Two-stage pre-treatment of coffee pulp waste to optimize the reducing sugar production using enzymatic hydrolysis

Robusta coffee (Coffee robusta L.) pulp waste has been known for its high cellulose and hemicellulose content which potentially could be utilized as a source of a reducing sugar feedstock. Unfortunately, it contains inhibitors such as lignin, tannin, caffeine, and total polyphenols that can inhibit the enzymatic hydrolysis process. Therefore, coffee pulp waste needs pre-treatment prior to its utilization in reducing sugar production. To optimize the pre-treatment condition, the two-stage of pre-treatment process was carried out using 0.2 M sulfuric acid and then organosolv using ethanol. Subsequently, the optimization was done using Response Surface Methodology (RSM), 2³ full factorial design, with the following input variables: ethanol concentration, temperature, and duration of pre-treatment. This study was subjected to determine the optimum conditions for organosolv pretreatment which resulted in inhibitor removal and high concentration of reducing sugar. The hydrolysis process was carried out for 60 hours using a mixture of enzymes with and without the addition of Tween 80 as a surfactant. The result indicated that the lignin removal percentage increased from 0.85% (w/w) to 16.905% (w/w) towards the remaining lignin grams, with the change of crystallinity index of cellulose from 17.23% into 16.43%. The concentration of obtained reducing sugar with the addition of Tween 80 was 2.402 mg/ml, 1.6 times higher than that obtained without the addition of Tween 80.