α-Glucosidase Inhibitory Activity of Tsuan-Kan Tea in Different Solid-State Aging Models: Phytochemical...

Tsuan-Kan tea (TKT) is a solid-state-aged mixture of citrus and tea leaves. This study examined how aging temperature, humidity, and time affect the properties of TKT. Solid-state aging (SSA) enriched active ingredients (total phenol content: 1.25-2.51-fold; total flavonoid content: 1.29-2.52-fold; DPPH radical scavenging activity: 7.16-18.35-fold; ferric-reducing antioxidant power: 5.06-11.96-fold) by avoiding traditional processing losses. Flavanone glycosides (narirutin: 2.81 mg/g; hesperidin: 37.33 mg/g), 5-hydroxymethylfurfural (0.20 mg/g), and tea polyphenols (catechin: 109.80 mg/g; theaflavin: 1.82 mg/g) increased substantially under high-temperature and humidity. Fourier-transform infrared spectroscopy suggested that hydrothermal environments promoted glycosidic bond cleavage (1078 cm^-1) and enhanced π-π stacking between polyphenols and flavanone glycosides, improving solubility. During SSA, aroma profiles shifted from fruity and floral to woody and aged. Partial least squares regression revealed that nonenzymatic browning products, particularly 5-HMF (variable importance in projection [VIP] = 1.806), strongly correlated with α-glucosidase inhibition. Although naphthalene exhibited statistical collinearity (VIP = 2.300) and theoretical binding affinity in molecular docking, its potential toxicity as a thermal processing byproduct designates it as a quality control marker for process intensity. Overall, SSA enriches functional phytochemicals (e.g., polyphenols and 5-HMF) via non-enzymatic pathways, providing preliminary in vitro evidence for developing functional beverages.