The effect of ethanol and heat on the functional hydrophobicity of casein micelles

Milk proteins are very important ingredients to the food industry. As new uses and applications for these proteins are developed, it becomes more important to understand their physicochemical properties when they are subjected to different treatments. It has been reported that casein micelles dissociate when heated in the presence of ethanol. The changes to the hydrophobicity of milk proteins during that process were evaluated by using the fluorescent hydrophobic probe 1-anilinonaphthalene-8-sulfonic acid (ANS). Raw skim milk, pasteurized skim milk, and whey protein isolate samples with ethanol concentrations of 0 to 60% (vol/vol) were heated from 20 to 60°C. The fluorescence of the samples with and without the addition of ANS was measured at an excitation wavelength of 390 nm and an emission wavelength of 400 to 500 nm. The results showed a decrease in the extrinsic fluorescence of the samples as the ethanol concentration and temperature increased, indicating competitive inhibition of the ANS-hydrophobic site interaction by ethanol. This inhibition was further enhanced by the addition of heat. This resulted in a reduction in the functional hydrophobicity of the milk proteins as ethanol rendered the hydrophobic sites unavailable for interaction.