The document discusses temperature management and cooling techniques for horticultural produce after harvest. It states that temperature control is the most important factor for maintaining quality. There are two phases to temperature management - an initial cooling phase to remove field heat from the product, followed by a holding phase at the optimal temperature during distribution. The rate of cooling depends on the temperature difference between the product and coolant, contact between them, and their thermal properties. Different cooling techniques can be selected based on the perishability of the product, such as room cooling, forced-air cooling, hydro-cooling, vacuum cooling, or package icing.

1. PRODUCT MANAGEMENTPRODUCT MANAGEMENT

2.  The control of temperature is the singleThe control of temperature is the single
most important tool available to maintainmost important tool available to maintain
quality after harvest.quality after harvest.
 Temperature management consist twoTemperature management consist two
phases.phases.
 The first is a cooling phase to removeThe first is a cooling phase to remove
product field that.product field that.
 The second phases involves holding theThe second phases involves holding the
product at the optimum temperature duringproduct at the optimum temperature during
distribution.distribution.

3.  The lower you can hold the temperature, withoutThe lower you can hold the temperature, without
causing an injury to the product, the greater thecausing an injury to the product, the greater the
effect on :effect on :
 the rate of respirationthe rate of respiration
 the rate of water lossthe rate of water loss
 pathological activitypathological activity
 insect activityinsect activity
 growth and development after harvestgrowth and development after harvest
 ethylene production, and the sensitivity of someethylene production, and the sensitivity of some
tissue to an ethylene exposuretissue to an ethylene exposure

4. The Principles of CoolingThe Principles of Cooling
Horticultural ProduceHorticultural Produce
 To ensure fast cooling and preventTo ensure fast cooling and prevent
rewarming, the coolroom or coolingrewarming, the coolroom or cooling
system must be able to effectively handlesystem must be able to effectively handle
the heat load it is exposed to.the heat load it is exposed to.
 The principles of heat exchange. When aThe principles of heat exchange. When a
substance changes from a solid to liquidsubstance changes from a solid to liquid
or a liquid to a gas, heat is absorbed.or a liquid to a gas, heat is absorbed.
When a substance changes from a gas toWhen a substance changes from a gas to
a liquid or a liquid to a solid, heat isa liquid or a liquid to a solid, heat is
evolved.evolved.

5.  During cooling, water in the product changesDuring cooling, water in the product changes
from a liquid to a gaseous state (water vapor),from a liquid to a gaseous state (water vapor),
absorbing heat from the product. The waterabsorbing heat from the product. The water
vapor evaporates (to an area of lower energyvapor evaporates (to an area of lower energy
level-the coolroom), cooling the product.level-the coolroom), cooling the product.
 The rate of cooling is determined by :The rate of cooling is determined by :
 the difference in temperature between the productthe difference in temperature between the product
and the coolant (air, water or ice)and the coolant (air, water or ice)
 the amount of contact that is achieved between thethe amount of contact that is achieved between the
product and the coolantproduct and the coolant
 the thermal conductivity of the product and thethe thermal conductivity of the product and the
packagepackage

6. The difference in temperatureThe difference in temperature
 The greater the difference in the temperature of theThe greater the difference in the temperature of the
product and the temperature of the coolant, the greaterproduct and the temperature of the coolant, the greater
the rate of cooling.the rate of cooling.
 The cooling curve shown above, has a initially rapid rateThe cooling curve shown above, has a initially rapid rate
of cooling but this tapers off over time as theof cooling but this tapers off over time as the
temperature differential decrease. For example, if thetemperature differential decrease. For example, if the
product’s temperature is 30product’s temperature is 30oo
C and the air temperature inC and the air temperature in
the coolroom is 4the coolroom is 4oo
C, there is a large temperatureC, there is a large temperature
differential and very rapid rate of heat exchanges, asdifferential and very rapid rate of heat exchanges, as
indicated by the slope of the curve. As the temperatureindicated by the slope of the curve. As the temperature
differential decrease, the rate of heat exchange slowsdifferential decrease, the rate of heat exchange slows
down.down.

8. The mechanical refrigerationThe mechanical refrigeration
systemsystem
 The mechanical refrigeration system used in aThe mechanical refrigeration system used in a
coolroom is based on the simple principle ofcoolroom is based on the simple principle of
heat exchange. That is, a liquid absorb heat toheat exchange. That is, a liquid absorb heat to
change to a gas.change to a gas.
 The warmed air from the coolroom passes overThe warmed air from the coolroom passes over
the liquid refrigerant at the evaporator. The liquidthe liquid refrigerant at the evaporator. The liquid
refrigerant absorbs the heat (that is coming fromrefrigerant absorbs the heat (that is coming from
the product and the other sources), changing thethe product and the other sources), changing the
refrigerant into a gaseous form. The refrigerantrefrigerant into a gaseous form. The refrigerant
gas then passes out to the compressor where itgas then passes out to the compressor where it
is condensed back into a liquid. The heat isis condensed back into a liquid. The heat is
released and expelled outside the coolroom.released and expelled outside the coolroom.

10. Sources HeatSources Heat
During cooling, there are several sources ofDuring cooling, there are several sources of
heat that the refrigeration system has toheat that the refrigeration system has to
cope with. Theses include :cope with. Theses include :
 Product heat (field and respiratory)Product heat (field and respiratory)
 Conducted heatConducted heat
 Infiltrated heatInfiltrated heat
 Other heat sourcesOther heat sources

11. 1.1. Product heatProduct heat
Product heat is made up of two components.Product heat is made up of two components.
The first is referred to as theThe first is referred to as the field heatfield heat of theof the
product. The product’s temperature at harvestproduct. The product’s temperature at harvest
will be similar too the ambient temperature .will be similar too the ambient temperature .
The second component of product heat isThe second component of product heat is
respiratory heatrespiratory heat . Different products respire. Different products respire
at different rates, therefore producing varyingat different rates, therefore producing varying
amounts of respiratory heat.amounts of respiratory heat.

12. 2. Conducted heat2. Conducted heat
Outside heat is conducted across theOutside heat is conducted across the
walls, roof and floor into the coolroom.walls, roof and floor into the coolroom.
Coolrooms have insulated walls and roofCoolrooms have insulated walls and roof
to minimize the conducted heat load. Theto minimize the conducted heat load. The
thicker the insulation in the walls and roof,thicker the insulation in the walls and roof,
the less the conducted heat load.the less the conducted heat load.

13. 3. Infiltrated Heat3. Infiltrated Heat
Whenever the door of a coolroom is opened,Whenever the door of a coolroom is opened,
heat infiltrates into the coolroom. If the door isheat infiltrates into the coolroom. If the door is
left open for long periods, the infiltrated heatleft open for long periods, the infiltrated heat
load on the coolroom is high.load on the coolroom is high.
4. Other heat sources4. Other heat sources
Other heat sources contribute to the heat load inOther heat sources contribute to the heat load in
a coolroom. Theses include humans, electrica coolroom. Theses include humans, electric
light fittings, electrical components of thelight fittings, electrical components of the
refrigeration system and forklifts.refrigeration system and forklifts.

14. Selecting a cooling techniqueSelecting a cooling technique
Room coolingRoom cooling Only products that have a low toOnly products that have a low to
very low perishibilityvery low perishibility
Forced –air coolingForced –air cooling Fruits, fruit vegetables, tubers, cutFruits, fruit vegetables, tubers, cut
flowers, floral vegetablesflowers, floral vegetables
Hydro-coolingHydro-cooling Stems, leafy vegetable, some fruitStems, leafy vegetable, some fruit
and fruit vegetablesand fruit vegetables
Vacuum coolingVacuum cooling Leafy vegetables, some stem andLeafy vegetables, some stem and
floral vegetablesfloral vegetables
Package icingPackage icing Roots, some floral vegetables,Roots, some floral vegetables,
stems, some leafy vegetablesstems, some leafy vegetables