This study describes the emulsion stabilizing properties of the licorice root (Glycyrrhiza glabra L.) derived saponin glycyrrhizin and its corresponding aglycone 18β-glycyrrhetinic acid to further increase the understanding between structure and functional behavior. For this, we prepared 10% oil-in-water emulsions and investigated the emulsion stabilizing properties regarding environmental stresses including extreme pH, ionic strength, and temperature. Glycyrrhizin and its aglycone formed nano-sized emulsion droplets at neutral pH that were stable across a broad range of pH-values (pH 5–9), ionic strength (0–200 mM NaCl), and temperature (up to 60 °C). In contrast, emulsions were unstable at low pH (pH <5), as well as high ionic strength (>200 mM NaCl, >5 mM CaCl₂) and temperature as well as after a freeze-thaw cycle. Thereby, the observed instability was mainly attributed to the reduction of electrostatic forces caused by the protonation of free carboxylic acid groups at low pH, screening of electrostatic forces at high ionic strengths, and thin interfaces causing coalescence during a freeze-thaw cycle. Overall, both molecules yielded remarkably stable emulsions at very low molecule-to-oil ratios, and therefore our results are relevant for ‘all-natural’ emulsion-based foods and beverages, as well as pharmaceutical and cosmetic products.

中文翻译：

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天然表面活性剂甘草甜素的结构-功能关系研究：乳液稳定性

该研究描述了乳液稳定的甘草根（的性质甘草L.）衍生的皂苷甘草甜素和其相应的糖苷配基18 β β-甘草酸以进一步增加结构和功能行为之间的了解。为此，我们制备了10％的水包油型乳液，并针对环境应力（包括极端pH，离子强度和温度）研究了乳液的稳定性能。甘草甜素及其糖苷配基在中性pH下形成纳米级乳剂液滴，可在广泛的pH值（pH 5–9），离子强度（0–200 mM NaCl）和温度（最高60°C）范围内保持稳定。相反，乳液在低pH（pH <5）和高离子强度（> 200 mM NaCl，> 5 mM CaCl ₂）和温度，以及经过冻融循环后。因此，观察到的不稳定性主要归因于在低pH下由游离羧酸的质子化引起的静电力的降低，在高离子强度下的静电力的筛选以及在冻融循环中引起聚结的薄界面。总体而言，这两种分子均以非常低的分子与油比产生了非常稳定的乳液，因此我们的结果与基于“全天然”乳液的食品和饮料以及药品和化妆品有关。