| Popis: |
The milk fat globule membrane (MFGM) plays an essential role in emulsifying milk fat. In recent decades, the behavior of MFGM polar lipids has been studied using Langmuir trough model systems, were the surface tension of lipid monolayers spread on the surface of a aqueous solution is analyzed as the surface is compressed or expanded. The problem with most Langmuir trough experiments reported in the literature that study MFGM lipid behavior is that they spread MFGM lipids on solutions that are not fully representative of milk. Pure milk naturally contains protein and lipid surfactants, which makes studying the behavior of isolated lipid monolayers spread on pure milk difficult. In this thesis, a process to produce non-surface-active (NSA) solutions from bovine skim milk and dairy-based beverages that have insignificant amounts of surfactants is described. The resulting NSA solutions from the process described here can be used to improve Langmuir trough experiments on MFGM lipids. The process to produce NSA solutions involves two steps. The first step is ultrafiltration of milk through an ultrafiltration membrane with a molecular weight cut-off of at least 10 kDa. The second step is washing the permeate obtained from ultrafiltration 4 times with water-immiscible solvents at a 10:1 permeate to solvent (v/v) ratio. Collected permeate is washed once with hexane, once with chloroform, and then twice with hexane before being filtered through activated carbon. Solvent washing removes proteins in the permeate by forming protein stabilized permeate-in-solvent emulsions that can be separated from the permeate in a separatory funnel. NSA solutions produced from skim milk had a significantly higher surface tension than pure skim milk, resulting from the loss of surfactants. There was no significant difference in the surface tension of the resulting NSA solutions when their surface was compressed, indicating a lack of adsorbed surfactants at their surface. Percent total solids were lower in the NSA solutions compared to bovine skim milk due to the removal of proteins, but the measured pH, conductivity, hardness, and alkalinity of the NSA skim milk permeates was not significantly different to that of pure skim milk. The process of producing NSA solutions from milk was also carried out for 3 different dairy-based beverages to show that the process is not limited to just milk. Lastly, the lift-off point, which is the mean molecular area at which the surface pressure significantly rises above 0 mN/m, was compared for milk ganglioside GM3 monolayers spread on ultrapure water to those spread on NSA bovine skim milk permeate. The difference in lift-off points for milk GM3 monolayers spread on the different subphases was found to be significantly greater than zero for each milk GM3 sample analyzed. In essence, the procedure described in this thesis to produce NSA solutions from milk and dairy-based beverages can improve Langmuir trough experiments studying interfacial MFGM lipid behavior by making the experiments resemble milk more realistically. |
