I have been told that Gallic Acid is water soluble only in the presence of other compounds – in the case of tea, notably thearubigin. Can you confirm/refute this? I wonder if you can also explain the mechanism if you believe it to be soluble? Lastly, what do you beleive are the implications for bio-availability?
Gallic acid has very low water solubility, and for it to dissolve in water, one of two situations are required:
1) The presence of another substance which acts as a solvent for gallic acid (eg propylene glycol)
2) The addition of an alkaline substance such as sodium hydroxide which, in this example, produces one of the sodium salts of gallic acid (several are possible) which will be more soluble in water.
Thearubigins are a class of compounds known as polymeric polyphenols, some of which are actually based on gallic acid, and I doubt that they contribute to solubilising gallic acid. I would guess that the very low concentrations of gallic acid present in tea are sufficiently soluble in water to have a measurable presence when the hot water is added to the tea leaves.
I am not quite sure what the questioner requires when asking about the mechanism of solubility and I have no idea of the bioavailability of gallic acid.
I agree that the question is about tea – maybe they are looking to put a tea extract in their formulation which isn’t unusual! Ok so this is what I understand of the tea process as background to answering this question:
Tea is rich in polyphenols which have antioxidant properties. Polyphenols can be further classified into different chemical families including tannins and flavonoids – Gallic Acid fits into the tannin family and thearubigin is a flavoniod.
Thearubigin is formed during the fermentation process that is required to make black tea where it makes up between 10-20% of the dry weight. To confuse matters further thearubigin degrades to form (among other things) gallic acid if the fermentation is allowed to progress too far. This has an adverse affect on the taste.
OK so as for the question, gallic acid is soluble up to 1.1g per 100g water and has a better solubility in alcohol. It’s solubilityalso increases with temperature. This info can be found here, meaning that the answer to the first part of the question is NO.
As for the implications for bioavailability this isn’t something that can be answered as the question is stated as one would need to know the dose being ‘taken’ or applied, the other properties of the formulation and the experimental conditions. As far as skincare actives go, being water soluble doesn’t give them a skin-penetration advantage as the skin barrier is lipidic.
The reader may have been getting confused between green, white and black teas.
It’s good to remind people that cosmetics aren’t supposed to interact with the body’s metabolism so the bio availability of the material should really be negligible if it’s going into a cosmetic. If it actually has a metabolic effect, then you’re creating a drug (at least in the US).