Effect of fixation on quality characteristics and volatile organic compounds in dried walnuts

In China, walnuts are traditionally baked and dried in their green husks. We explored the changes in the nutrient composition and flavor of walnuts that were pre-treated with hot air at 150 °C for 20–100 min (fixation) and then sun-dried. The results indicated that, fixation significantly increased the content of secondary metabolites (e.g., polyphenols (56.74–116.11 mg/kg) and squalene (7.31–19.86 mg/kg)). The composite scores of the six groups were calculated by principal component analysis and ranked. The composite scores after 20, 40, and 60 min of fixation were higher than Natural drying (ND). Gas chromatography-ion mobility spectrometry (GC-IMS) characterization of the volatile organic compounds (VOCs) of walnuts treated with different processes identified a total of 39 VOCs. The total amount of VOCs increased significantly after fixation, and walnuts obtained after different fixation times showed different flavor characteristics. Heterocyclic (22 %–35 %), aldehydes (13 %–19 %), and ketones (7 %–23 %) were more prevalent in the fixation groups. Orthogonal partial least squares discriminant analysis model was integrated with odor activity values (OAVs) to identify different treatment groups. Screening identified 13 compounds as the key VOCs affecting the flavor characteristics of walnut from different treatments. In summary, a suitable fixation treatment significantly improves the flavor and quality of walnuts. • Fresh walnuts were dried without fixation or after fixation for various times. • The effects of fixation on the overall quality of dried walnuts were measured. • GC-IMS distinguished the six drying methods. • Thirteen key VOCs were identified in walnut samples. • Fixation improved the flavor of walnuts.