And just to continue the discussion I would add that acidity of commercial vinegars varies greatly---by 3% or more in some cases. And then tomatoes carry their own acid content as well. So, I guess what we are all saying is that you aim for a balance of flavors.

I agree with all three of the responses, but suggest that you hold off on doing anything at all for a few weeks. Condiments always need a good long time to sit, undisturbed, before you have any idea how they've turned out. Can it up in quarts or larger, but put a bit into a jar in the fridge, or can a very small jar, and then let the ketchup sit for at least a fortnight. Then taste it. And correct a small amount, as recommended by DonnyG if necessary. I also would suggest that onions that have not been sweated in oil before being boiled with the liquid ingredients can also give condiments a rather bad-tasting, rough edge, and in particular, an unpleasant mouth-feel. In this case though, it really does sound like the reduction of the vinegar was the problem. I like pierino's idea for using sherry vinegar. (Brilliant!!) DonnyG's recommendation to try to correct a small bit at a time is good . . . it's something I have to do on a fairly regular basis when jamming, given that I'm always experimenting and ending up with results that need correcting in one way or the other. Thanks, thirschfeld for the insight into celery seed. I've never liked the stuff, probably for that reason. I've never been able to put my finger on it because, as you know, I'm crazy about a strong celery flavor. Great thread here! ;o)

I make ketchup every year towards the end of tomato season and I use 14 lbs of tomatoes to 2 cups vinegar which I add at the very end and I use 3/4 cup of light brown sugar. I have never had the vinegar overload so it might be the reduction and adding of more vinegar that caused this. Celery seed sometimes can do this to people as well, the back of the throat thing. I add a tablespoon to mine as well so I don't think it is to much. DonnyG has a good idea for a fix though so instead of throwing it out see if you can fix it.

I think you may be right about the vinegar. That does seem like a lot. You might be able to smooth it out next time by substituting a sherry vinegar which might be more mellow than the cider vinegar. And also reduce the quantity by 1/2 cup. Taste as you go because once it's in you can't take it out.

I wouldn't have reduced the vinager and only added to taste at the end. Try adding honey or sugar to a small bowl of your batch and see if that helps. Keep track of your portion ratio so that you know about how much to add to the rest of batch. The idea is balance, not to make it sweet. I hope this helps.