Evaluation of the usefulness of serial combination processes for drying of apples

Abstract

Serial combination drying processes are currently studied as alternatives to conventional drying processes. Compared to freeze-drying (FD), serial combination drying appears to be faster and less expensive, while providing better product quality than hot-air drying (HAD) and microwave-vacuum drying (MVD) as previously demonstrated for carrots. Using the example of carrots, it has also been shown that the drying front moves radially outwards over the course of FD. This unexpected behavior was suggested to originate from the carrots’ heterogeneous structure. It was hypothesized that apple pieces behave differently. Here, this hypothesis was proven by using micro-computed tomography measurements of partly freeze-dried apple pieces. In order to improve the drying process of apple pieces, several single and combination drying processes were carried out. Processes were evaluated by using drying time and sample quality as relevant parameters. Sample quality was determined by analyzing the 3D-structure, rehydration behavior, color, and ingredient retention. Results showed that single MVD is a well suitable drying technique for apple pieces, producing dried products of equal quality to FD. Different from carrots, serial combinations are not necessary to improve the quality of dried apple pieces. Nonetheless, especially a combination of HAD and MVD was useful to obtain specific structures such as puffed pores that did not result from single MVD.