The ultimate aim of food preservation is to prevent it from spoiling.Spoiling can be due to either the microbial growth or due to fats turning rancid. There are number of techniques used for preservation of food such as canning,pickling,drying,freeze drying,irradiation,pasteurization and many more.Drying is the ancient method for food preservation which refers to the removal of moisture from food product.Conventional drying which used hot air often lead to loss in the sensory characteristics as well as nutritional qualities.There comes in the picture freeze drying and lets read further about Freeze Drying Process.
Freeze Drying Definition
Lyophilisation or freeze drying is a process in which water is frozen followed by its removal from the sample initially by sublimation(primary drying) and then by sublimation(desorption).Since heat is not applied in this case.So,both nutritional qualities and sensory characteristics were better retained.Freeze drying has similar preservative effect by reduction in water activity without heat application.
The water vapor pressure of a food is held below 4.58 torr (610.5 Pa) and water is frozen.When the food is heated solid ice sublimes into vapor without melting.The pressure in food dried cabinet is below the vapor pressure at the surface of ice and a vacuum pump is present in the cabinet which removes the vapor and let it condense on refrigeration coils. Food products are dried in 2 stages: first by sublimation to 15% moisture content and then by evaporative drying (desorption) of unfrozen water to 2% moisture content.The rate of drying depends on resistance of the food to heat transfer and to a lesser extent on resistance to vapor flow from sublimation.
Properties Of Freeze Dried Food Products
- The moisture content of freeze dried food product is below 3%.Freeze dried product is lighter in volume/weight.
- There is very good retention of volatile flavour and aroma components with the process
- Freeze drying causes very little change in chemical components of food.There is very minute difference in the nutritive value of fresh food and freeze dried food product.
Related Reads: Conventional Drying Of Fruits and Vegetables
Freeze Drying Process
- Freezing–It is most critical process in whole freeze drying process.The product to be freeze dried is cooled below its triple point(lowest temperature at which liquid and solid phases of product coexist) which esures the sublimation.Freezing is done rapidly inorder to lower the material to its eutectic point quickly to avoid the formationof ice crystals.Generally the freezing temperature is between -50 to -80 degree Celsius.In lab,it is often doneby placing the product to be frozen in freeze drying flasks and the flasks are rotated in a bath called shell freezer which is cooled by mechanical refrigeration ,dry ice and methanol or liquid nitrogen.
- Primary Drying(Sublimation)- In this process temperature and pressure should be below the triple point of water ,i.e, 0.0098 degree Celsius and 4.58 m of Hg.So,in this process pressure is lowered and enough heat is supplied for the water to sublimate.The heat supplied is latent heat of sublimation.In this process moisture is removes upto 98-99%.Pressure is controlled by applying partial vacuum.The condenser plates resolidifies the water vapor into ice .Driving force for sublimation is vapor pressure difference between the evaporating surface and the condenser.
- Secondary Drying (Desorption)-It aims to remove unfrozen water molecules which could not be able to convrt in ice and sublime.In this process temperature is raised higher,around 50 degree Celsius to break all physico-chemical interactions (to remove bound water ) that have formed between water molecules and frozen material.Generally, Pressure is kept lowered to around 50 mm of Hg.Rate of secondary drying is slow and it takes around 10-12 hours.
- Packing-After drying,vacuum is replaced by filtered dry air or any inert gas such as nitrogen to establish atmospheric pressure.At the end of all these operations moisture content reaches 0.5%-4%.
Basic Components Of Freeze Drying-
- Freeze Drying Chamber-It contains the trays to hold the food during drying and heaters to supply latent heat of sublimation.
- Refrigerated Condenser-A condenser is a pipe coil system which condenses all the vapours from the product.The sublimated ice is accumulated in condenser and is manually removed at the end of the operaton.In lab scale,the condenser is cooled by means of carbon dioxide or liquid nitrogen .The refrigeration system should be able to maintain temperature of condenser substantially below temperature of product.
- Refrigeration Unit-During the freeze drying process,water contained in the material passes 3 stages.The product should be cooled,frozen and subjected to sublimation.During this stage ,heat must be applied to product to compensate for the sublimation.
- Vacuum Pump-It sucks all the air present in freeze drying chamber and maintains vacuum inside it.
Advantages Of Freeze Drying
- Since the water is removed from the food product it becomes very light and hence becomes easy for transportation(portable)
- Nutritional qualities and sensory characteristics of freeze dried product is retained.
- The products do not require refrigeration hence can last for months or years.
- Thermolabile food products can easily be dried.
Disadvantages Of Freeze Drying-
- Freeze dried food products are quite expensive due to specialized equipment needed for this process.
- The food product is prone to oxidation,due to high porosity and large surface area.Hence the product is packed in vacuum or in inert gas.
Shelf Life Of Freeze Dried Foods | How Long Does Freeze Dried Food Last
Since,the moisture present in freeze dried products is upto 2-3% ,therefore, these products can be stored for much longer period of time.In case of freeze dried fruits and vegetables it can be stored upto 25 years and freeze dried meat and meat products can lasts for several months Simply dehydrated fruits and vegetables can last for atmost 15 years.
Food types suitable for freeze drying
- Fruits and juice
- Fish and seafood
Small fruits and vegetables can easily be freeze dried whereas whole fruits and whole vegetables would not be suitable.It also applies to meat products,large meat pieces are difficult to freeze dry whereas small meat chunks can be dried very well.
Freeze Drying Process Video