm (0.18 inches) mercury absolute pressure.
Water evaporating from the surface removes field
heat from the product and condenses on evaporator
coils within the vacuum tube. Evaporator coils
increase cooling efficiency by removing water vapor
from the air, permitting faster evaporation of the
water from the surface of the product. Crated corn
can be vacuum cooled from 27.3oC (81F) to 3.4C
(380F) in about 50 minutes [9].
Measurement of sweet corn pulp temperature in
the vacuum cooler is important. A gauge that
measures absolute pressure in the chamber gives a
direct indication of the boiling temperature of
water in the chamber, and is probably the most
reliable guide to managing a vacuum cooler. A
remote temperature probe inserted into an ear of
corn and connected to an indicator outside the
chamber will indicate cooling of exposed corn, but
corn in the interior of the load may be considerably
warmer, particularly if air is present. The value of
a wet-bulb thermometer (which represents the
boiling point of water in a vacuum) is limited by air
leaks into the system which may raise the actual
boiling point of the water considerably above the
wet-bulb reading.
Packed corn can be cooled quickly and uniformly
in large loads by this method, but container walls
or other barriers can seriously slow cooling by
retarding water vapor escaping from the corn.
Failure to properly wet the corn before or during
the vacuum cooling cycle can result in a 1 percent
moisture loss for each 6oC (11 F) change in corn
temperature [7]; therefore, denting of kernels may
occur. The vacuum-cooled sweet corn must be
moved quickly to a cold room, or rapid rewarming
will occur.
Vacuum coolers are costly to purchase and
require skilled operators. To be economically
feasible, there must be a large daily and annual
output of cooled produce. Thus, a vacuum cooler
must either be located close to a long-season
production area or made portable so it can be
moved to locations where there is such production.


Hydrocooling
The use of cold water to quickly cool produce is
an old and effective method used for cooling a wide
range of fruits and vegetables in bins or in bulk
before packing or in containers after packing.
However, /ts use for packed commodities has
limitations because of the difficulty of achieving
sufficient water flow through the containers, and
because the containers must be water-tolerant.
Hydrocooling by showering or immersion in water
is the most common precooling method for sweet
corn. Hydrocooling removes heat at a slower rate
than vacuum cooling. The heat capacity of refriger-
ated water is greater than that for air, which
means that a given volume of water can remove
more heat than the same volume of air at the same
temperature.
Hydrocooling removes no water from the pro-
duce, and may even revive slightly wilted produce.
Effectiveness of this cooling method depends upon
low and constant water temperature 0-1oC (32-
34F), maximum surface contact of water with corn,
and sufficient time for heat removal. Hydrocooling
corn in bulk is more efficient than hydrocooling
crated corn due to improved contact between water
and corn. However, bulk handling may not fit into
the management scheme and some rewarming of
the corn will occur during the subsequent packing
operations. Crated corn on pallets may take over
an hour in a hydrocooler to cool from 28oC (82oF) to
7C (45oF) using 40C (400F) water [9].
Several types of hydrocoolers are used for sweet
corn in Florida. Conveyor hydrocoolers are the
most common type (Figure 2). In this type, corn in
bulk, individual containers, or in containers on
pallets stacked four or five layers high, is carried on
a conveyor through a shower of water. Large
overhead spray nozzles must be capable of dis-
charging a large volume of water over the
palletized crates to efficiently remove heat. If the
mass of produce is deep (a foot or more) the water
may "channel" (pour through larger openings where
least resistance to flow is encountered) and come in
contact with only part of the lower surfaces. Chan-
neling may be avoided by providing a heavy shower
over a shallow depth of produce, or by proportion-
ing the shower and the drainage from the bottom of
containers so that the containers will be partly or
entirely filled with water. Drainage must be suffi-
cient to keep the water in the containers moving,
and to remove all water before containers leave the
hydrocooler. Corn toward the center of the pallet is
most difficult to adequately cool.




















-Ir









Figure 2. Conveyor hydrocooler precooling palletized crates
of sweet corn.

The length of the hydrocooler conveyor is
important and must not be underestimated. If, for
example, a hydrocooler takes 45 minutes to cool
sweet corn from 34"C (94oF) to 4oC (40*F) with 1C
(340F) water, the hydrocooler must be large enough
to hold, at one time, the maximum quantity of corn
that will be loaded into it in 45 minutes. Also, for
greater cooling efficiency, the refrigeration capacity
must be sufficient to maintain a constant water
temperature of 1C (340F), regardless of the initial
corn pulp temperature.
Many commercial hydrocoolers are inefficient
simply because they lack insulation. Tests have
shown that less than half of the refrigeration
supplied to most conveyor-type hydrocoolers is used
to cool the produce-the rest is lost through insuffi-
cient insulation [7].
The room-type or "batch" hydrocooler has no
conveyors and so is more easily insulated. Contain-
ers to be cooled are stacked in rooms, normally
palletized, and may be left there in temporary
storage. At least two rooms are usually provided,
so that one can be precooling while the other is
emptied and filled. Even though only half the floor
space is used at any one time for precooling, floor
area in this type of operation is much less expen-
sive than floor area in a conveyor operation, and
the total space occupied is not necessarily more
than for the latter. Overhead spray nozzles capable
of discharging a large volume of water over the
palletized crates are located in the roof of the cold


room to efficiently remove heat. The system should
be designed so that only nozzles located above the
product requiring cooling are discharging cold
water. The cooling water returns to the refrigera-
tion unit through drains in the floor.
Sanitation of the hydrocooling water is critical,
since it is first refrigerated, drenched over the
product, collected, recooled, and drenched again
over the product. Decay organisms present on a
few vegetables can accumulate in the water,
inoculating subsequent product being hydrocooled.
Commodities which are hydrocooled must be
resistant to contact with waterborne pathogens and
be able to withstand the force of the water drench.
Potential limitations of hydrocooling must also
be considered. When the hydrocooler is operating
at capacity, arriving warm produce must remain at
ambient temperatures to await cooling. Attempts
to shorten the cooling (residence) time to increase
throughput of sweet corn will result in unsatisfac-
tory cooling. Furthermore, when cooling is com-
pleted, the product must be moved quickly to a cold
room, or rapid rewarming will occur. Cooling
efficiency may be low unless the hydrocooler is
operated continuously at maximum capacity or is
inside a cold room or insulated enclosure. Shower-
pan holes must be cleaned daily to avoid plugging
that can cause uneven water flow over the product.

Slush ice cooling
Slush-icing (liquid-icing), an improved form of
package-icing, is another method to precool sweet
corn. A slurry of refrigerated water and finely
chopped ice can be drenched over either bulk or
containerized produce. This slush-ice method is
becoming more widely adopted for commodities
tolerant of direct contact with water and, as noted
above, has been used commercially to precool sweet
corn in Florida (Figure 3). The water acts as a
carrier for the ice so that the resulting slush can be
pumped into a packed container. The rapidly
flowing slush causes the product in the container to
float momentarily and distributes the ice through-
out the container, achieving better ice/product
contact when the water drains out the container
vents. As the product settles in the container, the
ice encases the individual items by filling air voids
(Figure 4), thus providing good contact for field
heat removal. The residual ice continues the
cooling process and also maintains high relative
humidity within the container. Slush-icing is a
somewhat slower cooling method than vacuum
cooling and hydrocooling. Corn in wax-impreg-
nated fiberboard boxes which was placed in a 8.4C








(47.5oF) cold storage room (which is too warm) after
slush ice cooled from 23C (74oF) to 5C (42oF) in an
hour [9].






















Figure 3. Injection of water-ice slurry Into carton through the
hand hole during slush ice precooling of sweet corn.
---._7
--...


L


Figure 4. Carton cut open showing ice well-dispersed among
the ears of corn after slush icing.

Skilled management is required to control the
slurry mixture and insure adequate ice remains to
precool the corn. It is important to quickly store
the cartons in a cold room after precooling to insure
no reheating of cartons.
Slush-icing requires use of more expensive,
water-tolerant shipping containers (wax-impreg-
nated fiberboard boxes) and specialized slurry
mixing and pumping equipment, as well as ice-
making and handling equipment. The systems
used in Florida were batch systems and some
packinghouse operators question the ability of


slush-ice precooling for large volume operations.
However, equipment is commercially available for
automatically icing pallet loads of packed cartons.

Room cooling, storage, and shipping
The simplest and slowest cooling method is room
cooling, in which the bulk or containerized com-
modity is placed in a refrigerated room for several
hours or days. Air is circulated by the existing fans
from the evaporator coil in the room. Vented
containers and proper stacking are critical to
minimize obstructions to air flow and ensure
optimal heat removal. Room cooling is satisfactory
only for commodities with a low respiration rate,
such as mature potatoes and onions. For sweet
corn it should be used only after precooling for 7/8-
cooling time, during short-period storage prior to
shipment. Refrigerated trucks should be precooled
prior to loading the precooled sweet corn. After
precooling, top icing of wirebound-crated corn is
desirable during holding or transport to continue
cooling, remove heat of respiration, and keep the
husks green and fresh. The ice should be wind-
rowed to prevent interference with air movement.

Management guidelines


Sweet corn precooling experiments [9] which
illustrate important management points are
outlined in Table 1. Tests 1 through 4 were indi-
vidual evaluations for indirect comparisons of
precooling methods while Tests 5 and 6 were
designed for direct comparison of precooling meth-
ods. Various supersweet sweet corn varieties were
used depending on the cooperator and test date.
Standard wood crates were used for the
hydrocooling and vacuum cooling tests, while
specially-designed, cascaded waxed, corrugated
cartons were used for the slush ice tests. Slush-ice
precooling in Tests 1 and 2 was accomplished using
a commercial system with a three-pallet capacity:
one waiting injection; one being injected; and one
draining water. Two men operated nozzles (Figure
3), one on each side of the pallet, injecting a water-
ice slurry into two cartons at a time through the
hand holes, and working from the top to the bottom
of the pallet. Slushing required approximately two
minutes while draining and movement to cold
storage required an additional two minutes. Six
instrumented cartons were placed on a pallet of
corn and slushed in the same manner pallets were
normally slushed. The slush ice precooling in Test
5 was accomplished with a smaller, one-pallet
commercial slush system. During the test, only one
slush nozzle was used. Other system problems









Table 1. Commercial sweet corn precooling studies.
Test Location Type of precoolina Quality
number Slush ice Hydrocooling Vacuum evaluated
1 Homestead 3-Palletz
2 Homestead 3-Palletz
3 Zellwood Palletz
4 Alachua Short Tunnelz
5 Homestead 1-Pallety Single Crates" Before/After
6 Zellwood Modifiedy Long Tunnel'z. Palletz Before/After

zPallet YCarton "Crate "Commercial Shipment "Pallet cooling data lost


with ice delivery increased the time required to
slush and only the instrumented cartons were
slushed before movement to cold storage. The
slush ice precooling in Test 6 was not possible with
the slush system at the test location. To make a
direct comparison between precooling methods, a
modified slush-ice procedure was devised. Crushed
ice was mixed with water in a large tub and then
poured into the opened top of the waxed cartons
containing the corn and sensors.
The Test 4 hydrocooling was conducted in a two-
tunnel pallet (conveyor) hydrocooler (Figure 2) with
5C (41oF) refrigerated water continuously shower-
ing down from an overhead perforated storage pan
while the pallets were slowly pulled through the
tunnels by cleated chain drives. The hydrocooler in
Test 5 showered refrigerated water (1"C (34"F))
from an overhead perforated storage pan over
individual crates of sweet corn conveyed five
abreast through the hydrocooler on a slowly moving
belt. The hydrocooling in Test 6 was also a two-
tunnel pallet hydrocooler similar to, but much
longer than, the Test 4 hydrocooler.
The same vacuum cooler was used in Tests 3 and
6 (Figure 1). The pallets of corn were moved into
the vacuum tube on long wagons and sprayed with
water. The tube was sealed and sufficient vacuum
(5.2 mm (0.2 inches) mercury absolute pressure)
was applied to cool the corn to 3C (38oF). After
breaking the vacuum, and a brief rewetting and
draining period, the wagon of pallets was pulled
from the tube and exposed to ambient conditions
until each pallet was removed from the wagon by
forklift and transferred to a 7C (45"F) cold storage.
Test 5 involved a commercial shipment from
Homestead to Gainesville via Jacksonville. During
Test 5, the corn was transported from the packing-
house cold storage to the cold storage of a shipper,
then transported to Jacksonville by commercial
refrigerated trailer and placed in the receiver's cold
storage, then transported (air conditioned vehicle)
to Gainesville (approximately 116 km (72 miles)),


and stored at 5C (41oF) for a total of ten days after
harvest. During Test 6, the corn from the
hydrocooler was transported back to the vacuum
cooler packinghouse and stored overnight with the
vacuum-cooled and modified slush ice-precooled
corn. All three precooling treatments were trans-
ported (air conditioned vehicle) to Orlando (ap-
proximately 30 km (19 miles)) and stored at 5C
(41oF) for a total of 10 days after harvest.
Treatment conditions and final temperatures for
slush ice, hydrocooling, and vacuum precooling are
summarized (Table 2). The first entry indicates
corn with an initial average shank temperature of
23.6oC (74.5oF) was precooled using the 3-pallet
slush-ice unit (Test 1), placed in a 1.8C (35.2"F)
cold room for 1.8 hours, and achieved a final
average shank temperature of 5.7oC (42.3F). In
addition, the hydrocooling and vacuum cooling
entries indicate the precooling time and average
shank temperature after precooling. The
hydrocooling entries also provide the mean cooling
water temperature.
The concept of percent cooling is used to compare
different types of precoolers. The percent cooling is
based on the ratio of the temperature difference
between the initial and final corn temperature to
the temperature difference between the initial and
ideal corn temperature, 0C (32oF). The time to
achieve 50 and 75% for all precooling tests is
provided in Table 3. In terms of the definitions
above, when the cooling medium temperature
equals the ideal temperature, half cooling repre-
sents 50% cooling, while 7/8 cooling equates to 88%
cooling. For example, if corn with an initial tem-
perature of 28"C (82.4oF) is cooled for 30 minutes
using 0C (32"F) to a temperature of 14C (57.20F),
the percent cooling would be [(28-14)/ (28-0)] 100
= 50%6 and the half cooling time would be 30
minutes.
The three-pallet slush-ice Tests 1 and 2 indi-
cated very satisfactory performance of the slush-ice
6 [(82.4 57.2) / (82.4 32)] 100 = 50%










Table 2. Summary of treatment conditions for cooling sweet corn.
Initial Mean Cold
shank store Room Final
temp. air cool shank
(C) temp. (hr) temp.
(oC) (oC)
Slush-ice
Test 1 23.6 8.6 1.8 5.7
Test 2 18.7 3.3 17.2 0.7
Test 5 21.7 7.0z 22.6' 5.0Y
Test 6 26.7 6.4 19.2 5.5


Initial After Cold
shank Pre- Mean Cooling stor. Room Final
temp. cool water Shank air cool shank
(C) (min) temp. temp. temp. (hr) temp.
Hydrocooling (C) (C) (oC)

Test 4 27.3 17 5.0 19.3 9.1 21.6 8.2
Test 5 20.5 13 1.0 17.3 7.0 22.9 9.0
Test 6 28.0 60 4.7 7.2 6.4 19.2 6.8

Initial Cold
shank Pre- stor. Room Final
temp. cool Shank air cool shank
(C) (min) temp. temp. (hr) temp.

Vacuum cooling

Test 3 26.5 52 4.8 7.0 0.8 5.6
Test 6 27.3 53 3.4 6.4 19.2 6.8

during shipment Yminimum achieved followed by warming


Table 3. Summary of sweet corn precooling tests.

Time to achieve cooling (minutes)

Method % 75%
Slush-ice

Test 1,3-pallet 41 103
Test 2, 3-pallet 52 157
Test 5, 1-pallet 124 682
Test 6, Modified 135 669


Hydrocooling
Test 4, Short tunnel 111 1231 (70%)Y
Test 5, Single crates 228 1914(60%)Y
Test 6, Long tunnel 34" 67"


Vacuum cooling
Test 3 31 46
Test 6 27 39

Y75% not achieved during test
lost data; cooling time during test from initial and final temperatures measured with hand-held thermometer.









unit (Table 3, Figure 5). A carton of corn slush iced
during Test 1 was cut open to reveal 11 kg (24 lb) of
ice well dispersed among the ears of corn (Figure
4). After initial precooling, ice remained for addi-
tional cooling and could serve as insurance in the
event of a break in the cool environment during
shipment. The cooperator indicated his receivers
were pleased when cartons were opened upon
receipt and ice was still present. The one-pallet
slush-ice unit used during Test 5 did not perform as
well as the first unit (Table 3, Figure 5). The
modified slush ice procedure results in Test 6 were
similar to Test 5 (Table 3, Figure 5). The manage-
ment requirements for slush-icing are similar to
that needed for vacuum cooling but with more
variables to consider (uniform packing, rapid
handling, thorough injection, proper water-ice
slurry concentration, sufficient ice supply, proper
injection pressure, system plugging, etc.).
The short-tunnel hydrocooler (Test 4) involved a
moderate cooling water temperature and short
precooling time (Table 2), resulting in less cooling
than the three commercial slush ice-tests (Table 3,
Figure 5 and 6). Both reduced water temperature
and increased precooling time would improve the
cooling performance. The cooling water tempera-
ture for the single-crate hydrocooler in Test 5 was
ideal (Table 2) and the product was precooled in
single crates rather than on pallets, which would
increase the cooling rate. However, the inadequate
precooling time (Table 2) resulted in a cooling
performance less than that of the three commercial
slush-ice tests and the short-tunnel hydrocooling
system of Test 4 (Table 3, Figure 5 and 6). An
increase in the precooling time would provide
greater cooling. The cooling water temperature for
the long-tunnel hydrocooler in Test 6 was moderate
but the precooling time was ideal (Table 2). The
shank temperature of one ear of corn in one test
crate was measured before and after precooling
using a hand-held thermometer. A decrease in the
cooling water temperature would improve the
cooling rate and could allow shorter cooling time
with increased throughout. With adjustments to
operating conditions, this system also has the
potential to be an outstanding hydrocooling system
(Table 3, Figure 6).
The vacuum cooling tests (Tests 3 and 6) pro-
duced similar results during different harvest
seasons (Table 2, Figure 7). Vacuum cooling
provided the most rapid cooling rate of all the
methods (Table 3). The vacuum-cooled corn actu-
ally was warmed after placement in the cold
storage room (Table 2). Reducing the cold room


temperature would decrease the loss of precooling
benefits. Movement directly from the vacuum
cooler to the cold storage room, rather than expo-
sure to ambient conditions, is very desirable.
The results of the commercial shipping Test 5
(Figure 8 and 9) illustrate the importance of main-
taining the storage temperature after precooling.
During transport by refrigerated trailer to Jackson-
ville at 5C (410F), the hydrocooled corn reached a
low temperature (average shank temperature for
three crates) of about 10C (50F) (Figure 8), while
the slush-ice cooled corn continued to cool below
5C (41.0F) (Figure 9). During unrefrigerated
transport of the crates and cartons from Jackson-
ville to Gainesville (2.5 to 3 hours), the hydrocooled
corn cob temperatures rose to about 140C (570F)
(Figure 8), while the ice melted in the slush iced
corn and the cob temperatures rose to about 9C
(48oF) (Figure 9). This unrefrigerated transport
may have reduced potential quality differences
between these two precooling treatments. When
placed in 5C (41oF) cold storage in Gainesville, the
corn in slush ice cartons cooled more slowly than
the hydrocooled corn in crates (Figure 8 and 9).
The comparative Test 6 (Figure 10) lacked
complete data on hydrocooling although the opera-
tion appeared to be satisfactory. The modified
slush-ice treatment performed poorly, indicating
the importance of injecting sufficient water-ice
mixture of proper consistency into the cartons.
Unrefrigerated transport of the precooled corn
(after overnight cold room storage) occurred be-
tween Zellwood and Orlando prior to additional 5C
(41oF) cold storage.

Quality parameters
Tests 5 and 6 included initial and final quality
evaluations. Samples for measurement of mois-
ture, dry matter, pericarp, and sugar content of the
sweet corn kernels were taken initially, and after
up to 10 days storage at 5C (41F). Ratings of
appearance quality and eating quality after storage
were made on the same six ears per treatment
replicate used for the measurements described
above. Appearance ratings were given for husk
color, husk drying, silk appearance and kernel
appearance; eating quality ratings were given for
kernel taste and kernel texture. The ears were
rated on a 1 to 5 scale with 1 = poorest quality and
5 = best quality in each case.
Comparing hydrocooling to slush-ice precooling
in Test 5, there were no significant changes in
moisture, dry matter or pericarp contents during









storage (data not shown). Sugar content decreased
30 percent during 10 days of storage at 5oC (41F),
but there was no significant difference between the
two precooling treatments (data not shown). Husk
color was better and there was less drying in slush-
iced sweet corn after 2 days of storage but there
was no difference between the treatments after 10
days (Table 4). Silk appearance showed no treat-
ment effect but kernel appearance was better after
10 days of storage in the slush-ice treatment (Table
4).
When hydrocooling, slush-ice, and vacuum
cooling were compared (Test 6), moisture content
was lower and dry matter content higher in the
vacuum-cooled sweet corn after storage for 5 or 10
days at 5oC (41F) (Table 5). Pericarp content
increased during storage and sugar content de-
creased in vacuum-cooled and slush-iced sweet corn
but did not change significantly in hydrocooled
sweet corn (Table 5). Silk appearance was best in
slush-iced sweet corn, but husk color, husk drying,
kernel appearance and kernel taste were not
affected by the three precooling treatments (Table
6). Kernel texture ratings were lower in vacuum-
cooled sweet corn and higher in slush-iced sweet
corn.
The precooling methods tested resulted in
relatively minor differences in quality of the sweet
corn. Even though the vacuum-cooled sweet corn
was drenched with water prior to and after cooling,
there was still some indication of kernel drying.
The husks, however, appeared to remain in good
condition. Although hydrocooling had no effect on
pericarp content or sugar level compared to slush
ice in Test 5, the more thorough hydrocooling
treatment performed in Test 6 resulted in lower
pericarp and higher sugar levels. Slush ice precool-
ing appears to be capable of producing sweet corn
that has quality comparable to hydrocooling and
vacuum cooling. The greatest potential for slush
icing may be as a supplement to either
hydrocooling or vacuum cooling where the ice may
act as a buffer against suboptimal temperature and
humidity conditions encountered during handling.

Conclusion
This publication presented cooling requirements,
cooling methods, quality parameters, and manage-
ment guidelines for maintaining the quality of
Florida sweet corn. Studies conducted at various
commercial sweet corn precooling operations were
discussed. Recommendations to the packinghouse
operators concerning possible system performance


improvements were presented, such as increasing
residence time within a hydrocooler to achieve
better cooling or lowering the cold room tempera-
ture to prevent warming of vacuum-cooled corn.
The results of sweet corn quality evaluations
judged from a consumer perspective were also
presented.
The importance of precooling and proper tem-
perature management during subsequent handling
of sweet corn was illustrated. The three precooling
methods discussed are valuable first steps in
proper temperature management for sweet corn.
However, each has advantages and disadvantages.
Slush-ice cooling appears to be a viable precooling
alternative for precooling sweet corn. Results from
studies of precooling operations at commercial
packing houses indicate that most are doing a good
job of precooling. The performance of existing
systems can be improved if the operators make
adjustments suggested in this study. Additional
study is needed and planned to provide more
thorough advice to the packinghouse precooling
operators. In addition to the evaluation of cooling
efficiency, more information is needed for economic
and energy analysis in order to compare the perfor-
mance of various commercial precoolers.









Table 4. Appearance ratings for precooled sweet corn after 2 and 10 days of storage at 5C (41.00F) (Test 5).

Appearance ratings
Storage Husk Husk Silk Kernel
time Precool color drying app. app.
(days) method (1-5) (1-5) (1-5) (1-5)


2 Slush ice 5.0ay 4.8a 3.9a 5.0a
Hydrocool 3.6b 3.6b 3.7a 5.0a

10 Slush ice 2.5a 2.7a 2.6a 4.4a
Hydrocool 2.7a 2.3a 2.2a 3.2b

zBased on ratings from 6 ears per container, 3 containers per treatment: husk color, 1 =yellow, 5=green; husk drying, 1=dry,5=fresh, turgid;
silk, 1=limp, collapsed, 5=fresh, turgid; kernels, 1=indented, dull, 5=turgid, shiny.

YPairs of means within storage times followed by the same letter are not significantly different by t-test (P>0.05).


Table 5. Analysis of sweet corn kernels prior to precooling and after 5 and 10 days of storage at 50C (41.00F) (Test 6).

Kernel analysis
Storage Moisture Dry Pencarp Sugar
time Precool content matter content content
(days) method (% fr. wt.) (% fr. wt.) (% dry wt.) (% fr.wt.)


0 74.57 25.43 11.83 11.66

5 Vacuum 75.17 24.83 10.73 11.61
Slush ice 75.62 24.38 10.98 10.88
Hydrocool 75.57 24.43 11.95 10.51

10 Vacuum 74.83 25.17 13.21 9.64
Slush ice 76.12 23.88 13.20 8.99
Hydrocool 76.26 23.74 12.46 10.99

LSD(0.05) 0.37 0.37 0.78 1.11


Table 6. Quality ratings of precooled sweet corn after 10 days of storage at 5C (41.0F) (Test 6).

Appearance qualityz Eating quality

Husk Husk Silk Kernel Kernel Kernel Precool
color drying app. app. taste texture method
(1-5) 15)(1-5) (1-5) (1-5) (1-5)


vacuum .a z
vacuum z./a z./a 1.c i./a 3./a 2

Slush ice 2.8a 2.7a 3.0a 2.3a 3.9a 3

Hydrocool 2.6a 2.8a 2.4b 2.3a 3.7a 3.

zRatings as in Table 4.

YBased on ratings from 6 ears per container, 3 containers per treatment:kernel taste, 1 =bland, watery or starchy, 5=fresh, sweet corn taste;
kernel texture, 1 flaccidd, tough, 5=turgid, tender.

"Means in columns followed by the same letter are not significantly different by DMRT (P>0.05).


.9o

.5a

2ab









References


1. Appleman, C.O. and J.M. Arthur. 1919.
Carbohydrate metabolism in green sweet corn.
Jour. Agr. Res. 17:137-152.


2. Brecht, J. K. 1988. Effect of handling prac-
tices on sweet corn quality. IFAS Sweet Corn
Institute Proceedings. VEC 88-2. p. 12-15.


3. Brecht, J. K. 1990. Postharvest quality of
sweet corn cultivars. IFAS Sweet Corn Institute
Proceedings. VEC90-2. P13-16


4. Freie, R. L., and H. V. Young, Jr. 1990.
Florida Agricultural Statistics. Vegetable Sum-
mary 1988-89. Department of Agriculture and
Consumer Services. Florida Agricultural Statistics
Service. Orlando, Florida, April.


5. Hardenburg, R. E., A. E. Watada, and C. Y.
Wang. 1986. The commercial storage of fruits,
vegetables, and florist and nursery stocks. Agricul-
tural Handbook No. 66. U.S. Department of Agri-
culture. Washington, D.C.


6. Kader, A.A., R.F. Kasmire, F.G. Mitchell, M.S.
Reid, N.F. Sommer, and J.F. Thompson. 1985.
Postharvest technology of horticultural crops.
California Coop. Ext. Serv. Pub. 3311.


7. Mitchell, F.G., R. Guillou, and R.A. Parsons.
1972. Commercial cooling of fruits and vegetables.
California Agr. Expt. Sta. Man. 43.


8. Sargent, S.A., M.T. Talbot, and J.K. Brecht.
1988. Evaluating precooling methods for vegetable
packinghouse operations. Proc. Fla. State Hort. Soc.
101:175-182.


9. Talbot, M.T., S.A. Sargent, and J.K. Brecht.
1989. Evaluation of commercial precooling for sweet
corn. Proc. Fla. State Hort. Soc. 102:169-175.








SWEET CORN: SLUSH ICE COOLING
TEMPERATURE GRADIENT


0 10 20 30 40 50
ELAPSED


* Test 1
3-Pallet


60 70 80
TIME (MINUTES)


+ Test 2
3-Pallet


o Test 5
1 -Pall


S32
90 100 110 120

A Test 6
et Modified


Figure SA. Time-temperature relationships for sweet corn precooled using slush Ice (Tests 1, 2, 5, and 6).


i(

I-


02

t-


86


77



68
FT

59 >
;2
m
50



41


~







SWEET CORN: SLUSH ICE COOLING
PERCENT COOLING


0 10 20 30 40 50
ELAPSED
U Test 1 + Test 2
3-Pallet 3-Pallet


60 70 80
TIME (MINUTES)
o Test 5
1-Pallet


90 100 110 120

A Test 6
Modified


Figure 5B. Percent of initial temperature cooling curves for slush ice precooling (Tests 1, 2, 5, and 6).


100

90


80

70

60

50

40

50

20








SWEET CORN: HYDROCOOLING
TEMPERATURE GRADIENT


Hand-held thermometer reading,


0 15 30 45 60 75 90 105
ELAPSED TIME (MINUTES)


* Test 4
Short-tunnel


+ Test 5
Single-crate


o TEST 6
Long-tunnel


Figure 6A. TIme-temperiture relationships for sweet corn precooled using hydrocooling (Tests 4, 5, and 6).


20


16


12


8


86


77

-4
m
68
m

59
ci
m
50 +


41


120







SWEET CORN: HYDROCOOLING
PERCENT COOLING
100

90

80
8 0 ------------------------- -----------
70 -
0 70
60
O
o 50
- + + + +
z 40
w
0 5 0 ---- j-------------------





0
00






0 15 30 45 60 75 90 105 120
ELAPSED TIME (MINUTES)
Test 4 + Test 5 o TEST 6
Short-tunnel Single-crate Long-tunnel


Figure 6B. Percent of initial temperature cooling curves for hydrocooling precooling (Tests 4, 5, and 6).








SWEET CORN: VACUUM COOLING


TEMPERATURE GRADIENT


0 10 20 30 40 50 60 70 80 90 100 110


ELAPSED
. TEST 3


TIME (MINUTES)
+ TEST 6


Figure 7A. Time-temperature relationships for sweet corn precooled using vacuum cooling (Tests 3 and 6).


28


24


(D
F-

w
r


LJ
I-


86



77

-I
m
68
F-
m

59 -

m

50 3



41



32


120







SWEET CORN: VACUUM COOLING
PERCENT COOLING


0 10 20 30 40 50 60 70 80
ELAPSED TIME (MINUTES)
TEST 3 + TEST 6


90 100 110 120


Figure 7B. Percent of initial temperature cooling curves for vacuum precooling (Tests 3 and 6).


100

90

80

70







SWEET CORN HANDLING TEST

Hydrocooling


0 10 20
ELAPSED TIME (HR)


-- Ave. Crate Temp.


86


77

--H
68 F
-DT
m
59
c
C-

m
50
-n

41


40


--- Ambient Temp.


Figure 8. Time-temperature relationships for sweet corn precooling using hydrocooling during commercial shipment and storage (Test 5). KEY TO HANDLING STEPS: a,
precooled, placed In packer's cold room (3-40C (37-39F)); b, transported to shipper (ambient); c, held In shipper's cold room (8C (46F)); d, loaded refrigerated trailer,
transported to receiver (Jacksonville) (5C (410F)); e,f, unloaded trailer, placed in receiver's cold room (70C (450F)); g, transported to Vegetable Crops Dept. (Galnesvlle)
(ambient); h, placed in postharvest cold room (5C (41F)).


28


24


w
Ir


w
Q_

F-








SWEET CORN HANDLING TEST
Slush Ice Precooling


0 10 20 50
ELAPSED TIME (HR)


--- Ave. Carton Temp.


---- Ambient Temp.


Figure 9. Time-temperature relationships for sweet corn precooling using slush ice during commercial shipment and storage (Test 5). See Figure 8 for legend to handling
steps.


28

24


86


77

--I
68 m

m
59
--
C2

50
-n

41


52








SWEET CORN: COMPARATIVE COOLING TEST 6
TEMPERATURE GRADIENT


77



68 m



59



50



41


0 50 60


* VACUUM


90 120 150
ELAPSED TIME (MINUTES)
+ MOD. SLUSH ICE


180


210


240


o HYDROCOOL
Long-Tunnel


Figure 10A. Comparative time-temperature relationships for sweet corn precooling using modified slush Ice, hydrocooling (partial) and vacuum cooling (Test 6).







SWEET CORN: COMPARATIVE COOLING TEST 6
PERCENT COOLING


0 -m------
O Y
0 U

* VACUUM


6


0 90 120 150
ELAPSED TIME (MINUTES)
+ MOD. SLUSH ICE <


180


210


240


HYDROCOOL
Lonq-tunnel


Comparative percent of initial temperature cooling curves for sweet corn precooling using modified slush ice, hydrocooling (partial) and vacuum cooling (Test


100

90


rooc
- >


cj~c
0)-


(ii)j


Figure 10B.
6).



















































































COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste,
director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
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