I was recently told that corn starch will help break down tough cuts of meat. London Broils were on sale and what the heck, let's give it a try. Although told the meat needed to be sliced in strips, I put a London Broil roast in a gallon zip-lock and covered it well with a half box of corn starch. Two days in the garage fridge. Washed off all the corn starch, patted dry with a paper towel and rubbed both sides of the roast with olive oil then put my normal salt, garlic powder and ground black pepper coating on both sides. Got the grill to high heat, seared the first side, flipped and cooked until the second side was seared and I figured the center was hot enough. It was the most tender London Broil I've cooked. I'll give it three days with the corn starch next time just to further the experiment.