Effect of pH Condition on Natural Indigo (Indigofera tinctoria) Reduction by Yeast (Saccharomyces cerevisiae)

Ultimate goal of this research is to develop an efficient microbial system for indigo reduction by using yeast ( Saccharomyces cerevisiae ). For this purpose, we investigated the effect of pH condition on natural indigo ( Indigofera tinctoria ) reduction by yeast. We also checked the viability of yeast at high pH condition by observing yeast cell in the indigo reduction bath with an optical microscope. The pH of indigo reduction bath was initially set from 10.2 to 12.0 with the buffer solution. The pH value dropped drastically up to 5.5–6.5 at the initial stage of reduction. After adjusting pH once within 1–2 days, we measured some parameters related to reducing power such as the color yield ( K/S value) of ramie fabric dyed with the supernatant of bath, the oxidation/reduction potential (ORP), and the pH values changed. When the initial pH was set at lower than 11 (10.2, 10.7), the baths adjusted to pH 11 showed better reducing power than the baths of pH other than 11. And also, when the initial pH was set higher than 11 (11.3 and 12.0), the baths adjusted to pH 11 showed better reducing power than the baths of pH other than 11. It took 5–11 days to reach the maximum dye uptake after the initiation of reduction, and, during the corresponding time, the ORP was from -563 to -599 mV. All the initial and the adjusted pH values were not maintained but decreased continuously, and the maximum dye uptake reached at pH 9.17-9.43. The initial and the adjusted pH greatly affected the indigo reducing power, and the highest color yield was obtained with indigo reduction bath of initial pH 10.7. When the pH of the bath was adjusted to pH 11.0, the maximum reducing power was obtained most efficiently. Also, for the pH adjustment, use of sodium hydroxide showed higher reducing power than use of calcium hydroxide.