The hedge fund manager is likely to push for the company to cut costs or sell itself to a rival, according to The Wall Street Journal.
One potential suitor is the recently-merged Kraft Heinz company - now the fifth-largest food company in the world.
The merger of Kraft and Heinz was orchestrated by 3G Capital, which is helmed by Brazilian billionaire Jorge Lemann, and Warren Buffett's Berkshire Hathaway.
Ackman himself is an investor in 3G's funds.
The private-equity firm has wasted little time in shaking up the upper ranks of the newly-combined company, and has a reputation for savage cost-cutting.
Lemann, a Swiss-Brazilian, has gone from journalist to national tennis champion to banker and now billionaire investor. Buffett likes to call him, "Georgie Paolo."
"Money is simply a way of measuring if the business is going well or not, but money in and of itself doesn't fascinate me," Lemann said in January 2008, according to an interview published in HSM Management magazine.
Shows you that he's just in it for the love of the game.