Under refrigerated conditions, Gram-negative rods that are able to produce under such conditions ( psychrotrophic/cold-loving bacteria ) usually predominate, especially Pseudomonas spp.

This is especially so when the milk has a high initial infection of these bacteria and the milk is stored for 2-4 days before pasteurization - which is often the case these days, where milk is mostly being picked up from farms every second day only.

This deterioration also often influences the final products made from such milk, because although bacteria will mostly be killed by pasteurization, some produce exo-enzymes that survive pasteurization and even the UHT (Ultra High Temperature/Long Life) process in some cases.

An example of Pseudomonas attacking dairy products happen a lot with cottage cheese. Pseudomonades are widely distributed and aerobic. They tend not to need much carbohydrate, which help them a lot in attacking cottage cheese, because most of the lactose has been removed in the cheese making process. They live very well on high-protein foods like cottage cheese producing slime, pigments and odours. They also prefer high water activity ( high moisture), which makes cottage cheese pretty much the ideal food considering everything that has been said.

Many Pseudomonas spp. are psychrotrophic ( cold-loving ) bacteria although they will grow a lot faster from 15 degrees Celsius to temperate.

Thus a temperature shock during transport for example will give them a big boost - to cause even more destruction.