Laurent Fortin, 48, was detained in March in Shanghai at the trendy Farine bakery and coffee shop where he had been working since the end of 2016 after moving to China to take up a new job.
Health inspectors allegedly discovered out-of-date flour during a raid on the outlet and a company warehouse. Six Chinese staff were arrested as well as Fortin.
The owner of the group, a successful local French entrepreneur named Franck Pecol, was not in China at the time of the arrests and is believed to be in France.
“We demand the immediate liberation of Laurent as well as the involvement of the French government,” said a petition started on the change.org site by Fortin's family.
His brother, David Fortin, told AFP that the arrested baker had been held in tough conditions without a bed or chair at the start of his time in jail.
If convicted, he faces between a year and 15 years in prison “even though he was just an employee of the company,” his mother told the Paris Normandie newspaper.
The state-run Shanghai Daily newspaper said that Fortin was the production manager for the chain, which has four other outlets.
The shops run by Farine, which means “flour” in French, are popular hangouts for expats and wealthy Chinese consumers in Shanghai, offering traditional French breads and pastries.
The arrests came after a former employee at the chain posted allegations and video on the Chinese social media platform Weibo alleging that Farine was using out-of-date flour imported from France.