A Comparative Study of Vacuum-Freeze-Dried and Hot-Air-Dried Gannan Navel Orange Slices: Physical Characteristics,...

The Gannan navel orange (GNO) industry is large but constrained by limited deep-processing. This study employed vacuum freeze-drying (FD) and hot-air drying (HD) methods to prepare dried GNO slices, comparing their physical properties, volatile/non-volatile compounds, antioxidant activity, and sensory quality. Compared with the HD sample (ΔE= 10.11), the color changes were more pronounced in the FD sample (ΔE= 34.39), which appeared whiter and brighter. The FD method preserved more vitamin C (9.09% loss) than the HD method (27.28% loss). In contrast, HD samples contained significantly higher levels of total flavonoids, total phenols, hesperidin, narirutin and didymin, with respective increment percentages of 13.81%, 19.27%, 17.03%, 27.56% and 33.33% compared to FD samples. Volatile analysis revealed that drying treatments led to a decrease in terpene content compared with fresh slices (fresh 48.84% vs. FD 47.81% vs. HD 47.42%), while ester content increased (fresh 13.87% vs. FD 14.59% vs. HD 14.45%). Both methods reduced key monoterpenes (e.g., β-terpineol, terpinolene, 3-carene, α-terpineol, β-thujene and α-terpinene), possibly converting them into compounds such as p-mentha-1(7),8-diene-ol and thymol. Notably, HD samples contained much higher levels of harmful compounds 5-hydroxymethylfurfural and furfural. FD samples exhibited superior antioxidant activity and were preferred in sensory evaluation for appearance, aroma, texture, and taste. The findings indicate that FD offers advantages in terms of morphological retention, vitamin C preservation, antioxidant activity, safety and sensory perception, underscoring the market potential of FD navel orange slices as a high-value, healthy food product.