FOOD90023 · Food Microbiology
Microbial Metabolism
A microbe's metabolism is its energy business, and it is the chemistry behind both spoilage and fermentation. Catabolism breaks food molecules down to release energy as ATP; anabolism spends that ATP to build the cell. The way a microbe makes ATP — respiration (with an electron transport chain) or fermentation (without one, using an organic final electron acceptor and substrate-level phosphorylation) — sets both how fast it grows and what by-products it dumps into the food: acids, gas, alcohol, off-odours. The same reactions are spoilage when unwanted and fermentation when controlled. You need to define and not swap catabolism and anabolism, compare the ATP yields (aerobic respiration ~38 ATP vs fermentation ~2 ATP per glucose) and explain why, state the true defining feature of fermentation, match organisms to their oxygen class, and answer the recurring malolactic-fermentation question.
What this chapter covers
- 01Catabolism vs anabolism, coupled by the ATP↔ADP cycle
- 02Glycolysis (EMP pathway): glucose → 2 pyruvate, 2 ATP + 2 NADH
- 03Respiration vs fermentation — and the real defining feature of fermentation
- 04ATP yields: aerobic respiration (~38) vs fermentation (~2) and why
- 05Fermentation products and organisms (lactic-acid, alcoholic, malolactic)
- 06Oxygen / redox classes — aerobe, anaerobe, facultative, microaerophile, aerotolerant
- 07Malolactic fermentation (MLF) and its inhibitory factors
Worked example: ATP yield — aerobic respiration vs fermentation
- +1(a) Yields. Aerobic respiration nets about 38 ATP per glucose (glycolysis + the citric-acid cycle + oxidative phosphorylation). Fermentation nets only about 2 ATP per glucose — just the substrate-level ATP from glycolysis.
- +1(b) Why — the electron transport chain. Respiration passes the electrons from NADH down an electron transport chain to a final inorganic acceptor (O₂ in aerobes), driving oxidative phosphorylation and most of the ATP.
- +1(b) Why — fermentation skips it. Fermentation has no electron transport chain; it just regenerates NAD⁺ by dumping electrons onto an organic molecule (e.g. pyruvate → lactate or ethanol), so only the 2 glycolytic ATP are made.
- +1(c) Consequence — growth. Because fermenters extract so little energy per glucose, they consume substrate fast and to grow at all must process a lot of it — which is exactly why they pour out by-products.
- +1(c) Consequence — the food. Those by-products (lactic acid, ethanol, CO₂) are what we taste and smell — spoilage when unwanted, but harnessed deliberately to make yoghurt, beer, wine and sauerkraut.
Key terms
- Catabolism vs anabolism
- The two halves of metabolism. Catabolism breaks complex molecules down and releases energy (captured as ATP and NADH). Anabolism builds complex molecules and consumes energy. They are coupled by the ATP↔ADP cycle — catabolism charges ATP, anabolism spends it.
- Fermentation
- Energy metabolism that regenerates NAD⁺ using an organic final electron acceptor and substrate-level phosphorylation only (no electron transport chain). Its defining feature is that organic acceptor — 'no oxygen' is necessary but not the definition. It yields ~2 ATP per glucose and excretes acids, alcohol or gas.
- Glycolysis (EMP pathway)
- The first stage of glucose catabolism, common to respiration and fermentation: glucose (6C) → 2 pyruvate (3C), netting 2 ATP + 2 NADH in the cytoplasm. What happens to the pyruvate and the NADH electrons afterwards decides respiration vs fermentation.
- Oxygen (redox) class
- How an organism relates to oxygen: obligate aerobe (needs O₂), obligate anaerobe (killed by O₂), facultative anaerobe (uses O₂ if present, ferments if not), microaerophile (needs low O₂) and aerotolerant anaerobe (ignores O₂). The class tells you where in a food an organism can grow.
- Malolactic fermentation (MLF)
- A secondary fermentation, mainly in wine, in which lactic acid bacteria convert harsh malic acid to softer lactic acid (and CO₂), lowering acidity and changing flavour. It is inhibited by low pH, low temperature, high SO₂, high alcohol and low nutrient levels.
Microbial Metabolism FAQ
What is the real defining feature of fermentation — isn't it just 'no oxygen'?
No — that is the classic trap. Fermentation is defined by using an organic molecule as the final electron acceptor and making ATP by substrate-level phosphorylation only (no electron transport chain). The absence of oxygen is usually true but is not the definition: anaerobic respiration, for example, happens without oxygen yet still uses an electron transport chain and an inorganic acceptor, so it is respiration, not fermentation. State the organic-acceptor / no-ETC point to get the mark.
Why does the same organism sometimes spoil food and sometimes ferment it?
Because spoilage and fermentation are the same chemistry with a different value judgement. The acids, gas and alcohol a microbe excretes are spoilage when they appear in a food you wanted fresh, and fermentation when you deliberately set up the conditions and the starter culture to get them — the lactic acid that ruins milk is the same lactic acid that makes yoghurt.
How do I answer the malolactic fermentation (MLF) question?
Define it first: lactic acid bacteria convert sharp malic acid to softer lactic acid (plus CO₂) in wine, lowering acidity and rounding the flavour. Then list the inhibitory factors the question wants — low pH, low temperature, high sulfur dioxide (SO₂), high alcohol and low nutrients — because the marks are split between the definition and the factors that switch it on or off.
Do I need to memorise every step of glycolysis and the citric-acid cycle?
No. The exam rewards the energetics and logic, not every intermediate: glycolysis nets 2 ATP + 2 NADH and 2 pyruvate; respiration then uses the electron transport chain to reach ~38 ATP; fermentation skips the chain for ~2 ATP. Know that comparison, the defining feature of fermentation, and the named products, rather than the full pathway map.
Exam move
Build this chapter around one comparison and one recurring question. The comparison is respiration vs fermentation: rehearse the ATP yields (~38 vs ~2), the reason (electron transport chain and oxidative phosphorylation vs substrate-level only with an organic acceptor), and the food consequence (by-products = spoilage or fermentation). Make sure you can state the true defining feature of fermentation and never reduce it to 'no oxygen'. The recurring question is malolactic fermentation: pre-write its definition plus its inhibitory factors. Finally, be able to match a named organism to its oxygen class and to name the main fermentation products and the lactic acid bacteria or yeast behind them — cheap, high-frequency marks.
