Upgrading sugarcane straw into biopigments via enzymatic hydrolysis and fermentation by Monascus ruber: integrating statistical analysis and kinetic modeling

Abstract

In this work, the Kraft process was employed for lignin extraction from sugarcane straw, and the resulting cellulosic pulp was fractionated, chemically characterized, and subsequently subjected to enzymatic hydrolysis. Delignification was based on a modified Kraft process using an AU/E-20 Regmed rotary reactor, which allowed four simultaneous reactions with a useful volume of 1 L each. A solid loading of 10% (60 g biomass to 0.6 L alkaline solution containing 3.5 M NaOH and 0.61 M Na₂S) was applied under all tested conditions. The adopted sulfidity was 25%, and the active alkali charge relative to biomass was 9% (w/w), ensuring effective lignin solubilization while preserving the structural carbohydrates. Through mathematical modeling, the kinetic profile of enzymatic hydrolysis was monitored over time. A 2² factorial experimental design was applied to evaluate the effects of solid content (%, w/v) and enzyme loading (FPU g⁻¹) on sugarcane straw hydrolysis. Delignification resulted in approximately 60% lignin removal. The delignified pulp exhibited high enzymatic accessibility, with digestibility values exceeding 80% for both glucan and xylan, indicating a substantial improvement in biomass susceptibility to enzymatic attack and conversion efficiency. These results confirm the effectiveness of the modified Kraft process as a pretreatment strategy. The hydrolysate was subsequently used for biopigment production by the fungus Monascus ruber in submerged cultivation, evaluating the influence of hydrolysate concentration and cultivation temperature. Maximum pigment production reached 9.54 AU, 10 AU, and 14.65 AU for yellow, orange, and red pigments, respectively. Lower cultivation temperatures and reduced hydrolysate concentrations favored overall pigment production.