Competitive adsorption of bile salts and milk protein in oil-in-water emulsions
Baird, William J.
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The competitive adsorption of bile salts with milk proteins (whey protein concentrate (WPC) or sodium caseinate (SCN)) was studied in oil-in-water emulsions. The research confirmed previous findings that the degree of displacement of protein depends on the number of hydroxyl groups attached to the sterol ring of the bile salt, but also highlighted the importance of the presence (or absence) and nature of amino acid residues conjugated with the sterol ring. In general, decreasing the number of dihydroxy bile salts such as sodium deoxycholate (NaDC) displaced more protein from the oil droplet surface than trihydroxy bile salts such as sodium cholate (NaC). This is attributed to the greater hydrophobicity of the sterol ring of the NaDC. However, hydrophobicity is not the only factor that determines the ability of the bile salt to displace protein. If sodium taurocholate (NaTC) is used in the competitive adsorption experiments which contains a large charged hydrophilic sulphonate group attached to a taurine residue conjugated to a NaC molecule, the NaTC displaces much more protein from the emulsion droplet surface even though NaTC is less hydrophobic than NaC. A similar effect is seen with sodium glycodeoxycholate (NaGDC) when compared to NaDC. NaGDC is less hydrophobic than NaDC due to the conjugation of a glycine residue, but NaGDC is better at displacing protein from the oil-water interface. This observation is explained in terms of the greater steric hindrance that a conjugated bile salt (NaTC or NaGDC) experiences when it adsorbs to an oil-water interface. The charge amino acid group sits further into the aqueous side of the oil-droplet interface and disrupts the adsorbed protein layer to a greater extent than the non-conjugated bile salts. Another interesting feature of the competitive adsorption was the difference between degree of displacement for the different protein types SCN or WPC. For all bile salts SCN was significantly more resistant to displacement than WPC. The reason for this is not clear but may be due to either the greater surface activity of caseins, or to binding of bile salts to WPC or SCN proteins. To investigate further the adsorption of bile salt in emulsions the surface coverage of the bile salts was determined. Results indicated that NaC adsorb as a monomer over much of the concentration range used, whilst the other three bile salts adsorbed as micelles. The consequences of this for competitive displacement of protein are discussed. Finally, preliminary experiments were carried out to elucidate the effect of NaC concentration on lipase catalysed hydrolysis of the triglycerides in the emulsion droplets. The release of free fatty acids is observed to increase with increasing NaC concentration in the emulsion. This is discussed in relation to the effect of NaC on the WPC and SCN protein concentration at the droplet surface, and the likely effect of NaC micelles on the rate of lipase activity