## 3 Results and Discussion
### Antioxidant Activities of Yogurt Samples
#### 3.1.1 Radical Scavenging Assay during Storage  
The DPPH radical is one of the rare stable organic nitrogen radicals and can take up an electron or hydrogen radical to become a stable diamagnetic molecule. The stable DPPH radical scavenging model is a broadly used method to evaluate antioxidant activity in a short period of time, compared with other methods. The effect of antioxidants on DPPH radical scavenging is related to their ability to donate hydrogen [36]. The DPPH activities of probiotic and symbiotic yogurt samples were lowest after day 1 storage, when values, including control (S1-C), were below 50%. However, after 14 days of storage, the activities of probiotic (S2-P = 54.65%) and symbiotic (S3-Syn1 = 61.01%, S4-Syn1.5 = 62.73%, and S5-Syn2 = 65.51%) yogurt samples were significantly increased, compared with that of the control sample (S1-C = 26.44%), as shown in Table 2. Increased antioxidant activity in probiotic and symbiotic yogurt may result from bioactive (antioxidative) peptides released from protein digestion via bacterial fermentation. Similarly, the pH of yogurt, because of the fermentation of available lactose by yogurt culture, becomes acidic, which creates a favorable environment for _S. boulardi_ to utilize the remaining sugars available and produce more bioactive compounds.  
All the yogurt samples displayed radical scavenging capacities in descending order, i.e., S5-Syn2 < S4-Syn1.5 < S3-Syn1 < S2-P < S1-C. The addition of inulin in symbiotic yogurt also contributed to the higher antioxidant activities. In previous studies, [PERSON] et al. [37] reported that inulin had significantly higher antioxidant activity than other sugars, even after cooking and digestion. In another study, [PERSON] et al. [38] observed that probiotics in yogurt drinks stored at refrigerated temperature had higher antioxidant activity than those in yogurt drinks stored at room temperature. DPPH activity was increased in yogurt with the addition of an aqueous extract of _Matricaria recuttia_[39]. Hydrolysis of milk protein or organic acid production can be another reason for yogurt's antioxidant activity because of microbial metabolic activity during fermentation and storage under refrigeration [40].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name of Sample** & & & & **Storage Day** & \\ \hline
**S1-C** & 11.65 \(\pm\) 0.7 E & 20.38 \(\pm\) 0.36 E & 26.44 \(\pm\) 0.8 E & 38.74 \(\pm\) 0.25 E & 45.48 \(\pm\) 0.57 E \\
**S2-P** & 11.94 \(\pm\) 0.54 D & 22.44 \(\pm\) 0.5 D & 54.65 \(\pm\) 0.85 D & 64.57 \(\pm\) 0.43 D & 72.32 \(\pm\) 0.76 A \\
**S3-Syn1** & 12.07 \(\pm\) 0.32 C & 27.74 \(\pm\) 0.86 C & 61.01 \(\pm\) 0.28 C & 70.75 \(\pm\) 0.36 C & 70.23 \(\pm\) 0.65 D \\
**S4-Syn1.5** & 14.09 \(\pm\) 0.87 B & 29.50 \(\pm\) 0.4 B & 62.73 \(\pm\) 0.92 B & 71.26 \(\pm\) 0.84 B & 72.03 \(\pm\) 0.54 BC \\
**S5-Syn2** & 15.84 \(\pm\) 0.48 A & 32.80 \(\pm\) 0.74 A & 65.51 \(\pm\) 0.74 A & 72.75 \(\pm\) 0.27 A & 72.14 \(\pm\) 0.84 BC \\ \hline \hline \end{tabular} Different superscripted letters indicate significant differences (\(p\leq\) 0.05) within a column.  
\end{table}
Table 2: Influence of storage time on the antioxidant activity of different yogurt samples.