Protein degradation in bovine milk caused byStreptococcus agalactiae

Abstract

Streptococcus(Str.)agalactiaeis a contagious mastitis bacterium, often associated with cases of subclinical mastitis. Different mastitis bacteria have been evaluated previously from a diagnostic point of view, but there is a lack of knowledge concerning their effect on milk composition. Protein composition is important in achieving optimal yield and texture when milk is processed to fermented products, such as cheese and yoghurt, and is thus of great economic value. The aim of thisin vitrostudy was to evaluate protein degradation mainly caused by exogenous proteases originating from naturally occurringStr. agalactiae. The samples were incubated at 37°C to imitate degradation caused by the bacteria in the udder. Protein degradation caused by different strains ofStr. agalactiaewas also investigated. Protein degradation was observed to occur whenStr. agalactiaewas added to milk, but there were variations between strains of the bacteria. Caseins, the most economically important proteins in milk, were degraded up to 75% in milk inoculated withStr. agalactiaein relation to sterile ultra-high temperature (UHT) milk, used as control milk. The major whey proteins, α-lactalbumin and β-lactoglobulin, were degraded up to 21% in relation to the sterile control milk. These results suggest that different mastitis bacteria but also different strains of mastitis bacteria should be evaluated from a milk quality perspective to gain knowledge about their ability to degrade the economically important proteins in milk.