Food preservation by cold

Overview

The use of refrigeration for food is undoubtedly the most widespread technique. Low temperatures retard the development of micro-organisms, chemical and enzymatic reactions that cause spoilage. The enzymes and chemical reactions are slowed down considerably at lower temperatures (<5 ° C), while the majority of microorganisms do are more capable of metabolic activity at temperatures below -5 ° C. Some, such as coliform bacteria, even inactivated.

There are two processes that use this technique, refrigeration and freezing.

Refrigeration

Refrigeration is to store food at a low temperature near the freezing point, but still positive with respect to it. Typically, the refrigeration temperature is around 0 ° C. At these temperatures, the rate of growth of microorganisms in food is slowed down.

Refrigeration is used for preserving perishable foods in the short and medium term. The retention period varies from a few days to several weeks depending on the product, temperature, relative humidity and type of packaging. Table 1 shows some examples, but we must also take into account the regulations in this area.

Some basic rules must be respected in the application of refrigeration: refrigeration must be made as soon as possible after collection, it should apply to foods initially healthy and be continuous throughout the distribution chain.

Freezing

Freezing is to store food at temperatures below the freezing point, typically -18 ° C. It is used for food long-term (4 to 24 months).

During freezing, the metabolic activity of most pathogens and alteration is inhibited. However, the chemical weathering reactions are not completely arrested. The most important of these reactions are enzymatic oxidation of lipids, carbohydrates hydrolysis and lipolysis. To address this, manufacturers typically a whitening products (Frozen vegetables case) before freezing.

Kinetics of freezing

The kinetics of freezing is characterized by three phases (Figure 1). During the first phase, the speed of the product decreases rapidly and then stabilizes for a time, which corresponds to the second phase, at a level equivalent to that of ice formation. Then, the temperature returns to its descent to reach the desired final temperature, is the third phase.

Depending on the speed of freezing food, there are:

1. Freezing or quick freezing: during which the products are stabilized by rapidly lowering the temperature to -18 ° C to heart. This technique allows the formation of many small ice crystals that do not deteriorate the food. Only a small exudate occurs during thawing.
2. Slow freezing, which applies to products which, by their appearance or their manner of harvesting, can not meet certain requirements, eg speed of freezing that affect the frozen products. The cooling of the food is slow resulting in the formation of ice crystals of relatively large size compared to cells of the product. The sharp needles of ice crystals can puncture and tear the cell wall and low resistance encourage some exudation during thawing.

Freezing Techniques

Freezing of food products using several techniques. Among these techniques there are tunnel freezers, plate freezers and systems of direct freezing (cryogenic freezing).

The tunnel freezers using air as refrigerant intermediary. It is cooled through the evaporator of the refrigerating machine, the pulse produced by a fan, and then recycled to be cooled again. This system exists in continuous and discontinuous. The advantage of freezing tunnels lies in their flexibility. They are recommended when it comes to freeze several types of products, shape and size.

In the case of plate freezers, the product is cooled in contact with the surface plate in which circulates the refrigerant. This type of freezer can be horizontal or vertical plates as the plates are arranged horizontally (in form of shelves) or vertically. Their advantage lies in their efficiency. Unfortunately, they can not be used for product shape, before or after packaging, regular. Moreover, they are no longer able to operate continuously.

Cryogenic freezing (or direct) is to place the product in direct contact with a cold source, by spraying a liquid such as liquid nitrogen or CO₂liquid that evaporates on contact with the product. In such cases, the refrigeration unit is provided by the latent heat of vaporization of the liquid in contact with the product (liquid nitrogen evaporates at -196 ° C and liquid CO₂ evaporates at -54 ° C ). The freezing system has the advantage of being fast and not requiring refrigeration system itself. However, it is quite expensive and can not therefore be used for food products with high market value.