🦠 Food Spoilage and Microbiology

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Food Spoilage and Microbiology

Food spoilage is any undesirable change in food that makes it unfit for consumption or unacceptable to the consumer. It results in economic losses (estimated 30–40% of food produced in India is wasted) and poses potential health risks.

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Types of Food Spoilage

Microbial Spoilage

The most common and economically significant type. Caused by bacteria, moulds, and yeasts that use food as a substrate for growth, producing undesirable changes in colour, texture, flavour, and aroma (proteolysis → putrid odours; lipolysis → rancid odours; fermentation → off-acids, off-gases).

Chemical Spoilage

• Oxidative rancidity: unsaturated fatty acids react with atmospheric oxygen → free radicals → aldehydes, ketones, carboxylic acids → rancid odour and flavour; accelerated by light, heat, metal ions, pro-oxidant enzymes
• Hydrolytic rancidity: lipases (microbial or endogenous) hydrolyse triglycerides → free fatty acids → soapy/rancid taste (short-chain FFA especially offensive)
• Enzymatic browning: polyphenol oxidase (PPO) oxidises phenolic compounds → brown/black pigments; cut apple, potato, banana, mushroom; requires O2 and copper cofactor; prevented by blanching, acidification (lemon juice), or SO2
• Non-enzymatic browning: Maillard reaction (heat), caramelisation (high temperature)
• Vitamin degradation: Vitamin C, B vitamins progressively degrade during storage

Physical Spoilage

• Physical damage (bruising, cracking, compression) in fresh produce → cell damage → microbial entry and enzymatic activity
• Dehydration (moisture loss) → wilting, shrivelling in fresh produce; texture loss in baked goods
• Freezer burn: surface dehydration in frozen food stored without adequate packaging
• Insect/rodent damage → contamination, weight loss, mycotoxin introduction
• Rehydration of hygroscopic powders → caking, microbial growth

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Factors Affecting Microbial Growth in Food

Intrinsic Factors (Properties of the Food)

Factor	Effect on Microbial Growth
Water activity (Aw)	Most bacteria require Aw >0.91; moulds can grow at 0.70; see Lesson 5
pH	Most bacteria prefer pH 6.5–7.5; moulds/yeasts grow pH 2–8; acid foods (pH <4.6) much safer
Redox potential (Eh)	Aerobic organisms need high Eh; anaerobes need low Eh; vacuum packing lowers Eh
Nutrients	Rich media (meat, dairy) support rapid growth; simple sugars and starches support few organisms
Natural antimicrobials	Lysozyme (egg white, milk); allicin (garlic); eugenol (cloves); cinnamaldehyde (cinnamon)
Physical structure	Intact surfaces resist microbial penetration; cut surfaces vulnerable

Extrinsic Factors (Environmental Conditions)

Factor	Effect
Temperature	Most critical; see temperature classes below
Relative Humidity (RH)	High RH → surface moisture → mould/yeast growth; dried foods need low RH storage
Gaseous atmosphere	O2 required by aerobes; reduced O2 (MAP) suppresses most spoilage bacteria
Other microorganisms	Competitive exclusion; LAB producing bacteriocins inhibit pathogens

Temperature Classifications of Microorganisms

Group	Optimum Growth Temp.	Min.	Max.	Examples
Psychrophiles	10–15°C	−5°C	20°C	Rare; polar environments
Psychrotrophs	20–30°C	0–7°C	35°C	Pseudomonas, Listeria — key refrigeration spoilage
Mesophiles	30–40°C	10°C	45°C	Most pathogens (Salmonella, E. coli, S. aureus) — room temp most dangerous
Thermophiles	50–60°C	40°C	70°C	Bacillus stearothermophilus — canned food spoilage

Danger Zone: 5°C to 60°C — temperature range supporting rapid microbial growth; food should not remain in this zone for more than 2 hours.

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Key Food Spoilage Organisms

Bacteria

Organism	Food Type	Spoilage Sign
Pseudomonas spp.	Meat, fish, poultry, dairy	Off-odours (putrid), slime, green fluorescent pigment; psychrotroph
Clostridium spp.	Canned foods, cooked meats	Gas production, putrefaction; anaerobe; spore-former
Bacillus spp.	Bread, cereals, cooked rice	"Rope" in bread (B. subtilis); ropy, stringy texture; heat-resistant spores
Lactobacillus spp.	Dairy, meat, beer	Souring (lactic acid production); often desirable in fermented foods
Erwinia spp.	Vegetables, soft fruits	Soft rot of vegetables; pectinase → cell wall breakdown
Leuconostoc	Refrigerated meats, vegetables	Slime, off-odours at cold temperatures

Moulds

Genus	Food Type	Spoilage/Hazard
Aspergillus	Cereals, groundnut, spices, dried fruits	Aflatoxin production (A. flavus, A. parasiticus); green/yellow-green sporulation
Penicillium	Bread, citrus, cheese	Blue-green sporulation; patulin in apple juice
Rhizopus/Mucor	Bread, soft fruits	Fluffy white/grey growth; bread mould; soft rot of strawberries
Fusarium	Cereal grains (wheat, maize)	Fumonisin, DON (deoxynivalenol/vomitoxin) production
Alternaria	Tomato, fruits, cereals	Black rot; alternaria toxins

Yeasts

• Saccharomyces cerevisiae: desirable in bread, beer, wine; spoilage in fruit juices (fermentation)
• Candida, Rhodotorula: high-sugar foods (honey, jam), cold foods; pink/red pigmentation
• Zygosaccharomyces rouxii: extremely high-sugar and high-salt tolerance; spoils jams, syrups, soy sauce

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Foodborne Diseases

Classification

Foodborne infection: live microorganism ingested → colonises gut → illness after incubation period.

Foodborne intoxication: preformed toxin in food ingested → rapid onset illness; organism may not survive (e.g., S. aureus enterotoxin is heat-stable at 100°C/30 min).

Major Foodborne Pathogens

Pathogen	Classification	Food Vehicle	Incubation	Key Symptoms	Prevention
Salmonella spp.	Infection	Eggs, poultry, raw meat, vegetables	6–72 hours	Diarrhoea, fever, vomiting	Cook to 75°C; prevent cross-contamination
Staphylococcus aureus	Intoxication	Cooked foods handled after cooking, dairy	1–6 hours	Sudden vomiting, nausea; heat-stable toxin	Personal hygiene; temperature control; heat-stable toxin survives cooking
Clostridium botulinum	Intoxication	Home-canned foods, honey (infants), fermented fish	12–36 hours	Botulism: muscle paralysis, respiratory failure; most lethal toxin	Proper retort sterilisation (F₀); acidification; never feed honey to infants <1 year
Bacillus cereus	Both	Cooked rice (emetic), soups, stews (diarrhoeal)	Emetic: 1–5 hr; Diarrhoeal: 6–24 hr	Emetic: vomiting; Diarrhoeal: abdominal pain, diarrhoea	Refrigerate cooked rice immediately; do not hold at room temperature
E. coli O157:H7	Infection	Undercooked beef, raw vegetables, unpasteurised milk	3–8 days	Bloody diarrhoea, HUS (haemolytic uraemic syndrome)	Cook thoroughly; pasteurise milk
Listeria monocytogenes	Infection	Ready-to-eat meats, soft cheese, smoked fish	1–70 days	Flu-like symptoms; high risk in pregnancy (miscarriage); meningitis	Cold chain (but grows at 0–4°C!); avoid RTE foods in high-risk groups
Campylobacter jejuni	Infection	Poultry, raw milk, water	2–5 days	Diarrhoea (often bloody), cramps, fever	Cook poultry to 75°C; pasteurise milk
Vibrio cholerae	Infection	Water, seafood, raw vegetables	2 hours–5 days	Profuse "rice-water" diarrhoea; severe dehydration; cholera	Safe water; proper sanitation
Hepatitis A virus	Infection	Water, raw shellfish, raw vegetables	15–50 days	Jaundice, fever, nausea	Vaccination; handwashing; food handler hygiene

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Mycotoxins

Mycotoxins are toxic secondary metabolites produced by certain moulds (primarily Aspergillus, Fusarium, Penicillium). They are heat-stable — cooking does not destroy them. Prevention focuses on pre- and post-harvest management.

Aflatoxins — Most Important for India

• Produced by Aspergillus flavus and A. parasiticus
• Types: B1, B2, G1, G2 (B = blue fluorescence; G = green under UV); B1 is most toxic and carcinogenic
• Substrates: groundnut, maize, chilli, cottonseed, copra, spices — especially in hot, humid conditions
• Effects: acute aflatoxicosis (high dose) → liver failure; chronic (low dose) → hepatocellular carcinoma (liver cancer); immunosuppression; growth stunting in children
• IARC classification: Group 1 carcinogen
• FSSAI limits: 10 µg/kg (B1) and 15 µg/kg (total) in food; 10 ppb total in groundnuts for direct human consumption
• Export issue: India's groundnut and spice exports frequently rejected by EU (EU limit: 2 µg/kg B1) due to aflatoxin contamination

Other Important Mycotoxins

Mycotoxin	Producing Organism	Commodity	Health Effect
Ochratoxin A (OTA)	Aspergillus, Penicillium	Wheat, barley, coffee, grapes, dried fruits	Nephrotoxic (kidney damage); possible carcinogen (IARC 2B)
Fumonisin B1/B2	Fusarium moniliforme, F. verticillioides	Maize, maize products	Linked to oesophageal cancer (South Africa); equine leukoencephalomalacia
Deoxynivalenol (DON / Vomitoxin)	Fusarium graminearum, F. culmorum	Wheat, barley, oats	Vomiting, feed refusal in animals; immunosuppression
Zearalenone	Fusarium spp.	Maize, wheat	Oestrogenic effects; reproductive disorders
Patulin	Penicillium expansum	Apple juice (from mouldy apples)	Mutagenic; CODEX limit: 50 µg/kg in apple juice

Mycotoxin Prevention

• Pre-harvest: use resistant varieties; biological control (Afla-Guard® — atoxigenic A. flavus displaces toxigenic strains); crop rotation; timely harvest
• Post-harvest: rapid drying to <12% moisture for cereals and <8% for groundnut; proper storage (low humidity, <15°C, airtight); avoid mechanical damage; regular monitoring
• Decontamination: physical (density sorting, colour sorting by fluorescence); chemical (ammoniation — destroys B1 but industrial only); detoxification (clay adsorbents in animal feed)

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Indicator Organisms

Indicator organisms signal the potential presence of pathogenic contamination (especially faecal):

Indicator	Significance	Method	Standard
Total Plate Count (TPC)	Overall hygiene; general microbial load	Agar plate count (35°C, 48h)	Varies by food; milk <20,000/mL
Coliforms	Faecal contamination; sanitation failure	MacConkey agar; IMVIC tests	Absent in 100 mL drinking water
Faecal coliforms (E. coli)	Definitive faecal contamination	44.5°C incubation	Absent in 100 mL water (WHO)
Enterococcus	Faecal contamination; survives processing	Enterococcus selective agar	Supplement to coliforms

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HACCP — Hazard Analysis and Critical Control Points

HACCP is a systematic preventive approach to food safety that identifies, evaluates, and controls food safety hazards throughout the food production process. Developed by NASA and Pillsbury (1960s) for astronaut food; codified by Codex Alimentarius (1997).

Seven Principles of HACCP

1. Conduct a Hazard Analysis — identify all potential biological, chemical, and physical hazards at each process step; assess severity and likelihood
2. Identify Critical Control Points (CCPs) — steps where control measures can prevent, eliminate, or reduce hazard to acceptable levels; use CCP decision tree
3. Establish Critical Limits — measurable parameters (temperature, time, pH, Aw, etc.) that must be met at each CCP (e.g., pasteurisation: 72°C/15 sec)
4. Establish Monitoring Procedures — how and how often CCPs will be measured and by whom
5. Establish Corrective Actions — actions when CCP monitoring shows critical limits are not met (e.g., reprocessing, product destruction)
6. Establish Verification Procedures — confirm HACCP system is working (periodic testing, audits, review)
7. Establish Documentation and Record-keeping — written HACCP plan; monitoring records; corrective action records

12 Steps of HACCP implementation (5 preliminary + 7 principles): Assemble team → Describe product → Identify intended use → Construct flow diagram → Verify flow diagram → then 7 principles.

CCP examples:

• Poultry processing: cooking step (CCP — 75°C/15 sec)
• Dairy milk: pasteurisation (CCP)
• Canned food: retort sterilisation (CCP)
• Fresh produce: wash water chlorine level (CCP)

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Types of food spoilage (microbial, chemical, physical), key spoilage organisms, foodborne diseases, mycotoxins (aflatoxin, ochratoxin), HACCP principles, and indicator organisms.
Section context	Revise this lesson with the rest of Food Safety for stronger conceptual continuity.