The woman scooped up a mouthful of the steamy stew and bit down on the digit before spitting it out, Joy Alexiou, spokeswoman for the Santa Clara County Health Department, said Thursday.
Alexiou said the county coroner is attempting to find a fingerprint match in an electronic database.
Ben Gale, director of environmental health for the county, said officials are also conducting a "trace back," following the chili ingredients to their manufacturers.
Employees at the Wendy's were asked to show investigators their fingers after the Tuesday night incident. All employees' digits were accounted for, officials said, adding that the well-cooked finger may have begun its journey in a food processing plant.
Wendy's spokesman Denny Lynch said company officials met Thursday with authorities in San Jose to discuss the results of their internal investigation.
"All of our employees have 10 digits," Lynch said, adding that there have been "no finger-related" accidents reported to the Occupational Safety and Health Administration from any supplier of chili ingredients to Wendy's.
"By law, you can't hide that sort of stuff," he said. "All of our chili suppliers report no accidents."
"We have found nothing to support the allegations that we are the source" of the finger, Lynch added, noting that health officials will eventually discover where the digit originated.
"They'll be able to tell, as gross as it sounds, if it was from a cadaver or not, for example," Lynch said. "We want to find the truth."
However, Gale cautioned that officials will likely find where the finger entered the ingredients within three weeks, but he added there's no guarantee authorities will ever determine who lost the digit.
Health officials said the fingertip is approximately an inch-and-a-half long. They believe it belongs to a woman because of the long, manicured nail.
The county's health department, sheriff's department and environmental health department were working on the case Thursday.
Alexiou said the woman, who asked officials not to identify her, is at minimal risk of contracting illnesses from the finger.
"It's an extremely low chance because the chili was cooked at a very high temperature that would have killed anything in the finger," Alexiou said.
Nonetheless, she said health department officials would ask the woman's doctor to test her blood "to make sure nothing got passed to her."
"She was so emotionally upset once she found out what it was," Alexiou said. "She was vomiting."
The Wendy's restaurant was closed for a few hours Tuesday while investigators seized the remaining chili and its ingredients.
Asked whether customers were still buying chili Thursday, an employee at the Wendy's said only, "We always sell a lot of chili."