Freezing of Fruit and VegetablesSubmitted by: Abhishek ThakurSubmitted to : Dr. Somesh Sharma

Freezing Freezing is a phase transition in which a liquid

turns into a solid.

Principles of Freezing

• Does not sterilize food.• Extreme cold (0oF or -18oC colder):

– Stops growth of microorganisms and– Slows chemical changes, such as

enzymatic reactions.

Quality retention in frozen foods

The principal factors that effect the retention of quality in frozen foods are :

- the quality of the raw material used (variety, maturity, suitability for freezing and frozen storage).

- The treatment given prior to freezing (blanching, SO2, ascorbic acid).

Cont.- The freezing method and freezing rate.- The storage temperature, and temperature

fluctuations.- The storage time.

Treatment prior to freezing aimed at reducing deteriorative change during freezing and frozen storage include :

- Blanching of some fruits and most vegetables to inactivate peroxidase, catalase and brown enzymes, reduce cellular oxygen, reduce microbial numbers and improve color

- Addition of or dipping into ascorbic acid or sulphur dioxide solutions to retain color and reduce browning.

Freezing has main effects on : Enzymes Microorganisms tissues

Effect of freezing on enzymes. the enzymes must be inactivated or inhibited if

satisfactory quality is to be retained. During freezing and frozen storage the

concentration of cellular constituents including enzymes and their substrate increase, hence rates of enzymic activity in frozen tissues can be appreciable, despite the low temperatures

9Effect of freezing on microorganisms The growth of microorganisms in foods at temperatures

below about –12oC has been confirmed. Thus storage of frozen foods at about –18oC and below prevents microbiological spoilage.

Although microbial numbers are usually reduced during freezing but frozen foods are not sterile .

Effect of freezing on tissuesFoods do not have sharp freezing points, but freeze over a range of temperature depending on the water content and cell composition.Rapid freezing, and storage without wide fluctuations in temperature, lead to small intracellular ice crystals and maintenance tissues with minimum damage to cell membranes.

If fish is frozen too slowly, some of its cells may rupture and release nutrients into the liquid that drips from the fish when it thaws.Some flavours become weaker and some become stronger when food is frozen.

Advantages of Freezing

Many foods can be frozen. Natural color, flavor, and nutritive value retained. Texture usually better than other methods of food preservation. Foods can be frozen in less time than they can be dried or canned. Simple procedures. Adds convenience to food preparation. Proportions can be adapted to needs unlike other home preservation

methods. Kitchen remains cool and comfortable.

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Methods of freezingFreezing techniques include : the use of cold air blasts or other low temperature gases coming in contact

with the food, e.g. blasts, tunnel, fluidized bed, spiral, belt freezers.

Multipass tunnel freezer

Contd. Indirect contact freezing,

e.g. plate freezers, where packaged foods or liquids are brought into contact with metal surfaces (plate, cylinders) cooled by circulating refrigerant (multi-plate freezers).

Direct immersion of the food into a liquid refrigerant, or spraying liquid refrigerant over the food (e.g. liquid nitrogen, and sugar or salt solutions).

Vegetable Storage

Temperature0ºF5ºF10ºF15ºF20ºF25ºF30ºF

Length of Storage1 year

5 months2 months1 month2 weeks1 week3 days

Disadvantages of Freezing (Deterioration in food product) Changes in colour (loss of natural colour constituents, e.g.

chlorophyll pigments, development of off colour) Changes in texture (loss of cloud destruction of gels, protein

denaturation, toughening) Changes in flavour (loss of natural flavour, development of off-

flavour, rancidity) Changes in nutrients, such as ascorbic acid in fruits and vegetables,

unsaturated lipids, essential amino acids. Initial investment and cost of maintaining freezer is high. Storage space limited by capacity of freezer.

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Thank you……………