Evaluation of apple pomace biochemical transformation to biofuels and pectin through a sustainable biorefinery

The waste apple pomace, rich in free sugars and structural carbohydrates, can prove to be a good feedstock for biofuels and value-added chemicals production. In this work, apple pomace was used for the production of pectin and biofuels by applying dilute sulfuric acid (1% w/v) treatment. The treatment factors, i.e., temperature and residence time, were optimized to maximize the pectin and biofuels (bioethanol and methane) yields. The solid loaded in the treatment step was 10 g of dried apple pomace per 140 ml of sulfuric acid (1% w/v). The liquor resulting from the dilute acid treatment, containing solubilized sugars and pectin, was subjected to fermentation and pectin recovery. The resulting treated solids were anaerobically fermented to produce biomethane. Furthermore, the environmental and economic aspects of the process developed were evaluated. The highest pectin extraction yield of 164 g pectin per kg of dried apple pomace was obtained after 60 min acid treatment at 94 °C. The pectin extracted with the highest yield had a degree of esterification and galacturonic acid content of 71.3% and 61.2%, respectively. On the other hand, the maximum sugar yield of 411.4 g fermentable sugars (glucose and fructose) per kg of dried apple pomace was obtained at 140 °C after 30 min acid treatment, which was anaerobically fermented to 136.3 g ethanol per kg of dried apple pomace. The highest methane yield (>120 mL/g volatile solids) was obtained at mild conditions. However, the yield decreased with treatment severity. The process developed in this work has a potential to produce over 131.1 million liter of bioethanol in Iran (in 2020). The estimation showed that mixing 5% ethanol (E5) with gasoline can reduce nearly 189.2 k ton of greenhouse gas (GHG) emission in comparison with a gasoline-fueled vehicle in 2020.