PRE-COOLING IN HORTICULTURAL CROPS

Abstract

The importance of precooling can be clearly recognised as it is portrayed as an intricate and essential part of the proper temperature management of all horticultural crops. Precooling is in essence the removal of heat or the reduction in temperature of the perishable produce as soon as possible after harvest. This process slows the respiration rate and minimises other decorative processes and thus helps to maintain quality at a high level. Precooling in conjunction with the proper storage or transportation allows for the extension of shelf or vase life of the horticultural produce which results in more satisfied customers at all levels of purchase. Within precooling a variety of different techniques exist for use in the horticultural industry. Hydro-cooling, vacuum cooling, room cooling, icing, forced air cooling, and cryogenic cooling are the principal methods in commercial use at present. Each of these individual techniques also have many variations, leading to a great diversity of perishable produce which may be precooled. As consumer awareness and sophistication are ever increasing due to the growing fear of chemical residues and the uncertainty surrounding genetically modified foods presently, and with the change to organic products continuing, alternative techniques of extending shelf life and maintaining high level of quality are being

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PRE-COOLING IN HORTICULTURAL
CROPS
1Nandkishor Madhavrao Kanade*, 2Sujayasreee.O.J and
3Pradeep Kumar Vishwakarma
1,3Ph.D Scholar, Division of Fruits Crops, ICAR-Indian Agricultural
Research Institute, New Delhi.
2Ph.D Scholar, Division of Postharvest Technology, ICAR-Indian
Agricultural Research Institute, New Delhi
*Email-nandkishorukd123@gmail.com
ABSTRACT
The importance of precooling can be clearly recognised as it is
portrayed as an intricate and essential part of the proper temperature
management of all horticultural crops. Precooling is in essence the
removal of heat or the reduction in temperature of the perishable
produce as soon as possible after harvest. This process slows the
respiration rate and minimises other decorative processes and thus
helps to maintain quality at a high level. Precooling in conjunction
with the proper storage or transportation allows for the extension of
shelf or vase life of the horticultural produce which results in more
satisfied customers at all levels of purchase. Within precooling a variety
of different techniques exist for use in the horticultural industry. Hydro-
cooling, vacuum cooling, room cooling, icing, forced air cooling, and
cryogenic cooling are the principal methods in commercial use at
present. Each of these individual techniques also have many variations,
leading to a great diversity of perishable produce which may be
precooled. As consumer awareness and sophistication are ever
increasing due to the growing fear of chemical residues and the
uncertainty surrounding genetically modified foods presently, and with
the change to organic products continuing, alternative techniques of
extending shelf life and maintaining high level of quality are being

772
investigated. Precooling is one of the techniques which adheres to
this ethos and should be applied widely throughout the entire
horticultural industry to attain its true potential.
Keywords: Precooling, field heat, perishables
1. Introducation
Pre-cooling is the key component in the preservation of quality
for perishable fresh produce in post-harvest systems. Pre-cooling is also
very closely linked to the other operations such as handling and storage.
The practice of pre-cooling fruits and vegetables after harvest has existed
for many years. In that time, several methods and techniques of precooling
were developed, primarily to meet requirements of the large producers
and markets. Postharvest cooling is essential to ensure that product quality
is maintained from harvest to retail. For kiwifruit, the maximum storage
potential is achieved when fruit is efficiently cooled to near 0 0C, shortly
after harvest [2]. Kiwifruit, kept at 0 C and 90–95 % relative humidity can
have a storage period of 3–5 months [3]. This affords market flexibility
and eliminates the need to transport to market immediately after harvest.
However, improper cooling can lead to hot or cold spots within the package
or pallet, and consequently quality loss in horticulture produce during
storage [4].
Although produce may be pre-cooled in a cold storage facility,
pre-cooling differs from cold storage. In cold storage, the temperature is
simply maintained at a predetermined low temperature. If the cold storage
facility is to double as a pre-cooling facility, higher refrigeration capacity
is required as well as appropriate provisions for pre-cooling and handling
of the produce.
1.1. Requirementof Precooling
Fresh produce starts to deteriorate immediately following harvest.
Respiration due to enzymatic oxidation in the growing produce continues
after harvest. This process results in the consumption of sugars, starches
and moisture without replenishment by the plant. Carbon dioxide and other
gases along with heat are generated in the process. If the heat is not removed,
the process is accelerated. Growth of molds and the loss of moisture from
the produce are also accelerated by heat. Bruising of the produce further
accelerates these processes, resulting in the loss of texture, firmness, colour,
flavour and appearance. In addition, some nutritional value may also be
lost. When these losses occur, the produce is generally considered to have
lost its freshness and quality.

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Pre-cooling refers to the rapid removal of field heat shortly after
the harvest of a crop. Field heat can be defined as the difference in
temperature between the temperature of the crop harvested and the optimal
storage temperature of that product. In general the temperature should be
cooled down till it reaches 88% of the existing difference in temperature
and its optimal storage temperature. Field heat should be removed as fast
as possible since, for most produce, an hour delay at field conditions of
about 35°C will lead to a loss in shelf-life of about 1 day – even at optimal
storage conditions.
Nevertheless, due to biological factors, the importance of rapid
pre-cooling varies. According to the Indian Board of Horticulture, especially
grapes, mandarins, berries, cherries, leeches, melons, stone fruits, sapotas,
okra, tomatoes, capsicum, chilli peppers, eggplant, cucumbers, green beans,
peas, and spinach should be rapidly pre-cooled, whereas other, less
perishable produce is made up of mangoes, papaya, guava, green bananas,
pomegranates, radish, cabbage, cauliflower and carrots. More detailed
information about adequate pre-cooling methods for various fresh produce
items can be found here.
Commercially important fruits that need immediate precooling
include apricots; avocados; all berries except cranberries; tart cherries;
peaches and nectarines; plums and prunes; and tropical and subtropical
fruits such as guavas, mangos, papayas, and pineapples. Tropical and
subtropical fruits of this group are susceptible to chilling injury and thus
need to be cooled according to individual temperature requirements. Sweet
cherries, grapes, pears, and citrus fruit have a longer postharvest life, but
prompt cooling is essential to maintain high quality during holding. Bananas
require special ripening treatment and therefore are not precooled.
According to the FAO, precooling is “amongst the most efficient
quality enhancements available” and is regarded “as one of the most value-
adding activities in the horticultural chain”.
1.2. Pre-Colling Benefits Include:
I. Lowering the required workload of a cold storage since optimum
storage temperature is reached more quickly
II. Restricting and minimizing respiratory activity, thereby
conserving the weight of the produce, and enzymatic degradation
of the produce harvested; thus preventing softening, water loss
and wilting

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III. Preventing microbial growth, such as bacteria and fungi thereby
decreasing the rate of decay
IV. Decreasing rate of ethylene production and the impact on ethylene
sensitive produce
V. Delaying chilling injuries for certain fruits
VI. Increasing the daily intake into storage facilities which should
not exceed 10% of its cooling capacity if produce is not pre-
cooled.
1.3. Factors Affecting Selection of Precooling Methods
I. Produce characteristics: characteristics of produce, such as
chilling sensibility or the need for rapid heat removal, lead to
differing cooling requirements making methods more or less suited.
Products also differ in their flow capacity; the faster products can
be cooled down, the better. Some methods cannot be tolerated by
some fruits and vegetables, e.g. if they cannot get in contact with
water
II. Packaging: the way produce is being packaged makes precooling
methods more or less suitable
III. Scale: size of operations/amount of produce to be cooled
IV. Efficiency: depending on the circumstances some methods will
be more energy efficient than others
V. Skilled labor: methods require various levels of skilled and trained
personnel. The availability of such trained personnel has to be
considered
VI. Economic viability: the price of precooling methods differ and
have to be considered. This is true with regards to investment as
well as running costs, e.g. electricity. In general, the cost of the
pre-cooling method has to justifiable with regards to product
volume and the increase in product value in order to make economic
sense
Regardless of which method is used, the process should always be
monitored in order to ensure that precooling is achieved in the most efficient
way. Depending on method and product at hand, produce will cool at
different rates.

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1.4. Methods of Procooling
In general, there are three different methods to pre-cool produce,
using either air, water or the creation of a vacuum.
A. Forced Air Cooling
Forced-air cooling involves forcing refrigerated air through packed
fresh produce, via openings in the packages stacked upon pallets, by
generating a pressure difference across the pallets. Of the different air
flow systems available the tunnel cooler is the most common [1].
The typical operating conditions recommended in industry for the
forced-air cooling of non-polylined horticultural produce is a flowrate range
of 0.5–2.0 L kg1 s1 [5] and a pressure drop range of 60–750 Pa [1]. The
efficiency of the forced-air cooling process is determined by the rate and
uniformity of product cooling in comparison to the energy input required
[6]. The pressure drop across a pallet is also dependent on the vent area of
the packages [7,8, 9, 10, 11] showed that when the aerodynamic resistance
of packages of horticultural packages, undergoing forced-air cooling, was
lowered the energy requirement to maintain the airflow through the pallet
was reduced.
B. Room Cooling
Precooling produce in a cold-storage room or precooling room is
an old well-established practice [8. This widely used method involves the
placing of produce in boxes (wooden, ®berboard or plastic), bulk containers
or various other packages into a cold room, where they are exposed to cold
air. Typically the cold air is discharged into the room near the ceiling, and
sweeps past the produce. Field heat can be removed from an individual
bunch of unpacked cut flowers in 20 min by room cooling, the reduction in
temperature of packaged cut flowers is a slow process requiring two or
more days which is a relatively long period for a commodity with such a
short shelf life containers to return to the heat exchangers [12].
C. Hydrocooling
It is used because of its simplicity and effectiveness that
hydrocooling is a popular precooling method [18]. Hydrocooling essentially
is the utilisation of chilled or cold water for lowering the temperature of a
product in bulk or smaller containers before further packing. Identified
hydro cooling as being achieved by fooding, spraying, or immersing the
product in/with chilled water [13]

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D. Vacuum Cooling
Rapid cooling of horticultural produce can be carried out with
vacuum cooling. Vacuum cooling is achieved by the evaporation of moisture
from the produce. The evaporation is encouraged and made more efficient
by reducing the pressure to the point where boiling of water takes place at
a low temperature [19, 20, 21].
E. Cryogenic Cooling
The use of the latent heat of evaporation of liquid nitrogen or solid
CO2 (dry ice) can produce ‘boiling’ temperatures of -196 and -78 0C
respectively. This is the basis of cryogenic precooling. In cryogenic cooling,
the produce is cooled by conveying it through a tunnel in which the liquid
nitrogen or solid CO2 evaporates. However, at the above temperatures the
produce will freeze and thus be ruined as a fresh market product. This
problem is prevented by careful control of the evaporation rate and conveyor
speed [22].
F. Contact Icing
Package icing involves direct placement of slush, flaked, or crushed
ice over the product in shipment containers. This method is efficient where
it is used; however, it can result in uneven cooling because the ice generally
remains where it was placed until it has melted [23]. Icing can be effectively
used to cool products such as collards, kale, Brussels sprouts, broccoli,
radishes, carrots and onions. Flowers may also be cooled by means of ice
cubes inserted into flower boxes in foil sacks. However, reported that icing
is less effective, since a much longer period is needed for cooling and
lowering flower temperature from 20 to 5C requires a large amount of ice
(i.e. about 25% of the weight of the flowers) [14].
1.5. Calculation Method
A. Heat Load
Total heat load comes from product, surroundings, air infiltration,
containers, and heat-producing devices such as motors, lights, fans, and
pumps. Product heat accounts for the major portion of total heat load, and
depends on product temperature, cooling rate, amount of product cooled
in a given time, and specific heat of the product. Heat from respiration is
part of the product heat load, but it is generally small. [17].
B. Precooling Time Estimation Methods
Accurate estimations of precooling times can be obtained by using

777
finite-element or finite-difference computer programs, but the effort
required makes this impractical for the design or process engineer. In
addition, two- and three-dimensional simulations require time-consuming
data preparation and significant computing time. Most research to date
has been in the development of semi analytical/ empirical precooling time
estimation methods that use simplifying assumptions, but nevertheless
produce accurate results. [16].
C. Fractional Unaccomplished Temperature Difference
The semi logarithmic temperature history curve consists of an initial
curvilinear portion, followed by a linear portion. Simple empirical formulas
that model this cooling behavior, such as half cooling time and cooling
coefficient, have been proposed for estimating the cooling time of fruits
and vegetables [16].
D. Half-Cooling Time
A common concept used to characterize the cooling process is the
half-cooling time, which is the time required to reduce the temperature
difference between the commodity and the cooling medium by half [15].
E. Cooling Coefficient
The cooling coefficient is minus the slope of the ln(Y ) versus
time curve, constructed on a semi logarithmic axis from experimental
observations of time and temperature [15].
1.6. Other Pre-Cooling Tips
a) Do not load pre-cooling facility beyond its optimum capacity.
b) When stacking produce, allow adequate air-circulation to ensure
all vegetables can be evenly cooled.
c) Use proper receptacles (such as vented boxes and baskets for
forced-air cooling, and waxed cartons or Styrofoam boxes for
hydro-cooling).
d) Transfer vegetables out from the pre-cooling facility immediately
after pre-cooling, to avoid overcooling or dehydration of the
vegetables.
e) Use potable water for precoolers to minimise any food safety
concerns.
f) Separate ethylene-sensitive vegetables from ethyleneproducing

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ones.
g) If a chiller is used for precooling, keep it closed at all times to
minimise temperature and relative humidity fluctuations.
2. Conclusion
One of the most important factors affecting the postharvest life
and quality of horticultural crops is temperature. Quality loss after harvest
occurs as a result of physiological and biological processes, the rates of
which are influenced primarily by product temperature. As the maintenance
of market quality is of vital importance to the success of the horticultural
industry, it is necessary not only to cool the product but to cool it as quickly
as possible after harvest. The process of precooling is the removal of field
heat which arrest the deteriorative and senescence processes so as to
maintain a high level of quality that ensures customer satisfaction. . As
consumer awareness and sophistication are ever increasing due to the
growing fear of chemical residues and the uncertainty surrounding
genetically modified foods presently, and with the change to organic
products continuing, alternative techniques of extending shelf life and
maintaining high level of quality are being investigated. Precooling is one
of the techniques which adheres to this ethos and should be applied widely
throughout the entire horticultural industry to attain its true potential
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