What is the main reason why coffee beans change color during baking?

1, light the lightest frying, no fragrance and concentration can be used to test the degree of mild cinnamon frying, leaving a strong sour taste, the western United States likes light beans cinnamon color 3, medium moderate frying. Mellow, moderate sour taste for comprehensive coffee moderate 4, high sour taste neutralized, began to feel bitter taste loved by Japanese and Nordic people