Two things have a major effect on the shelf life of a product. One is bacterial growth and one is oxidation.
Firstly, what is the purpose of a preservative? it is to prevent the growth of bacteria. In any environment exposed to water, bacteria WILL grow. There is no exception to this rule. Even with refrigeration, eventually bacterial contamination of some kind will happen. Some of it is innocuous but some is dangerous, particularly to immunocompromised individuals, such as MRSA. And even if you have a healthy immune system, rubbing this antibiotic resistant strain of bacterial directly on your skin is not advised, amirite? Any product that contains water or water based ingredients needs a preservative. Period.
Some essential oils are anti microbial or antifungal, but not at the level of concentration used in products meant to be put on skin. For an essential oil to be a effective preservative, you wouldn't be able to use the product on your body. This doesn't negate their potential benefits, such as tea tree oil for dandruff, it just doesn't make them an effective primary preservative.
Secondly, what is the purpose of an antioxidant? An antioxidant prevents oils from oxidizing, i.e. going rancid when being exposed to oxygen. Some examples of antioxidants are mixed tocopherols (vitamin e), rosemary extract, and grape seed extract.Certain oils and butters are naturally high in antioxidants such as vitamin e. This means they have a longer shelf life than oils low in antioxidants.
THE TAKEAWAY: oxidation has nothing to do with microbial growth. Just because your product will not readily oxidize doesn't mean bacteria won't grow in it. And just because you can't see the bacteria doesn't mean it's not there. Therefore, all good products will have both a preservative system and either naturally occurring or added antioxidants.
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