Food stamp recipients in 17 states lost access to the electronic system used by stores to verify their benefits on Saturday, leaving many unable to buy groceries, the company that manages the system said. People enrolled in the government food assistance program use plastic vouchers similar to debit cards. Starting at about 11 a.m. EDT (1500 GMT), some of those cards stopped working, Xerox spokesman Kevin Lightfoot said. A power outage that started the problem was fixed within 20 minutes, Lightfoot said, but shoppers continued to run into difficulties throughout the day. By early evening, the problem still had not been fixed.