Evaluation of different preservation techniques on the storage potential of Kefir grains.

Kefir is an acidic, mildly alcoholic dairy beverage produced by the fermentation of milk with a grain-like starter culture (Koroleva, 1988). These grains usually contain a relatively stable and specific balance of microbes that exist in a complex symbiotic relationship (Obermann & Libudzisz, 1998; Witthuhn et al. 2004). The different groups of microbes present in the grains are active at different stages of the fermentation (Koroleva, 1982). The lactococci, including Lactococcus lactis subsp. lactis, Lc. lactis subsp. cremoris and Lc. lactis subsp. diacetilactis provide rapid acid development during the first hours of the fermentation (Litopoulou-Tzanetaki & Tzanetakis, 2000). As the acidity of the milk increases it provides favourable conditions for the growth of the lactobacilli (Rea et al. 1996). The yeasts, acetic acid bacteria and the aroma-producing microbes, mainly leuconostocs, have a much slower growth rate than the lactic acid producers, resulting in the slow production of the aroma compounds and the gradual increase in the concentration of these substances in the later stages of the fermentation (Koroleva, 1982). In the past the preservation of the microbial populations present in the traditional Kefir grains was achieved by methods including freezing (Garrote et al. 1997), lyophilisation (Oberman & Libudzisz, 1998), air-drying (Kroger, 1993) and refrigeration (Marshall, 1993). Research has shown that traditional Kefir grains preserved by air-drying and lyophilisation retain their activity for up to 12-18 months (Oberman & Libudzisz, 1998). Frozen grains stored at -20 degrees C were found to maintain the microbial activity for up to 7-8 months, whereas grains stored at refrigerated temperatures showed a decreased activity after about 10 d (Oberman & Libudzisz, 1998). The aim of this study was to evaluate the impact of four different preservation techniques on the activity of mass cultured Kefir grains (Schoevers & Britz, 2003). The activity of the grains was evaluated at different time intervals using four activity measurements, including changes in substrate pH, titratable acidity (TA), lactose and lactic acid levels of the final Kefir beverage.