Valorization of tomato pomace by sequential lycopene extraction and anaerobic digestion

Tomato pomace, a major byproduct of tomato paste production, is an abundant solid waste stream from food processing in California. Tomato pomace is a rich source of lycopene, a red carotenoid and antioxidant, and lignocellulose, the recalcitrant but energy-rich polysaccharide matrix that comprises plant cell walls. Harvesting both of these co-products could add substantial value to the pomace and potentially reduce waste. In this study, lycopene was extracted from tomato pomace using a mixed organic solvent approach. Yields of lycopene from the tomato pomace tended to be higher than most literature values reported for raw tomatoes, and consistent with many reported values for lycopene in tomato pomace and other products. However, review of the current literature indicates that reported lycopene content of tomatoes products varies by roughly two orders of magnitude, which suggests a need for investigation of the factors responsible for this unusually wide range. After lycopene extraction, direct bioconversion to methane via anaerobic digestion and pretreatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate ahead of anaerobic digestion were explored. Under certain conditions, especially 100 °C for 1 h, pretreatment was beneficial to enzymatic digestion of cellulose. Extraction resulted in a statistically significant reduction in methane yield compared to raw pomace after 90 days of anaerobic digestion. However, supplementation of extracted pomace with the non-lycopene-containing aqueous fraction from the extraction is expected to restore the methane yield to that of raw pomace based on measured values for chemical and biochemical oxygen demand. Ionic liquid pretreatment decreased methane production of extracted pomace.