Resource recovery in the food processing industry: Simultaneous production of dietary fiber and xylose for xylitol fermentation from oat hulls

Waste streams of the food processing industry contain considerable quantities of carbohydrates that could be fermented to value-added products. This thesis includes work that describes the development of an organosolv process that was designed to produce dietary fiber from oat hulls as well as recover a xylose-rich hemicellulose fraction for bioconversion to xylitol. Optimal results were obtained by treating oat hulls for 15 min at 200°C or 30 min at 190°C with an ethanol-water mixture (60:40) using 0.025 N sulfuric acid as a catalyst. The resultant fiber yield was 46 to 48% of the oat hull input, with the fibers adsorbing water (5.3 to 6.0 g H2O/g fiber) at the high end of the commercially available range; 50% of the available xylose was also recovered. Inhibitory compounds produced during the organosolv process compromised the ability of Candida tropicalis ATCC 9968 (30 g/L inoculum) to ferment the hemicellulose solution that contained 90 g/L xylose to xylitol. By comparison, C. tropicalis produced xylitol from a model substrate of similar sugar composition at a volumetric rate of 3.44 g/L/h and a yield of 0.75 g/g xylose. Six consecutive treatments with powdered activated charcoal (50 g/L) removed most of the inhibitory fractionation by-products from the hemicellulose solution, while reducing the concentration of hemicellulose sugars by less than 10%. As a result, C. tropicalis completely consumed the xylose (104 g/L) and produced xylitol at a rate of 1.0 g/L/h and a yield of 0.55 g/g xylose. This is the first demonstration wherein hemicellulose sugars produced by organosolv fractionation were bioconverted to xylitol. An economic analysis was done to determine the price at which the fiber fraction would have to be sold in order to make the organosolv fractionation of oat hulls profitable. Based on the process parameters from the above analysis, the price for the fiber would be at least {dollar}0.26 to {dollar}0.62 per kg, depending on the scale of the operation.