Metmyoglobin is a naturally occurring pigment found within muscle tissue, responsible for the color of meat. This protein contains a heme group, which is a complex iron-containing compound that interacts with oxygen. The state of this iron within the heme group directly influences the visual appearance of meat. Metmyoglobin’s presence contributes to the brown or grayish-brown hues often observed in meat.
Understanding Meat Color Changes
The vibrant red color commonly associated with fresh meat comes from myoglobin, a protein abundant in muscle cells. Myoglobin’s iron atom is in a reduced state (Fe2+) and can bind reversibly with oxygen, forming oxymyoglobin, which gives meat its bright cherry-red appearance. This oxygenated state is what consumers look for in fresh cuts.
Over time or under certain conditions, myoglobin undergoes a chemical change known as oxidation. During this process, the iron atom in myoglobin loses an electron, transitioning from a reduced (Fe2+) to an oxidized (Fe3+) state. This oxidized form, known as metmyoglobin, cannot bind oxygen, leading to the characteristic dull brown or grayish-brown color.
Factors Leading to Metmyoglobin Formation
Exposure to oxygen is a primary driver of metmyoglobin formation, particularly when oxygen levels are either too low or fluctuating. While a small amount of oxygen initially creates the desirable red oxymyoglobin, prolonged exposure or an insufficient supply can lead to oxidation. Temperature also plays a significant role; higher temperatures increase the rate at which myoglobin oxidizes into metmyoglobin, meaning meat stored at room temperature will brown much faster than refrigerated meat. The duration of storage directly correlates with metmyoglobin accumulation, as the oxidative process is continuous over time. Additionally, slight changes in the meat’s pH can influence myoglobin’s stability, making it more susceptible to oxidation.
Is Metmyoglobin-Containing Meat Safe to Eat?
The presence of metmyoglobin is a natural chemical change and does not inherently indicate spoilage. This color alteration is a result of the iron in the meat’s pigment oxidizing, similar to how an apple browns after being cut.
Meat that has developed a brown color solely due to metmyoglobin formation is safe for consumption, provided other signs of spoilage are absent. Consumers should not rely on color alone to determine meat safety.
Instead, it is important to assess other spoilage indicators, such as an off-odor, a slimy or sticky texture, or the presence of mold. A sour, putrid, or ammonia-like smell is a strong indicator of spoilage, regardless of the meat’s color. If any of these spoilage signs are present, the meat should not be consumed, even if it still appears somewhat red.
Preventing and Minimizing Metmyoglobin Formation
Proper storage is an effective way to slow down the formation of metmyoglobin in meat. Storing meat in airtight packaging minimizes its exposure to oxygen, which is a key factor in the oxidation process. Vacuum-sealed bags are particularly effective as they remove most of the air, significantly extending the meat’s bright red color.
Maintaining a consistent low temperature, ideally at or below 40°F (4°C) in the refrigerator, slows metmyoglobin formation. For longer storage, freezing meat at 0°F (-18°C) or below largely halts the oxidation process, preserving its quality and appearance for extended periods. When thawing, it is best to do so slowly in the refrigerator to minimize temperature fluctuations that can encourage browning.