Yeast in Food Production

Yeast is a single-celled fungus. Each yeast cell is a complete living organism that can feed, respire and reproduce on its own. Because yeast cells are so small, billions of them can be used in a bowl of dough or a vat of brewing liquid.

Like all living things, yeast needs energy, which it releases by respiration. The special thing about yeast is that it can respire without oxygen. Respiration without oxygen is called anaerobic respiration, and in yeast (and some other microorganisms) it is given a special name: fermentation.

When yeast ferments, it breaks down glucose (a sugar) and releases two waste products: ethanol (a type of alcohol) and carbon dioxide (a gas). Both of these products are extremely useful to humans, which is why yeast has been used to make food and drink for thousands of years:

• The carbon dioxide is the useful product when making bread.
• The ethanol is the useful product when brewing beer and wine.

Exam tip. Examiners want two facts about yeast itself: it is a single-celled fungus, and it respires anaerobically (fermentation). Calling yeast 'bacteria' is a common and costly mistake.

The reaction that makes yeast so useful is its anaerobic respiration, or fermentation. You must be able to write the word equation for it:

glucose → ethanol + carbon dioxide

A few points to fix in your mind:

• The starting material (reactant) is glucose — a sugar that yeast feeds on.
• The products are ethanol and carbon dioxide.
• There is no oxygen in this equation, because fermentation is anaerobic (oxygen is not needed).
• The reaction also releases a small amount of energy for the yeast cell. (Anaerobic respiration releases far less energy than aerobic respiration, because the glucose is only partly broken down.)

Here is the same idea shown as a diagram:

Exam tip. Write the equation exactly: glucose → ethanol + carbon dioxide. Do not add oxygen, and do not write water — those belong to aerobic respiration. Spelling 'ethanol' correctly is expected.

Bread-making is the classic example of yeast in food production, and the part of spec 5.5 examiners ask about most.

When making bread, the baker mixes flour, water and yeast (often with a little sugar to feed the yeast) to make a dough. The dough is left in a warm place. Here is what happens:

1. The yeast ferments the sugar (glucose), respiring anaerobically.
2. This produces carbon dioxide gas and ethanol.
3. The carbon dioxide is trapped in the stretchy dough as bubbles.
4. The bubbles make the dough expand and rise, so it becomes light and full of air pockets.

The dough is then baked in a hot oven. During baking:

• The heat makes the carbon dioxide expand further, so the bread rises a little more, then the dough sets firm around the bubbles.
• The ethanol evaporates (boils off) in the oven's heat, so the finished bread does not taste of alcohol.
• The high temperature kills the yeast, stopping fermentation.

The result is a light, risen loaf full of small holes left by the carbon dioxide bubbles. Without yeast (or another raising agent), bread would be flat and dense.

The warm conditions matter: yeast respires faster when it is warm, so the dough rises more quickly. If it is too cold, the yeast works slowly; if it is too hot before baking, the yeast is killed and the dough will not rise.

Exam tip. The two facts that score the marks are: carbon dioxide makes the dough rise, and the ethanol evaporates during baking. Do not say the carbon dioxide makes it taste nice — its job is to make the dough rise.

Yeast is also used in brewing — making alcoholic drinks such as beer and wine.

In brewing, the same fermentation reaction takes place (glucose → ethanol + carbon dioxide), but this time the useful product is the ethanol (the alcohol). The carbon dioxide simply bubbles off (although in fizzy drinks like beer some of it gives the drink its fizz). So:

• In bread-making, we want the carbon dioxide (to make the dough rise).
• In brewing, we want the ethanol (the alcohol in the drink).

This is a neat point to remember: it is the same yeast carrying out the same fermentation, but a different product is collected depending on what we are making.

Conditions that help yeast work

Yeast ferments best when it has:

• A supply of sugar / glucose to respire.
• A warm temperature — yeast respires faster when warm, so fermentation speeds up. (Very high temperatures kill the yeast, and cold temperatures slow it down.)
• No oxygen (or very little) for fermentation, since it is anaerobic respiration. In a sealed brewing vessel, oxygen is soon used up and fermentation continues anaerobically.

These are the same reasons bread dough is left somewhere warm to rise.

Exam tip. If a question gives you brewing, the useful product is ethanol; if it gives you bread, the useful product is carbon dioxide. Read the question and pick the right product — examiners design questions to catch students who mix them up.