Comparative optimization of apple pomace pretreatment conditions to maximize sugar generation for dark fermentative biohydrogen production

Emerging pretreatment techniques like autoclave-assisted extraction (AAE), heat-assisted extraction (HAE), and ultrasound-assisted extraction (UAE), which utilize water instead of organic solvents, are particularly attractive due to their minimal energy requirements and reduced environmental impact. The Box–Behnken design was employed to optimize the operational parameters for AAE, HAE, and UAE, resulting in total sugar (TS) yields of 0.39, 0.54, and 0.53 g/g of dry apple pomace, respectively. The HAE showed higher TS and micronutrient yields and lower concentrations of inhibitors, and fermentation was conducted with apple pomace hydrolysate (APH) produced under HAE against glucose employing Clostridium tyrobutyricum. Biohydrogen, lactic, acetic, propionic, and butyric acids were monitored daily. The hydrogen production rates were 433.92 mL/L·day for glucose and 744.51 mL/L·day for APH. Re-assimilation of acetic acid contribute to increased butyric acid production. The findings highlight apple pomace as feedstock for biohydrogen and butyric acid production, supporting waste valorization and carbon neutrality.