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Why is -18 °C the ideal freezer temperature?
Food producers recommend -18 °C as the ideal temperature for frozen food. Read on to discover why so, and what the American Fruit and Vegetable coalition had to do with this.
Do you still remember buying your first ever refrigerator or freezer? When I bought my first freezer, a few years back, I received the full-on sales pitch during which all the fantastic features of my new freezer-to-be were praised in the most exaggerated way imaginable and I was told that the freezer temperature should ideally be set to -18 °C. On my asking why -18 °C was the magic temperature, the sales consultant was bewildered – unable to give an answer, and so, I set about finding out the answer to this question myself…
Initially, at the outset of modern freezing, -10 °C was regarded as the safe temperature for storing frozen food. However, as we discovered more and more about the nature and benefits of freezing, lower temperatures were recognised as being more suitable. In the late 1930s, the American Fruit and Vegetable coalition advocated that a freezing temperature of 0 °F (equivalent to -17.8 °C) be maintained, largely on the basis that 0 °F was a nice round number, rather than for any specific scientific reasons. It was quite some time later that the rounded temperature of -18 °C became accepted as the standard food freezing temperature in Europe.
EC frozen food directive adopted in 1989
In 1964, the International Institute of Refrigeration recommended a minimum temperature of -18 °C for frozen food. Both national and international committees agreed with this and firmly adopted the figure in standards, norms and laws. Then, on the basis of the 1967-adopted ‘Code of practice for frozen foods’, the EC Commission in turn created its own Directive for quick-frozen foodstuff and, in 1989, prescribed a minimum temperature of -18 °C for the storage of frozen food.
Essentially, lower temperatures slow down reactions and the propagation of microorganisms. However, there is no uniform reduction in the speed that chemical reactions occur as temperature is lowered, although there is a certain adherence to Van’t-Hoff’s rule (the velocity of chemical reactions is increased twofold or more for each rise of 10 °C in temperature).
Experts have found that when stored at between -30 °C and -18 °C, the reaction rate for fruit and vegetables is two to three times slower. For sensitive substances in particular, this means that their vitamin content deteriorates significantly faster at higher temperatures. After one year of storage at -12 °C, the vitamin C content in vegetables is only about 20% of the vitamin C content of those stored for the same period at -18 °C.
The colder the storage environment, the better the quality of the frozen food. However, because maintaining lower temperatures involves greater energy usage, -18 °C represents an optimum compromise between food quality and energy consumption.