PRESERVATION BY LOW TEMPERATURE

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

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

4. 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

5. 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 .

6. 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.

7. 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.

8. TYPES OF FREEZING:
A.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.

9. 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.

10. 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.

11. • 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.

12. 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 .

13. 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.

14. 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.

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

16. THANK YOU
-SAI KIRAN .K
111718012046