This is probably an unsustainable charge. Battery is commission of an uprivileged harmful or offensive touching. The "touching" here would have been discharged gas settling on the officer's nasal passages or entering through his mouth. There probably is no case law on the subject. There was an extension of the law years ago, when a horse, or was it an elephant, dumped on a patron at a circus.
It would seem that the courts would be reluctant to extend the idea of battery to include the result of an involuntary, or was it voluntary act, the expulsion of gas. What if some person just suffered from good old b.o., body odor, because of deficient hygienic practices? Would the mere emanation of the smell be sufficient to constitute a battery, or must a "discharge" occur? I walked by that person, and his odor offended me?
What about some unfortunate farting in an elevator? It is usually considered against public policy to extend the scope of litigation. But the courts have been inclined to do so in the pursuit of right to sue, establishing new and novel causes of action, such as granting a transsexual class status under anti-discrimination laws that did not include such a class.
If you were on a jury, would you engage in jury nullification and acquit this putative criminal? Or would you make him suffer his just desserts and languish in prison for being gross?
How many times a day does an average person fart? How many of those farts are uncontrollable? How many times within a stipulated period does a drunk fart? How many times a day does a person who has eaten spicy and/or gaseous food, fart? What if some unfortunate had shit in his pants? Aha. the diarrhea defense.
I can see some lawyer hanging out a shingle declaring that he specializes in assault and battery litigation with his practice limited to farting defense. Oh well, he may handle plaintiff's cases also. He stinks. You smell him. Sue the bastard for intrusion upon your close.