Innovative drying methods of kiwi fruit: Effects on phenolic content, antioxidant activity, and microstructural properties

Abstract

This study investigates the effects of different drying methods and temperatures on the total phenolic content (TPC), antioxidant activity, and microstructural properties of kiwi fruit. Fresh samples were subjected to hot air drying (HAD), vacuum drying (VD), ultrasound-assisted vacuum drying (UAVD), and freeze drying (FD) at 50°C, 60°C, and 70°C. Drying time significantly decreased with increasing temperature across all methods, with the shortest duration recorded for VD at 70°C (210 min), while UAVD at 60°C presented an optimal balance of dry-ing efficiency and quality retention (300 min). TPC was strongly influenced by drying conditions; FD samples showed the highest TPC (5216.7-mg gallic acid equivalent [GAE]/100-g dry matter [DM]), followed by UAVD at 60°C (4760.9-mg GAE/100-g DM). Antioxidant activity measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) also peaked in UAVD70 (38.6 mg Trolox equivalent [TE]/100-g DM) and FD (35.1 mg TE/100-g DM), showing a strong positive correlation with TPC (R² > 0.85). 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) values remained more stable but declined slightly, compared to fresh samples, with UAVD70 (8.30 mg TE/100-g DM) performing comparably to FD (8.76 mg TE/100-g DM). Among phenolic acids, chlorogenic acid was best pre-served in UAVD50 (14.92 µg/mL), while FD maintained the highest caffeic acid content (0.72 µg/mL). Color anal-ysis showed significant differences: FD resulted in the brightest product ((L* = 71.2), while HAD70 yielded darker tones and the highest a* shift because of chlorophyll degradation. The environmental scanning electron micros-copy imaging confirmed that FD retained the best cellular integrity, while HAD70 caused pronounced structural collapse. UAVD-treated samples maintained porosity and structure, confirming the beneficial effect of ultrasound on preservation of microstructure. Overall, FD proved superior in preserving nutritional and structural quality, while UAVD, especially at 60°C, emerged as a promising and efficient alternative for producing high-quality dried kiwi fruit.