Effects of gastrointestinal pH conditions on the stability of the β-lactoglobulin/vitamin D3 complex and on the solubility of vitamin D3

β-Lactoglobulin (βlg) is the major bovine milk protein with important biological and functional properties including a transport role for small hydrophobic ligands. However, βlg is prone to structural changes triggered by modification of its environment such as pH variation. An unfavorable environment during formulation, manufacture or storage of food products or transit in the gastrointestinal track can have a dramatic impact on the stability of the βlg/ligand complex, resulting in the deterioration of its binding ability and premature release of the ligand. This can impair the biological properties of the ligands and reduce their beneficial effects on health. In the present study, vitamin D3 (D3) was used as a nutraceutical ligand model to study the pH-stability of the βlg/D3 complex as well as the consequences of the complex formation on the solubility of the vitamin. Fluorescence spectroscopy was used to monitor the stability of the βlg/D3 complex at pH1.2, 2.0, 3.0, 5.0, 6.8, 7.0 and 8.0. U-HPLC was used to evaluate the solubility of D3 by preparing the βlg/D3 complex in different ratios using a static concentration of D3 and increasing concentrations of βlg. The binding of D3 to βlg was not significantly affected by pH. Furthermore, the data allowed determination of the fractional residual fluorescence representing the fraction of βlg not bound to the ligand, which indicated that the βlg/D3 complex remained stable at all pH values. Therefore, D3 might be retained during formulation and storage of food products at different pH values and during passage in the stomach, which has important implications for the food industry. The solubility of D3 was also significantly increased as a result of binding to βlg, which confirms its role as a carrier to improve the uptake of D3 and consequently the beneficial effects of D3 on health. © 2013 Elsevier Ltd.