Preparation of Non-Surface-Active Langmuir Trough Subphases from Milk

Milk polar lipid interfacial behavior continues to be analyzed using Langmuir trough experiments, but the reported Langmuir trough subphases commonly used are not fully representative of milk. A method to transform liquids of biological origin, such as milk, into appropriate Langmuir trough subphases does not currently exist, which hinders the applicability of Langmuir trough experiments to nature. Here, a procedure to manufacture milk-derived Langmuir trough subphases with insignificant amounts of surfactants from bovine milk is presented. Ultrafiltration is used to remove the bulk of protein surfactants from milk followed by the creation of solvent-induced emulsions that remove trace proteins and lipids from collected skim milk permeates. Change in surface tension upon compression of resulting washed permeates from the surfactant removal process was ≤0.1 mN/m, terming the permeates non-surface-active (NSA). NSA permeates (72.4 ± 0.2 mN/m) had a surface tension similar to that of ultrapure water (72.6 ± 0.1 mN/m), but their pH, conductivity, percent total solids, Brix percentage, alkalinity, and hardness were not the same, with NSA permeates being more compositionally similar to skim milk than water. The lift-off points of milk ganglioside GM3 monolayer surface pressure-area isotherms spread on NSA permeates and ultrapure water subphases were significantly different when compared for the same sample, indicating that surface tension measurements obtained on ultrapure water are not the same as with NSA milk permeate. Overall, surfactants were removed from bovine milk without the addition of exogenous compounds, allowing for the production of a NSA solution derived from milk that can be used in the Langmuir trough experiment to more realistically resemble the natural environment of milk polar lipids. The procedure described here was able to produce NSA solutions for other dairy beverages aside from milk, indicating that it can be applicable to other biological fluids.