# Thai Rice Vinegars: Production and Biological Properties
## 3 Results and Discussion
### Solid State Fermentation of Rice Vinegar  
Four types of Thai rice--polished rice (Hom Mali), black fragrant rice (Hom Nil), glutinous rice (Kiaw Ngu) and black glutinous rice (Niew Dam)--were used as raw materials for vinegar fermentation. The fermentation broth was sampled to determine the glucose, ethanol and total acid content during fermentation period; the results are shown in Figure 2.  
The glucose content in fermentation broth was determined after 3 days of saccharification. Subsequently during alcohol fermentation, glucose content was detected at low levels due to the recent addition of water to the fermenter; thus, a small amount of glucose from saccharification was dissolved in the broth. On the next day, the maximum glucose content was detected on the first day of alcohol fermentation. The fermentation broth of glutinous rice exhibited the highest content of glucose at 12.66% (\(w/v\)), followed by black glutinous rice, black fragrant rice and polished rice with maximum glucose contents of 11.42%, 10.07% and 9.80% (\(w/v\)), respectively. The glucose content then decreased rapidly during the next period, and the ethanol content increased. These results suggested that glucose from saccharification was consumed by yeast cells and converted to ethanol. The maximum ethanol contents in the four types of rice samples were detected on days 7 and 8 of fermentation and were found to decrease due to the conversion of ethanol to acetic acid. The fermentation broth of glutinous rice exhibited the highest content of ethanol at 6.21% (\(v/v\)), followed by black glutinous rice, black fragrant rice and polished rice with maximum glucose contents of 5.64%, 5.27% and 4.95% (\(v/v\)), respectively. Ethanol productivity during fermentation varied by glucose content from hydrolyzed rice starch [5]. During ethanol fermentation, a critical high concentration of ethanol can cause toxicity to yeast cells [21]. When the cell membrane of _S. cerevisiae_ attaches to the accumulated  
Figure 2: Total acid content in fermented broth of polished rice (**A**), black fragrant rice (**B**), glutinous rice (**C**) and black glutinous rice (**D**) during vinegar fermentation. ethanol, the composition of the cell membrane or organelle structure of _S. cerevisiae_ is changed for adaptation, leading to a higher tolerance to ethanol stress. [PERSON] et al., (2015) found that _S. cerevisiae_ showed a greater variation of the cell membrane in stationary phase and log phase than lag phase [22]. These results indicated that the stationary phase and log phase of _S. cerevisiae_ exhibited higher ethanol tolerance. _S. cerevisiae_ has been widely used in brewing and ethanol production industries, demonstrating a high tolerance to ethanol levels ranging approximately from 14% to 20% (\(v/v\)) of the brewing products [23]. However, the ethanol content detected in this study was lower, so the yeast should have remained alive and active in the fermentation system. Ethanol was oxidized to acetic acid by _A. pasteurianus_ TISTR102 after adding the cell starter culture to the fermentation broth. In this step, the total acid content was determined and increased throughout fermentation, while the detected ethanol content was decreased (Figure 2). The increase in total acid content in the last period of fermentation was slow or remained constant. Glutinous rice contained the highest total acid content at the end of acetic acid fermentation, followed by black glutinous rice, black fragrant rice and polished rice, with values of 6.77%, 5.44%, 4.10% and 3.83% (\(w/v\)), respectively. [PERSON] et al., (2015) reported the conversion of 6.28% (\(w/v\)) alcohol from rice wine to an acetic acid content of 6.6-7.8% (\(w/v\)), which indicated a high rate of ethanol conversion to acetic acid [24]. Ethanol, glycerol and Na-DL lactate are the best carbon sources for _Acetobactor_ sp., especially ethanol, which is used for acetic acid production [25]. Consequently, the ethanol from the alcohol fermentation step is necessary for acetic acid fermentation.