Apple pomace (AP), the residue generated in cider and apple juice making factories, is used mainly as animal feed or for compost. The general objective of the ULTREIA project is the multiproduct valorization of AP for the extraction of polyphenolics, the production of enzymes and the biological conversion into acetone-butanol-ethanol (ABE).
Figure 1. Scheme of the multi-product biorefinery for the valorization of apple pomace envisaged in the ULTREIA project.
The main bioactive ingredients of apple pomace include procyanidins, flavonols, dihydrochalcones, hydroxycinnamic acids and catechins. The strong evidence from in vitro and in vivo studies revealed the potential use of apple phenolics as effective food supplements or nutraceuticals for the management of chronic diseases. In a first stage of the project, water and ethanol will be used to extract polyphenolics from apple pomace at different temperatures, using conventional and a non-conventional technology (ultrasonic assisted extraction). Particular attention will be given to the potential release of sugars in the polyphenolic fraction. In this sense, the minimization of monomeric sugars release is desired. This is one of the challenges of this proposal. Up to now, single objectives have been pursued: either the extraction of polyphenolics or the sugar release. Hence, our double objective goes beyond previously published research in this topic.
After polyphenolic extraction the remaining solid fraction will be subjected to enzymatic hydrolysis, leading to a solution with high concentration of fermentable sugars. Enzyme cost is one of the main constraints in the cellulosic biofuel production and it has been reported that on site enzyme production is a promising option to reduce enzyme cost. For this reason, cellulases used in this step will be produced on site, using AP and the different streams derived from the valorization process.
Subsequently, the AP hydrolysate will be used as substrate for ABE fermentation by Clostridia species. Biobutanol could be a preferable option to bioethanol because of its higher energy content, lower water miscibility and hygroscopicity, and better compatibility with existing vehicle engines. It has been described the importance of pretreatment methods in the fermentability of apple pomace, hence it is expected that the ABE productivity will depend on the different conditions applied to extract the polyphenolics. The reduced concentration of phenolics (known inhibitors of Clostridia) in the hydrolysate could favor the production of ABE. The integration of the fermentation process with an in-situ product removal system is one of the solutions described to overcome the problem of butanol toxicity on Clostridia. In the present project, an in-situ liquid-liquid extraction approach will be evaluated. The role of Universidade de Santiago de Compostela in the ULTREIA project will be to assess AP as a source of polyphenolics and biobutanol.