Are The Burgers Done? KSU's Hunt Knows

July 15, 1996

If hamburger patties turn brown after being cooked, then that's supposed to be a good sign they're done and ready to eat - or are they? Maybe they are, but consumers should not rely on color alone to determine if the burgers are really ready. Premature browning can mislead people into believing their burgers are done when in fact they are not fully cooked, which is necessary to insure that any harmful bacteria are killed.

Food Safety Consortium researchers at Kansas State University advise that the one reliable way to be sure a patty is finished is to use a small probe thermometer and take the temperature of the center of the patty. If it's 160 degrees Fahrenheit, it's done regardless of what color it is.

"Some patties will still be pink at 160 degrees. Some will be pink even at 170," said Melvin Hunt, a KSU animal sciences professor and Consortium principal investigator. "Color can mislead you on both ends of the scale."

Premature browning is partly a function of how much oxygen hamburger meat gets before it is cooked, and that can be traced to the way it is packaged. Scientists at KSU originally thought that the use of Vitamin E in cattle feed might have influenced premature browning because the vitamin helps extend color life. That turned out not to be the case.

"What actually happens is there are other factors that are so much more powerful than the Vitamin E antioxidant capability that we think are related to premature browning that they totally wiped out any effects that Vitamin E might have had," Hunt said. "The bottom line was that Vitamin E meat can premature brown or can be normal. It depends on how the product was handled ahead of time."

So if premature browning is a given condition with some patties, researchers wondered if there was a way to set a certain cooking time that would guarantee they were done, regardless of how soon they browned. It wasn't possible, Hunt said, because of the numerous variables.

"Low-fat patties cook differently than high-fat patties. If they cook from a frozen state versus a thawed state, the cooking times are different. So it makes it difficult to give recommendations to people in their homes on how long to cook ground beef."

Hunt recommended placing a thermometer in the middle of the patty near the end of the cooking, not throughout the process.

Consumers shopping for ground beef can make some educated guesses about the likelihood that some burgers would brown prematurely, thanks to conclusions determined by the KSU researchers. The key is in myoglobin, the pigment present in meat that carries oxygen to the cells. Myoglobin can appear on the surface of meat in three basic colors, Hunt said.

"If there is no oxygen present, as would be the case in a vacuum package, the meat ought to appear purple red," Hunt said. The meat has a pigment called deoxymyoglobin.

"If the meat has been exposed to oxygen, it's going to appear bright red; we call that oxymyoglobin. If meat has been discolored of if the chemical state of the pigment is oxidized, then it can appear tan to brown" because of the metmyoglobin pigment.

The question then becomes which of the three types of pigment is present in the middle of the patty during cooking. "If it is the purple color - which you would typically expect of some patties if you have oxygen excluded from the center of the patty - then it will give you a red to pink to brown cooked color formation that is reliable.

"But if it is either bright red on the inside or has some degree of brownness on the inside, then the cooked appearance is not particularly reliable," Hunt said. "Deoxymyoglobin, the purple one, will give you a reliable cooked color indicator. If it is oxymyoglobin or metmyoglobin, that's where the premature browning will occur."

Hunt warned that ground beef purchased at a grocery store may have all three pigment forms in it, appearing red on the top with possibly some brown layers underneath and purple in the middle. If a consumer takes a pound of ground beef and divides it into four quarter-pounders, it would not be unusual for the segments to have all those pigments.

The problem in determining whether a particular patty is likely to brown prematurely during cooking depends on what pigment is predominant. "That's the part we don't have fully defined yet: how much of that purple pigment must be there to give a reliable indicator," Hunt said. "Our bottom line is to try to get a measure on the temperature."

Color would be a reliable indicator if ground beef were vacuum packaged for a sufficient amount of time so that it becomes purple with the deoxymyoglobin pigment. But vacuum packaging is not likely to catch on as a trend in the industry.

"There's an extremely large tonnage of ground beef that's not sold in the vacuum package and the industry isn't moving toward that," Hunt said. "The disadvantage is that it's hard to get patties that are completely reduced to the deoxymyoglobin state because the grinding process itself is an oxygen-incorporating process. When you grind it, it becomes very bright red all the way through and the only way to get it back to the purple state is to put it in a vacuum for a long enough period of time so the pigment can burst back to a purple state."

But that process is not easy to perform and probably won't be implemented, Hunt noted, because "the machinery is not out there."

The final recommendation from the KSU team and the USDA Food Safety and Inspection Service remains that consumers should monitor the endpoint temperature of patties to be sure that potentially harmful bacteria are killed.

Food Safety Consortium

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