Hello... I need to extend the shelf life of my finished product. It is a basic cookie dough - butter, egg yolk, flour, sugar, baking soda, vanilla extract. Would like to get this to last about 7 days after baked. I have one friend who tells me I should add guar gum. Tried it with no positive results. Is there a general type additive/preservative that can be purchased by the home baker? Thank you for any suggestions you have.