Twin-screw extrusion pretreatment of defective Arabica coffee press cake for carbohydrates recovery

Abstract

Defective Arabica coffee beans generate large amounts of low-value residues, such as coffee press cake (a byproduct of coffee oil extraction) rich in polysaccharides — mainly mannan — but still underexploited as a raw material. In this study, defective coffee press cake was processed by twin-screw extrusion followed by enzymatic hydrolysis using a crude mannanase-rich extract to recover monosaccharides. To the best of our knowledge, this is the first study to apply extrusion pretreatment optimization to defective coffee beans aiming to enhance the recovery of fermentable monosaccharides. The extrusion pretreatment was optimized using response surface methodology, identifying the best conditions as 80 °C and 20 rpm. Increasing the number of extrusion cycles enhanced enzymatic accessibility, yielding 116.2 ± 0.5 mg.g⁻¹ of mannose after 5 cycles and enabling up to 50% mannose recovery within 24 h of hydrolysis. Under optimized enzymatic conditions (40 IU.g⁻¹ enzyme loading, 48 h), mannose recovery increased to 64% relative to acid hydrolysis, corresponding to 147.1 ± 3.9 mg.g⁻¹. At high solids loading (30%), concentrated mannose syrups containing up to 33.8% mannose were produced. Overall, this integrated process demonstrates the feasibility of valorizing defective coffee residues through a sustainable approach aligned with biorefinery and green chemistry principles, generating mannose-rich syrups with potential applications in the food, pharmaceutical, and cosmetic industries.