Comparative optimization of apple pomace pretreatment conditions to maximize sugar generation for dark fermentative biohydrogen production
Description
This study compared autoclave-assisted extraction (AAE), heat-assisted extraction (HAE), and ultrasound-assisted extraction (UAE) techniques for producing fermentable sugars. The Box–Behnken response surface experimental design was employed to optimize the independent variables of AAE (apple pomace particle size-Dp, pH, hydrolysis temperature-T, hydrolysis time-t), HAE (Dp, pH, T), and UAE (Dp, ultrasonication power-P, t) in order to maximize the dependent variables of total sugars (TS) yield and supernatant recovery (SR). The optimum conditions for AAE, HAE, and UAE resulted in TS yields of 0.39 g/g, 0.54 g/g, and 0.53 g/g of dry apple pomace, respectively. Micronutrients essential for the fermentation process, such as boron, manganese, iron, copper, zinc, and molybdenum, were analyzed from extracts produced under optimal conditions for each hydrolysis technique. Potential microbial inhibitors such as carboxylic acids, furfural, ketones, and phenols in the hydrolysate were also analyzed. The HAE optimum conditions (Dp=170.8 μm, pH=3.73, T=99.1 °C), showed significantly higher yields of TS and essential elements, and lower concentrations of potential microbial inhibitors. As a result, it was selected as the best hydrolysis technique and used to produce apple pomace hydrolysate (APH) for the fermentation. Batch fermentation was then conducted with APH and glucose (control) substrates employing Clostridium tyrobutyricum. Biohydrogen, lactic, acetic, propionic, and butyric acids production were monitored daily. The reassimilation of acetic acid for enhanced butyric acid production was observed. The volumetric hydrogen production rates (VHPR) were 433.92 mL/L·day for glucose and 744.51 mL/L·day for APH. The findings highlight apple pomace waste's potential as a feedstock for biohydrogen and organic acid production, supporting waste valorization, carbon neutrality, and the circular economy.