Exopolysaccharides are carbohydrate polymers that can be produced by several microorganism, consisting of a long linear or branched chains of monosaccharides as glucose, galactose, manose, glucuronic acid, rhamnose; among others (Welman and Maddox 2003; Botelho et al. 2014). Kefiran is the most abundant exo-polysaccharide found in kefir, and is present in the cell free fraction (De Moreno de LeBlanc 2006).
Organic acids as orotic, citric, pyruvic, lactic, uric, acetic, propionic, butyric, and hippuric acids can be found in kefir beverage (Guzel-Seydim et al. 2000; Ferreira 2014). These acids directly influence the flavor properties and directly affect the sensory characteristics. The organic acids are natural preservatives, associated to the inhibition of certain food pathogens.
The power of microorganism inhibition is related to different mechanisms of mode of action of organic acids. The undissociated forms of organic acids improve the inhibitory power against pathogens by the kefir grains (Guzel-Seydim et al. 2000). The undissociated forms of lactic acid and acetic acid founded in kefir beverage implies a modification of permeability of cell membranes through the acidification of cytoplasm and degradation of enzymes inhibiting the growth of some pathogenic species like Escherichia coli and Bacillus cereus (Guzel-Seydim et al. 2000; Garrote et al. 2000; Ferreira 2014).
The down grade of the pH promoted by organic acids produced during kefir fermentation influence an increase in inhibition of pathogenic bacteria. However, the organic acids, except for lactic acid, changes very little during kefir storage (Guzel- Seydim et al. 2000).