Antioxidants — Playing the star and supporting roles in cosmetic formulations
As a cosmetic chemist, I love it when an ingredient is multi-functional because I can get more benefits without increasing the cost of my formula. Anti-oxidants are a great example.
Skin is exposed to a variety of external aggressors every day. Exposure to UV light, environmental pollutants, and exposure to microbes are a just few examples. The skin mounts inflammatory response to these aggressors generating reactive oxygen species (ROS). ROS have an unpaired electron in the outer shell of an atomic orbital and are highly reactive because the molecule will seek to gain additional electrons through bonding to fill that shell.
Skin aging is accelerated by an accumulation of ROS induced damage to organic molecules like lipid, proteins and nucleic acids. The skin has its own antioxidants and defense mechanisms to neutralize ROS, including enzymes like superoxide dismutase (SOD). But oxidative stress can overwhelm these mechanisms.
External application could supplement the body’s own supply of antioxidants preventing formation and proliferation of ROS. Ascorbic acid (Vitamin C), tocopherol (Vitamin E), isoflavones and polyphenols are examples of cosmetic antioxidants. Additionally, plant extracts rich in antioxidant compounds have become popular. While these antioxidants can be broken down and essentially used up in the process, peroxidases are enzymatic antioxidants are not consumed. Stability, efficacy thresholds, and active molecular form are important parameters when formulating with antioxidants.
Antioxidants in the supporting role
Cosmetic formulations contain fragrances, natural fats and oils that can all be subject to auto-oxidation by exposure to air causing off-odors and other instabilities. Raw material specifications often contain a test for peroxide value to measure the degree of oxidation and it is important to consider storage and handling procedures for these materials because heat and high temperatures can catalyze the oxidation reaction. Because of this instability, stored materials should be checked periodically to evaluate acceptability.
Adding antioxidants to the raw material or the formulation is a way to preserve the stability. Some susceptible ingredients have antioxidants already added by the supplier, while others like sunflower seed oil can be natural sources of antioxidants. Typically, a raw material with a peroxide value below 5 can be aided by the addition of antioxidants but once the number has progressed beyond 10 there is little that can be done.
To aid in formula stability, antioxidants should be soluble in the ingredients they are intended to protect. BHT (butylated hydroxytoluene) and BHA (butylated hydroxyanisole) are two common oil soluble antioxidants. BHT is a bit more effective than BHA. Tocopherols (Vitamin E derivatives) are weaker antioxidants than BHT or BHA but are much more label friendly for their associated skin benefits. Propyl gallate is an example of a water soluble antioxidant.