Quillaja triterpenic saponins can act as natural foaming agents to offer a real alternative to synthetic surfactants. L Rigano, N Lionetti & R Otero explain how
Many plants contain saponins, a wide category of natural surfactants. Their industrial application is growing, firstly under the pressure of the green wave, but also for their specific properties, which differentiate them from common synthetic surfactants.
The Quillaja saponaria (soapbark tree) saponins, extracted from the plant growing in arid areas of Chile, exhibit surfactant properties because of their lipophilic triterpenoid structure, with side chains of hydrophilic carbohydrates. Quillaja (synonym: bois de Panama) extracts are obtained by aqueous extraction of the milled inner bark or wood of pruned stems and branches of Quillaja saponaria Molina (family Rosaceae). The word ‘quillay’ is derived from the native Mapuche word ‘quillean’ meaning ‘to wash’.
Saponins are good foaming agents, with very stable foams, but only the purified quillaja saponins reach cosmetically acceptable levels. The purified extracts are also used in beverages to provide the foamy head. In cosmetic applications, acne prevention, anti-inflammatory and anticariogenic activity have been reported. The sustainability of raw materials derived from plants is increasingly considered important. Recently, quillaja extracts (from Desert King International) have been made available in an inorganic certified version produced from eco friendly and sustainable FSC certified forests. This represents an additional key element for the potential applications in modern cosmetics.
Quillaja triterpenic saponins, having an average molecular weight of 1800 - 2000 Dalton, consist predominantly of glycosides (2-5 sugar units per each side chain) of quillaic acid. They are non-ionic surfactants, resistant to salt and heat and stable to acid pH. Careful purification of saponins leads to a spray-dried purified aqueous extract of the Chilean soap bark tree (commercial name Andean QD ultra), a light beige free-flowing powder with the average composition:
Moisture content 2.0 - 7.0%
Fibre 0.01 - 0.5%
Proteins 3.5 - 7.0%
Ash 6.5 - 12.0%
Fats 0.01 - 0.5%
Carbohydrates 73.0 - 87.94%
It is soluble in water, giving a yellow-brown appearance. When taste is not important, a less pure grade is also available.
COSMETIC POTENTIAL
Quillaja extract can be formulated with many common cosmetic ingredients. The increasing awareness of the potential harmful effects on the mouth mucosa of the traditional synthetic surfactant sodium lauryl sulphate has fuelled the need for its replacement with milder surfactants, with an acceptable taste in the mouth. Quillaja saponins are generally compatible with hydrotropes used in toothpastes, such as xylitol, glycerine and sorbitol, up to 20%.
Purified quillaja triterpenic saponins, used alone, have an average foaming power which is about 40% lower than for most high-foaming surfactants. Yet a blend prepared with coco-glucoside, in the ratio 80:20 with saponins, has foaming power better than that prepared with SLES. Foam quality remains optimal, ie small, homogeneous bubbles.
To verify the compatibility of purified triterpenic quillaja saponins with polymers used in toothpastes, solutions containing 10% saponins and a series of natural and synthetic polymers at 1% solids were prepared.
Xanthan gum, hydroxyethylcellulose and carrageenan show enhanced thickening effects. Lower (and higher) amounts of saponins have been tested, keeping constant the polymer amount (figure 1).
A positive synergic interaction exists between natural polymers and quillaja triterpenic saponins, especially for carrageenan. The foam stability of such systems – 10% purified quillaja saponins + 1% xanthan gum or Chondrus crispus extract – noticeably increases, while they reach quite interesting rheology values.
In the case of hydrated silica, saponins (tested at 0.5, 1 and 2% respectively) slightly decrease the silica viscosity, making it possible to control the flow stability of such gels, which is frequently a problem in toothpastes formulated with them. In order to get optimum results it is advisable to first wet the hydrated silica powder by 20% w/w glycerine. In another set of trials, a 25% solution of saponins with 5% tocopherol forms an opaque gel that turns into a stable milk when diluted in water. Foundation formulations, where the pigment wetting properties of quillaja saponins were exploited, produced interesting skin adhesion and a natural looking appearance.
EMULSION STABILITY
Several experiments have been carried out to establish the possibilities for using quillaja saponins in innovative emulsions. Triglycerides like almond and rice oil, caprylic/capric triglyceride, jojoba oil and a proprietary blend of meadowfoam seed oil with shea butter extract (Fancol VB) can be emulsified at 20%. For the largely hydrophilic properties of quillaja saponins, they need to be blended with lipophilic emulsifiers. Among the many tested, the best performers were steareth-2, polglyceryl-3 stearate and sucrose distearate, at 3.75% in formula, blended with 0.75% saponins. Cetyl alcohol at 1% and xanthan gum at 0.2 were also added as long-term formula stabilisers.
Stable systems were obtained both by using the mentioned individual oils alone as fatty phases but also with their 1:1 blends. In general, blends had better resistance in the stability trials at high temperatures (43°C). The vegetal oils reported above can also host 25% of the total fatty phase as constituted by other oils (as isostearyl alcohol and hydrogenated polydecene) that, when used alone, induced instability in previous experiments. Trials aimed to reduce the amount of internal oil phase in the formulae were unsatisfactory.
In contrast, using 30% oil phase, 4.7% emulsifier and 1% saponins provided generally stable emulsions, even at high temperatures. Quillaja saponins seem suitable to emulsify very high amounts (30% and more) of vegetal oils that are often difficult to introduce in emulsions. They also provide a light skin feel during application. In many cases, co-emulsifiers from natural sources like polglyceryl-3 stearate and sucrose distearate can be used, making it possible to follow an organic formulation strategy. Many formulations have been prepared and are now in stability testing.
APPLICATIONS
Purified quillaja triterpenic saponins, which provide skin compatible and environmentally friendly surfactants with a low toxicity profile but with innovative performances, seem to represent the long searched for alternative to synthetic surfactants. They have potential applications in cleansing systems, toothpastes and mouthwash. Interaction with thickeners, surfactants and sparingly soluble molecules, especially when they exhibit glycosidic groups in their structure, can open the way to many new formulation experiments.
Applications in emulsions seem to offer the most potential in complex blends. Formulations of normalising and anti-dandruff shampoo, conditioners, cleansing fluids and oral care have been prepared, examples of which are shown here.
