How Tiny Microbes Turn Milk Into Curd and Dough Into Bread

Microbes are often thought of only as germs, yet many of them work for us every day — souring milk into curd, raising bread, flavouring cheese, making beverages and producing life-saving medicines.

This part covers microbes in household products, fermented beverages, antibiotics, and industrial products such as acids, enzymes and bioactive molecules.

Lactic acid bacteria such as Lactobacillus turn milk into curd, the yeast Saccharomyces cerevisiae leavens bread dough, microbes ferment palm sap into toddy, and particular microbes give Swiss cheese its holes and ripen Roquefort cheese.

Curd:

- Lactobacillus and other lactic acid bacteria (LAB) grow in milk and produce acids that coagulate and partly digest milk proteins
- A little curd added to fresh milk acts as a starter, supplying huge numbers of LAB
- LAB also **increase vitamin B${}_{12}$, improving nutritional value, and in the stomach they help check disease-causing microbes

Dough and batter:

- Dosa and idli batter is fermented by bacteria, and its puffed-up texture comes from carbon dioxide
- Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae

Toddy. A traditional drink in some parts of southern India, made by fermenting palm sap with microbes.

Cheese:

- Swiss cheese — its large holes come from carbon dioxide released by the bacterium Propionibacterium sharmanii
- Roquefort cheese — ripened by growing particular fungi on it, which give its distinctive flavour

An everyday example. Setting dahi at home overnight with a spoonful of the previous day's curd uses exactly this LAB starter.

The substance. The holes in Swiss cheese are trapped gas, not missing cheese** — bacterial carbon dioxide gets caught as the cheese sets.

Brewer's yeast, Saccharomyces cerevisiae, ferments malted cereals and fruit juices to produce ethanol; wine and beer are made without distillation, while whisky, brandy and rum are made by distilling the fermented broth, which concentrates the alcohol.

Fermentation:

- Yeast breaks down sugars without oxygen, producing ethanol and carbon dioxide
- The same species, Saccharomyces cerevisiae, is used for both baking and brewing
- Different beverages come from different raw materials and processing

With and without distillation:

- Not distilled — wine and beer
- Distilled from the fermented broth — whisky, brandy and rum

Worked example. In fermentation, one glucose molecule gives two molecules each of ethanol and carbon dioxide:

$${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to 2{\text{C}}_2{\text{H}}_5\text{OH}+2{\text{CO}}_2$$

So $180$ g of glucose can give at most $2\times 46=92$ g of ethanol, with the remaining $88$ g released as carbon dioxide.

An everyday example. Bubbles rising from fruit juice left too long in a warm kitchen are carbon dioxide from wild yeasts fermenting its sugar.

The substance. Distillation does not make alcohol — it only separates and concentrates the ethanol that fermentation has already produced.

Penicillin is obtained from the mould Penicillium notatum, and the work of Alexander Fleming, Ernst Chain and Howard Florey established it as an effective antibiotic; antibiotics kill or stop the growth of disease-causing bacteria, and they must be used judiciously because misuse selects for resistant bacteria.

Antibiotics:

- Chemicals produced by some microbes that kill or retard the growth of other, disease-causing microbes
- The name means against life — against the life of disease-causing organisms

Penicillin:

- Produced by the mould Penicillium notatum
- Inhibits the growth of Staphylococcus bacteria
- Penicillin works by blocking the building of bacterial cell walls, so it harms bacteria but not human cells

What antibiotics achieve. They have greatly improved our ability to treat deadly diseases such as plague, whooping cough, diphtheria and leprosy.

Why judicious use matters:

- Overuse and misuse, such as taking antibiotics for viral infections or stopping a course early, favour the survival of resistant bacteria
- Resistant bacteria multiply and spread, making infections hard to treat
- Antibiotics do not act on viruses, such as those that cause the common cold

An everyday example. Asking a chemist for antibiotics to treat a cold is a misuse — a cold is caused by a virus, which antibiotics cannot kill.

The substance. Resistance develops in the bacteria, not in the patient — it is the microbes that change.

Microbes are grown industrially to make organic acids such as citric acid from Aspergillus niger, acetic acid from Acetobacter aceti, butyric acid from Clostridium butylicum and lactic acid from Lactobacillus; enzymes such as lipases, pectinases and proteases; and bioactive molecules such as streptokinase, cyclosporin A and statins.

Organic acids:

- Citric acid — Aspergillus niger, a fungus
- Acetic acid — Acetobacter aceti, a bacterium
- Butyric acid — Clostridium butylicum, a bacterium
- Lactic acid — Lactobacillus, a bacterium

Enzymes:

- Lipases — added to detergents to remove oily stains from laundry
- Pectinases and proteases — make bottled fruit juices clearer

Bioactive molecules:

- Streptokinase — from Streptococcus, modified by genetic engineering; a clot buster that dissolves blood clots in patients who have had heart attacks
- Cyclosporin A — from the fungus Trichoderma polysporum; an immunosuppressant for organ-transplant patients
- Statins — from the yeast Monascus purpureus; lower blood cholesterol by competitively inhibiting the enzyme that makes cholesterol

An everyday example. Clear packaged fruit juice owes its transparency to pectinases and proteases that break down the cloudy material.

The substance. Cyclosporin A deliberately suppresses the immune system — that is exactly what stops the body rejecting a transplanted organ.