Abstract
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.
Highlights
One of the major trends within the food and beverage industry is the replacement of synthetically-derived food additives such as surfactants by ‘natural’ alternatives
The emulsion stabilization was attributed to the hydrophobic interactions between the surfactant molecules originating from the high van der Waals surfaces and to the high net negative charges contributing to the electrostatic repulsion between the emulsion droplets [11]
We have carried out a systematic study of the emulsion stabilizing properties of the naturally-derived, food-grade saponin glycyrrhizin and its corresponding aglycone 18β-glycyrrhetinic acid
Summary
One of the major trends within the food and beverage industry is the replacement of synthetically-derived food additives such as surfactants by ‘natural’ alternatives. The group of amphiphilic saponins has attracted the Saponins are glycosides composed of one or several hydrophilic sugar chains attached via glycosidic bonds to a hydrophobic backbone, which is of triterpene, steroid alkaloid or steroid nature. The licorice (Glycyrrhiza glabra L.) derived saponin is composed of the hydrophobic aglycone 18β-glycyrrhetinic acid (Fig. 1a) attached to a di-glucuronic acid sugar chain via an ether bond, and is referred to as glycyrrhizin (glycyrrhizic acid, Fig. 1b). This amphiphilic saponin is the characteristic ingredient of licorice and is used as food additive E958 mainly because of the 30–50 times higher sweetness compared to sucrose [5]
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