Exploring Deep Eutectic Solvent Pretreatment and Laccase Biobleaching on Oat Hulls in Efforts to Overcome Recalcitrance

Sammanfattning: Oat production produces an abundant side stream product called oat hulls, formally regarded as a waste product. These oat hulls are nutritionally packed, containing insoluble dietary fibres. The solubilization and subsequent enzymatic hydrolysis of insoluble fibres, i.e. xylan, can potentially be developed into prebiotic oligosaccharides. However, insoluble fibres are tightly bound, making it difficult for enzymatic conversion to soluble fibres. Current methods to degrade oat hulls require harsh chemicals which are concerning for the environment and are cost intensive. In this study greener retreatment methods on oat hulls using two deep eutectic solvents (DES), choline chloride: glycerol (1:2) and choline chloride: ethylene glycol (1:2), were explored. Pretreated oat hulls were subjected to two individual xylanases to observe the digestibility efficiency and properties of the pretreated oat hulls. Soluble fibre yields obtained following enzymatic treatment for the DES pretreated oat hulls were significantly low, between 1.1-1.4 % of the total enzymatic treated samples. The hydrolysates were analysed to determine mono- and oligosaccharide composition. Based on these findings, one can conclude that DES treatment is not feasible on oat hulls. Furthermore, unknown impurities were observed in the enzyme hydrolysis control samples and monosaccharide and oligosaccharide chromatograms, applying a cocktail of biomass-degrading enzymes working in synergy to convert oat hulls to fermentable sugars. Exploration studies on the potential of using laccase NS51003 as an enzyme for biobleaching treatment on oat hulls were carried out. It is presumed by applying a biobleaching step before hydrogen peroxide bleaching the amount of chemicals needed would be reduced. The purpose of bleaching is to produce white powders from oat hulls that can be used as a food additive in various food products, i.e. white pastas. Laccase treatment was shown not to improve the brightness and the CIELAB colour space L value, which refers to the whiteness of oat hulls, after hydrogen peroxide bleaching. However, optimization techniques that aided the bleaching process are discussed.