A low-energy, cost-effective approach to fruit and citrus peel waste processing for bioethanol production

Large quantities of fruit waste are generated from agricultural processes worldwide. This waste is often simply dumped into landfills or the ocean. Fruit waste has high levels of sugars, including sucrose, glucose, and fructose, that can be fermented for bioethanol production. However, some fruit wastes, such as citrus peel waste (CPW), contain compounds that can inhibit fermentation and should be removed for efficient bioethanol production. We developed a novel approach for converting single-source CPW (i.e., orange, mandarin, grapefruit, lemon, or lime) or CPW in combination with other fruit waste (i.e., banana peel, apple pomace, and pear waste) to produce bioethanol. Two in-house enzymes were produced from Avicel and CPW and were tested with fruit waste at 12-15% (w/v) solid loading. The rates of enzymatic conversion of fruit waste to fermentable sugars were approximately 90% for all feedstocks after 48 h. We also designed a d-limonene removal column (LRC) that successfully removed this inhibitor from the fruit waste. When the LRC was coupled with an immobilized cell reactor (ICR), yeast fermentation resulted in ethanol concentrations (14.4-29.5 g/L) and yields (90.2-93.1%) that were 12-fold greater than products from ICR fermentation alone.