What differentiates lactic acid from glycolic acid? Which skin conditions can benefit from the use of mandelic and malic acids? And what is pyruvic acid? Knowing the ins and outs of chemical exfoliation and when to use certain solutions begins with a knowledge of each exfoliant and how it interacts with the skin. This knowledge can help ensure safe administration of treatments, as well as optimal, condition-specific results.

To exfoliate, by definition, is to “wash with a granular cosmetic preparation in order to remove dead cells from the skin’s surface.” This definition signifies the most familiar form of exfoliation known to clients as physical exfoliation. Physical exfoliation has been around for eons, with the first recorded uses dating back to ancient Egyptians, and, while this form of exfoliation can provide some benefits to the skin, its uses on many skin conditions and the results provided are limited. Overuse of this form of exfoliation, which many clients can be prone to, can exacerbate various skin conditions. It can irritate open, active acne or sensitive and rosacea-prone skin types, leaving aestheticians and many clients with these conditions feeling as though there are no options for them. Fortunately, there are options for everyone in the form of chemical exfoliation.

Whether they know it or not, every person exfoliates regularly through a process called desquamation. Though it does not sound very appealing, throughout the day, skin breaks down a multitude of protein-based bonds known as corneodesmosomes, which hold corneocytes (the outermost layer of skin cells) together. It may help to visualize the skin as a brick wall, with the process of desquamation breaking down the cement or glue holding the bricks together.

Chemical exfoliation offers a solution by breaking down this thicker layer of dead skin cells and glue, providing resurfacing action without physical exfoliation. That is not to say over-exfoliation cannot happen with chemical-based treatments, as these active ingredients could potentially be harmful if misused, but it also means there are new, powerful ingredients that allow professionals to work throughout deeper layers in the skin. It can also provide more effective results for what clients care about the most.

WHERE TO BEGIN?

Choosing the best chemical exfoliant for different skin conditions, along with remembering which acid is safe for each skin type and tone, is a daunting task for aestheticians. Once professionals understand how these chemical exfoliants work physiologically in the skin and their various innate properties, they unlock an invaluable tool. These tools provide phenomenal results for all skin conditions, including hyperpigmentation, acne, rosacea, and aging.

Understanding chemical exfoliation begins with knowing what differentiates each category of acids from each other, as well as what separates acids from enzymes. Part of this is learning which components of the skin’s matrix they break down, in addition to their solubility (oil or water soluble). The skin’s matrix is primarily composed of water, oil, and protein. Each category of acids and enzymes work as construction crews that break down and remodel different parts of the skin’s framework. Alpha hydroxy acids break down the water portion, beta hydroxy acids soften protein on the surface of the skin and break down oil, and enzymes digest the protein.

The strength and level of exfoliation are controlled by percentage and pH, which stands for potential hydrogen. As exciting as higher percentage peels sound, pH is even more influential on how strong a chemical peel is. The more acidic a peel is, the more tissue becomes destroyed and coagulated, creating an inflammatory reaction in the skin that initiates new cell generation and repair.¹ Less acidic peel formulations provide more control over the peel and provide less of a chance for injury or irritation. Every acid used in skin care varies in pH levels and, through formulation, the natural pH levels can be changed, allowing each acid to be amplified or buffered, depending on the desired effect.

Another defining aspect of acids is their molecular weight. Every acid has a different molecular weight that defines which layers of the skin the acid works on and potentially plays a role in the acid’s reaction time. The higher the weight, the slower and more superficial the acid will penetrate; the lower the weight, the faster and deeper the acid penetrates. Various acids can also be used in combination with each other to amplify effects and provide increased results for multiple skin conditions.

ALPHA HYDROXY ACIDS

There are six alpha hydroxy acids used in skin care including glycolic, lactic, malic, mandelic, tartaric, and citric acids. Alpha hydroxy acids are water-soluble, typically derived from fruits, sugars, or milk, and primarily provide various age management benefits to the skin. Some alpha hydroxy acids also provide antibacterial properties, giving potential benefit to acne-prone skins. Alpha hydroxy acids are formulated into all categories in homecare products, as well as light and medium depth peels for in-treatment room usage.

Lactic Acid    
Of all the alpha hydroxy acids, lactic acid has been utilized in skin care the longest. Cleopatra bathed in milk with roses to achieve radiant, soft, and supple skin. What most do not realize is this milk was fermented or soured to produce the natural lactic acid and give its beneficial exfoliating properties. Although typically derived from milk, lactic acid has recently been produced by other fermented plant sources, offering a fantastic solution to clients seeking vegan-friendly lactic products.

Lactic acid is unique in its ability to increase the skin’s natural moisturizing factor (NMF) and ceramides effectively increase barrier function and help build hydration.² Lactic acid also possess the ability to inhibit tyrosinase, a key enzyme involved in the creation of hyperpigmentation. Due to its slow reaction time and higher molecular weight than glycolic acid, lactic acid offers solutions gentle enough for virtually every skin type, including rosacea.

Glycolic Acid
Compared to lactic acid, glycolic acid is a relatively new acid utilized in skin care. While glycolic acid is usually derived from cane sugar, fruit sources are also available to derive this powerful chemical exfoliant. Glycolic acid has the lowest molecular weight of all alpha hydroxy acids, which allows it to penetrate the deepest and most rapidly into the skin. As glycolic acid penetrates the skin, it begins thinning the top layers of the epidermis quickly, all while helping to restructure the deeper layers of the epidermis, providing multiple benefits to many skin conditions.

As with lactic acid, glycolic acid inhibits tyrosinase and boosts hydration, but does not provide these benefits nearly as gently. Glycolic acid in higher percentage peel formulations is not an appropriate choice for sensitive skin types or higher Fitzpatrick skin tones due to its lower molecular weight and rapid reaction time. For those with non-sensitive skin types and Fitzpatrick I through III skin tones, glycolic peels offer a rapid road to results for multiple skin conditions, including hyperpigmentation, dry acne, rough textured, and aging skin types.

Mandelic and Malic Acids
Another acid relatively new to the skin care world is mandelic acid, which is typically derived from bitter almonds. Mandelic acid has the second highest molecular weight of all alpha hydroxy acids, meaning it penetrates slowly into the skin and is gentle enough for all skin types and skin tones on the Fitzpatrick scale. The highlight of mandelic acid is its ability to provide exceptional pigment lightening and inhibiting benefits for Fitzpatrick IV through VI skin tones, which traditionally have to steer clear of stronger acid peels.^3,4 This acid also contains incredible natural antibacterial properties, functioning as a natural antibiotic on the skin, making it an active acid to treat and prevent breakouts. Mandelic acid is often combined with salicylic acid in peels to provide an increase in efficacy for the treatment of both hyperpigmentation and acne.⁴

Like mandelic acid, malic acid has a higher molecular weight, allowing it to be used on all skin types and tones. Malic acid is most commonly sourced from apples and has native collagen, elastin, and hydration boosting properties. These properties make the epidermis more flexible and resilient, allowing the skin to bounce back easier from folding. In addition, malic acid has pigment lightening and inhibitory properties, offering an additional solution to those that suffer from hyperpigmentation. Mandelic and malic acid work wonderfully together in both homecare and peel formulations to augment each other’s effect in treating hyperpigmentation.⁵

Citric and Tartaric Acids
While not typically seen as the primary acids in peels, both citric and tartaric acids play a supportive role in peels and homecare products, offering multiple benefits for many skin types. Often found in citrus fruits, citric acids provide antioxidant, detoxifying, and astringent benefits to the skin. Tartaric acids can promote tightening, neutralize free radicals through antioxidants, and are thought to have other potential antiaging benefits to the skin, but more research is needed to discover their full potential.

BETA HYDROXY ACID
Beta hydroxy acid has a smaller molecular structure, is oil-soluble, and softens keratin (the proteins in skin), leading to increased shedding of skin cells from the epidermis. The most common form of beta hydroxy acids used in skin care is salicylic acid and, due to its oil solubility, it is the most prevalent and studied acid in treating acne-prone skins with great efficacy. It is often found in various formulations to treat acne, blackheads, and excess oil, including acne washes, exfoliants, toners, masks, serums, moisturizers, and professional peels.

In addition to its oil solubility, salicylic acid also benefits acne-prone skin types through regulation of hormones and inflammatory mediators involved in the creation of inflammation in the skin. By regulating these inflammatory irritants, it reduces the chance of post-inflammatory hyperpigmentation or scarring.

Though not thought of as the first solution for hyperpigmentation, salicylic acid also provides a multitude of proven benefits in treating hyperpigmentation, specifically in the case of melasma and post-inflammatory hyperpigmentation.^6,7 Through its same exfoliating and anti-inflammatory properties imparted to acne, salicylic acid increases exfoliation and removal of irregular melanin and regulates inflammatory mediators that can trigger the creation of hyperpigmentation.

NEXT GENERATION AND NOVEL ACIDS
 Discoveries of new ingredients in skin care are frequent and, as time goes on and science evolves, these new ingredients allow professionals to achieve incredible results without harsh side effects.

Pyruvic Acid
Pyruvic acid is gaining a lot of attention in the aesthetics industry, as it can treat many skin conditions with less irritation than other acids, depending on the formulation. In the body and skin, pyruvic acid naturally provides energy to cells through the Krebs cycle. In skin care, pyruvic acid is derived from flowers and is a precursor to lactic acid. Pyruvic acid is considered neither an alpha hydroxy acid or a beta hydroxy acid, but instead an alpha-keto acid, with both water and some oil solubility. Due to its dual solubility, it has similar properties to salicylic acid, in its ability to soften proteins and break down oil, but also gives the benefits of glycolic acid for aging skin types.

Pyruvic acid mirrors the effect of alpha hydroxy acids, including increased hydration and collagen production, as well as the lightening of hyperpigmentation.⁸ In addition, pyruvic acid reduces inflammation and modulates sebum production (thanks to its oil solubility), lending benefits to rosacea and acne-prone skin types, all while having a more general tolerability than glycolic acid.⁹

Azelaic Acid
Azelaic acid is another acid garnering attention both in the consumer and professional skin care world. Found naturally occurring on the skin and in nature, azelaic acid is a dicarboxylic acid that is produced by indigenous yeast on the skin. Though not used as a primary acid in peels, azelaic acid is used as a supportive ingredient and as the single active ingredient in topical treatments, including prescription-only and over-the-counter products.

Azelaic acid benefits multiple skin conditions, including the treatment of inflammatory rosacea, acne, and hyperpigmentation. In clinical studies, azelaic acid showed more efficacy than 2 percent hydroquinone for the treatment of hyperpigmentation, without hydroquinone’s observed negative effect. This effect for hyperpigmentation is due to azelaic acid’s ability to inhibit tyrosinase and energy production of hyperactive pigment cells or melanocytes.¹⁰ In rosacea, azelaic acid regulates the inflammatory mediators responsible for creating the cascade reaction of redness and flushing seen in the skin, as well as treating the breakouts associated with inflammatory rosacea.¹¹ As an effective treatment for acne, azelaic acid inhibits bacteria, lowers inflammation, and increases exfoliation, which helps clear the pores and reduce infection that can lead to breakouts.^12,13

ENZYME TYPES AND USES
As mentioned previously, enzymes in skin care digest and break down proteins in the skin and the most common enzymes used in skin care are bromelain (pineapple), papain (papaya), and pumpkin. These all fall into the category of protease and proteolytic enzymes.

Enzymes have been used throughout the ages to beautify the skin, with ancient civilizations crushing up fruits and vegetables, then applying them to achieve radiant, supple, hydrated skin. Modern-day enzymatic treatments usually include peels, exfoliants, masks, and serums, both professional and over-the-counter.

Breaking down and digesting skin cells with enzymes causes an irritation that triggers a repair reaction in the skin involving the barrier function and fibroblasts, or collagen-producing cells. This reaction increases the production of new collagen and elastin, as well as the removal of the old, non-functioning collagen and elastin.

In essence, enzymes boost the skins natural removal of junk. Each enzyme differs in their properties and level of activity, with papaya enzymes tending to be more active in breaking down the skin’s protein than pineapple. As a result, papaya enzymes have a higher chance of irritating the skin, whereas pineapple enzymes have, surprisingly, been shown to provide anti-inflammatory benefits, making pineapple appropriate for more sensitive skin types.

The enzyme in pumpkins offers additional benefits for rough textured skin and hyperpigmentation. Pumpkin enzymes latch onto old melanocytes and melanin contained within the epidermis, pulling these cells to the surface and accelerating their removal through increased cellular turnover. In this case, pumpkin enzymes effectively work as a stain-lifter, as these cells can cause uneven skin tone and superficial hyperpigmentation.

Thank goodness the science of skin care has come a long way from the days of exfoliating the skin with sharp pieces of salt or nutshells and bathing in sour milk.

References

1 Berson, D.S., J.L. Cohen, M.I. Rendon, et al. “Clinical role and application of superficial chemical peels in today’s practice.” Journal of Drugs in Dermatology no. 9 (2009): 803-811.

2 Rawlings, A.V., A. Davies, M. Carlomusto, S. Pillai, K. Zhang, R. Kosturko, P. Verdejo, C. Feinberg, L. Nguyen, and P. Chandar. “Effect of lactic acid isomers on keratinocyte ceramide

synthesis, stratum corneum lipid levels and stratum corneum barrier function.” Archives of Dermatological Research 288, no. 7 (1996): 383-90.

3 Hassan, K.M. and A.V. Benedetto. “Facial skin rejuvenation: ablative laser resurfacing,chemical peels, or photodynamic therapy? facts and controversies.” Clinics in Dermatology 31, no. 6 (2013): 737-740.

4 Sarkar, R., V. Garg, S. Bansal, et al. “Comparative evaluation of efficacy and tolerability of glycolic acid, salicylic, mandelic acid, and phytic acid combination peels in melasma.” Dermatologic Surgergy 42, no. 3 (2016): 384-91.

5 Taylor, M.B., J.S. Yanaki, D.O. Draper, J.C. Shurtz, and M. Coglianese. “Successful short-term and long-term treatment of melasma and post-inflammatory hyperpigmentation using vitamin C with a full-face iontophoresis mask and a mandelic/malic acid skin care regimen.” Journal of Drugs in Dermatology 12, no. 1 (20131): 45-50.

6 Ahn, H.H. and I.H. Kim. “Whitening effect of salicylic acid peels in Asian patients.” Dermatologic Surgery 32 (2006): 372-375. 

7 Ejaz, A., N. Raza, N. Iftikhar, et al. “Comparison of 30% salicylic acid with Jessner's solution for superficial chemical peeling in epidermal melasma.” Journal of College of Physicians and Surgeons Pakistan 18 (2008): 205-208.

8 Berardesca, E., N. Cameli, G. Primavera, and M. Carrera. “Clinical and instrumental evaluation of skin improvement after treatment with a new 50% pyruvic acid peel.” Dermatologic Surgergy 32, no. 4 (2006): 526-31.

9 Dréno, B., T.C. Fischer, E. Perosino, et al. “Expert opinion: efficacy of superficial chemical peels in active acne management—what can we learn from the literature today? evidence- based recommendations.” Journal of the European Academy of Dermatology and Venereology 25, no. 6 (2011): 695-704.

10 Breathnach, A.S. “Melanin hyperpigmentation of skin: melasma, topical treatment with azelaic acid, and other therapies.” Cutis 57, no. 1 (1996): 36-45.

11 Two, A.M. and J.Q. Del Rosso. “Kallikrein 5-mediated inflammation in rosacea: clinically relevant correlations with acute and chronic manifestations in rosacea and how individual treatments may provide therapeutic benefit.” The Journal of Clinical and Aesthetic Dermatology 7, no. 1 (2014): 20-5.

12 Gollnick, H.P., K. Graupe K, and R.P. Zaumseil. “Azelaic acid 15% gel in the treatment of acne vulgaris. Combined results of two double-blind clinical comparative studies.” Journal der Deutschen Dermatologischen Gesellschaft 2, no. 10 (2004): 841-7.

13 Holland, K.T. and R.A. Bojar. “Antimicrobial effects of azelaic acid.” Journal of Dermatological Treatment 4, no. 1 (1993): 8-11.

Brian Goodwin is an international educator for Eminence Organic Skin Care. As a master aesthetician, master herbalist, and consultant with over 13 years of experience in the industry, Goodwin brings his love and enthusiasm for skin care to the masses with his passionate, fun, and educational training. Thousands of spa professionals have attended Goodwin’s master classes at tradeshows and on-site training internationally, with a curriculum focusing on guiding spas towards finding success in both educational and business growth. In addition, Goodwin has been published as an author for feature articles in multiple industry leading publications, including DERMASCOPE.