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Dilute acid pretreatment of lignocellulosic biomass at higher temperatures (>160 °C) solubilizes/removes hemicelluloses (xylan, arabinan, mannan, galactan) and acidsoluble lignin (ASL), but, it does not remove acid-insoluble lignin (AIL). During acid pretreatment, the condensation and redeposition of coalesced lignin over cellulose fibers reduces the access of cellulose to cellulases. For higher delignification, various multistage pretreatments are available, however, all these are energy/chemical intensive methods. Therefore, an effective pretreatment which provides increased cellulose accessibility by enhanced removal of hemicelluloses and lignin in a single-step would be preferred. Our investigation reports a novel assisted single-step acid pretreatment (ASAP) process for enhanced delignification of biomass under acidic conditions. Pretreatment of rice straw (particle size of 20 mm) with H 2 SO 4 (0.75% v/v) + boric acid (1% w/v) + glycerol (0.5% v/v) (solid/liquid (S/L), 1:5) at 150 °C for 20 min removed hemicelluloses completely, 44% of the lignin, and ∼48.5% of the silica leaving a solid consisting of 69 ± 1.5% glucan, 0.7 ± 0.06% ASL, 20 ± 2.0% AIL, and 12 ± 1.5% silica. The C/L (cellulose/ lignin) ratio of solids resulted from ASAP was found to be > 3.00, while it was < 2.00 for acid only and untreated solids. Enzymatic hydrolysis of ASAP treated biomass with enzyme loadings of 20 FPU g −1 at 15% (w/v) solids concentration gave about 72% glucan conversion to glucose. This amount of glucose was around 2.6 times higher than obtained with enzymatic hydrolysis of acid-only-pretreated solids and 4.2 times higher than untreated rice straw (control). Therefore, the assisted-acid pretreatment dramatically enhanced delignification of rice straw and thereby glucan-to-glucose conversion.

Glycerol has attracted the attention of scientific and industrial communities due to its generation in bulk quantities as a byproduct of biofuel industries. With the rapid growth of these industries in recent years, glycerol is frequently treated as a very low-value byproduct or even a waste product with a disposal cost associated to it. Glycerol is not only abundant and inexpensive but also can generate more reducing equivalents than glucose or xylose. This unique characteristic of glycerol offers a tremendous opportunity for its biological conversion to valuable products at higher yield. This review focuses on research efforts to utilize glycerol as a carbon source for the production of a variety of fuels and chemicals by both native and metabolically engineered microorganisms.

BackgroundE. coli is a robust host for various genetic manipulations and has been used commonly for bioconversion of hexose and pentose sugars into valuable products. One of the products that E. coli make under fermentative condition is ethanol. However, availability of limited reducing equivalence and generation of competing co-products undermine ethanol yield and productivity. Here, we have constructed an E. coli strain to produce high yield of ethanol from hexose and pentose sugars by modulating the expression of pyruvate dehydrogenase and acetate kinase and by deleting pathways for competing co-products.ResultsThe availability of reducing equivalence in E. coli was increased by inducing the expression of the pyruvate dehydrogenase (PDH) operon under anaerobic condition after replacement of its promoter with the promoters of ldhA, frdA, pflB, adhE and gapA. The SSY05 strain, where PDH operon was expressed under gapA promoter, demonstrated highest PDH activity and maximum improvement in ethanol yield. Deletion of genes responsible for competing products, such as lactate (ldhA), succinate (frdA), acetate (ack) and formate (pflB), led to significant reduction in growth rate under anaerobic condition. Modulation of acetate kinase expression in SSY09 strain regained cell growth rate and ethanol was produced at the maximum rate of 12 mmol/l/h from glucose. The resultant SSY09(pZSack) strain efficiently fermented xylose under microaerobic condition and produced 25 g/l ethanol at the maximum rate of 6.84 mmol/l/h with 97% of the theoretical yield. More importantly, fermentation of mixture of glucose and xylose was achieved by SSY09(pZSack) strain under microaerobic condition and ethanol was produced at the maximum rate of 0.7 g/l/h (15 mmol/l/h), respectively, with greater than 85% of theoretical yield.ConclusionsThe E. coli strain SSY09(pZSack) constructed via endogenous pathway engineering fermented glucose and xylose to ethanol with high yield and productivity. This strain lacking any foreign gene for ethanol fermentation is likely to be genetically more stable and therefore should be tested further for the fermentation of lignocellulosic hydrolysate at higher scale.

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