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How do microorganisms turn milk into yoghurt, dough into bread, and sugar into beer?

Describe how microorganisms are used in food and drink production through fermentation

A focused answer to the O-Level outcome on food biotechnology. Fermentation by yeast and bacteria, how bread, yoghurt and alcoholic drinks are made, and the conditions needed.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome asks you to describe how microorganisms are used to make food and drink through fermentation. It is the oldest branch of biotechnology, and the key is to know which microorganism is used, what it ferments, and which product, gas or acid or alcohol, actually matters in each food.

The answer

What fermentation is

Fermentation is the breakdown of sugars by microorganisms to release energy, producing useful products such as carbon dioxide, ethanol or lactic acid. Different foods rely on different products.

Bread

Bread uses yeast:

  • The yeast ferments the sugars in the dough, producing carbon dioxide and a little ethanol.
  • The carbon dioxide is trapped in the dough, making it rise.
  • The ethanol evaporates during baking, so the useful product here is the gas.

Alcoholic drinks

Beer and wine use yeast:

  • The yeast ferments sugars (from grain or grapes) to produce ethanol and carbon dioxide.
  • Here the useful product is the ethanol (alcohol); the carbon dioxide is mostly allowed to escape.

Yoghurt and cheese

These use bacteria:

  • A starter culture of lactic-acid bacteria ferments the milk sugar (lactose), producing lactic acid.
  • The acid lowers the pH, making the milk proteins clot and thicken, giving yoghurt its texture and tangy taste. Cheese-making follows a related principle.

Conditions that must be controlled

For good, safe fermentation, several conditions matter:

  • A suitable temperature so the microorganisms grow and ferment well.
  • The right microorganism (starter culture).
  • Sterility, keeping out contaminating microorganisms, for example by heat-treating the milk and using clean equipment.

Examples in context

Example 1. Soy sauce and tempeh. Across Asia, fungi and bacteria ferment soybeans to make soy sauce and tempeh, developing flavour and improving digestibility. These traditional fermented foods show food biotechnology deeply embedded in everyday cooking.

Example 2. Industrial yoghurt production. In a factory, the same lactic-acid fermentation is scaled up under tightly controlled temperature and hygiene, with carefully chosen starter cultures, to make consistent yoghurt in huge quantities. It shows a traditional process industrialised.

Try this

Q1. Name the useful product of yeast fermentation in (a) bread-making and (b) brewing. [2 marks]

  • Cue. (a) Carbon dioxide, which makes the dough rise. (b) Ethanol (alcohol).

Q2. Explain how bacteria turn milk into yoghurt. [2 marks]

  • Cue. They ferment the milk sugar (lactose) into lactic acid, which lowers the pH and makes the milk proteins clot and thicken.

Q3. State two conditions that must be controlled when making yoghurt. [2 marks]

  • Cue. A suitable warm temperature and keeping the process free of contaminating microorganisms (the right starter culture and time are also acceptable).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksExplain how yeast is used to make bread and how it is used to make beer, comparing the products that matter in each case.
Show worked answer →

Examiners want the same organism producing different useful products for different purposes.

In both cases, yeast carries out fermentation, breaking down sugars to release energy and producing ethanol and carbon dioxide.

In bread-making, the useful product is the carbon dioxide gas. As the yeast ferments the sugars in the dough, the carbon dioxide is trapped, making the dough rise. The small amount of ethanol evaporates during baking.

In brewing beer, the useful product is the ethanol (alcohol). The yeast ferments the sugars from the grain, producing alcohol, while the carbon dioxide is largely allowed to escape (though some gives the drink its fizz).

What markers reward: yeast fermenting sugars to make ethanol and carbon dioxide in both, with the carbon dioxide being the useful product in bread (making dough rise) and the ethanol being the useful product in beer.

Original5 marksDescribe how yoghurt is made from milk using bacteria, and state two conditions that must be controlled.
Show worked answer →

The answer should describe lactic-acid fermentation and give two conditions.

Milk is first heat-treated to kill unwanted microorganisms. A starter culture of suitable bacteria (such as lactic-acid bacteria) is added. The bacteria ferment the milk sugar (lactose), producing lactic acid.

The lactic acid lowers the pH, which makes the milk proteins clot and thicken, turning the milk into yoghurt and giving it a tangy taste.

Two conditions to control: a suitable warm temperature, so the bacteria grow and ferment well; and keeping the process free of contaminating microorganisms (for example by heat-treating the milk and using clean equipment). Time and the right starter culture are also acceptable.

What markers reward: adding a bacterial starter culture that ferments lactose into lactic acid, the acid lowering pH so the milk thickens into yoghurt, and two valid controlled conditions such as temperature and sterility.

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