Microbes make food
What is in your lunchbox today? Is there bread, cheese or a yoghurt? If there is, then you have got microbes to thank for them. Let's have a look at how microbes are used to make these three foods.

The raw materials for making bread are flour, sugar, salt, yeast and milk or water. The microbe in this list is yeast. It is a fungus. Before the yeast can play its part in bread making the ingredients have to be mixed. When they are mixed, a sticky dough is made. It is flattened and folded many times in a process called kneading. This helps the gas to collect in bubbles later in the bread making process.

After the dough has been kneaded, it is left to stand for an hour. During this time the yeast becomes active and it feeds on the sugar in the mixture. As it feeds it also produces alcohol and carbon dioxide. The carbon dioxide collects in gaps in the dough and forms bubbles. As the yeast feeds the bubbles grow in size and make the dough rise and expand. At the end of an hour the size of the lump of dough has doubled.

The dough is then cut up and placed in tins. The tins are placed in a warm cupboard for half an hour. During this time the yeast continues to feed and produce alcohol and carbon dioxide so the dough rises even higher. The baker then places the tins in a hot oven for about twenty minutes. Microbes, like yeast, become more active when they are warmed but when they are heated too strongly they die. The heat in the oven is too strong for the yeast but it has done its job.

When a gas is heated it expands and the carbon dioxide in the bread is no exception. The heat makes the bubbles expand and stretch the dough even more but the expansion does not continue for long. The reason for this is that the dough undergoes a permanent change. It turns into bread, which will no longer be stretched by the carbon dioxide. However the bread is strong enough to hold the shape of the bubbles so that it does not collapse when the loaf leaves the oven.

Alcohol is a liquid and at the surface of every liquid evaporation takes place. Heat speeds up evaporation and in the oven it is so hot that all the alcohol turns to vapour quickly and none is left in the bread. When the loaf is brought out of the oven it is cooled before it is sliced and wrapped.

Cheese is made from milk but not all the milk is converted into cheese. Early in the cheese-making process bacteria produce chemicals which change the milk into two substances called curd and whey. The curd is a solid substance, which is used to make the cheese, while the whey is a watery substance, which is used to make other dairy products. The curds are pressed into lumps and stored for some time so that bacteria and sometimes fungi can feed inside them and give the cheese its texture and flavour.

Yoghurt is also made from milk. Two different kinds of bacteria are added to the milk and the mixture is heated to between 40 and 45°C for three to six hours. During this time the two kinds of bacteria work together to make the yoghurt become a thick liquid and give it a pleasant taste.

Next time you take a bite of bread and cheese or have a yoghurt think of how the microbes worked to give you these foods. Think also how you are rewarding the microbes - you are eating them.

Is it not harmful to eat the microbes that make bread, cheese and yoghurt?
No, it is quite safe. Your body digests them with the rest of the food and the protein in the microbes may be used to make the muscles in your arm that let you move the computer mouse.

Is the mould that grows on damp bread like yeast?
No. Although yeast and mould are both fungi they are quite different. If you look at a piece of fresh yeast it looks and feels a little like putty. You cannot see any structure in it. If you put some yeast under the microscope, you would see that a yeast has a tiny almost spherical body. When it feeds, it grows and breeds. It has an unusual way of breeding. It produces a bud, which is a small copy of itself. The bud stays on its parent and may even produce a bud before it is full-grown. Yeast may form groups of up to six or seven individuals before they break up. Moulds grow by producing threads, which grow through the bread and digest it. When a mould breeds some of the threads rise above the bread and develop swollen ends. Spores develop in these swollen ends and are released into the air.

Is yeast used to make alcoholic drinks?
Yes, it is. Yeast is used in the making of beer and wine.

How is yeast used in beer making?
Beer making begins with barley grains. These are allowed to germinate until the roots are about three quarters of a centimetre long. During the germinating process the plant inside the seed breaks down the food into its food store and uses it for growth. After a short period of germination some of the foods are still left and they are in a state which can be easily digested by yeast. When the barley grains have short roots they are mashed up and boiled then cooled and mixed with yeast. The yeast feeds on the broken up contents of the grains and makes alcohol. This process of making alcohol is called fermentation and lasts for up to ten days. When it is over, the liquid is filtered before being put in cans and bottles.

Is there anything unusual about the filtering of beer?
Yes. The beer is passed though a filter made from diatoms - a kind of algae. A diatom is unusual because it makes a shell made of silica. When diatoms die, their shells gather at the bottom of the sea and form rock. It is the rock made of diatom shells which has holes in it which are small enough to trap the yeast but let the beer pass through.

How is yeast used in wine making?
Wine is made from grapes. They are crushed and mixed with yeast. When the grapes are crushed, the sugar that they contain is released and the yeast can feed on it in the process of fermentation. This process is stopped when the amount of alcohol reaches a certain level. The amount of alcohol in a wine is measured as a percentage. Most wines have an alcohol content of between 8 and 12%. If the alcohol content reaches 14% it kills the yeast and fermentation stops. The fermented wine is filtered to remove the yeast before it is bottled.

Why do some cheeses have green or blue moulds growing in them?
These moulds help to give the cheese its flavour and texture. Camembert cheese is made by spraying the surface of newly made cheese with spores of the mould. They then break open and the mould grows through the cheese. Roquefort cheese is made by using special needles to inject the mould spores deep into the freshly made cheese. The spores then break open inside the cheese and the mould begins to grow.

Can any microbes be eaten as a food?
Yes, they can. In the salty lakes in Chad and Mexico there grows an algae which has long threads. When the threads are collected from the water they form tangled mats. These mats are allowed to dry in the Sun and form a material, which can be eaten like a biscuit. A fungus which makes threads (unlike yeast) is grown in a mixture containing glucose syrup. The threads are collected from the mixture, dried, then flavoured and coloured to make it look like meat. It contains less fat and more fibre than beef. In Britain the name of this product is Quorn.