Study on drying kinetics, antioxidant activity, total bioactive compounds, physicochemical properties and microstructural characteristics of dehydrated star fruits (Averrhoa carambola) by different drying methods†

Abstract

Drying ensures an uninterrupted supply of highly perishable and seasonal fruits such as star fruit (Averrhoa carambola). In this study, the effect of different drying methods-hot air drying (HAD), freeze drying (FD) and microwave drying (MWD; 100, 450 and 800 W) on drying kinetics, antioxidant content, physicochemical properties and surface microstructure properties of star fruits was thoroughly investigated. Among seven different thin-layer drying models, the two-term model is the best-fitted model with the lowest SSE (0.00358–0.000058) value. The FD sample exhibited the least deviation in textural attributes from fresh samples, as a low temperature and pressure gradient maintained the smooth, granular and porous structure of fresh fruits. In contrast, high temperature dried star fruits demonstrated a hard and brittle structure with low resilience and adhesiveness due to the fast moisture removal process and high pressure gradient during drying. High temperature dried products also exhibited a darker color characteristic of phenolic degradation products than the FD sample. Among all dried samples, the HAD star-fruit possessed a higher total phenolic content (1139.67 ± 4.07 mg GAE/100 g dm) and total flavonoid content (4.71 ± 0.05 mg QE/100 g dm), DPPH (95% inhibition) and FRAP (ferric reducing antioxidant potential) radical scavenging activity (148.52 ± 0.05 mmol g-1) as well as the highest content of chlorogenic acid (79.34 ± 2.38 mg/100 g), gallic acid (15.13 ± 0.45 mg/100 g), kaempheral (48.85 ± 1.46 mg/100 g), myricetin (106.38 ± 3.19 mg/100 g), and trans-cinnamic acid (186.85 ± 5.60 mg/100 g). The HAD method was adjudged to be the best drying technique for star fruits.