Check your fridge's temperature with a thermometer to make sure it's staying as cool as it should.

Your refrigerator seems like a simple enough appliance. Plug it in, let it cool, set the fridge and freezer temperatures, and fill it with food. From there on it should do its job, but things aren't always that easy or simple.

Something as simple as how you organize your refrigerator can affect how long your food stays fresh. And making sure your fridge and freezer are set to the correct temperature is more important than you might think.

The ideal temperature for refrigerators and freezers

Spoiled food isn't the only concern with a refrigerator or freezer that isn't cooling to the correct temperature. Food-borne illness is the bigger threat.

To avoid food spoiling prematurely and keep it safe to eat, the US Food and Drug Administration (FDA) recommends keeping your refrigerator at or below 40 degrees Fahrenheit (4 degrees Celsius). Your freezer should be set to 0 degrees Fahrenheit (-18 degrees Celsius). You want to avoid the "danger zone" between 40 degrees an 140 degrees Fahrenheit (60 degrees Celsius), where bacteria can grow rapidly on food and make you sick.

Some modern refrigerators have built-in digital thermometers in the refrigerator, freezer or both. With those you can set exact temperatures. Many refrigerators have only dials with a numerical scale or one that goes from "cold" to "coldest."

No matter what kind of controls yours has, it's best to periodically check the temperature with a separate thermometer to ensure not only that the units are cooling as they should, but also to double-check the accuracy of the built-in thermometers.

How to check the temperature of your freezer and refrigerator

The thermometer you use for checking the temperature does matter. For a refrigerator, you should use a bulb-style or candy thermometer. You can also use an instant-read meat thermometer.

To check the temperature of a refrigerator, it's best to use food or liquid that has been in the compartment for at least 24 hours. The most common practice is to place a glass of water in the refrigerator (but not in the door) and let it sit for a day. Then place the thermometer in the glass to get a reading. If you're using an instant-read thermometer, you'll have your reading, well, instantly. For other types of thermometers, close the door and check back in a few hours, after internal temperature of the refrigerator has stabilized and the thermometer has had a chance to get a reliable reading.

Since the internal temperature of a refrigerator isn't consistent throughout, you may want to repeat the process and place the glass of water in different places around the refrigerator.

For the freezer, using a glass of water obviously won't work. Instead, simply place the thermometer in the freezer between two pieces of already frozen food in the center of the freezer cabinet. You can also place the thermometer in ice cream. Close the door and wait at least 12 hours to allow the thermometer to get a reading.

When you make an adjustment to the temperature of the refrigerator or freezer, you should give it at least 24 hours for the new setting to stabilize before taking another reading. Repeat the process until you reach the desired temperatures.