The officer should have asked you if you had anything in your mouth before he let the 15 minutes pass. Can you prove you had the chew in your mouth? Its not a good test if you had something in your mouth.
If you performed the chemical test or preliminary alcohol screen test with any foreign object in your mouth then the results of those tests come into question. While the chewing tobacco by itself might not contain alcohol, the longer it remains in your mouth would potentially allow any mouth alcohol to become absorbed in the tobacco, thereby compromising the integrity of any breath alcohol result. Further, the officer was not in compliance with essential standards if in fact you performed any test with tobacco in your mouth.
For a complete answer we will need more facts. Blowing into the breath machine while having chewing tobacco in your mouth can potentially affect the results, but we need to compare the in field breath results with the station chemical test results that I assume you blew into after you were arrested (unless you refused or you chose blood). You will need to compare the results between the breathalyzer and the chemical test to see if there was any inconsistency. Also there is a mouth alcohol reader on the chemical test at the station (usually) so you have to consider that too. This is not meant to be legal advice, I am not your lawyer, you should consult with an attorney.