Anhidrous milk-fat phospholipid mixturebehavior at the air water interface

ADELV TURINA; DA GARCIA,; JM.SANCHEZ; MA.PERILLO

Universidad de Quilmes. Bernal Bs. As. Argentina

Workshop; First MERCOSUR workshop on biomembranes; 2000

SAB

Introduction                 At present we are searching for appropriate lipidic  mixtures forming stable liposomes in milk. Anhydrous milk fat (AMF) was found as an interesting possibility due to its availability (it is produced in local dairy industries) and the fact that its addition would not introduce significant changes in the chemical composition of milk.                 AMF contains between 96 and 99% of triacylglycerol. The fatty acid composition of its total lipids include 16:0 (@ 30wt%), 18:1 (@ 20wt%), 18:0 and 14:0 (@ 10wt%) among the major components (Timmen and Patton, 1988). Triglycerydes are neutral lipids which partition primarily into phases made up of other nonpolar or weakly polar lipids. They orient at the air-water interface with the polar glyceryl portion interacting with water, both in the pure form and in mixtures with phospholipids and exhibit finite solubility in phosphatidylcholine vesicles (Hamilton, 1989). In the present work the miscibility of anhydrous milk fat (AMF) with phosphatidylcholines of different sources was studied at the air-water interface. Methods       Mixtures of dipalmitoyl phosphatidylcholine (dpPC), egg- phosphatidylcholines (egg-PC) (Avanti Polar lipids), or soybean PC (PCsb) with AMF (from Nestle, Villa Nueva, Pcia.de Cordoba) were prepared in chloroform with molar fractions of AMF (xAMF) varing between 0 and 1. The molecular areas at different surface pressures were measured in a compartment with an initial surface area of 247.5 cm2. The absence of surface-active compounds in the pure solvents and in the subphase solution was checked. Lipid monolayers were spread by depositing 30 ml of those solutions onto the aqueous subphase. After 5 min. the monolayer was compressed at a constant rate of 7.5 cm2/min at 22oC ± 1oC. Reproducibility was within ±0.1 Å2 and ± 0.004 mN/m for molecular area and surface pressure, respectively. Mean molecular weight values used for calculations were: 760 for egg-PC and PCsb, and 825.32 for AMF. Results and Discussion                 The expanded isotherms obtained with AMF monolayers (Fig.1)   resemble those of unsaturated and a variety of mixed saturated triglycerides containing 14, 16 and 18 carbon atoms which form expanded monolayers at room temperature (Gaines, 1966). The behavior of the mixtures AMF-PCs is also similar to that of triglycerydes-PC (Smaby and Brockman, 1984). One or two significant points were detected in the surface pressure?molecular area isotherms:one point corresponded to the collapse pressure of the pure compound either AMF or PC (at xAMF 0  and 1 respectively) and a second point, which was composition dependent, was found at a higher molecular area and was always accompanied by another one coincident with the collapse pressure of the PC present in the mixture (Fig.1). So, the isothermal phase diagrams for the mixtures at 22ºC were characterized by two compositional regions (Fig.2). At the limit of miscibility with lower mol fractions of AMF, the surface pressure was composition-independent, but above a mixture-specific stoichiometry, surface pressure at the limit of miscibility was composition-dependent (Fig.2a). We suggest that at low mol fractions of AMF, the surface consisted of a mixture of  phospholipid and AMF, whereas, above certain AMF mol fraction, the surface phase consisted of complex and excess PC. That mixture was ideal with egg-PC (Fig.2b) and dpPC (not shown) and showed positive deviations from ideality with PCsb (not shown). The segregation of  the excess PC occured at AMF mol fraction of  0.14, 0.093 and 0.113 with egg-PC, PCsb and dpPC respectively.