Freeze-drying is a popular food dehydration method that is also known as lyophilization. The dehydration process preserves the highly perishable items from the attack of microbes. Usually, foods if not preserved or cured properly, get contaminated by the growth of bacteria resulting in the food not suitable for human consumption. Freeze-drying method uses the techniques of freezing and drying the foods and makes them easy to store for longer and to transport.
History of Freeze-drying Food Preservation Method
Freeze-drying was first introduced during World War II as a commercial method to preserve the ‘serum’ to be used for the medical treatment of injured soldiers. The serum supplies sent to Europe during the war were spoiling due to the lack of refrigeration. In that situation the freeze-drying method proved highly beneficial in rendering the chemical composition of the serum stable.
Shortly, after this invention, the process of lyophilization became popular for penicillin as well. Since then the application of this preservation method has been associated with many products including food, pharmaceuticals, bacterial cultures and even for preserving the body tissues used for medical analysis.
Freeze-Drying Food Preservation: Process
The freeze-drying process has been carried away in four stages. All these stages are discussed below:
- Pretreatment – this stage usually involves the treatment of foods prior to freezing. It may include curing of food with salt, vinegar and spices. Any food especially meats and fish are cured in brine made up of these ingredients so that flavor penetrates deep inside the food. It also includes the cutting and peeling of vegetables, if using to freeze.
- Freezing – this is the most vital stage in the process of food-preservation method. The cured or treated food is kept in the large freezers to freeze. In such a way the flavors and the moisture of the food locked inside it and at the coldest temperature the bacteria present in the food gets killed.
- Primary drying – this stage involves the pressure drying of the frozen food. It works on the principle of ‘sublimation’ that evaporates the water content of the frozen food and dehydrates the food. A reduced level of pressure and heat is maintained around the food that does not allow the frozen food to extract water, but the frozen water directly transformed into gas and the food gets dried. It is suggested to keep the heat level low at this stage, as excess heat may deteriorate the shape and texture of the food.
- Secondary drying – this is the final stage of the freeze-drying process and it mainly involves the removal of water molecules remaining in the food, if any. Although frozen water is removed in the primary drying phase, the water molecules can be removed by slightly raising the temperature surrounding the food.
These four stages are normally followed in commercial food-preservation; however, this drying process can be carried out at home as well for certain foods like apple, carrot or potato. The normal freezer of home refrigerator can be used to freeze the paper thin slices of these foods. Within few hours the slices become frozen and with in 1-2 weeks the frozen water of the slices will be sublimed. The freeze-dried foods will be ready at home within few weeks.
Freeze-Dried Foods: Properties
- The food preserved with freeze-drying method is usually lightweight and a popular choice for hikers.
- This type of food is ideal for astronauts travelling in spacecrafts.
- This food has the advantage of having ‘long life’ as compared to wet food that that tends to get bacteria contamination very fast.
- The reduced moisture level in this type of food inhibits the growth of bacteria and preserves the food from spoiling.
- The nutrition, flavor and even smell of this dried food remain the same after the dehydration.
- The best feature of this food is the ability to get rehydrated. The microscopic pores left in the dried product after the sublimation of ice allows the water to rehydrate the food very easily.
Commonly Freeze-Dried Foods
Almost any kind of food can be freeze-dried, but some of the most common such foods are as follows:
- Desserts – Most commonly the ice-creams are preserved with this method that allows easy transportation while hiking, camping and even in space crafts.
- Herbs – Various culinary herbs are dehydrated with this method. However, air-dried herbs are most commonly used in cooking.
- Instant coffee – Quite often with the use of expensive driers, the coffee is vaporized in hot air to produce freeze-dried coffee.
- Fruits – Commonly freeze-dried to be added in breakfast cereals and in instant dessert mixes.
- Meats – This is the most popular food that is preserved with freeze-drying method to increase its shelf-life.
Freeze-dried ice-cream introduced under the contract with NASA by Whirlpool Corporation. This ice cream is preserved with freeze-drying and ready to eat dessert choice for astronauts in space.