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

Abstract

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), 2 3 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 pre-treatment 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.