Interfacial Characterization of β-Lactoglobulin Networks: Displacement by Bile Salts

Abstract

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.

Effect of Gum Arabic, Gum Ghatti and Sugar Beet Pectin as Interfacial Layer on Lipid Digestibility in Oil-in-Water Emulsions

“…This might suggest that it is more difficult for bile salts to displace GG from the interface than GA and SBP. It was reported that bile salts was able to disrupt the interfacial protein network at very low concentration (0.005 mg/mL) [20]. The higher resistance of GG against interfacial displacement was presumably attributed to its higher binding affinity with bile salts [1,33,[42][43][44][45].…”

“…Bile salts play a crucial role in lipid digestion by displacing initial adsorbed materials from the interface and permitting lipase/colipase complexes to act on the bile salts coated oil droplets. Interfacial studies suggest that this displacement occurs via an orogenic mechanism [20]. The addition of bile salts to a phospholipid stabilized emulsion destroyed the packed phospholipid adsorption layer, allowing greater binding of the lipase to the oil-water interface [21].…”

Changes in WPI-Stabilized Emulsion Interfacial Properties in Relation to Lipolysis and ß-Carotene Transfer During Exposure to Simulated Gastric–Duodenal Fluids of Variable Composition

“…For example, it was shown that the rate of lipolysis is greater for emulsions stabilized with proteins compared to with PL or low molecular weight surfactants (i.e., Tween 20). This was explained by an increased ability of BS to displace proteins from an oil-water interface compared to PL and low molecular weight surfactants [17]. More recently, it was also reported that BS could penetrate and disrupt a β-lactoglobulin film formed at an oil-water interface [17].…”

Relating Food Emulsion Structure and Composition to the Way It Is Processed in the Gastrointestinal Tract and Physiological Responses: What Are the Opportunities?

“…As discussed by Borgström and coworkers, the adsorbed protein layer of a food emulsion stabilized by food proteins is readily displaced in the small intestine by bile salts, after which the kinetics of digestion by small intestinal lipases will be determined by the bile salt covered interface. 153 As a typical example, the displacement of an adsorbed layer of β-lactoglobulin by bile salts was shown to follow the general mechanism of orogenic displacement by low molecular weight surfactants, 157 for which the surface pressure is the controlling parameter.…”