Dairy Microbiology, Hygiene and Sanitation
Domain 6 tests milk microbiology, indicator organisms, spoilage and cleaning-sanitation. You need to distinguish hygiene indicators from pathogens and know what pasteurisation can and cannot do.
Core rules
- Standard plate count measures total viable bacteria; coliforms indicate poor hygiene or post-pasteurisation contamination; MBR and resazurin tests estimate bacterial activity.
- Pasteurisation reduces, but does not sterilise, vegetative pathogens; spores survive. Psychrotrophs grow at refrigeration temperatures and cause lipolysis and proteolysis.
- CIP sequence: rinse, alkaline wash, acid wash, rinse, sanitise; use correct concentration, temperature and contact time.
- Biofilms on gaskets and dead legs protect bacteria from cleaning.
Worked example
A pasteuriser reduces a count from 10^5 CFU/mL to 10^2 CFU/mL. Log reduction = log10(10^5) - log10(10^2) = 5 - 2 = 3 logs, or 99.9% reduction. The reasoning is that each log is a tenfold reduction; a 3-log drop leaves one-thousandth of the original.
Traps
- Coliforms are hygiene indicators, not proof that a specific pathogen is present.
- Pasteurisation does not sterilise milk.
- Too-strong sanitizer can damage equipment and taint milk.
- A visually clean surface can still harbour biofilm.