Fruit Quality and Antioxidant Content in Durian (Durio zibethinus Murr.) cv. ‘Monthong’ in Different Maturity Stages

Durian (Durio zibethinus Murr.) is a major economic crop in Thailand, with the ‘Monthong’ cultivar being particularly valued for its commercial significance and extensive cultivation in northern Thailand. However, the thick, hard shell of durian complicates ripeness assessment based on external appearance, often leading to premature harvesting and unripe fruit sales. Variations in consumer preferences for different ripeness stages present challenges in meeting market demands. Due to the absence of a definitive harvest index for ‘Monthong’ durian, this study aims to (1) evaluate the potential of fruit shell color composition as an indicator of maturation stage and (2) assess the impact of harvest maturity on fruit quality and antioxidant content. A completely randomized design (CRD) was employed in the experiment. Fruits were collected at intervals of 15 days from 15 to 135 days after full bloom (DAFB). The results showed that fruit circumference and length increased progressively with age, with maximum fruit size observed at 90–135 DAFB. Fruit weight, firmness, dry matter, total phenolics, flavonoids, β-carotene, lycopene, and antioxidant activity peaked at 120 DAFB. The values recorded at this stage were: fruit weight (3652.30 g), firmness (42.08 N/cm2), dry matter (37.13%), total phenolics (43.98 mg/100 g fresh weight (FW)), flavonoids (8.33 mg catechin/100 g FW), β-carotene (1.35 mg/100 g FW), lycopene (53.98 mg/100 g FW), and antioxidant activity (6.32 mg TE/100 g FW). The highest total soluble solids (TSS) content was observed at 135 DAFB, with a value of 25 °Brix. These findings indicate that: (1) maturation stages can be effectively differentiated using shell color; (2) ‘Monthong’ durians reach their maximum size at 90 DAFB; (3) fruits harvested at 90–105 DAFB exhibit high firmness and low sweetness, making them suitable for markets prioritizing texture; (4) fruits harvested at 105–120 DAFB exhibit lower firmness and higher sweetness, making them preferable for direct consumption; and (5) total soluble solids, acidity, phenolics, flavonoids, β-carotene, lycopene, and antioxidant activity increase with maturation. These insights provide a valuable reference for optimizing harvest timing to meet specific market and consumer preferences.