# Thai Rice Vinegars: Production and Biological Properties
## 3 Results and Discussion
### Biological Properties of Rice Vinegars
#### 3.3.2 Determination of Antibacterial Activity  
Thai rice vinegars were shown to possess antibacterial activity by the agar disc diffusion method. Gram-negative (_Escherichia coli_) and Gram-positive bacteria (_Staphylococcus aureus_ and _Staphylococcus epidermidis_) were selected as the tested microorganisms. Sterilized deionized water was used as a negative control, and antibiotics consisting of 0.24 mg/mL of tetracycline-HCl served as a positive control because of the wide antibiotic resistance of both of Gram-negative and Gram-positive bacteria [31]. After 24 h of incubation, the diameter of the inhibition zone (clear zone) was determined. Thai rice vinegar samples, commercial vinegars and acetic acid inhibited bacterial growth similarly to tetracycline-HCl. The antibacterial activities of the vinegar samples are shown as the diameter of the inhibition zone (DIZ) in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples (Acctic Acid Content)**} & \multicolumn{3}{c}{**Diameter of Inhibition Zone (mm)**} & \multicolumn{2}{c}{**IC\({}_{\text{50}}\) Value (ug/mL)**} \\ \cline{2-7}  & _E. coli_ & _S. aureus_ & _S. epidermidis_ & **L929** & **HT-29** \\ \hline \multicolumn{1}{c}{Polished rice vinegar (3.78\%)} & 13.7 \(\pm\) 0.6 \({}^{\text{Dd}}\) & 13.7 \(\pm\) 0.6 \({}^{\text{Dd}}\) & 10.0 \(\pm\) 1.0 \({}^{\text{Ee}}\) & 133.46 & 110.92 \\ \hline \multicolumn{1}{c}{Black fragrant rice vinegar (4.06\%)} & 14.3 \(\pm\) 0.6 \({}^{\text{Dd}}\) & 14.0 \(\pm\) 0.0 \({}^{\text{Dd}}\) & 11.0 \(\pm\) 1.0 \({}^{\text{Dde}}\) & 135.25 & 81.66 \\ \hline \multicolumn{1}{c}{Glutinous rice vinegar (6.68\%)} & 17.3 \(\pm\) 0.6 \({}^{\text{Bb}}\) & 17.7 \(\pm\) 0.6 \({}^{\text{Bb}}\) & 13.0 \(\pm\) 0.0 \({}^{\text{BChc}}\) & 121.84 & 90.17 \\ \hline \multicolumn{1}{c}{Black glutinous rice vinegar (5.33\%)} & 17.3 \(\pm\) 0.6 \({}^{\text{Bb}}\) & 16.0 \(\pm\) 0.0 \({}^{\text{Cc}}\) & 12.0 \(\pm\) 0.0 \({}^{\text{CDed}}\) & 171.06 & 74.02 \\ \hline \multicolumn{1}{c}{0.24 mg/mL Tetracycline-HCl} & 21.0 \(\pm\) 0.0 \({}^{\text{Aa}}\) & 21.0 \(\pm\) 0.0 \({}^{\text{Aa}}\) & 16.7 \(\pm\) 0.6 \({}^{\text{Aa}}\) & - & - \\ \hline \multicolumn{1}{c}{Commercial fermented rice vinegar (4.20\%)} & 16.0 \(\pm\) 0.0 \({}^{\text{Cc}}\) & 16.0 \(\pm\) 1.0 \({}^{\text{Cc}}\) & 12.7 \(\pm\) 0.6 \({}^{\text{BChc}}\) & - & - \\ \hline \multicolumn{1}{c}{Commercial distilled vinegar (5.00\%)} & 17.3 \(\pm\) 0.6 \({}^{\text{Bb}}\) & 17.3 \(\pm\) 0.6 \({}^{\text{Bb}}\) & 13.7 \(\pm\) 0.6 \({}^{\text{Bb}}\) & - & - \\ \hline \multicolumn{1}{c}{Acetic acid (4.00\%)} & 14.7 \(\pm\) 0.6 \({}^{\text{Dd}}\) & 14.3 \(\pm\) 0.6 \({}^{\text{Dd}}\) & 12.0 \(\pm\) 1.0 \({}^{\text{Ded}}\) & - & - \\ \hline \multicolumn{1}{c}{Sterilized deionized water} & - & - & - & - & - \\ \hline \hline \end{tabular} Values are expressed as the mean \(\pm\) S.D from triplicate assessments. \({}^{\text{A-E}}\) Columns followed by different letters are significantly different in terms of the diameter of inhibition zone of Thai rice vinegars compared by acetic acid content (\(p<0.05\)). \({}^{\text{A-E}}\) Columns followed by different letters are significantly different in terms of the diameter of inhibition zone of Thai rice vinegars compared by type of sample (\(p<0.05\)).  
\end{table} 
Table 3: Diameter of the inhibition zone of Thai rice vinegars and cytotoxicity of rice vinegars toward the cancer cell lines. Figure 3: Total phenolic contents in rice vinegar during fermentation. The antimicrobial activity of the samples was classified according to the diameter of the inhibition zone as weak inhibition (7 mm \(<\) DIZ \(<\) 10 mm), medium inhibition (11 mm \(<\) DIZ \(<\) 14 mm) and strong inhibition (DIZ \(>\) 15 mm) under the statistical analysis described below Table 3[31]. Glutinous rice vinegar was the most effective Thai rice vinegar for the inhibition of bacterial growth demonstrating strong inhibition against _E. coli_ (DIZ = 17.3 \(\pm\) 0.6 mm) and _S.