If you think those shopper loyalty cards weighing down your wallet or key chain are only good for special discounts, you’ve overlooked a completely different sort of potential benefit.

Those colorful bits of bar-coded plastic also give stores a way to track your purchases that can help federal investigators hunt down the source of foodborne illness. And they can be used by the retailers to quickly let you know if food you bought has been recalled.

But there are broad differences among stores on how they use the cards to shore up food safety. And two consumers, backed by the Center for Science in the Public Interest, are putting Safeway‘s Club Card to the test.

They filed a lawsuit Wednesday in a California court alleging that Safeway failed to notify them about tainted peanut butter and eggs they bought, despite having access to their contact information through their club cards.

One mom, Dee Hensley-Maclean of Montana, bought peanut butter crackers and cookies from Safeway that were part of a nationwide recall of products tainted with salmonella. She says Safeway never contacted her about them.

“If Safeway knows that there is a problem, and they know how to get in touch with me, quite frankly I’m astonished that they wouldn’t try to spare me or my children from a preventable foodborne illness,” she says in a statement released by CSPI.

How did Hensley-Maclean find out about the recall? She bought some similar snacks at Costco, which contacted her through her membership card.

(Hensley-Maclean is no relation to our blogger Scott Hensley, although he is prone to talking up his latest Costco finds around the office.)

A Safeway spokeswoman declined to discuss the details of the specific case, but said that company does notify customers about serious recalls in a variety of ways. “As a policy we do and have used club card data to contact customers,” spokeswoman Teena Massingill tells Shots.

One problem, she says, is that not all retailers’ shopper cards work the same way. Some, like Safeway’s, don’t require a contact number, she says. Costco, on the other hand, requires a membership and generally requires people to fork over more personal data.

Even with adequate access to personal data, the systems don’t always work smoothly. Our NPR colleague Cathy Duchamp tells us she purchased some fresh spinach from the local Giant grocery store last Sunday and swiped her shopper loyalty card at checkout.

She received a robocall from the store, alerting her to a recall of the spinach on Wednesday night. That call came one day after the recall due to listeria contamination was announced by FDA.

By then, though, Duchamp’s family had already eaten most of the spinach. Luckily, no one got sick. “To me, if they’re collecting the information, they should have to use it,” she says.

The new food safety law will require grocery stores to do a better job notifying customers about recalls, and we’re betting those shopper loyalty cards will play a big role. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.