starts to deteriorate or spoil from the time that it is harvested or slaughtered.
The process commences with enzyme action which occurs as the cells in
the food die. It continues with action by a range of spoilage bacteria
and, in some foods, moulds and yeasts. The food will develop off flavours,
odours and a breakdown in texture, providing recognisable signs that the
food is no longer fit to eat.
order to slow down the spoilage process and thus keep foods for a longer
period, a wide range of preservation methods are applied to foods. Most
methods involve the removal or control of factors which affect bacterial
control - the use of low or high temperatures
control - dehydration
general, more than one method of preservation will be employed to achieve
an acceptable shelf-life for products. A combination, for example, of
some form of heat processing combined with the use of a chemical preservative,
will often produce a more durable product than if a single preservation
method were used.
use of different preservation methods will be influenced not only by the
shelf-life required but also by the demand for quality in terms of taste,
smell, appearance, nutritional value, etc. of the food as some methods
may significantly affect these aspects.
use of low temperatures slows down the speed of enzyme reaction and also
prevents the growth of most pathogens and food spoilage bacteria.
methods using low temperatures include:
- storage between 0°C and 5°C.
- storage at -18°C; there are a variety of commercial techniques
to freeze foods which include fluidised-bed freezing, air-blast freezing,
plate freezing, cryogenic freezing and the 'pellofreeze' system.
is used to destroy both pathogenic and spoilage bacteria; some heat processes
will also destroy spores.
methods in this category include:
- time and temperature combination to destroy active pathogens and some
spoilage bacteria in the product, e.g. for milk 72°C for 15 seconds.
- destruction of all micro-organisms using temperatures above 100°C.
treated (UHT) - used for some products to destroy micro-organisms without
causing significant change to flavour, relying upon high temperature
for a very short time.
- usually used to improve palatability rather than to improve storage
heating - involves use of high-voltage electric current through foods
to destroy micro-organisms.
- involves use of heat to appropriate temperature and for a prescribed
time to destroy micro-organisms, including Cl. Botulinum spores. Process
also includes removal of oxygen and hermetic sealing of containers to
avoid post-process contamination.
of moisture prevents micro-organisms from growing.
methods in this category include:
methods such as sun-drying.
methods using hot air including tunnel drying, roller drying
and spray drying.
which uses warm air after food has been quickly frozen.
are a wide range of chemical additives used for food preservation which
affect the growth of micro-organisms in different ways.
commonly used for food preservation include:
- makes water unavailable for use by micro-organisms.
- acts in a similar manner to salt.
and lactic acids - lower the pH below the normal growth range of
acid and sorbic acid - inhibit mould and yeast growth in acid foods.
use of chemical preservative is the subject of specific regulations which
control the types and quantity of chemicals which may be used in foods.
physical methods of preservation are used, including:
atmosphere packaging - air around the food product has a reduced oxygen
content and increased level of nitrogen and carbon dioxide which will
slow down growth of many micro-organisms.
packing - removes oxygen to inhibit growth of aerobic bacteria but does
not prevent anaerobes from thriving. Often combined with refrigeration
to give foods a reasonable shelf-life.
irradiation - uses ionising radiation to destroy micro-organisms but