A piece of roast beef I took out of the deli bag looked green when I looked at it. The beef looked like it had a rainbow on it when I held it up to the light.
This shiny display of colors made me wonder if the meat had gone bad. We ate it anyway because it didn’t smell bad or feel sticky, which are two signs that meat has gone bad.
Later, when I told our Senior Science Editor Paul Adams about my worry, I found out that this iridescence has a cool reason for happening, and a lot of other people have also been curious about it.
You just pulled a beautiful roast from the oven. The outer crust is a perfect golden brown. But when you slice into it the meat has an odd green iridescent sheen. This can be alarming at first but there’s no need to panic. The greenish cast is simply a natural process of gases, iron and pigments in the beef reacting to heat. Read on to learn exactly why roast beef can turn green.
Myoglobin Causes Redness in Meat
Raw beef has its characteristic red color thanks to a protein called myoglobin. This protein stores oxygen in muscle cells, similar to how hemoglobin in blood carries oxygen. Myoglobin makes up 1-3% of the compounds in meat.
In living muscle, myoglobin is a rich dark purple when oxygenated. After slaughter when oxygen decreases, the myoglobin becomes a vibrant cherry red.
Heat and pH Changes Alter Myoglobin
When meat is cooked, the structure of myoglobin changes. Heat causes the globin portion of the molecule to release, leaving just the iron-containing heme portion.
The iron atom in heme can change oxidation states in response to heat. This causes shifts in color from red to tan to brown.
Changes in pH from aging and curing also lead to color changes. Acidic environments cause more denaturation of myoglobin into hemichrome, leading to graying.
Green Coloration Comes from Gases
Two naturally occurring gases are mainly responsible for the greenish tinge in cooked meat
Hydrogen Sulfide – This gas results from sulfur-containing proteins breaking down with heat. It has a distinct rotten egg odor.
Carbon Monoxide – Formed when carbon dioxide interacts with hot meat. Responsible for the green/gray hue.
When these gases interact with iron atoms in myoglobin, they form new pigment compounds that reflect light differently, causing a greenish iridescence on the meat surface.
Why Do Some Cooked Meats Not Turn Green?
The amount of gas penetration impacts greening. Conditions like cooking method, meat pH, and moisture all affect this:
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Dry cooking methods like grilling allow more gases to interact with meat.
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Lower pH from aging reduces sulfides, lowering greening.
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Added moisture prevents gas penetration into meat.
So meat that is aged, cooked sous vide or braised in liquid is less likely to appear green. But dry roasted beef is prone to more sulfide and carbon monoxide reactions.
Fat Content Also Plays a Role
Higher fat content in a roast can also increase the likelihood of greening:
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Fat reduces moisture at the meat surface.
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Fat crackling leads to small cavities that allow more gas penetration.
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More unsaturated fat leads to increased carbon monoxide binding to myoglobin.
So leaner roasts will often stay redder, while fatty prime rib is more susceptible to a green/gray tint after cooking.
Is Green Meat Safe to Eat?
The green iridescence on cooked roast beef is harmless and purely cosmetic. Since gases cannot penetrate deep into the meat, the green color is just on the surface. The interior meat maintains its normal color.
Meats cured with nitrates like corned beef can also take on a greenish cast when cooked. Again, this is harmless and expected.
As long as the meat reaches proper cooking temperatures to kill bacteria, the green portions are completely safe to eat. The odd color does not indicate spoilage or contamination.
Preventing Green Coloration in Roast Beef
If you prefer your roast beef without the greenish cast, there are a few preventative measures you can take:
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Cook at lower temperatures (200°F vs. 350°F) to reduce sulfides.
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Roast meat in a covered pan to limit gas exposure.
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Wrap in foil after cooking for better moisture retention.
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Limit aging time to 2-3 weeks maximum to prevent excessive myoglobin breakdown.
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Choose leaner cuts of meat over prime cuts.
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Opt for wet cooking methods like braising.
While green beef may not be visually appealing, keep in mind it does not affect flavor or safety. But following these tips can help you achieve the perfect red roast beef you crave.
When to Worry About Green Meat
Green meat isn’t always harmless. In rare cases, it can indicate bacterial spoilage:
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Unnaturally bright or intense green color.
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Slimy mucus-like texture.
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Strong sulfur or ammonia odor.
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Mold growth.
These signs point to spoilage by hydrogen sulfide-producing bacteria. The meat would smell rotten and be unsafe to eat.
So while green roast beef is normal, very green raw meat prior to cooking should be discarded. Always inspect meat for other indicators of spoilage.
The Takeaway on Green Roast Beef
Seeing green meat can be unsettling, but in a properly cooked roast it is merely cosmetic and harmless. Gases interact with myoglobin to cause the odd coloration. While you can take steps to prevent it, a greenish tinge does not make the beef unsafe to eat.
With a better understanding of the causes, you can rest assured that vibrant green roast is just a quirk of meat chemistry and not a health risk when thoroughly cooked. So sit back and enjoy a special holiday prime rib without concern over its appearance.
[Meat contains iron, fat, and many other compounds. When light hits a slice of meat, it splits into colors like a rainbow. There are also various pigments in meat compounds which can give it an iridescent or greenish cast when exposed to heat and processing.]
Frequency of Entities:
roast beef – 20 times
green – 18 times
meat – 12 times
myoglobin – 5 times
gases – 4 times
color – 6 times
iron – 3 times
heat – 3 times
What Makes a Shimmery Rainbow Appear on Meat?
Usually, color—including the red and brown colors of meat—is caused by the presence of pigment. But iridescence on meat has nothing to do with pigment. The shimmery rainbow is actually due to a phenomenon called structural color. This means the physical structure of an object causes it to diffract light into a spectrum of colors.
When light hits beef muscle fibers at the microscopic level, it can bend and create a rainbow on the cut surface of the meat.
In a similar way, light can also bend when it hits other smooth, flat surfaces, like butterfly wings and peacock feathers.
These rainbows on meat are so common—and so many people are mystified and/or concerned about them—that the USDA has even posted an explanation on its website.
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Why Does Meat Go Rainbow Coloured? | Food Unwrapped
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