Evaluation of Processing Parameters Affecting Antioxidant Activity and Sensory Quality of Wild Bitter Melon Leaf Tea

Abstract

Wild bitter melon (Momordica charantia var. abbreviata) leaves represent an underutilized resource rich in polyphenols and flavonoids. However, the processing parameters for producing a palatable and bioactive herbal tea from these leaves remain unclear. This study evaluated three critical factors-harvest age, vacuum-drying temperature, and stevia blending ratio-to maximize phenolic retention, antioxidant capacity, and sensory quality. Leaves of wild bitter melon (Momordica charantia var. abbreviata) cultivated at Kien Giang University (Vietnam) were harvested at 60, 80, 100, and 120 days of age. Processing was performed at the university’s Practice–Laboratory Center. The leaves were vacuum-dried at 45, 55, or 65 °C (70 kPa, 24 h; final moisture ~6–7%). Total polyphenols and flavonoids were determined by Folin-Ciocalteu and NaNO₂–AlCl₃–NaOH colorimetry, respectively, while antioxidant activity was evaluated by the DPPH assay with half-maximal inhibitory concentration expressed as mg dry weight per mL extract (mg/mL). Sensory evaluation of the teas brewed from leaf–stevia blends (9.5:0.5 to 7.5:2.5, w/w) followed TCVN 3218:2012 using a trained panel (n = 10). Harvesting at 80 days yielded the highest Total polyphenol content (29.36 mg gallic acid equivalents/g dry weight) and the lowest Half-maximal inhibitory concentration (1.34 mg/mL). Drying at 55 °C maximized total polyphenol content (47.67 mg gallic acid equivalents/g dry weight) and total flavonoid content (123.48 mg quercetin equivalents/g dry weight) while minimizing half-maximal inhibitory concentration (1.23 mg/mL). The 8.5:1.5 leaf–stevia blend achieved the highest sensory score (16.27; “Good”), balancing bitterness and sweetness. These integrated parameters provide a scientific basis for valorizing wild bitter melon leaves as a functional herbal tea with enhanced bioactivity and consumer acceptance.