Psychrotrophic bacteria are bacteria that are capable of surviving or even thriving in extremely cold environment. They provide an estimation of the product's shelf life, also they can be found in soils, in surface and deep sea waters, in Antarctic ecosystems, and in foods. They are responsible for spoiling refrigerated foods.
Psychrotrophic bacteria are of particular concern to the dairy industry. Most are killed by pasteurization; however, they can be present in milk as post-pasteurization contaminants due to less than adequate sanitation practices. According to the Food Science Department at Cornell University, psychrotrophs are bacteria capable of growth at temperatures at or less than 7 °C (44.6 °F). At freezing temperatures, growth of psychrotrophic bacteria becomes negligible or virtually stops.