Differential scanning calorimetry application to freeze-dried milk and milk fractions

The thermal profiles of whole freeze-dried raw milk, obtained by differential scanning calorimetry (DSC) upon heating, were compared to those of their concentrate fractions (cream, skimmed milk, acid casein and whey) in order to associate the thermal peaks with the related components. Two peaks associated with fat melting, a glass transition attributed to caseins and a complex exothermic peak associated with lactose and its interactions with the other milk components were observed, in a close relation to the values of water activity of the samples. Freeze drying is the least invasive technique for drying milk, thus the results of this study may be attributed to the thermal transitions of milk components in their native state, unlike what is observed on roller- or spray-dried milk. The DSC technique is confirmed as an effective tool for the evaluation of the thermophysical properties of milk, as modified by different industrial processes

Physical characterization of whole and skim dried milk powders

The lack of updated knowledge about the physical properties of milk powders aimed us to evaluate selected physical properties (water activity, particle size, density, flowability, solubility and colour) of eleven skim and whole milk powders produced in Europe. These physical properties are crucial both for the management of milk powder during the final steps of the drying process, and for their use as food ingredients. In general, except for the values of water activity, the physical properties of skim and whole milk powders are very different. Particle sizes of the spray-dried skim milk powders, measured as volume and surface mean diameter were significantly lower than that of the whole milk powders, while the roller dried sample showed the largest particle size. For all the samples the size distribution was quite narrow, with a span value less than 2. The loose density of skim milk powders was significantly higher than whole milk powders (541.36 vs 449.75 kg/m3). Flowability, measured by Hausner ratio and Carr’s index indicators, ranged from passable to poor when evaluated according to pharmaceutical criteria. The insolubility index of the spray-dried skim and whole milk powders, measured as weight of the sediment (from 0.5 to 34.8 mg), allowed a good discrimination of the samples. Colour analysis underlined the relevant contribution of fat content and particle size, resulted in higher lightness (L*) for skim milk powder than whole milk powder, which, on the other hand, showed higher yellowness (b*) and lower greenness (-a*). In conclusion a detailed knowledge of functional properties of milk powders may allow the dairy to tailor the products to the user and help the food processor to perform a targeted choice according to the intended use