Microorganisms and Food

This topic involves how microorganisms can be helpful in food production and how they can harm us if we don't take care during food storage and preparation.

Useful Microorganisms

Bacteria are used to ferment milk into yogurt and cheese. The acid produced by the bacteria thickens the milk in yogurt making. In cheese making, rennet is added to separate the milk into solid curds and liquid whey. The curds can then be pressed into solid cheese.

Yeast is used in bread making. When the yeast is activated and begins to respire, the carbon dioxide bubbles spread through the dough and make it rise. Any alcohol produced evaporates in cooking.

Yeast is also used in wine and beer-making. Wheat is ground up to form a mash in beer-making, which is fermented with yeast to produce alcohol. Hops are added for flavour. In wine-making, grape juice is fermented with yeast to produce wine. In both cases, carbon dioxide can escape from the mixture through one-way valves which allow the gas to escape before the pressure builds up to a dangerous level inside the fermentation chamber. The one-way valve prevents the entry of microbes into the mix.

Food Poisoning

Food poisoning usually occurs when food is contaminated with a disease-causing bacterium, then left for long enough in suitable conditions for that bacterium to multiply to dangerous levels.

For example, it's a definite fact that you are carrying bacteria around on your hands right now. If just one of those is a salmonella bacterium and you put your hand in your mouth and swallow it... you'll be perfectly fine. Your stomach acid would deal with it quickly enough.

But imagine you touched a piece of chocolate... which you left in a warm place for a few days. The one bacterium would quickly multiply into millions of bacteria (assuming it has moisture and a food source) and then when you ate the chocolate you could get food poisoning.

We avoid cases of food poisoning by people who handle food having good personal hygiene (caterers need food safety certificates) and by storing food in conditions that prevent mirobial growth (see earlier section).

Preserving and Storing Food

Food spoils when it is broken down by microorganisms. In order for microorganisms to multiply to the levels needed to break down food, they need the following conditions:

1. Warmth

2. Moisture

3. Food Source

4. Suitable atmosphere

5. Suitable pH

There are many methods of avoiding spoilage:

1. Freezing - this slows the growth of microorganisms to almost a complete stop.

2. Refridgeration - again, this slows the growth of microorganisms though not as much as freezing.

3. Salting - The salt draws out moisture from the food and makes it unavailable to microorganisms

4. Pickling - e.g. in vinegar - conditions are too acidic for microbia growth

5. Freeze-drying - all water is removed from the food and it is sealed. No moisture is available to microorganisms.

6. Canning - food is heat-treated and sealed in cans/jars. Any microorganisms are killed by the intense heat and no others can enter.

7. Storing in air-tight containers avoid microorganisms getting onto food.

8. Sealing in bags containing only nitrogen or sulfur dioxide - microorganisms literally have no oxygen for respiration and die. Others die from the poisonous sulfur dioxide.

9. Pasteurisation - heating to a high temperature to reduce the number of bacteria.

10. Sterilisation - heating to a high temperature for a longer period of time to kill all bacteria.