Sugarcane bagasse-a resource rather than a waste

Sugarcane bagasse is an important, renewable, abundant and cheap or even having negative value agricultural waste in many countries (Bustos et al., 2003; Molina Junior et al., 1995; Rodrigues et al., 2001; Van Haandel, 2005). Composition of the fibrous residue may vary based on its different verities, age of cane at the time of harvesting and efficiency of milling operation for extracting the juice

Besides the compositional analysis, different fractions of bagasse can be separated employing suitable techniques. For example, Bustos et al. (2003), while describing sugarcane bagasse hydrolysis with HCl have mentioned 128°C, 2% HCl and 51.1 minutes as optimal conditions. At these conditions they obtained 22.6 g xylose, 3.31 g arabinose, 3.77g glucose, 3.59 g acetic acid and 1.54 g furfural/L.

Concerning the bioconversion of the substrate, products such as alcohol, alkaloids, mushrooms, protein enriched animal feed, enzymes L-glutamic acid, fruity aroma, and xylitol have been reported to be obtained from the waste sugarcane bagasse (Alonso et al., 2007; Christen et al., 1994; Liu et al., 2006, 2007; Martinez et al., 2000; Pereira et al., 2007; Sasaki et al., 2003; Van Haandel, 2005). Besides the above mentioned diverse and usually bench scale utilities of the substrate Meunchang et al. (2005) have rightly commented that one of the under utilized sources of organic materials, is the sugarcane industry. Global sugar production from sugarcane releases large amounts of sugar mill by-products as filter cake and bagasse. It had been reported that in Brazil at the end of last century during the ethanol production season more than 60×106 tons of sugarcane bagasse containing 50% moisture were produced annually.

Like any other lignocellulosic material the sugarcane bagasse is a complex and stable substrate. Any significant and efficient utilization would require first its hydrolysis. Following acid, enzymatic or microbial hydrolysis of the sugarcane bagasse the monosaccharides yield can find many applications in different bioconversion processes. One consideration is their conversion into biofuel, the ethanol. Sugarcane bagasse has relatively earlier been considered a source of fermentable carbohydrates (Du Toit et al., 1984). However, pretreatment of the bagasse has been found useful for the microbial attack, which may results into its saccharification, fermentation or the both processes simultaneously. Chemical as well as microbial enzymatic pre-treatments have been described by various workers (Chaudhary & Qazi, 2006a; Dominguez et al., 1996; Laser et al., 2002; Lavarack et al., 2002; Martin et al., 2002; Zheng et al., 2002). Martin et al. (2002) have described that sugarcane bagasse is a potential lignocellulosic feedstock for ethanol production, since it is cheap, readily available and has a high carbohydrate content. These workers performed different pretratments of the substrate at 205°C for 10 minutes followed by its hydrolysis using cellulolytic enzymes. They found highest yield of xylose (16.2 g/100g dry bagasse), arabinose (1.5 g/100g) and total sugar (59.9 g/100g) in the hydrolysis of the SO2- impregnated bagasse. The H2SO4 impregnated bagasse gave highest glucose yield (35.0 g/100g) but the lowest total sugar yield (42.3 g/100g). Sulfuric acid impregnation led to a three-fold increase in the concentration of the fermentation inhibitors, the furfural and 5- hydroxymethyl furfural and a two fold increase in the concentration of inhibitory aliphatic acids (formic, acetic and levulinic acids) compared to the without any impregnation and sulfur dioxide impregnation yields. They found no major differences in the content of inhibitors in the hydrozylates obtained from SO2-impregnated and non-impregnated bagasse. When Martin and colleagues studied fermentability of the three hydrolyzates with a xylose utilizing Saccharomyces cerevisiae with and without nutrient supplementation they found that the H2SO4 impregnated bagasse fermented considerably poorer than the situations found in the other two categories of the bagasse. Cheng et al. (2007) have reported the ethanologenic fermentation of sugarcane bagasse hemicellulose hydrolyzates, pretreated by over-liming as well as electrodialysis and supplemented with nutrient materials employing Pachysolen tannophilus DW 06. These workers found that compared with detoxification by over-liming, detoxification by electrodialysis decreased the loss of sugars and increased the acetic acid removal. This lead to better fermentability and the Cheng’s team found that a batch culture employing electrodialytically pretreated hydrolyzate substrate gave 21g ethanol L-1 with a yield of 0.35 g L-1 sugar and productivity of 0.59 g L-1 h — 1. For better yield of the produce ethanol from sugarcane hydrolyzates, the above described studies highlight two important notions. That is detoxification of inhibitory substances, that may emerge within the hydrolyzates, of different nature and varying levels depending upon the specific pre-treatment employed. Secondly bagasse hydrolyzate would mainly consists of carbohydrates content, its supplementation with a suitable nutritive material is likely to enhance the growth and/or fermentative potential of the microorganism(s).

Fermentation inhibitors can be tackled at two levels i. e., their removal/detoxification or employing the inhibitors’ resistant fermentative microorganisms. Martinez et al. (2000) reported that hemicellulose syrups from dilute sulfuric acid hydrolyzates of hemicellulose contain inhibitors that prevent efficient fermentation by yeast and bacteria. These workers have optimized overliming treatments for sugarcane bagasse hydrolyzates and found a substantial reduction in furfural, hydroxymethyl furfural and three un-identified high — performance liquid chromatography peaks. They further demonstrated that the extent of furan reduction correlated with increasing fermentability, although furan reduction was not found to be the sole cause of reduced toxicity. Rodrigues et al. (2001) studied the influence of pH, temperature and drgree of hydrolyzate concentration on the removal of volatile and non-volatile compounds from sugarcane bagasse hemicellulosic hydrolyzate treated with activated charcoal before and after the vacuum evaporation process. They found that furfural and 5-hydroxymethyl furfural were almost totally removed irrespective of pH, temperature and whether the charcoal was added before or after the vacuum evaporation process. Adding activated charcoal before the vacuum evaporation process favoured the removal of phenolic compounds for all values of pH. Acetic acid was most effectively removed when the activated charcoal was added after the vacuum evaporation process at an acid pH (0.92).

Regarding the use of fermentation inhibitory products’ resistant microorganisms, Morita & Silva (2000) reported the fermentation of precipitated sugarcane bagasse hemicellulosic hydrolyzate containing acetic acid, employing Candida guilliermondii FT 120037 under different operational conditions for the production of xylose. At pH 7.0 and Kla of 35/h (4.5 vvm), the acetic acid was rapidly consumed and that the acetic inhibition was not important. They concluded that the acetic acid assimilation by the yeast inidicates the ability of this strain to ferment a partially detoxified medium and makes possible the utilization of the sugarcane bagasse hydrolyzate in this manner.

For simultaneous bioconversion of cellulose and hemicellulose to ethanol, need of xylose fermenting microorganisms has been established (Chandrakant & Bisari, 1998; Sedlak & Ho, 2004; Toivari et al., 2001; Yang et al., 1997). De-Castro et al. (2003) have described a new approach for the utilization of hemicellulosic hydrolyzates from sugarcane bagasse. They diluted the conventional feedstock, sugarcane juice; by the bagasse hydrozylate to the usual sugar concentration of 150 gm per liter that is employed for industrial production of ethanol. These workers used a pentose fermenting yeast strain, and achieved ethanol productivity of about 11.0 gm per liter per h and overall sugar conversion of more than 95%. Katzen and Fowler et al. (1994) reported first commercial application of unique fermenting organism capable of converting five carbon sugars and oligmers of cellulose directly to ethanol. These worker described conversion of hemicellulose content of sugarcane bagasse to the five-carbon sugar by mild acid prehydrolysis, followed by fermentation of the 5- carbon sugar extract with recombinant Escherichia coli. The process also recovered the majority of sucrose normally lost with the bagasse fibers to ethanol. Sun & Cheng (2002) have described the benefits of simultaneous saccharification and fermentation that it effectively removed glucose, which is an inhibitor to cellulase activity thus increasing the yield and rate of cellulose hydrolysis.

Various workers have reported different protocols and models for fermenting cellulosic biomass to ethanol and considered it the cleanest liquid fuel alternatives to fossil fuels (Gray et al., 2006; Lawford & Rousseau, 2003; Lin & Tanaka, 2006; Sun & Cheng, 2002). From above cited literature it appears that relatively recently considered renewable resource, the sugarcane bagasse process much potential for the bioethanol production. In Brazil sugarcane cultivtion and its dependent sugar industry is well developed. Consequently, a huge amount of bagasse is generated. It consists mainly of 37% cellulose, 28% hemicellulose and 21% lignin (Bon, 1996). A reasonable number of cellulose saccharifying and/or ethanologenic bacteria as well yeast have been isolated and characterized in our laboratory (Chaudhary & Qazi, 2006a; Saeed, 2005).

Above referred studies suffice to highlight different achievements and areas that require more research concerning the developments of bioprocesses to utilize sugarcane bagasse lignocellulosic material for obtaining bioethanol at economically feasible levels. As in all such bioprocess developments subsidiary supports are very important. Benefits derived from appropriate utilization of auxiliary products/often process wastes, have an influential bearings on the main process economics. In this regard Pandey et al. (2000) pointed out an important aspect. Accordingly, developing associated or complimentary technologies, during the fuel ethanol production from sugarcane bagasse which could produce other value-added by-products would improve the overall economy of ethanol production. It is pertinent here to mention that the non-fermentable residues of variously processed sugarcane bagasse would contain the microorganisms employed for the saccharification and/or fermentation of the substrate. Thus the residue may attain the levels of protein (due to single cell protein) that may render them to the status of animal feed / supplement. This may bring additional support to the process economics. Following is a brief review of single cell protein in connection with microbiological utilization of lignocellulosic materials including sugarcane bagasse.

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