## 2 Results and Discussion
### Sensory Assessment of Kefir
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Attributes** & **Lactose-Free Kefir** & **Traditional Kefir** \\ \hline \multicolumn{3}{c}{**Aroma**} \\ Fermented & 2.62 \(\pm\) 0.18 & 3.78 \(\pm\) 0.25 \({}^{*}\) \\ Creamy & 7.29 \(\pm\) 0.32 & 2.41 \(\pm\) 0.13 \({}^{*}\) \\ Sour & 1.80 \(\pm\) 0.16 & 4.55 \(\pm\) 0.27 \({}^{*}\) \\ Yeasty & 0.74 \(\pm\) 0.11 & 1.63 \(\pm\) 0.13 \({}^{*}\) \\ Sweet & 2.88 \(\pm\) 0.26 & 0.42 \(\pm\) 0.09 \({}^{*}\) \\ Fruity & 4.47 \(\pm\) 0.35 & 1.36 \(\pm\) 0.11 \({}^{*}\) \\ \hline \multicolumn{3}{c}{**Taste**} \\ Sour & 2.81 \(\pm\) 0.24 & 7.33 \(\pm\) 0.41 \({}^{*}\) \\ Milky & 6.19 \(\pm\) 0.33 & 3.61 \(\pm\) 0.23 \({}^{*}\) \\ Sweet & 4.78 \(\pm\) 0.36 & 0.67 \(\pm\) 0.10 \({}^{*}\) \\ Bitter & 0.42 \(\pm\) 0.08 & 2.14 \(\pm\) 0.12 \({}^{*}\) \\ Refreshing & 4.67 \(\pm\) 0.21 & 5.28 \(\pm\) 0.35 \({}^{*}\) \\ \hline \multicolumn{3}{c}{**Mouthfeel**} \\ Mouthcoat & 6.95 \(\pm\) 0.40 & 4.73 \(\pm\) 0.26 \({}^{*}\) \\ Airy & 3.22 \(\pm\) 0.19 & 5.60 \(\pm\) 0.31 \({}^{*}\) \\ Astringent & 2.16 \(\pm\) 0.14 & 4.38 \(\pm\) 0.24 \({}^{*}\) \\ \hline \hline \end{tabular} \({}^{*}\)Significantly (\(p<\) 0.05) different from the lactose-free kefir.  
\end{table} 
Table 3: Mean values (\(\pm\)SD) of the intensities of aroma, taste and mouthfeel attributes of kefir assayed by trained panel (n = 10) (no intensity—0; very high intensity—10). 2,3-butanedione (Table 2). In fermented dairy products, various compounds with four carbon atoms: diacetyl, acetoin, and 2,3-butanediol are responsible for the typical butter-like taste [53]. These compounds can be generated by glycolysis or by citrate metabolism of several lactic acid bacteria, e.g., Lactococcus and Leuconostoc species [53]. Among these C4 compounds, diacetyl is the most important flavor compound due to its low threshold. The important effect of diacetyl on the milk aroma has been known since 1929, when it was shown that the distinctive aroma of fermented milk could be sensed when the concentration of diacetyl reached 1 mg/kg [53]. Both _Streptococcus thermophilus_ and _Lactobacillus bulgaricus_ are able to produce diacetyl, and strains of _Lactococcus lactis_ subsp. _lactis_ biovar. _diacetyl lactis_ may accumulate significant amounts of diacetyl due to their high capacity to metabolize citrate. Acetoin is the reduced form of diacetyl, and its flavor is considerably weaker than that of diacetyl. However, acetoin reduces the harshness of diacetyl and contributes to the mild creamy flavor. 2,3-Butanediol is the reduced form of acetoin, contributing somewhat to the creamy or buttery attribute [53]. Unlike lactose-free kefir, traditional kefir was distinguished by highly scored sour attribute (Table 3). High intensity of sourness of traditional kefir resulted from higher counts of lactic acid bacteria than in the lactose-free kefir, which convert lactose to lactic acid [53]. Regarding mouthfeel attributes, the mouthcoat of the lactose-free kefir was rated as more intense and smoother than of the traditional one (scores 6.95 and 4.73, respectively; Table 3). This is of high importance for dairy products as it may reflect product ability to form a film coating on the tongue and palate during consumption [54]. This could also be affected by much higher ratings of milky/creamy attributes of lactose-free kefir. Interestingly, airy attribute was scored significantly higher in traditional (scores) than in the lactose-free kefir (scores 5.6 and 3.22, respectively), probably due to the refreshing sensations and slightly higher level of carbonation (data not shown). The astringent mouthfeel was found to be about two times higher in the traditional kefir samples as compared with lactose-free kefir (scores 4.38 and 2.16, respectively). This may have resulted from the sourness and fermented attributes perceived as more intense in traditional kefir (Table 3). Mouthfeel is regarded as of high importance for elderly subjects, who commonly experience taste loss and olfactory dysfunction [55; 56].