# Thai Rice Vinegars: Production and Biological Properties
## 3 Results and Discussion
### Biological Properties of Rice Vinegars  
#### 3.3.1 Total Phenolic Content of Rice Vinegars  
Black glutinous rice vinegar exhibited the highest content of total phenolics (133.68 \(\pm\) 4.52 mg GAE/100 mL of rice vinegar), followed by black fragrant, polished and glutinous rice vinegar, providing a total phenolic content of 102.07 \(\pm\) 3.44, 82.15 \(\pm\) 2.67 and 76.81 \(\pm\) 2.58 mg GAE/100 mL of the rice vinegar, respectively. The large amounts of total phenolic contents of pigmented rice vinegars were due to pigments in the rice grain such as anthocyanins [13]. Furthermore, among eight rice samples in the reviewed study, black glutinous rice was the pigmented rice containing the highest anthocyanin amounts. The anthocyanins detected in the black glutinous rice were cyanidin 3-glucose and pe-onidin 3-glucose [15]. Moreover, phenolic compounds such as ferulic acid, p-coumaric, sinapic acid and vanillic acid were found in rice [30]. These phenolic compounds in rice grains have shown biological activities such as antimutagenic, anticancer, allergic reaction inhibitory, cardiovascular disease preventative and plasma cholesterol level-reducing effects [14; 16; 17].  
The variation of total phenolic contents during fermentation processes is shown in Figure 3. The total phenolic contents of rice vinegars increased continuously over the 32 days of fermentation. These phenolic compounds could be extracted from rice grain  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Rice Vinegars** & **Reducing Sugar Content** & **Ethanol Content** & **Total Acid Content** & **Acetic Acid Content** \({}^{\star}\) \\  & **(\%\(w\)/\(v\))** & **(\%\(w\)/\(v\))** & **(\%\(w\)/\(v\))** & **(\%\(w\)/\(v\))** \\ \hline Polished rice & 0.60 \(\pm\) 0.34 & 0.33 \(\pm\) 0.06 & 3.84 \(\pm\) 0.07 \({}^{\circ}\) & 3.78 \(\pm\) 0.18 \({}^{\mathrm{d}}\) \\ \hline Black fragrant rice & 0.63 \(\pm\) 0.36 & 0.38 \(\pm\) 0.06 & 4.10 \(\pm\) 0.17 \({}^{\mathrm{c}}\) & 4.06 \(\pm\) 0.27 \({}^{\mathrm{c}}\) \\ \hline Glutinous rice & 0.68 \(\pm\) 0.44 & 0.44 \(\pm\) 0.07 & 6.77 \(\pm\) 0.19 \({}^{\mathrm{a}}\) & 6.68 \(\pm\) 0.10 \({}^{\mathrm{a}}\) \\ \hline Black glutinous rice & 0.66 \(\pm\) 0.44 & 0.37 \(\pm\) 0.07 & 5.44 \(\pm\) 0.10 \({}^{\mathrm{b}}\) & 5.33 \(\pm\) 0.31 \({}^{\mathrm{b}}\) \\ \hline \end{tabular}
\begin{tabular}{c c c c} \hline
**Values are expressed as the mean \(\pm\) S.D from triplicate assessments. \({}^{\mathrm{\star}}\)\({}^{\mathrm{\star}}\) Columns followed by different letters are significantly different in each Thai rice vinegar product (\(p<0.05\)). \({}^{\star}\) Acetic acid content of polished rice, black fragrant rice, glutinous rice and black glutinous rice vinegar was 91.69%, 98.84%, 98.79% and 97.90% of the total acid, respectively.**  
\end{table}
Table 2: Reducing sugar, ethanol, total acid and acetic acid contents in Thai rice vinegars in 32 th day of fermentation at room temperature.  
by ethanol with an acidified organic solvent [16], such as anthocyanin extract from black rice [12]. Thus, during ethanol and acetic acid production, phenolic compounds consisting of rice substrates were extracted and released in the fermentation broth.