J. Kenji Lopez-Alt, chef and food scientist, thinks he's figured out the ingredients for the chain’s famous burger. He deduced that the secret sauce is made by combining 2 tablespoons plus 2 teaspoons of mayonnaise, 1 tablespoon of ketchup, and 2 teaspoons of pickle relish, a splash of vinegar, and a pinch of sugar.
Not only is its secret sauce compared to Thousand Island, but also McDonald’s famous Big Mac sauce. A closer look at the ingredient list shows one significant difference between the two: while McDonald's, like In-N-Out, includes sweet pickle relish and mayonnaise in their secret sauce, McDonald's doesn't include ketchup or any tomato products at all.