Maximising high solid loading enzymatic saccharification yield from acid-catalysed hydrothermally-pretreated brewers spent grain

Stuart Wilkinson,Katherine A Smart,David J Cook,Sue James

doi:10.18331/brj2016.3.2.7

Stuart Wilkinson, Katherine A Smart + Show 2 more

Open Access

https://doi.org/10.18331/brj2016.3.2.7

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Abstract

Enzyme saccharification of pretreated brewers spent grains (BSG) was investigated, aiming at maximising glucose production. Factors investigated were; variation of the solids loadings at different cellulolytic enzyme doses, reaction time, higher energy mixing methods, supplementation of the cellulolytic enzymes with additional enzymes (and cofactors) and use of fed-batch methods. Improved slurry agitation through aerated high-torque mixing offered small but significant enhancements in glucose yields (to 53 ± 2.9 g/L and 45% of theoretical yield) compared to only 41 ± 4.0 g/L and 39% of theoretical yield for standard shaking methods (at 15% w/v solids loading). Supplementation of the cellulolytic enzymes with additional enzymes (acetyl xylan esterases, ferulic acid esterases and α-L- arabinofuranosidases) also boosted achieved glucose yields to 58 – 69 ± 0.8 - 6.2 g/L which equated to 52 - 58% of theoretical yield. Fed-batch methods also enhanced glucose yields (to 58 ± 2.2 g/L and 35% of theoretical yield at 25% w/v solids loading) compared to non-fed-batch methods. From these investigations a novel enzymatic saccharification method was developed (using enhanced mixing, a fed-batch approach and additional carbohydrate degrading enzymes) which further increased glucose yields to 78 ± 4.1 g/L and 43% of theoretical yield when operating at high solids loading (25% w/v).

Highlights

Production of bioethanol from brewers spent grains (BSG) is a current area of research interest as higher value uses are sought for this co-product derived from the beer brewing process
Whilst high % theoretical yields would indicate a high degree of process efficiency, they do not provide a measure of the ‘usability’ of a feedstock
A feedstock with a high % theoretical glucose yield but low g/L yield would not be viable to use for biofuel production

Summary

Introduction

Production of bioethanol from brewers spent grains (BSG) is a current area of research interest as higher value uses are sought for this co-product derived from the beer brewing process. Maximising the operational solids loading used (a high solid to liquid ratio) during the enzymatic saccharification step, whilst minimising the required enzyme dose are likely to be key factors which need to be addressed in order to produce high glucose concentrations cost effectively and minimise water usage (Hodge et al, 2008; Kristensen et al, 2009). Operation at high solids loading would likely drop the conversion efficiency of cellulose to glucose resulting in % theoretical yields well below acceptable limits of process efficiency This is possibly due to rheology related mass transfer limitations because of the high viscosity of the media impeding enzymatic access to all of the available substrate (cellulose) and impeding dispersal of the hydrolysis products which may feedback inhibit the cellulases (Gan et al, 2003; Chundawat et al, 2008). This would suggest that an upper limit exists with regards to the maximum functional solids loading that can be used during the enzyme saccharification step as going beyond this would detrimentally limit the available free water present

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