## 3 Results and Discussion
### Volatile Compounds of Symbiotic Yogurt
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & & & & & & & & \\ \hline
**Volatiles** & **RT a** & **RI b** & **CAS** & **Identification** & **S1** & **S2** & **S3** & **S4** & **S5** \\ \hline
**Acids** & & & & & & & & & \\ Acetic acid & 2.05 & 610 & 64-19-7 & RI, MS & n.d & n.d & 88.19 & n.d & n.d \\ Octanoic acid & 12.18 & 1178 & 124-07-2 & RI, MS & 28.41 & 25.80 & n.d & n.d & n.d \\ Hexanoic acid & 6.35 & 937 & 142-62-1 & RI, MS & 26.27 & 51.13 & 80.66 & 305.09 & 46.26 \\ \hline
**Esters** & & & & & & & & \\ Formic acid, isopentyl ester & 2.39 & 831 & 110-45-2 & RI, MS & n.d & 99.30 & n.d & n.d & n.d \\ Butanoic acid, 3-methyl-, & 9.82 & 1105 & 659-70-1 & RI, MS & n.d & n.d & n.d & n.d & 29.09 \\ Octanoic acid, methyl ester & 10.42 & 1123 & 111-11-5 & RI, MS & n.d & n.d & n.d & 22.93 & 20.22 \\ Octanoic acid, ethyl ester & 12.8 & 1197 & 106-32-1 & RI, MS & 34.48 & 53.46 & 101.09 & 207.18 & 180.43 \\ Decanoic acid, ethyl ester & 19.2 & 1395 & 110-38-3 & RI, MS & n.d & n.d & n.d & n.d & 29.01 \\ Hexadecanoic acid, ethyl ester & 35.43 & 2093 & 628-97-7 & RI, MS & log & log & log & 42.37 & 16.40 \\ \hline
**Aldehydes** & & & & & & & & & \\
3-Furaldehyde & 3.41 & 835 & 498-60-2 & RI, MS & n.d & n.d & 50.57 & 51.43 & 48.52 \\
5-Hydroxymethylfurural & 13.85 & 1224 & 67-47-0 & RI, MS & n.d & n.d & 115.41 & n.d & n.d \\ \hline
**Alcohols** & & & & & & & & \\
3-Furanmethanol & 3.85 & 864 & 4412-91-3 & RI, MS & n.d & n.d & 80.89 & 85.54 & 82.12 \\ Phenylethyl alcohol & 10 & 1110 & 60-12-8 & RI, MS & n.d & n.d & 128.23 & 75.12 & 51.70 \\ Maltol & 10.13 & 1115 & 118-71-8 & RI, MS & n.d & n.d & 62.50 & 70.33 & 77.12 \\
2-(2-Butoxyethoxy)-ethanol & 12.52 & 1188 & 112-34-5 & RI, MS & n.d & n.d & n.d & 20.32 & 18.01 \\ \hline
**Ketones** & & & & & & & & \\
2-4-Dihydroxy-2.5- & & 6.3 & 989 & 10,230-62-3 & RI, MS & n.d & n.d & 84.78 & 80.09 & 78.22 \\ dimethyl-3(2H)-furan-3-one & & 11.6 & 1146 & 28,564-83-2 & RI, MS & n.d & n.d & 258.34 & 21.92 & n.d \\ \hline
**Pyrazole** & & & & & & & & & \\
3-Methoxycarbonylpyrazole & 9.16 & 1083 & 15,366-34-4 & RI, MS & n.d & n.d & 82.43 & 85.44 & 90.23 \\ \hline
**Others** & & & & & & & & & \\ Methoxy-phenyl- oxime & 4.32 & \(\ldots\) & MS & 45.61 & log & 228.48 & 229.47 & 40.80 \\ Naphthalene & & 12.2 & 1178 & 91-20-3 & RI, MS & n.d & n.d & n.d & 136.91 & 140.44 \\ \hline \hline \end{tabular}  
* RT— Retention time; b— Retention index was determined with HP-5 column by injection of a mix on \(n\)-alkane (6-c23). MS, mass spectra; Si—control plain yogurt, S2—yogurt with _S. boulardi_, Si—yogurt with _S. boulardi_ + 1% inulin, S4—yogurt with _S. boulardi_ + 1% inulin, S4—yogurt with _S. boulardi_ + 15% inulin, S5—yogurt with _S. boulardi_ + 2% inulin, n.d—not detected, log—low of quantitation.  
\end{table} 
Table 2: Volatile compounds of the yogurt samples identified by HS-SPME-GC-MS.  
Aldehydes are considered as important aroma compounds contributing to the volatile profile of fermented dairy products with lactic acid bacteria [43; 44]. In the symbiotic yogurt of this study, 5-hydroxymethylfurfural (115.41 ug/L) and 3-furaldehyde (about 50 ug/L) were detected at relatively high levels. Acid compounds were generally present in various fermented dairy products [45; 46]. Among the three acid volatiles identified, hexanoic acid was found in all the yogurt samples at a concentration ranging from 26.27 ug/L (S1) to 305.09 ug/L (S4), while octanoic acid was detected in the yogurt samples (S1, S2) without inulin, and acetic acid only in one symbiotic yogurt sample (S3). Ester volatiles were generally produced at low concentration in dairy products when lactose was fermented by LAB [47]. Octanoic acid ethyl ester and hexadecanoic acid ethyl ester were detected in all the yogurt samples of this study, but formic acid isopentyl ester was detected only in the probiotic yeast yogurt. Other three esters were found in the symbiotic yogurt samples. Ketone compounds play a key role in the creamy flavor of dairy products [48].