Influence of heat on miscibility of Quillaja saponins in mixtures with a co-surfactant

• The impact of heating on Quillaja saponin–co-surfactant mixtures was studied.
• Na-caseinate, pea protein, rapeseed, and egg lecithin were used as co-surfactants.
• Mixed systems at different concentration ratios were assessed at pH 3, 5, and 7.
• Heating caused structural changes in Quillaja saponin–protein mixtures.
• Heating did not lead to dramatic changes in Quillaja saponin–lecithin mixtures.

Thermal treatment of mixed surfactant systems can have a major impact on their phase behavior through modified interactions between the surfactants. In this study, we investigated the miscibility behavior of aqueous binary surfactant systems composed of Quillaja saponin extract and sodium caseinate, pea protein, rapeseed lecithin, or egg lecithin at different concentration ratios (0–5% w/v) at pH 3, 5, and 7 upon heat treatment (25–75 °C). The results revealed that the heat-treated Quillaja saponin–sodium caseinate mixtures at pH 7 remained miscible when the ratio of Quillaja saponins was equal or higher to the ratio of caseinate, otherwise the mixtures flocculated due to increased hydrophobic interactions. At pH 3, the aggregation of Quillaja saponin–sodium caseinate structures was intensified by heating mainly through self-association of casein molecules. In Quillaja saponin–pea protein mixtures as well as in pure pea protein samples heating led to weakening of the gel structures at all tested pH values. In contrast, heating did not affect Quillaja saponin–rapeseed lecithin mixtures, which stayed miscible independent of pH due to electrostatic repulsive forces. Furthermore, the flocculated (pH 5, 7) or aggregated (pH 3) Quillaja saponin–egg lecithin mixtures were only slightly affected by heating. These results are important for understanding the interactions of binary surfactant systems when subjected to heating, which is a common processing step in many food applications.