## 3 Results and Discussion
### Goat's Milk and Goat's Milk with Permeate with GOS as a Raw Material for the Production of Kefir  
The kefir taste and odor evaluation values are presented as multidimensional exploration lines (Figure 1). This allowed the identification of the interaction between components of the kefir flavor profile of milk and the permeate mixture. Three categorical ranges were used for the values of the descriptors being evaluated. The majority of samples contained a kefir-like odor and kefir-like taste. A smaller but statistically significant number of samples were evaluated as having a sour odor, sour taste, and aftertaste. Other flavors and odors had the least effect on the sample characteristics of kefir. However, the enzymatic hydrolysis and transgalactosylation of lactose made kefirs made from this raw material sweeter in taste and have an aftertaste. The intensity of the bitter taste decreased. This was noticeable in both kefir with goat's milk and its mixture. [PERSON] et al. [41] showed, using kefir as an example, that the more sour and bitter it was, the less sweet it was. The taste of lactose-free kefir was sweeter and milkier compared to kefir with typical lactose content. The sweetness of kefir could be due to the presence of the monosaccharides glucose and galactose, which are 80% and 35% sweet, respectively. It should also be mentioned that the sweetness of GOS is 35% [41]. The addition of permeate to milk intensified the cream odor in kefir, but after hydrolysis and transgalactosylation, this odor was almost imperceptible. Quite the opposite was found in the case of milk kefir. In addition, the presence of permeate in kefir increased the buttermilk taste, and the presence of GOS intensified it even more. On the other hand, the presence of GOS in kefir from the mixture of milk and permeate increased the sour odor and weakened the kefir-like odor. It was also observed that GOS formed as a result of the enzymatic conversion of lactose, and the change in the casein-to-serum-protein ratio from 4.2 in goat's milk to 1.1 in goat's milk after microfiltration may have a positive effect on the composition of microorganisms characteristic of kefir obtained from goat's milk with permeate (Table 5). It was shown that after 14 days of refrigerated storage, the mesophilic LAB and yeast counts were 8.2-fold and 2.1-fold higher, respectively, in the sample of milk subjected to the enzymatic conversion of lactose (KM/GOS) than in the non-reacted milk (KM). Additionally, a greater number of mesophilic LAB were observed in milk with permeate. The process of the enzymatic conversion of lactose resulting in the formation of GOS in goat's milk with permeate (KM + P/GOS) resulted in about 6.6-fold more mesophilic LAB than in the kefir sample without the (KM + P) reaction. These parameters may change with the further expiration of the refrigerated storage time. The mesophilic LAB count is similar to those in other studies. [PERSON] et al. [42] reported that the number of _Lactococcus lactis_ subsp. _lactis_, _Lactococcus lactis_ subsp. _cremoris_, _Lactococcus lactis_ subsp. _lactis_ biovar. _diacetylactis_, _Streptococcus thermophilus_, _Lactobacillus kefir_, and _Lactobacillus acidophilus_ was stable during refrigeration and amounted to approx. 9.3 log cfu/mL. In our research, the number of yeast cells in all kefir samples (on average, approx. 3.66 cfu/mL) was higher compared to the result reported by [PERSON] et al. [42], where it was approx. 2.3 log cfu/mL. As reported by the Codex Standard for fermented milk, the number of yeast cells in kefir should be at least 4 log cfu/mL [43]. Figure 1: Categorizing the kefir flavor profile of goat’s milk and goat’s milk with permeate after enzymatic conversion of lactose. From left to right: KM—kefir from goat’s milk; KM/GOS—kefir from goat’s milk after enzymatic conversion of lactose; KM + P—kefir from goat’s milk with permeate; KM + P/GOS—kefir from goat’s milk with permeate after enzymatic conversion of lactose.