Greetings, fellow formulators!
Ok, so here is my story for today--
I've created an emulsified product using phospholipid-based emulsifiers, and also including lysine, vitamin C, and several botanical oils and botanical extracts including St. John's Wort and arnica. The pH of the water soluble phase is around 5.5 (adjusted up with NaOH). I've been "field testing" this product, basically subjecting it to the abuse of life in the summer in the south, leaving it in my hot car, in the sun, etc. The emulsion is a lamellar one and although I have not seen signs of separation, I have noticed that the product darkens noticeably over a period of about 4 weeks.
The product is preserved with BHT as an antioxidant and benzylalcohol DHA as well as two essential oils (tea tree and peppermint) as anti-microbials. I have done simple dip-stick testing for on the most abused samples and they come out clean for total aerobic and yeasts and molds. What do you think could be causing the darkening?
Thanks for your spirit of collaboration!