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• The pH dependence of the casein micelle was investigated using SAXS and USAXS. • SAXS profiles were analyzed using a structure model including water domains. • The size of a micelle and CCP were changed in response to changes in pH. • pH affected the voluminosity and other physical properties in a micelle. • The structure of the casein micelles varied sensitively with pH changes. Changes in milk pH significantly influence the behavior and physical properties of casein micelles; however, the effects of these changes on casein micelle structures are still unclear. The aim of this study was to elucidate the effect of changes in pH range from 5.9 to 7.1 on the structure of casein micelles in milk using small-angle X-ray scattering (SAXS) and ultra small-angle X-ray scattering (USAXS). The casein micelles formed one-dimensional aggregates. The micelle radius decreased with decreasing pH, whereas the size of the water domain increased. The distance between colloidal calcium phosphates (CCP) remained unchanged, whereas the CCP radius decreased with decreasing pH. Voluminosity, which was calculated from scattering intensities, increased at increased pH. In conclusion, the micelle structure changed significantly in response to changes in pH. Our findings help to understand the changes in the physical properties of milk at various pH levels in terms of the microscopic structure. [ABSTRACT FROM AUTHOR]

• The temperature-dependence of the casein micelle was investigated by in-situ SAXS. • SAXS profiles were analyzed using a structure model including water domains. • Inner structure of casein micelles varied sensitively with temperature changes. • The structure changes were thermoreversible. Milk is used and processed under various environmental temperature, and its physicochemical properties are also strongly affected by temperature. Therefore, it is important to reveal the structure of milk at variable temperatures. In this study, the temperature dependence of the inner structure of bovine casein micelles in the temperature range of 10–40 °C was investigated by in-situ small-angle X-ray scattering (SAXS) method. The micelle size calculated from the SAXS profiles using a micelle model including water domains was almost independent of temperature. The water domain expanded and the distance between the colloidal calcium phosphates (CCP) decreased with increasing temperature. The number of CCPs in a micelle increased, because CCPs were newly formed by the transfer of calcium and inorganic phosphate from serum into the micelle. These structural changes occurred during the cooling process. Therefore, in the temperature range of 10–40 °C, the structure of the casein micelle varied sensitively with the temperature, and these structural changes were thermoreversible in nature. [ABSTRACT FROM AUTHOR]