Potatoes -- McDonald’s uses potatoes called russet Burbank. These potatoes have a nice oval appearance and just the right amount of sugar. Excess sugar can impair taste and add an aroma of burnt but that is impossible because the company uses a process called bleaching. During that process excess sugar is removed.
Vegetable Oil -- In the past McDonald’s fried the potatoes in beef tallow, but because reducing the saturated fat in food was inevitable, they started using vegetable oil instead. Fries received two doses of vegetable oil. One dose was received during the frying process at the factory and the second dose during cooking in fast food restaurants. Corn oil and TBHQ, (additive which in large amounts can cause discomfort and can even cause stomach cancer), is added to the restaurant food.
Sodium Pyrophosphate -- potatoes react to iron, so, after frying they can get ugly color. Sodium pyrophosphate ions which prevent the reaction with iron ions are added so the potatoes keep their beautiful color over the whole process.
Dextrose -- Dextrose is better known as glucose. The usage of glucose undoes the process that previously removes the sugar (bleaching) and allows the sweet taste to be returned. All this is resulting as a homogeneous coating which caramelize the fries equally. When you add ketchup, you are actually adding more sugar to the potatoes as well.
Salt -- In the end, just before serving, pretty good amount of salt is added to the potatoes. It creates a hedonistic combination of fat, salt and sugar.