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The tannase (from Aspergillus niger) was immobilised by glutaraldehyde conjugation to amino-functionalised chitosan-coated magnetic nanoparticles (Fe3O4-CS nanoparticles). Fourier-transform infrared spectroscopy and thermo-gravimetric analysis showed that chitosan was coated on the surface of magnetic nanoparticles. Transmission electron microscopy indicated that the synthesised nanoparticles (Fe3O4-CS) were almost spherical or ellipsoidal with an average diameter of 5.97 ± 1.25 nm. The stability and functionality of free and immobilised tannase were compared. Both forms of tannase exhibited the same optimal temperature of 30 °C, whereas the optimal pH value of immobilised tannase (pH 4.5) was lower than that of the free tannase (pH 5.5). The pH and thermal stabilities of immobilised tannase were significantly better than those of free tannase. Immobilised tannase retained over 50% of its initial activity after repeated utilisation for eight cycles. Furthermore, the immobilised tannase effectively improve the clarity and colour of black and green tea infusions. These results showed that amino-functionalised Fe3O4-CS nanoparticles are an efficient carrier for immobilising tannase, and immobilised tannase can be used in the clarification of tea infusion.

Small molecules with beneficial biological activities can be generated by controlled fermentation of natural food products. In this study, the fermentation products of tannic acid by Aspergillus carbonarius FCYN212 were isolated and identified. Two major chemical structures were obtained: gallic acid (GA) and 3,4,6-tri-O-galloyl- d -glucose (TGG). GA and TGG exhibited stronger free radical scavenging ability and ferric-reducing power than butylated hydroxyanisole (BHA). Additionally, TGG demonstrated significant dose-dependent enzyme inhibitory activities against α-amylase and α-glucosidase with IC50 values of 334.6 ± 8.9 and 46.5 ± 1.6 μM, respectively. Notably, TGG was a mixed type inhibitor of α-amylase and α-glucosidase according to a Dixon plot. This study revealed the antioxidant and enzyme inhibitory potential of TGG isolated from a fermented extract of tannic acid. TGG may therefore serve as an antioxidant and antidiabetic ingredient in functional or medical foods.