Various raw materials as a most affecting parameter for biobutanol production are being investigated to find cheaper and highly available alternatives [48-51]. On the basis of different varieties of raw materials, biofuels are classified in two categories

(i) first-generation biofuels (ii) second-generation biofuels. The biofuels of these categories are produced by the consumption of food-related (sugarcane and cereal grains) and non-food (lignocellulosic and wastes) materials, respectively [9,10,27].

Due to the food insecurity worldwide, second-generation biofuels indicates toward the sustainable production of fermentation-based liquid fuels [1].

Previously, cereal grains and sugarcane were the common raw materials for ABE fermentation (Fig. 7.1) [16], but in present world the consumption of these substrates has been criticized because of shortage and prices hiking of edible materials. In present era, only few countries such as Brazil and U. S. have enough

production of food-based materials for the production of first-generation biofuels [13, 21, 52]. Therefore, the main focus of the research turned towards the non-food materials such as lignocellulosic materials. For utilizing lignocellulosic materials, firstly these materials are converted in monomers such as glucose, fructose, mannose, sucrose, lactose, starch, dextrin, galactose, xylose, arabinose, raffinose, melezitose, inulin, minnitol, trehalose, ramnose, malibiose, and glycerol. During the fermentation studies, it was found that glucose, fructose, mannose, sucrose, lactose, starch, and dextrin were completely consumed by clostridial bacteria in butanol production. While, galactose, xylose, arabinose, raffinose, melezitose, inulin, and minnitol were partially consumed, but later on it was observed that xylose and arabinose were also utilized completely by some strains. It could be considered as a milestone point because there is a significant amount of pentose sugar (xylose and arabinose) along with hexose sugar are produced from ligno- cellulosic materials on hydrolysis process [15].

Various starch — (sago, defiberated-sweet-potato-slurry, degermed corn, extruded corn, liquefied corn starch, and cassava) [35, 36, 44, 53] and lactose — (whey per­meate) [38, 54] containing substrate were examined for ABE fermentation. For instance, utilization of liquefied corn starch (a product of corn processing industry) showed significant results when there was a proper removal of product (by gas stripping) and Na2S2O5 (inhibitor for fermentation) was being used through the fermentation process [53]. Lactose-containing substrates (wastes of dairy industry like cheese whey) has also been investigated as feedstocks for butanol production using C. acetobutylicum DSM 792 and C. acetobutylicum AS 1.224. Results pos­tulated that cheese whey produced higher yield than direct lactose solution [54].

Recently, researchers uncovered the suitability of Clostridial bacteria to ferment the lignocellulosic materials as some of them have saccharolytic ability (Fig. 7.2) [55]. Rest of the strains have shown efficient performance toward hexose and pentose sugars producing from lignocellulosic materials through hydrolysis process. Lig — nocellulosic biomass is most abundant renewable source on the Earth [2, 56].

For instance, a developing country like India produces huge amount of biomass (over 370 million tons every year) in the form of direct plants, rice husk from rice mill, saw dust from saw mill, bagasse from sugar mills, etc. [28]. The potentiality of biomass such as wood forestry residues, corn stover, wheat straw, corn fibers, barley straw, and switch grass has been examined at laboratory scale for biobut­anol production (Table 7.1) [3, 22-24, 26, 58, 59]. Still, the endeavors are required for optimizing the processes including hydrolysis and removal of the inhibitors from fermentation broth for utilizing lignocellulosic biomass.