Fermentation is the anaerobic breakdown of organic molecules by microorganisms — primarily yeast and bacteria — to produce useful substances such as ethanol, carbon dioxide, lactic acid, and pharmaceutical drugs. At industrial scale, the process takes place in carefully controlled vessels called fermenters, where temperature, pH, and oxygen supply are regulated to maximise yield.
What is fermentation?
Fermentation is a type of anaerobic respiration in which microorganisms partially break down organic molecules (usually sugars) without using oxygen, releasing energy and producing useful chemical by-products.
Two main types are relevant at GCSE:
| Type | Organism | Equation | Products used for |
|---|---|---|---|
| Alcoholic | Yeast (Saccharomyces) | Glucose → ethanol + carbon dioxide | Alcoholic drinks, biofuels, bread |
| Lactic acid | Bacteria (Lactobacillus) | Glucose → lactic acid | Yoghurt, cheese, silage |
Equation for alcoholic fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (glucose → ethanol + carbon dioxide)
The yeast carries out this process anaerobically (without oxygen). When oxygen is present, yeast prefers aerobic respiration, which produces more energy but no ethanol.
How is ethanol produced industrially?
Bioethanol is produced as a fuel and industrial solvent by fermenting sugars derived from crops:
- Raw material: sugar cane (juice pressed out directly), sugar beet (sucrose extracted), or starch crops (wheat, maize) where the starch is first broken down to glucose by amylase enzymes.
- Fermentation: yeast is added to the sugar solution in the fermenter. Optimal conditions: temperature ~30–37 °C (too hot denatures yeast enzymes; too cold slows the reaction), slightly acidic pH (~4.5), anaerobic atmosphere.
- Product: a dilute ethanol solution (~5–15%).
- Purification: the ethanol is separated from water and other impurities by fractional distillation (ethanol boils at 78.4 °C, water at 100 °C).
Bioethanol is considered a renewable fuel because the CO₂ released when it is burned was recently absorbed by the crop during photosynthesis — making it broadly carbon-neutral over its lifecycle (though fertiliser production and machinery use introduce some net emissions).
What is an industrial fermenter and how is it designed?
An industrial fermenter is a large, sealed vessel in which microorganisms are grown under controlled conditions to produce a desired product. Key design features:
| Feature | Purpose |
|---|---|
| Water jacket or cooling coils | Maintain optimal temperature (remove heat generated by microorganism metabolism) |
| pH meter + acid/alkali addition | Keep pH within the optimal range for the enzymes involved |
| Nutrient inlet | Supply glucose, minerals, and vitamins continuously or in batches |
| Stirrer / paddles | Mix the culture to ensure even nutrient distribution and prevent settling |
| Air supply / filter (if aerobic) | Sterile air for aerobic fermentations (e.g. penicillin); excluded for anaerobic |
| Sterile conditions | Prevents contamination by unwanted microorganisms that would compete with the production strain |
| Sampling port | Allow monitoring of product concentration, pH, and organism health |
Sterility is critical: any contaminant microorganism could outcompete the production strain, consume the nutrient supply, or produce unwanted by-products. Fermenters and all incoming materials are sterilised, usually with steam (autoclaved).
How is penicillin produced by fermentation?
Penicillin is an antibiotic produced by the mould Penicillium — one of the most significant pharmaceutical products of the 20th century. Its discovery by Alexander Fleming in 1928 (observing that the mould killed bacteria on a Petri dish) is now backed by industrial-scale fermentation.
Production steps:
- Penicillium is grown in a large fermenter in an aerobic medium rich in nitrogen and glucose.
- Temperature is maintained at ~25 °C.
- After a growth phase, the mould begins producing penicillin as a secondary metabolite (a compound produced when nutrients become limited, not during primary growth).
- The broth is filtered to remove the mould mycelia.
- Penicillin is extracted and purified by solvent extraction and precipitation.
One fermenter can produce hundreds of kilograms of penicillin, enough to treat thousands of patients.
How is biotechnology used to produce other useful substances?
Beyond ethanol and antibiotics, fermentation and genetic engineering are used to produce:
| Product | Organism used | Application |
|---|---|---|
| Insulin | Genetically modified bacteria or yeast | Treatment of diabetes mellitus |
| Citric acid | Aspergillus niger fungus | Food preservative and flavouring |
| Yoghurt | Lactobacillus bacteria | Food production |
| Biofuels (biogas) | Anaerobic bacteria | Methane from organic waste |
| Chymosin (rennet) | GM bacteria | Cheese-making (replaces animal rennet) |
Human insulin was one of the first products of genetic engineering (early 1980s): the human insulin gene was inserted into E. coli bacteria, which then produced human insulin during fermentation. This replaced insulin extracted from pig and cow pancreases, which caused immune reactions in some patients.
Frequently asked questions
Why does the concentration of ethanol eventually stop increasing during fermentation?
Yeast cells are killed by high ethanol concentrations — typically above about 14–18% by volume, depending on the yeast strain. As fermentation proceeds, ethanol accumulates and reaches a concentration that is toxic to the yeast, killing the cells and stopping the reaction. This is why wine and beer produced by simple fermentation reach a natural ceiling of roughly 12–16% alcohol by volume; spirits such as whisky and vodka must be produced by distillation after fermentation to reach higher concentrations.
What is the difference between aerobic and anaerobic fermentation?
Strictly speaking, fermentation is always anaerobic — it does not use oxygen. However, in industrial biotechnology, the term "fermentation" is often used loosely for any large-scale cultivation of microorganisms, including aerobic ones (such as Penicillium for penicillin production). The key distinction is: true anaerobic fermentation (yeast → ethanol + CO₂; bacteria → lactic acid) does not require oxygen; aerobic processes (penicillin, mycoprotein production) do require a sterile oxygen supply to the fermenter.
Why are fermentation conditions (temperature and pH) so important?
The enzymes inside yeast and bacteria cells are proteins with specific optimal temperatures and pH values. Too high a temperature denatures the enzymes irreversibly, killing the microorganism. Too low a temperature slows enzyme-catalysed reactions dramatically. Similarly, moving too far from the optimum pH changes the shape of enzyme active sites, reducing or stopping the fermentation. Industrial fermenters invest heavily in monitoring and control systems because a few degrees or half a pH unit can significantly reduce yield.
What are the advantages of using biotechnology to produce drugs such as insulin?
Producing human insulin using genetically modified bacteria has several advantages over older methods: (1) the product is genuine human insulin, not pig or cow insulin, so it is less likely to cause immune reactions; (2) the supply is not limited by the availability of animal pancreases; (3) it can be produced in very large quantities at relatively low cost in sterile fermenter conditions; (4) genetic engineering allows precise control over the protein's structure. The main regulatory challenge is ensuring purity — the product must be rigorously tested before use in patients.
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