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.