Heat Resistant Proteases Produced in Milk by Psychrotorophic Bacteria of Dairy Origin

Abstract

Production of heat resistant proteases by psychrotrophs growing in milk, resist- ance of such proteases to ultrahigh tem- perature treatments and action of these enzymes on milk were studied. All of the psychrotrophs obtained from raw milk produced proteases that survived 149 C for 10 s. Seventy to ninety percent of the raw milk samples contained psychro- trophs capable of producing heat resistant proteases. The protease chosen as a model was resistant to heat treatments at 110 to 150 C, and the inactivation parameters suggested that thermal destruction of heat resistant proteases would damage the milk severely. The casein content and pH of normal milk were suitable for protease action, and the protease was quite active at normal and elevated room tempera- tures. The protease rapidly spoiled sterile milk with the development of bitter flavor, clearing, or coagulation; and the susceptibility of sterile milk to protease increased during storage of the milk.