Does James contradict Jesus when he says some foods should not be eaten?

In Matthew 15 , we read: > [Jesus] called the people to him and said to them, "Hear and understand: it is not what goes into the mouth that defiles a person, but what comes out of the mouth; this defiles a person ... Are you also still without understanding? Do you not see that whatever goes into the mouth passes into the stomach and is expelled? But what comes out of the mouth proceeds from the heart, and this defiles a person. For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander. These are what defile a person. But to eat with unwashed hands does not defile anyone." We see in Mark's account that "thus He declared all foods clean." Now, when we move to James and the church elders in Acts 15, we find them writing a letter in which, among other things, they impose food requirements on the Gentile churches: > For it has seemed good to the Holy Spirit and to us to lay on you no greater burden than these requirements: that you abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality. If you keep yourselves from these, you will do well. Farewell." Do these statements contradict, or can they be correlated? If they contradict, then who gave James and the elders the authority to overrule the teachings of Jesus?