Solvent extraction pulls a wanted compound out of plant or animal material by soaking it in a liquid that dissolves that compound and little else: water takes tannins, some alkaloid salts and many pigments, alcohol takes both water-soluble and oil-soluble compounds, and warm oil takes oil-soluble ones. Several small washes recover more than one large wash of the same total volume. Choose the solvent by what you want, then keep the work outdoors if the solvent burns.
Part of Organic Chemistry
Dissolving target compounds out of natural material with water, alcohol or oil, used when heat would destroy them.
Match The Solvent To The Compound, Because Like Dissolves Like
The carrier sets the pattern: use solvents at lower temperatures for compounds that do not survive steam distillation. It names three solvents and what each gives.
- Water gives teas, decoctions and infusions, and suits tannins, some alkaloids and many pigments.
- Alcohol (distilled spirits, soaked for days to weeks) dissolves both water-soluble and oil-soluble compounds and gives tinctures, the basis of most herbal medicine. Strength is the lever: see Tincture Making and Tinctures and Extracts.
- Oil (olive, coconut or animal fat), warm, takes oil-soluble compounds and gives infused oils for topical use and cooking.
The rule behind the table is that polar solvents (water, alcohol) dissolve polar compounds such as sugars, salts and many acids, while non-polar solvents (oil) dissolve non-polar ones such as resins and fats. Alcohol sits in the middle, which is why it is the most useful single solvent.
What You Need
- Solvent: water, ethanol of known strength, or clean oil. Never use petrol, paint thinner or an unidentified solvent on anything meant for the body.
- Vessels: glass, glazed ceramic or enamel with a tight lid for alcohol (see Storage Containers for why solvent stores are special).
- Filter: cloth or paper, and a press for squeezing the marc.
- Ground or chopped material, dry where possible.
Method: One Batch By Maceration
Step 1: Reduce the material to small pieces so more surface meets the solvent.
Step 2: Cover with solvent in a sealed vessel, away from heat and sunlight.
Step 3: Wait. Alcohol extraction takes days to weeks per the carrier; shake daily.
Step 4: Strain and press the residue to recover liquid held in the marc.
Step 5: Recover or store. An alcohol extract is stored in sealed glass. Recovering solvent by distillation uses the Distillation Apparatus.
Worked Example: Why Two Washes Beat One
Suppose 1 L of watery liquid holds 10 g of a compound that prefers an organic solvent four times over (a distribution ratio K = 4; the value is illustrative, not measured).
- One wash with 1 L of solvent. Fraction taken = K × V_s ÷ (K × V_s + V_w) = 4 × 1 ÷ (4 × 1 + 1) = 4 ÷ 5 = 0.80, so 8 g.
- Two washes of 0.5 L each. First wash: 4 × 0.5 ÷ (4 × 0.5 + 1) = 2 ÷ 3 = 0.667, so 6.67 g, leaving 3.33 g. Second wash: 0.667 × 3.33 = 2.22 g. Total 6.67 + 2.22 = 8.89 g.
Splitting the same solvent into two portions recovers 8.89 g instead of 8 g. The gain is real but modest; the principle (many small washes) holds for any K.
Common Mistakes
- Using heat on a flammable solvent. Warm ethanol gives off vapour that ignites; use a water bath outdoors, never a flame beneath the vessel.
- Assuming a stronger solvent is better. Pure alcohol can fail to wet dry material and can pull unwanted compounds as well. Strength is chosen for the target.
- Concluding the extract has a safe dose. Extraction changes strength unpredictably; see Standardization.
Safety
Solvent vapour burns, and some solvents poison
Ethanol and most organic solvents are flammable; handle them outdoors with no flame near. Methanol, petrol and many industrial solvents are toxic through skin and lungs, and most solvent vapours sink into low spots (see Fire Safety). Keep stocks in sealed, labelled containers, and keep extracts made for medicine apart from extracts made for other uses.
What’s Next
- Learn next: Tinctures and Extracts — solvent extraction applied to herbal preparations
- Combine with: Pressing and Rendering — the mechanical alternative with no solvent