Effect of gamma radiation as pretreatment of rice for bioethanol production: Inhibition of inhibitors

Gelatinized rice solution was pretreated with gamma radiation followed by α–amylase and glucoamylase to release free sugar for bioethanol production. 86 % (w/w) of rice carbohydrates converted into fermentable sugar at a dose of 2 kGy plus incubation of 3 h with 5 U (per g of rice) of each enzyme at 55 °C. However, during fermentation by employing Saccharomyces cerevisiae var. bayanus, it was observed that sugar consumption, ethanol yield, and residual sugar were 41 %, 37 %, and 43 %, respectively. Obtained poor fermentability was due to the dose dependent increase in concentration of carboxylic acids like lactic acid, succinic acid, etc. Metals ions catalyzed the biomass conversion into carboxylic acids, a class of fermentation inhibitors. Thus, naturally present Mg, Ca, and K ions in rice samples were precipitated at pH 9 before irradiation. 61 % of Mg, 30 % of Ca, and 19 % of K were precipitated in alkaline medium. This further resulted in 45 % (w/w) less formation of carboxylic acids when samples were irradiated to 2 kGy at pH 9. However, the amount of free sugar released during pretreatment remained unaffected by the change in pH. Inevitably, the overall fermentability at pH 5 of the optimized pretreated sample was improved to 84 %, 81 %, and 3 % for sugar consumption, ethanol yield, and residual sugar, respectively. In the present study, the synthesis of fermentation inhibitors during radiation processing was successfully inhibited by precipitating metal ions in alkaline conditions is shown for the first time.