Extraction solvents: what is used and what is controlled
The solvent decides which compounds leave the plant and what stays in the product. It is the variable that changes an extract the most and the one that shows up least in purchasing conversations.
What each solvent carries
Water extracts the polar fraction: polysaccharides, glycosides, tannins, mineral salts. It is the safest solvent and the one that performs worst with lipophilic compounds.
Ethanol, alone or mixed with water, covers the middle range and is the industry workhorse: adjusting the proportion decides which fraction comes along. A 30 % hydroalcoholic and a 70 % of the same material give clearly different extracts.
Non-polar solvents — acetone, hexane — are used for lipid fractions and resins. They give high yields in those cases and bring far stricter control requirements with them.
Supercritical CO2 behaves as an adjustable non-polar solvent and evaporates without leaving residue. It is excellent for essential oils and oleoresins, and it is the wrong technique for water-soluble polyphenols.
The ICH classes and why they matter
The ICH Q3C guideline, adopted in one form or another by most pharmacopoeias, sorts solvents into three classes by toxicity.
Class 1 — benzene, carbon tetrachloride — must not be used. Class 2 — methanol, acetone, hexane, dichloromethane — is admitted within specific parts-per-million limits. Class 3 — water, ethanol, ethyl acetate, isopropanol — is considered low toxicity and its limits are wide.
What matters to a buyer is not memorising the table, but knowing that the solvent class determines which analysis has to appear on the certificate. A process using class 2 solvents without a residual test on the lot is an incomplete file.
What to demand per lot
A declaration of which solvents took part in the process, including those in the purification steps, not just the main extraction.
A residual solvent test on the delivered lot whenever any class 2 solvent was involved, with the method and the limit of quantification.
And coherence: if the data sheet says hydroalcoholic extraction and the certificate reports acetone, something needs explaining before the lot is accepted.
The effect that is not on the data sheet
Two extracts of the same plant at the same marker percentage, obtained with different solvents, do not behave the same in a formula. Colour changes, smell changes, solubility changes and stability against light and oxygen changes.
That is why, when a second supplier is qualified for a reference already in production, the solvent is one of the parameters that has to match. A change of solvent is a change of material, even when both sheets declare the same number.