Apple peel will cause sore throat. I tested and it was true to me. Why?

I knew apple may cause my sore throat or sore throat worse. Then I learned that apple peel would be the cause. I tested and only apple peels caused terrible sore throat or irritation. This started after I developed allergies a few years ago. When I ate apple peel, I tasted tart or bitter. I guess that tart or bitter part which I didn`t taste on peeled apple may cause the sore throat. What is the tart and bitter part? Why does it cause the effect?