Comparison of Casein Micelles in Raw and Reconstituted Skim Milk

Shift of CN and whey proteins from the serum to the micellar phase during heat treatment and water removal. Profiles represent the intensity of stained protein bands along lanes of a single SDS-PAGE gel for samples of noncentrifuged raw milk (○) and supernatants of centrifuged raw milk (), medium-heat-treated milk (solid line) and medium-heat skim milk protein reconstituted for <1h at 20°C with gentle mixing (●). Due to the high protein concentration required for detailed investigation of the serum CN, the intensity of the most concentrated bands is saturated. The nonlinear response may distort the apparent ratio of total CN to whey and proportion of CN in the serum phase.

Transmission electron microscopy images of CN micelles from raw milk, medium-heat-treated milk, reconstituted medium-heat skim milk protein (SMP; <10min, gentle mixing at 20°C), and reconstituted high-heat SMP (<10min, gentle mixing at 20°C) viewed at 65,000× magnification. Image exposure was adjusted using Adobe Photoshop (Adobe, San Jose, CA) to remove background noise.

Turbidity and pH of reconstituted skim milk with reconstitution time. Turbidity of low-heat skim milk protein (SMP) was reconstituted at the following temperatures: 20°C with gentle mixing (), 40°C with gentle mixing (), and 20°C with high-shear mixing (). Turbidity of medium-heat SMP was reconstituted at the following temperatures: 20°C with gentle mixing (♢), 40°C with gentle mixing (□), and 20°C with high-shear mixing (○). Turbidity of high-heat SMP was reconstituted at the following temperatures: 20°C with gentle mixing (♦), 40°C with gentle mixing (■), and 20°C with high-shear mixing (●). Turbidity of raw skim milk was reconstituted at 20°C for low-heat (thin solid line), medium-heat (short-dash line), and high-heat (long-dash line) powders. The pH of low-heat (*), medium-heat (), and high-heat (+) SMP was reconstituted at 20°C with gentle mixing. The thick solid line indicates the pH of raw skim milk.

Shift of CN and whey proteins from the micellar phase back to the serum during reconstitution of (A) high-heat skim milk protein (SMP) and (B) and medium-heat SMP. Profiles represent the intensity of stained protein bands along lanes of SDS-PAGE gels for samples of noncentrifuged raw milk (○) and supernatants of SMP reconstituted at 20°C with gentle mixing for <1h (●), 7h (), and 24h (solid line).

Turbidity of medium-heat skim milk powder reconstituted at the following temperatures: 20°C with gentle mixing (♢) and 4°C with gentle mixing (▵). Turbidity of samples was taken during reconstitution at 4°C and was remeasured after being stabilized at 20°C for 15min (▴). Turbidity of raw skim milk was reconstituted at 20°C (dashed line) and 4°C (solid line). Similar trends were found for low-heat and high-heat powders reconstituted at 4°C.