Food Contamination and Food Spoilage

 FOOD CONTAMINATION AND FOOD SPOILAGE

FOOD CONTAMINATION

Source of Contamination	How Contamination Occurs	Common Microorganisms
Green Plants & Fruits	Naturally on surface, damaged tissues, during harvesting & transport	Molds (Penicillium, Aspergillus), Yeasts, Lactic acid bacteria, Bacillus
Animals	Skin, hair, intestines, hides, slaughter, milking, egg laying	Salmonella, Staphylococcus, Streptococcus, E. coli
Soil	Contact with soil, unwashed vegetables, root crops, eggs, meat	Clostridium, Bacillus, Mucor, Rhizopus, Actinomycetes
Water	Washing, cleaning, processing, contaminated ice	E. coli, Shigella, Salmonella, Vibrio
Sewage	Leakage, irrigation of crops, water contamination	Pathogens – E. coli, Salmonella, Viruses, Parasites
Air	Dust particles, spores settling on food	Molds (Aspergillus, Penicillium), Yeasts, Bacillus spores
Handling & Processing	Humans, equipment, utensils, packaging	Staphylococcus aureus, E. coli, Bacillus

FOOD SPOILAGE

·       Food spoilage - Undesirable changes in food making it unfit for consumption.

·       Food spoilage causative agents - Microbial growth, enzymes, chemical reactions, physical damage.

Classification of Foods Based on Spoilage

Category	Examples	Reason	Spoilage Rate
Perishable Foods	Milk, meat, fish, fruits	High moisture, rich nutrients	Spoils very fast
Semi-perishable Foods	Potatoes, nuts, some fruits	Moderate moisture	Spoils slowly
Non-perishable Foods	Cereals, flour, sugar, biscuits	Low moisture	Very slow spoilage

 

Intrinsic Factors Affecting Spoilage

Intrinsic Factor	Meaning	Effect on Microbes
pH	Acidity/Alkalinity of food	Low pH → bacteria inhibited; yeasts & molds grow
Water Activity (aw)	Available water for microbes	aw < 0.6 prevents growth
Nutrients	Proteins, carbs, fats, vitamins	Determines which microbes grow (proteolytic, lipolytic, etc.)
Biological Structures	Skin, shell, rinds, membranes	Natural barriers reduce contamination
Natural Inhibitors	Lysozyme (egg), benzoic acid (fruits), lactenin (milk)	Inhibit microbial growth

Extrinsic Factors Affecting Spoilage

Extrinsic Factor	Effect on Food/Microbes
Temperature	Cold → slows growth; Warm → fast spoilage
Relative Humidity	High RH → mold growth on surface
Oxygen Availability	Aerobes on surface (molds); anaerobes in canned foods

Microbial Spoilage Mechanisms

Type of Spoilage	Process	Organisms Involved	Result
Carbohydrate Decomposition	Fermentation of sugars	Yeasts, lactic acid bacteria	Souring, gas, alcohol
Protein Decomposition (Putrefaction)	Break down of proteins	Proteolytic bacteria – Pseudomonas, Bacillus, Clostridium	Foul odor, ammonia, H₂S
Fat Degradation	Lipolysis of fats	Molds, Pseudomonas	Rancidity, off-odors
Pigment Formation	Pigment-producing microbes	Serratia (red), Pseudomonas (green)	Discoloration

Spoilage Signs in Common Foods

Food	Signs of Spoilage	Typical Microorganisms
Milk	Souring, curdling	Lactic acid bacteria
Meat	Slimy surface, odor, color change	Pseudomonas, Bacillus
Bread	Mold growth (black/green)	Rhizopus, Aspergillus, Penicillium
Fruits & Vegetables	Soft rot, color change	Molds, Erwinia
Canned Food	Bulging cans, sour smell, gas	Clostridium, Bacillus