Use of low temerature and high temperature for food preservations.

Preservation of food by use of high temperature and low temperature:

Among the various methods of food preservations, these two methods i.e. use of high temperature and low temperature is best techniques of food preservations without altering the nutritional value of food.

1. Preservation by use of high temperature.

2. Preservation by use of low temperature.

• Preservation by the uses of high temperature:

Use of high temperature in food brings about denaturation of proteins, thermal breakdown of plasma membrane and inactivation of enzymes that require for microbial metabolism. Heat treatment employed should be such that either it kill an organism or inhibit that growth of surviving spoilage organisms. The various types of or degree of heating used in food must be classified as below:

1. Heating below 100*C (Pasteurization)
2. Heating at about 100*C
3. Heating above 100*C

• Heating below 100*C (Pasteurization):

Pasteurization is heat treatment at temperature below 100*C. It kills any apart (pathogenic or spoilage causing or undesirable) but not (like lactic and bacteria) microorganisms present in food. Heating may be by means of steam, hot water, dry heat or electric current. Pasteurization is applied under following conditions:

1. When microorganisms are very heat resistant such as yeast in fruit juices.
2. When major aim is to kill undesirable microorganisms as with market milk.
3. When more rigorous heat treatment might harm the food product or changes the quality of food as with market milk.
4. When main spoilage organism arenot very heat resistant.
5. When any surviving spoilage organisms will be taken care by additional preservative method to be employed.
6. When competing organisms are to be killed and allow the growth of desire organism like addition of starter culture during fermentation.

Time and temperature used in pasterurization depands on method employed and product treated as :

1. Low Temperature Time Holding (LTH)
2. High Temperature Short Time (HTST)

• HTST: It indicates application of high temperature for a short period of time. HTST for pasteurization of milk is 71.7*C for 15second.
• LTH: It indicates application of low temperature for a long period of time. For example: LTH for pasteurization of milk is 62.8*C for 30 minutes.

• Heating about 100*C (Boiling):

A temperature of about 100*C is obtained by boiling of liquid food, by immersion of container of food in boiling water, by expososure to hot steam. Most home canners use pressure-cookers at about 100*C or less for some acidic food. Heating at 100*C may kill most of vegetative cell but spores are quite resistant. During baking interval temperature of bakery product cannot reach to 100*C but surface is with 100*C or above. Similarly, roasting, frying, cooking, simmering also involved the heat treatment of food at about 100*C.

• Heating above 100*C : Temperature above 100*C are often obtained by means of steam under pressure in steam-pressure sterilizer or retorts. The temperature in the retorts increase with rise in steam pressure. Ultra high temperature (UTH) is one of the process that require heating temperature above 100*C for sufficient holding period and results in the proper sterilization of food and food products

#Canning:

Canning is defined as the preservation of food in sealed container. It implies heat treatment as the principle factor in the prevention of spoilage/ preservationof food. Canning is mostly done in tin canes madeup of tin coated steel, in glass containers made up of alumunium, plastic, etc. Canning is also refered as hermetically (air tight) 'sealed container'. Nicolas Appert reknowed as Father of Cannin. He heated canned food product for hours in boiling water and often the result that favour long term preservation. He then established this phenomenon as Appertixation. Today with his idea, many canned foods are preserved for a long period of time. Special cans are prepared for specific food products.

During process of canning at first raw food is harvested and washed with clean water. Then the food is chopped into smaller pieces and then placed in can. The can is then sealed and heated either placing in boiling water or by steam or by placing in oven. There are two system of canning, they are:

1. Hot Pack
2. Cold Pack

Flow chart of Canning (Hot Packed and Cold Packed)

Proper canning is important for preservation of food. Defective canning leads to spoilage of food. During canning, can must be completely filled with food to replaced most of the air from the canes or evacuate the unfilled space. During canning, the can must be heated at sufficient temperature to kill vegetative cells and spores. Can must not be leaked type, if can is leaked, it facilitates contamination of can through leak and helps in germinate of surviving spores.

• Use of low temperature:

Low of temperature is mainly prefered method of food preservations. It is used most commonly as housedhold method and rapidly in food industries too. The use of low temperature in food for its preservations mainly retards:

1. Microbial growth an activities.
2. Chemical reaction and action of food enzymes.
3. Biochemical reaction.
4. Insect and pest activities.

The lower the temperature, the slower the chemical reaction, enzymatic action and microbial growth. Preservation of food at low temperature varies with microorganisms . Most pathogenic or spoilage causing organisms failed to grow at temperature below 5*C but some microorganisms like Clostridium botulinum Type E and Yersinia enterocolitica are reported to grow even at lower temperature.

Preservation of food with drop of 10*C may stop the growth of some organisms and slow the growth of other organisms. Afurther decrease of 10*C in temperature, food further stop growth of more organisms and slow down growth of other.Therefore storage of food at low temperature influence the important environmental factors for preservation of food. Various temperature employed in low temperature are:

1. Chilling storage
2. Comon or cellar storage
3. Cold storage
4. Frozen storage.

Chilling storage:

Chilling storage is done at a temperature around freezing and involves cooling by ice or mechanical refrigerator. It is a temporary storage process. Food like dairy product like fish, meat, some vegetables and fruits can be hold in chilling storage. This method slower the enzymatic and microbial changes in food.

Common or cellar storage:

Refer to use of cold temperature above freezing. This temperature is generally in between 10-15*C. Roots craps (under root), potatoes. tomato cabbage, apple and similar food are stored for limited period in common or cellar storage. Persiable (food which spoiled fastly) food like fruits and vegetables can be stored for certain time period. Common storage doesnot prevent the spoilage of fruits and vetebales by their own enzyme system or microorganisms but only lower their activities. In place where mechanical refregirator is only available common storage is applicable.

Cold storage : Refer to use of low temperature i.e. temperature above or below freezing with the replication of mechanical refregiration. During cold storage food is generally stored above 0*C.

Frozen storage :

It is a storage of food in frozen state at or below -18*C. Under the usual condition of storage a frozen food shows prevention in microbial growth entirely and the action of food enzyme is greatly retarted. The rate of freezing of food based on number of factors such as method used, the temperature applied, circulation of air, size and shape of food, etc. Two basic ways to achieve the freezing to food are:

The rate of freezing of food based on number of factors such as method used on number of factors such as method used, the temperature applied, circulation of air, size and shape of package, kinds of food, etc. Two basic ways to achieve the freezing to food are:

1. Quick freezing.
2. Slow freezing.

• Quick freezing:

Process by which the temperature of food is lower to about -20*C within 30 minutes or less. It is accomplished by one of following method:

(A) Direct immersion of food in refrigererator: (-20*C / -70*c /-80*C) (-20*C ice cube forming) (-4*C in another chamber of freeze)

(B) Air blast freezing: In this method air at -17.8*C to 34*C is blown across the food. Slow freezing process by which desired temperature is achieved within 3-72 hours. It is effective to kill microbial cell. This type of freezing is applied in home freezer.

• Slow freezing: Process with desire temperature is acheived within 3 to 70 hours. It is more effective in killing microbial cells. This type of freezing is applied in home freezer.

# Difference between fast freezing and slow freezing :

Refrence:

1. Jay, MJ. Modern Food Microbiology. 3rd. Delhi: CBS Publications and Distributors, 1987.
2. Frazier, Wc and Westhoff DC. Food Microbiology. 3rd. New-Delhi: Tata MCGraw-Hill Publishing Company Limited, 1986.