How do u keep pressed flowers from yellowing?

"...popular mixture for drying is made of equal parts of borax and white cornmeal. The cornmeal is light weight and has less tendency to flatten flowers. It also makes boxes easier to handle and move after filling. The main function of these materials is to hold the petals in place while they dry naturally. Good aeration is important for rapid drying.

"Some people feel that the addition of about three tablespoons of non-iodized salt per quart of mix helps petals retain color during drying. The ratio of borax to sand or cornmeal varies widely, but the quality of drying does not seem to differ greatly between them. Ratios varying from 1:1 to 1:6 have been used (first number refers to borax)."