When choosing a RTE food, the criteria that a consumer considers besides convenience, is that the meal should, be palatable, have a reasonable likeness to freshly cooked food and should be safe to eat. Fitting the bill are ready meals, prepared by “freeze-drying” process, which are comparatively much better in terms of taste, freshness and ease of preparation before consumption. At the outset it is important to know that freeze-drying, also known as Lyophilisation is a scientifically proven physical technique used for preservation of biological products. It is not just dehydration but a three-step process involving freezing the food at extremely low temperatures, followed by dehydration in vacuum chambers under low pressure and finally sealing in airtight packages.

The process of Freeze drying

Though a lot of parameters like temperature and pressure differ with the kind of food to be freezer dried, a general outline of the process can be summarized as follows

  1. All the raw materials undergo quality check and appropriate cleaning processes. Vegetables, chicken, meat, etc are sliced into appropriately sized pieces so as to ensure proper and uniform freeze drying. They are then mixed in the required proportions and pre-cooked accordingly.
  2. The precooked food is then laid in trays and placed in cold chambers maintained at temperature as extremely low as -25 C. This results in rapid freezing of the food and the water in the food turning into ice crystals.
  3. In the next step the frozen food is exposed to vacuum chambers maintained at low pressure, for a predetermined amount of time. During this process the temperature of the food is increased to around 40 C which results in the ice crystals directly turning into vapour in a process called sublimation. This results in the formation of tiny pores in the food in the place of ice crystals. These pores help to maintain the shape and size of the food and also later facilitate its rehydration.
  4. In the third step the food is subjected to secondary drying under high temperatures, wherein any remaining water in the food is removed. This way 98%of water is removed from the food.
  5. The final step which is equally important is sealing the food, in predetermined serving quantities, into airtight packages to ensure that the food doesn’t absorb moisture. Several methods are available to achieve this, like vacuum packing or use of nitrogen gas to replace oxygen in the packages.

Origin of freeze drying technique

Freeze drying technique dates back to the time of world war-ll, when it was used to preserve blood plasma in an unrefrigerated condition without damaging its natural constituents. The technique then gained entry into the food sector given its ability to preserve food for very long periods without refrigeration.

Most fruits and vegetables, liquids, chicken, meat, and a variety of sea food can be freeze-dried. However the food items have to be sliced into freezable chunks before the process.

Advantages of freeze drying over other methods of preservation

  1. Removes 98-99% of water content as compared to just dehydration in which 95 to 96 % water is removed(and yes that 2% is a difference that matters!)
  2. Since there is extremely low moisture content, microbial growth is not possible which can otherwise lead to contamination and spoilage of food.
  3. The most vital advantage of freeze drying is that chemical preservatives are not required to be added. So it is a healthier option.
  4. The process of freeze drying doesn’t cause much damage to the organic constituents of the food thus maintaining its nutritional value.
  5. Freeze drying renders the food to be stored unrefrigerated with a longer shelf life.
  6. After rehydration, the appearance, taste and size of freeze dried foods is close to the original nature of freshly prepared food.
  7. These foods require minimal preparation before consumption which includes simple rehydration of the food with hot water.(For best results Manufacturers’ instruction should be followed)
  8. Freeze-drying causes the food to weigh less and can be easily packed in small pouches and very conveniently carried in one’s luggage.