Surface accumulation of milk proteins and milk protein hydrolysates at the air–water interface on a time-scale relevant for spray-drying

Accumulation of surface-active compounds may significantly alter the surface composition of food powders compared to the bulk composition of the product and therefore affects the functional properties. Through modification of a commercial contact angle meter surface accumulation of milk proteins and protein hydrolysates could be analysed via surface tension measurements in a time interval relevant for atomisation of emulsions during microencapsulation. The different caseins showed a slower surface occupation compared to industrially processed sodium caseinate and casein hydrolysate with β-casein being more surface-active than α- and κ-casein. In a similar manner, β-lactoglobulin showed a lower surface activity than whey protein isolate and whey protein hydrolysate. It is concluded that molecular weight profile plays an important role for the surface activity of milk proteins, but other factors like surface hydrophobicity, number of ionisable groups and state of aggregation also need to be considered.