Hot Infusion

Hot soaking in a water or a hydrocarbon solvent becomes necessary when some of the compounds you want to extract are not volatile, slow to dissolve at room temperature, or are bound up in dense material. Solvents become more powerful as the temperature rises. Hot infusion of leaves, bark, seeds or roots is exactly the same as process as making coffee or tea. You can, in fact make coffee or tea with cold water, but the extraction takes several hours, rather than a few minutes with hot water. In the same way, many botanical materials will extract rapidly with added heat, but might take weeks at room temperature.

When making a hot infusion, you must be careful that the more volatile compounds don't evaporate. This is not distillation, and the temperature should not reach the boiling point of either the solvent or the volatile compounds being extracted.

A form of extended hot soaking is used in the preparation of Italian walnut liqueur. Green walnuts (before the shells harden) are packed into large jars and covered with sugar, herbs and grappa at about 80% ethanol. The sealed jars are then placed out in the full sun, where they get quite hot every day. In a few months the contents mature to a delicious, dark liqueur. The water in unripe walnuts reduces the strength of the spirits from around 80% to a final strength of about 40%.

Hot soaking is a technique that's worth trying when cold soaking is ineffective. In general, you should try the simplest method first - i.e. try cold soaking before applying heat. Experiment with small quantities before committing yourself to a large batch!

As compounds begin to dissolve in the solvent, they reduce its capacity to dissolve any more of the compound. Many sparingly soluble compounds require so much solvent to extract them that doing so is impractical or uneconomical. Franz von Soxhlet was a German chemist who solved this dilemma, and the device named after him (the Soxhlet extractor) is a mainstay of analytical laboratories to this day. Like most clever inventions, it is startling in its simplicity, and once you understand how it works, it is easy to make one of your own.

The solvent is boiled in the small container below, and the vapor travels upward through the bypass tube into the extraction tube and then into the condenser. The hot condensed vapor then drips into the extractor body, which contains the sample. If the material being extracted is crumbled or powdered, it is usually held in a "thimble" made of some porous material, so the solvent can pass through it.

As pure, hot solvent drips into the extractor body, the level of liquid rises until all the material is submerged. The compounds we are extracting begin to dissolve in the pure solvent, and the liquid level continues to rise until it reaches the top of the "U" tube outside the main body. When this happens, a siphon is established, and all the liquid drains back into the boiling flask.

The genius of the soxhlet extractor is that it constantly re-purifies the same small amount of solvent (by distillation!), and re-uses it. The extracted materials are kept in the boiler while the sample in the extraction chamber is constantly bathed in fresh, pure, hot solvent. This greatly increases the extraction rate and the concentration of the final product.

Needless to say, volatile compounds do not need the kind of concentrated attack provided by the soxhlet extractor, but it does an excellent job of rapidly extracting difficult compounds, and is particularly useful when dealing with small quantities, such as seeds.

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