Food and Dairy Microbiology
Domain 9 covers why food spoils, how preservation slows or stops it, and the microbiology of milk and fermented products.
Spoilage and preservation
Intrinsic factors include water activity, pH, nutrients, redox potential and antimicrobial compounds. Extrinsic factors include temperature, humidity and atmosphere. Spoilage organisms include Pseudomonas and lactic acid bacteria, moulds such as Aspergillus and Penicillium, and yeasts. Preservation methods are refrigeration, freezing, drying, salting, sugaring, acidification, canning, irradiation, modified atmosphere and chemical preservatives. Clostridium botulinum is anaerobic; its toxin is heat-labile but its spores are heat-resistant, so low-acid canned foods need pressure canning. Staphylococcus aureus toxin is heat-stable.
Milk and dairy
Milk is a good microbial medium with pH near 6.7 and high water activity. Pasteurisation reduces pathogens; sterilisation kills all microbes. The methylene blue reduction test decolourises faster when the microbial load is high. The phosphatase test detects incomplete pasteurisation. Standard plate count and coliform count are hygiene indicators. Fermented products include curd and yoghurt (Lactobacillus, Streptococcus), cheese (rennet and starter cultures), butter, ghee and buttermilk. Probiotics are live beneficial microbes.
Worked example
Milk decolourises methylene blue in 30 minutes. Reasoning: rapidly growing microbes consume oxygen and reduce the dye; a short reduction time indicates a high bacterial load and poor quality. Good-quality fresh milk retains the blue colour for hours.
Traps
Pasteurisation does not sterilise. Boiling does not destroy all spores. Mould growth does not always mean the remaining food is safe. Coliforms indicate possible faecal contamination. Curd setting is due to acid production, not rennet.