|MadSci Network: Chemistry|
I doubt that there is a chemical reaction between orange juice and toothpaste. Rather, if there is anything at all happening it may (?) be that brushing your teeth cleans all that yuckie placque off and some of it is still around. Also brushing your teeth may stimulate the taste buds on your tongue (do you brush your tongue occasionally - you should). Orange juice is of course rather acidic and can taste quite tart. Taste, bitterness, etc will vary a lot from batch to batch even in frozen and canned oj. You might try brushing your teeth after breakfast instead of before. That way you have clean teeth for a longer time (until lunch or your next snack) and avoid the oj/toothpaste/brushing problem all together ;-}
Try the links in the MadSci Library for more information on Chemistry.