Investigation of the Yeast Flora in Dairy Products: A Case Study of Kefyr
Marie-Therese Wyder*, Hans Spillmann, Zdenko Puhan
Laboratory of Dairy Science, Institute of Food Science, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland
Article history:
Received September 10, 1997
Accepted November 24, 1997
Key words:
kefyr, yeast flora composition, electroplioretic karyotypes, identification
Summary:
The traditional starter for manufacture of kefyr is kefyr grain which consists of a gelatinous matrix in which lactic acid bacteria, yeasts and mostly also acetic acid bacteria are embedded. The typical microflora of kefyr is, however, not exactly defined. The aim of this project was to isolate and characterise yeasts from kefyr grains and kefyr in order to learn about the diversity of the yeast flora. The isolated yeasts were characterised and grouped by electrophoretic karyotyping in the contour-clamped homogeneous electric field and by traditional methods. In the five investigated kefyr grains and kefyr obtained with them, the following yeast species could be identified: Candida kefyr, Kluyveromyces marxianus, Candida colliculosa, Torulaspora delbrucckii, Saccharomyces unisporus, Brettanomyces anomalus and a still unknown yeast U. The composition of the yeast flora was strongly dependent on the production procedure. In grains with at least two different species yeast U always dominated; only when absent, Sacch. unisporus was predominant. But during fermentation, the lactose fermenting yeasts K. marxianus and C. kefyr could catch up with yeast U. When kefyr was taken as inoculum, the composition of yeast flora changed completely during fermentation. Where K. marxianus or C. kefyr were present, they outweighed all other species; if absent, however, Sacch. unisporus dominated, yeast U disappeared and the total yeast count decreased. It can be concluded that the composition of yeast flora in traditional kefyr is quite homogeneous and depends strongly on the inoculum, i.e. whether kefyr grain or kefyr is used. Three out of five isolated yeast species were non lactose fermenting. Karyotypes for yeasts other than K. marxianus or C. kefyr were species specific.
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