Activist investor Bill Ackman is paying approximately $5.5 billion for a 7.5 percent stake in snack maker Mondelez International.Ackman's Pershing Square Capital Management LP is buying about 120.3 million shares of the Deerfield, Illinois-based company. He hasn't yet said what, if anything, he intends to urge the maker of Oreo cookies and Cadbury chocolate to do, but the stake raised the possibility of further deal-making among big packaged food makers.Ackman is not the only activist investor with a stake in Mondelez. Nelson Peltz's Trian Fund Management LP holds a 3.1 percent stake in the company, according to FactSet. Peltz is a Mondelez board member. Mondelez, the maker of products including Oreo cookies, Trident gum and Cadbury chocolate, split off from Kraft in 2012. Kraft kept the North American grocery brands and Mondelez took the snacks. Kraft has combined with H.J. Heinz to form The Kraft Heinz Co., creating one of the world's biggest food companies with brands such as Jell-O, Oscar Mayer, Velveeta, Ore-Ida and their namesakes.Heinz' owners, Warren Buffett's Berkshire Hathaway and the Brazilian investment firm 3G Capital, engineered the deal.