Microorganisms that spoil common foods
Each food has its own spoilers: moulds on bread and fruits, bacteria on meat, fish, eggs and milk, and yeasts in juices and pickles.
Bread, fruits and vegetables
| Food | Type of spoilage | Organisms | Signs |
|---|---|---|---|
| Bread | Mouldiness | Rhizopus stolonifer (black bread mould), Penicillium (blue-green), Aspergillus, Mucor, Neurospora sitophila (pink-orange) | Cottony black, green or pink patches |
| Bread | Rope | Bacillus subtilis and B. licheniformis; their spores survive baking | Soft, sticky, stringy crumb with a fruity, over-ripe smell |
| Bread | Red ('bloody') bread | Serratia marcescens | Red spots on the crumb |
| Fruits | Mould rots | Penicillium (blue and green mould of citrus), Botrytis cinerea (grey mould), Rhizopus (soft rot), Aspergillus niger (black mould), Colletotrichum (anthracnose of mango) | Soft or dry rotten patches with coloured mould |
| Vegetables | Bacterial soft rot | Erwinia carotovora (now Pectobacterium), Pseudomonas | Mushy, foul-smelling tissue in onion, cabbage, potato |
Meat, fish and eggs
| Food | Organisms | What you see or smell |
|---|---|---|
| Meat: surface (with air) | Pseudomonas, Acinetobacter, Moraxella; Lactobacillus and Leuconostoc | Surface slime, souring, greening, colour change; mould spots |
| Meat: deep inside (no air) | Clostridium | Putrefaction: foul smell of hydrogen sulphide, ammonia and amines; bone taint |
| Fish | Pseudomonas, Shewanella putrefaciens, Acinetobacter-Moraxella, Flavobacterium | Fishy smell of trimethylamine (TMA, made from TMAO in the muscle), slime, soft flesh; salted fish turns pink from salt-loving bacteria |
| Eggs | Pseudomonas, Proteus, Serratia (enter through shell pores); moulds Penicillium and Cladosporium | Green rot (Pseudomonas fluorescens), black rot (Proteus), red rot (Serratia), colourless rot; mould 'pin spots' |
Milk, juices and pickles
| Food | Change | Organisms |
|---|---|---|
| Milk | Souring and curdling | Lactic acid bacteria: Lactococcus (Streptococcus) lactis, Lactobacillus |
| Milk | Gas production | Coliforms (Enterobacter, E. coli), Clostridium |
| Milk | Sweet curdling and bitterness (protein breakdown) | Bacillus cereus, Pseudomonas |
| Milk | Ropiness (stringy milk) | Alcaligenes viscolactis |
| Milk | Rancidity (fat splitting) | Pseudomonas and some moulds |
| Milk | Blue, red or yellow colour | Pseudomonas syncyanea (blue), Serratia marcescens (red), Pseudomonas synxantha (yellow) |
| Fruit juices | Fermentation (gas, alcohol), souring, vinegar taste, mould mats | Yeasts (Saccharomyces, Zygosaccharomyces); lactic acid bacteria (Lactobacillus, Leuconostoc); acetic acid bacteria (Acetobacter, Gluconobacter); moulds (Penicillium, Byssochlamys) |
| Pickles | Softening, white surface scum, darkening, slipperiness, hollow pieces | Pectin-splitting moulds and bacteria; film yeasts (Debaryomyces, Pichia, Candida) that eat up the acid; Bacillus species |
The pattern behind the list
Moist, low-acid foods (meat, fish, egg, milk) are spoiled mainly by bacteria. Acid foods (fruits, juices, pickles) and sugary or dry foods (bread, jam) are spoiled mainly by yeasts and moulds, which tolerate low pH and low water activity. Pseudomonas leads the spoilage of chilled meat, fish and milk because it keeps growing in the cold. Egg is protected by its cuticle, shell, membranes, lysozyme and the high pH (about 9) of its white, so bacteria must first get through the shell.
Knowing the spoiler tells a food service manager the fix: chill milk, meat and fish (bacteria); keep bread and pickles dry and covered (moulds and film yeasts); use a clean, dry spoon for pickles. Ropy bread and rotten eggs must be thrown away, not trimmed.
2 marks: “What is rope in bread?” Bacterial spoilage by Bacillus subtilis or B. licheniformis, whose spores survive baking; the crumb turns soft, sticky and stringy with a fruity smell.
5 marks: “Write a note on the spoilage of milk.” Souring by lactic acid bacteria, gas by coliforms, sweet curdling and bitterness by Bacillus, ropiness by Alcaligenes, rancidity by Pseudomonas, coloured milk; prevent by pasteurising and chilling.
10 marks: “Explain the microorganisms involved in the spoilage of bread, meat, fish and eggs.” For each food give organism, type of spoilage and signs (a table helps); explain why each food suits that group (pH, aw, oxygen) and how to prevent it.
Moist and neutral → bacteria; acid, sweet or dry → yeasts and moulds. Cold meat, fish and milk → Pseudomonas.