How to Keep Food Cool for a Picnic: Tips & Tricks

A tone mapped HDR image of a picnic setup on t...

Picnic (Photo credit: Wikipedia)

Summer is winding down, but the weather is still nice. One of my favorite warm-weather related activities involving food is a picnic. But the hot weather can cause a lot of problems for keeping your food cool and safe to eat. Fortunately, there are a few simple steps to keeping your food cool on it’s way to your picnic.

Unless you are going to picnic in your backyard, the traditional image of a all your food in a big wicker picnic basket just isn’t a good idea. The basket isn’t going to stay much cooler than the surrounding air. And having things like meat or mayonnaise sitting out in 80+ degrees is a recipe for food poisoning.

Use a Cooler to keep food cool

The first, most absolutely necessary step is to get some sort of insulated container such as an Igloo cooler. The simple act of keeping your food insulated will go a long way in keeping it cool. If you are just taking your food a short distance, this should be sufficient.

Use ice to keep food cool

In the bottom of your cooler, put a layer of ice. By using ice, you are lowering the ambient temperature inside the cooler from the ~40 degrees of your refrigerated food to below 32 degrees. Plus, it takes a LOT of energy for ice to melt. As heat penetrates the cooler, it will first have to overcome the “heat of fusion” to melt the ice before it can start warming the food.

If you are going to need to keep your picnic basket cool for a couple hours, definitely throw in a 5 or 10 pound bag of ice to keep the food cool.

Use salt to keep food cool

The next step in keeping food cool during transportation is salt. Not salt in the food, but salt in the cooler. After adding the ice, pour in a small quantity of water and salt. It seems counter-intuitive. Don’t you put salt on your driveway to melt the ice? Yes, the ice melts, but it cools the mixture down.  Salt water has a lower freezing point than fresh water. As the outside of an ice crystal melts, it reveals the inside of the crystal, which is usually cooler. The upshot is that now you are creating an ambient temperature of around 0 degrees, taking even more time to warm up to a dangerous level.

My boss once left over the weekend a cooler outside filled with a salt-ice-water mixture and cartons of milk. On Monday, there was still ice and the milk was still fresh! Use this method if you are taking an overnight trip and need the food to stay cool for a day or more.

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