The suitability of Brewers' Spent Grain (BSG) for 2nd generation bioethanol production: a combined biochemical and economic assessment
H αξιολόγηση του Στερεού Αποβλήτου Ζυθοποιίας (Brewers' Spent Grain - BSG) στην παραγωγή βιοαιθανόλης 2ης γενιάς: μία συνδυαστική βιοχημική και οικονομική ανάλυση
KeywordsBrewers’ Spent Grain (BSG) ; Zymomonas mobilis 8b ; Single Vessel Approach (SVA) ; Separate Hydrolysis - Fermentation (SHF) ; Pretreatment ; Acid ; Alkali ; Optimization ; Cost ; Hydrolysis ; Fermentation
The present work represents a comprehensive evaluation of the suitability of Brewers’ Spent Grain (BSG) derived from Greek breweries, as a substrate for the production of 2nd generation bioethanol. The study incorporates the first comparative optimization between mild alkaline and mild acid pretreatment of BSG with respect to its susceptibility against enzymatic hydrolysis, in addition to the evaluation of the performance of the ethanologenic bacterium Zymomonas mobilis 8b in the fermentation of the produced sugars. Additionally, a cost-perspective analysis is carried out for the determination of the optimal pretreatment parameters. In contrast to other pretreatment optimization attempts, where only two independent parameters are used for the optimization, in our approach the optimization was performed with respect to three independent variables, namely, pretreatment temperature, time and alkali/acid load, with the sugar yield produced by the subsequent enzymatic hydrolysis as the dependent variable. In a second, innovative, approach, the obtained experimental results were used in order to access the pretreatment optimization process from an economical perspective, using the specific sugar yield - the amount of sugars produced per unit capital - as a dependent variable. Depending on the optimization target (sugar yield or specific sugar yield), a different set of optimum pretreatment conditions was obtained. This was true for both alkaline and acid pretreatment although for the latter, the corresponding optima differ only with respect to the acid load. Acid pretreatment, proved relatively more cost effective compared to the alkaline, with slightly higher overall sugar yields. The differences that have risen between the two optimization strategies, proves that our approach may result in accountable reductions in the cost per kg of reducing sugar obtained. The second part of our work concerned the evaluation of the genetically modified strain of Ζ. mobilis 8b for the production of ethanol from BSG hydrolysates. Two operational modes for the hydrolysis and fermentation process were evaluated; Separate Hydrolysis and Fermentation (SHF) and Single Vessel Approach (SVA). The most important result obtained from our fermentation data, is the verification that Z. mobilis 8b is able to ferment the sugars in the BSG hydrolysates in SVA mode, especially after alkaline pretreatment where the microorganism ferments the BSG derived sugars at levels equal to the maximum theoretical. Oddly enough, the sugars obtained from BSG, hydrolyzed and fermented in SHF mode, proved to be the less suitable substrates for ethanol production by Z. mobilis 8b, despite the fact that the hydrolysate was fortified by the necessary salts. In overall, our results prove that ethanol production from BSG, represents a potentially economically viable perspective, where its outcome is strongly dependent on the experimentally verified selection of pretreatment conditions, microorganism, and overall biorefinery approach (SVA or SHF).