Microorganisms and Food Spoilage
Many microorganisms obtain their nutrition by digesting the organic matter around them. In doing so, they leave behind waste products.
There are four main ways that microorganisms spoil our food.

Visible growth of microorganisms on food. This is most obvious when fungi grow on food. For example, colonies of moulds Mucor and Pencillium often grow on bread. The mould has usually been growing for a few days before it becomes noticeable as either black (Mucor) or blue/green (Pencillium) mould.
Microorganisms use an external digestion process. They release enzymes into the food and absorb the nutrients released by breakdown of the food molecules. When this happens, food often smells sweet as sugars are released from carbohydrate molecules. The food will eventually be reduced to a mush by the action of enzymes.
The bacterium Clostridium botulinum produces a toxin called botulin. This causes botulism. If these bacteria are growing on food, the toxin will be present. It is one of the most toxin substances known.
The presence of the microorganisms in food can cause infection. For example, Salmonella bacteria, sometimes present in poultry products, attack the lining of the stomach and the digestive system.

How do we prevent food spoilage?

cooking - heat denatures enzymes and other proteins and kills microorganisms
pasteuring - this involves heating to 72 degrees for 15 seconds and then cooling rapidly to 4 degrees killing harmful microorganisms
drying, salting and coating in sugar - these processes dehydrate any microorganisms as water leaves them by osmosis
smoking - the food develops a hardened, dry outer surface, and smoke contains antibacterial chemicals
pickling - this uses an acid pH to kill microorganisms by denaturing their enzymes and other proteins
irradiation - ionising radiation kills the microorganisms by disrupting their DNA structure
cooling and freezing - these do not kill microorganisms, but retard enzyme activity so their metabolism, growth and reproduction is very slow

Methods to prevent further contamination include:

canning - food is heated and sealed in airtight containers
vacuum wrapping - air is excluded so microbes cannot respire aerobically
any plastic or paper packaging

Using microorganisms to make food
Microorganisms have been used for many years in the manufacture of food. Many traditional food are made with the help of microrganisms.

Yoghurt is milk that has been affected by Lactobacillus bacteria. The Lactobacillus uses the lactose in the milk to make lactic acid, which causes the milk protein to thicken. The bacteria partially digest the milk, making it easily digestible by humans. Yoghurt can help to ensure that our digestive system contains the non-pathogenic bacteria it needs to aid digestion.
Cheese is made from milk that has curdled. The solid portion of the milk is acted upon by Lactobacillus bacteria. The cheese can be given additional flavour by contamination with fungi such as Penicillium to produce blue cheese.
Bread is made to rise by yeast, which respires anaerobically to release carbon dioxide. Bubbles of gas collect in the dough, causing th dough to rise.
Alcohol is another product of the anaerobically respiration of yeast. Cereal grains containing the sugar maltose can be used to brew beer, as the yeast respires the sugar. Grapes contain the sugars fructose and glucose, which the yeast uses in making wine.

	Created by siobhan.quirk almost 11 years ago

Undergraduate Biology (Food and Health) Note on Microorganisms and Food, created by siobhan.quirk on 28/05/2013.

Microorganisms and Food