I took this from yahoo news.

When to shop: If possible, go grocery shopping as your last errand before you head home. If you must run other errands, put a cooler in your car and buy a bag of ice to keep the perishables chilled until you get back home. In fact, in hotter climates, you need a cooler in the car even if shopping is your last errand of the day. Forty-five minutes in summertime traffic can render meat rancid, milk spoiled and butter liquid.

While you’re shopping: Put meats or fish in plastic bags before you stick them in your cart so they don’t drip on the vegetables or pantry items.

Read more here.