## 2 Results and Discussion
### Sensory Assessment of Kefir  
As anticipated, traditional kefir distinctly differed from lactose-free kefir in the intensity of aroma and taste attributes (Table 3), all of them differing significantly (\(p<0.05\)) between both types of kefir. The intensity of sour aroma was perceived as dominating in traditional kefir, probably resulting from higher abundance of carboxylic acids, mainly acetic, (Table 2). The substantial impact of sourness in aroma profile of traditional kefir was also reported by others [48]. The sensory scores of sour aroma may be also associated with the low pH values of kefir [17]. In traditional kefir, the fermented aroma was perceived as more intense than in the lactose-free one, probably due to about 3-fold higher amounts of alcohol compounds in traditional kefir (14.23%) than in the lactose-free one (5.49%). Some authors indicated that alcohols, especially ethanol and carbon dioxide, may contribute to the formation of fermented and alcoholic aroma of traditional kefir [12; 49]. Furthermore, a substantial content of acetic acid, perceived as vinegar aroma compound, may have contributed to higher scoring of fermented aroma in traditional kefir [11]. Lactose-free kefir had a much higher intensity of creamy aroma than the traditional one (scores 7.3 and 2.4, respectively). The differences in sensory scoring of milky/creamy aroma intensity of the lactose-free kefir were supposedly due to about 3-6-fold higher contents of 3-hydroxy-2-butanone and of 2,3-butanedione than in the traditional one (Table 2); these volatile compounds were reported responsible for buttery aroma of fermented milk products [15; 44]. It should be noted that 3-hydroxy-2-butanone dominated in the volatile profile of lactose-free kefir. We suppose that this compound has a crucial impact on scoring aroma of the lactose-free kefir. The results of rating creamy aroma of traditional kefir are consistent with those published elsewhere; in our study, this aroma attribute was found less perceptible in the traditional kefir, as compared with sour aroma [30; 48]. The fruity aroma of lactose-free kefir had a much higher intensity than the traditional one (scores 4.47 and 1.36, respectively). According to literature data, the abundance of acetaldehyde, nonanal, and terpenes may alter the fruity aroma of kefir [11]. The sourness of traditional kefir, which scored 2.5-fold higher than the lactose-free one, was due to a high content of carboxylic acids, especially acetic acid, and that may have masked the fruity aroma of traditional kefir. A similar intensity of sour taste was also reported for sheep milk kefir stored for 7 days [40]. Traditional kefir was rated as more refreshing and bitter than the lactose-free one, the bitter taste differing remarkably (scores 2.14 and 0.42, respectively). As reported by [PERSON] et al. [27], sharp acid and yeasty flavor, together with prickly sensation due to carbon dioxide produced by yeast, can be considered as the typical kefir flavor. Contrary to traditional kefir, the taste of lactose-free kefir was rated as sweeter and milkier (Table 3). Higher sweetness of lactose-free kefir was undoubtedly due to higher contents of glucose and galactose, compared with traditional kefir (Table 1). Considering sucrose sweetness as a reference (100%), the sweetness of glucose amounts to 70-80%, that of galactose--35%, and of lactose--20% [50]. Increased intensity of sweet taste was scored in kefir manufactured from goat milk, in which lactose was enzymatically bioconverted into galactooligosaccharides (kefir contained 0.1-0.2 g/kg lactose) [51]. Enzymatic bioconversion of lactose into galactooligosaccharides (GOS) makes the product sweeter, as the sweetness of GOS is 35% [52]. We found that the more sour and bitter was kefir, the less sweet was perceived. The intensity of milky taste of lactose-free kefir was much higher than that of traditional kefir (scores 6.19 and 3.61, respectively), maybe due to a high content of diacetyl and