Acoustic attenuation spectroscopy and helium ion microscopy study of rehydration of dairy powder

Complete hydration is essential for the production of structured dairy products from powders. It is essential that the ingredients used hydrate completely. Determination of an end point of rehydration is non-trivial, but ultrasound-based methodologies have demonstrated potential in this area and are well suited to measuring bulk samples in situ. Here, acoustic attenuation spectroscopy (AAS) is used to monitor rehydration of skim milk powder, and recombined systems of micellar casein isolate with lactose and whey protein isolate. Dynamic light scattering, zeta-potential measurements and AAS as a function of pH characterise each component around its isoelectric point to assess its functionality. Scanning helium ion microscopy was used to image the dry powders, without any conductive coating, producing resolution equivalent to scanning electron microscopy, but with much larger focal lengths and fewer imaging artefacts. Imaging the powders provides information on particle size and morphology which can affect dissolution behaviour. Reconstituted skim milk powder and recombined samples were monitored showing there are changes occurring over several hours. Attenuation coefficients are shown to predict the end point of hydration. Model fitting is used to extract volume fractions and average particle sizes of large and small particle populations in recombined samples over time. AAS is demonstrated to be capable of tracking the dynamics in rehydrating dispersions over time. Physical parameters such as the volume fraction and particle size of the dispersed phase can be determined.