This study explored the rehydration behavior of apple powder produced using different drying technologies: hot air drying, and freeze drying, along with two dissolving procedures (self-dissolving and forced-dissolving). In the freeze-drying process, samples were previously frozen by two different freezing methods: blast freezing (− 30 °C) and liquid nitrogen freezing (− 196 °C). A non-destructive low-field nuclear magnetic resonance (LF-NMR) was used to identify the different proton pools and their mobility during the rehydration process. T 2 relaxometry revealed that both drying and dissolving procedures significantly influenced the water dynamics and distribution in the powders. The major proton population was observed under the immobilized peaks (at T 22 ), while a minor proton population was observed for the bound water peaks (at T 21 ) in all the powders. The consistent, narrower, and uniform peaks across all the forced-dissolved powders highlighted homogeneous rehydration, while self-dissolved powders exhibited broadened peaks, indicating heterogeneity. In the forced-dissolved process, the peak area decreased in the order of powders obtained by hot air drying (HAD) > freeze-dried with blast freezing (FD1) > freeze-dried with liquid nitrogen freezing (FD2), correlating with structural changes in the powders resulting from the drying technologies. Weighted average relaxation times in the forced-dissolved procedure were shorter than in the self-dissolved procedure, indicating greater water absorption due to increased accessibility of water-binding sites during forced dissolving. These findings emphasize the critical role of drying and dissolving procedures in determining the rehydration properties of apple powders, offering valuable insights for optimizing their use in food processing applications.
Sultana et al. (Thu,) studied this question.