Respiration · Process
Fermentation and what we use it for
Bread, yoghurt, cheese, soy sauce, vinegar, chocolate, and every alcoholic drink there has ever been. All of them are the waste products of something respiring without oxygen, and we built a civilisation on them before we knew that.
Start here
A loaf of bread is full of holes. Something put them there.
Cut a slice and count them: hundreds of bubbles, each one a pocket of gas trapped in dough that set around it in the oven. The dough went in flat and came out twice the size. Nothing was pumped into it.
What made the holes?
Carbon dioxide, breathed out by yeast. Yeast is a living single-celled fungus, and in dough it is short of oxygen, so it respires anaerobically — releasing carbon dioxide, which inflates the dough, and ethanol, which boils off in the oven. Bread is risen by a waste gas, and the alcohol leaves through the crust.
At the bench · one vessel, four dials
Set the conditions, see what you have made
nothing changed yet
The same four dials that a brewer, a baker and a yoghurt maker all set differently. Some settings give you nothing at all, which is just as informative.
Organism
Oxygen
Temperature
Sugar
no reaction
Rate 0% of maximum
Carbon dioxide
none
Ethanol
none
What you have made
Nothing, and nothing will happen now
At 80 °C the organism’s enzymes are denatured and the cells are dead. Cooling the vessel will not bring them back — this is the same permanent change you met in Enzymes in digestion, and it is why a baker uses warm water rather than hot.
no reaction
Rate 0% of maximum
Lactic acid
none
Gas produced
none — this route makes no gas
What you have made
Nothing, and nothing will happen now
At 80 °C the organism’s enzymes are denatured and the cells are dead. Cooling the vessel will not bring them back — this is the same permanent change you met in Enzymes in digestion, and it is why a baker uses warm water rather than hot.
no reaction
Rate 0% of maximum
Carbon dioxide
none
Ethanol
none
What you have made
Nothing — no fuel
A living organism with nothing to respire. Fermentation is respiration, and respiration needs a substrate: no sugar, no products, however perfect the other three dials are.
no reaction
Rate 0% of maximum
Lactic acid
none
Gas produced
none — this route makes no gas
What you have made
Nothing — no fuel
A living organism with nothing to respire. Fermentation is respiration, and respiration needs a substrate: no sugar, no products, however perfect the other three dials are.
glucose + oxygen carbon dioxide + water
Rate 100% of maximum
Carbon dioxide
100 units
Ethanol
none
What you have made
Fast growth, and no alcohol
With oxygen available, yeast respires aerobically instead — it gets far more energy per glucose, so it grows and divides quickly, and produces carbon dioxide and water rather than ethanol. This is exactly how yeast itself is manufactured, in open stirred tanks. It is also why a brewer seals the vessel: to force the organism down the route that makes the product.
contaminated
Rate 100% of maximum
Lactic acid
none you could use — the milk does not set
Gas produced
none — this route makes no gas
What you have made
Poor conditions for these bacteria
Lactic acid bacteria of this kind do their work without oxygen, and an open stirred vessel also invites in every other organism in the room. Seal it if you want yoghurt rather than a science experiment.
glucose ethanol + carbon dioxide
Rate 12% of maximum
Carbon dioxide
12 units
Ethanol
12 units
What you have made
Slow fermentation — a sourdough in the fridge
At 4 °C the yeast is alive and unharmed, and everything is happening slowly — molecules collide less often. Bakers use this deliberately: an overnight cold rise gives more time for flavour to develop while the dough inflates gently.
glucose ethanol + carbon dioxide
Rate 100% of maximum
Carbon dioxide
100 units
Ethanol
100 units
What you have made
Beer, wine, or a rising loaf
A brewer lets the gas out through an airlock and keeps the liquid; a baker keeps the gas in the dough and lets the ethanol boil off in the oven. Same reaction, opposite product wanted.
glucose lactic acid
Rate 12% of maximum
Lactic acid
12 units
Gas produced
none — this route makes no gas
What you have made
Barely anything — this is why yoghurt lives in the fridge
The bacteria are alive and almost inactive. This is exactly why a finished yoghurt is refrigerated — not to stop the bacteria being there, but to slow them almost to a halt so it does not keep souring.
glucose lactic acid
Rate 100% of maximum
Lactic acid
100 units
Gas produced
none — this route makes no gas
What you have made
Yoghurt
Sealed and warm with sugar available. Lactic acid is accumulating, the pH is falling, and the milk protein is curdling into a thick set. Left too long it becomes unpleasantly sour, so the maker chills it to stop the reaction where they want it.
Two fermentations, two products
Which organism decides what you get.
A single-celled fungus
Yeast
- glucose ethanol + carbon dioxide
Two products, one of them a gas. Which one you want decides what you build: a baker wants the gas and lets the ethanol evaporate, a brewer wants the ethanol and lets the gas escape through an airlock.
Bread, beer, wine, cider, and ethanol for fuel.
Bacteria
Lactic acid bacteria
- glucose lactic acid
One product, no gas. The acid curdles milk protein, which is what thickens yoghurt, and it drops the pH low enough that spoilage organisms cannot grow.
Yoghurt, cheese, sauerkraut, kimchi, sourdough’s sour taste, and salami.
Key fact
Fermentation is anaerobic respiration in micro-organisms. In yeast, glucose gives ethanol + carbon dioxide; in bacteria such as those in yoghurt, glucose gives lactic acid. What we call the food is the organism's waste.
Think again
“Fermenting is just food going off in a controlled way.”
It is closer to the opposite. Milk left on a windowsill spoils because whatever lands in it grows; milk turned into yoghurt is deliberately given one chosen organism and the conditions that organism likes, and the lactic acid it produces drops the pH far enough that the spoilage organisms cannot grow. That is why yoghurt keeps longer than the milk it was made from, why sauerkraut survived European winters, and why every traditional cuisine has fermented foods in it. Alcohol does the same job by different means. The pattern is worth holding onto: a fermented food is one where we picked the micro-organism first, and the waste product it makes is exactly what keeps everything else out.
“Yeast is a powder — a raising agent, like baking powder.”
Baking powder is a chemical that releases carbon dioxide when it gets wet and warm, and it works in a bowl of anything. Yeast is a living organism: a single-celled fungus, of the kind you met in Unicellular organisms, dried into a state where it survives on a shelf and revives in warm water. Everything a baker does follows from that. You give it sugar because it needs feeding; you use warm water because it is a living thing with enzymes and an optimum temperature; you do not use boiling water, because that denatures the enzymes and kills it, exactly as in Enzymes in digestion; and you wait, because it is respiring at its own pace and cannot be hurried. A packet of dried yeast contains billions of dormant cells, and a dead one raises nothing.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · The yeast equation
What does yeast produce when it respires anaerobically?
Rung 2 · The one that catches people
Dough contains yeast, which produces ethanol. Why is there almost no alcohol in the finished loaf?
Rung 3 · Explain the yoghurt
Explain how warm milk becomes yoghurt, and why the yoghurt keeps for longer in the fridge than the milk it was made from.
Rung 4 · Take it somewhere new
Design an investigation to find the temperature at which yeast ferments fastest. Say what you would measure, what you would keep the same, and what shape of result you would expect — and explain what happens above the optimum.
Key note
Fermentation is anaerobic respiration carried out by micro-organisms. Yeast, a single-celled fungus, converts glucose to ethanol and carbon dioxide — the gas raises bread and the ethanol makes beer and wine. Bacteria convert sugars to lactic acid, which is how yoghurt, cheese and sauerkraut are made and why they keep. Both need sugar, warmth and no oxygen, and both stop if the organism is killed.
Going further
The same vessel, scaled up, is one of the workhorses of modern industry. Almost all the insulin used by people with diabetes is now made by micro-organisms — bacteria or yeast — carrying an inserted human gene, grown in fermenters of several thousand litres; before that it was extracted from the pancreases of pigs and cattle, which was expensive, in short supply, and not quite human insulin. Antibiotics including penicillin are grown the same way, from a fungus. Quorn is a fungus grown in a fermenter and harvested as food. Ethanol for fuel is made by yeast on an industrial scale from sugar cane, and in Brazil a large share of cars run on it. What every one of these has in common is a tank, a chosen organism, a food supply, and careful control of temperature, pH and oxygen — the four dials on the bench above, with a bigger vessel.
Before this lesson
Connects to
At GCSE this becomes
- Industrial fermenters and their control systems, biotechnology, and genetically modified organisms making human proteins.
Where to next
Ask Mr Badmus AI
Want to work out what your dials would produce?
The bench is a simplified model: rate is shown as a percentage of the best case with one organism at a time, and real fermentation also depends on pH, on how much alcohol or acid has already built up, and on the strain being used. Yoghurt bacteria work on lactose, the sugar in milk, which the bench treats as sugar in general.
Lesson content © MrBadmusAI.