Invited review: Recent developments in understanding the rehydration characteristics of high-protein dairy powders

The use of dairy-based ingredients is increasingly prominent in the food industry due to their functional and nutritional benefits. High-protein powders are highly attractive due to their superior nutritional (e.g., high protein, calcium) and functional benefits (e.g., gelation, emulsification, foaming). Complete rehydration is essential for achieving optimum functionalities. However, poor wettability, slower rehydration, and the presence of insoluble particles are observed in high-protein dairy powders, especially in casein-rich powders, such as milk protein concentrate and micellar casein concentrate. The low solubility of these powders can cause processing difficulties, such as clogged filters and processing lines, leading to a loss of nutritional and functional properties and increased operating costs. Therefore, it is crucial to measure rehydration characteristics before application. Characterizing the different phases of rehydration helps in identifying the rate-limiting stages and strategies to improve them. Various methods for measuring each rehydration stage are discussed in detail in this review. Methods based on changes in conductivity, turbidity, particle size, and rheological properties during powder reconstitution are examined. Various imaging techniques, ultrasound-based methods, and nuclear magnetic resonance techniques that have been used for characterizing rehydration behavior are also expanded upon here. Apart from these techniques, the methods suitable for in-line application in the industry are also mentioned. In addition to dairy powders, the rehydration of infant formula is discussed. The rehydration of infant formula is important for ease of end-user application and for delivering proper nutritional values. This review discusses common methods such as the insolubility index and wetting time, as well as some newer techniques based on focused beam reflectance measurement, electric resistance tomography, and Lumisizer. Overall, this review elucidates the existing industrial methods and recently developed techniques for characterizing rehydration, their applicability for in-line measurement, and their advantages and disadvantages.