L-lactic acid production from cassava pulp through high-solids simultaneous saccharification and fermentation: Development of fed-batch strategy based on substrate degradation kinetics

• Use of a first-order kinetic model to describe changes in solids concentration. • Use of the first-order rate constant to estimate optimal substrate feeding times. • Excessive accumulation of unhydrolyzed solids was avoided, thus improving L-LA titer. • Comparable L-LA titer achieved with lower biomass than other high solids systems. • Double-feeding fed-batch process significantly improved medium-based economic yield. High-solids simultaneous saccharification and fermentation (SSF) processes have received increasing attention for their improved product titer compared to low-solids systems. However, despite being crucial for mixing efficiency and determining additional substrate additions, the concentration of unhydrolyzed solids accumulating in the system has rarely been monitored. Without such monitoring, multiple substrate additions could lead to excessive solids accumulation and poor process performance. This study addressed this issue by monitoring residual solids concentration and modeling it using a first-order kinetic model to estimate optimal substrate feeding times. Based on kinetic analysis, batch SSF using Enterococcus gallinarum TSKKU D-6 and 150 g/L of cassava pulp (CP) produced 77.0 ± 1.2 g/L of L-lactic acid (L-LA) within 60 h, with productivity and yield of 1.28 ± 0.02 g/(L·h) and 0.51 ± 0.01 g/g-CP, respectively. The kinetic model estimated the optimal feeding time to be around 36 h. A single-feeding fed-batch process was subsequently conducted, with CP addition at 36 h to raise the solids concentration to 150 g/L, producing 99.2 ± 2.0 g/L of L-LA at 96 h, with productivity and yield of 1.03 ± 0.02 g/(L·h) and 0.46 ± 0.01 g/g-CP, respectively. Using the same approach, a double-feeding process with CP additions at 36 and 72 h achieved 109.9 ± 0.2 g/L of L-LA at 96 h, with productivity and yield of 1.15 ± 0.01 g/(L·h) and 0.42 ± 0.01 g/g-CP, respectively. Although L-LA yield, fermentation efficiency, and productivity slightly decreased with increasing solids loading, the double-feeding process significantly improved medium-based economic yield and produced comparable L-LA titers to systems employing much higher solids loadings, demonstrating its potential for enhanced L-LA production with reduced biomass consumption.