Functional Smoothie Development with Fisetin, Avocado Seed Flour, and Kiwifruit Essential Oil Obtained Using Solvent-Free Microwave Extraction (SFME)

Abstract The aim of this study was to investigate the physicochemical, antioxidant, and sensory properties of fisetin-enriched fruit smoothies and to explore their potential as functional and health-promoting beverages. The dry matter content of the samples ranged from 12.52% to 13.03%, while the °Brix values ranged from 17.97% to 18.60%. Specifically, statistically significant increases ( p < 0.05) were observed in smoothies enriched with fisetin. Sample S5, containing 4.5% fisetin, had the highest ash content (0.39%). The water activity values of the smoothies (0.30–0.38) remained below the microbial growth threshold (0.60), confirming product safety. pH values ranged from 3.46 to 3.98, and titratable acidity ranged from 0.25% to 0.43%, with sample S5 exhibiting the most acidic profile. Sample S5 had the highest viscosity and pectin values at 3057 cP and 0.85%, respectively. Sugar content was balanced with the addition of stevia and ranged from 7.55 to 11.42 g/100 g. Total phenolic content was significantly higher in sample S5 (316.52 mg GAE/L). Antioxidant capacities were highest in sample S5 with the following values: CUPRAC (8.73 mMoL Fe 2 ⁺/100 g fw), FRAP (2.84 mMoL Trolox/100 g fw), ORAC (6.12 mMoL Trolox/100 g fw), DPPH (1.94 mMoL Trolox/100 g fw), and ABTS •+ (3.13 mMoL Trolox/100 g fw). Furthermore, sample S5 contained the highest levels of ascorbic acid (0.94 mg/100 g) and anthocyanins (18.2 mg/100 g) compared to sample S1 that served as the control group without fisetin. In contrast, sample S3, containing 1.5% fisetin, had the highest carotenoid content (62.5 µg/100 g), while sample S2, containing 1% fisetin, and sample S4, containing 3% fisetin, were superior in terms of flavonoid content compared to the other samples. Sensory evaluation revealed that sample S3 received the highest scores in terms of color and overall acceptability, while samples S4 and S5 were superior in terms of flavor and fruit taste. The research findings highlight that smoothies enriched with fisetin, particularly sample S5, could be a promising functional beverage.