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I believe you are refering to Casein which is a compund found in Milk and is called the Nicotein of fast food. It gives the brain an Opiate like affect and makes the food more addictive.
They also add Monosodium Glutamate which flavors the food and makes the food very tasty. It is used a lot in Fast foods and Chinese foods. This link goes over the Casein and MSG and why they make the food more addictive: http://www.livestrong.com/article/369937-addictive-ingredients-in-fast-food/
I hope this helps and is what you are looking for as far as why fast food is addictive. Joan