Refrigerating traditional brewed Kikkoman soy sauce after opening is not strictly mandatory because the high salt and fermentation content acts as a natural preservative, preventing harmful bacterial growth. However, storing opened bottles in the refrigerator is strongly recommended by manufacturers and culinary experts if you wish to preserve its complex, delicate flavor profile, vibrant color, and rich umami aromatics over a prolonged period, as exposure to ambient room temperature and air can gradually oxidize and degrade the sauce's optimal taste.