Effect of Acidification and Heating on the Rheological Properties of Oil-Water Interfaces with Adsorbed Milk Proteins

The behavior of casein and whey proteins at the oil-water interface was studied using a dynamic drop tensiometer (DDT). The dilational modulus of the interface was measured for aqueous solutions of skim milk powder (SMP) and whey protein concentrate (WPC) with various additions (salt, calcium, lactose) and (order of) various processing steps. Acidification or heating was performed before or after creation of the interface. The elastic properties of oil-water interfaces with adsorbed milk proteins could partly determine the rate of partial coalescence and resulting product instability. For WPC, preacidification slows down the adsorption, but the modulus is not affected. This is probably because, although the whey proteins change conformation more slowly at the interface, still a homogeneous film is formed. If postacidification is applied, coarsening of the protein film leads to loss of interfacial rigidity. Preheating of the aqueous phase with WPC leads to denaturation and aggregation, but the aggregates formed are still surface active and give high moduli. If preheating of a WPC solution is followed by postacidification, the resulting modulus is high (approximately 60 mN/m). The oil-water interfacial properties of SMP are only minimally affected by preheating or by choice of powder (low, medium, or high heat). At low pH, however, aggregates are formed that are less surface active, and interfacial moduli are lower. If measurements are performed at high temperature (i.e., if postheating is applied), for both SMP and WPC systems, moduli became much lower (approximately 10 mN/m). This is probably because of accelerated rearrangements, leading to the formation of inhomogeneous film structures.