Microstructure analysis as a tool for understanding mechanical behavior and polyphenol transport in fruit tissue induced by combined impregnation techniques: prototypes with high potential as antioxidant source

Abstract

Alternatives to traditional vacuum impregnation technology (VI) have been proposed to enrich plant-based foods with bioactive compounds. As a model system apple slices and elderberry extract were selected, and four combined blanching-vacuum impregnation treatments were used to enhance the phenolic profile of the fruit matrix. The work aimed to study the role that plant tissue alterations at microscopic level plays in relation to compositional and mechanical behavior induced by the proposed techniques. The elderberry-apple slices exhibited a fivefold increase in polyphenol content, which was considerably improved in blanched apples (up to eightfold). The variations in comparison with VI process were observed in cyanidin glucosides, rutin, and mainly catechin. Dry impregnated samples showed a more brittle structure compared to control ones, but the blanching stage provoked a slight loss of firmness and crispness enhanced by applying long periods of impregnation at atmospheric pressure. Under processing conditions of blanching combined with vacuum-atmospheric impregnation, disruption of cell walls and membranes improved the transfer rate of bioactive compounds inside the tissue, affecting the mechanical properties of the final matrix. The observation of tissue ultrastructure allowed the mechanical behavior and bioactive retention to be better understood. Substantial improvement of the phenolic profile and specific physical properties related to crispness and appearance could be achieved using combined treatments. Additionally, according to a qualitative analysis of consumer’s perception, the best freeze-dried product in terms of bioactive potential could be considered a potential prototype to be launched as antioxidant source.