McDonald's says onions from Taylor Farms in Salinas are the source of the E. coli food poisoning that sickened 49 people in 10 states. One person has died.
Federal investigators believe the raw sliced onions are to blame for the deadly outbreak.
The U.S. Centers for Disease Control and Prevention said the quarter pounders are linked to the 49 cases of E. coli food poisoning.
McDonald's pulled the quarter pounder burgers off the menu at 20% of its stores in the U.S. And now, other fast food restaurants, including Taco Bell, Pizza Hut, KFC and Burger King are pulling onions from some menus.
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According to McDonald's officials, Taylor Farms of Salinas sent onions to one distribution facility.
Taylor Farms has recalled peeled whole and diced yellow onions for potential E. coli contamination.
The recalled onions went from Salinas to a Taylor Farms facility in Colorado, according to a U.S. foods spokesperson.
Taylor Farms issued a statement Thursday saying: "We test both raw and finished products for pathogens and have found no traces of E. coli. We have never seen E. coli O157:H7 associated with onions in the past."
At the Thursday farmer's market at San Francisco's Ferry Building, a local grower talked about how production at small farms differs from larger ones, such as Taylor Farms.
RELATED: 1 dead in multistate E. coli outbreak tied to McDonald's Quarter Pounders, CDC says
Ramon Rojas is with Rojas Family Farms in Tulare by Fresno.
"We produce very small amounts of fruit, which gives a us a lot more time, more attention to detail," Rojas said. "What we try to do is as much quality control as we can. Look over the crops. And again, in big companies, you may not have that same attention to detail."
"E. coli hasn't been linked to onions before," said Dr. Peter Chin-Hong, professor of Medicine at UCSF. He said this E. coli outbreak is an important reminder of the importance of rigorous food safety standards for all producers.
"In 1993 when big outbreak in meat and Jack-in-the-Box, which led to more improvements in meat production. Now with more vegetables being implicated, we need to think of other things and have new strategies, like thinking of the irrigation system to keep our food supply as safe as possible," Chin-Hong said.