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FIND OUT MORE. Digestion 144–145 • Micro-organisms 85

The use of microbes (micro-

organisms) to produce and process

materials is called biotechnology.

Bacteria and yeasts are used to

produce products, such as

yoghurt, cheese, and wine,

by. FERMENTATION.

HOW DO WE USE MICROBES?

Microbes are like tiny chemical factories. They

make chemicals called enzymes (biological

catalysts) to break down chemicals in their

surroundings for food. As microbes feed and

multiply, they produce chemical by-products. Yeast

cells make an enzyme that turns sugar into alcohol.

HOW ARE GRAPES MADE INTO WINE?

Natural yeast on grape skins ferments the fruit sugars

to produce alcohol. Fermentation stops after 10 to 30

days, when all the sugars have been used up, or when

the alcohol content is 12 to 15 per cent, which stops

the yeast cells working. The wine is sealed in barrels.

If it is exposed to the air, the alcohol is oxidized to

acetic acid (vinegar), and the wine turns sour.

WHAT ARE ANTIBIOTICS?

Some microbes produce chemicals that kill disease-

spreading bacteria. These chemicals are antibiotics.

They either destroy the bacteria completely, or stop it

multiplying. The first antibiotic, penicillin, was made

from a mould that grows on bread. Its antibiotic

action was discovered by chance in 1928.

HOW IS MILK TURNED INTO CHEESE?

An enzyme called rennin is added to milk. It ferments

the milk sugars to produce lactic acid. The acid

separates the milk into solid curds and liquid whey.

The curds are pressed into cheeses. Microbes continue

to work in the cheese as it ages, making its flavour

stronger and changing its texture.

Fermentation is the action of yeasts and bacteria on the sugars

in fruit, grains, milk, and other food. Yeast cells added to bread

dough feed on its natural sugars, turning them into carbon dioxide

and water. This makes the dough rise to make light, fluffy bread.

4 PENICILLIN GROWTH

This macrophotograph (close-up) shows

a disc-shaped culture (growth) of the

green penicillin mould growing on agar jelly

in a glass petri dish. Penicillin is an antibiotic

substance made by the mould Penicillium notatum.

It was among the first antibiotics to be discovered,

and is still widely used to treat infections.

4 WINE-MAKING

In the traditional champagne method for making

sparkling (fizzy) wine, the wine continues to

ferment after it has been bottled. The bottles are

turned upside down to allow the yeast to settle

onto the cork. Finally the cork is loosened briefly,

and the gas pressure shoots the sediment (settled

particles of yeast) from the bottle.

FERMENTATION

4 CHEESE-MAKING

The type of cheese made depends

on the variety of microbes

added, and the conditions of

fermentation. As the milk curdles,

the curds are separated from the

whey. This batch will become

Baby Swiss cheeses.

Biotechnology

1 YEAST
This electron microscope picture
shows individual yeast cells. Some
of the cells are reproducing by
budding (producing two cells
from one).

Yeast cells

sink downwards

Carbon dioxide gas

produced during

fermentation gives

champagne its fizz

SIR ALEXANDER FLEMING

Scottish, 1881-1955

Fleming won the Nobel Prize

for Medicine in 1945 for the

discovery of penicillin. While

searching for compounds that

would kill bacteria without

harming the body, he noticed

that bacteria cultured (grown)

in a glass dish had died

around spots of mould.

The mould had blown in

through a window. Fleming

extracted the antibiotic

substance from the mould

and called it penicillin.

biotechnology