Ackman’s Pershing Square Capital Management made its announcement after the close of trading Thursday. The Wall Street Journal reported a person familiar with Ackman’s thinking as saying that he believes the company can go beyond the plans it has already announced to cuts costs and grow revenues, and that it could also sell itself to a rival.

The news is the latest example of big-name investors putting pressure on the processed food industry’s long-established blue chips as they struggle to adapt to a change in consumer tastes towards fresher and healthier products. It comes only four months after Warren Buffett and Brazil’s 3G (in which Ackman is also an investor) teamed up to force Kraft Food Group and H.J. Heinz into a merger.

The merged Kraft Heinz (KFT), which started trading last month, is already being touted as a possible buyer for Mondelez. If so, that would reverse the 2012 deal that created Mondelez by carving it out of the Kraft empire.

For Mondelez, with its globally familiar portfolio of snacks such as Ritz crackers and Cadbury’s chocolate, it’s a case of that deja vu feeling all over again. It’s only 18 months since it wrestled another big-hitting activist investor, Nelson Peltz, into a truce by offering him a seat on its board. Peltz had wanted Mondelez to merge with the snacks division of Pepsico Inc (PEP). He dropped that demand as part of the truce.

It’s not clear how much Ackman spent to accumulate his position, which is built mainly on options and forward contracts. As of last night, Pershing has bought only 3% of Mondelez’ stock directly. However, the position was worth some $5.5 billion at Wednesday’s closing price in New York, and Ackman is likely already sitting on a sizeable profit on paper. Mondelez’ stock has risen by over a third since March, a development which is explained to some degree by last night’s announcement.