Sunflower - Bioenergy Crop Plants - page 726

et al. (2002) performed concentrated acid hydrolysis (26% H 2 SO 4 ) of mixed wood chips and
achieved almost 80% sugar recovery. Agriwastes such as vegetable wastes, pawpaw, etc. have been
saccharified by acid or enzymatic hydrolysis yielding 7.8 g/L and 7.6-13.6 g/100 g fermentable
sugars (Akin-Osanaiye et al. 2005; Campo et al. 2006).
30.6.2 E nzymatic S accharification
The pretreated lignocellulosics are vulnerable to enzymatic saccharification. Interest in the
enzymatic hydrolysis of lignocellulosic wastes to get ethanol has increased because this involves
milder conditions than acidic hydrolysis. Extensive studies on the physiological aspects, kinetics,
and economics of enzymatic saccharification have been reviewed (Wyman 1994; Bothast and Saha
1997; Lynd et al. 2002). It has been suggested that for a cellulase preparation to be commercially
successful, it should produce 1100 FPU/L per hour, but so far only half of it has been obtained
practically (Bon and Ferrara 2007). Hence, many recent studies have been with ready-made enzyme
concentrates from different companies such as Dyadic, Genencor, and Novozymes for enzymatic
saccharification (Ruiz et al. 2008; Vaithanomsat et al. 2009). On the other hand, research on
isolation of new cellulases and hemicellulases from bacterial and fungal sources has continued, and
efforts are being made to improve cellulase titers (Aro et al. 2005).
The rate and the extent of saccharification is affected by many factors, such as source of enzyme,
nature of the substrate, methods of pretreatment, enzyme and substrate concentration, product
inhibition, and enzyme stability, the optimization of which is important for process economics.
Tewari et al. (1988) investigated the enzymatic hydrolysis of various agricultural lignocellulosic
residues and observed that an increase in enzyme concentration (1-4 IU/mL) and reaction duration
(12-72 h) improved saccharification, but an increase in the substrate concentration (>5.0% w/v)
had an inhibitory effect. Soto et al . (1994) studied the enzymatic saccharification of alkali-treated
sunflower hulls using a commercial cellulase (Celluclast) supplemented with cellobiase (Novozyme)
at a cellobiase/cellulase activity ratio of 0.25 and obtained maximal saccharification with an enzyme
concentration of 50 FPU/g, with higher concentrations giving a negligible further increase. Overall
glucose yield represented conversion of 60% of the cellulose or 39% of the polysaccharides. We
optimized 25 FPU/g at 50°C, pH 4.8, and an incubation period of 72 h from T. reesei to obtain
a saccharification of 49.73% of pretreated sunflower stalks (Sharma et al. 2002b). Ruiz et al.
(2006) used a commercial cellulase and obtained 72% saccharification, corresponding to a glucose
concentration of 43.7 g/L from the pretreated sunflower stalks. Vaithanomsat et al. (2009) also
used a commercial cellulase (Celluclast 1.5 L) and optimized saccharification conditions as 50°C
temperature, pH 4.8, and a sunflower stalk pretreated pulp/buffer ratio of 1:6 to obtain 11.97%
glucose.
Won Park and Kajiuchi (1995) modified cellulases with alpha-allyl-omega-methoxypolyoxy-
alkyline (POA) and maleic acid anhydride (MAA). In comparison with native cellulase, modified
cellulase was more stable toward temperature, pH, and organic solvents and gave greater conversion
of substrate. Cellulase modification also facilitated strong adsorption of cellulase onto substrate.
Kaar and Holtzapple (1998) demonstrated the role of Tween-20 in improving the enzymatic
saccharification of corn stover. Addition of Tween-20 improved the conversion of cellulose, xylose,
and total polysaccharides by 42, 40, and 42%, respectively. They concluded that Tween-20 acts
as an enzyme stabilizer, lignocellulose disrupter, and enzyme effector. A similar effect was also
studied by us for saccharification of pretreated rice straw (Kocher et al. 2008). Hang and Woodams
(1999) evaluated the efficacy of three commercial fungal enzyme preparations (namely, Celluclast
1.5 L, Rapidase Pomaliq, and Clarex ML) as an enzyme source for the production of soluble
sugars from corn husks whereby Rapidase Pomaliq, derived from Aspergillus niger and T. reesei ,
produced the highest yield of soluble sugars from corn husks. Nikolov et al. (2000) reported 80%
saccharification of pretreated cellulose fibers from the paper industry by the cellulase complex
of T. T. reesei.