Valorization of Chinese cabbage waste into caproic acid via anaerobic fermentation: Effects of cellulase and pre-fermentation

The effects of different pretreatments and their synergies on caproic acid production from Chinese cabbage waste (CCW) via anaerobic fermentation were investigated. The results showed that cellulase hydrolysis combined with ethanol pre-fermentation (group sy-CE) significantly enhanced electron donor (ED) supply, raising ethanol concentration to 7,424 mg COD/L, an increase of 232% over the single cellulase hydrolysis group (group C). This group achieved a peak caproic acid concentration of 8,191 mg COD/L, representing 52.4% and 41.8% increases compared to group C and ethanol pre-fermentation group (group E), respectively. The enrichment of chain-elongating bacteria such as Caproiciproducens and Clostridium further supported efficient caproate synthesis in the sy-CE group. In the cellulase hydrolysis coupled with lactic acid pre-fermentation group (group sy-CL), an excessively high initial lactic acid concentrations induced propionic acid accumulation, which significantly reduced caproic acid production to 2252 mg COD/L, only 27.5% of that in group sy-CE. These findings provide critical theoretical foundations for optimizing the bioconversion processes of carbohydrate-rich wastes.