Dairy Microbiology, Starter Cultures and Quality Control
Dairy Microbiology, Starter Cultures and Quality Control tests microbial groups, culture activity, indicator tests and the difference between hygiene indicators and pathogens. The exam favours interpretation of colony counts and test results.
Core microbiology
Raw milk contains lactic acid bacteria, coliforms, psychrotrophs, spore formers and possible pathogens. Pasteurisation destroys most vegetative pathogens but not all spores. Starter cultures must be active, free from phage and used at the correct temperature. Coliforms indicate poor hygiene; standard plate count indicates overall viable load; phosphatase indicates pasteurisation efficiency; MBRT and resazurin tests estimate microbial quality.
Worked example
A 1 mL sample of 10^-3 dilution gives 30 colonies on a standard plate. CFU/mL = 30 × 10^3 = 3.0 × 10^4 CFU/mL. Reasoning: the dilution factor restores the original concentration. If two duplicate plates give 28 and 32, use the average 30 before multiplying.
Traps
Do not call every colony a pathogen. Coliforms are hygiene indicators, not proof of pathogens. Do not use a negative phosphatase test as proof of sterility. Antibiotic residues inhibit starter cultures and can cause false low counts. Phage can stop acid production without visible contamination.