ABSTRACT
A nutrient agar medium containing 0.1% of a low melting point fraction of butterfat was shown to be suitable for detection, enumeration and isolation of lipolytic bacteria from milk. Bacterial growth was not inhibited by the butterfat and lipolytic reactions were clearly visible and easily interpreted. Lipolytic counts on the butterfat agar compared favourably with lipolytic counts obtained with other commonly used media.
Subject(s)
Agar , Butter , Milk/microbiology , Animals , Cattle , Culture Media , In Vitro TechniquesABSTRACT
Raw milk samples were stored for 1-4 d and examined for bacterial growth and lipase activity. Thirty-six samples in which an increase in the heat-stable lipase activity was observed during storage were selected for further study. From these raw milk samples 205 lipolytic psychrotrophic strains were selected using butterfat agar and subsequently characterized with 86 taxonomic tests. Complete linkage cluster analysis of the taxonomic data produced two major and six minor clusters at the 83% similarity level. Pseudomonas fluorescens and Ps. fragi accounted for 63.9 and 31.2%, respectively, of the isolates.
Subject(s)
Food Microbiology , Milk/microbiology , Pseudomonas fluorescens/classification , Pseudomonas/classification , Animals , Cattle , Cold Temperature , LipolysisABSTRACT
The lipolytic floras of 36 raw milk samples showing lipolytic defects were dominated by pseudomonads. Representative lipolytic isolates were selected and tested for growth, lipase activity and lipolysis in ultra-heat-treated milk at temperatures ranging from 5 degrees to 30 degrees C. Pseudomonas fluorescens was the most frequently encountered species but Ps. fragi was found to cause more severe lipolytic defects in both single and mixed strain milk cultures. A representative strain of Ps. fragi multiplied faster in cold-stored milk than did three representative strains of Ps. fluorescens. The lipases produced by Ps. fragi strains were more heat-stable than those produced by Ps. fluorescens strains.
