# Thai Rice Vinegars: Production and Biological Properties
## 3 Results and Discussion  
### Comparison of the Starter Culture in Alcohol Fermentation  
The yeast suspension of _Saccharomyces cerevisiae_ TISTR 5049, non-enrichment and enrichment of baker's dried yeast activated in 1.8% (\(w/v\)) glucose were used as a starter to compare ethanol productivity in alcohol fermentation. The results showed that the maximum ethanol content was produced by enrichment of baker's dried yeast fermented with average content of four types of rice sample over 7 days of fermentation (Figure 1). The yeast suspension and non-enrichment of baker's dried yeast used as starter cultures could produce a lower amount of ethanol than enriched baker's dried yeast. The ethanol productivity using yeast suspension and non-enrichment of baker's dried yeast as a starter culture in four types of rice fermentation showed no statistically significant difference (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Yeast Culture**} & \multicolumn{4}{c}{**Ethanol Content (\%\(v\)/\(v\))**} \\ \cline{2-5}  & **Polished Rice** & **Black Fragment Rice** & **Glutinous Rice** & **Black Glutinous Rice** \\ \hline Yeast suspension & 3.17 \(\pm\) 0.07 \({}^{\text{cB}}\) & 3.46 \(\pm\) 0.13 \({}^{\text{cB}}\) & 4.12 \(\pm\) 0.08 \({}^{\text{aB}}\) & 4.07 \(\pm\) 0.12 \({}^{\text{bB}}\) \\ \hline [PERSON]’s dried yeast (non-enrichment) & 3.58 \(\pm\) 0.04 \({}^{\text{cB}}\) & 3.22 \(\pm\) 0.07 \({}^{\text{cB}}\) & 4.45 \(\pm\) 0.12 \({}^{\text{aB}}\) & 3.84 \(\pm\) 0.10 \({}^{\text{bB}}\) \\ \hline [PERSON]’s dried yeast (enrichment) & 4.95 \(\pm\) 0.12 \({}^{\text{cA}}\) & 5.16 \(\pm\) 0.12 \({}^{\text{cA}}\) & 5.98 \(\pm\) 0.09 \({}^{\text{aA}}\) & 5.54 \(\pm\) 0.12 \({}^{\text{bA}}\) \\ \hline \hline \end{tabular} Values are expressed as the mean \(\pm\) SD from triplicate assessments. \({}^{\text{A-B}}\) Columns followed by different letters are significantly different in the starter culture of yeast (\(p<\) 0.05). \({}^{\text{aA}}\)\({}^{\text{cB}}\) Rows followed by different letters have significantly different ethanol contents (\(p<\) 0.05).  
\end{table} 
Table 1: Ethanol production of each rice substrate using various starter cultures. The glucose content was decreased after the first day of alcohol fermentation due to conversion to ethanol (Figure 1). The released glucose from saccharification was converted to ethanol by yeast, and it was consumed for cell division and growth of yeast culture in the fermentation broth. These results showed that the glucose content was diminished immediately after the first day of fermentation. In a previous study, baker's yeast and loop-pang (a Thai traditional fermentation starter culture) were used in the hydrolysis of oil palm empty fruit bunch aqueous phase enriched with glucose culture for ethanol fermentation. The results showed that the ethanol content produced by baker's yeast was higher than that by loop-pang [19]. Furthermore, alcohol fermentation of commercial dried yeast was studied. Commercial New Aule alcohol yeast and New Aule baker's instant dried yeast were used for bioethanol production in sugarcane molasses. The New Aule baker's instant dried yeast produced more ethanol than its alcohol yeast with an ethanol content of 102.854 g/L and 74.8 g/L, respectively. Commercial yeast strains have many advantages compared with natural yeast strains, such as a longer shelf life (more than one year), high cell viability (up to 4.6 \(\times\) 10\({}^{10}\) yeast cells/g), a high efficiency for culture in the short term, low or no contamination from other microbes, lower cost and predominant properties such as high thermal, sugar, alcohol and acid tolerance [20]. Thus, baker's dried yeast was used as a starter culture in the alcohol fermentation step due to the simple and more convenient. The ethanol productivity from enrichment of baker's dried yeast also showed a high efficiency in the further step of acetic acid fermentation.