Freezing is one of the oldest, well established and widely used technology for preservation of food. The technology and methods have undergone a lot of changes with improvements taking place over the years, but the basic idea has continued to remain the same – to maintain the food at a temperature low enough, so that the microbes cannot thrive, chemical and enzymatic reactions are reduced and metabolic reactions delayed, thereby increasing the shelf life of a food that has been stored. Frozen food consists of frozen fruits, vegetables, fish & marine produce, meat, ready meals, bakery products, soups and appetizers that provide domestic consumption and export opportunities. Some of the frozen product brands in India are Yummiez, McCain, Wendy’s, ITC, Amul etc.
During freezing, temperature of food is reduced below its freezing point by removal of heat and a portion of water undergoes a change in state to form ice crystals. All frozen products are supposed to be stored at a temperature of -18 °C in order for the product to obtain an extended shelf life. The temperature regime covering the freezing process, the cold-store temperatures (-18 °C), distribution temperatures (-15 °C) and retail display (-12 °C) are given as legal standards.
Freezing of food starts when the food is placed in contact with a cold medium. The freezing process is initiated by the formation of a nucleus or a seed. This initial stage is called as nucleation. It is required as a base upon which the ice crystals grow further. As the ice crystals start growing, the phase changes from liquid to solid. Freezing point is defined as the temperature at which the first ice crystal appears and the liquid at that temperature is in equilibrium with the solid. The rates of freezing determine the type, size and distribution of ice formation. There are two types of freezing, one being slow and the other being fast freezing.
Slow Freezing: As the product takes longer time to cool, the ice crystals formed are less and of a larger size, which on thawing might rupture the cell walls and thus might result into deformation of the cell structure. It also increases the drip loss.
Fast Freezing: The ice crystals formed in this process are smaller in size and are uniformly distributed, thus keeping the cell structure intact on thawing.
According to International Institute of Refrigeration (IIR), the freezing process is basically divided into three stages based on major temperature changes in a particular location in the product, for pure water and food respectively. Pre-freezing – Freezing until first Ice crystal appears. Freezing Period – Transformation of water into ice. Complete Freezing – Most of the freely available water is converted into ice and reduced to the storage temperature and maintained.
The quality of the food will decrease if there is a formation of larger ice crystals, further resulting in greater drip loss. Drip loss is loss in flavor and tissue juices of the food product on thawing. This causes nutrient loss and dehydration in frozen food products. If the storage temperature has alterations, the ice crystals formed in the food product might increase in size, causing damage to the tissue and thus damaging the texture of the food product. The maximum recommended storage temperature at which microbial spoilage ceases is registered between -18°C and -12°C. Although, microbiological spoilage can be avoided at these temperatures, the enzymes present in the product will still play a part in spoilage. Hence, hygienic conditions or heat processing (blanched or cooked) will increase the shelf life. Blanching of vegetables is commonly done prior to freezing. Addition of sugars is an important pre-treatment for fruits prior to freezing, since the treatment has the effect of excluding oxygen from the fruit, which helps to retain colour and appearance. Moisture loss by evaporation from the surface of a product leads to freezer burn. It is usually in the form of patches of light-colored tissues, produced by evaporation of water, which leave air pockets between meat fibers.
Thawing: The processes that are involved in heating the frozen food products. Improper thawing might result in risk of microbial damage. Microbiologically safe thawing process includes thawing inside a refrigerator at temperatures below 5°C, heating in microwave oven or heating as a part of the cooking treatment.
Freezing Equipment: More than one type of freezing equipment might be suitable for a particular product. The selection of freezing equipment can be done based on batch or in-line operation. The cost benefit analysis considering the economical and functional characteristics of the equipment are important. Feasibility in terms of plant location, processing area, type of product and its dimensions, cleaning and hygiene maintenance is an important consideration.
Different types of freezing equipment use different heat transfer systems, namely air, contact, cryogenic.
New freezing techniques or combinations are being developed for their potential benefits, technical and economic advantages as well as quality enhancements. Some of the new technologies include high pressure assisted freezing, ultrasound assisted freezing, radio-frequency assisted freezing. In Pressure Shift Freezing (PSF), the food is cooled under high pressure to sub-zero temperatures. These processes result in generation of small and uniform ice crystals, thus improving the storage quality of the frozen food products.
- Handbook of Food Preservation by freezing- Mohammad Shafiur Rahman and Jorge F. Velez-Ruiz
- Frozen Food Science and Technology Edited by Judith A. Evans Food Refrigeration and Process Engineering Research Centre (FRPERC) University of Bristol, UK
- Introduction to Freezing- fao.org
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M. Tech, Food Technology, Shoolini University