• Berkshire Hathaway's 27% stake in the food giant shrank in value to $8.7 billion from $9.8 billion between Thursday and Friday.
  • Kraft Heinz reported sharp declines in fourth-quarter sales and profits on Thursday.
  • Fitch cut the credit rating on Kraft Heinz bonds to "junk" on Friday.
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Warren Buffett lost more than $1 billion this week after Kraft Heinz stock plummeted 11% in two days.

The famed investor's Berkshire Hathaway conglomerate owns about 27% of the food giant, according to Bloomberg data. The value of its stake plunged to $8.7 billion from $9.8 billion between Thursday and Friday, after a one-two punch of disappointing earnings and a credit-rating cut pummeled Kraft Heinz shares.

On Thursday, the maker of Heinz ketchup and Kraft macaroni and cheese revealed a 5% drop in fourth-quarter net sales and a 14% slump in adjusted earnings per share. CEO Miguel Patricio said the performance was "disappointing" in a press release .

On Friday, Fitch cut Kraft Heinz's credit rating to BBB- from BB+, downgrading its bonds to non-investment grade or "junk," Bloomberg reported . The credit-rating agency cited the company's large debt pile, dwindling profits, and negative outlook.

Kraft Heinz stock has nosedived almost 75% from its peak in February 2017, underscoring the magnitude of Buffett's mistake.

The so-called "Oracle of Omaha" partnered with private-equity group 3G Capital to buy Heinz for about $28 billion on Valentine's Day in 2013. The pair then cofinanced Heinz's $50 billion mega-merger with Kraft Foods in 2015. Kraft-Heinz's market capitalization is currently less than $33 billion.

"We try to buy good businesses at a decent price, and we made a mistake on the Kraft part of Kraft Heinz," Buffett said at Berkshire's annual meeting last year."We paid too much money."

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