viernes, mayo 17, 2002

Exopolysaccharides Produced by Lactic Acid Bacteria of Kefir Grains

Ginka I. Frengovaa, Emilina D. Simovaa,*, Dora M. Beshkovaa and Zhelyasko I. Simovb a Laboratory of Applied Microbiology, Institute of Microbiology, Bulgarian Academy of Sciences…


Exopolysaccharides produced by lactic acid bacteria have generated increasing attention among researchers for the last few years. The lactic acid bacteria are food-grade organisms, and the exopolysaccharides that they produce contribute to the specific rheology and texture of fermented milk products and may have application in nondairy foods. When added to food products, polysaccharides function as thickeners, stabilizers, emulsifiers, gelling agents, and water binding agents (Giraffa,1994; Crescenzi, 1995). Kefir Ð a unique product among the cultured milk varieties Ð is produced with an original native starter (kefir grains). Kefir is defined as the yogurt of the 21st century (Gorski,1994). The kefir grains consist of slimy materials in which yeast and bacterial cells are firmly embedded.
The polysaccharide matrix, forming the structure of the kefir grain, is kefiran, identified by a number of researchers (La Riviere et al., 1967; Neve, 1992; Pintado et al., 1996). The lactic acid bacteria, yeast and polysaccharide “kefiran” that make up the kefir grains have been described as a symbiotic community that impart unique properties to kefir (Margulis, 1996).
Investigations into the active producers of kefiran are controversial. Although La Riviere et al. (1967) reported that Lactobacillus brevis, now regarded as Lactobacillus kefir, was responsible for kefiran production, Kandler and Kunath (1983) concluded that Lactobacillus kefir was not a kefiran producer. According to other authors, the principal producer of the kefiran polymer in kefir grains is Lactobacillus kefiranofaciens and several other unidentified species of Lactobacillus (Mitsue et al., 1998, 1999; Yokoi et al., 1990; Toba et al., 1987). Thus it remains undecided which microorganism is responsible for kefiran production in kefir grains. Exopolysaccharide production is an important feature of lactic acid bacteria characterization in forming starter cultures for fermented milk products with suitable texture and specific rheology.
The present paper reports on kefiran production by lactic acid bacteria, isolated from kefir grains, and selection of an active producer of kefiran with view of including it in a kefir starter. There is no information available about production of exopolysaccharides by single strain cultures and kefir starter cultures during kefir fermentation and storage.

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