When neither cold nor warm soaking is fully effective, heating the solvent to its boiling point often does the trick. This is particularly so when the flavor and aromatic compounds are tightly bound in a dense matrix of plant material which hinders access by the solvent. Seeds, bark and root material can all fall into this category and extraction using a boiling solvent is often the only practical way of getting a good yield. This process is limited to use with products that are not affected by heat, though. Once again, it makes good sense to experiment with a small quantity before attempting a large-scale extraction. Many chemical and herbal handbooks contain lists of compounds and botanicals that are either resistant to, or destroyed by heat.
A technique that can work well with heat-resistant compounds is to boil plant material in a solvent and collect the distilled vapor. The argument is that the act of distillation will "purify" the product, only the most volatile compounds being carried over in the vapor. This is one of the traditional methods of making gin and many liqueurs. Curacao, for example, is the distilled extract of bitter orange peels.
Since many plant materials are degraded by heat, the results are not always what you might hope for. The best advice we can give is to compare the results you get with methods of gradually increasing severity, and choose the one you like best
Extracting with liquids creates a dilemma: some compounds require high heat or extended time to fully extract, while those very conditions can ruin some desired compounds or extract unwanted ones. There are some materials that you simply can't successfully extract with liquids.
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