Öz
Milk spoils due to microorganisms transmitted from environments, equipment and personnel starting from milking to process. Lactic acid bacteria, psychrotrophic Gram negative bacteria, Gram positive spore-forming bacteria and pathogenic bacteria are present in the milk microbiota. The activity of these bacteria and enzymes changes the sensory and structural properties of milk, causing it to spoil. Milk microbiota is one of the most important parameters affecting the quality and safety of both milk and dairy products. Bacteria found in milk that cause milk spoilage are Pseudomonas, Acinetobacter, Brevundimonas, Flavobacterium and some coliform members. Besides contamination before and after pasteurization, microorganism activity also plays an important role in spoiling milk. Especially, Bacillus species with psychrotrophic thermophilic character are responsible for the deterioration that occurs after pasteurization. In this article, information on the ways of contamination of milk with microorganisms, the characteristics of microorganisms as well as their roles in milk spoilage are presented.