Microbiology of Raw and Processed Foods

 

Category	Examples	Typical Microorganisms	Main Source of Contamination	Type of Spoilage	Notes
Raw Plant Foods	Fruits, vegetables, grains, nuts	Pseudomonas, Erwinia, Bacillus, yeasts (Saccharomyces), molds (Penicillium, Aspergillus)	Soil, air, water, harvesting & handling	Soft rot, souring, discoloration, mold growth	High moisture → rapid spoilage
Raw Animal Foods	Meat, fish, milk, eggs	Pseudomonas, Lactobacillus, Salmonella, Staphylococcus, Vibrio, molds	Slaughtering, skin, intestine, equipment	Off-odors, slime, rancidity, putrefaction	High protein & aw → supports bacterial growth
Pasteurized Foods (Heat processed)	Pasteurized milk, soups, juices	Surviving thermoduric bacteria, post-contamination coliforms, yeasts	Improper heat / post-processing contamination	Sour, gas, curdling, surface growth	Reduces pathogens but requires cold storage
Canned Foods	Vegetables, meat, fish, fruits	If spoiled: Clostridium botulinum, Bacillus stearothermophilus	Under-processing or leakage	Gas production, can swelling, flat sour	Commercially sterile if properly processed
Refrigerated Foods	Stored meat, fish, vegetables	Psychrotrophs: Pseudomonas, Lactobacillus, molds	Air, handling, packaging	Slime, souring, surface mold	Cold slows growth but doesn’t stop it
Frozen Foods	Frozen meats, vegetables, ice cream	Surviving spores & psychrotrophs after thawing	Pre-freezing contamination	Spoilage only after thawing	Freezing lowers aw, halts metabolism
Dried Foods	Milk powder, grains, dried fruits, spices	Aspergillus, Penicillium, osmophilic yeasts, Bacillus spores	Air, storage humidity	Mold growth, off-flavors	aw < 0.85 prevents bacteria
Fermented Foods	Curd, cheese, pickles, sauerkraut, dosa batter	LAB: Lactobacillus, Lactococcus; yeasts: Saccharomyces	Added cultures or natural flora	Acidification, CO₂, aroma compounds	Safe due to acid, alcohol, antimicrobials
High Sugar Foods	Jams, jellies, syrups, honey	Osmophilic yeasts, molds	Air & storage	Fermentation, yeast growth	High sugar inhibits bacteria
High Salt Foods	Pickles, salted fish, soy sauce	Halotolerant bacteria & yeasts	Brining, handling	Turbidity, gas, film yeast	Salt reduces aw
Smoked Foods	Fish, meat	Smoke-resistant molds, yeasts	Storage environment	Surface mold growth	Smoke contains natural antimicrobials
Chemical Preserved Foods	Soft drinks, pickles, sauces	Resistant yeasts & molds	Bottling contamination	Gas, cloudiness, film layer	Preservatives inhibit most bacteria