FOOD PRESERVATION
When food is available more than the present
use, it is preserved for future consumption. Foods such as fruits and
vegetables have a short growing season and preservation makes them available
for use throughout the year and avoids wastage of surplus crops.
Principles of Food Preservation
Prevention or delay of microbial decomposition.
1.
By keeping out microorganisms - asepsis
2.
By removal of microorganisms - filtration
3.
By hindering the growth and activity of
microorganisms - low temperature, drying.
4. By killing the microorganisms - heat or radiation.
Prevention or delay of self decomposition of food -By destruction or
inactivation of food enzymes, (eg) blanching.
Prevention of damage caused by insects, animals and mechanical causes.
Methods of Food Preservation
All methods used for preserving foods are based upon the general
principle of preventing or retarding the causes of spoilage. - microbial
decomposition, enzymatic reactions and damage from mechanical causes insects
and rodents.
Preservation by low Temperature
i. Freezing
Freezing
may preserve foods for long periods of time provided the quality of food is
good to begin with and the temperature is maintained in freezers. In slow
freezing process or sharp freezing the foods are placed in refrigerated rooms
at temperatures ranging from - 4 o C to 29 o C. In quick freezing process the lower
temperatures used - 32 o C to -40 o C freeze foods so rapidly that fine crystals
are formed and the time of freezing is greatly reduced over that required in
sharp freezing.
ii. Dehydrofreezing
Of fruits and vegetables consists of drying the food to about 50 percent
of its original weight and volume and then freezing the food to preserve it.
Preservation by high Temperature
i. Pasteurisation
Pasteurisation is a heat treatment that kills part but not all the
microorganisms present and usually involves the application of temperatures
below 100 o C. The heating may be by means of steam, hot water or dry heat and
the products are cooled promptly after the heat treatment. Usually milk is
pasteurized. Three general methods are used now-a-days.
a. Holding or Batch system (Holder method)
This consists in bringing the milk or cream to a temperature usually 65 o
C and holding at that point for at least 30 minutes followed by rapid cooling.
b. High Temperature Short Time Method (HTST)
(Flash method)
This consists of raising the temperature of the milk to at least 72 o C
for 15 seconds followed by quick cooling.
c. Ultra High Temperature System (UHTS)
In this
system, milk is held for 3 seconds at 93.4 o C. After pasteurisation the milk is
cooled rapidly to 7 o C or lower.
ii. Canning
Canning involves the application of temperatures to food that are high
enough to destroy essentially all microorganisms present plus air tight sealing
in sterilised containers to prevent to contamination. Fruits, vegetables and
flesh foods are preserved by this method.
Preservation Using Preservatives
Preservatives has been defined as chemical agents which serve to retard,
hinder or mask undesirable change in food. These changes may be caused by
microorganisms, by the enzymes of food or by purely chemical reaction. The PFA
classifies them as class I preservatives and class II preservatives. Class I
preservatives are salt, sugar spices, vineger, honey and edible vegetable oils.
Class II preservatives are benzoic acid and its sodium and potassium salts,
sorbic acid and its sodium and potassium salts. The preservatives generally
used in fruit and vegetable products may be broadly classified as organic and
inorganic preservatives.
The organic preservatives are benzoic acid,
chlorobenzoic acid or salicylic acid. The only permitted inorganic preservative
is sulphur-di-oxide which is generally used in the form of sulphites. The
preservatives permitted in fruit and vegetable products in India are sodium
benzoate, sulphites and sorbic acid. Sulphur-di-oxide, sulphites and
metabisulphites are used in confectionary, fruits and fruit juices and wines.
Sodium or potassium metabisulphites are used in the preservation of fruit
products. Sodium benzoate is used as a preservative in grape crushes and tomato
ketchup.
Preservation by high Osmotic Pressure
By the
principle of osmosis, jams, jellies and pickles are preserved.
i. High Concentration
of Sugar
Apples, guavas, grapes and pineapples are suitable for making jams and
jellies. For making jams and jellies, the fruit should be just ripe because the
pectin content is high in such fruits.
ii. High Concentration
of Salt
Salt is used to control microbial population in foods such as butter,
cheese, vegetables, meats, fish and bread. Spices and other condiments have
bacteriostatic effect. In addition to salt and several spices, oils are used in
making pickles. Aerobic bacteria and mould growth are prevented by covering the
top with oil. Properly prepared and stored pickles can last upto a year or more
without spoilage. The important preservative agents in pickles are salt,
vinegar, sugar, oil, spices and condiments.
Preservation by
Dehydration
Dried foods are preserved because the available moisture level is so low
that microorganisms cannot grow and enzyme activity is controlled. Dehydration
processes are used commercially for many foods including dried milks, eggs,
coffee, tea, fruit drinks, dessert mixes and traditional dried fruits,
vegetables, meat and fish. As a result of heat applied during the drying
process many of the organisms present in the food are destroyed. Bacteria
require 18% available moisture for their growth, yeast 20% or more and moulds
13-16 %. It is therefore essential in the preservation of food by drying to
reduce moisture as much as possible without damaging the essential quality of
the food.
Food Irradiation
Food irradiation is a process of food preservation in which food is
exposed to ionizing energy - radio isotope cobalt - 60. The potential uses of
food irradiation are
To avoid
the use of harmful chemical compounds like methyl bromide and ethylene oxide
for insect disinfestation in stored products and microbial decontamination of
spices.
1.
To extend shelf life of meat, poultry and sea
foods by killing microorganisms causing their spoilage.
2.
To enhance safety of food by killing food borne
pathogenic microorganism and parasites.
Food
irradiation reduces post harvest storage losses. Irradiation at the appropriate
level does not change the flavour, taste, smell, texture and mineral contents
of foods. Irradiated foods are safe and wholesome for human consumption.
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