**2. Ethanol production from rice straw**

Ethanol production is primarily centered around the lignocellulosic fraction of the plant biomass. Among all the left-over waste of crop species, rice straw is the cheapest and most abundant source of lignocellulosic feed stock. Rice straw, possessing considerable amount of cellulose (32–47%), hemicellulose (19–27%), with relatively less lignin (5–24%), is considered as one of the potent bioenergy sources [3, 5]. Various enzymes have been identified in the biosynthesis of these polymers (**Figures 1**–**3**) which determine the type and amount of polymer production in the plant cells. The cell wall polymers form a complex network by crosslinking with each other inside the cell walls. Hence, various pre-treatments are employed in order to break these complexes, to reduce crystallinity of cellulose (crt), degree of polymerisation (DP), increase in

**Figure 1.** *Cellulose biosynthesis pathway in microbes and plants.*

*Breeding Rice for Sustainable Bioenergy Production DOI: http://dx.doi.org/10.5772/intechopen.98572*

#### **Figure 2.** *Hemicellulose monomer biosynthesis pathway.*

biomass surface area, and breaking the lignin seal. Chemical pre-treatment of rice straw is practiced to enable enzymatic saccharification for ethanol production [13–26].

Biological pretreatment, an eco-friendly method, overcomes the disadvantages of chemical pretreatment. White-rot fungi (*Pleurotus ostreatus*) of class Basidiomycetes are most promising microorganisms [27]. Basidiomycetes degrades lignin fraction in lignocellulosic biomass in rice straw. Patel and co-workers [28] in a study on rice straw reported that pretreatment involving *Aspergillus niger* and *Aspergillus awamori*, followed by *Saccharomyces cerevisiae* aided fermentation and recorded highest ethanol yield of 2.2 g/l. Cellulose upon hydrolysis produces glucose while hemicellulose produces hexose and pentoses [29]. Use of steam pretreatment or hydrolysis of rice straw using H2SO4 has also been reported [30, 31]. Pretreatment with *Aspergillus niger* increased the glucose yield from 43 to 87% [32].

Cellulose contain glucans while hemicellulose is composed of polymers of xylan, mannan, glucan, galactan and arabinan. The general process of ethanol

#### **Figure 3.**

*Lignin biosynthesis pathways. The various enzymes are PAL [phenylalanine ammonia-lyase]; TAL [tyrosine ammonia-lyase]; C4H [cinnamate 4-hydroxylase]; C3H [4-hydroxycinnamate 3-hydroxylase]; COMT [caffeic acid 3-O- methyltransferase]; F5H [ferulate 5-hydroxylase]; 4CL [4-coumarate: CoA ligase]; CCoA-3H [coumaroyl-CoA 3-hydroxylase]; CCoAOMT [caffeoyl-CoA O-methyltransferase]; CCR [cinnamoyl-CoA reductase]; CAD [cinnamyl alcohol dehydrogenase]; LAC [laccase]; and PDX [peroxidase] (modified from Furtado et al. [11]; Vermerris and Abril [12]).*

production involves conversion of cellobiose to ethanol by a series of steps of involving pre-treatment, enzymatic saccharification and fermentation as described earlier. These steps may include simultaneous saccharification and fermentation (SSF) or separate enzymatic hydrolysis and fermentation (SHF). SSF is generally used as cost incurred in the process is less [33]. In this process also, higher yield of ethanol is obtained. However, some drawbacks are observed in this process such as requirement of optimum temperature (40-50°C) for enzymatic hydrolysis, which the microorganisms cannot tolerate. This problem can be tackled by using thermophilic microorganisms such as *Kluyveromyces marxianus, Candida lusitamiae and Zymomonas mobilos* or mixed culture of *Bettanomyces clausenii* and *Saccharomyces cereviseae* [34, 35]. Shengdong and co-workers [36] employed the SSF of alkali and alkali/microwave pretreatment to generate ethanol using cellulase from *Trichoderma reesei* and *Saccharomyces cereviseae*. The ethanol concentration was 29.1 g/l and yield were 61.3% under optimum condition. Chada and co-workers [37] mentioned that SSF was superior to traditional saccharification in production of ethanol as it can improve the ethanol content by removal of end product inhibition by saccharification process. In the fermentation process alcohol is mixed with the straw to produce fermentable sugars and this is referred to as mash. This mash is fed into fractional distillation unit which differentiates alcohol from other components. The alcohol thus produced is cleaned and dehydrated to remove the water content. After cleaning and drying bioethanol is produced with a purity of 99.7% V/V.

These chemical processes for saccharification are harmful to the environment. Hence now research should be focused towards minimising or eliminating these steps by developing rice genotypes with higher saccharification efficiency (SE).

#### **3. Role of plant breeding and biotechnology to enhance SE**

As mentioned earlier the lignocellulosic biomass is primarily a complex network of various cellular constituents including cellulose, hemicellulose, lignin and interaction of a wide array of compounds like chlorophyll, waxes, oils, terpenes

#### *Breeding Rice for Sustainable Bioenergy Production DOI: http://dx.doi.org/10.5772/intechopen.98572*

and phenolics, called extractives [38, 39]. It is beneficial to have knowledge on the genetics as well as correlation between biomass traits and these cellular constituents. A greater insight into the composition, structure and the synthesis of cellular constituents will help in designing suitable breeding strategies for the genetic modification of cell wall architecture and in turn development of high energy efficient rice genotypes.
