Yeast, Bread and Fermentation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 5.5 — The role of yeast in food production, including bread
• 5.6 — Practical: investigate anaerobic respiration by yeast in different conditions (required practical)

Specification section 5(a) Food production — also taught in Increasing Crop Yield, Yoghurt and the Industrial Fermenter, Fish Farming and Selective Breeding.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Yeast is a single-celled fungus, and everything on this page follows from one equation you already know. When oxygen is short, yeast respires anaerobically: glucose is broken down into ethanol and carbon dioxide, releasing a small amount of energy. Two products, and two entire industries — brewing wants the ethanol, and baking wants the carbon dioxide.

The equation is worth writing out and keeping in front of you: glucose → ethanol + carbon dioxide. Notice what is missing from it — there is no oxygen on either side, which is what makes it anaerobic, and there is no water among the products, which is how it differs from the aerobic equation. In symbols it is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The process is also called FERMENTATION.

How yeast makes bread rise

1. Flour, water, sugar and yeast are mixed into a dough and kneaded, and the kneading develops a network of stretchy gluten proteins through it.
2. The dough is left in a warm place. The yeast respires the sugars anaerobically, producing carbon dioxide and ethanol.
3. The carbon dioxide cannot escape through the stretchy dough, so it collects in thousands of small bubbles.
4. Those bubbles expand and the dough RISES — the process bakers call proving.
5. The loaf is baked. The heat kills the yeast, so no more gas is produced, and it evaporates the ethanol, so no alcohol remains in the bread.
6. The heat also makes the trapped gas expand further and sets the gluten around the bubbles, leaving the permanent holes you can see in a slice.

Both products are therefore doing something in a loaf: the carbon dioxide raises it and the ethanol contributes to the flavour before being driven off in the oven. It is worth noticing what the holes in a slice of bread actually are — they are the fossilised record of individual bubbles of gas made by a fungus respiring, held in place by a protein mesh that set around them.

Think of it like inflating balloons inside a rubber sheet

Gas produced inside something stretchy pushes outwards and the material gives, so the whole thing expands rather than the gas escaping. Heat it and the gas expands further while the rubber sets hard, and you are left with the shape permanently. Now change one thing: use a sheet that is not stretchy, and the gas simply escapes and nothing rises. That is bread made from flour with too little gluten, and it is why the kneading matters as much as the yeast.

Brewing uses the other product. Yeast is added to a sugary liquid — from barley for beer, from grapes for wine — and respires anaerobically, so ethanol accumulates. The container is sealed to keep oxygen out, because with oxygen the yeast would respire aerobically instead and produce carbon dioxide and water rather than ethanol. Fermentation eventually stops on its own when the sugar runs out, or when the ethanol concentration becomes high enough to kill the yeast that produced it.

That second stopping point is worth pausing on because it is a genuinely striking piece of biology: yeast is poisoned by its own waste product, at around 15 % ethanol. Anything stronger has to be concentrated afterwards by distillation, which is why no naturally fermented drink is spirits-strength — a limit set by the tolerance of a fungus.

Finally, the reason the practical works. You cannot see anaerobic respiration happening, but you can count the carbon dioxide leaving as bubbles, and the rate of bubbling is a measure of the rate of respiration. That is what makes yeast such a convenient organism for investigating how temperature, sugar concentration or pH affect a living process — the whole thing reduces to counting.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

glucose → ethanol + carbon dioxide

Conditions: Anaerobic respiration (fermentation) in yeast. Learn the word equation; the symbol equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Learn this definition · Fermentation

Anaerobic respiration in yeast, in which glucose is broken down into ethanol and carbon dioxide, releasing a small amount of energy.

The words this topic is built on

Yeast

A single-celled fungus, used in baking and brewing because it respires anaerobically.

Anaerobic respiration

The release of energy from glucose without oxygen. In yeast the products are ethanol and carbon dioxide.

Gluten

The network of stretchy proteins developed by kneading dough, which traps the carbon dioxide as bubbles.

Proving

Leaving dough in a warm place so that the yeast respires and the trapped gas makes it rise.

Optimum temperature

The temperature at which an enzyme works fastest — about 40 °C for the enzymes of respiration in yeast.

Statement 5.5 — how yeast makes bread rise

1. Flour, water, sugar and yeast are mixed into a dough and kneaded, developing a network of stretchy gluten proteins.
2. The dough is left in a warm place, and the yeast respires the sugars anaerobically.
3. Carbon dioxide and ethanol are produced.
4. The carbon dioxide is trapped as bubbles by the stretchy dough, and the bubbles expand so the dough rises.
5. Baking kills the yeast so gas production stops, evaporates the ethanol so no alcohol remains, and sets the gluten around the bubbles.

The two industries built on the same reaction

Required practical 5.6 — investigating anaerobic respiration by yeast

1. Mix dried yeast with glucose solution in a boiling tube, and add a thin layer of oil on the surface to keep oxygen out so that respiration is anaerobic.
2. Seal the tube with a bung and delivery tube leading into limewater, or into an inverted measuring cylinder of water to collect the gas.
3. Place the tube in a water bath at the first temperature and allow five minutes for the contents to reach it before starting to record.
4. Count the bubbles produced per minute, or measure the volume of gas collected in a fixed time.
5. Repeat at a range of temperatures — for example 20, 30, 40, 50 and 60 °C — keeping everything else the same.
6. Plot rate of gas production against temperature. The rate rises to an optimum around 40 °C and then falls sharply as the enzymes are denatured.

Variables

Independent (changed) — temperature of the water bath
Dependent (measured) — rate of carbon dioxide production — bubbles per minute, or volume of gas per minute

Sources of error

Interpreting the temperature graph — what to say about each part

• As temperature rises to about 40 °C the rate INCREASES: the enzymes and substrate have more kinetic energy, so there are more successful collisions per second
• The rate is highest at the OPTIMUM temperature, around 40 °C for yeast
• Above the optimum the rate FALLS SHARPLY: the enzymes are denatured, their active sites change shape and the substrate no longer fits
• The fall is not a reversal — denaturation is permanent, so cooling the tube again does not restore the rate
• At very low temperatures the rate is not zero, only very slow — molecules still collide, just far less often

Why the oil layer is on the surface of the tube

A. IT EXCLUDES OXYGEN, so the yeast respires anaerobically and produces ethanol and carbon dioxide — which is what the investigation is about.

B. WITHOUT IT the yeast would respire aerobically, producing carbon dioxide and water and releasing far more energy per glucose molecule, so the results would not be measuring what was intended.

Not this: Bread contains alcohol, because yeast produces ethanol as it rises.

This: Ethanol is produced during proving, but baking evaporates it. The heat also kills the yeast, which is why a baked loaf does not go on rising.

Model answer [4 marks]

Explain how yeast causes bread dough to rise. [4]

The yeast respires the sugars in the dough anaerobically, producing carbon dioxide and ethanol. The dough contains a network of stretchy gluten proteins, so the carbon dioxide cannot escape and is trapped as bubbles. The bubbles expand, which makes the dough increase in volume and rise. During baking the heat kills the yeast so gas production stops, and it sets the gluten around the bubbles, leaving holes in the finished loaf.

Model answer [2 marks]

In the yeast practical, explain why a layer of oil is added to the surface of the mixture. [2]

The oil forms a barrier that prevents oxygen from the air dissolving into the mixture. This ensures that the yeast respires anaerobically, producing ethanol and carbon dioxide, which is the process the investigation is designed to measure.

Model answer [4 marks]

Explain the shape of a graph of gas production by yeast against temperature between 20 °C and 60 °C. [4]

Between 20 °C and about 40 °C the rate of gas production increases, because the enzymes and substrate molecules have more kinetic energy and so collide successfully more often. The rate is highest at the optimum temperature of about 40 °C. Above this the rate falls sharply, because the enzymes are denatured — their active sites change shape so the substrate no longer fits. By about 60 °C the rate is close to zero, and cooling the mixture would not restore it because denaturation is permanent.

Mark-losing trap. Learn the equation: glucose → ethanol + carbon dioxide. No oxygen appears on either side.

Mark-losing trap. Baking bread drives OFF the ethanol and kills the yeast. Bread does not contain alcohol.

Mark-losing trap. The gas is trapped by the STRETCHY GLUTEN. Saying 'the dough traps it' does not explain why.

Mark-losing trap. The oil layer excludes OXYGEN, so respiration is anaerobic. It is not there to stop the gas escaping.

Mark-losing trap. Above the optimum the enzymes are DENATURED, and cooling does not undo it. Never say the yeast 'slows down'.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which two substances does yeast produce when it respires anaerobically?
2. Grade 7 · Explain [2 marks] — Explain why a baked loaf of bread contains no alcohol, even though the yeast produced ethanol while the dough was rising.
3. Grade 8 · Explain [5 marks] — Explain how yeast causes bread dough to rise, and what happens to the dough during baking. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [4 marks] — In the yeast practical, 42 cm³ of carbon dioxide is collected in 5 minutes at 30 °C, and 78 cm³ in 3 minutes at 40 °C. Calculate how many times faster the rate of respiration is at 40 °C than at 30 °C.
5. 9+ · Analyse [5 marks] — A student investigating yeast finds the rate of gas production rises from 20 °C to 40 °C and then falls almost to zero by 60 °C. They conclude that yeast prefers warm conditions and becomes sluggish when too hot, and that cooling the tube back to 40 °C would restore the rate. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who proved fermentation is done by a living thing, and the one who weighed a fermentation and balanced it. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Louis Pasteur — the one who proved fermentation is done by a living thing

This whole page assumes that fermentation is respiration carried out by a living organism, and that was fiercely disputed until Pasteur settled it. The leading chemists of his day held that fermentation was an ordinary chemical decomposition that happened to occur in the presence of a residue; Pasteur showed that it required living yeast cells, that different microbes gave different products, and that the process was what happens when they are deprived of air — 'life without oxygen', as he called it. He also worked out why wine spoiled and invented gentle heating to prevent it.

• “How did you show that fermentation needs a living organism?”
• “What did the chemists of your day believe instead?”
• “What did you mean by calling fermentation life without air?”
• “How did you work out why wine was spoiling?”
• “Why did so many people resist the idea that microbes were responsible?”

Antoine Lavoisier — the one who weighed a fermentation and balanced it

Lavoisier was the first to treat fermentation as a chemical equation rather than a mysterious transformation. He measured the sugar going in and the alcohol and carbon dioxide coming out, and showed that the masses balanced — that nothing was created or destroyed, only rearranged. He was wrong about the details, having no idea that a living organism was responsible, but the method he brought is the reason the equation on this page can be written at all.

• “How did you measure what fermentation produced?”
• “Why does it matter that the masses balance?”
• “What did you think was causing the change?”
• “What could your measurements not tell you?”

Carry on through the course

• Previous lesson: Increasing Crop Yield
• Next lesson: Yoghurt and the Industrial Fermenter
• Increasing Crop Yield — Everything a grower does is one of two things — remove whatever is limiting the crop, or remove whatever is competing with it.
• Yoghurt and the Industrial Fermenter — Bacteria turned into a manufacturing process — and the vessel built to give them the best conditions they will ever have.
• Fish Farming and Selective Breeding — Growing food by controlling every loss of energy — and by doing deliberately what the environment does by accident.
• Genetic Engineering — Cutting a gene out of one organism and making another organism read it — and the reason that works at all.
• GM Insulin and GM Crops — The same technique used twice — once to brew a human hormone in a tank, and once to change what a field can grow.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification