In a household environment, food is frozen by placing it in a freezer and allowing heat transfer to occur by conduction. This process can take several hours, depending on the type of food being frozen.
Other forms of freezing have been developed in the food industry to speed-up the time taken to freeze a product. This ensures that products are frozen in the shortest time possible, reducing the risk of micro-organism growth.
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Blast freezing: Food is subjected to a constant, steady stream of cold air (-40oC or lower) in a tunnel or cabinet.
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Scraped heat exchange: Products, such as ice cream, are frozen this way to reduce the formation of large ice crystals. The product is scraped against a cooled surface and then immediately scraped away.
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Cryogenic freezing: Liquid nitrogen (or CO2) is sprayed directly onto small food items such as soft fruits or prawns. It is due to the liquids' extremely low temperatures, -196oC and -78oC, which make freezing as a result almost instant.
c) Drying
Micro-organisms need water in order to grow and multiply. When moisture is reduced in food, micro-organism growth is reduced. Dehydration reduces the water activity level, weight and bulk of the food and helps to preserve the product.
Dehydration is the reduction of water to prevent micro-organism activity. Many products, such as vegetables, are diced before drying - to increase their surface area - making water loss more rapid.
Although using the sun is one of the most traditional methods of drying, it is slow and only practical in hot, dry climates (so the UK wouldn’t be suitable!). The food may be vulnerable to contamination through pollution and vermin from rodents and flies to make matters worse.
Of course, the food industry has researched and developed other forms of.
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Spray drying: suitable for dried milk and coffee powder. A fine spray of liquid is injected into a blast of hot air in a chamber. Water evaporates in seconds, leaving the solid part of the product behind in a powdered form.
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Tunnel drying: hot air is blown over a food such as vegetables in a tunnel.
d) Irradiation
Basically, this process of preserving food exposes it to carefully controlled amount of ionising energy.
This process is most commonly used to inhibit sprouting of vegetables, delay ripening of fruits or reduce numbers of micro-organisms which cause food poisoning.
e) Chemical Preservatives
This includes;
Pickling
Pickling vegetables and fruits with vinegar prevents the growth of micro-organisms. This is due to the food being placed in a low pH solution in which micro-organisms cannot grow.
Sugar and Salt
The addition of large quantities of sugar inhibits the growth of micro-organisms by making water unavailable. Jams, marmalades and jellies use this principle in their manufacture.
Coating food in salt or placing it in a salt solution (brine) reduces the moisture content of the food, i.e. it reduces the availability of water to micro-organisms. With little moisture, micro-organism growth is reduced. However, the taste of the food may well be changed as a consequence.
