Food Spoilage and Microorganisms

What Are Microorganisms?

Every time you leave a bowl of leftover chicken curry on the worktop overnight, you are giving microorganisms exactly what they need to multiply at speed. Microorganisms are invisible to the naked eye, but their effects — a furry grey patch on a bread loaf, a fizzing punnet of strawberries, the sour smell of yoghurt left too long — are very familiar. The three types you need to know are bacteria, yeasts and moulds. Each behaves differently and responds to conditions differently.

Bacteria are single-celled organisms that can divide roughly every 20 minutes under ideal conditions — meaning one bacterium can become millions in just a few hours. Yeasts are single-celled fungi that reproduce by budding and are responsible for fermentation (the process that makes bread rise and wine alcoholic). Moulds are multicellular fungi that grow as thread-like filaments called hyphae; you can see them on an over-ripe peach or a forgotten corner of bread.

What Microorganisms Need to Grow

Think of a microorganism’s needs as a simple checklist: food, moisture, warmth and time. Remove or limit any one factor and you slow or stop growth. This is the entire logic behind every food preservation and storage method you will study.

• Food (nutrients): Bacteria and moulds thrive on protein and carbohydrates. High-protein moist foods — cooked chicken, shellfish, soft cheeses, cooked rice — are prime targets.
• Moisture: Water availability (measured as water activity, aw) is critical. Dried foods like pasta, flour and biscuits have a very low aw, which is why they keep for months. Moist cooked foods have a high aw and spoil quickly.
• Warmth: Most harmful bacteria multiply fastest between 5°C and 63°C — the temperature danger zone. Below 5°C growth slows dramatically; above 63°C most bacteria begin to die.
• Time: Even in ideal conditions, it takes time for numbers to reach a dangerous level. A portion of cooked rice left at room temperature for two hours is much riskier than one left for twenty minutes.

Two additional factors affect growth: pH and oxygen. Most bacteria prefer a neutral pH (around 6.5–7.5). Very acidic foods like lemon juice or vinegar (low pH) slow bacterial growth — which is why pickling and marinating help preserve food. Some bacteria need oxygen (aerobic); others grow without it (anaerobic), which is why vacuum-packed meats can still carry a risk if temperature control fails.

High-Risk Foods

High-risk foods are those that are ready to eat, moist and high in protein — meaning they support rapid bacterial growth and will not be cooked again before being eaten. The absence of that final heat treatment is what makes them dangerous: any bacteria already present will be consumed alive. Classic examples include cooked poultry, cooked rice, custards and cream-based desserts, soft cheeses, cooked shellfish and egg-based dishes such as quiche. A plate of leftover roast chicken that has been sitting at room temperature is a textbook high-risk food scenario.

Enzymes and Food Spoilage

Enzymes are not microorganisms — they are biological catalysts produced by the cells of plants, animals and microorganisms themselves. In a freshly cut apple, enzymes called polyphenol oxidases react with oxygen in the air to produce brown pigments. This is enzymic browning, and it is why your apple slices turn brown within minutes of cutting. The same process causes bananas to go from yellow to brown and bruised pears to discolour.

Enzymic browning is not caused by bacteria or moulds — it is a purely chemical process. However, it is still spoilage because it affects the appearance, texture and sometimes the flavour of the food. You can control it by:

• Acids: Squeezing lemon juice over cut fruit lowers the pH and deactivates the enzymes. This is why a fruit salad with a citrus dressing stays fresher-looking for longer.
• Blanching: Briefly plunging vegetables into boiling water (blanching) denatures the enzymes by heat before freezing them. Without blanching, frozen peas or green beans would turn mushy and brown during storage because the enzymes would still be active even at –18°C.
• Covering and minimising oxygen exposure: Wrapping cut fruit in cling film slows the reaction by limiting oxygen contact.

Signs of Food Spoilage

Food spoils through three main routes: enzymic action, mould growth and yeast action. The signs are often obvious if you know what to look for.

• Enzymic action: Ripening and softening of bananas (starch converts to sugars), browning of cut fruits and vegetables, softening of meat as proteases break down muscle fibres after slaughter.
• Mould growth: Visible fuzzy patches (blue-green, white or black) on the surface of bread, soft cheeses, fruit and leftovers. Moulds produce thread-like hyphae and release spores; the visible fuzzy area is only the fruiting body — the hyphae may extend deeper into soft foods. Eating mouldy food is generally not recommended because some moulds produce toxic compounds called mycotoxins.
• Yeast action: A fermenting, bubbling or fizzy quality, often with an alcoholic or sharp smell. This is why a punnet of squashed strawberries left in a warm kitchen develops a fizzing quality, or why opened fruit juice left for several days starts to ferment.

Understanding that the same organisms that spoil food can also be harnessed to make it is an important distinction. The difference lies entirely in control: controlled conditions, starter cultures and careful monitoring of temperature, pH and moisture turn potential spoilage into valuable food products.