How long you spend shopping in stores could put your fresh purchases at risk.
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These days, grocery shopping is less and less of a "one and done" activity. Competition is extra strong right now thanks to discount stores, but finding the best deals often means divvying up your shopping list between two or more places—which saves money, but takes time. 76 percent of shoppers visit upwards of three stores per shopping trip, with low prices at the top of the list for why they do so, according to recent retail research.

But that often means your food spends more time in the car—and in the summer, when temperatures are at record highs in almost every state, that can be more than just an inconvenience.

The temperatures inside your car (which can rise by nearly 20 degrees every 10 minutes in 70 degree weather, according to data from San Francisco State University) can aid the growth of foodborne bacteria and germs on perishable groceries. How long is too long when it comes to leaving groceries in your car while you shop elsewhere, you might ask?

The United States Department of Agriculture advises that raw meat, for example, shouldn't be left outside of your fridge or freezer for more than two hours. If the weather outside is warmer than 90 degrees, however, officials say perishable purchases need to be refrigerated within an hour after you leave the grocery store.

More on keeping your groceries safe:

Leaving a car loaded with fresh groceries in a parking lot just for 30 minutes in 80 degree weather means the car will be upwards of 140 degrees by the time you return from check out. The kind of bacteria linked to foodborne illnesses thrive in these conditions—heat and humidity—and the USDA says proper food spoilage can easily occur at any temperature under 140 degrees.

Consumer Reports has published a guide to help shoppers keep their purchases safe from spoiling in the heat, mostly comprised of straightforward tips—like parking in the shade if possible, and planning your day carefully so you don't dash off to run an errand that you didn't plan for. They also advise you to purchase insulated bags—varieties with thicker linings such as The Premium Thermal Bag—and to bring them inside with you so they remain cool while you shop. If you're going to be shopping for longer than two hours, some experts suggest taking the time to stock your perishable groceries in a cooler lined with ice.

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But one expert asks shoppers to take the extra time to check their meat and fish, as these items are the most risky when it comes to foodborne illneses. "Any increase in temperature allows bacteria to multiply, especially the type that causes food poisoning," says James E. Rogers, Ph.D., Consumer Reports' director of food-safety testing and research. If you take any precautionary measures, focus on meat and fish first: milk and dairy products aren't as risky given that they've been pasteurized, Rogers says. And consider scheduling your trip so those are the last things you buy.

Another way to reduce spoilage is to bag your frozen goods together, separating meat and fish to avoid any cross contamination. And one important tip for those who drive cars equipped with partitioned trunks—always put your groceries in the main cabin, especially if it's air conditioned.