Lactic acid production from sugarcane bagasse and harvesting residues

Koekemoer, Tunet (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die potensiaal van suikerrietbagasse en suikerriet oesreste, wat van verskillende Suid Afrikaanse suikermeule verkry is, en met verdunde suur katalise behandel is, is vir bioraffinadery-gebaseerde etanol-, melksuur- en elektrisiteitproduksie in hierdie studie geassesseer. Op ʼn droëmassabasis het suikerrietbagasse 38% glukaan, 15% arabinoxilaan, 27% lignien, 7% ekstraktiewe, 9% asetiel groep en 3% as bevat. Daarteenoor het suikerriet oesreste 33% glukaan, 17.5% arabinoxilaan, 20% lignien, 16% ekstraktiewe, 5% asetiel groep en 9% as bevat. Na aanvanklike keuringseksperimente om die verskille tussen response vanaf die twee voerstowwe te assesseer, is ʼn roteerbare, sentrale saamgestelde ontwerp gebruik om die xilose vanaf hemisellulose, glukose vanaf sellulose en die gekombineerde suikeropbrengs ná verdunde suurbehandeling en ensimatiese hidrolise te optimeer, waar temperatuur, swaelsuurkonsentrasie en tydsduur die onafhanklike veranderlikes was. Op grond van ʼn geoptimeerde regressiemodel met ʼn 95% vertroue vlak het al drie faktore ʼn statisties beduidende effek op die behandeling van suikerrietbagasse gehad, terwyl slegs temperatuur en swaelsuurkonsentrasie ʼn beduidende effek op die behandeling van die oesreste gehad het. Modelvoorspellings het tot die optimale produksie van 24.5 g xilose, 32.4 g glukose en 63 g gekombineerde suiker per 100 g DM vir suikerrietbagasse, en 17.4 g xilose, 42.9 g glukose en 66.7 g gekombineerde suiker per 100 g DM vir oesreste gelei. Stoombehandeling is gebruik om voldoende hoeveelhede, hemisellulose-ryke hidrolisaat vir melksuurfermentasie eksperimente te produseer, waar ses verskillende melksuurbakterieë vanaf verskillende navorsingsgroepe en kultuurversamelings gebruik is. Dié stamme is op grond van hul vermoëns om (i) xilose, arabinose en glukose gelyktydig te fermenteer, (ii) by matig tot hoë temperature te funksioneer en (iii) bestand te wees teen hoë inhibitorkonsentrasies wat tydens stoombehandeling geproduseer word, geselekteer. Die natuurlike weerstand van elk van die bakteriële stamme teen inhibitoriese verbindings wat in hemisellulose hidrolisate aangetref word, is onder anoksiese asook aërobiese toestande getoets. Laasgenoemde is ingesluit ten einde vas te stel of lae suurstofspannings in skudfleskulture die fermentasie van vyf-koolstofsuikers negatief beïnvloed, aangesien dié suikers gewoonlik via die pentose-fosfaat weg geassimileer word, en waar lae suurstof tot redoks wanbalanse kan lei. Oor die algemeen was melksuurkonsentrasies onder anoksiese toestande heelwat hoër in fermentasiesop wat met 75% (v/v) hidrolisaat aangevul was. Bacillus coagulans P38 het onderskeidelik 4.18 en 20.42 g/L melksuur vanaf die hidrolisate van bagasse en suikerriet oesreste geproduseer terwyl Bacillus coagulans MXL-9 onderskeidelik 5.58 g/L en 16.97 g/L melksuur van die twee substrate geproduseer het. Daarteenoor het Lactoccocus lactis IO-1 onderskeidelik 8.68 g/L en 17.44 g/L melksuur geproduseer. Die resultate het die belang van bakteriële stamseleksie beklemtoon wanneer relatief toksiese substrate vir die produksie van ʼn ekonomies-belangrike platformverbinding soos melksuur gebruik word.

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