## 1 Introduction  
Kefir is a fermented, refreshing milk beverage, traditionally prepared by inoculation of raw milk with kefir grains. Kefir grains contain diverse species of lactic acid bacteria (Lactobacillus, Lactococcus, Leuconstoc, and Streptococcus), yeasts (Kluyveromyces, Candida, Saccharomyces and Pichia), acetic acid bacteria, and mycelial fungi in a protein/polysaccharide matrix [1, 2]. Lactic acid bacteria and yeasts exist in a complex symbiotic relationship and are responsible for alcoholic and lactic fermentation, respectively [3]. Kefir contains more than 50 species of probiotic microorganisms, acting beneficially in the gastrointestinal tract via adhesion to the intestinal mucus and interference with pathogenic bacteria [2, 4]. The microbial fermentation of kefir produces many valuable organic compounds, such as bioactive peptides, amino acids, exopolysaccharide, bacteriocins, antibiotics, hydrogen peroxide, vitamins (B\({}_{1}\), B\({}_{12}\)), and calcium [2, 4]. These compounds may act independently or interact, bringing about diverse health benefits attributed to kefir consumption [5]. For example, the synergistic effect of carbonyl compounds, histone, and cathelicidin with organic acids was recently reported [6]. Many authors observed antimicrobial activity of kefir against enteric bacterial pathogens, including _Escherichia coli_, _Listeria monocytogenes_, _Salmonella enterica_, _Salmonella_ species (_Typhimuriumium_ and _Arizona_), _Shigella flexneri_, _Yersinia enterocolictica_, _Staphylococcus aureus_, and _Enterococcus faecalis_[6; 7; 8; 9]. Due to the potential health benefits conferred by probiotic microorganisms and/or their metabolites, the consumption of kefir is popular in Europe, Asia, and South and North America [10]. Microbial species in kefir grains also play a crucial role in generating volatile compounds that contribute to the formation of the unique flavor properties of traditional kefir [11]. The use of gas chromatography-mass spectrometry (GC-MS) enabled detecting numerous volatile compounds in the traditional kefir, mainly carboxylic acids, alcohols, aldehydes esters, and ketones [11; 12; 13; 14; 15]. Acetaldehyde, ethanol, acetoin, diacetyl, and carbon dioxide are major compounds that contribute to the creamy, refreshing, and fermented aroma. Lactic acid provides a slightly sour taste of kefir [16; 17]. Olfactory and taste perceptions get reduced with aging [18]. Since odor and taste perceptions strongly depend on the volatile components of foods, diminished olfactory function decreases their perception of elderly consumers [18]. It was found that the elderly have higher optimum preferred flavor concentrations than young people [19]. Aging is also associated with digestion dysfunctions and absorption of nutrients [20]. Some conditions causing maldigestion or malabsorption seem to be more common in elderly patients [20]. These include exocrine pancreatic insufficiency, small intestinal bacterial overgrowth, enteropathies, vascular diseases, diabetes, and certain intestinal infections [20; 21]. Changes of gut microflora in the elderly appear to bring about a reduction of beneficial bacteria (Lactobacillus and Bifidobacterium), and an increase of potentially pathogenic species [22]. Many researchers reported that some probiotic strains may help to keep health in old people, suggesting both health and cost-saving benefits in offering fermented dairy products. These benefits include establishment of balanced intestinal microflora; improving colonization resistance and/or prevention of diarrhea; reduction of fecal enzymes; reduction of serum cholesterol; reduction of potential mutagenes; reduction of lactose intolerance; synthesis of vitamins; and predigesting of proteins [22]. Thus, nutritive and rich in microbial species kefir seems to be appropriate beverage for the elderly. However, lactose intolerance restricts the consumption of traditional kefir by the elderly.