Preservation of food by low temperature

PRESERVATION BY LOW TEMPERATURE

1. FOOD SCIENCE ASSIGNMENT TOPIC: Preservation of food by low temperature

2. CONTENT: Introduction Principle Chilling or refrigerated storage Freezing Types of freezing

3. INTRODUCTION: Use of low temperature is easiest, most convenient and leat time consuming method of preserving foods. Refrigration / freezing do not sterilize foods or destroy the microrganisms that cause spoilage It simply slows the growth of microrganisms and the chemical and enzymatic changes that effect quality or cause spoilage Use of freezing as a method of food preservation is that microrganisms are only deactivated and not killed

4. PRINCIPLE: Low temperature retard chemical reaction and action of food enzymes and action of food enzymes and slowdown or stop the growth and activity of microorganisms in foods. Pathogen growth is halted below -45 degrees and spoilage microrganisms don’t grow below -10 degrees .

5. CHILLING OR REFRIGERATED STORAGE: Refrigerated storage of food is generally practised both at home or industry level. In chilling the temperature of food is generally reduced to between -1 degree and 7 degree . Thus subsequent storage at refrigerated temperature extends the shelf life of both the fresh and processed foods.

6. FREEZING: Freezing is the removal of heat from the packaged or whole foods resulting in the temperature between slightly below the freezing point of food to -18 degrees . Frozen foods last many months without spolling . Freezing prevents the growth of microranisms due to reduced water activity.

7. TYPES OF FREEZING: Freezing system based on time taken for freezing: Based on time requirement to freeze the foods can be classified into two types i.e. Slow freezing Quick freezing.

8. SLOW FREEZING: Slow freezing occurs when food is directly placed in freezing rooms called “ shrap freezers”. This method involves freezing by circulation of air by convention i.e. through a specially insulated tunnel. Either naturally or with aid of fans . The temperature ranges from -15 to 29 degrees and may take from 3 to 72 hours. QICK FREEZING: Vigorous circulation of air enables freezing to proceed at a moderately rapid rate. In this process the temperature is kept between -32 to -40 degrees and the food attains the stage of maximum ice crystal formation in 30 min or less.

9. B. FREEZING SYSTEM BASED ON MODE OF OPERATION: BATCH FREEZING: This type freezing is mostly used for small operations . If a variety of products are to be frozen a batch freezer may be selected over continuous as they are more versatile. This freezing system is used for products with longer freezing time.

10. CONTINOUS FREEZING: This process of freezing is used in large scale production lines. This freezers are best used for freezing individual portion , such as small vegetables. Continuous freezing allows quick handling after freezing and a quick transfer to the cold store. BATCH /CONTINOUS FREEZING: This type of freezers are usually batch type of freezers operated with trollys which are loaded in sequence at fixed time intervals rather than all at one time as in the truly batch freezer.

11. C. FREEZING SYSTEM BASED ON CONTACT WITH PRODUCT : DIRECT CONTACT FREEZING: In case of direct contact freezers, the product to be frozen is completely surrounded by the freezing medium i.e. the refrigerant which maximizes the efficiency of the heat transfer. INDIRECT CONTRACT FREEZING : In this type of freezer food materials to be frozen are seperated from the refrigerant by a conducting material usually a steel plate. The food product is indirectly exposed to the freezing medium .

12. D.FREEZING SYSTEM BASED ON HEAT TRANSFER SYSTEMS : AIR : In general airis used as the freezing medium in the freezing either as still air or forced air packaged or unpackaged solid foods can be frozen in air at temperature ranging from -18 to -40 degrees . STILL AIR /SHRAP /FREEZING: This method involves freezing by circulation of air by convention through a specially in sulated tunnel,either naturally or with aid of fans.

13. CABINET FREEZING: This method involves circulation of cold air in a cabinet where the product is placed in a tray . The moisture pick up from the product may deposit on the cooling coils as frost ,which acts as insulation.

14. TUNNEL FREEZING: In this process ,the products are placed in trays in a long tunnel and cool air is circulated over the product.

15. THANK YOU -SAI KIRAN .K 111718012046 NAG- A.