Food gives us energy and nutrition to do our daily activities. We all love to eat and, if it is something we really enjoy, then we may like to save some for later. Instinctively, we will resort to saving it in a refrigerator. That is because we are aware of low temperature increasing the shelf life of food. But, how was it eons ago? And what are the other ways of preserving food? Let us find out.
The methods used for preserving foods are based on the general principle of preventing or retarding the cause of spoilage – microbial decomposition, enzymatic and non-enzymatic chemical reactions and damage from mechanical causes, insects and rodents.
There are different methods preserving food, classified based on categories.
Food Preservation at cold temperature
Freezing: Freezing inhibits the growth of microorganisms. This, in turn, helps preserve food for a longer period. However, we have to keep in mind that the food we store or freeze is in a good condition (fresh and edible) and the enzymes present in the food have been destroyed by heating it before it is frozen.
Slow freezing – This is also known as sharp freezing. Food is kept in a refrigerated room at temperatures ranging from -40C to -290C. At this temperature, it takes between 3 to 72 hours for the food to get frozen.
Quick freezing – Large quantity of food can be frozen in a short period using this method. This uses temperature between -32OC to -40OC.
Dehydro freezing – Dehydro freezing of fruits and vegetables is done by drying food to about 50% of its original weight and volume and then freezing the food, to preserve it.
Food Preservation at high temperature
Pasteurisation – Pasteurisation is a heat treatment that kills few, but not all, the microorganisms present and usually involves the application of temperatures below 100OC. The heating can be done by means of steam, hot water, dry heat or electric currents and the products are cooled promptly after the heat treatment.
Blanching – Blanching is a treatment in which food is boiled for 2 to 5 minutes at 100OC and then transferred to cold water bath to stop the cooking process. Blanching enhances the colour of the food and neutralises the enzymes in the food.
Preservation by dehydration
Dehydration is done by drying food, using different sources like sun, electricity, osmosis and heat. There are different methods of drying and are mentioned below.
Freeze drying – Removal of water from a product while it is frozen by sublimation is called freeze drying. The ice in the frozen water changes to water vapor and is carried away by circulating heated air, thus reducing the moisture content of the food.
Sun drying – It is a simple and slow process where food is exposed to direct sun until it dries off. It has limitations, though – the drying process can only be done on sunny days.
Osmosis drying – This process uses heavy salt for drying. The moisture is drawn out from all cell tissues making it unavailable to the microorganisms to survive.
Mechanical drying – Using hot air oven helps in removing water from the food items. The temperature used for drying are 52OC to 60OC and the duration for drying is 6 to 15 hours for vegetables and 6-24 hours for fruits. Spray drying is used for liquid food items like milk. Spray drying changes liquid milk to powdered milk.
Smoke drying – This process is usually used on meat by exposing it to smoke. Smoke drying does not only remove moisture from the meat, it also adds flavour and colour to the meat.
Preservation by preservatives
Preservation can also be done by adding preservatives. Preservatives are chemical agents which serve to retard, hinder or mask undesirable change in food. They are classified as class I and class II preservatives. Class I preservatives comprise of salt, sugar, spices, vinegar, honey and edible vegetable oils. Class II preservatives are benzoic acid, potassium metabisulphite and sorbic acid.
Preservation by High-Pressure Carbon Dioxide
New methods of preservation have been found that can preserve raw food materials like meat, egg and even juice. It is called high-pressure carbon dioxide. It is also known as super critical carbon dioxide. High-pressure carbon dioxide (HPCD) or Super critical carbon dioxide is an alternative cold pasteurization technique for foods. This method presents some advantages because it allows processing at much lower temperature than the ones used in thermal pasteurization. Despite intensified research efforts, the last couple of years, the HPCD preservation technique has not yet been implemented on a large scale by the food industry. However, it has been found out to be useful in the food industry.
Preservation by Cold Plasma Technology
Cold plasma technology is a novel technology that is also used de-activate endogenous enzymes which are responsible for browning reactions, particularly polyphenoloxidase and peroxidases. Several research studies showed a reduced growth of microorganism through different mode of actions by etching phenomenon and cell disruption by electrophoration.
Preservation of food is important because it reduces food wastage. Raw vegetables and fruits are changed to another form of food by means of value addition. Value addition makes the food products fun to eat such as jams, jellies and pickles. As you can see, a number of methods are available and it is up to us to choose the kind of preservation we prefer. Some are low cost (mechanical drying), no cost (sun drying) and high cost (Cold plasma technology, high-pressure carbon dioxide). It is our choice to select a preservation method that is relevant and convenient to us.
Have you tried any of these methods? Do share with us your experience.