Transcript Food Bacteria Groups

Food Bacteria Groups
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Food temperature
Both heating and cooling food can change
its susceptibility to microorganisms, and
therefore its ability to stay fresh.
Cooling
Keeping foods such as dairy produce,
cooked foods and raw food ingredients in a
refrigerator at a temperature from 0°C to
3°C will slow the growth of microorganisms
such as bacteria, but won't stop it entirely.
Storing Food Separately
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Freezing
Many different types of food are quickly frozen in factories
to maintain high standards of freshness and safety.
Products are stored in commercial freezers at between 18°C and -29°C. Home freezers store products at -18°C.
Freezing does not kill bacteria, but will keep them
dormant.
Cook-chilled foods
Cook-chilled foods can be cooked from frozen without
defrosting. The downside is that cook-chilled foods have a
short lifespan and have to be thoroughly reheated before
eating - to at least 72°C - to avoid the risk of food
poisoning. Leftovers should be disposed of quickly.
In the factory, the foods are prepared by cooking them to
70°C for two minutes to destroy microorganisms. Food is
packaged into portions and blast-chilled. Here the
temperature is instantly lowered and stored at 0°C to 4°C.
Ultra Heat Treatment (UHT)
UHT is used for products such as "long-life" milk. The
product is heated to a temperature of 132°C to 140°C for
one second to destroy all bacteria. Then the food is
rapidly cooled.
Microorganisms in food
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Microorganisms in food
Three types of microorganisms affect food:
bacteria, moulds and yeasts. By a natural
chain of events, these microorganisms will
cause food to change - often, but not always,
making food "bad" or unpalatable. The
graphic should help you to remember the
difference between bacteria, moulds and
yeasts.
Microorganisms in food
Food acidity and microorganisms
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Food acidity and micro-organisms
Some foods, such as citrus fruits, are acidic;
others, such as sodium bicarbonate, are
alkaline. The more acid the food is, the
lower it will be on the pH scale. The more
alkaline the food is, the higher it will be on
the pH scale. Foods that are neither acid nor
alkaline (for example pure water) are called
neutral and have a pH value of 7.
Food acidity and microorganisms
Acidity or alkalinity
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Acidity or alkalinity of food is important for
two reasons: it affects the taste of a product
and it affects the rate at which
microorganisms grow within and upon food.
For example, lemon juice can be used to
reduce the pH level of yoghurt, to a pH below
4.5. This inhibits the growth of
microorganisms, making the yoghurt last
longer, but it will also give it a sharp taste.
Oxidation of foods
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Food can also be affected by the oxygen in
air. When some fresh fruit, such as an apple
or pear, is cut or peeled, the outside surfaces
darken and turn brown, making the food
seem unattractive. The browning is caused by
enzymes in the fruit reacting to the oxygen
in the air. By adding an acid (eg lemon juice)
or sugary solution (eg syrup), the enzymic
browning or discolouring action is slowed
down.
Oxidation of foods