UBC Theses and Dissertations
The identification, characterization, and utilization of the ropy lactic streptococci strains and their extracellular excretion Macura, Dragan Mirkov
Traditional cultured milk products have been a source of beneficial lactic acid bacteria whose potential ls constantly being exploited in health and nutrition by modern scientists. One such source is the Scandinavian ropy sour milk, whose constituent microflora produces a unique extracellular excretion which prevents syneresis and graininess, and produces a thicker, smoother, and creamier product than normal sour milk. The industrial potential of these bacteria lies in that they could be used in modern cultured dairy products instead of the bacterial and plant gum stabilizers, or extra butterfat, which are currently used to alter the rheological properties. However, when grown in pure culture, these bacteria gradually lose their ropy character, indicating culture instability. This research project attempted to provide the basic information needed to control the ropy cultures so that reliable ropiness can be assured, and thus large scale industrial applications be made possible. It described the growth characteristics of four ropy isolates, identified their species, isolated and characterized the viscous material produced by one of the ropy isolates, investigated the antimicrobial activity against the major milk spoilage bacteria, studied the compatibility of one ropy strain with buttermilk starter bacteria, and demonstrated the beneficial contribution to the viscosity of commercial buttermilk by this ropy strain. It was found that of four pure culture ropy isolates, one (LLF) was a ropy variant of S. lactls, previously named S. lactis longi. The remaining three (L416, 701 and 705) were, thus far unnamed, variants of S. cremoris, with a suggested name S. cremoris longi. Very good ropiness was produced in liquid milk, neutralized cheese whey, M 17 medium of Terzaghi and Sandine (1975), and a complex casein digest medium, as well as on tomato juice agar and skim milk agar. The culture instability was found to be due to the serial culture transferring, and due to the prolonged product incubation at growth temperature. The analyses of the viscous material showed that it was composed of 47% total protein, 20% methylpentoses, 9.3% protein-bound hexoses, 2.8% sialic acids, and no hexosamines. The rest of the molecle remains to be identified. The ropy culture L416 was found to be inhibitory to ATCC strains of E. coli, S. aureus, and to P. fluorescens biotype A, while it showed no deleterious effects against the standard bacteria of buttermilk starter. Addition of the ropy culture to the commercial buttermilk starter resulted in significant contribution ([sup α] ≤ 0.05) to the equilibrium apparent viscosity, and the consistency of the product.
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