Investigation of an effective acid pre‑treatment method for the valorisation of Canola fines

Abstract

Canola fines are the straw-like material and other residues removed from incoming Canola crops before processing for seed oil. In South Africa, this lignocellulosic biomass is abundant and non-seasonal, making it an ideal substrate for recovery of sugars for valorization to higher value products. Pre-treatment for release of sugars from lignocellulose should be optimized for a given substrate. In this study, mechanical pre-treatment to reduce the size of Canola fines was followed by: (i) steam-assisted acid pre-treatment (0–2% H₂ SO₄ % v/v) at different temperatures (20–55 °C), and biomass loading rates (2–8% w/v), and (ii) enzyme hydrolysis, using a central composite experimental design approach. The sugars in the hydrolysates obtained from stages (i) and (ii) were measured quantitatively and qualitatively as outputs that were used to assess process efficiency and to obtain predictive models. The raw and spent Canola fines from each phase were fully characterized in terms of chemical and structural changes at each stage of pre-treatment.

The Canola fines contained 23.0% w/w lignin, 21.4% w/w cellulose, and 15.9% w/w hemicellulose. The maximum yield of total sugars after steam-assisted acid hydrolysis (18.6% w/w) was increased after enzyme hydrolysis (28.3 % w/w) and was higher than reported for other studies pre-treating rapeseed and Canola straws (10.2–11.8% w/w). The sugar profiles in the hydrolysates after steam-assisted acid hydrolysis (xylose>arabinose>glucose) differed from those obtained after acid-assisted enzyme hydrolysis (xylose>glucose>cellobiose>arabinose). The microscopic and chemical analyses of the Canola fines showed sequential breakdown of the lignin, cellulose, and hemicellulose, with some lignin still intact at the end of the pre-treatment. For application of this technology, no heating would be required for acid pre-treatment as temperature was not a significant variable. In addition, the optimal H₂ SO₄ concentration was low (1.7% v/v), minimizing the use of hazardous chemicals as well as the risk of releasing microbial and/or enzyme inhibitors during the process. Enzyme hydrolysis should only be required if the downstream application requires higher sugar concentrations and/or different sugar profiles.