Exopolysaccharide pullulan production from enzymatic hydrolysate of quinoa stalks via citric acid–assisted hydrothermal pretreatment

Quinoa waste is often overlooked due to the limited scientific understanding of its potential applications. However, the carbohydrates it contains can be hydrolyzed and utilized in biotechnological processes to produce valuable products, such as pullulan, an exopolysaccharide used in the food and pharmaceutical industries. This approach can also help mitigate pollution from waste incineration. To effectively extract sugars from quinoa stalks, pretreatment is essential to modify their structural properties. In this context, citric acid–assisted hydrothermal pretreatment (CA-HTP) was evaluated to enhance the enzymatic digestibility of the carbohydrates, allowing for the subsequent production of pullulan from the extracted sugars. A central composite design (CCD) was employed to assess the impact of pretreatment time (20 to 60 min) and citric acid loading (3 to 12% w/w) at 180 °C. Under the optimized condition of CA-HTP (37.12 min, 9.43% w/w CA loading), 59 g of glucose per 100 g of pretreated biomass (94% hydrolysis yield) was achieved after 24 h of enzymatic process. Low CA loading during HTP produced 6.8 g of xylose per 100 g of biomass and 10 mg/g of XOS (xylobiose + xylotriose). Finally, using the enzymatic hydrolysate, 8.9 g/L of pullulan (yield 0.51 g/g) was achieved. CA-HTP enhances biomass susceptibility to enzymatic hydrolysis, releasing a high concentration of fermentable sugars and prebiotics. This process could be applied in the valorization of other organic residues, contributing to the development of a circular bioeconomy.