Production of fermentable sugar, ethanol, D-lactic acid, and biochar from starch-rich traditional Chinese medicine decoction residues

Roots and stems comprise a large proportion of traditional Chinese medicines and often serve as the energy storage units of plants. However, their decoction residues still contain a significant amount of starch, and direct landfilling, incineration, or carbon disposal results in a wastage of resources. In this study, five types of starch-rich traditional Chinese medicine decoction residues (TCMDRs)c, namely, Radix Isatidis Rhizoma Dioscoreae, Rhizoma Corydalis and Fritillaria Thunbergii. Radix Paeoniae Alba were screened and hydrolyzed using amylase-glucoamylase to produce fermentable sugar. The resulting glucose yields were 87.54%, 84.51%, 85.14%, 82.55%, and 87.75%, respectively. The enzymatic hydrolysate, after flocculation-decolorization treatment, was used to produce D-lactic acid and ethanol, resulting in a concentration and yield of 121.11 g/L (0.97 g/g) and 54.17 g/L (0.49 g/g), respectively. When single or mixed starch-rich TCMDRs were directly used as feedstocks for ethanol production via simultaneous saccharification and fermentation (SSF), they exhibited similar ethanol fermentability, with yields ranging from 0.33 to 0.43 g/g. The SSF residues were thermochemically transformed into biochar with a specific surface area of 89–459 m²/g to reduce secondary waste generation. The utilization value of starch-rich TCMDRs was significantly improved through the implementation of enzymatic hydrolysis to produce fermentable sugars, anaerobic fermentation to produce D-lactic acid and ethanol, and the utilization of fermentation residues for biochar production.