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Solubility of plant extracts ingredients



Solubility in a given solvent is influenced by the polarity of a molecule’s skeleton and functional groups, the molecular size, temperature, the pH of the solution, and other factors. The solubility of an isolated compound is easy to determine, but this may change in the complex matrix of a medicinal plant extract. The general principle: like dissolves like. A high-polarity solvent will pull high-polarity molecules out of the herbal material; a low-polarity solvent will select for low-polarity molecules.


1. Carbohydrates

1) Monosaccharide’s (e.g., glucose, fructose): highly water soluble

2) Organic acids (e.g., citric acid, formic acid; generally water soluble, varies with pH)

3) Disaccharides (e.g., sucrose, maltose: highly water-soluble)

4) Oligosaccharides (e.g., inulin, FOS: water-soluble, esp. in hot water)

5) Heteropolysaccharides (e.g., mucilages, gums: highly water-soluble)

6) Homopolysaccharides: (starch: hot-water soluble; cellulose: insoluble)

Dietary fiber: soluble (includes heteropolysaccharides such as mucilages, gums, and pectins) and insoluble (e.g., cellulose, lignin)

Carbohydrates generally stay dissolved in low-ethanol (~ 20 – 30%) preparations and in glycerites

Carbohydrates generally will precipitate out at higher ethanol ratios

Tannins can bind to and precipitate complex carbohydrates


2. Lipids

1) Fatty acids (e.g., Omega 3s, EFAs, PUFAs, DHA, EPA): mostly oil-soluble; slight solubility in ethanol

2) Triglycerides (= triacylglycerols: oils like Olive or Almond, also known as ‘fixed oils,’ are mostly composed of triglycerides): very slightly soluble in ethanol, not in water; will dissolve many oil-soluble compounds

3) Phospholipids (e.g., lecithin, phosphatidyl choline, PS) are emulsifiers

4) Waxes (e.g., beeswax): soluble in warm oils but not in cold

5) Alkamides (e.g., the tingly isobutylamides in Echinacea & Spilanthes): soluble in mid- concentration ethanol (around 40 – 60 %)

6) Polyacetylenes (e.g., arctinal from Burdock; PHT from Bidens) similar to alkamides

Unsaponifiable matter (anything dissolved in an oil, such as Olive, which is not a fatty acid or triglyceride; includes oil-soluble vitamins, phytosterols, carotenoids, etc.)

Essential oils are not lipids, but rather are composed mainly of small terpene compounds; soluble in mid-to-high percentage ethanol, fixed oils, slightly soluble in water

Lipids generally soluble in other lipids


3. Amino acids & derivatives

1) Free amino acids at pH ~ 7 are zwitterions; very water-soluble; generally soluble in low-percentage EtOH

2) Some sulfur-containing amino acid derivatives (e.g., ajoene and sulfides from Garlic) are oil-soluble; some sulfides have limited water solubility

3) Cyanogenic glycosides (e.g., amygdalin, prunasin) soluble in water, more so in hot water, somewhat soluble in cold EtOH, more so in hot EtOH

4) Amines: like alkaloids, generally more soluble in acidic media; amine salts water-soluble (ionic)

5) Methylxanthines (e.g., caffeine): hot-water soluble, less so in cold water

6) Peptides: (e.g., glutathione) generally water-soluble; depends on pH (lose solubility at the isoelectric point)

7) Proteins: generally water-soluble; depends on pH (lose solubility at the isoelectric point)

8) Enzymes: generally water-soluble; depends on pH (lose solubility at the isoelectric point)

Proteins and enzymes denature in EtOH

Tannins can bind and precipitate proteins


4. Phenolic compounds

Structures of phenolic compounds (polyphenols) vary widely, and many can occur as either glycosides or aglycones. Glycosides are generally more water-soluble than the aglycones. Depending on pH, the hydroxyl groups may be ionized; this enhances water-solubility.

1) Phenolic acids (solubility varies with structure and pH)

2) Phenylpropanoids (solubility varies with structure and pH)

3) Coumarins (glycosides generally water- and ethanol-soluble; aglycones less so)

4) Lignans (generally soluble in acetone/water; Flaxseed lignans are not oil-soluble)

5) Phenylpropanoid derivatives (e.g., capsaicin, curcumin, gingerol: soluble in high-percentage ethanol, fixed oils)

6) Stilbenoids (e.g., resveratrol; soluble in EtOH)

7) Xanthones (generally soluble in EtOH and oils)

8) Styrylpyrones (e.g., kavalactones; soluble in high-percentage EtOH, acetone)

9) Flavonoids (solubility varies, but many are water and EtOH-soluble)

Anthocyanins: water- and EtOH-soluble

OPCs: water- and EtOH-soluble

Tannins: smaller tannins soluble in hot water, EtOH; larger tannins relatively insoluble

10) Isoflavonoids: soluble in EtOH; low water-solubility; however, the glycosidic forms are more water-soluble

11) Benzofurans: (e.g., usnic acid; soluble in hot EtOH; solubility in water and EtOH is very low)

12) Chromones (e.g., khellin; soluble in hot water, hot EtOH)

13) Quinones (e.g., anthraquinones; solubility varies; most soluble in water and EtOH; bianthraquinones in Hypericum are oil-soluble)

14) Phloroglucinol derivatives (e.g., hyperforin, cannabinoids: soluble in oil and high-percentage EtOH)


5. Terpenoids (terpene compounds)

As a general rule, terpenoids tend to be oil- and high-percentage EtOH soluble. Triterpene and steroidal saponins are water-soluble because of their sugar groups. The smaller terpenoids (in essential oils) have very limited solubility in water, and are soluble in fixed oils and EtOH. Small terpenes include:

1) Hemiterpenes

2) Monoterpenes

3) Sesquiterpenes

The larger terpenes are not volatile and tend to be oily or resinous substances. In general, they are soluble in fixed oils or high-percentage EtOH. Many resinous materials are composed of larger terpenes (and/or polyphenolic aglycones):

4) Diterpenes

5) Triterpenes

5) Tetraterpenes (carotenoids)

The aglycones of triterpenoid saponins (sapogenins) are practically insoluble in water or EtOH, while their glycosidic forms are highly water-soluble and amphiphilic (soapy/foamy).


6. Steroids

Plant steroids are oil-soluble, with the exception of glycosidic forms (e.g., steroidal saponins and cardiac glycosides); the latter are slightly water-soluble.

1) Phytosterols (soluble in fixed oils, high-percentage EtOH)

2) Steroidal saponins (aglycones are not water-soluble, glycosidic forms are)

3) Cardiac glycosides (slightly water-soluble; more so in dilute alcohol; aglycones oil-soluble)


7. Alkaloids

The pH of the solution influences the solubility of alkaloids. In general, alkaloids are more oil-soluble in high pH (alkaline) solutions. Conversely, they are more water-soluble in acidic solutions. Most alkaloids are soluble in mid- to high-percentage ethanol/water solutions.

Capsaicinoids (pseudo-alkaloids) are oil-soluble.

The purple and yellow betalain alkaloids (found in Beet, Pokeberries, Prickly Pear Cactus, Amaranth, etc.) are very soluble in water and will remain dissolved in low-to-mid ethanol solutions.

The isoquinoline alkaloids are soluble in moderate-to-high ethanol solutions with the exception of berberine, which is far more water-soluble than the others.

The N-oxide forms of pyrrolizidine alkaloids (PAs) are very water-soluble. PAs are soluble in low-pH solutions, but insoluble in high-PH solutions. They are also soluble in ethanol and in organic solvents such as acetone. They are nearly insoluble in fixed oils.


Polarity of solvent solutions

The overall polarity of a solvent mixture depends on the ratios of ethanol and water (and glycerin if you are using it) in it. Adding glycerine (glycerol) could either raise the overall polarity of an ethanol/water solvent mixture, or could lower it, depending on this ratio.

One way to measure the polarity of a compound is to give it a number called the 'dielectric constant':

Water 80

Glycerol 46

Ethanol 25

You can determine the polarity of a solvent mixture by multiplying the volume fraction of each solvent times its dielectric constant and summing.For example, If you are using 50% ethanol and 50% water, without any glycerol, your overall polarity would be 52.5: (.50)(25) + (.50)(80) = 52.5