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Growth studies on the lactic streptococci: III. Observations on continuous growth behaviour in reconstituted skim-milk

Published online by Cambridge University Press:  01 June 2009

A. R. Keen
A. R. Keen
Affiliation:
New Zealand Dairy Research Institute, Palmerston North, New Zealand
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Summary

The behaviour of Streptococcus cremoris AM1 and AM2, and Streptococcus lactis ML8 in continuous growth in reconstituted skim-milk medium under controlled conditions of pH has been investigated.

At 20°C under aerobic conditions small quantities of oxygen were found to stimulate the growth rate of Str. lactis ML8 between pH 5·05 and 5·45. Growth between pH 5·45 and 6·35 became erratic due to the accumulation of hydrogen peroxide. Above pH 6·35 hydrogen peroxide could not be detected in the continuously growing culture and below pH 5·05 the growth rate of the culture was not affected by small quantities of oxygen. An effect of direct oxygen toxicity on growth, distinct from that of hydrogen peroxide accumulation, was observed over the limited pH range investigated. After 48 h growth in the presence of small concentrations of oxygen the effect disappeared.

At 30°C under anaerobic conditions a region in the dilution rate curves was observed with Str. cremoris AM1 and AM2 and Str. lactis ML8 which suggested the occurrence of a growth-limiting component. This region of slow growth could be correlated with diminished soluble nitrogen values, and was eliminated by enrichment of the medium with soluble nitrogen.

An unusual feature of the investigation was the dilution rates, indicative of extremely fast specific growth rates, experienced for Str. lactis ML8. It was thought unlikely that wall growth contributed appreciably to the dilution rate as a similar phenomenon was not observed with Str. cremoris AM1 or AM2.

Type
Research Article
Information
Journal of Dairy Research , Volume 39 , Issue 1 , February 1972 , pp. 151 - 159
Copyright
Copyright © Proprietors of Journal of Dairy Research 1972

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