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Enzymatic hydrolysis at high solids loading has the potential to reduce both capital and operational expenditures. Here, pretreatment of rice straw (PRS) with dilute acid was carried out at a pilot scale (250 kg per day) at 162°C for 10 min and 0.35% acid concentration, followed by enzymatic hydrolysis at different total solids loadings. It showed that although the total sugar concentration increased from 48 to 132 g/l, glucan conversion reduced by 27% (84–66.2%) with increasing solids from 5 to 20% in batch mode. Therefore, two different fed-batch approaches were evaluated to improve the glucan conversion by the sequential addition of a substrate and/or enzyme. At 20% solid loadings and a 3 filter paper units/g enzyme dosage, the highest glucan conversion obtained was 66% after 30 h of hydrolysis in batch mode. However, in an optimized fed-batch approach, the glucan yield was improved to 70% by simply dividing the substrate feeding into three batches, that is, 50% at 0 h, 25% each after 4 h, and 8 h of hydrolysis reaction. The addition of surfactant (Ecosurf E6) further improved the conversion to 72% after 30 h. The role of critical factors, that is, inhibitors, enzyme–lignin binding, and viscosity, was investigated during the course of hydrolysis in the batch and fed-batch approaches. This study suggests a sustainable approach for improved hydrolysis at high solids loadings by fine-tuning a simple process.

In the transportation sector, the most commonly used biofuel is ‘bioethanol’ to reduce greenhouse gases. Ethanol production at the industrial level is employed by many yeast, bacteria, and fungi. But Saccharomyces Cerevisiae is most employed yeast. Wide range of substrates has been used for ethanol production such as lignocellulose, molasses, sweat sorghum cane extract, starch based substrate and other wastes. Lignocellulosic hydrolysates contain many inhibitors that can be reduced by treatment with activated charcoal and reducing agents, repeated sequential fermentation, over-liming, evaporation, anion exchanger, enzymatic treatment using peroxidase and laccase, and in-situ detoxification with fermenting microbes. Co-culturing of S. Cerevisiae with other microbes is targeted for optimization of ethanol production, short fermentation time, and for reduced process cost. Yeast cell immobilization has been considered as a potential alternative to enhance ethanol productivity. This paper also reviews the effects of various factors on yeast fermentation for ethanol optimization.