Sunflower - Bioenergy Crop Plants - page 728

not so for the pentoses. The latter which may constitute upto 30% as in corn stover, if fermented to
ethanol can improve the overall economy of ethanol production from lignocellolosics (Sakai et al.
2007). Ethanol production from lignocellulosic materials has been reviewed (Sun and Cheng 2002;
Hahn-Hägerdal et al. 2007; Sakai et al. 2007).
S. cerevisiae is the most widely used yeast for hexose fermentation to ethanol that provides
high yields and productivities in addition to remarkable ethanol tolerance, but pentoses have to be
converted to xylulose before fermentation to ethanol (Deng and Ho 1990; Sakai et al. 2007). The
pentose fraction in lignocellulosic hydrolysates consists mainly of d-xylose, which can be converted
to ethanol by a number yeasts like Pachysolan tannophilus , Pichia stipitis , Candida shehatae , etc.
The first step in xylose degradation is conversion to xylulose through xylitol, involving a xylose
reductase that reduces xylose to xylitol and a xylitol dehydrogenase to convert xylitol to xylulose
(Bothast and Saha 1997). However, these yeasts have a relatively low ethanol yield and inhibitor
tolerance. P. stipitis is able to ferment glucose, xylose, mannose, galactose, and cellobiose and has
the ability to produce cell mass from l-arabinose, but not ethanol (Agbogbo and Coward-Kelly
2008). Moreover, the genome sequence of P. stipitis has recently been published. The sequence
showed numerous genes encoding xylanase, endo-1,4-β-glucanase, exo-1,3-β-glucosidase,
β-mannosidase, and α-glucosidase (Jeffries et al. 2007). The presence of these genes in P. stipitis
suggests the presence of useful traits for the SSF of cellulose and hemicellulose (Berson et al. 2005).
Even S. cerevisiae strains have been genetically engineered to use xylose (Hahn-Hägerdal et al.
2007; Chu and Lee 2007).

30.7.1 E thanol p roduction
The production of ethanol has been studied in a number of pretreated and saccharified lignocellulosics.
In our laboratories, we standardized fermentation conditions for ethanol production using sunflower-
stalk- and hull-saccharified worts as substrates, in which a fermentation period of 24 h, temperature
of 30°C, pH of 5, and inoculum size of 3% (v/v) were found to be optimal. These conditions when
scaled-up in 1- and 15-L fermenters revealed ethanol yields of 0.439 and 0.437 g/g from sunflower-
stalk-saccharified worts, respectively, and 0.449 and 0.446 g/g, from sunflower hulls, respectively
(Sharma et al. 2002a, 2004; Table 30.2). Okur and Saracoglu (2006) reported 0.41 g/g ethanol from
35 g of reducing sugars obtained from acid hydrolysis of sunflower hulls. Vaithanomsat et al. (2009)
reported a maximal ethanol yield of 0.028 g/100 g sunflower stalks. Jargalsaikhan and Saracoglu
(2009) optimized sunflower-hull hemicellulosic hydrolysate using P. stipitis and reported an ethanol
yield of 0.32 g/g.

taBle 30.2
ethanol Production from enzymatic hydrolysate of sunflower stalks and hulls in 1- and
15-l Fermenters by S. cerevisae var. ellipsoideus

sunflower stalks sunflower hulls

Fermentation
efficiency (%)

ethanol
yield (g/g)

Fermentation
efficiency (%) ethanol yield (g/g)

Fermentation
time (h)

1 l 15 l 1 l 15 l 1 l 15 l 1 l 15 l

6 0.89 0.090 17.45 17.64 0.113 0.111 22.16 21.76

12 0.264 0.266 51.76 52.16 0.272 0.274 53.33 53.73

18 0.439 0.437 86.08 85.68 0.449 0.446 88.04 87.45

24 0.425 0.426 83.33 83.53 0.436 0.437 85.49 85.68

Fermentation conditions: sugars 40 g/L, temperature 30°C, pH 5.0, aeration 1 L/min for first 10 h, and agitation 150 rpm.