## 2 Results and Discussion
### Profile of Volatile Compounds
Ketones constituted one of the major chemical families of volatile compounds of lactose-free kefir; among the six identified ketones, the 3-hydroxy-2-butanone (acetoin) and 2,3-butanedione (diacetyl) dominated, their content being about 6- and 3-fold higher, respectively, than in the traditional kefir (Table 2). Both compounds are derived from pyruvate, the product of sugar (glucose or lactose) and citrate metabolism [15]. Most Lactobacillus species are able to co-metabolize citrate in the presence of another energy source, such as glucose or lactose, which resulted in the production of the C4 aroma compounds [39]. We suppose that the synthesis of C4 aroma compounds was more effective in lactose-free kefir than in the traditional one, probably due to a higher level of glucose in the lactose-free kefir (Table 1). Some citrate-positive lactic acid bacteria strains, e.g., _Leuconostoc_ spp., are also known to produce ketone compounds from citrates, e.g., 3-hydroxy-2-butanone (acetoin) and 2,3-butanedione (diacetyl) [14]. Much higher contents of those ketones in the lactose-free kefir than in the traditional one (Table 2) may thus have been due to higher counts of _Leuconostoc_ spp. in the former (Table 1). As was previously stated, 2,3-butanedione (diacetyl) may be easily converted to 3-hydroxy-2-butanone (acetoin) by the diaacetyl reductase, probably resulting in the higher domination of 3-hydroxy-2-butanone (acetoin) compared with 2,3-butanedione (diacetyl) in the lactose-free kefir [15; 43]. These two compounds are considered the main contributors to the \"buttery\" flavor of fermented milk [15; 44]. Lactose-free kefir also contained substantial amounts of 2-butanone (7.45%), the product of reduction of 2,3-butanedioI, derived from acetoin [12]. 2-Butanone is a characteristic volatile active component in yoghut and is associated with buttery and sour flavors [12]. 2-Butanone was also previously detected in substantial amounts in kefir samples and was observed from beginning and through fermentation [43]. Among the ketone compounds, only the 2-heptanone content was alike in both types of kefir (Table 2); that compound was reported as one of the key aroma compounds that contribute to \"creamy, fresh, fruity\" flavor of fermented milk [14; 15]. The lactose-free kefir contained significantly more of all other ketones; 2-propanone was rarely detected in fermented milk beverages [14]. The abundance of the abovementioned ketones, plus 2-nonanone, resulted from lipid metabolism by microorganisms [11; 14]. The two types of kefir greatly differed in the number and contents of volatile alcohols (Table 2). Ethanol in kefir is the product of yeasts, e.g., _Saccharomyces cerevisiae_, _Candidia kefir_, _Kluqueromyces lactis_[29]. Higher relative abundance of ethanol in traditional kefir than in the lactose-free one was associated with higher yeast counts in traditional kefir (Table 1). Some Lactobacillus and Lactococcus species are known to produce ethanol (and CO\({}_{2}\)) by the alcohol dehydrogenase-mediated acetaldehyde reduction [13; 15; 29]. This may explain a higher relative abundance of ethanol and lower relative abundance of acetaldehyde in the traditional kefir compared with the lactose-free one (Table 2) [15]. High ethanol share may be associated with yeast flavor present, to some extent, in the traditional kefir [29]. However, the contribution of ethanol to overall aroma and flavor is not clear in the literature. Probably, ethanol provides a complementary flavor and its higher amounts are not desirable [14]. Traditional kefir contains a substantial relative abundance of 2,3-butanediol, regarded as a reduction product of 2,3-butanedione through acetoin [13]. Some other alcohols were also detected in the traditional kefir only, resulting probably from amino acid catabolism [14; 45]. The levels of aldehydes were low compared with other compounds in both types of kefir, ranging from 2.02% to 2.97%.