Pre-Cooling and cool storage of fresh produce.


Original Article by: Cees Nijssen,
President of Nijssen Koeling b.v., Leiden -The Netherlands


It is the nature of all freshly harvested produce, at ambient temperatures, to deteriorate and die very quickly, within hours for some products.
The post-harvest function of pre-cooling and cold storage is designed to gain time by extending this life cycle long enough for the sensitive produce to arrive on the consumer’s table in fresh and attractive condition.
The natural decline that sets in immediately upon harvesting can never be completely stopped, at least not within our present capabilities, but our knowledge of how to control it is well advanced and is expanding quite rapidly.

The entire thrust of all post-harvest handling is concerned with time.
Essentially, to increase the “time” available to maintain an orderly harvesting, packing, storage, transportation, and marketing program.
Probably the most significant link in this entire chain of events, is the lapse in time between the moment of harvest and pre-cooling. It has been said that every hour saved from the moment of harvest and removal of the field heat can add a day to the useful shelf life of the product.

This fact contributes to the trend towards more pre-cooling facilities being located at the point of harvest, usually without storage for more than one day, designed for this purpose.

The current trend is to pre-cool the product at or close by the point of harvest, with the pre- cooling facility designed to match the rate of harvesting and to pre-cool the product in a matter of several hours or less. In many instances these products are loaded and rolling in less than six hours from the time they have been harvested. This is a far cry from cooling for a day or more.

Pressure cooling or forced air cooling is one of the fastest and most popular method and an extremely useful method of pre-cooling by a positive forcing of air through stacks of palletised produce. The labour factor with this type of plant is easy and low, all types of produce can be handled through the system and as a rule there is no need to handle individual packages.

To achieve fast cooling rates, it is essential that adequate cross venting of the packing be provided, hence close co-operation between the designed packing and the engineer is of the utmost significance. The list of products that can be handled includes most of the fresh produce from leafy vegetables, avocados. grapes, citrus, stone or tree fruit, etc., whether
pre-packaged or in bins.

The most important control parameters for optimal cool storage are temperature, lighting, relative humidity, and gas conditions. From all these factors is temperature the most important one.

There is no single temperature that could be said to be proper for storage of all types of produce, but until a lowest recommended optimal, every degree lower from the ambient temperature will rapidly extend the shelf life of the product. It reduces the rate of respiration mostly to less than 5 percent of the rate at ambient temperatures, also the vigour and rate of of spread of the always present ground diseases is greatly inhibited at low temperatures. unfortunately, the lowest optimal temperature is very secure. Only one degree below that optimum gives an enormous damage to fresh produce, and the damage often goes undetected until the product reaches the shelf, where it takes its toll.

A relative humidity on the order of 95% is preferred for most of the products, it is mistaken To belief that high humidity is associated with mold growth and decay. The reluctance probably arises from confusion between free moisture and highly humidified air, which is a true gas.

High humidity levels, without free moisture, can be automatically maintained by cooling with Filacell, a safe and efficient contact medium that air in direct contact brings with chilled water.

Produce that has been cooled to a temperature of the order 00 C and exposed to ambient air temperatures, will rise in temperature 4 to 5 times faster than would be the case of warm fruit being cooled down. The principal reason for this is the condensing of moisture by the cold
product from the warmer air to which it is exposed. A commonplace analogy is the bathroom mirror accumulating a film of moisture when the shower is being used. This condensation on the fruit not only causes the temperature to rise very quickly, but adds a film of free water to the product, this accelerating the incurrence of mold and decay, as well
as creating a soggy condition within the cartons. This condition, common as it seems to be, is easily prevented by proper cold store-to-carrier
loading arrangements, a cooled airlock, and or a dock shelter to minimize problems.

Temperatures that fluctuate several degrees in a storage atmosphere can cause condensation to take place on the fruit, for the same reason. Even though the averaged 24 hour temperature of such a room appears to be satisfactory there are often frequent hour to hour temperature fluctuations of 50 or more. These fluctuations can usually be traced to coil defrosting and cycling of the refrigeration system, to doors remaining open to the ambient atmosphere, or to overload of the refrigeration system due to warmer incoming fruit. Only with careful plant and equipment design and operation can these problems be avoided. It should be kept in mind that for each one-degree temperature rise of the air leaving the cooling surfaces, the humidity level can be decreased on the order of 4 percent.

Controlled or modified atmosphere of the environment involves the reduction of the normal level of oxygen and carbon dioxide content, and their replacement with nitrogen. Although very useful for long term storage of hard fruit it is requiring precise controls and is somewhat limited in application.