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Big Food Sees Big Changes

Three years after the old Kraft Foods split in half with the backing of activist investors, one of the activists is suggesting those businesses ought to reunite. The move is a testament to the seismic changes that have turned the food industry upside down in that short time.

The notion of a reunion is partly behind William Ackman’s investment in Mondelez International Inc., MDLZ 1.12 % the global snacks giant that Kraft became after spinning off its North American groceries business in 2012. Mr. Ackman’s Pershing Square Capital Management LP revealed late Wednesday it has built a $5.5 billion stake in Mondelez, 7.5% of the company.

Mr. Ackman believes Mondelez must cut costs significantly or sell itself to a rival, according to people familiar with the matter. They identified as one potential buyer Kraft Heinz Co. KHC -0.89 % , the company formed last month when H.J. Heinz merged with the old Kraft grocery-brand business.

The wild card is 3G Capital Partners LP, the Brazilian private-equity firm that acquired control of Heinz in 2013 and engineered the Kraft-Heinz merger.

3G is unlikely to explore a merger of Mondelez and Kraft Heinz immediately, said people familiar with the matter, given that Kraft and Heinz closed their merger in July. Kraft Heinz has just begun its integration process, which is expected to involve huge changes to the former Kraft operations that had revenue last year of $18.2 billion. Even given 3G’s ambitious bent, it could be some time before it would be ready to digest the far larger Mondelez, whose sales last year topped $34 billion, they said. One person said there was no pressure for 3G to rush to weigh Mr. Ackman’s thesis.

The recent run-up in Mondelez’s shares could make it too pricey for the investors, some of the people added. On Thursday, Mondelez’s trailing 12-month price-to-earnings ratio was 37, compared with Heinz’s valuation of 19 the day before its takeover was announced, and Kraft’s respective multiple of 35.

Shares of Mondelez edged up 1.1% Thursday, indicating investors had a...