## 3 Results and Discussion
### Antioxidant Activities of Yogurt Samples
#### 3.1.2 Total Phenol Content of Yogurt Samples during Storage
_boulardii_ in common. Among these samples, S3-Syn1 had the highest limonene concentration (162.28 \(\pm\) 7.30 mg/100 g), while S5-Syn2 had more \(L\)-limonene concentration (102.29 \(\pm\) 5.66 mg/100 g) in weeks 4 and 2, respectively. Limonene is generally considered safe as a food additive or flavoring and a fragrance additive. Previous studies in the literature have revealed that limonene can present antimicrobial, antifungal, antimalarial, and antitumoral activities [49]. A total of six volatile compounds in the control S1-C were formed--namely, one alcohol (ethanol), one ketone (acetoin), and four acids (acetic acid, butanoic acid, hexanoic acid, and octanoic acid) at the end of storage. In the fourth week, only four volatiles (ethanol = 1.72 \(\pm\) 0.66 mg/100 g, acetoin = 36.16 \(\pm\) 3.09 mg/100 g, hexanoic acid = 9.60 \(\pm\) 0.91 mg/100 g, and octanoic acid = 14.37 \(\pm\) 1.40) were present. Among alcohols, ethanol was the predominant compound present in all yogurt samples, but the concentration was lower in S1-C, ranging from 0.83 \(\pm\) 0.15 at week 1 to 1.72 \(\pm\) 0.66 mg/100 g at week 4, compared with probiotic S2-P, ranging from 28.13 \(\pm\) 2.76 at week 1 to 59.89 \(\pm\) 4.33 mg/100 g at week 4, and much higher concentration in symbiotic yogurt samples S3-Syn1, ranging from 93.32 \(\pm\) 7.34 at week 1 to 306.81 \(\pm\) 22.08 mg/100 g at week 4 of the refrigerated storage period. These results indicated that it is a low alcohol product, compared with kefir fermented with yeast and lactic acid bacteria. In terms of the final fermentation products, kefir lactic acid and ethanol were the main ones, whereas only traces of acetic acid were detected. The concentration of ethanol was \(\sim\)0.9% \(w/v\) (\(\sim\)9 g/L), as reported by [PERSON] et al. [50]. Similarly, [PERSON] et al. [51] also reported that ethanol was one of the main components in volatiles, in their study of the effect of lactic acid bacteria and yeasts on the structure and fermentation properties of Tibetan kefir grains. The other alcohol (phenylethyl alcohol) was only found in the symbiotic yogurt samples (S4-Syn1.5 = 13.09 \(\pm\) 0.98 and S5-Syn2 = 16.51 \(\pm\) 2.11 mg/100 g at week 3), having a higher concentration of inulin. Yeast cells are capable of producing phenyl ethyl alcohols via normal metabolism. This can be synthesized through two independent routes in yeast cells, either de novo via the Shikimate pathway or via the Ehrlich pathway [52]. Alcoholic compounds also contribute to flavor improvement in fermented dairy products. A total of four acids (acetic acid, butanoic acid, hexanoic acid, and octanoic acid) were identified in the yogurt samples having higher concentrations in the descending order of acetic acid \(\leq\) hexanoic acid \(\leq\) octanoic acid \(\leq\) butanoic acid found in symbiotic samples, followed by probiotic and control. Acetic acid was one of the main acids in yogurt samples, and significant differences were observed among symbiotic, probiotic, and control samples in terms of its concentration. Acidity is a decisive factor for flavor acceptance of yogurt and is maintained near pH 4.5 [53]. Acid compounds are generally present in various fermented dairy products [54]. Two short-chain fatty acid compounds (propanoic acid and 2-methyl-butanoic acid) were present in probiotic and symbiotic samples having \(S\). _boulardii_ in common. However, they were not found in control plain yogurt. This might be credited to the ability of \(S\). _boulardii_ CNCM I-745 to secrete alkaline phosphatase, which causes dephosphorylation of phospholipid substrates to generate phosphate and other short-chain fatty acids [55,56]. Three esters (1-butanol-3-methyl-acetate, butanoic acid-3-methyl butyl ester, and octanoic acid ethyl ester) were identified in only probiotic and symbiotic yogurt samples.