Effects of Psychrotrophic Bacteria Acinetobacter genomospecies 10 and Serratia liquefaciens on Raw Milk Quality

This study was conducted to investigate effect of psychrotrophic bacteria on the quality of raw milk. Acinetobacter genomospecies 10 was selected as lipolytic species, and Serratia liquefaciens as proteolytic species. Lipase present in inoculated raw milk with Acinetobacter genomospecies 10 did not affect total solid and fat contents. However, the free fatty acid (FFA) content, especially short chain FFAs, of milk with Acinetobacter genomospecies 10 was dramatically increased. FFAs produced by lipolysis of milk fat are important in flavor of dairy products, excessive lipolysis occurring in milk and dairy products could cause off-flavor, and produced FFAs may have an underiable effect on their flavor. In addition, protease influenced the quality of inoculated raw milk with Serratia liquefaciens. In degradation patterns of casein by SDS-PAGE analysis from inoculatred raw milk with Serratia liquefaciens, casein content was gradually decreased during storage at 4oC, and extensive degradation of κ-casein was observed on the storage day of 13. The free amino acids such as leucine, valine, arginine, and tyrosine were dramatically increased, which causes bitter taste in raw milk. These excessive peptides in dairy products, produced by psychrotrophic bacteria, can be possible to develop off-flavors and be responsible for gelling of milk by degradation.