Simultaneous Production of Caffeic Acid and Ferulic Acid from Spent Coffee Grounds Driven by a Cold-Adapted Feruloyl Esterase

Coffee grounds are a major by-product of coffee processing and consumption, yet they are typically underutilized or discarded, causing resource loss and environmental burden. In this study, a gene encoding a feruloyl esterase was identified from a metagenome-assembled genome and heterologously expressed in Escherichia coli, yielding a recombinant enzyme with a molecular weight of approximately 27 kDa. The feruloyl esterase showed optimal activity at pH 7.0 and 30 °C and retained high activity over a low-temperature range of 0–30 °C, indicating its cold-adapted property. The recombinant enzyme was further applied for the enzymatic bioconversion of waste biomass. Caffeic acid and ferulic acid were simultaneously released from spent coffee grounds, with yields of 1.85 and 0.46 mg/g, respectively. The caffeic acid yield obtained in this experiment was relatively high compared with those from previously reported single-enzyme catalytic systems using spent coffee grounds. This study provides the first evidence showing that the cold-adapted feruloyl esterase can efficiently catalyze the bioconversion of this waste biomass under mild conditions and achieve the co-production of two high-value phenolic acids.