Food Preservation: KS3 Biology

Why does food decay?

Food decay is caused by microorganisms — mainly bacteria and moulds (and sometimes yeasts) — that colonise food surfaces and then grow by feeding on the nutrients inside. They are decomposers: they secrete enzymes onto or into food, breaking down proteins, fats and carbohydrates into smaller molecules that they can then absorb.

As microorganisms digest food, they produce waste products that cause:

• Changes in smell and taste (e.g. rancid fat from fatty acid breakdown, acidic smell from bacterial fermentation)
• Changes in texture (soft, slimy, or mushy food as cell structures are broken down)
• Changes in colour (moulds producing coloured spores; bacteria producing pigments)
• Potentially harmful toxins — some bacteria produce toxins that cause food poisoning even if the food does not look obviously "off" (e.g. Staphylococcus aureus, Clostridium botulinum)

For microorganisms to grow on food, they need:

1. Water (moisture) — for chemical reactions in their cells
2. Warmth — optimal temperature for enzyme activity (~20–40 °C for most food-spoilage bacteria)
3. Nutrients — the food itself provides these
4. A suitable pH — most spoilage microorganisms prefer near-neutral conditions
5. For aerobic bacteria and moulds: oxygen

Preservation methods target one or more of these requirements.

How does refrigeration and freezing preserve food?

Refrigeration (typically 1–5 °C):

• Slows the growth and reproduction of bacteria and moulds — most cannot grow rapidly at low temperatures.
• Slows the activity of food enzymes that cause ripening and browning (e.g. browning of cut apples or bananas).
• Does NOT kill microorganisms — it only slows them. This is why food taken out of the fridge still eventually goes off, and why re-refrigerating thawed food is risky (bacteria that grew during thawing have now multiplied).

Freezing (below −18 °C):

• Water in the food freezes, becoming unavailable for microbial metabolism.
• Microbial growth essentially stops.
• Some bacteria survive (they are dormant, not dead) and resume growth when the food is thawed.
• Enzymatic activity also slows dramatically at freezing temperatures.

Properly frozen food can be kept safely for months to years.

What is pasteurisation and how does it work?

Pasteurisation was developed by Louis Pasteur in the 1860s to prevent spoilage of wine and beer, and is now applied universally to milk, fruit juices, and other beverages.

The process:

1. The liquid is heated rapidly to a target temperature — typically 72 °C for 15 seconds (High-Temperature Short-Time method).
2. It is then cooled rapidly.

Why it works: most pathogenic bacteria (disease-causing bacteria) and spoilage microorganisms are killed at 72 °C. Heat denatures their enzymes and proteins, killing the cells. The short time at high temperature is sufficient to kill pathogens without significantly changing the flavour or nutritional content of the liquid.

What it does not do: pasteurisation does not sterilise the liquid — heat-resistant spores and some bacteria survive. Pasteurised milk still has a limited shelf life (days to weeks) and must be refrigerated. Ultra-High Temperature (UHT) sterilisation (135 °C for 2–4 seconds) kills all microorganisms and spores, giving milk a shelf life of months at room temperature — at the cost of some flavour change.

How do salting, sugaring, and pickling preserve food?

These methods work by osmosis — removing water from microbial cells or food tissue.

Salting: Salt draws water out of microbial cells by osmosis (water moves from the cell cytoplasm, where salt concentration is lower, to the high-salt environment outside). This plasmolysis (shrinking of the cell contents) prevents bacteria from growing. Examples: salted fish, bacon, cured meats, sauerkraut.

Sugaring: A high concentration of sugar has the same osmotic effect on microbial cells as salt. Examples: jams, marmalades, crystallised fruit. The high sugar concentration dehydrates and kills most bacteria and moulds.

Pickling: Food is stored in vinegar (acetic acid, pH ≈ 2.4) or brine. The low pH inhibits or kills most bacteria and moulds, which cannot function in strongly acidic conditions. Examples: pickled onions, gherkins, pickled eggs.

How does drying preserve food?

Drying (dehydration) removes the water that microorganisms need to grow. Food can be dried by:

• Sun-drying (traditional — herbs, apricots, fish)
• Hot-air drying (industrial — pasta, cereals, powdered milk)
• Freeze-drying: food is frozen, then water is removed by sublimation under vacuum (instant coffee, camping food) — preserves flavour and texture particularly well

Without moisture, bacteria and moulds cannot multiply, even though the dried food may still contain living (dormant) microorganisms. Rehydrating dried food can allow growth to resume — hence the need to use reconstituted dried products promptly.

Summary table: preservation methods and how they work

Frequently asked questions

Why is food safe in the fridge but still eventually goes off?

Refrigeration slows but does not stop microbial growth. At 4 °C, bacterial doubling times are much longer than at room temperature — a bacterium that divides every 20 minutes at 30 °C might divide only every few hours at 4 °C — but division still occurs. Over several days, microbial populations reach levels sufficient to cause visible spoilage or produce enough toxins to cause food poisoning. Additionally, food-spoiling moulds (such as Penicillium species) can grow slowly at refrigerator temperatures. This is why refrigerated food has a "use by" date, not an indefinite shelf life.

How does vinegar preserve food when bacteria need an acidic environment to do some things?

Vinegar has a pH of about 2.4 — far more acidic than the pH 6–7 range optimal for most food-spoilage bacteria. At this pH, bacterial enzymes are inhibited and proteins are denatured, preventing bacteria from metabolising or reproducing. A few acid-tolerant (acidophilic) bacteria can survive, but the combination of low pH and salt in many pickling recipes makes conditions inhospitable for almost all food-spoilage organisms. The same principle applies to your stomach: gastric acid (pH 1–2) kills most bacteria you swallow with food.

Why must jars for jam or pickles be sterilised before use?

If bacteria or mould spores are already present in the jar when the food is sealed, they can grow and spoil the food even if the preservation conditions (high sugar, low pH) are correct — because they start with a head start. Sterilising jars (by heating in an oven, boiling, or using a dishwasher on a hot cycle) kills microorganisms on the glass surfaces before the food is added. Additionally, filling jars with hot liquid and sealing them creates a partial vacuum as the contents cool, which further discourages microbial growth by removing oxygen needed by aerobic spoilage organisms.

What is the difference between food going off and food poisoning?

Food that has gone off is noticeably spoiled — it looks, smells, or tastes bad due to microbial breakdown products. Most people would not eat it, reducing the risk of illness. Food poisoning is illness caused by consuming food contaminated with harmful bacteria (e.g. Salmonella, Campylobacter, E. coli O157:H7) or their toxins (e.g. toxins from Staphylococcus aureus or Clostridium botulinum). The dangerous feature of food poisoning bacteria is that the food often looks, smells, and tastes normal — there is no obvious warning sign. This is why proper food safety (correct storage temperatures, cooking to the right temperature, avoiding cross-contamination) matters even when food appears fine.

---

Professor Darwin at aitutors.me can walk you through the biology of microorganisms, connect it to food science, and help you tackle any KS3 question on decay and preservation.