Lard! Tacos, enchiladas and refried beans all need lard to taste right. It is the secret of French fries of course. Animal fat, it’s why we love bacon so much.
Actually, there were two secrets to McDonald’s French Fries.
First, as mentioned was the type of cooking oil used.
Second was a process called ‘blanching’. The fries were dipped into the hot oil for a short time, then hung on racks to drain.
When needed, they were then dropped back in the oil to cook.
This made them nice and golden brown on the outside and soft (but still cooked) on the inside.