The company employs around 250 workers but has no union at United Wholesale since it was created in 2001.
The Scottish Labour leadership hopeful said no union had ever asked to set up a collective pay bargaining unit at the company.
According to Labour’s general election manifesto in June said that if the party was in power it would ensure public contracts only went to companies meeting the “high standards we should expect of all businesses”, including “recognising trade unions”.
The Herald, who initially reported the story, could leave the leadership hopeful under fire, with the latest development raising further concerns about the credibility of the candidate.
The Glasgow list MSP owns almost a quarter of the shares in UWS, a cash and carry giant which pays staff below the “real living wage” of £8.45 an hour promoted by Labour.
However, he is not a director and has no role in the running of the business.
Despite being questioned, Mr Sarwar did avoid questions on the subject before later releasing a statement.
He said: “I have consulted with UWS, and what they tell me is there are several members of their employee base who are members of trade unions.
“No trade union has requested recognition. But if any trade union did request formal recognition that of course would be welcomed by the company.”
Companies need not recognise unions, but a union can apply for compulsory recognition.
An SNP spokesperson said: “These damaging revelations about Anas Sarwar’s business interests suggest he is more interested in making a big profit for himself than advancing the rights of working people.
“You cannot credibly claim to support a real living wage and trade union recognition for workers when you don’t provide it at your own company.”