Ultrasonic treatment of brewer’s spent grains for bioethanol production

Technical Session 21: Spent Grains Session
Jason Bennett, University of Abertay, Dundee, Scotland UK
Co-author(s): Graeme Walker and David Bremner, University of Abertay, Dundee, Scotland

ABSTRACT: Bioethanol (fuel ethanol derived through fermentation) is now the leading biofuel alternative to fossil-based liquid transportation fuels. Current production is dominated by U.S. corn-based and Brazilian sugarcane-based processes. However, more sustainable future bioethanol production needs to be based on non-food substrates that use lignocellulosic biowastes. The brewing and distilling industry sectors are uniquely placed to exploit the conversion of lignocellulose to bioethanol, through the utilization of spent grains. Bioconversion of brewer’s spent grains (BSG) to fuel alcohol represents an attractive but challenging opportunity for sustainable bioethanol production. In addition to the technological and scientific challenges in bioethanol production from spent grains, there are also constraints relating to economics and energy balances. For example, enzyme costs need to be lowered, particularly considering cellulolysis of feedstock. Any innovations to decrease cellulase enzyme dosage are a distinct advantage. We have evaluated the influence of ultrasonic irradiation (at varying frequencies between 382 and 1,174 kHz) on cellulolytic enzymatic digestion of pre-treated BSG. Results have shown that ultrasonic irradiation during enzymolysis increases the total sugar release rate from BSG. In particular, results from exposure of enzymolysis to ultrasound at a frequency of 998 kHz shows that ultrasound holds the potential to significantly reduce the dosing rates of cellulose enzyme required for the hydrolysis of lignocelluloses. Different yeast species, including Saccharomyces cerevisiae, Pichia stipitis, Kluyveromyces marxianus, Pachysolen tannophilus, and Candida shehatae, have been evaluated for their ability to ferment the mix of five and six carbon sugars liberated following ultrasonic pretreatment and during enzymatic hydrolysis of BSG. Results have indicated that while Saccharomyces cerevisiae can ferment hexose sugars within BSG hydrolysates, it lacks the ability to ferment pentose sugars. Pichia stipitis, Kluyveromyces marxianus, Pachysolen tannophilus, and Candida shehatae exhibited the ability to ferment the full range of both hexose and pentose sugars within BSG hydrolysates. However, sugar utilization between species varied greatly, with Pichia stipitis and Kluyveromyces marxianus displaying the best fermentation performance. Research conducted during this study has shown that the application of ultrasonic technology during the enzymolysis of BSG has the potential to significantly reduce the costs associated with cellulolytic enzyme dosing during the bioconversion of lignocellulosic substrates to bioethanol.

Jason Bennett graduated with a B.S. degree in biotechnology from the University of Abertay Dundee in 2008, with a thesis titled “The Application of Ultrasound in Yeast Biotechnology.” He is currently completing his Ph.D. degree with a thesis titled “The Application of Ultrasound in Bioconversion of Brewer’s and Distiller’s Spent Grains to Bioethanol.” In January 2012 he commenced a new post within the university, focusing on developing sustainable solutions for dealing with the co-products produced during malt whisky distillation.