Converting Citrus Waste to Ethanol and Other Co-Products
Conversion of citrus processing waste (CPW) generated during juice production into value added co-products is an important aspect of the juice industry as it offers a solution to waste disposal issues. Currently the practice of drying citrus waste to produce citrus pulp pellets (CPP) for use as cattle feed is profitable. However, until the recent rise in value, CPP value was marginal and often did not meet production costs. Another concern has been volatile organic emissions during CPP production. Only one third of the residual peel oil present in citrus waste is recovered during CPP production with most being vented to the atmosphere during drying and is a growing environmental concern. Improvements in limonene recovery and development of alternative value added co-products obtained from CPW could add substantial value to the citrus crop. For current CPP production, the energy required to dry CPW is the major cost involved and approximately 25 lb of limonene are obtained per ton of CPP produced. Since limonene is recovered during evaporation/concentration of pressed peel juice using a waste heat evaporator, little additional cost is associated with limonene recovery. The concentrated citrus molasses produced may be added back to the press cake or fermented to make ethanol, but only contains a third of the sugars in CPW that are fermentable by conventional yeast. While utilizing the entire CPW stream for ethanol using hydrolysis and fermentation is more involved, three times the amount of ethanol can be obtained compared to using press liquor alone. Most of the limonene must be removed as it inhibits fermentation. In the process developed 85–95% of the limonene contained in CPW can be removed and recovered by steam stripping. This greatly reduces concerns associated with the release of volatile organic compounds (VOCs) during processing of CPW and the limonene recovered has a value equal or greater than stripping costs. Using a mixture of enzymes and yeast, the CPW is then hydrolyzed and fermented simultaneously to produce ethanol followed by distillation to remove and recover the ethanol. Enzyme costs to hydrolyze and liquefy CPW have been reduced to less than a dollar per gallon of ethanol produced, and the economics for distillation are still being optimized. The distillation residues contain half the solids of raw citrus waste that can still be utilized as a CPP product. Other uses for the residues such as incorporation of the pectic materials into building product and paper additives, and ion exchange materials for wastewater remediation are also in development. Paper published with permission.