Sanjeet Kumar
sanjee.biotech@gmail.com
Food protection and food preservation have one aim in common
they are intended to prevent contamination and spoilage of foods by bacterial
infections. Many of the methods of food protection and preservation used today
are of ancient origin. Having an understanding of food microbiology is
important for food protection and preservation practice. Food protection methods are
measures taken to protect food from being contaminated by any agent. All food
must be protected at all times during storage and preparation from the
following contaminants:
Ø Any water that is not known to be
safe, including overhead leaks and drips
Ø Dirty hands
Ø Coughing and sneezing
Ø Dust and soot
Ø Flies, rodents and other vermin
Ø Insecticides and other chemicals
Ø Unclean utensils and work surfaces
These factors either affect the food directly to make it unsafe
or, like coughs and sneezes or insects, contaminate the food with bacteria. The
most important way of preventing contamination is by adopting good food handlers’ hygiene. This is the
term for a group of practices that should be followed at all times by anyone
handling food at any stage of the food supply process. Food handlers’ hygiene
in retail and commercial premises where food and drink is sold to customers is
of critical importance. The importance of promoting good food handlers’ hygiene
is:
v To prevent food contamination and
spread of disease caused by bacterial food infections.
v To ensure the good health of people
eating the food.
v To protect the health of the food
handler.
There
are other general principles for preventing food contamination:
- All
water used in food preparation should be wholesome.
- All
dishes, glasses and utensils must be kept clean by regular washing in
clean water, and clean utensils should be kept covered.
- All
surfaces that come into contact with food should be meticulously clean.
- Food
storage, preparation and serving areas should be free of pets, rats, mice
and insects.
- Food
should be covered, and kept separate from chemicals and poisons.
- Cloths
that come into contact with dishes and utensils, and that are used to
cover food, need to be changed daily and boiled before use.
Food is particularly vulnerable to
contamination while it is being prepared for eating. It is important to
remember food handlers’ hygiene and to ensure that all surfaces and utensils
are clean. Foods intended to be eaten raw, such as fruit and some vegetables,
must be washed carefully in clean, safe water. Food that is to be cooked must
be cooked thoroughly to kill all pathogenic microorganisms. All parts of the
food must reach a temperature of at least 70oC.
Food preservation includes a
variety of techniques that allow food to be kept for extended periods of time
without losing nutritional quality and avoiding the growth of unwanted
microorganisms. There are three basic objectives for the preservation of foods:
- Prevention
of contamination of food from damaging agents.
- Delay
or prevention of growth of microorganisms in the food.
- Delay
of enzymic spoilage, i.e. self-decomposition of the food by naturally
occurring enzymes within it.
Technique simply means to prevent
contamination of the food by spoilage agents or by contact with them. The word
‘aseptic’ means free from harmful bacteria, viruses etc. The technique requires
either using an artificial covering for the food, or keeping its natural
protective covering if there is one. Examples of natural coverings are the
shells of eggs, fat or skins in animals, and/or the skin or peel of fruits.
Leaving the natural covering of the food intact, or applying a clean artificial
cover, can prevent microorganisms from entering or dropping on to the food.
Bacteria can be physically removed
from food, or their numbers reduced, by techniques like washing, trimming,
sieving and filtration. For example, vegetables and fruit should be washed in
clean water; any damaged or dirty parts of vegetables should be trimmed off
with a clean knife; flour can be sieved to remove any unwanted contaminants. Heat is one of the oldest methods of destroying
microorganisms in food processing and preservation. The greatest advance in
food hygiene was inadvertently made when humans discovered the advantage of
boiling, roasting, baking and other heat treatments of food, hence preserving
the food for longer periods. Food is also rendered safe by the application of
heat because most pathogenic microorganisms are comparatively heat-sensitive.
Some of the methods of heat treatment used for food preservation are discussed
below.
Boiling is the process of applying heat to
water until the temperature reaches about 100°C. Boiling foods in water cannot
completely destroy all microorganisms, but the vegetative cells of bacteria,
yeasts and moulds are generally quickly destroyed at temperatures of 100°C or
above. Spores of some bacteria are extremely resistant to heat and are not
killed at this temperature, although their growth is prevented. For this
reason, boiling food can rarely be relied upon to ensure complete destruction
of all organisms. However, most pathogens are killed, provided that sufficient
exposure time is maintained. Although the spores of Clostridium botulinum,
which causes botulism, are extremely heat-resistant, the toxin produced by this
organism is readily destroyed by boiling. However, some toxins produced by
other bacteria such as staphylococci are not easily inactivated. Thermophilic
(heat-loving) organisms may survive the effects of boiling and can cause food
spoilage if environmental conditions are favourable for them.
Bacterial destruction by heat is
affected by time and temperature variation. The higher the temperature, the
more rapid is the destruction. On the other hand, as the temperature is
lowered, the time of exposure (holding
time) needs to be longer. Cooking
can have some disadvantages. It can damage the food’s appearance, texture and
flavour, and may also destroy some important vitamins. Nevertheless, the
advantages of cooking outweigh the disadvantages because it inhibits spoilage
and possible disease transmission.
Pasteurisation is named after its
inventor, Louis Pasteur, a French chemist. Pasteurisation is a process of heat treatment of milk, beer and
some other beverages. It requires sufficient holding time to assure the thermal
destruction of pathogens and organisms responsible for spoilage, without
altering the nutritional value. It involves heating the food to a specific
temperature for a specific time and then cooling rapidly. Pasteurisation kills most but not all of the
microorganisms present. It is a very useful method when more rigorous heat
treatment could harm the quality of the product, as in the case of milk, and
when the aim is to kill only the pathogens that are not very heat-resistant. The
temperature applied and the holding time of pasteurisation vary with the
equipment available and the type of food product. In milk pasteurisation, the
time-temperature combination is selected on the basis of the thermal death time
of the most resistant pathogens (TB bacilli) that may be present in raw milk,
and the maximum temperature and time at which the taste, palatability and
nutritive value of milk are maintained. Normally milk is pasteurised at 62.8°C
for at least 30 minutes or at 71.7°C for at least 15 seconds, or, if using
ultra-high temperature (UHT), at 135°C for 1–2 seconds. UHT milk is sterilised,
meaning all forms of life are destroyed. This extends its storage time but does
affect the taste.
Blanching is a mild pre-cooking operation
which can reduce the bacterial load on vegetables by 90%. It means the
application of boiling water or steam for a short time. It wilts some bulky
vegetables and prevents discolouring of others. It cleans peas of the moist and
sticky material around them. Blanching vegetables prior to canning, freezing or
drying helps to remove soil, insects and microorganisms, and destroys or slows
the action of enzymes. It sets the green colour and generally facilitates
dicing, peeling and packing. Canning is one of the most widely used
modern methods of processing and preserving food. It involves the careful
preparation of food packed into a sealed tin, glass or plastic container which
is subjected to defined high temperatures (above 100ºC) for an appropriate
period of time, and then cooled. Following the thermal (heat) processing, the
sealed container must be cooled immediately to a temperature of about 38ºC to
prevent unnecessary adverse effects of heat on the texture, flavour or colour of
the food. The canning method involves
the following steps: sterilising the food to be canned, packing it in sterile,
air-tight stainless metal, glass or plastic containers, and then hermetically
sealing (i.e. with a complete, airtight seal) the containers to prevent
contamination during handling and storage. In the heat process, all vegetative
bacteria are destroyed and spores cannot grow. Any can that is damaged or
swollen should not be used. A swollen, bulging can indicates that gas is being
produced on the inside and demonstrates there is microbial activity in the
food, so it would not be safe to eat.
Source:http://www.open.edu/openlearnworks/mod/oucontent/view.php?id=196&printable=1
