Browsing by Author "Koekemoer, Tunet"
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- ItemLactic acid production from sugarcane bagasse and harvesting residues(Stellenbosch : Stellenbosch University, 2018-03) Koekemoer, Tunet; Gorgens, Johann F.; Van Rensburg, Eugene; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH SUMMARY: Sugarcane bagasse and harvesting residues collected from different sugars mills across South Africa were evaluated for potential use in a biorefinery for ethanol, lactic acid and electricity co-production after pretreatment using dilute sulphuric acid as catalyst. On a dry mass basis, sugarcane bagasse consisted of 38% glucan, 15% arabinoxylan, 27% lignin, 7% extractives, 9% acetyl groups and 3% ash. By comparison, harvesting residues consisted of 33% glucan, 17.5% arabinoxylan, 20% lignin, 16% extractives, 5 % acetyl groups and 9% ash. Following pre-screening experiments to appraise the differences between the responses of the two feedstocks, a central composite, rotatable design was used to optimise the xylose from hemicellulose, glucose from cellulose and combined sugar yield after pre-treatment and enzymatic hydrolysis, where temperature, sulphuric acid concentration and residence time were the independent variables. Based on optimised regression at a 95% confidence level, all three factors had a significant effect on the pre-treatment of sugarcane bagasse, whereas only temperature and sulphuric acid concentration were significant during the pre-treatment of harvesting residues. Based on model predictions, optimal conditions resulted in the production of 24.5 g xylose, 32.4 g glucose and 63 g combined per 100 g DM for sugarcane bagasse and 17.4 g xylose, 42.9 g glucose and 66.7 g combined sugar per 100 g DM for harvesting residues. Steam pre-treatment was used to produce sufficient quantities of hemicellulose-rich hydrolysate for lactic acid production during fermentation using six different lactic acid bacteria obtained from various research groups and culture collections. These strains were selected based on the ability to (i) ferment xylose, arabinose and glucose simultaneously; (ii) operate at moderately to high temperatures, and (iii) were tolerant to inhibitor compounds produced during pre-treatment. The innate tolerance of each strain to inhibitory compounds found in hemicellulose hydrolysates were tested under anaerobic and micro-aerobic conditions. The latter was included to determine if the low oxygen tensions in shake flask cultures negatively affected fermentation of five-carbon sugars, usually assimilated via the pentose phosphate pathway where the absence of oxygen could lead to redox imbalances. Higher lactic acid concentrations were generally observed under anaerobic conditions in a fermentation broth supplemented with 75% (v/v) hemicellulose hydrolysate where Bacillus coagulans P38 produced 4.18 and 20.42 g/L lactic acid from the hydrolysates of sugarcane bagasse and harvesting residues, respectively. By comparison, Bacillus coagulans MXL-9 produced 5.58 g/L and 16.97 g/L lactic acid from the hydrolysates of bagasse and harvesting residues, respectively, and Lactoccocus lactis IO-1 produced 8.68 g/L and 17.44 g/L lactic acid from the respective substrates. These results accentuated the importance of bacterial strain selection when using complex and relatively toxic substrates for production of lactic acid as an economically important platform chemical.