As we have seen over and over since Matthew 15:11, the word "defile" is misleading here. The idea really means "to communicate" and "make common." It means "make common knowledge." What Jesus is saying here is that what comes from our hearts determines who we are to others and to God.
The infinitive here, "to eat" is used as a noun, preceded by an article. In English, we use the gerund, not the infinitive for this, "the eating.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.