Multi-index Assessment of Chili Pepper Drying Methods: Drying Efficiency, Quality Characteristics, and Flavor Profiles

In this study, six drying methods—vacuum freeze drying (VFD), hot air drying (HAD), heat pump drying (HPD), microwave vacuum drying (MVD), combined infrared-heat pump drying (IR-HPD), and combined microwave vacuum-hot air drying (MVD-HAD)—were comparatively investigated for their effects on the drying characteristics, nutritional content, antioxidant capacity, color, texture, and flavor of bird’s eye chili. The aim was to identify effective drying strategies for improving both the efficiency and quality of chili peppers. The results indicated that MVD and MVD-HAD achieved the highest drying efficiency, with drying times of only 26 and 170 min, respectively; their maximum drying rates were about 35 and 40 times that of HAD, respectively. VFD performed best in preserving heat-sensitive components such as vitamin C, polyphenols, and carotenoids, with retention rates all exceeding 84%. MVD and MVD-HAD significantly outperformed HAD and HPD in retaining vitamin C, capsaicin, polyphenols, and antioxidant capacity. VFD-dried peppers exhibited the highest lightness. Furthermore, MVD-HAD and MVD showed no significant difference in redness retention compared with the fresh samples. The hardness of chili peppers decreased significantly after drying, irrespective of the drying method used, while MVD-HAD best preserved their elasticity. A total of 54 chromatographic signal peaks were detected using gas chromatography-ion mobility spectrometry (GC-IMS), with aldehydes representing the largest proportion of the total, ranging from 30.89% to 41.05%. Drying reduced the contents of aldehydes and alcohols but increased those of ketones, esters, and heterocyclic compounds. This weakened the grassy aroma and enhanced the nutty, buttery, and caramel notes, imparting a characteristic roasted spicy flavor to the product. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) revealed significant differences in flavor composition of chili pepper subjected to different drying methods. Additionally, 19 key differential flavor compounds were identified. Taken together, MVD-HAD is an optimal drying strategy that balances both the efficiency and quality of chili peppers. The findings of this study provide a scientific basis for the optimization of chili drying processes.