Effects of pulsed vacuum drying conditions on drying characteristics and flavor quality in jujube slices

This research aims to systematically investigate the influence of pulsed vacuum drying (PVD) parameters on the drying kinetics, physicochemical attributes, and volatile flavor profiles of red jujube slices. Various PVD parameters were evaluated, including drying temperatures (65, 70, and 75 °C), vacuum holding durations (10, 15, and 20 min), and atmospheric holding durations (2, 4, and 6 min). Hot air drying (HAD) was utilized as the control group. The experimental results demonstrated that the total drying duration was shortened significantly as the drying temperature escalated, whereas it was moderately extended with the prolongation of both vacuum and atmospheric holding times. The drying kinetics were attributed to the complex interplay between thermal energy transfer and the pressure-driven moisture diffusion in the PVD process. Color preservation was one of the most critical quality indicators for fruit processing. Once the treatment group was subjected to a drying temperature of 70 °C, a vacuum holding time of 15 min, and an atmospheric holding time of 6 min (designated as PVD-70 °C-15:6), the color preservation was characterized by the highest luminosity (L* value of 55.50) and the minimum total color difference (∆E* value of 1.97), indicating the most favorable appearance similar to the fresh samples. The macro-level observations were further supported by microstructural analysis via scanning electron microscopy. While the HAD-treated slices displayed a densely packed and collapsed cellular arrangement, leading to maximum hardness and diminished crispness. By contrast, the PVD-70 °C-15:6 samples shared a well-developed, uniform honeycomb-like porous structure to facilitate a better balance between low hardness and moderate crispness after the mitigation of structural shrinkage. Furthermore, the landscape of volatile flavor was dominated by the drying parameters. Radar plot analysis and Principal Component Analysis (PCA) indicated that there were significant (P＜0.05) variations in the aromatic profiles under the different treatments. According to the headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS), a total of 77 volatile compounds were identified and then quantified, including 11 acids, 14 esters, 42 hydrocarbons, 2 alcohols, 3 aldehydes, 1 ketone, and 4 other compounds; Among them, organic acids, with the relative content from 16.66% to 27.92%, were identified as the predominant contributors to the characteristic aroma of the red jujube slices. Notably, the PVD-70 °C-15:6 treatment demonstrated that the relative content of aldehydes and acids substantially increased by 33.55% and 21.82%, respectively, compared with the HAD control. A relatively stable acidic profile was obtained to promote the synthesis and retention of key aldehyde-based flavor precursors after the fluctuations of cyclic pressure in the PVD process. In the PVD-70°C-15:6 treatment, the optimal overall quality was achieved to effectively balance the drying efficiency with the preservation of the structural, aesthetic, and aromatic integrity of the red jujubes. The findings can also provide a robust theoretical and technical framework for pulsed vacuum drying in high-quality dehydrated functional foods.