I've heard cream of tartar can work (along with other acid, like buttermilk). (kitchenistadiaries.com)
The standard recipe for homemade salad dressing follows a 3 to 1 ratio — that's 3 parts oil to 1 part vinegar (or other acid like citrus juice). (blog.kitchenaid.com)
Cooking foods containing other acids, such as vinegar, red wine, lemon or lime juice, in an iron pot can also increase the iron content of the final mixture. (naturallysavvy.com)