## 2 Results and Discussion
### Profile of Volatile Compounds
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Compounds**} & \multicolumn{3}{c}{**Peak Relative Abundance (\%)**} \\ \cline{2-4}  & **Rt** & **L lactose-Free Kefir** & **Traditional Kefir** \\ \hline \multicolumn{2}{c}{_Alcohols_} & \\ Ethanol & 1.67 & 1.52 \(\pm\) 0.11 & 2.20 \(\pm\) 0.16 * \\ Ethanethiol & 1.85 & 1.83 \(\pm\) 0.36 & 2.37 \(\pm\) 0.24 * \\
1-Pentanol & 4.30 & 0.44 \(\pm\) 0.18 & 1.69 \(\pm\) 0.05 * \\
3-Methyl-1-butanol & 4.41 & nd & 1.17 \(\pm\) 0.02 \\
2,3-Butanediol & 6.27 & 1.08 \(\pm\) 0.33 & 5.94 \(\pm\) 0.51 * \\
2-Pentanol & 7.07 & nd & 0.35 \(\pm\) 0.05 \\
1-Hexanol & 11.39 & nd & 0.25 \(\pm\) 0.00 \\
2-Heptanol & 13.47 & 0.61 \(\pm\) 0.00 & 0.27 \(\pm\) 0.02 * \\ _Total_ & & 5.49 \(\pm\) 0.17 & 14.23 \(\pm\) 0.20 * \\ \hline \multicolumn{2}{c}{_Acids_} & \\ Accite acid & 2.58 & 24.99 \(\pm\) 1.28 & 43.30 \(\pm\) 1.56 * \\ Butanoic acid & 6.78 & 1.27 \(\pm\) 0.23 & 1.27 \(\pm\) 0.39 \\ Hexanoic acid & 19.37 & 3.57 \(\pm\) 0.39 & 4.10 \(\pm\) 0.21 \\ Octanoic acid & 26.86 & 0.48 \(\pm\) 0.05 & 0.52 \(\pm\) 0.06 \\ _Total_ & & 30.30 \(\pm\) 0.49 & 49.18 \(\pm\) 0.84 * \\ \hline \multicolumn{2}{c}{_Aldehydes_} & \\ Acetaldehyde & 1.49 & 0.47 \(\pm\) 0.05 & 0.22 \(\pm\) 0.03 * \\
3-Methylbutanal & 2.81 & 1.09 \(\pm\) 0.12 & 2.59 \(\pm\) 0.11 * \\ Benzaldehyde & 16.80 & 0.08 \(\pm\) 0.03 & 0.02 \(\pm\) 0.02 * \\ Nonanal & 23.86 & 0.39 \(\pm\) 0.02 & 0.12 \(\pm\) 0.02 * \\ _Total_ & & 2.02 \(\pm\) 0.06 & 2.97 \(\pm\) 0.04 * \\ \hline \multicolumn{2}{c}{_Ketones_} & \\
2-Propanone & 1.77 & 4.35 \(\pm\) 0.40 & 8.96 \(\pm\) 0.55 * \\
2,3-Butanedione & 2.19 & 7.05 \(\pm\) 0.86 & 2.23 \(\pm\) 0.23 * \\
2-Butanone & 2.25 & 7.45 \(\pm\) 0.05 & 10.93 \(\pm\) 0.78 * \\
3-Hydroxy-2-butanone & 3.73 & 39.26 \(\pm\) 2.27 & 6.95 \(\pm\) 0.40 * \\
2-Heptanone & 12.74 & 0.69 \(\pm\) 0.18 & 0.78 \(\pm\) 0.06 * \\
2-Nonanone & 23.35 & 0.96 \(\pm\) 0.00 & 0.36 \(\pm\) 0.02 * \\ _Total_ & & 59.75 \(\pm\) 0.80 & 30.22 \(\pm\) 0.47 * \\ \hline \multicolumn{2}{c}{_Terpenes_} & \\ \(\alpha\)-Pinene & 15.14 & 0.18 \(\pm\) 0.01 & 0.35 \(\pm\) 0.18 * \\ \(\beta\)-Pinene & 17.54 & 0.29 \(\pm\) 0.02 & 0.40 \(\pm\) 0.21 \\
3-Carene & 19.37 & 0.22 \(\pm\) 0.02 & 0.32 \(\pm\) 0.16 \\ _m_-Cymene & 20.10 & 0.08 \(\pm\) 0.02 & 0.15 \(\pm\) 0.06 * \\ D-Limonene & 20.30 & 0.33 \(\pm\) 0.00 & 0.52 \(\pm\) 0.37 * \\ Copaene & 33.81 & 0.07 \(\pm\) 0.01 & 0.12 \(\pm\) 0.02 \\ _Total_ & & 1.17 \(\pm\) 0.02 & 1.86 \(\pm\) 0.15 * \\ \hline \multicolumn{2}{c}{_Other compounds_} & \\ Toluene & 5.23 & 0.11 \(\pm\) 0.03 & 0.16 \(\pm\) 0.01 \\ Dimethyl disulfide & 4.54 & 0.91 \(\pm\) 0.35 & 0.82 \(\pm\) 0.20 * \\
2-Methyltetrahydro- & 18.21 & 0.24 \(\pm\) 0.07 & 0.56 \(\pm\) 0.25 \\ _Total_ & & 1.27 \(\pm\) 0.18 & 1.54 \(\pm\) 0.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Volatile compounds identified in kefir and their relative abundance (means \(\pm\) SD, n = 9). (30.30% and 49.18% in the lactose-free and traditional kefir, respectively). The presence of acetic acid in fermented beverages could be attributed to heterofermatterative lactic acid and acetic acid cultures present in kefir grains microflora [3]. Acetic acid may contribute to acidic, vinegar, and pungent odor notes of kefir [14]. Heterofermatterative species convert lactose into lactic acid, acetic acid, ethanol, and CO\({}_{2}\)[41]. Acetic acid may have resulted from the metabolism of ethanol. Three other acids, butanoic, hexanoic, and octanoic, i.e., fatty acids, were detected in kefir samples in much lower quantities than acetic acid. Their contents did not differ significantly between the two types of kefir (Table 2). These acids probably originated from milk fat lipolysis [42]. Hexanoic acid can contribute to the sweaty, cheesy, and acidic aroma of kefir [11].