Effect of water activity and light exposure on the stability of bioactive compounds in matcha (Camellia sinensis) powder during storage periods.

This study quantitatively evaluated the combined effects of water activity (Aw) and light exposure on the degradation kinetics and storage stability of major bioactive compounds in matcha. Matcha samples were stored under controlled water activity conditions (Aw 0.11-0.93) and either light-shielded conditions or continuous exposure to standard laboratory LED fluorescent lighting (approximately 450-500 lx), and changes in physicochemical properties, antioxidant capacity, and key bioactive compounds were evaluated. Higher Aw increased the water solubility index and decreased the water absorption index, indicating structural loosening. Total polyphenol and flavonoid contents, as well as antioxidant activities (ABTS and DPPH), declined progressively, particularly under high Aw and light exposure. Kinetic analysis revealed that catechin derivatives and vitamin C degraded markedly during storage, with gallated catechins (EGCG and ECG) exhibiting short half-lives of approximately 30-40 days under light exposure. In contrast, gallic acid remained relatively stable. These findings demonstrate that both humidity and light critically influence the functional stability of matcha, and that storage under low water activity (Aw ≤ 0.53) and light-shielded conditions significantly improved the retention of bioactive compounds under the experimental conditions examined.