Effects of Common Japanese Cooking Methods on the Flavonoid Content, Total Polyphenol Content, and Antioxidant Activity of Food Ingredients

Polyphenols, particularly flavonoids, are known for their antioxidant and anti-inflammatory effects. Cooking methods influence flavonoid content; however, the impact of Japanese cooking methods on flavonoid content has not been adequately studied. We cooked onion, tofu, bok choy, komatsuna, spinach, and orange by grilling, stir-frying, boiling, simmering, steaming, and microwaving and then quantified flavonoids, total polyphenols, and antioxidant activity. The flavonoid content decreased significantly (p < 0.05), with onions being most impacted in stir-fry (quercetin decreased to 39.8% of raw content). Total polyphenol content decreased significantly (p < 0.05), with oranges showing the largest reduction after simmering (from 54.4 to 22.1 mg GAE/100 g). However, antioxidant activity remained relatively unaffected, with retention factors of 17.5–107.5% and significant changes observed in only specific combinations. This study provides insights into the alterations of functional components in Japanese cuisine, contributing to accurate estimations of their intake from cooked meals.