Extracts that are designed specifically for cosmetic products come in many forms. They are usually liquid extracts in a cosmetically friendly solvent or solvent blend such as water, butylene glycol, glycerin, vegetable oil, or cosmetic ester. Some of them are standardized to a marker compound, but many are not. Many non-standardized extracts are designed simply to add a botanical name to your ingredient deck, but others have in vitro or clinical data from the vendor indicating various benefits to skin. Sometimes these tests have been carried out by an independent lab and sometimes the vendor’s own testing facilities have produced the data. It is important to remember that, unlike academic journal articles, this type of data is not peer-reviewed and outside labs do not often repeat the testing to confirm the results. Some larger cosmetic companies will in fact do their own ingredient testing to confirm vendor claims before they choose to use the ingredient in a personal care product.
In most cases, the goal of producing an extract is to increase the potency of the botanical by concentrating the biologically active constituents. Although it is not uncommon for one particular constituent from the plant to be predominantly responsible for the therapeutic benefit derived from the plant, frequently there will be a series of closely related compounds, or in some cases unrelated compounds, each of which
contributes to some degree to the beneficial properties of the plant. For oral supplements, another benefit to producing an extract is that it reduces the total quantity of plant material that must be ingested to achieve an efficacious dose. In the case of topical applications, as in cosmetics, where the benefit of the digestive process is not available, extraction of the biologically active compound increases the bioavailability of the compound to the skin.
Many types of extraction processes are used commercially to produce extracts. For instance, volatile products, such as essential oils, are often extracted by steam distillation. Lipophillic compounds, such as carotenoids, xanthophylls, and, once again, essential oils or their constituents, are more and more often extracted by CO2 super critical fluid extraction. This process uses pressure and low heat to convert CO2 into a fluid phase, or physical state, in which it acts as an excellent non-polar solvent. The polarity can be modified, to some extent, by adding more polar solvents, such as ethanol, to the extraction. A major advantage to this process is that it is “green”; no harmful organic solvents are released into the environment. Enzymes such as cellulases can be used to break down the cell walls of the plant tissue causing the cells to expel their contents into the enzyme solution. Despite the many options, solvent extractions are probably still the most common method for extracting small molecules such as the secondary metabolites of plants.
The general approach for solvent extraction is “like dissolves like.” In other words, non-polar solvents will extract non-polar constituents and polar solvents will extract polar constituents. So an extraction process needs to be based on the particular compound, or class of compounds, that provide the beneficial properties of the plant in question.
For a high concentration of the target compound(s) in the final extract, the solvent selection and extraction process development must be based upon optimal extraction and purification of the desired compound. Factors such as solvent choice, extraction duration, temperature, etc. of the extraction process must all be evaluated. While applying heat during the extraction process can reduce the required duration of the process and produce a more exhaustive extraction, some compounds are heat labile. Light and oxygen can also cause degradation of certain compounds. If the target compound is highly labile, reducing the extraction temperature may be required. Nitrogen blanketing may decrease degradation, or the addition of antioxidants can also help to protect some highly labile compounds.
A common extraction process would include macerating, or often refluxing, the dried, milled plant material in an alcoholic solvent or water-alcohol mixture. The benefits of alcohol are that it is a powerful solvent capable of extracting many types of molecules, it can preserve the extract and so eliminate the risk of microbial degradation, and alcohol is relatively volatile and is removed fairly easily once the extraction is complete, by vacuum distillation or evaporation.
Alcohol extracts are generally very complex mixtures of constituents. Due to the fact that alcohol is a powerful solvent, it extracts a wide range of constituents from the plant material. A second purification step is often performed in order to obtain a high level of the biologically active compound in the final extract product. The alcohol may be distilled off and a second purification step applied. A simple and common approach is to perform a second extraction on the dried extract using a different solvent. For example, the alcoholic extract of Centella asiatica may be dried down to a paste consistency. The paste is then extracted with acetone and the precipitate is recovered by filtration and dried. The precipitate is assayed and adjusted to 40% saponins, milled and packaged as a commercial extract (15). For other types of extracts, the target compound might be recovered from the filtrate instead of the precipitate. Ideally, a solvent will be found in which the desired constituent is only partially soluble. At elevated temperatures the constituent will dissolve in the solvent and any insoluble material can be filtered out. As the extract is cooled
the solubility of the target compound goes down and, under the right conditions, it crystallizes out of solution. The relatively pure compound is then recovered by filtration Other commonly used purification techniques include ultrafiltration, nanofiltration, and column chromatography. Ultrafiltration and nanofiltration are excellent purification methods for water-based extracts. These methods use membranes to separate compounds based on molecular size. Column chromatography is a procedure by which solutes are separated by differential migration properties in a system consisting of two or more phases. One phase moves continuously in a given direction and the individual compounds within the phase exhibit different mobilities by reason of differences in adsorption, partition, solubility, molecular size, or ionic charge density. The second phase, a stationary phase, may act through adsorption, as in the case of adsorbents such as activated alumina, silica gel, and ion-exchange resins, or it may act by dissolving the solute thus partitioning it between the stationary and mobile phases.