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Growth of an extracellular proteinase-deficient strain of Pseudomonas fluorescens on milk and milk proteins

Published online by Cambridge University Press:  01 June 2009

Joan P. Torrie ,
Hilaire Cholette ,
Debbi A. Froehlich  and
Robin C. McKellar
Joan P. Torrie
Affiliation:
Food Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1a 0C6, Canada
Hilaire Cholette
Affiliation:
Food Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1a 0C6, Canada
Debbi A. Froehlich
Affiliation:
Food Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1a 0C6, Canada
Robin C. McKellar
Affiliation:
Food Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario K1a 0C6, Canada
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Summary

An extracellular proteinase-and lipase-deficient mutant of a psychrotroph, Pseudomonas fluorescens strain 32A, has been isolated and the absence of the proteinase enzyme confirmed by growth on differential media, enzyme assay and polyacrylamide gel electrophoresis. Competition between the parent and the mutant was observed when equal numbers of the 2 strains were inoculated together into raw skim-milk at 6 °C. Bitterness was detected at 6 °C in pasteurized skim-milk inoculated with the parent cells concurrent with the detection of proteolytic activity. In the case of the mutant, slight bitterness which did not increase with increasing cell numbers was detected in the absence of proteolysis. Mutant cells failed to grow on Na caseinate as the sole source of carbon. It was concluded that the extracellular proteinase, while not essential for growth in milk, does provide a selective advantage to the producer organism. This enzyme is, however, essential for growth on milk proteins and contributes to the development of bitterness in pasteurized milk.

Type
Original Articles
Information
Journal of Dairy Research , Volume 50 , Issue 3 , August 1983 , pp. 365 - 374
Copyright
Copyright © Proprietors of Journal of Dairy Research 1983

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