Comparative analysis of drying methods for fresh in-hull walnuts: Impact on sensory quality, physical and chemical properties and flavor characteristics

The quality of dehydrated products, including physical, nutritional, and sensory attributes, is influenced by the drying method used. However, the effects of different drying methods on walnuts are not yet clear. To investigate a better drying method for fresh in-shell walnuts, five drying methods (natural drying, hot air drying, vacuum freeze-drying, vacuum drying, and microwave drying) were analyzed and compared for their effects on the sensory quality, physicochemical properties, and flavor characteristics of walnuts. The results indicated that, at the sensory level, the microwave drying group and the vacuum freeze-drying group were closer to fresh samples. In terms of physicochemical properties, vacuum freeze-drying had the highest protein content (21.41 ± 0.73 mg/g) and total phenolic content (212.08 ± 2.06 mg/g), with better antioxidant activity (84.11 ± 0.88 %) and optimal rehydration performance (90.61 %). Samples dried with hot air and microwave drying had more volatile compounds, resulting in better overall flavor quality of the walnuts. Compared to the microwave drying group, the hot airdrying group had the highest content of amino acids and fatty acids, emitting a pleasant baked aroma. A total of 47 volatile compounds were identified by gas chromatography-ion mobility spectrometry (GC-IMS), including 16 alcohols, 12 aldehydes, 8 ketones, 7 esters, 1 furan, 1 ether, 1 acid, and 1 aromatic hydrocarbon. OPLS-DA analysis identified 15 volatile flavor compounds, including 5 esters, 6 alcohols, 2 aldehydes, 1 ketone, and anisole. In summary, all five drying methods can alter the flavor of walnuts, and the drying method for walnuts should be appropriately selected based on different storage requirements and processing methods to create better products. • Fresh in-hull walnuts were dehydrated by five drying methods. • Hot air drying (HAD) is the highest-quality thermal drying process for fresh in-hull walnuts. • GC-IMS effectively distinguished between the five drying methods. • VFD will be a promising technology for drying fresh in-hull walnuts. • The effects of five drying methods on the sensory quality of fresh in-hull walnuts were explained.