ISIS scientists, in collaboration with Unilever, the University of Edinburgh, Oxford, John Innes and Croda, have studied molecules derived from tea seed, liquorice root and horse chestnut seed to characterise their structures and how this relates to their performance. This work builds on previous studies and was supported by the InnovateUK Industrial Biotechnology Catalyst scheme.
Surfactants are used in a wide range of home and personal care products, foods, beverages and cosmetics; our explainer article gives more information on their structure and properties. They are often added to these products to stabilise the mixtures of other active ingredients, and their stabilisation capability is closely linked to their structure.
To remove fossil carbon from product formulations, as part of their 'Sustainable Living Programme', Unilever are investigating the feasibility of using a type of naturally occurring surfactants known as saponins which are found in a variety of plants. These are already used in some traditional medicine, vaccines and in some foods and beverages. These molecules are often present in residual waste from food sources and could therefore be an ideal feedstock for a wide range of products. For instance, the tea seed saponin is derived from the waste of seeds used to create tea seed oil, a premium culinary oil used throughout China.
The three molecules investigated in their recent studies have been escin, extracted from horse chestnut seed, glycyrrhizic acid (GA), from liquorice root, and tea seed saponin described previously. Although similar in structure, these three molecules have small differences that impact the way they aggregate in solution, and form layers at the air/water interface. As well as studying the molecules on their own, the group has also looked at how they behave when mixed with each other.
By using small angle neutron scattering on SANS2D, the researchers were able to carry out a detailed characterisation of GA. They found that the number of carboxyl groups, as well as the number of saccharide groups, present in the head group of the molecule played a key role when determining the shape of the aggregates, known as micelles, formed in solution. The effect of the carboxyl groups was confirmed when comparing GA with the escin and tea saponins. The larger head group of the tea saponin dominated its interactions and the molecules formed smaller, more globular micelles.
However, although both GA and escin formed more elongated micelles, the researchers found that the interaction was much more complex than just being determined by the size of the head group; the number of the carboxyl groups in GA as well as the number of saccharide groups gave the head group a larger effective area, and this changed its aggregation in solution and its adsorption at the air/water interface.
On mixing the saponins, it was found that adding escin or GA to tea seed saponin had the effect of diluting the domination of the large head group interactions and they observed more elongated micelle structures. However, the mixture of escin and GA was more complicated. Although both biosurfactants form elongated micelles when separate, mixing them together causes smaller, more globular micelles to be formed.
These results show that it is possible to mix saponins to get different properties, and shows how similar they are to conventional detergent molecules which are frequently used in combination. This is an important finding, as the ability to manipulate the behaviour of these molecules is crucial if these biosurfactants are to be used more widely in industry.
The detailed characterisation of GA can be found at DOI: 10.1016/j.jcis.2021.03.101
The study of the saponin mixtures can be found at DOI: 10.1016/j.colsurfa.2021.127420