High-Level Expression of a Novel Tannase from <i>Penicillium oxalicum</i> in <i>Aspergillus niger</i> for Enzymatic Production of Punicalin from Pomegranate Peel

Pomegranate peel, a polyphenol-rich byproduct, is a valuable resource for punicalin production. A novel tannase (<i>Po</i>TanA) from <i>Penicillium oxalicum</i> was expressed in <i>Aspergillus niger</i> FBL-B, producing a maximum activity of 450.6 U mL^-1 through high cell density fermentation. <i>Po</i>TanA exhibited optimal activity at pH 6.0 and 45 °C, with remarkable stability under acidic conditions (pH 2.5-6.5) and temperatures up to 45 °C. It demonstrated broad substrate specificity, showing highest specific activity toward methyl gallate (224.1 U mg^-1). The tannase was further used to directly hydrolyze the pomegranate peel for punicalin production. When applied to pomegranate peel hydrolysis under the optimized conditions (pH 6.0, 40 °C, and 20 U mL^-1), <i>Po</i>TanA produced punicalin at a yield of 20.2% with a conversion rate of 90.5%, the highest reported to date. These results highlight <i>Po</i>TanA as an efficient biocatalyst for sustainable valorization of pomegranate waste into high-value punicalin.