First it was tomatoes. Now it's beef.

Fourth of July picnic tables are getting a careful look as familiar hamburgers are feared to be among the tainted ingredients in separate food safety scares.

On Wednesday, The Kroger Co. expanded its voluntary recall of some ground beef products to its stores in more than 20 states, saying the meat may be contaminated with E. coli.

The nation's biggest traditional grocer also urged customers to check the ground beef in their refrigerators and freezers to determine whether it is covered by the recall.

The warning comes as federal investigators try to pinpoint the source of a separate salmonella outbreak linked to tomatoes that has sickened nearly 900 people, raising more questions about the nation's food safety system.

While insisting that tomatoes remain the leading suspect, investigators are looking at other produce but remain mum on exactly what vegetables are getting tracked.

But don't call off your Fourth of July plans just yet.

Dr. Manny Alvarez, managing health editor for FOXNews.com, offered these tips for safe summer barbecues:

— Keep your meat separate from your vegetables

— Keep uncooked meats separate from cooked meats

— Clean all cooking surfaces

— Wash your hands with hot, soapy water before and after handling food

— Heat the coals on your grill for 20 to 30 minutes before cooking meat

— Use a meat thermometer to make sure the food reaches a safe internal temperature

— Don't give raw meat to children, seniors or people with chronic diseases - they should only eat meat that is well done.

"I would still argue that the American food supply is one of the safest," Alvarez said. "But, some of the responsibility has to be shared with the consumer. People need to develop good hygenic processes."

Kroger's recall stems from meat obtained from one of Kroger's suppliers, Nebraska Beef Ltd., that has been linked to illnesses reported in Michigan and Ohio between May 31 and June 8 caused by E. coli bacteria.

Nebraska Beef has recalled from wholesalers and other processing companies nearly 532,000 pounds of ground beef produced on five dates between May 16 and June 24.

Kroger said Wednesday that as a precaution it removed from stores all ground beef supplied by Nebraska Beef marked with sell by dates of May 21 or later.

"Ground beef in stores today comes from other suppliers not involved in the recall," Kroger spokeswoman Meghan Glynn said Wednesday.

The Cincinnati-based company initiated a recall June 25 for Kroger stores in Michigan and in central and northern Ohio. The expanded recall includes ground beef sold at Fred Meyer, QFC, Ralphs, Smith's, Baker's, King Soopers, City Markets, Hilander, Owen's, Pay Less and Scott's with overlapping sell-by dates from mid-May through mid-July.

In some stores, the recall includes products in Styrofoam tray packages wrapped in clear cellophane or purchased from an in-store service counter. It does not include ground beef sold in 1-, 3-, or 5-pound sealed tubes or frozen ground beef patties sold in the frozen food section of its stores.

Kroger is notifying customers about the expanded recall by placing signs in stores in meat departments. It also is using its register receipt notification system.

Kroger can track purchases by customers who use the company's loyalty card, which entitles customers to certain discounts. Sometimes those customers receive information about products the next time the card is used and a receipt is issued, Glynn said.

In other cases, Kroger is able to call customers who used the loyalty card to purchase a tainted product, and it is doing that with the ground beef recall, Glynn added.

Dr. Marc Seigel, a FOX News medical contributor said 76 million Americans get food poisoning each year.

Click here to watch Seigel discuss food safety on America's Newsroom.

"Make sure the meat isn't turning gray," Seigel cautioned, "and smell is a big thing."

Seigel said the following are symptoms of food poisoning:

— Fatigue and weakness

— Diarrhea and/or vomiting

— Abdominal cramping

— Fever

— Dehydration

"If any of these symptoms are persistent, it's important to see your physician," Seigel said. "Antibiotics might be necessary. But, it's important to keep well-hydrated, especially in the heat."

The Associated Press contributed to this report.