Natural Preservative for Soap

I want to try adding lavender, dried flowers, and citrus zest to my soap. What can l add to keep them from turning brown, other than grape seed extract? It is very expensive. Thank you.

ANSWER: I have never found a way to keep natural botanicals, etc... from turning brown. It's just a natural process. I don't feel like it detracts from the final product.

Thanks for Asking!
Sydne