AHA, BHA, fruit acid and polyhydroxy acids in skincare

Belinda Carli, 11/01/2022

FORMULATING WITH ACIDS IN GENERAL

While there are some acids that require specific treatments to be added, and stable, in formulations (like salicylic acid), all, unless particularly stated by suppliers, require a low pH, around 3.5, to be ‘bioavailable’ and effective.  

All too often we see formulas ‘boasting’ a high input of acids, but the final pH is around 4.5 or even 5 – at this pH, there is unlikely to be any activity from the acid remaining in the formula.

When an acid is added to a formula, it pulls the pH down. The more acid that is added, the lower that pH becomes. All acids should be added with regulatory restrictions in mind, not just to comply with cosmetic regulations, but to ensure consumer safety, as that is how the regulations are determined. When a company ‘adds’ a lot of acid to a formula, but then the final product is pH adjusted back up, they are effectively turning the acid to its inactive salt form.

In other words, rather than using a lot of acid and then neutralizing it to achieve a higher pH, you may as well use less acid and not neutralize it, to save on costs, reduce potential instability/poor shelf life, and ensure it works as intended.

Even partially neutralized, the portion that has been neutralized may as well not be present at all.

This means that any preservatives, gums, emulsifiers or even surfactants that are intended to be added to the formula with these acids present, must be compatible with the low pH required to ensure their effectiveness. Even though the formulas have a low pH, a preservative is still needed in a formula anytime it’s pH is above 3.

Finally, while some polyhydroxy acids provide humectant benefits, all acid formulas can benefit from the presence of extra humectant for its moisturizing benefits.

Regulatory restrictions on hydroxy acids

Regardless of what you read on the internet, the main alpha-hydroxy acids and beta-hydroxy acids are regulated by input. We are not listing limits here, as these limits are country dependent and can change at any time. In other words, before formulating with acids, you really need to learn how to search the regulatory limits and make sure you are referencing the most up to date inputs to ensure safe and suitable used in your cosmetic formulations.

See our links at the end of this blog to find courses where you can learn cosmetic ingredient regulations for yourself.

As an example, salicylic acid is usually limits to 2%w/w for stability reasons – it can be very hard to stabilize above this and often recrystallizes; but please don’t just ‘assume’ your country will allow this input or material without checking.

Likewise, an input up to 5%w/w glycolic acid is pH dependent in some countries, and additional warnings are required on the label before it can be sold.

So called ‘clinical’ or ‘therapeutic’ strength products are exactly that – and a different set of regulations apply to those products. They should not be considered ‘cosmetic’ as they are then ‘quasi-drug’, or ‘therapeutic’ as their description suggests, and special labelling and training on their use is then required.

This video also provides a general overview of how to add acids to cosmetic formulas:

Alpha-hydroxy acids

Common alpha-hydroxy acids include: lactic acid, glycolic acid, citric acid, tartaric acid, mandelic acid, malic acid and several fruit acids and even the acid from sugar cane extract.

Glycolic acid is undoubtedly the most well know, but also the most irritating. With a molecular weight of just 76 g/mol, it is extremely small. Because of its low molecular weight, it is able to penetrate the outer layers of the epidermis and this is why it creates significant irritation.

‘Fruit acid’ materials are usually based on the extract of the fruit acid combined with citric and lactic acid, and often have naturally occurring or added glycolic acid to improve their performance. To ensure the key acids have the required performance, they also need a low pH. Watch how to make a fruit acid peel here: 

Mandelic acid is a popular AHA at the moment – it has less irritating effects than glycolic acid, but still great efficacy, and it has been shown to be particularly effective on hyperpigmented and even darker skin tones. Watch how to make a mandelic acid serum here:

Beta-hydroxy acid

Salicylic acid a beta-hydroxy acid and is popular especially amongst acne sufferers. It is much milder than glycolic acid, but still provides effective keratolytic effects.

To use salicylic acid in a formula, you usually need to dissolve it first in ethanol, or in larger proportions of propylene glycol or propanediol, and ensure the pH remains low (around 3.5 – 3.8, if regulations permit) to prevent it from recrystallizing when added to the remainder of your formula. Watch how to make a salicylic acid face scrub here:

Polyhydroxy acids

Polyhydroxy acids have, as the name suggests, multiple hydroxyl groups on the molecule – this makes them effective humectants and with a much larger molecular weight, less irritating than the very small glycolic acid.

Polyhydroxy acids include: galactose, gluconolactone and lactobionic acid.

They will tend to pull the pH of a formula down, so will generally need to be ‘buffered’ with a small input of sodium citrate, otherwise they can cause long term stability and viscosity issues within the formulation.

Watch how to buffer a formula with gluconolactone with our video hyaluronic acid gel with sodium PCA here:

Learn how to formulate with acids and check their regulatory inputs.

Learn how to check cosmetic ingredient regulations as part of our:

• Diploma of Personal Care Formulation: the most comprehensive global regulatory learning for ingredient searches, this is also the qualification that enables you to earn the title ‘Cosmetic Chemist’.
• Certificate in Advanced Cosmetic Science: focus specifically on regulatory searches of cosmeceutical active ingredients.
• Certificate in Cosmetic Regulatory Essentials: teaches labels and claims compliance as well as comprehensive ingredient compliance searches and product evaluations for evidence and claims substantiation.