Microbes at work: fermentation, industrial enzymes and the bioreactor
By the end of this lesson you will be able to
- Define biotechnology and outline how it uses cellular and biomolecular processes to make products and solve problems.
- Explain how yeast makes bread rise and how bacteria turn milk into yoghurt, using word and symbol equations.
- Describe the features of an industrial fermenter and explain the purpose of each, and interpret a growth curve and a product curve.
- Outline how microorganisms make fuels (bioethanol, biogas), chemicals (citric acid, penicillin) and treat wastes (sewage).
- Explain why proteases, lipases and pectinase are used in industry, and calculate yields and percentage changes from fermentation data.
You met this before
- Aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy released). Yeast without oxygen: glucose → ethanol + carbon dioxide (+ a little energy). Muscles and lactic acid bacteria without oxygen: glucose → lactic acid (+ a little energy).
- Yeast is a single-celled fungus (eukaryote) that reproduces by budding; bacteria are prokaryotes that divide by binary fission, as often as every 20 minutes in good conditions.
- Enzymes are proteins that speed up reactions; each has an optimum temperature and pH, and is denatured when its active site loses its shape (Unit 3).
- A population of microbes in a closed vessel grows in stages: lag, exponential (log), stationary, death — because nutrients are used up and wastes build up.
Write the symbol equation for fermentation of glucose by yeast, and state the two products bakers and brewers each want.
Reveal
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
Bakers want the carbon dioxide (it inflates the dough; the ethanol evaporates in the oven). Brewers want the ethanol (the carbon dioxide gives the fizz, or escapes).
Revise it in full: Year 2 · Cell functions · Lesson 1 — Cells at work: where the energy comes from · Year 2 · Cell functions · Lesson 5 — Cells in systems and the frontier: from Hooke’s cork to meat grown in a tank · Year 2 · Infectious diseases · Lesson 4 — Fighting disease: soap, disinfectant, antiseptic, antibiotic, antiviral
The puzzle
In 1942, the whole United States had enough penicillin to treat fewer than a hundred patients. By D-Day in June 1944, American factories were making about 2.3 million doses for the Allied invasion alone.
Nothing about the drug had changed. It was still made by a mould, Penicillium, that Alexander Fleming had noticed on a plate in 1928. What changed was the way the mould was grown.
Chemists had been growing it on the surface of liquid in thousands of milk bottles. Engineers moved it into steel tanks as big as a house, stirred, bubbled with sterile air and cooled by water jackets — and chose a better strain, found on a mouldy melon in a market in Peoria, Illinois.
How can the same organism make a thousand times more product just because of the container it lives in?
A mould growing on the surface of still liquid in a bottle is compared with the same mould stirred and bubbled with air in a tank. Predict three things that limit the mould in the bottle and that the tank removes.
Compare your answer
Oxygen: in still liquid only the surface gets oxygen, and Penicillium respires aerobically; bubbling sterile air through a stirred tank supplies every cell.
Nutrients: without mixing, cells near the food use it up and the rest starve; stirring keeps the sugar evenly spread and the wastes diluted.
Temperature: respiration releases heat; in a large still mass the middle overheats. A cooling jacket and stirring keep it at the optimum for the mould’s enzymes. The tank also allows sterile, sealed conditions, so no other microbe competes.
The science — structured notes
1.1 What biotechnology is
Biotechnology is the use of living organisms, their cells, or molecules taken from them, to make products or solve problems. The IB puts it in one line: biotechnology uses cellular and biomolecular processes to solve problems and create products.
“Cellular” means using whole cells — yeast fermenting sugar, bacteria digesting sewage. “Biomolecular” means using molecules made by cells — an enzyme in washing powder, a DNA polymerase in a test tube.
The word was coined in 1919 by the Hungarian engineer Károly Ereky, who fattened pigs on sugar beet on an industrial scale, but the practice is as old as farming. People brewed beer and baked leavened bread in Mesopotamia and Egypt more than 5 000 years ago without knowing that microbes existed.
The difference between old and new biotechnology is control. In this unit you move from using organisms as they are (Lesson 1), to choosing their genes (Lessons 2 and 5), to copying DNA and cells (Lessons 3 and 4).
| Area | What it does | Examples in this unit |
|---|---|---|
| Food | Microbes change raw food into new products | Bread, yoghurt, cheese, sorghum beer, mageu, mycoprotein |
| Fuels and chemicals | Microbes convert sugar or waste into useful substances | Bioethanol, biogas, citric acid, penicillin |
| Environment | Microbes break down wastes | Sewage treatment, composting, oil-spill clean-up |
| Enzymes | Purified enzymes speed up an industrial reaction | Proteases and lipases in detergents, pectinase in fruit juice |
| Genetics and medicine | Genes are changed, copied or read | GM crops, insulin from bacteria, PCR, cloning, CRISPR, mRNA vaccines |
“Biotechnology began in the 1970s, when scientists learned to change genes.”
Reveal answer
False. Genetic engineering (1973 onwards) is the newest branch. Brewing, baking, cheese-making and fermenting vegetables are biotechnology thousands of years old: they use living microbes to make a product. What changed in the 1970s was that people could alter the organism’s DNA directly instead of simply choosing the best strain.
1.2 Food from microbes: bread and yoghurt
Bread. Flour, water, salt and baker’s yeast (Saccharomyces cerevisiae) are kneaded into dough. Amylase enzymes in the flour break starch into sugars, and the yeast respires them.
At first there is air in the dough, so respiration is aerobic; the oxygen is soon used up, and the yeast ferments. Both processes release carbon dioxide. Kneading has built a stretchy network of the protein gluten, which traps the gas in bubbles, so the dough rises.
In the oven the heat kills the yeast, the bubbles expand, the ethanol evaporates, and the gluten and starch set around the bubbles. The holes in a slice of bread are the fossil of fermentation.
Yoghurt. Milk is heated to about 85–90 °C to kill other bacteria and change the milk proteins, cooled to about 43 °C, and inoculated with two lactic acid bacteria, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus.
Over 4–6 hours they ferment the milk sugar lactose to lactic acid. The pH falls from about 6.7 to about 4.5. At that pH the milk protein casein loses its charge and coagulates into a soft gel, which is why yoghurt is thick. The acid also stops most spoilage bacteria growing, so yoghurt keeps for weeks.
During yoghurt-making the pH of the milk falls from 6.7 to 4.5. The pH scale is logarithmic: each fall of 1 unit is a tenfold rise in the concentration of hydrogen ions. Calculate how many times more acidic (in H⁺ concentration) the yoghurt is than the milk.
Fall in pH = 6.7 − 4.5 = 2.2
Factor = 10^2.2
Factor = 158, about 160 times
So the bacteria have raised the hydrogen-ion concentration about 160-fold. That is why casein coagulates, and why the yoghurt is hostile to most spoilage bacteria.
A baker’s dough rises well at 30 °C, slowly at 15 °C, and not at all when the yeast was dissolved in water at 70 °C. Explain all three observations.
Reveal answer
At 30 °C the enzymes of respiration are near their optimum, the yeast respires fast and releases carbon dioxide quickly, which the gluten traps, so the dough rises well.
At 15 °C the molecules have less kinetic energy, so enzyme–substrate collisions are less frequent and respiration is slower: less CO₂ per minute, slow rising.
At 70 °C the yeast’s enzymes (and membranes) are denatured — the active sites lose their shape — so the cells are killed and release no CO₂ at all; the dough does not rise however long it waits.
1.3 Local ferments and the chemistry they share
Southern Africa has a long fermentation tradition. Madila, Botswana’s sour milk, is milk fermented by lactic acid bacteria in a sack or calabash, the whey drained off. Mageu (mahewu) is thin maize porridge soured by lactic acid bacteria into a drink. Bojalwa, sorghum beer, uses both: lactic souring, then yeast fermentation of the malted grain’s sugars.
Around the world, injera in Ethiopia, kimchi in Korea, soy sauce, cheese and chocolate all depend on fermenting microbes.
| Product | Organism | Sugar used | Main product | Why it matters |
|---|---|---|---|---|
| Bread | Yeast (fungus) | Glucose, maltose from starch | Carbon dioxide (+ ethanol) | Gas leavens dough |
| Beer, sorghum beer, wine | Yeast | Maltose (malted grain), glucose, fructose | Ethanol (+ CO₂) | Alcohol, flavour; preserves the drink |
| Yoghurt, madila | Lactic acid bacteria | Lactose | Lactic acid | Thickens; low pH preserves |
| Mageu, sauerkraut | Lactic acid bacteria | Glucose from maize or cabbage | Lactic acid | Sour taste; preserves |
| Cheese | Lactic acid bacteria + the enzyme chymosin | Lactose | Lactic acid; chymosin clots casein | Turns perishable milk into a food that keeps for months |
“In yoghurt, yeast ferments glucose into lactic acid and oxygen. The acid kills all the bacteria, including the yoghurt bacteria, so yoghurt is sterile.” Find three errors.
Reveal answer
(1) Yoghurt is made by lactic acid bacteria, not yeast. (2) The sugar in milk is lactose, and fermentation releases no oxygen — the only product is lactic acid.
(3) The acid does not kill everything: the yoghurt bacteria tolerate pH 4.5 and stay alive in the pot (that is what “live yoghurt” means). The low pH stops most spoilage bacteria from multiplying, which is why it keeps; it is not sterile.
1.4 The industrial fermenter
A fermenter (or bioreactor) is a sealed vessel in which microorganisms are grown under controlled conditions to make a product. Industrial fermenters hold from a few thousand to several hundred thousand litres. Every feature exists to keep the microbes at their best and to keep every other microbe out.
| Feature | What it controls | Why it matters |
|---|---|---|
| Steam sterilisation of vessel, medium and air filters before use | Contamination | A single foreign bacterium doubling every 20 minutes outgrows the culture and can spoil a batch worth millions |
| Water jacket or cooling coils, temperature probe | Temperature | Respiration releases heat; enzymes work best at an optimum and denature above it |
| Stirrer (impeller) | Mixing | Keeps cells in suspension, spreads nutrients and heat, breaks air into small bubbles with a large surface area |
| Sparger and oxygen probe | Dissolved oxygen | Aerobic microbes (Penicillium, many bacteria) need oxygen for aerobic respiration and growth |
| pH probe, acid and alkali pumps | pH | Microbes release acids (or ammonia) as they grow; pH shifts would slow or stop enzymes |
| Nutrient inlet | Food supply | Sugar, a nitrogen source (ammonium salts, corn-steep liquor), minerals; in fed-batch it is added gradually |
Two ways of running one. In a batch culture everything is added at the start, the microbes grow, and the vessel is emptied when the product peaks — used for most beer. Penicillin is made by fed-batch culture: a batch to which sugar is added gradually (section 1.5).
In a continuous culture fresh medium flows in as broth is drawn off, holding the microbes in exponential growth for weeks — used for mycoprotein, the fungal protein sold as Quorn since 1985.
1.5 Reading a fermenter: growth curves and product curves
In a batch fermenter the number of cells follows the same four phases as any microbe population in a closed space. The product may follow the cells, or come later. A primary metabolite such as ethanol or lactic acid is made by respiration while the cells grow, so its curve rises with the population.
A secondary metabolite such as penicillin is not needed for growth. The mould makes it mostly once growth slows and nutrients run short, so its curve rises during the stationary phase. That is why a penicillin fermenter is fed sugar slowly — to hold the mould in a long stationary phase.
| Phase | What the cells are doing | Why |
|---|---|---|
| Lag | Few divisions; cells make enzymes for the new medium | Adjusting to new food, temperature, pH |
| Exponential (log) | Number doubles every generation time | Plenty of food and space, few wastes |
| Stationary | Births equal deaths — no net growth | Nutrients running short, wastes (acid, ethanol) building up |
| Death | Deaths exceed divisions | Food exhausted, toxic waste concentrations |
A 2.0 dm³ batch contains 100 g/dm³ of glucose. When the sugar is used up, the ethanol concentration is 46 g/dm³. The equation C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ shows that 180 g of glucose can give at most 92 g of ethanol. Calculate the theoretical mass of ethanol, the actual mass, and the percentage yield.
Glucose in the vessel = 100 g/dm³ × 2.0 dm³ = 200 g
Theoretical ethanol = 200 g × 92 ÷ 180
Theoretical ethanol = 102 g
Actual ethanol = 46 g/dm³ × 2.0 dm³ = 92 g
Percentage yield = 92 g ÷ 102 g × 100
Percentage yield = 90 %
The missing 10 % went into making new yeast cells and into other products (glycerol, a little acid). No fermentation reaches 100 %, because the organism must use some of the sugar to stay alive and grow.
In the November 2021 Biology exam, a graph showed bacteria growing in a test tube of nutrient liquid in three stages. Explain stage 3, in which the population stays constant, and state what the markscheme refused.
Reveal answer
In stage 3 there is no net population growth because as many bacteria are dying as are being produced by division [1], because limiting factors are present — nutrients are running out and waste products are building up [1].
The markscheme did not accept “the bacteria stop reproducing”: they are still dividing; it is the balance of births and deaths that holds the number steady.
A penicillin fermenter is run as a batch: all the sugar is added at the start. A second fermenter is fed-batch: the sugar is dripped in slowly. Predict which gives more penicillin, and explain using the idea of a secondary metabolite.
Reveal answer
The fed-batch fermenter. Penicillin is a secondary metabolite, made mostly when the mould’s growth slows because a nutrient is short. With all the sugar at the start, the mould grows fast, then the sugar runs out and the culture dies back after a short stationary phase.
Dripping the sugar in keeps the mould alive but growing slowly for days — a long stationary phase — so it goes on making penicillin much longer.
1.6 Fuels, chemicals and cleaning up wastes
Bioethanol. Yeast fermentation of sugarcane juice (in Brazil) or of glucose from maize starch (in the United States) produces ethanol, which is distilled and blended with petrol. The carbon in it was taken from the air by the crop a few months earlier, so burning it adds far less fossil carbon than petrol does. Growing, fertilising and processing the crop still uses energy, and the land could grow food instead.
Biogas. In a sealed anaerobic digester, bacteria and archaea break down manure, sewage sludge or food waste in stages; the last group, the methanogens, release methane. Biogas is typically 50–75 % methane, the rest mostly carbon dioxide. It is burned for cooking, heating or electricity. The digested slurry is a fertiliser.
Chemicals. Almost all the world’s citric acid, used in soft drinks and foods, is made by the mould Aspergillus niger in fermenters. Antibiotics such as penicillin, the amino acid glutamate, and vitamin B12 are also fermentation products.
Treating wastes. In the activated sludge process, invented in Manchester in 1914, sewage is bubbled with air in tanks full of aerobic bacteria and protists. They respire the dissolved organic matter to carbon dioxide and water and clump into flocs that settle out.
The sludge is then digested anaerobically, making biogas. Microbes are also used to break down oil spills (bioremediation) and to compost garden and food waste.
A dairy farm digester produces 400 m³ of biogas a day, 60 % of which is methane. Burning 1 m³ of methane releases 36 MJ. A household cooker uses 10 MJ a day. Calculate how many households the digester could supply with cooking energy.
Volume of methane = 0.60 × 400 m³ = 240 m³ per day
Energy = 240 m³ × 36 MJ/m³
Energy = 8 640 MJ per day
Households = 8 640 MJ ÷ 10 MJ
Households = 864 (about 860)
This assumes the gas burns completely and none leaks. Methane is a powerful greenhouse gas, so leaks cancel much of the benefit — a point worth making in any evaluation of biogas.
An activated sludge tank’s air pumps fail for two days. Predict what happens to the bacteria, to the smell, and to the quality of the water leaving the works.
Reveal answer
Dissolved oxygen falls to zero; the aerobic bacteria can no longer respire aerobically, so they stop breaking down organic matter efficiently and many die.
Anaerobic bacteria take over and release foul-smelling gases such as hydrogen sulfide. Organic matter passes through untreated, so the water leaving the works has a high biochemical oxygen demand; in a river it would be decomposed by river bacteria that use up the river’s oxygen, killing fish.
1.7 Enzymes as tools: detergents and fruit juice
Biotechnology also uses molecules without the cells. Industrial enzymes are made by growing a bacterium or fungus in a fermenter, then purifying the enzyme it secretes. You met detergent enzymes, lactase and amylase in Unit 3; here are the ones the IB names, and why.
| Enzyme | Substrate → product | Use | Why an enzyme |
|---|---|---|---|
| Protease | Protein → amino acids and short peptides | Biological detergents (blood, egg, grass stains) | Insoluble protein stains become small soluble molecules that wash away |
| Lipase | Fat → fatty acids and glycerol | Biological detergents (grease, butter, lipstick) | Fats are broken down at low temperature |
| Pectinase | Pectin → smaller soluble sugars | Fruit-juice extraction and clarification | Releases more juice from crushed fruit and removes the haze |
| Chymosin | Clots casein in milk | Cheese-making | Now mostly made by GM microbes instead of calf stomachs (Lesson 2) |
Why biological detergents save energy. A non-biological powder removes protein and fat stains mainly by heat and scrubbing, and works best at 60 °C or more. Detergent enzymes, from bacteria such as Bacillus, work at 30–40 °C, so a wash at 30 °C cleans as well. Heating less water saves electricity. The enzymes are also made to survive the high pH of detergent.
Why pectinase makes more juice. Plant cells are glued together by pectin in the middle lamella between their walls, and pectin is also in the walls. When fruit is crushed, pectin holds much of the juice inside cells and makes the juice cloudy and thick.
Pectinase, usually from Aspergillus, hydrolyses pectin into smaller soluble molecules. The cells separate and their walls weaken, so more juice runs out when the pulp is pressed, and the juice becomes clear.
100 g samples of apple pulp were left for 20 minutes at 40 °C, then filtered for 10 minutes. Without enzyme, 14.0 cm³ of juice was collected; with 2 cm³ of pectinase, 23.8 cm³. Calculate the percentage increase.
Increase = 23.8 cm³ − 14.0 cm³ = 9.8 cm³
Percentage increase = 9.8 cm³ ÷ 14.0 cm³ × 100
Percentage increase = 70 %
Always divide by the original value (the control without enzyme), not by the new one: 9.8 ÷ 23.8 would give a wrong 41 %.
A juice factory processes 20 tonnes of apples a day. Without pectinase it extracts 0.60 dm³ of juice per kilogram; with pectinase, 0.72 dm³/kg. The enzyme costs 18 pula per tonne of apples; juice sells to the bottler for 4.50 pula per dm³.
Calculate the extra daily income from using the enzyme after paying for it, and suggest one scientific reason the factory might still choose a lower dose than the one giving the most juice.
Reveal answer
Apples = 20 t = 20 000 kg
Extra juice = 20 000 kg × (0.72 − 0.60) dm³/kg
Extra juice = 2 400 dm³
Extra income = 2 400 dm³ × 4.50 pula/dm³
Extra income = 10 800 pula
Enzyme cost = 20 t × 18 pula/t
Enzyme cost = 360 pula
Net gain = 10 800 pula − 360 pula
Net gain = 10 440 pula per day
A lower dose may be chosen because the yield curve of an enzyme reaction levels off (a plateau): once almost all the pectin is hydrolysed, doubling the enzyme releases very little more juice but doubles its cost. The best dose is where the extra juice from the last unit of enzyme just pays for it.
Try it — the fermenter: ethanol against time
In this lab a 2-litre stirred fermenter holds 100 g/dm³ glucose and 5 g of yeast. You set the time since the yeast was added, and the ethanol sensor reads the concentration in the broth. You can also change the water-jacket temperature and whether air is bubbled through — the two things a brewer or a bioethanol plant must decide.
Mean readings at 30 °C, sealed: 0 h → 0.0; 8 h → 3.0; 16 h → 17.0; 24 h → 37.5; 32 h → 44.6; 40 h → 45.8; 48 h → 46.0 g/dm³. Calculate the mean rate between 16 and 24 h, the final yield as a percentage of the theoretical 51 g/dm³, and describe the shape.
Reveal answer
Rate = (37.5 − 17.0) g/dm³ ÷ (24 − 16) h
Rate = 20.5 g/dm³ ÷ 8 h
Rate = 2.6 g/dm³ per h
Yield = 46.0 g/dm³ ÷ 51 g/dm³ × 100
Yield = 90 %
Shape: a slow start (only 3.0 g/dm³ by 8 h — the lag while the yeast population builds), the steepest rise between about 16 and 24 h, and a plateau from about 36 h at 46 g/dm³, when the glucose is used up. It is not linear and not proportional: equal 8-hour steps give rises of 3.0, 14.0, 20.5, 7.1, 1.2 and 0.2 g/dm³.
Common mistakes to avoid
- “Yeast produces air / oxygen” — it releases carbon dioxide, by aerobic respiration and by fermentation.
- Ethanol from humans or yoghurt bacteria — they make lactic acid; only yeast (and a few bacteria) make ethanol.
- “The fermenter is heated to keep the microbes warm” — industrial fermenters are mostly cooled; respiration releases heat.
- “In the stationary phase the bacteria stop reproducing” — they reproduce, but deaths equal divisions.
- Forgetting sterility — the first job of a fermenter is to keep other microbes out.
- Percentage increase divided by the new value — always divide the change by the original (control) value.
- “Biological detergents kill germs” — the enzymes digest stains; they do not sterilise.
Study smart: how this is examined
Study routine. Today: draw the fermenter from memory and label six features with one reason each; write both fermentation equations. In two days: sketch a growth curve with a product curve for ethanol and one for penicillin, and explain the difference. In a week: complete the fermenter lab as a full investigation, take the quiz, and time yourself on the stretch question in 1.7.
How we found out — from a living ferment to a steel tank
For thousands of years brewers knew that fermentation worked but not what did it. In the 1830s three scientists — Charles Cagniard de la Tour, Theodor Schwann and Friedrich Kützing — each saw yeast under the microscope and proposed that it was alive and that it budded. The leading chemists, Justus von Liebig among them, mocked the idea: fermentation, they said, was a chemical breakdown.
Louis Pasteur settled it in a series of studies from 1857. He showed that lactic fermentation and alcoholic fermentation were each caused by a particular living microbe, that sterile broth did not ferment, and that yeast grew without oxygen by fermenting.
Then a twist. In 1897 Eduard Buchner ground yeast with sand, squeezed out a juice with no living cells in it, and found that the juice still fermented sugar to ethanol and carbon dioxide. Fermentation was done by molecules the cell made — “zymase”, a mixture of enzymes. Buchner won the 1907 Nobel Prize in Chemistry; both sides had been half right.
The steel tank came out of war. Alexander Fleming noticed in 1928 that a Penicillium mould killed the bacteria around it on a plate. Howard Florey, Ernst Chain and Norman Heatley at Oxford purified the drug and, in 1941, treated the first patients — with so little penicillin that they recovered it from one patient’s urine.
In 1941 Florey and Heatley took the work to the United States Department of Agriculture laboratory at Peoria. There corn-steep liquor was found to boost yields, and in 1943 a strain from a mouldy cantaloupe melon in a local market gave far more. Pharmaceutical companies, led by Pfizer, then scaled up deep-tank, aerated, stirred fermentation. Fleming, Florey and Chain shared the 1945 Nobel Prize.
Explain why Buchner’s cell-free juice changed the argument between Pasteur and Liebig, and why neither of them was entirely wrong.
Reveal answer
Pasteur held that fermentation needed living cells; Liebig that it was ordinary chemistry. Buchner’s juice contained no living cells but still fermented sugar, so fermentation is chemistry — but chemistry done by enzymes that only living cells make.
Pasteur was right that yeast causes fermentation (no yeast, no zymase); Liebig was right that the reaction itself needs no life once the molecules exist. The result founded biochemistry, and it is the basis of every enzyme product in this lesson.
Where it is used now
- Food in southern Africa. Madila, mageu and sorghum beer are made commercially as well as at home; South Africa’s commercial mageu and sorghum-beer industries use the same lactic and yeast fermentations, in steel tanks with temperature control.
- Fuel. Brazil runs much of its car fleet on sugarcane ethanol blends, and the United States blends maize ethanol into most of its petrol. Several African countries, including Malawi and Zimbabwe, have long blended molasses ethanol into petrol.
- Waste into energy. Small biogas digesters fed with cattle manure supply cooking gas to farm households in Kenya, Ethiopia, India and elsewhere; large digesters at sewage works generate electricity.
- Medicines. Antibiotics, human insulin, vaccines and many enzymes for diagnostic tests come out of fermenters — the same stirred tank, now with genetically modified organisms (Lesson 2).
- Laundry. Most laundry detergents in many countries contain several enzymes (proteases, lipases, amylases), allowing effective washes at 30 °C.
Science now — and the scientist you need to be
What is happening in 2026
The fastest-growing corner of this lesson is precision fermentation: microbes given a gene so that, in a fermenter, they make a single animal or plant protein. Whey proteins made this way by fungi have been sold in ice creams and protein drinks in the United States since about 2019. Egg-white proteins and the fats and flavours of meat are in development.
The global enzyme industry is dominated by a few companies; the largest, Novonesis, was formed in 2024 by the merger of Novozymes and Chr. Hansen, the maker of yoghurt and cheese cultures.
Two trends to watch over the next decade. The first is AI-designed enzymes: since the 2024 Nobel Prize in Chemistry for protein structure prediction and design, companies and universities have been designing enzymes that do not exist in nature, for example to break down plastics such as PET.
The second is the circular bioeconomy: turning waste — food scraps, crop stalks, brewery grain — into fuels, feed and chemicals by fermentation. Careers here include bioprocess engineer, fermentation scientist, food technologist and wastewater engineer.
The mindshift
What you need to be able to do
Name the organism, the substrate and the product for any fermented food; explain each feature of a fermenter; give a reason for every phase of a growth curve; calculate yields and percentage changes; and argue the benefits and costs of fuels and foods made by microbes.
See an example
For “clear apple juice” the assistant correctly named pectinase from Aspergillus and said it breaks down pectin to increase yield and clarity. It then claimed the enzyme is “removed by boiling” — partly right: juice is pasteurised, which denatures the enzyme, but the enzyme protein is not removed, it is inactivated. A good check on its wording.
Key terms
| Biotechnology | The use of living organisms, their cells or their molecules to make products or solve problems. |
| Fermentation | Anaerobic breakdown of sugar by microbes (ethanol and CO₂ by yeast; lactic acid by bacteria). In industry, any growth of microbes in a fermenter. |
| Fermenter (bioreactor) | A sealed, sterile vessel in which microbes are grown under controlled temperature, pH, oxygen and nutrients. |
| Batch / continuous culture | All nutrients at the start and harvest at the end / nutrients added and broth removed continuously. |
| Lag, exponential, stationary, death phase | The stages of microbial growth in a closed culture. |
| Primary / secondary metabolite | Made during growth (ethanol, lactic acid) / made mainly after growth slows (penicillin). |
| Anaerobic digester, biogas | Sealed tank where microbes break down organic waste; the gas, 50–75 % methane. |
| Activated sludge | Sewage treatment in which aerobic microbes, supplied with air, break down dissolved organic matter. |
| Protease, lipase | Enzymes that digest proteins and fats; used in biological detergents. |
| Pectinase | Enzyme that breaks down pectin, increasing juice yield and clarity. |
Think like a scientist
Criterion C A bioethanol pilot plant compares two 2.0 dm³ fermenters at 30 °C, each with 100 g/dm³ glucose and 5 g of yeast. Fermenter S is sealed; fermenter A has air bubbled through it throughout. Ethanol concentration (g/dm³) was measured in three samples at each time.
| Time / h | S: trial 1 | S: trial 2 | S: trial 3 | A: trial 1 | A: trial 2 | A: trial 3 |
|---|---|---|---|---|---|---|
| 0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
| 8 | 2.9 | 3.1 | 3.0 | 4.2 | 4.0 | 4.1 |
| 16 | 16.6 | 17.3 | 17.1 | 17.5 | 18.2 | 17.7 |
| 24 | 37.8 | 37.1 | 37.6 | 29.3 | 28.8 | 29.2 |
| 32 | 44.3 | 45.0 | 44.5 | 31.5 | 25.1 | 31.9 |
| 48 | 46.2 | 45.7 | 46.1 | 32.4 | 32.0 | 32.2 |
(a) Identify the anomalous result and suggest a specific cause. [2]
(b) Calculate the mean for each fermenter at each time, to 1 decimal place, excluding the anomaly. [3]
(c) Describe how you would plot both sets of results on one graph. [2]
(d) Compare the two curves, quoting values, and calculate each final concentration as a percentage of the theoretical maximum of 51 g/dm³. [4]
(e) Explain the difference between the plateaus using respiration and growth. [3]
(f) The plant manager concludes: “Air makes the yeast work faster, so we should aerate.” Evaluate this conclusion. [2]
Model answer
(a) A, trial 2 at 32 h: 25.1 g/dm³, lower than the 24 h value and about 6.6 below the mean of the other two trials (31.5, 31.9) [1].
Cause: that sample was taken from a poorly mixed region near the air inlet, or was diluted by condensate in the sampling port, or the sensor was not rinsed between readings — anything that lowered only that one reading [1].
(b) S: 0.0, 3.0, 17.0, 37.5, 44.6, 46.0 g/dm³ [1].
A: 0.0, 4.1, 17.8, 29.1, 31.7 (mean of 31.5 and 31.9 only), 32.2 g/dm³ [1 for excluding the anomaly, 1 for the rest correct].
(c) Time / h on the horizontal axis from 0 to 48; mean ethanol concentration / g dm⁻³ on the vertical axis from 0 to 50; even scales using more than half the grid [1]. Two sets of points with different symbols and a key; a smooth curve of best fit through each, not joined dot-to-dot [1].
(d) The aerated fermenter starts faster (4.1 against 3.0 g/dm³ at 8 h; 17.8 against 17.0 at 16 h) [1], but levels off earlier and much lower: 29.1 against 37.5 at 24 h and 32.2 against 46.0 at 48 h [1].
S: percentage = 46.0 ÷ 51 × 100
S: percentage = 90 % [1]
A: percentage = 32.2 ÷ 51 × 100
A: percentage = 63 % [1]
(e) With oxygen, the yeast respires part of the glucose aerobically, releasing much more energy per glucose, which it uses to grow and divide — hence more cells and the faster early rise [1].
Glucose respired aerobically becomes carbon dioxide and water, and glucose built into new cells becomes biomass; neither becomes ethanol [1]. Ethanol is still made because this yeast ferments at high sugar even with oxygen, but less of the fixed 200 g of glucose is left for it, so the plateau is lower [1].
(f) Partly supported: aeration does make the early rise faster (more yeast) [1]. But the aim of a bioethanol plant is ethanol, and aeration cuts the final yield from 90 % to 63 % of theoretical; the better decision is to aerate briefly at the start to grow the yeast, then seal — or not to aerate at all [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Define biotechnology.
Reveal answer
The use of living organisms, or their cells or molecules (such as enzymes), to make products or solve problems [1].
Explain how yeast causes bread dough to rise.
Reveal answer
Yeast cells respire the sugars in the dough (aerobically at first, then anaerobically) [1]; respiration releases carbon dioxide gas [1]; the gas is trapped in bubbles by the stretchy gluten network, so the dough expands [1].
Select the correct words: “In yoghurt-making, bacteria ferment (lactose / sucrose) to (lactic acid / ethanol), which (raises / lowers) the pH and makes the milk proteins (dissolve / coagulate).”
Reveal answer
lactose; lactic acid; lowers; coagulate. All four [2]; two or three [1].
State the purpose of four features of an industrial fermenter.
Reveal answer
Any four [1 each]: water jacket — removes heat released by respiration to keep the optimum temperature; stirrer — mixes cells, nutrients and oxygen evenly; sparger — supplies sterile air for aerobic respiration; pH probe with acid/alkali — keeps the optimum pH for enzymes; steam sterilisation and filters — prevent contamination by other microbes; nutrient inlet — supplies sugar and nitrogen source.
Explain why a biological detergent allows effective washing at 30 °C.
Reveal answer
It contains proteases and lipases that hydrolyse protein and fat stains into small soluble molecules that wash away [1]; these enzymes work at their optimum around 30–40 °C, so the water need not be heated to 60 °C, saving energy [1].
Outline how sewage is treated by microorganisms in the activated sludge process, and how biogas can be obtained from the sludge.
Reveal answer
Air is bubbled through the sewage so that aerobic bacteria and protists respire the dissolved organic matter to carbon dioxide and water [1]; the microbes clump and settle out as sludge [1]; the sludge is digested by anaerobic bacteria in a sealed tank, where methanogens release methane — biogas — which can be burned [1].
7. A yeast strain can tolerate at most 12 % ethanol by volume. Ethanol has a density of 0.79 g/cm³. Calculate the maximum glucose concentration (g/dm³) worth adding to a sealed fermenter, if the fermentation achieves 90 % of the theoretical 0.511 g of ethanol per gram of glucose. Explain what would happen to any glucose above this.
Reveal answer
Ethanol volume in 1 dm³ = 0.12 × 1 000 cm³
Ethanol volume = 120 cm³
Ethanol mass = 120 cm³ × 0.79 g/cm³
Ethanol mass = 94.8 g
Ethanol per gram of glucose = 0.90 × 0.511 g
Ethanol per gram of glucose = 0.460 g
Glucose = 94.8 g ÷ 0.460
Glucose = 206 g per dm³, about 210 g/dm³ [3]
Above this the ethanol reaches the concentration that denatures the yeast’s enzymes and damages its membranes, so fermentation stops and the extra glucose is left unfermented — wasted, and making the product sweet [1].
8. Bioethanol is often described as “carbon neutral”. Evaluate this claim, using photosynthesis, respiration and the fermenter in your answer.
Reveal answer
For: the carbon in the ethanol and in the CO₂ released during fermentation was taken from the air by the crop’s photosynthesis a few months earlier, so burning it returns carbon recently removed rather than adding fossil carbon [1].
Against: fossil fuel is used to make fertiliser, run tractors, transport the crop and heat the distillation that separates ethanol from the broth; fertiliser releases nitrous oxide; clearing land for the crop can release stored carbon [1].
Judgement: it is lower-carbon than petrol, not neutral; how much lower depends on the crop and the energy source of the plant [1]. Sugarcane ethanol whose distillery is powered by burning the crop residue (bagasse) saves far more than maize ethanol heated by fossil fuel [1].
Summary
- Biotechnology uses organisms, cells or their molecules to make products or solve problems; food fermentation is thousands of years old.
- Yeast respires sugar, releasing CO₂ that gluten traps (bread) and ethanol (brewing). Lactic acid bacteria ferment lactose to lactic acid; the low pH coagulates casein (yoghurt) and preserves the food.
- Fermenters are sterile and control temperature (cooling jacket), mixing (stirrer), oxygen (sparger), pH and nutrients. Growth has lag, exponential, stationary and death phases; primary products follow growth, secondary products (penicillin) come later.
- Microbes make fuels (bioethanol, biogas), chemicals (citric acid, antibiotics) and treat wastes (activated sludge, anaerobic digestion).
- Proteases and lipases in detergents allow low-temperature washing; pectinase increases juice yield and clarity by breaking down pectin.
Next time → Choosing the genes: selective breeding over centuries, genetic engineering in months — Bt maize, golden rice, insulin from bacteria, and how to weigh the benefits against the risks.