By Wailin Wong | Sara Lee Corp. made a surprise announcement Friday that Brenda Barnes, its chairwoman and chief executive, is going on a temporary medical leave effective immediately.

The Downers Grove-based food company declined to provide further information on Barnes’ condition or the expected length of her absence.

“We wish Brenda a speedy recovery and look forward to her return,” James Crown, lead independent director at Sara Lee who will be taking over as chairman of the board, said in a statement. “Out of respect for Brenda’s privacy, we will not be commenting further.”

Sara Lee’s shares were unchanged in after-hours trading.

Marcel Smits, the company’s chief financial officer, will serve as interim CEO. Sara Lee also said it has created an Office of the Chairman “to ensure the effective execution of strategic initiatives.” In addition to Crown and Smits, the group includes Christopher John Fraleigh, who is CEO of North American Retail and Foodservice. Meanwhile, Mark Garvey, Sara Lee’s corporate controller and senior vice president of global business services, will become interim CFO.

“It seems their growth team — their key C-level team — is taking over, and they all know the strategy and are employing it,” said Christopher Shanahan, an analyst at Frost & Sullivan. The full impact of Barnes’ absence will likely depend on how long she’s gone, Shanahan added, but “in the short term, there’s not going to be a whole lot of changes.”

Barnes, 56, joined Sara Lee as president and chief operating officer in July 2004 and became CEO in February 2005. She put in place a multiyear turnaround plan to boost revenues and profit margins.

Timothy Ramey, an analyst at D.A. Davidson, credits Barnes for having “gotten this company on track.”

“It’s always disruptive to have a quality CEO have to step aside for whatever reason,” Ramey said. “But hopefully it’s something she can deal with and resume (working).”

Under Barnes’ watch, the company has divested a number of business lines in the last several years. It has also launched a major stock buyback program this year that will total about $1 billion in 2010 and between $2.5 billion and $3 billion over three years.

Last week, Sara Lee reported quarterly results that beat analysts’ expectations.

“It’s really the outcome of several years of work,” Barnes said in the earnings call. “And at the time, I realized there was some question whether or not we could get there, and I’d like to say I think we’ve gotten there. And I think we have the model that actually can sustain itself.”

Shanahan noted that “the ball never stopped rolling” at Apple Inc. even when CEO and co-founder Steve Jobs –- arguably the CEO whose individual persona is most intricately linked with a company’s image –- took a medical leave of absence in early 2009.

Even if “we don’t know the degree to which (Barnes) is away. … It doesn’t mean her influence is not still felt,” Shanahan said.