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Aramark, which operates businesses including food service, uniforms, and facilities management, in August accepted a $6.3 billion buyout offer from a group led by the company's chairman and several investment funds.
In addition to higher leverage, Fitch says it expects significantly lower free cash flow after the buyout.
Aramark last month sold $1.78 billion of notes to help fund the buyout.
Fitch lowered Aramark's issuer default rating two notches to "B," five levels below investment grade, from "BB-minus."
The outlook is stable, however, indicating that Fitch does not expect to make further rating changes in the next one to two years.
Aramark's bonds did not trade on Friday, according to MarketAxess.