Kefir is a refreshing fermented milk with a slightly acidic taste obtained by incubating milk with kefir grains (Saloff-Coste, 1996). Kefir grains are a complex mixture of lactic acid bacteria and yeasts in a strong and specific association. They are characterized by an irregular form, a folded and uneven surface and a white or slightly yellow colour. They are tough and resilient and have a characteristic acid taste (Bottazzi et al. 1994). The basic microflora contains lactococci, homofermentative and heterofermentative lactobacilli, yeasts and acetic acid bacteria (Bottazzi et al. 1994; Rea et al. 1996). Among the yeasts isolated from grains and identified are Candida kefir, Saccharomyces cerevisiae, Sac. delbrueckii, Cand. holmii, Sac. unisporus and Sac. lipolytica (Angulo et al. 1993; Marshall, 1993; Garrote et al. 1997). Lactobacillus brevis, Lb. viridescens, Lb. casei, Lb. kefir, Lb. kefiranofaciens, Lb. kefirgranum, Lb. parakefir, Leuconostoc spp. and Lactococcus lactis are among the lactic acid bacteria present in the grains (Marshall et al. 1984; Toba et al. 1991; Takizawa et al. 1994; Garrote et al. 1997). The mixed microflora of yeasts and bacteria is held together by a matrix containing (g/kg dry weight) protein 340, polysaccharide 470 (Ottogalli et al. 1973).

The study of kefir grains in milk has been centred on the characteristics of the polysaccharide produced by lactobacilli within the grain (Yokoi et al. 1991). This polysaccharide, named kefiran, is composed of glucose and galactose (Yokoi et al. 1991). It has been suggested that proteins are incorporated from the growth media (Bassette & Acosta 1988), but no details about structure and composition are available.

Soyabeans are an important component of the diet in many countries and have been used to obtain fermented products such as sogurt (Mann, 1991). Special attention has been given to the growth of, and sugar utilization by, Bifidobacterium spp., Lb. delbrueckii subsp. bulgaricus and Streptococcus thermophilus in soya milk (Buono et al. 1990; Murti et al. 1993; Ankenman Granata & Morr, 1996). To our knowledge, there have been no attempts to ferment soya milk with kefir grains.

The aim of this study was to investigate the growth of kefir grains in soya milk and the composition of these grains, focusing on the matrix proteins.