Why Does Frozen Food Keep Well?
Frozen food keeps for a long time because the freezing of the water inside the food forces the bacteria, which cause it to decompose, into inactivity. Like all living things, bacteria need water in order to thrive. Bacteria are microscopic organisms, or forms of life which occur in air, water and soil all over the world.
But they flourish and multiply particularly wherever organic matter is present. Some may cause disease, others are harmless, or even beneficial, but their activity causes organic matter, including food, to decompose.
Modern discoveries have enabled sub-zero temperatures to be obtained by cooling air to about−300 °C (−508 °F) by compressing it and passing it into low pressure chambers through fine nozzles. The result is a sudden and violent expansion, causing the air to be drastically cooled.
In home refrigerators Freon-12 gas is used instead of air, and the temperatures are much less drastic. The temperature in the freezing compartment of a domestic refrigerator is about −4 °C or 25 °F and that of a deep-freezer about −15 °C or 5 °F.
Preserving food has an ancient history. The salting and smoking of fish and meat have been carried out for centuries. Another long used method of preserving food is to change its form, for example turning milk into butter and cheese, and grapes into wine. More recently preservation has been affected by canning, heat being used to kill bacteria, or dehydration.
Most fresh food contains 75 to 90 percent of water. When this liquid is removed, great savings in packaging, storage and transport are made. Potatoes, milk, eggs, tea and coffee are among the well- known products now sold as dry powders that need only the addition of water to reconstitute them.
The freezing kinetics is important to preserve the food quality and texture. Quicker freezing generates smaller ice crystals and maintains cellular structure. Cryogenic freezing is the quickest freezing technology available due to the ultra low liquid nitrogen temperature −196 °C (−320 °F).
The freezing technique itself, just like the frozen food market, is developing to become faster, more efficient and more cost-effective. Mechanical freezers were the first to be used in the food industry and are used in the vast majority of freezing / refrigerating lines. They function by circulating a refrigerant, normally ammonia, around the system, which withdraws heat from the food product. This heat is then transferred to a condenser and dissipated into air or water. The refrigerant itself, now a high pressure, hot liquid, is directed into an evaporator. As it passes through an expansion valve, it is cooled and then vaporizes into a gaseous state. Now a low pressure, low temperature gas again, it can be reintroduced into the system.
Cryogenic or (flash freezing) of food is a more recent development, but is used by many leading food manufacturers all over the world. Cryogenic equipment uses very low temperature gases – usually liquid nitrogen or solid carbon dioxide – which are applied directly to the food product.
Frozen products do not require any added preservatives because microorganisms do not grow when the temperature of the food is below −9.5 °C (15 °F), which is sufficient on its own in preventing food spoilage. Long-term preservation of food may call for food storage at even lower temperatures. Carboxymethylcellulose (CMC), a tasteless and odorless stabilizer, is typically added to frozen food because it does not adulterate the quality of the product.