Food Engineering

Lesson 35. PRINCIPLE OF FREEZE DRYING

Module 4. Freeze dehydration

Lesson 35

PRINCIPLE OF FREEZE DRYING

35.1 Introduction

Preservation of food products by drying is an established method. However, when heat is used as a method of removing moisture, certain heat sensitive food products are likely to get damaged. Moisture removal from food products can also be accomplished by freeze drying, which involves the sublimation of water from the frozen state directly to the vapor state. The advantage of freeze dehydration process is that moisture removal or dehydration can be accomplished without exposing the product to high temperatures. This now one hundred year old system involves eliminating the water contained in food or other products in order to keep it stable at room temperature and facilitate its storage.

35.2 Definition

Freeze drying defines the process of preserving food products by freezing them, and then evaporating the water (in the form of ice) directly into vapour by sublimation. Freeze drying produces one of the highest quality food products obtainable through any drying method, and allows for rapid and near complete re-hydration.

Freeze-drying a product consists of three stages. First, the product is frozen so that the water contained in it becomes ice. Second, the ice in the product is made to vaporize without passing through the liquid phase; this process is known as sublimation. This vapour is collected by use of condensers, and, once removed from the freeze-drying chamber, becomes ice again. Finally, secondary desiccation involves eliminating the water that was not converted into ice during the first stage.

35.3 Principle

Freeze drying also called sublimation drying is the drying of a product in frozen state. The ice evaporates without passing through the liquid state i.e., it sublimes. Conditions of sublimation of pure ice therefore exists below the triple point (0.01 ^oC and 610.8 Pa) at which all three states of matter can exist. Freeze drying is always done under vacuum. The sublimation temperature is determined by the value of the reduced pressure. It should be noted that sublimation process is only possible below the so-called ‘eutectic’ point.

The first stage of freeze-drying is to freeze the food in conventional freezing equipment. Rate of freezing affects the quality of product as well as rate of drying. Rapid freezing produce small ice crystal, which do little damage to the structure of the product but, they leave behind very small pores and capillaries, which offer a greater resistance to the passage of the vapour. Slow freezing produces large crystals and they make rapid drying possible. But, disadvantage of large crystals is that they damage cells; produce separation and denaturation of protein.

If the water vapour pressure of a food is held below 610.8 Pa and the water is frozen, when the food is heated the solid ice sublimes directly to vapour without melting. Water vapour is continuously removed from food by keeping the pressure in the freeze dryer cabinet below vapour pressure at the surface of the ice, removing vapour with a vacuum pump and condensing it on refrigeration coils. As drying proceeds ‘sublimation front’ moves into food. Latent heat of sublimation is either conducted through the food to the sublimation front or produced in the food by microwaves. After vapour travels out of the food through channels formed by the sublimed ice and is removed.

Foods are dried in two stages; first by sublimation to approximately 15% moisture content (wet basis) and then by evaporative drying (desorption) of unfrozen water to 2% (wet basis). Desorption is achieved by raising the temperature in the drier to near ambient temperature while retaining the low pressure. The rate of drying depends on the resistance of the food to heat transfer and to a lesser extent on the resistance to vapour flow through the sublimation front.

In common with almost all materials, water can exist in three different states of phases; a liquid, a solid, or gas. By plotting a graph of pressure versus temperature over an appropriate range, the boundaries of the three states can be seen. This is called phase diagram. The triple point is the state where ice, liquid and water vapour can exist together in equilibrium.

Advantages of Freeze Drying:

1. Minimum damage to, and loss of activity in, heat labile materials

2. Creation of porous, friable structure

3. Speed and completeness of rehydration

4. The ability to sterile filter liquids just before dispensing (if needed)

Disadvantages are as follows:

1. High capital cost of equipment

2. High energy cost

3. Lengthy process time

4. Possible damage to products due to change in pH and tonicity when solutes concentrate as pure water freezes into ice

Freeze-drying is slower than conventional dehydration. Energy costs for refrigeration are high and production of partial vacuum is an additional expense. This together with high capital investment results in high production costs. Hence freeze-drying is used for expensive foods, microbial cultures, coffee, tea, egg, milk products, fruit, fruits juices, certain vegetables, etc.