The objective of freezing a commodity is to preserve and prolong its life. There are various methods of freezing and various kinds of refrigerants used by the industry. O.S Gautam explains here some of the conventional freezing methods.
Microflora, inbuilt enzymes and oxidation can spoil any agricultural commodity or food product. All these processes reduce with the reduction in temperature and virtually stop completely at –60 degree Celsius. It is practically difficult to obtain a freezing point of –60 degree Celsius and more difficult to maintain it. It is found that freezing a commodity to –18 degree Celsius and storing it at the same temperature levels till consumption, is enough to keep the product safe at least for a year.
There are various methods of freezing and various kinds of refrigerants used by the industry.
Contact freezers are used for freezing products in bulk and in blocks. The product is filled in trays or pans with water that acts as a freezing media. It gives better contact with freezers plates and gives fast freezing. After freezing product gets embedded inside ice which also acts as a barrier in between product inside and oxygen in the air to arrest putrefactive oxidation and dehydration from dry cold store air. Ammonia (NH3), Freon 104, Carbon dioxide (CO2) gases are generally used as refrigerants in this system. The problem in this method of freezing is that customer has to thaw whole block even when he needs only a fraction of it. Customer is forced to refreeze remaining quantity, which is not a recommended practice as product quality suffers during refreezing and product deteriorates gradually on repetition of this process. However it is a good freezing system for keeping raw materials in bulk for later commercial uses.
In blast freezing it is air, which is cooled and blasted or circulated over the product. Cold air is blown over the product, which cools the product and takes off its heat. This is a kind of exchange. Cold air is blown from one end generally top of the chamber and hot air is sucked from below to pass through the cooling coils. Cooling coils have circulating refrigerant inside which takes heat from the air in order to get converted into gaseous form. The refrigerant gas is passed through the condensers and converted again into liquid and sent again to cooling coils to cool air. This process is repeated again and again till core of the product attains temperature of –18 degree Celsius. This is a conventional and most economical method but takes longer time for freezing. Moreover product is gradually cooled from ambient temperature to freezing temperature, passing through full range degree by degree, causing slow freezing, higher drip loss and up to some extent some quality loss also. Therefore conventional blast freezers have gone through several technological upgradations and various quick freezing models (IQF Freezers) are available now which include continuous belt freezers, spiral freezers, flow freezers, fluidized belt freezers etc. where freezing time is cut down only to minutes from hours. However, most of the upgraded freezers are meant for individual quick freezing (IQF) type of production where product is small in size and thickness and the individual piece is frozen separately. All retail consumer packs are preferred this way as it brings convenience to customer for end use. Customer can pick up whatever quantity, he /she needs.
Though brine-freezing technology is considered to be old and obsolete yet it is still in use for commercial ice production in several developing countries. Freezers have a tank fitted with cooling coils. Brine made of common salt (sodium chloride) or calcium chloride is filled into the tank and cooled with the help of passing refrigerant. The product to be frozen is submerged in the cold circulating brine and gets frozen. The product however comes in the contact of brine that alters its natural taste and makes it salty or even bitter if brine is made of calcium chloride.
This is the most efficient freezing method. However, it is not commercially viable. Liquid Nitrogen is used as a refrigerant and sprayed directly over the product. The temperature of liquid nitrogen is –90 degree Celsius, therefore it freezes the product instantly without giving any time and chance for putrefactive and biochemical processes to set in. The cost of nitrogen is much higher in comparison to other refrigerants and its single-use makes it unviable. Availability, handling and storage of nitrogen are other hurdles making its use more difficult. It is therefore not adopted for commercial food production. however, its use in medical surgeries, preservation of organs for transplant, etc is still continuing.
Slow Freezing vs Quick Freezing
As mentioned earlier product can be frozen by several means some of them very slow like keeping some commodity in cold store directly or in deep freezers, moderately slow like in blast freezer or frozen quickly in IQF and Cryogenic freezers. When the process is very slow like freezing in cold stores, it may take several days or even weeks to freeze a commodity depending on its size, thickness and girth. A commodity is considered frozen when its temperature reaches to –18 degree Celsius or below at its core. Biochemical changes and putrefactive activities keep working although at a slow pace and if time is longer especially when a commodity is thick or too big, they may render it unsuitable for human consumption by the time its core temperature reaches to –18 degree Celsius. Another activity, which attributes to the above, is the loss of nutrients and drip loss during slow freezing due to the translocation of electrolytes.
Loss of Nutrients and Translocation of Intracellular Electrolytes Due to Slow Freezing
Plant and animal tissues have 80 to 90% water. Biological water filled inside cells and intercellular spaces (plasma), blood, lymph everywhere is water and they are carriers of nutrients, chemicals, proteins, carbohydrates, salts and minerals into the cells and excretes back carbon dioxide and other break down products from metabolic and catabolic activities. When temperature of a commodity reaches below 0 degree Celsius (–2 to –4 degree Celsius), water wants to get frozen and convert into ice as freezing point of water is 0 degree Celsius. Salts, minerals and other nutrients are dissolved in biological water and form a kind of mild brine, which freezes at a much lower temperature. When freezing is slow it gives enough time for some of the water to get dissociated from brine and converted into ice. This process starts in intercellular plasma in the intercellular spaces. Separation of a portion of water from brine increases its ionic concentration, which sets up a process called ex-osmosis as a result electrolytes, and other nutrients flow out from the cells (intracellular plasma). This process may become vigorous when freezing is very slow and may damage cell structures very profusely and as a result all vital nutrients and chemicals are lost into intercellular spaces that flow out when the product is thawed for end-use. This is also the reason for drip or weight loss during freeze-thaw process.
Therefore the freezing process has to be designed to cross –2 to –4 degree critical temperature range soonest possible without giving much time to set above nutrient migration process, This defines quick freezing.