Microbial forms and types
Bacteria are tiny single cells (about 1 to 5 micrometres) with no true nucleus. Shapes: cocci (round), bacilli (rods), spirilla (spirals). Examples in food: Lactobacillus (curd), Acetobacter (vinegar), Salmonella (poisoning).
Yeasts are single-celled fungi, round or oval, 5 to 10 micrometres. They multiply by budding. Saccharomyces is used for bread and alcohol.
Moulds are fungi made of long threads called hyphae; a mat of hyphae is a mycelium. They spread by spores. Examples: Rhizopus (bread mould), Aspergillus (koji for soy sauce, but some moulds make toxins), Penicillium (cheese rind).
Microbial nutrition and physiology
Microbes cannot swallow food. They release enzymes outside, then absorb small molecules through the cell wall and membrane. What they need:
- Carbon source: sugars, starch, fats.
- Nitrogen source: proteins, amino acids, ammonium.
- Minerals and vitamins in small amounts.
- Water (measured as water activity; very dry or salty foods keep well).
Conditions: temperature (mesophiles like 20 to 45 °C, most food microbes), pH (bacteria like near neutral, yeasts and moulds tolerate acid), and oxygen: aerobes need it, anaerobes cannot use it, and facultative microbes (such as yeast) grow with or without it.
Microbial growth and genetics
Growth means more cells, not bigger cells. Bacteria divide by binary fission: 1 → 2 → 4 → 8. The time for the count to double is the generation time. Count after n generations = starting count × 2n.
A growth curve has four phases: lag (getting ready), log (fast doubling), stationary (food runs low, count steady) and death (cells die).
Genetics. A microbe's DNA (genes) decides its enzymes and traits. When DNA copies, small errors called mutations sometimes happen. Bacteria can also swap genes by conjugation, transformation or transduction. Food scientists use mutation and selection to get better microbes (for example a yeast that makes more bread gas).
Microbial enzymes and types
Enzymes are protein catalysts that speed up reactions. Each enzyme fits one kind of molecule, like a key in a lock (its substrate). Microbes make many enzymes, and industry uses them.
- Amylase: starch → sugars (bread, brewing, syrup).
- Protease: proteins → amino acids and peptides (cheese, soy sauce, meat tenderising).
- Lipase: fats → fatty acids (cheese flavour).
- Pectinase: breaks pectin, so juice clears.
- Lactase: milk sugar (lactose) → glucose + galactose (lactose-free milk).
- Invertase: sucrose → glucose + fructose.
Enzymes work best at an optimum temperature and pH and stop when heated too much (denatured).
Microbial metabolism
Metabolism is all the chemical changes in a cell. Catabolism breaks food down to release energy (ATP); anabolism uses that energy to build new cell parts.
- Aerobic respiration (with air): glucose + O₂ → CO₂ + H₂O + a lot of energy.
- Alcoholic fermentation (yeast, no air): glucose → ethanol + CO₂ + a little energy. The CO₂ makes bread rise and beer fizz.
- Lactic acid fermentation (Lactobacillus): glucose → lactic acid + a little energy. The acid sets milk into curd and keeps it safe.
- Acetic acid fermentation: ethanol + O₂ → acetic acid (vinegar).
Key formulas and definitions
- Count after n doublings = start × 2ⁿ, n = time ÷ generation time
- Aerobic: glucose + O₂ → CO₂ + H₂O + energy (much)
- Alcoholic: glucose → ethanol + CO₂ + energy (little)
- Lactic: glucose → lactic acid + energy (little)
- Key terms: hyphae, mycelium, spore, budding, binary fission, generation time, mutation, enzyme, substrate
Worked examples
1. Bacteria double every 30 minutes. Starting with 1 cell, how many are there after 3 hours?
3 hours = 6 generations. 2⁶ = 64 cells.
2. A yeast culture starts with 500 cells and doubles every 1.5 hours. How many cells after 6 hours?
6 ÷ 1.5 = 4 doublings. 500 × 2⁴ = 500 × 16 = 8,000 cells.
3. Name the enzyme and the microbe group used to make lactose-free milk and say what happens.
Lactase (from yeast or fungi) splits lactose into glucose and galactose, so people who cannot digest lactose can drink the milk.
4. Why does dough rise but a yeast-in-water mix with a lot of air smell less of alcohol?
With plenty of air, yeast respires: it makes CO₂ and water and grows more cells. Without air it ferments and makes alcohol. Dough has little air deep inside, so yeast ferments there.
Common mistakes
- Thinking that growth means each cell gets larger. In microbes it means more cells.
- Mixing up the two fermentations: yeast makes alcohol + CO₂; Lactobacillus makes lactic acid.
- Forgetting that the count doubles each generation, not adds the same number each time.
- Calling all moulds harmful. Some make cheese and soy sauce; some make toxins.