Total phenolic content and product quality of air-dried, oven-dried and stir-fried mint leaves (Mentha suaveolens)

Drying is essential to prolong the shelf-life of the food but the loss of moisture during drying can accelerate the food quality degradation. This study aims to compare the total phenolic content, color quality and chemical groups of air-dried, oven-dried and stir-fried mint leaves. Air dried leaves were produced after 24 hours drying in an open air condition (25ºC). The mint leaves were processed at 180-200°C for 5 minutes under the stir-frying treatment while oven-dried leaves were produced at 40°C and 60°C for 3 hours in an oven respectively. Total phenolic content, color, and chemical groups of the air-dried, oven-dried and stir-fried mint leaves were determined. Stir-fried mint leaves had the significantly highest total phenolic content (315.09 mg/g) compared with other treatments. As for product quality, the color of mint leaves degraded after thermal treatments and the best drying treatment to preserve the color was oven drying at 60℃. The intensity of the bands at 1598.02-1597.31 cm-1 for aromatic C=C was much higher for the mint leaves under oven-drying at 60°C and stir-frying than for the air-drying and oven-drying at 40°C. High heating temperature in stir frying increased total phenolic content of mint leaves whereas the color quality significantly declined.