Food spoilage: meaning, causes and factors
Food spoilage is any change that makes food unfit or unacceptable to eat. Microbes, the food's own enzymes, pests, chemical reactions and physical damage cause it.
What spoilage means
Food spoilage is any change in a food's colour, odour, taste, texture or nutritive value that makes it unfit or undesirable for people to eat. Signs include sour or putrid smell, slime, gas, mould patches, softening and colour change. Spoiled food is not always unsafe, and unsafe food is not always spoiled: milk carrying disease germs can look and smell normal. So preservation must deal with both spoilage organisms and disease-causing (pathogenic) organisms.
Causes of food spoilage
| Cause | What happens | Example |
|---|---|---|
| Microorganisms (bacteria, yeasts, moulds) | Grow on food and break down its protein, carbohydrate and fat, producing acid, gas, slime, off-odours and sometimes toxins | Curd turning too sour; mould on bread; slimy leftover rice |
| The food's own enzymes | Keep working after harvest or slaughter: over-ripening, softening, browning, fat splitting | Cut brinjal or apple turning brown (polyphenol oxidase); banana over-ripening |
| Insects, pests and rodents | Eat and dirty the food and open it to microbes | Weevils in rice and dal; rats in grain godowns |
| Chemical reactions without enzymes | Oxidation of fats (rancidity), non-enzymic (Maillard) browning, loss of vitamins | Rancid smell in old groundnut oil or namkeen; browning of stored milk powder |
| Physical factors | Heat, cold, moisture, dryness, light, air and time; bruising and crushing | Wilted greens; bruised tomatoes rotting fast; freezer burn on meat |
Factors that control microbial spoilage
| Factor | Inside or around the food | Effect on spoilage |
|---|---|---|
| Water activity (aw) | Intrinsic (inside) | Most spoilage bacteria need aw of at least 0.91, yeasts 0.88, moulds 0.80. Salt-loving bacteria grow down to 0.75, dry-loving (xerophilic) moulds to 0.65 and sugar-loving (osmophilic) yeasts to 0.60 (Frazier & Westhoff) |
| pH (acidity) | Intrinsic | Most bacteria prefer near-neutral pH (6.5-7.5); yeasts and moulds tolerate acid, so they spoil fruits and pickles |
| Nutrients | Intrinsic | Protein-rich, moist foods (meat, fish, milk, egg) support fast growth |
| Oxidation-reduction potential | Intrinsic | Air-loving (aerobic) microbes spoil surfaces; anaerobes such as Clostridium grow deep inside meat or in cans |
| Natural antimicrobials and covers | Intrinsic | Lysozyme in egg white; essential oils in clove, garlic and cinnamon; shell, peel and skin keep microbes out |
| Temperature | Extrinsic (around) | Psychrophiles grow in the cold, mesophiles best at about 30-37 °C, thermophiles at about 50-60 °C; most food bacteria grow fast between about 5 and 60 °C |
| Relative humidity | Extrinsic | Damp storage lets dry foods pick up water and grow mould |
| Gases | Extrinsic | Oxygen helps moulds and aerobes; carbon dioxide and nitrogen in packs slow them |
Water activity made simple
Water activity (aw) = vapour pressure of water in the food ÷ vapour pressure of pure water at the same temperature. Pure water has aw 1.0. Fresh milk, meat, fish and fruits are close to 1.0, so they spoil quickly. Drying, adding sugar or salt, and freezing all lower aw because they remove water or tie it up. That is why moist foods are spoiled mainly by bacteria, while dry or sugary foods such as bread and jam are spoiled by moulds and yeasts.
In a hospital or hostel kitchen, knowing the causes of spoilage tells you what to control: keep cooked food out of the danger zone (about 5-60 °C), store dry goods in pest-proof bins, and use FIFO (first in, first out) so stock does not age. Spoilage also destroys nutrients such as vitamin C.
2 marks: “Define food spoilage.” Any change in a food's colour, odour, taste, texture or nutritive value that makes it unfit or undesirable for people to eat.
5 marks: “What is water activity? How does it affect food spoilage?” Define aw as the vapour pressure of water in food ÷ that of pure water (pure water = 1.0); give limits (bacteria 0.91, yeasts 0.88, moulds 0.80); moist foods spoil by bacteria, dry and sugary foods by moulds and yeasts; drying, sugar, salt and freezing lower aw.
10 marks: “Discuss the causes of food spoilage.” Define spoilage; explain microbes, food enzymes, insects and rodents, chemical reactions and physical factors with examples; add intrinsic and extrinsic factors (aw, pH, temperature, oxygen); close with how preservation controls each cause.
Spoilage causes = MEIC-P: Microbes, Enzymes, Insects and rodents, Chemical reactions, Physical factors.