Pre-cooling of Agricultural Products

Overview

Precooling 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 produce. 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.

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”. Precooling benefits include:

• lowering the required workload of a cold storage since optimum storage temperature is reached more quickly
• 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
• preventing microbial growth, such as bacteria and fungi thereby decreasing the rate of decay
• decreasing rate of ethylene production and the impact on ethylene sensitive produce
• delaying chilling injuries for certain fruits
• increasing the daily intake into storage facilities which should not exceed 10% of its cooling capacity if produce is not pre-cooled

There are several methods that can be chosen to precool produce. Which method is the most suitable choice depends on various factors of which some are listed below:

• 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
• Packaging: the way produce is being packaged makes precooling methods more or less suitable
• Scale: size of operations/amount of produce to be cooled
• Efficiency: depending on the circumstances some methods will be more energy efficient than others
• Skilled labor: methods require various levels of skilled and trained personnel. The availability of such trained personnel has to be considered
• 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.

Methods

In general, there are three different methods to precool produce, using either air, water or the creation of a vacuum. Below each method and possible variations are shortly explained:

Shade

The simplest, most low cost and low technology option to reduce field heat is to move produce to a deeply shaded area directly after harvest. The temperature in packing houses, with roofs that are light colored or reflective, can reduce temperatures as much as 20°C.

Air cooling

Use of refrigerated air in a conventional cold storage room, a special pre-cooling room, a funnel cooler, or a forced air cooler. Air cooling can be subdivided in the methods of room cooling and forced-air cooling:

Room cooling: produce is put into a cold (refrigerated) room

Advantages	Very energy efficient if designed properly
Challenges	Very slow
	Produce has to be stacked properly so that the cold air can flow through the stacks
Adequate for	not for produce requiring immediate and rapid cooling
	fruits rather than vegetables
	not for produce which is stored in bulk bins or containers since the temperature of produce located towards the middle of the storage bin will increase because heat generation through respiration is likely to outpace the speed heat removal citrus, apples stored under controlled atmosphere conditions, which however, will not have optimal quality if this method of pre-cooling is used

Forced-air cooling: a fan is used to drive air through packed produce within a refrigerated room. The usage of the fan increases the cooling rate compared to basic room cooling. Produce should be packed and stacked in a way that allows air to flow through fast

Advantages	Faster than room cooling
Challenges	Time required for cooling depends on air temperature
Adequate for	Most widely used and adaptable form of pre-cooling
	Used for fruits and fruit-type vegetables
	avocado, cucumber, melon, pumpkin, banana, mango, coconut, eggplant, okra, grapes, orange, strawberry, brusselsprouts, grapefruit, papya, squash, carambola, guava, passion fruit, tangerine, cassava, kiwi, pepper tomato, cherimoya, persimmon, pineapple, pomegranate, litchi, breadfruit

Water cooling

Uses water to take up the heat from harvested produce. Cold or chilled water is sprayed on produce or the produce is dipped into it. Water should be able to flow through the produce rather than around it.

Advantages	Heat is removed rapidly and about 15 times faster than if air was being used since water is a better heat transfer medium
	can be done with a refrigerated CaCl2 solution to prevent storage disorders in fruits
	no moisture loss
	least expensive pre-cooling method
Challenges	Need for clean and sanitized water to prevent microbial contamination
	Energy efficiency is lower than with room and forced air cooling
Adequate for	Commodities that can tolerate becoming wet
	Generally used for stem and flower type as well as root vegetables, and some tree fruits celery, peas, asparagus, Chinese cabbage pomegranate, beet, cucumber, broccoli, eggplant, radish, Brussels sprouts, green onions, spinach, cantaloupe, kiwi fruit, squash, carrot, leek, sweet corn, cassava, orange, melon and cauliflower
Equipment required	Packaging equipment: wire-bound wooden crates, waxed fibreboard cartons, mesh poly bags and bulk bins, pallets (need to be stacked in a way that water can enter packages)

Ice cooling

Ice is used to remove heat from fresh produce.

Advantages	Advantageous if packages are too dense to be cooled with air
	Removes heat rapidly
	Continues to remove heat through absorption while melting
	Usable during transport, distribution and storage
Challenges	Ice used must not contain any chemical, physical or biological hazards
	Inefficient since about half of the cooling effect is lost to heat exchange with the environment instead of actually contributing to cooling the produce (USAID, 2009)
	Depending on the proximity of the ice-making facility and the availability of insulating packaging, losses during the transport to the cooling facility will occur (USAID, 2009)
Adequate for	Produce with high respiration rates
Equipment required	Ice
	Storage and package containers which are suited for icing applications has to remain stable if being wet, such as waxed fibreboard cartons, wooden wire-bound creates, baskets, hampers and perforated plastic liners

Top icing: crushed ice is applied on the produce either manually or mechanically

Adequate for

Brokkoli, green onions, leafy greens, brussel sprouts, leeks, cantalpupes, parsley, carrots, peas, chinese cabbage

Liquid icing: ice and water mix is injected through vents or handholds into packages. This is done without opening packages and without removing them from pallets

Advantages:	Can be used for small and large operations alike
Challenges:	Produce gets in contact with water increasing the threat of microbial development. Sanitation is crucial.

Individual package icing: crushed ice is added manually on top of the packaged produce

Advantages	Very simple
Challenges	There is the danger of produce being cooled unevenly since ice is simply put on top of packages/cartons
	Application of ice on every single package is labor intensive and slow and is therefore mainly applied to small operations

Ice bottles: ice bottles wrapped in paper are placed in between produce

Advantages	Produce has no direct contact with ice
	Can be used during transport

Vacuum cooling
temperature is reduced by placing produce in a metal cylinder in which the atmospheric pressure is reduce which leads to evaporation and a cooling effect.

Advantages	Fast and uniform cooling method
Challenges	Costly for small scale enterprise
	High water loss which can be prevented by adding water during the cooling process
Adequate for	Produce having a high surface to volume ratio
	Produce which cannot get in contact with water and for which using air is difficult
	For leafy vegetables
	Brussels sprouts, carrot, peas, cauliflower, snap beans, celery, spinach, chinese cabbage, sweet corn, leek, Swiss chard

The table below offers and overview of the various pre-cooling methods described summarizing their cooling time, suitability for produce, cost and efficiency.