How do I stop oils going rancid?
Short answer
Use stable oils, add an antioxidant like vitamin E or rosemary CO2 extract, and store products cool and dark.
Explanation
Signs of rancidity in the skincare book: a sharp smell like old cooking oil, a sticky or stringy texture, sediment or strong cloudiness, or darkening, and for a cleansing oil, no longer turning milky in water. Throw it away.
In soap, oils high in unsaturated fats raise the risk of rancid 'dreaded orange spots'. That's reflected in the calculator's iodine value (ideally under 60). Fresh food additives shorten shelf life too, so the book suggests an antioxidant or chelator and a 6–12 month use-by date.
What to do
- Protect oxidation-prone oils like grape seed with vitamin E.
- Bottle facial oils in amber or cobalt glass.
Source: The Big Book of Homemade Skin Care, The Big Book of Homemade Soap — Omega-Renewal Facial Elixir · Interpreting Soap Calculator Quality Values