I have this a little book that helps with that kind of stuff:Yeah I've just decided to put my 4yr old on a preservative free diet to see if it improves his behaviour. Took me hours to do the food shopping to find stuff that didn't have any of the nasty numbers in it!
http://www.possibility.com.au/shoppingc/index.html
But yeah, unfortunately it is time consuming.
Now if I only could remember to take it shopping with me!