08.08.2015   BIOETHANOL

Fermentation

Hydrolyzates obtained from sorghum fiber are solutions rich in both hexoses and pentoses (Kurian et al., 2010). Production of ethanol from these mashes is possible only with the use of osmotolerant and pentose fermenting yeast or bacterial strains (Table 2).

Ballesteros et al. (2003) obtained 16.2 g ethanol/L when hydrolyzates obtained from sweet sorghum bagasse were fermented with Kluyveromyces marxianus. On the other hand, Kurian et al. (2010) working with Pichia stipitis obtained 38.7 g ethanol/L with a theoretical conversion of 82.5%. In Fig. 3, a flowchart of ethanol production from sorghum bagasse is depicted. A yield of 158 L ethanol/ ton biomass (wet basis) can be obtained after a sulfuric acid hydrolysis. The process yielded 110 kg of lignin and other non-fermentable materials. Almodares & Hadi

(2009) and Gnansounou et al. (2005) reported that the cellulase used in Simultaneous

Microorganism

Characteristics

Clostridium acetobutilicum

Useful in fermentation of xylose to acetone and butanol; bioethanol

produced in low yield

Clostridium thermocellum

Capable of converting cellulose directly to ethanol and acetic acid. Bioethanol concentrations are generally less than 5 g/l. Cellulase is strong inhibition encountered by cellobiose accumulation

Escherichia coli

Native strains ferment xylose to a mixture of bioethanol, succinic, and acetic acids but lack ethanol tolerance; genetically engineered strains predominantly produce bioethanol

Klebsiella oxytoca

Native strains rapidly ferment xylose and cellobiose; engineered to ferment cellulose and produce bioethanol predominantly

Klebsiella planticola ATCC 33531

Carried gene from Zymomonas mobilis encoding pyruvate decarboxylase. Conjugated strain tolerated up to 4% ethanol

Lactobacillus pentoaceticus

Consumes xylose and arabinose. Slowly uses glucose and cellobiose. Acetic acid is produced along with lactic in 1:1 ratio

Lactobacillus casei

Ferments lactose, particularly useful for bioconversion of whey

Lactobacillus xylosus

Uses cellobiose if nutrients are supplied: uses glucose, D-xylose and L — arabinose

Lactobacillus pentosus

Homolactic fermentation. Some strains produce lactic acid from sulfite waste liquors

Lactobacillus plantarum

Consumes cellobiose more rapidly than glucose, xylose, or arabinose. Appears to depolymerize pectins; produces lactic acid from agricultural residues

Pachysolen tannophilus Saccharomyces cerevisiae ATCC 24 860

Co-culture of S. cerevisiae and strains resulted in the best ethanol yield

Pichia stipits NRRL Y-7124, Y — 11 544, Y-11 545

NRRL strain Y-7124 utilized over 95% xylose based on 150 g/L initial concentration. Produced 52 g/L of ethanol with a yield of 0.39 g ethanol per g xylose

Pichia stipits NRLL Y-7124 (floculating strain)

Maximum cell concentration of 50 g/L. Ethanol production rate of 10.7 g/L. h with more than 80% xylose conversion. Ethanol and xylitol yield of 0.4 and 0.03 g/ g xylose

Saccharomyces cerevisiae CBS 1200

Candida shehatae ATCC 24 860

Co-culture of two yeast strains utilized both glucose and xylose. Yields of 100 and 27% on glucose and xylose, respectively

Table 2. Native and engineered microorganisms capable of fermenting xylose to bioethanol1

1 With data from: Balat et al. (2008) and Lee (1997).

Saccharification and Fermentation (SSF) can be added directly or from material previously deviated from pretreatment and inoculated along with Trichoderma reesei or other fungi such as Neurospora crassa and Fusarium oxysporum. These microorganisms were capable of directly fermenting cellulose (Mamma et al., 1996). F. oxysporum was used in a SSF along with S. cerevisiae, yielding 5.2 to 8.4 g ethanol per 100 g of fresh sorghum. The efficiency was calculated based on soluble sugars and not in total polysaccharides (Mamma et al., 1996).