So you think your hamburger is safely cooked when it is brown all the way through? Forget it.

Hamburger meat turns brown for reasons that have nothing to do with cooking, according to tests done by the federal government. And that means it can hold bacteria that make you sick.Researchers at the Agriculture Department bought 240 pairs of packages of ground beef from stores around the country. Some patties were frozen and then cooked, and some were cooked without freezing.

The patties were cooked to one of four temperatures: 135 degrees, 150 degrees (both too low to assure safety), 160 degrees (the recommended cooking temperature) and 175 degrees, at which a hamburger has lost most of its juices.

Variations in color occurred even in patties that were cooked at the same temperature.

The researchers found that freezing ground beef and thawing it overnight in the refrigerator greatly increased its chances of turning brown before it was thoroughly cooked.

The simple message here is that you cannot tell by looking if a hamburger is done. If you don't already have an instant-read thermometer (the big ones meant for roasts will not do), invest in one.