what is pre-cooling

Introduction

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 pre-cooling were developed, primarily to meet requirements of the large producers and markets in the United States and Europe.

• minimized production losses
  Weather variability during the harvest season can cause produce to mature earlier or later than planned. If the produce is not harvested at the optimum time, losses will occur. Late harvesting may also occur if market opportunities are not available at the desired time. Pre-cooling and cold storage are valuable tools that better allow produce to be harvested on time and sold when markets become available.
• improved economics of harvest operations
  The daily harvest may be increased with the assurance that produce quality will be preserved. This confidence permits harvesting to be done over a longer period, thus spreading out workloads. Daily harvest hours could also be extended because the effect of ambient temperatures on the produce would be lessened. This flexibility results in better use of equipment and personnel.
• minimized losses during marketing
  Some types of produce spoil very quickly if not pre-cooled immediately after harvest. These types have to be sold within a day or two of harvesting to be of acceptable quality. Strawberries are a good example. Other examples are leafy vegetables that require trimming to maintain a fresh appearance. Pre-cooling extends the shelf life and thus the opportunity for sale before the produce is no longer marketable. See Table 1.
• improved utilization by consumer
  Consumers can be supplied with top quality produce with a longer shelf life through pre-cooling. This extended shelf life lessens the urgency to consume or process produce quickly after it is purchased. Consumers are more likely to make larger purchases, enjoy lower handling costs and have more timely product utilization.
• expanded market opportunities
  Retail marketers require produce of the highest quality that has longest practical shelf life. A highly perishable product that is not pre-cooled, kept cold and handled with care will lose quality and have difficulty competing with a quality imported product. It is also difficult for the lower quality product to meet grading standards. Pre-cooling is, therefore, the key for perishable produce to enter and compete in retail markets.

Factors contributing to spoilage

Different types of fruits and vegetables spoil naturally at different rates. Table I shows the length of time fresh produce (not rapidly pre-cooled) will last in a refrigerator at 4 degrees centigrade.
Rapid pre-cooling and storage at a stable temperature will extend this shelf life for most produce subject to rapid spoilage. For higher storage temperatures, it is a generally accepted rule of thumb that "deterioration of fresh produce doubles for every 10 degrees centigrade above the optimum storage temperature." Types of produce with high spoilage rates are often of higher value than those with low spoilage rates. This aspect makes control of spoilage an especially important factor for them.
Produce such as onions, potatoes and winter squash spoil rapidly if not cured as soon as possible after harvest. Curing at an elevated temperature heals cuts and bruises and forms a tight outer skin that resists further deterioration.
Atmospheres in storages also affect the spoilage rate. Excess carbon dioxide and insufficient oxygen result in rapid deterioration and off flavours. Ethylene produced by some fruits accelerates the ripening of many fruits. It is therefore important to consider the gases that will be associated with the cold storage.

Table 1. Shelf-life of Fruits and Vegetables Refrigerated at 4°C for Best Flavour and Nutrition

(Adapted from Publication 1695, Agriculture Canada).
Significant moisture loss affects the appearance and firmness of produce. Table 2 shows the maximum amount of moisture the produce may lose before losing its appearance and firmness. Room cooling, forced air cooling and vacuum cooling all remove some moisture from produce during pre-cooling. The amount of moisture lost will depend on design and operation of the system. More moisture will be lost if the produce is held for extended periods before cooling.
Water used in direct contact with the produce can affect produce quality as well. Rapid deterioration and off flavours can be triggered by water containing iron, high levels of minerals, bacteria and other organic material.
Contaminated water used on fresh produce can also mean that people who consume it become ill. The cleanliness, quality and sanitation of water used in produce facilities is therefore very important.
Table 2. Percentage of Weight (Moisture) Loss from Fruits and Vegetable that Affects Produce Quality after Harvest

(Adapted from Postharvest Physiology of Perishable Plant Products by Kays, S.J., 1991).
In preparing fresh produce for long term storage, remove free surface moisture on the product. This moisture can support the growth of spoilage bacteria. This situation is particularly true in bruised and cut areas of produce such as root crops. If mechanical damage is minimized and the tops are removed with a clean cut, the removal of free moisture is easier.
The spoilage rate of produce and its contributing factors are the most important considerations when selecting the type of pre-cooling and handling system to be used.