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The reputations of some of the world’s most powerful investors rest on a huge but humdrum challenge: getting consumers excited again about Kraft macaroni and cheese.

Kraft Heinz, the food giant that also makes Jell-O and Oscar Mayer meats, surprised Wall Street on Thursday when it announced poor earnings, a multibillion-dollar write-down and an accounting investigation by the Securities and Exchange Commission. Kraft’s shares plunged 28 percent on Friday, lopping more than $16 billion off the company’s stock market value.

3G Capital, the Brazilian investment firm that led the merger of Kraft and Heinz in 2015 and steers the combined company, has taken a big financial hit on its 22 percent stake in Kraft. Warren E. Buffett’s Berkshire Hathaway has an even bigger stake in Kraft and lost an estimated $4 billion on Friday.