FREEZE DRYING

Freeze Drying• Freeze-drying also called as Lyophilization

• It is a process in which material is freeze then high-pressure vacuum is applied to sublimate the water in the form of vapour

• Process was first applied to food products after the Second World War

• Coffee was first freeze dried food products to be produced, but now vegetables, fruits, meats, fish, dairy products, herbs and food flavorings can be freeze dried

Principle

• Based on Sublimation, where water passes directly from solid state (ice) to the vapor state without passing through the liquid state.

• Sublimation of water can take place at pressures and

temperature below triple point i.e. 4.579 mm of Hg and 0.0099 degree Celsius.

• Material to be dried is first frozen and then subjected under a high vacuum to heat; frozen liquid sublimes leaving only solid ,dried components.

* Lyophilization is carried out below the triple point to enable conversion of

ice into vapor, without entering the liquid phase (known as sublimation).

Phase diagram of water

Steps in Freeze Drying

Pretreatment

Freezing

Primary Drying

Secondary Drying

• Pretreatment - method of treating the product prior to freezing. include concentrating the product, formulation revision, decreasing a high-vapor-pressure solvent, or increasing the surface area.

• Freezing - The product is frozen. This provides a necessary condition for low temperature drying. freezing temperatures are between −50 °C and −80 °C

• Primary Drying - pressure is lowered, and enough heat is supplied to the material for the ice to sublime. This phase may be slow because, if too much heat is added, the material's structure could be altered.

• Secondary Drying - to remove unfrozen water molecules, since the ice was removed in the primary drying phase. In this the temperature is raised higher than in the primary drying phase, to break any physico-chemical interactions that have formed between the water molecules and the frozen material

Equipment

The main components of freeze drying equipment are:

Refrigeration System Vacuum System Control System Product Chamber or Manifold Condenser

• Refrigeration system - cools the condenser located inside the freeze dryer. It also cool shelves in the product chamber for the freezing of the product.

• Vacuum system - consists of a separate vacuum pump connected to an airtight condenser and attached product chamber.

• Control systems - include temperature and pressure sensing ability. Advanced controllers use the programming of a complete “recipe” and monitor freeze drying process. It depends on the application and use.

• Product chambers - a larger chamber with a system of shelves on which to place the product.

• Condenser – It is to attract the vapors being sublimed off of the product. The sublimated ice accumulates in the condenser. temperature of the condenser substantially below the temperature of the product.

Heat & Mass transfer There are 3 methods of heat transfer

Through frozen layerThrough dried layerHeating by microwaves

When heat reaches sublimation front, it raises temperature and water vapour pressure of ice, vapour then moves through dried food to region of low vapour pressure in drying chamber.

Application

• Fruits & Vegetables• Beverages• Dairy Products• Meat• Fish• Spices & herbs• Bakery & Confectionery

Advantages

• Lightweight, which reduces the cost of shipping.

• Requires no refrigeration, storage costs are reduced.

• Physical structure of the food is not altered, food retains much of its color, shape, texture, and flavor when it is prepared for consumption.

• Reconstituted quickly and easily by adding water.

• Have a long shelf life when packed properly.

Conclusion

• The freeze-drying process is expensive, which restricts its application on wide range in the food industry.

• New technical solutions are applied to modify the freeze-drying process and make it time- and cost-effective.

• It is suitable for high value products with specific biological and phytochemical properties

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