Oxidant–Antioxidant Status of Commercial Matcha Beverages According to Flavor Category, Preparation Medium, and Preparation Temperature: A Comparative Evaluation

Background: Matcha is increasingly consumed in flavored and milky forms, but the effect of preparation conditions on its antioxidant–oxidant profile remains unclear. Methods: Twenty-six commercial matcha products (unflavored and various flavors) were each evaluated under eight preparation conditions comprising four preparation media (water, cow’s milk, almond milk, and oat milk) and two temperatures (25 °C and 80 °C), yielding 208 condition-specific preparations. Thus, the eight preparations derived from each commercial product represented repeated evaluations of that product. Total antioxidant status (TAS) and total oxidant status (TOS) were measured, and the oxidative stress index (OSI) was calculated from their ratio. Temperature-related differences were evaluated using the Mann–Whitney U test, whereas differences across flavor categories, preparation media, and combined grouping variables were evaluated using the Kruskal–Wallis test; corresponding effect sizes were reported. Results: Only TAS and TOS differed significantly by flavor type (p < 0.001). Commercial products categorized as beetroot-flavored showed the lowest TAS and TOS values; these findings reflect differences among the commercial formulations evaluated and should not be interpreted as an independent effect of beetroot flavoring. TAS, TOS, and OSI differed significantly by preparation medium (p < 0.001), with large effect sizes (ε2 = 0.557, 0.637, and 0.638 for TAS, TOS, and OSI, respectively). Temperature differed significantly across all three parameters. The highest TAS was observed in samples prepared with water at 25 °C; the lowest TOS and OSI values were observed in samples prepared with cow’s milk at 25 °C. Among the combined-group comparisons, the largest effect sizes were observed for the combined temperature–preparation medium groups (ε2 = 0.654–0.746) and combined preparation medium–flavor category groups (ε2 = 0.723–0.780). Conclusions: The assay-detectable oxidant–antioxidant profile of the commercial matcha beverages varied across the preparation conditions and commercial product categories evaluated. These findings represent matrix- and assay-dependent analytical differences and should not be interpreted as evidence of physiological oxidative stress or health effects.