The effect of sorghum grain decortication on bioethanol production technologies

Nkomba, Edouard Yves (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Due to issues surrounding the burning of fossil fuels such as the effect of greenhouses gases on the climate and the threat energy security poses to non-producing nations, biofuels are being promoted for their potential local availability and carbon neutrality. Depending on the materials used, biofuels can be qualified as first (edible) or second (non-edible) generation. Whereas second generation technologies are still not economically viable, first generations biofuels (such as bioethanol from starch) will hold a major share of renewable liquids fuels in the short to medium term. The recent commercialization of enzymes with marked activity towards non-gelatinized or raw starch (cold processing), and their subsequent expressions by genetically modified organisms (Consolidated bioprocessing) could potentially cut the costs and energy requirements of the conventional high temperature processing, which involve cooking or gelatinizing starch. Hence, alternatives such as low temperature cold processing are being investigated for industrial application, while processes to improve the performance of the consolidated bioprocessing are being explored. Furthermore given that biofuels production is continuously increasing, the availability of the main co-product of the conversion process known as distillers dried grains with solubles (DDGS), is following the trend. It has been shown that sorghum grains decortication (removal of bran) prior entering the conversion process could significantly improves the DDGS quality, by reducing the fibre content thereof, hence increasing its market value. Furthermore, the bran components in grains have been shown to negatively affect starch hydrolysing enzymes. In this study, three bioethanol conversion processes (conventional warm, cold and consolidated bioprocessing) and the effect of decortication on key performance measures was assessed using sorghum grains. When using whole grains, the cold and conventional processing achieved similar ethanol concentration (130.4 and 132.1 g/L), productivity (1.55 and 1.51 g.L-¹.h-¹) and ethanol yield as a fraction of the theoretical maximum (89.7 % and 89.03 %). Although a slight decrease in the ethanol yield from consumed glucose was observed in slurries containing decorticated grains, performance of the cold processing was not significantly affected. However, the ethanol productivity of the conventional warm processing decreased with decortication (1.25 g.L-¹.h-¹). The performance of the cold processing using decorticated grains could match the whole grains process, while using 11.7 wt% less enzymes. The DDGS obtained from decorticated grains had higher average protein content (26%) and lower crude fibre content (30.7 %), compared to DDGS from whole grains processing. The acid and neutral detergent fibres contents in DDGS from both types of grains were on average decreased by 17.6 and 26.7% respectively by the cold processing relatively to the conventional processing. The performance of the consolidated bioprocessing could not match the enzyme-based processing, mostly due to limited production of starch-hydrolysis enzymes. The low ethanol tolerance of the recombinant yeast (approximately 90 g/L) prevented consumption of all of the glucose released in the very high gravity slurry. Furthermore, the CBP yeast inoculum size did not have a significant effect on the rate of starch hydrolysis and ethanol productivity, despite design of a fermentation process with high yeast biomass and yeast-produced enzyme concentrations in the starch slurry. Further improvements to the inoculum production, to increase biomass and enzyme concentrations, can be considered, although CBP yeast still lacks sufficient amylase production to achieve efficient starch grains conversion without supplementation with enzymes.

AFRIKAANSE OPSOMMING: Verskeie kwessies rondom die verbranding van fossielbrandstowwe, soos die effek van groenhuisgasse op die klimaat en energie-onsekerheid in nie-olie-produserende lande, promoveer biobrandstof as potensiële alternatiewe energiebron, weens koolstof neutraliteit en plaaslike beskikbaarheid van roumateriale. Afhangend van die aard van die roumateriaal kan biobrandstof in twee kategorieë verdeel word, nl. eerste (eetbare) en tweede (nie-eetbare) generasie biobrandstof. Gegewe dat tweede generasie biobrandstof nog nie ekonomies lewensvatbaar is nie, is die vooruitsig dat eerste generasie biobrandstof steeds die grootste aandeel van vloeibare, hernubare brandstofmark in die kort- tot mediumtermyn sal beslaan. Die onlangse kommersialisering van ensieme wat beduidende aktiwiteit tot ongegelatiniseerde, oftewel rou stysel, toon (koue prosessering), en die uitdrukking van hierdie ensieme deur geneties gemodifiseerde mikro-organismes (gekonsolideerde bioprosessering), het die weg gebaan om kostes en energiebehoeftes aansienlik te besnoei, vergeleke met konvensionele hoë-temperatuurprosesse waar stysel eers gekook en gegelatiniseer moet word om die amilose vir ensiemvertering toeganklik te maak. Derhalwe geniet alternatiewe prosesse soos lae temperatuurprosessering vir industriële toepassing baie aandag, terwyl die werkverrigting van gekonsolideerde bioprosessering in diepte ondersoek word. In pas met die toenemende produksie van biobrandstof, volg die produksie van ʼn hoof byproduk, nl. distilleerders droë korrels en oplosbares (DDKO), ʼn soortgelyke tendens. Daar is verder bewys dat ʼn sorghumgraan semelverwyderingstap, wat die stysel na etanol omskakelingsproses voorafgaan, ʼn beduidende verhoging in die kwaliteit van die DDKO teweeg kan bring, omdat die verlaging in veselinhoud die markwaarde van die finale produk verhoog. In hierdie studie is drie bio-etanol produksieprosesse, nl. die konvensionele warm proses, die koue proses, asook gekonsolideerde bioprosessering, ondersoek. Die invloed van semelverwydering van die sorghumgraan is ook op sleutel prestasie maatstawwe bepaal. Heel graan (graan waarvan semels nie afgeskil is nie) het onderskeidelik tot soortgelyke etanol konsentrasies (130.4 en 132.1 g/L) in die koue en konvensionele warm prosesse gelei, waar soortgelyke klein verskille ook in die produktiwiteit (1.55 en 1.51 g.L-¹.h-¹) en opbrengs as persentasie van die teoretiese maksimum (89.7% en 89.03%) waargeneem is. Alhoewel ʼn klein afname in die etanol opbrengs van suspensies met semel-vrye graan bespeur was, het hierdie prosesstap geen beduidende invloed op die werkverrigting van die koue proses gehad nie. Semelverwydering het wel tot ʼn afname in die produktiwiteit (1.25 g.L-¹.h-¹) van die warm proses gelei, maar die werkverrigting van die koue proses kon dié van die warm proses ewenaar deur 11.7 massa% minder ensiem te gebruik. Daar is bevind dat die gemiddelde proteïeninhoud in die DDKO van die semel-vrye graan 26% hoër en die kru veselinhoud 30.7% laer was as dié van graan waarvan die semels nie afgeskil is nie. Daar is ook bevind dat relatief tot die warm proses, die suur-gewaste en neutraal-gewaste vesel in die DDKO van beide tipes graan onderskeidelik 17.6 en 26.7% laer in die koue proses was. Die werkverrigting van die gekonsolideerde bioprosessering benadering was aansienlik laer as dié van prosesse waar kommersiële ensieme gebruik is (warm en kou prosesbenaderings), hoofsaaklik weens beperkings in die produksie van ensieme wat die rou stysel kon hidroliseer. Daarbenewens is ook bevind dat die geneties gemodifiseerde gis ʼn laer etanol toleransie (ongeveer 90 g etanol/L) gehad het wat die opname van alle beskikbare glukose in hoë-gravitasie suspensies verhoed het. Die grootte van die inokulum van die geneties gemodifiseerde gis het geen beduidende invloed op die tempo van stysel hidrolise of etanol produktiwiteit gehad nie, ten spyte van eksperimente wat vir hoë biomassa- en ensiemkonsentrasies ontwerp is. Verdere ontwikkelingswerk vir inokulum voorbereiding ten einde die biomassa- en ensiemkonsentrasie van die rekombinante gis te verhoog is aangedui. Die rekombinante gis se amilase produksievermoëns bly egter vir effektiewe stysel omskakeling onvoldoende, wat daarop dui dat eksterne ensiem byvoeging steeds benodig word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97809
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