615. Colour changes in heated and unheated milk V. The effects of temperature of measurement, pH, and the addition of certain ions on the reflectance of separated milk

Variation in the temperature of measurement, pH, and concentration of calcium, citrate and phosphate ions have been consistently found to affect the reflectance of separated milk in such a way that the entire curve of reflectance versus wave-length in the visible spectrum is raised or lowered. At temperatures between 10 and 50°C. the reflectance rises reversibly with temperature. The change is not instantaneous, and if the temperature is changed abruptly the reflectance change to the new value is exponential, with a time constant of about 40 min.Reflectance is independent of pH in the range 5·8–6·6, but for greater values it drops sharply.Added calcium ions increase the reflectance of separated milk, and added citrate and phosphate ions decrease it, independently of the simultaneous changes in pH.It is suggested that the reflectance changes which have been found to occur are all caused by changes in the sizes of the casein particles in milk which are brought about by the factors considered. The results given by other authors on viscosity and bound water content, and on direct electron-micrographic observation of casein, are cited to support this contention.It is further suggested that the change in casein particle size with temperature may be due to a change in the calcium-ion distribution between solid and liquid phases.The viscosity and reflectance results for separated milk heated to 70°C. and above are apparently anomalous, but they can be explained on the hypothesis that denatured soluble proteins aggregate into particles having a high ration of length to breadth.