Effects of hydrolysis and bleaching conditions on the efficiency of cellulose microfibrils extraction from coffee parchment through a design of experiments

Cellulose microfibrils were isolated from coffee parchment through acid hydrolysis, alkaline hydrolysis, and bleaching. Factorial design experiments allowed studying the influence of the chemical precursor concentrations and reaction times on the mass losses. TGA, SEM, XRD, and FT-IR techniques allowed for characterizing the coffee parchment hydrolyzed and bleached. Obtained results suggest that acid hydrolysis breaks down the lignocellulosic compounds from the coffee parchment, and alkaline hydrolysis allows the solubilization of lignin and hemicellulose. Lignocellulosic compound dissolution produces a reduction/disappearance of some vibrational bands. This dissolution enhances the crystalline index and decreases the microfibril’s diameter. However, in coffee parchment, the microfibrils are twisted, giving the appearance of fibers with a minor diameter. The design of the experiment results suggests that the main factors during acid and alkaline hydrolysis are the concentration of the chemical precursors. In the bleaching process, a variation in the factors does not significantly influence the response variable. However, for brightness, the precursor concentration affects the cellulose quality. The optimal conditions for cellulose extraction from coffee parchment are 5% (v/v) of HNO₃ by 2 h, 3% (m/v) NaOH by 60 min, and a 1:1 mass of NaClO:CH₃COOH by 45 min for bleaching.