Freeze drying is a process that dehydrates frozen foods placed inside a vacuum. The moisture in the foods converts into a gaseous vapour that leaves the food during processing. This removal of water content from frozen food is called sublimation. The freeze drying process allows the food to keep its original size and shape with minimum cell damage. Shrinkage is therefore eliminated or minimized and the food remains preserved for long periods in a near perfect shape. The freeze drying process is carried out in a special machine called a freeze-dryer, which has a large chamber for freezing and a vacuum pump for removing moisture.
The freeze-dried process was developed commercially in World War II when it was used to preserve blood plasma and penicillin for wounded soldiers. Freeze-dried coffee which was created in 1938 is the best-known freeze-dried product. Nestle company invented freeze-dried coffee called Nescafe and later other powdered food products were freeze dried. Over 400 different types of freeze-dried food products have been commercially developed since the 1960s. Although freeze-drying is used to preserve food in the 1970s agricultural crops such as peanuts and tobacco were also freeze dried.
Commonly heat is used in crop and food processing which alters the structure and chemistry of the product. The reason why freeze-drying is used is because it preserves crops and food the closest to the natural structure and chemistry. The process became popular when it was used to create freeze-dried ice cream which was taken into space as astronaut food. Freeze-dried food lasts longer than other preserved food and is very light weight, which makes it perfect for space travel. It is also widely used to produce essences or flavourings to add to food.
Steps in the freeze drying process
- The food product is first frozen so that a condition can be created for low temperature drying.
- After freezing the product is placed under vacuum. This enables the frozen solvent in the product to vaporize without passing through the liquid phase, a process known as sublimation
- Heat is applied to the frozen product within the vacuum to speed up sublimation or removal of water
- Low-temperature condenser plates remove the vaporized solvent from the vacuum chamber by converting it back to a solid.
The vacuum is maintained sufficiently high, usually within a range of about 0.1-2 mm Hg so that sublimation is done speedily. Heat is controlled in a way that does not melt the ice but allows the moisture to turn into vapour that leaves the food speedily. Sublimation or removal of water takes place from the surface of the ice on the frozen food. As the sublimation continues the ice recedes towards the centre of the food. This means that drying of the food takes place from the surface to the centre till moisture in the food is lowered to below 5 per cent. Since the frozen food remains rigid during sublimation, escaping water molecules leave voids behind them, resulting in a porous sponge like dried structure. Mostly heat used for sublimation is infrared and microwave radiations which make it easy to pass through already dried and porous food layers and then into the ice core.
Foods that are Freeze-dried
Freeze drying can be used to dehydrate sensitive, high-value liquid foods such as coffee and juices. It is especially used for drying high value foods such as strawberries, whole shrimp, diced chicken, mushroom slices, and sometimes food pieces as large as steaks and chops. These types of foods have special flavours and colours and textural appearances that cannot be preserved by any other current drying method except freeze-drying. A whole strawberry, for example, is soft, fragile, and almost water so cannot be preserved well unless it is freeze dried. Freeze dried foods are shelf-stable at room temperature for up to twenty-five years or more, if canned, and between 6 months to 3 years if stored in a poly-bag container.
The Benefits of Freeze-Drying
- Freeze drying is considered and expensive process but is used at it makes carrying the food easy especially to space.
- The food retains its original form, size, colour, taste, texture and nutrients
- When placed in water the food regains its fresh flavour, aroma, texture, and appearance
- Food has a long shelf life at room temperature and does not require a cold storage
- The weight of the freeze-dried products is reduced by 70 to 90 percent, with no change in volume and so is easy to handle
- Shipping costs are reduced because of the light weight and lack of refrigeration
- Low water activity virtually eliminates microbiological concerns
- Any type of food or ingredient, whether solid or liquid, can be freeze-dried