A food production company in Sherwood Park, HQ Fine Foods, is trying to determine how their product was contaminated with Listeria.

"We're working diligently to try to identify that, if we can," said Steve Sposari, president and CEO of parent company Premium Brands Holdings' sandwich division.

On Dec. 28, the Canadian Food Inspection Agency recalled 36 varieties of sandwiches produced at HQ Fine Foods.

That followed an initial recall of 19 varieties of sandwiches a week earlier.

The mainly submarine-style sandwiches are sold under the labels Hygaard, Quality Fast Foods, Made To Go, Best Value and Fresh and Local, mainly at convenience stores in Alberta.

Sposari said the company believes the contamination is restricted to the Edmonton plant, and that the bacteria was detected on one assembly line.

"We think we have a pretty good idea but we have not had that finalized."

Since the recall, Sposari said, all tests have come back negative.

The recalled sandwiches carry a best-before date of Feb. 1, 2018.

The recall is nationwide because the products are shipped across the country, said Aline Dimitri, executive director of food safety science directorate at the CFIA.

Listeria is a worrisome bacteria. - Aline Dimitri, CFIA

"We detected it in a sample sandwich," Dimitri said. Though she didn't know exactly where, she noted CFIA inspectors test either at the company itself or at a retail store.

From there, she said, they work to unravel the origin of the contamination.

She said the bacteria doesn't change the smell or look of the food.

"Listeria is a worrisome bacteria," she said.

Anyone who may have eaten the products and is experiencing symptoms such as vomiting, nausea, persistent fever, muscle aches, severe headache and neck stiffness, should go to the doctor, she said.

The recalled products should be thrown away or returned to the store where they were bought, the CFIA advises.

Sposari said the company is trying to reassure customers the safety processes are robust.

"Our track record speaks for itself," he said. "We will ensure and maintain the highest quality standards."

The company faced similar Listeria recalls in 2009, 2011 and 2015.

Sposair said about 200,000 sandwiches are shipped out in one day around Canada.

"Super unfortunate that we're dealing with this right now but it's just a reality for anybody that's in the food business that this can happen."

Dimitri said the CFIA could expand the recall further as more tests are done.

There have been no reported illnesses linked to the products.