Optimization Study of Citrus Wastes Saccharification by Dilute-acid Hydrolysis

The effects of time, acid concentration, temperature and solid concentration on dilute-acid hydrolysis of orange peels were investigated. A central composite rotatable experimental design (CCRD) was applied to study the individual effects of these hydrolysis factors and also their interdependence effects. The enzymatic hydrolysis of the peels by cellulase, β-glucosidase, and pectinase enzymes resulted in 72% dissolution of the peels, including 18.7% galacturonic acid and 53.3% of a total of glucose, fructose, galactose, and arabinose. Dilute-acid hydrolysis up to 210°C was not able to hydrolyze pectin to galacturonic acid. However, the sugar polymers were hydrolyzed at relatively low temperature. The optimum results were obtained at 116°C, 0.5% sulfuric acid concentration, 6% solid fraction, and 12.9 min retention time. Under these conditions, the total sugars obtained at 41.8% dry peels and 2.6% of total hexose sugars were further degraded to hydroxymethylfurfural (HMF). No furfural was detected through these experiments from decomposition of pentoses.