No abstract

Producing biofuels such as ethanol from non-food plant material has the potential to meet transportation fuel requirements in many African countries without impacting directly on food security. The current shortcomings in biomass processing are inefficient fermentation of plant sugars, such as xylose, especially at high temperatures, lack of fermenting microbes that are able to resist inhibitors associated with pre-treated plant material and lack of effective lignocellulolytic enzymes for complete hydrolysis of plant polysaccharides. Due to the presence of residual partially degraded lignocellulose in the gut, the dung of herbivores can be considered as a natural source of pre-treated lignocellulose. A total of 101 fungi were isolated (36 yeast and 65 mould isolates). Six yeast isolates produced ethanol during growth on xylose while three were able to grow at 42 °C. This is a desirable growth temperature as it is closer to that which is used during the cellulose hydrolysis process. From the yeast isolates, six isolates were able to tolerate 2 g/L acetic acid and one tolerated 2 g/L furfural in the growth media. These inhibitors are normally generated during the pre-treatment step. When grown on pre-treated thatch grass, Aspergillus species were dominant in secretion of endo-glucanase, xylanase and mannanase.

Cellulases are a group of hydrolytic enzymes that break down cellulose to glucose units. These enzymes are used in the food, beverage, textile, pulp, and paper and the biofuel industries. The aim of this study was to isolate fungi from natural compost and produce cellulases in submerged fermentation (SmF). Initial selection was based on the ability of the fungi to grow on agar containing Avicel followed by cellulase activity determination in the form of endoglucanase and total cellulase activity. Ten fungal isolates obtained from the screening process showed good endoglucanase activity on carboxymethyl cellulose-Congo Red agar plates. Six of the fungal isolates were selected based on high total cellulase activity and identified as belonging to the generaTrichodermaandAspergillus. In SmF of synthetic media with an initial pH of 6.5 at 30°CTrichoderma longibrachiatumLMLSAUL 14-1 produced total cellulase activity of 8 FPU/mL and endoglucanase activity of 23 U/mL whilstTrichoderma harzianumLMLBP07 13-5 produced 6 FPU/mL and endoglucanase activity of 16 U/mL. The produced levels of both cellulases and endoglucanase byTrichodermaspecies were higher than the levels for theAspergillus fumigatusstrains.Aspergillus fumigatusLMLPS 13-4 produced higherβ-glucosidase 38 U/mL activity thanTrichodermaspecies.

Efficient conversion of pentose sugars to ethanol is important for an economically viable lignocellulosic bioethanol process. Ten yeasts fermenting both D-xylose and L-arabinose were subjected to an adaptation process with L-arabinose as carbon source in a medium containing acetic acid. Four Meyerozyma caribbica-adapted strains were able to ferment L-arabinose to ethanol in the presence of 3 g/L acetic acid at 35°C. Meyerozyma caribbica Mu 2.2f fermented L-arabinose to produce 3.0 g/L ethanol compared to the parental strain with 1.0 g/L ethanol in the absence of acetic acid. The adapted M. caribbica Mu 2.2f strain produced 3.6 and 0.8 g/L ethanol on L-arabinose and D-xylose, respectively, in the presence of acetic acid while the parental strain failed to grow. In a bioreactor, the adapted M. caribbica Mu 2.2f strain produced 5.7 g/L ethanol in the presence of 3 g/L acetic acid with an ethanol yield and productivity of 0.338 g/g and 0.158 g/L/h, respectively, at a KLa value of 3.3 h−1. The adapted strain produced 26.7 g/L L-arabitol with a yield of 0.900 g/g at a KLa value of 4.9 h−1.

Banana pseudostem (BPS) is an agricultural waste with a high holocellulose content, which, upon hydrolysis, releases fermentable sugars that can be used for bioethanol production. Different pretreatment methods, namely, 3% (w/v) NaOH, 5% (v/v) H2SO4, and liquid hot water, applied on the BPS resulted in the availability of 52%, 48%, and 25% cellulose after treatment, respectively. Saccharification of the pretreated BPS with 10 FPU/g dry solids (29.3 mg protein/g d.s) crude enzyme from Trichoderma harzianum LMLBP07 13-5 at 50°C and a substrate loading of 10 to 15% released 3.8 to 21.8 g/L and from T. longibrachiatum LMLSAUL 14-1 released 5.4 to 43.5 g/L glucose to the biomass. Ethanol was produced through separate hydrolysis and fermentation (SHF) of alkaline pretreated BPS hydrolysate using Saccharomyces cerevisiae UL01 at 30°C and 100 rpm. Highest ethanol produced was 17.6 g/L. Banana pseudostem was shown as a potentially cheap substrate for bioethanol production.