Canker sores are generally due to a deficiency of L-lysine or vitamin A/carotenoids. I would try taking a carotenoid complex like the one from Country Life. This helps to maintain healthy mucous membranes. I haven't seen that before, but it may just be a part of your body's detoxification process. I think that one canker sore a year is too many, so you've had an ongoing imbalance in the mucous membranes if you've been having 2-3 a year.

I'm not familiar with undecenoic acid's effectiveness against klebsiella oxytoca. Its similarity in structure to caprylic and lauric acids may mean that it might also be effective, or not.