Food spoilage and food-borne pathogens
Which microbes spoil food and which make people ill, the factors that control their growth, the major food-borne pathogens, and a growth sum.
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Food is food for microbes too. Bacteria, yeasts and moulds that grow in milk, meat, fish, fruit, bread and cooked meals cause spoilage: off-odours, slime, sourness, gas, mould growth and texture changes that make food unacceptable. A smaller but more dangerous group, food-borne pathogens, cause illness, sometimes without any sign of spoilage. The World Health Organization estimated that unsafe food causes around 600 million illnesses and 420,000 deaths a year, with children under five bearing a large share.
Food microbiology, spoilage and pathogens are central topics in food science, dairy technology and public health exams.
How microbes grow in food
Under favourable conditions, bacteria multiply by binary fission. Starting from a few hundred cells per gram, numbers can reach millions or more within hours at warm temperatures, which is why cooked foods left at room temperature are a common cause of outbreaks.
Factors controlling growth
Intrinsic factors (properties of the food)
- Water activity (aw): most bacteria need aw above about 0.91; many yeasts and moulds grow down to about 0.80 to 0.70; dry foods and those high in sugar or salt resist bacteria.
- pH: most bacteria prefer near-neutral pH; below about 4.6, Clostridium botulinum cannot grow or produce toxin, which is why acidity defines canning rules; yeasts and moulds tolerate acid foods such as fruit juices.
- Nutrients: protein-rich, moist foods such as meat, fish, milk, eggs and cooked rice support rapid growth.
- Redox potential and oxygen: aerobic, anaerobic and facultative microbes favour different foods and packages.
- Natural antimicrobials: lysozyme in egg white, essential oils in spices, and organic acids.
Extrinsic factors (the environment)
- Temperature: the most important control. The "danger zone" of roughly 5 to 60 °C allows rapid growth of many pathogens. Psychrotrophs, such as Pseudomonas and Listeria monocytogenes, grow slowly even in refrigerators.
- Relative humidity of storage.
- Gas atmosphere: carbon dioxide and low oxygen slow many spoilage organisms.
These factors are combined deliberately in hurdle technology.
Spoilage organisms by food
- Milk: lactic acid bacteria (souring), psychrotrophic Pseudomonas (bitter, rancid flavours from heat-stable enzymes), and spore-forming Bacillus in pasteurised milk.
- Fresh meat and fish: Pseudomonas, Shewanella and other Gram-negative bacteria (slime, off-odours) aerobically; lactic acid bacteria in vacuum packs.
- Fruits and vegetables: moulds such as Penicillium, Botrytis and Rhizopus, and soft-rot bacteria such as Pectobacterium.
- Bread and bakery goods: moulds such as Penicillium and Aspergillus; rope spoilage by Bacillus.
- Juices and soft drinks: yeasts and acid-tolerant bacteria such as Alicyclobacillus.
- Canned foods: spore-forming bacteria if processing or seams fail, causing swollen cans.
Major food-borne pathogens
| Pathogen | Typical sources | Notes |
|---|---|---|
| Salmonella | poultry, eggs, meat, raw produce | among the most common bacterial causes worldwide |
| Campylobacter | undercooked poultry, raw milk | very common cause of bacterial diarrhoea |
| Shiga toxin-producing E. coli (such as O157:H7) | undercooked beef, raw milk, sprouts, leafy greens | can cause kidney failure in children |
| Listeria monocytogenes | ready-to-eat foods, soft cheese, deli meats | grows at refrigeration temperatures; dangerous in pregnancy |
| Staphylococcus aureus | handled foods | heat-stable toxin; rapid vomiting |
| Clostridium perfringens | cooked meats and gravies cooled slowly | spores survive cooking |
| Bacillus cereus | cooked rice held warm | emetic and diarrhoeal toxins |
| Clostridium botulinum | improperly canned low-acid foods | rare but deadly neurotoxin |
| Vibrio species | raw seafood | V. parahaemolyticus, V. cholerae |
| Norovirus, hepatitis A | food handlers, shellfish, produce | viruses; hand hygiene is critical |
| Parasites | undercooked meat, contaminated water and produce | Toxoplasma, Taenia, Cryptosporidium |
Some cause infection (the microbe grows in the gut), others intoxication (a toxin formed in the food, such as staphylococcal toxin or botulinum toxin), and some a combination.
How fast can bacteria grow in food?
A dish of cooked rice contains 100 Bacillus cereus cells per gram after cooking and is left at a warm room temperature, where the bacteria double every 30 minutes after a short lag.
- In 4 hours there are 4 × 60 ÷ 30 = 8 doublings.
- Count = 100 × 28 = 100 × 256 = 25,600 per gram.
- In 8 hours (16 doublings): 100 × 65,536 ≈ 6.6 million per gram, high enough to form toxin.
In a refrigerator at 4 °C, B. cereus barely grows. Cooling cooked rice quickly, within about an hour, and keeping it chilled prevents most "fried rice syndrome" outbreaks.
Five keys to safer food
WHO's Five Keys to Safer Food summarise everyday prevention: keep clean (hands, surfaces, utensils); separate raw and cooked foods; cook thoroughly, especially meat, poultry, eggs and seafood; keep food at safe temperatures, chilling promptly and not leaving cooked food at room temperature for more than about two hours; and use safe water and raw materials. They apply equally to homes, street vendors and canteens.
Key points
- Spoilage microbes make food unacceptable; pathogens make people ill, sometimes without visible spoilage.
- Growth follows lag, log, stationary and death phases; numbers rise rapidly in the log phase.
- Intrinsic factors: water activity, pH, nutrients, redox potential and antimicrobials; extrinsic: temperature, humidity and atmosphere.
- The danger zone is about 5–60 °C; Listeria grows even in refrigerators; acid foods below pH 4.6 prevent botulinum toxin.
- Major pathogens: Salmonella, Campylobacter, STEC, Listeria, S. aureus, C. perfringens, B. cereus, C. botulinum, Vibrio, norovirus and hepatitis A.
- Count after n doublings = initial count × 2n.
What the exams ask
- School-leaving and entrance papers: causes of food spoilage and common food poisoning organisms.
- Degree and research-fellowship entrances: intrinsic and extrinsic factors, spoilage of specific foods, characteristics of major pathogens, infection versus intoxication, and growth calculations.
- Food safety examinations: outbreak investigation, microbiological criteria, and controls in HACCP plans.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can I tell if food is unsafe by smelling it?
No. Spoilage often smells, but many pathogens, such as Salmonella, Listeria and botulinum toxin, do not change how food looks or smells.
Does reheating make spoiled food safe?
Not always. Heat kills many bacteria but does not destroy heat-stable toxins such as those of S. aureus or the emetic toxin of B. cereus.
Why is Listeria special?
It grows at refrigerator temperatures, survives in factory environments, and causes severe illness in pregnant women, newborns, the elderly and people with weak immunity.
Sources
- World Health Organization. WHO Estimates of the Global Burden of Foodborne Diseases. WHO, 2015.
- Jay, J. M., Loessner, M. J. and Golden, D. A. Modern Food Microbiology, 7th edition. Springer, 2005.
- Doyle, M. P., Diez-Gonzalez, F. and Hill, C. (eds) Food Microbiology: Fundamentals and Frontiers, 5th edition. ASM Press, 2019.
- International Commission on Microbiological Specifications for Foods. Microorganisms in Foods 5: Characteristics of Microbial Pathogens. Blackie Academic, 1996.
- Gram, L. et al. "Food spoilage: interactions between food spoilage bacteria." International Journal of Food Microbiology 78, 79–97 (2002).
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