Microwave subcritical water pre-treatment and enzymatic hydrolysis of geographical identification (GI) tag Indian black rice (Chakhao Poireiton) straw for fermentable sugar production

Abstract

Rice straw is an important feedstock for second-generation biorefineries, the majority being conventional white rice. This research investigated the potential of the straw of black rice Chakhao Poireiton, a rice variety of increasing importance, for biorefining. Pre-treatment of black rice straw was carried out in a pressurized microwave reactor and free sugar release was measured following digestion with Cellic® CTec3. This was compared to white rice straw. Pre-treatment (100 -200° C for 5 min) did not drastically impact the sugar composition but brought about an enhanced release of glucose after enzymatic hydrolysis from 25 to 50% (black rice) and from 26 to 55% (white rice). For xylose digestibility, the increase was around seven-fold for black rice (8 to 57%) and five-fold for white rice (13 to 64%). This improvement in digestibility of the straw samples could have come from modification in the lignocellulose structural features making the polysaccharides more susceptible to hydrolysis. Black rice could be a suitable feedstock for a combined biorefinery -food and nutraceuticals from grain (much higher than white rice) and the straw for fermentable sugars comparable to white rice straw for biofuels.

Comparative Study of Effective Pretreatments on the Structural Disruption and Hydrodepolymerization of Rice Straw

“…The pH was 3.9, and the solution was acidic after the hydrothermal pretreatment because of the acid generated in situ by the RS under high-temperature conditions (Table 5b) [55]. The pH after microwave pretreatment was 6.4, indicating that temperature largely influenced the in situ acid formation during RS pretreatment as microwave pretreatment was performed at 100 • C, whereas hydrothermal pretreatment was conducted at 180 • C. Similar research reported by Moirangthem et al [36] indicated that when RS was pretreated using microwave radiation at 100 • C for 5 min, the pH value was 6.1, whereas at 180 • C for 5 min, the pH was 4.7. They reported a decrease in both the pH of the hydrolysate liquor and the recovered weight of the solid residue, along with subtle changes in the recovered RS residue.…”

“…Microwave pretreatment (MP) is a convenient and high-speed method. Microwave heat disrupts the waxy surface, degrades the lignin-hemicellulose complex, partially dissolving the lignin and hemicellulose in RS, which is beneficial for subsequent hydolyzation [36]. Cryocrushing pretreatment (CP) is a novel method of freezing materials and crushing them into powders using sub-zero temperatures.…”

Enhancement of Biological Pretreatment on Rice Straw by an Ionic Liquid or Surfactant

“…Moirangthem et al pretreated rice straw pressurized microwave and digestion with enzymes. However, the rice straw require pretreatment above 160 °C for 5 min to obtain a significant impact on the subsequent zyme hydrolysis [23]. Compared with our study, biological pretreatment could occur der mild conditions but with a longer reaction time.…”