Why Ethanol is the Preferred Solvent for Botanical Extraction

Spirt Extracts Spirit

One of the many aliases of ethanol is spirit. Purportedly, in the early history of distillation, the “spirit” of the liquid was observed to leave the fermentation brew. The distillate – or spirit – could then be used by early chemists as an extraction solvent for botanicals. It was an early method for researching, concentrating, and preserving botanicals for medicinal or flavor use. By the principle of “like dissolves like,” spirit extracts spirit: ethanol was observed to extract the spirit, or essence, of the botanical. This fundamental use of ethanol is still critical for today’s chemists, because you can’t put a blueberry in a mass spectrometer, but you can indeed inject the spirit of the blueberry. Before we had analytical instrumentation like mass spectrometers, “spirit” had to be something observable. That could mean an observable color change, but more especially a flavor and aroma change. We always understood on some level that flavor was important: we evolved to seek certain flavors, and after that (during our lifetime) we were conditioned to seek flavors. Flavor is essential to our physical health because it signifies the presence of nutrients and bioactive compounds, and to our emotional health because it resonates with our memories. The compounds that are, and always have been interesting to us, are the ones that are biologically relevant. Flavor was the empirical observation for the underlying mechanism.

To illuminate the importance of botanical extracts, let’s consider one of the most ubiquitous ethanol extracts: vanilla. How important is it to you if you have genuine vanilla extract or synthetic? Most consumers prefer real vanilla extract, even though imitation vanilla may be an order of magnitude cheaper. Is it truly because they can taste the difference? It’s improbable: though natural vanilla is composed of hundreds of compounds to give it a complex aroma, there are just a few major volatile organic compounds responsible for the signature smell – which means that in most practical contexts, the imitation vanilla is indistinguishable from the authentic version. Still, given the option between a real vanilla extract and a chemically-identical synthetic replica (assuming analytical flavor chemistry were advanced to that point), most of us would still prefer to have authentic vanilla, for reasons we can’t quite explain. Could it be that we still suspect the presence of the spirit of vanilla? It is interesting, after all, that we still haven’t truly replaced the original extraction solvent. What is it about ethanol that makes it such an irreplaceable solvent? Chemically, ethanol has a polar end (-OH), that looks and behaves like water, but also a carbon chain that gives ethanol nonpolar characteristics. Because that carbon chain is not very long (only 2 carbons), ethanol is still miscible with water, and can also dissolve a wide range of molecules – both slightly polar and slightly nonpolar. Hence it is an effective solvent for many flavor compounds, coloring agents, and bioactive or medicinal compounds. Culturally, ethanol has a long history of being in our food, so its continued use as a solvent is more palatable than less familiar and less studied compounds. Ethanol can be metabolized by the human body, and need not be removed in subsequent processing out of safety concerns. Ethanol is considered a “green” solvent, because of its relatively low toxicity, but also because bioethanol (including all food grade ethanol) is derived from crops with carbon-offsets.

Ethanol Extraction

Ethanol extraction serves multiple aims. It can be used to obtain botanical extracts for consumer purposes (e.g. vanilla extract) and as sample preparation for instrumental analysis. An effective extraction separates desired compounds into a less complex matrix, leaving behind the undesired compounds, like off-aromas, or analytical/biological interferences.

The simplest extraction method is direct extraction in ethanol, called solid-liquid extraction (SLE), and is performed much as it would have been for hundreds of years to extract the essence of botanicals. The plant, macerated, is soaked in solvent so that the compounds having an affinity for the solvent are transferred into the liquid. The spent solids of the plant can be discarded as waste. Parameters such as time, temperature, and pressure can be manipulated to control the extraction. In pressurized liquid extraction (PLE), higher extraction efficiency is obtained with the trade-off that undesirable compounds will no longer be excluded. Another long-standing technique that tends to make use of ethanol in food and botanical applications is Soxhlet extraction, which allows for continuous solvent extraction in a closed system. For analytical purposes, solid phase extraction (SPE) is a versatile sample preparation technique that may employ ethanol. It operates on the same principle as a chromatography column, in that analytes are separated based on their relative affinity for a solid phase material (usually modified silica) compared to an eluting solvent (such as ethanol). Super critical fluid extraction (SFE) with carbon dioxide (SC-CO2) is a popular alternative extraction method for botanical compounds, since it does not require the use of solvents and has lower environmental impact. CO2 becomes a supercritical fluid (something like a dense fog) at high pressure and temperature, and is used like a solvent to extract compounds from a plant (e.g. cannabinoids in hemp, or caffeine to decaffeinate coffee). The CO2 extract is then depressurized so that the CO2 separates from the solute. Still, even SC-CO2 tends to benefit from the addition of a co-solvent in small quantities (approximately 20-fold lower than that of solid-liquid extraction by some reports), resulting in improved speed and higher yields. The use of food grade ethanol in these lower quantities maintains the low solvent cost and the food grade integrity of the product – there is no need to worry about future purification from a safety standpoint. Even with today’s technological advancements in extraction chemistry, ethanol is the preferred solvent for botanical extraction. The efficacy of ethanol as a solvent for some of our most biologically-relevant compounds is something that was intuited long before it was understood mechanistically. It endures not because of tradition, but because of chemistry.

References

• Selective extraction of bioactive compounds from plants using recent extraction techniques: A review
• Food waste: a potential bioresource for extraction of nutraceuticals and bioactive compounds
• A comprehensive review on vanilla flavor: Extraction, isolation and quantification of vanillin and others constituents
• Supercritical Fluid Extraction of Plant Flavors and Fragrances