Every day most Americans—usually without realizing it—eat food that Cargill has touched in some way. With $120 billion in sales, it is the largest private company in the U.S. and has its hand in almost every part of the food chain, from providing feed to livestock producers to selling ingredients to manufacturers. MacLennan is remaking the 151-year-old company to suit shifting consumer preferences. Cargill has reduced its use of antibiotics in turkeys and cattle, and created a GMO-free corn syrup, among other things. The shake-up is needed: Results have slumped in part because of a slowdown in emerging markets.