How drying technologies shape digestive fate of polyphenols in fruits and vegetables: A critical evaluation

Drying is widely applied to fruits and vegetables to extend shelf life, reduce post-harvest losses, and produce stable, bioactive-rich ingredients. However, the nutritional value of dried products is still often assessed solely based on polyphenol retention, despite the fact that drying-induced changes in tissue architecture, oxygen exposure, enzyme activity, and matrix integrity can strongly affect gastrointestinal release and bioaccessibility. This critical evaluation synthesizes 17 studies examining how different drying technologies influence the release behaviour, bioaccessibility, and antioxidant activity of polyphenols in dried fruits, vegetables, and herbs using in vitro gastrointestinal digestion models. Across matrices, the oral phase contributed minimally to polyphenol release, whereas the gastric phase generally represented the main liberation step. The intestinal phase displayed compound-dependent behaviour, acting either as a secondary release stage for bound or matrix-associated phenolics or as a stage of degradation and reduced recovery for anthocyanins and other labile flavonoids. Phenolic acids were generally the most digestion-recoverable polyphenols, while flavan-3-ols, flavonols, and anthocyanin derivatives showed greater dependence on matrix structure, oxidation, glycosylation/acylation, and pH stability. Drying methods that balanced moderate structural disruption with low oxygen exposure, such as vacuum, microwave–vacuum, ultrasound-assisted vacuum, infrared, and low-temperature vacuum drying, often improved bioaccessibility compared with conventional hot-air or freeze-drying methods, although matrix-specific exceptions occurred. Measured antioxidant activity reflects not only total polyphenol release but also compound transformations, degradation products, and the inherent sensitivity of the analytical assay. Overall, the review highlights the need to move beyond retention-based evaluation toward bioaccessibility- and bioavailability-oriented process design, supported by harmonized digestion protocols and compound-specific profiling.