You may sometimes depend on the color of cooked meat to indicate whether it is done or not. While looking at meat as it cooks is a good rule of thumb, other factors can affect meat's color and texture. When exposed to heat, beef undergoes chemical changes that transform the meat fibers from deep red to pink to brown or gray. Other factors can also affect the finished color of beef sausage.
My, Oh, Myoglobin
Meat is made up of muscle fibers. The color of these fibers comes from the types of protein present in the cells. Because muscles used for walking or standing require higher amounts of oxygen than muscles used for, say, breathing or chewing, they contain higher amounts of the oxygen-delivering protein, myoglobin. This richly pigmented protein gives raw beef its dark red color.
Applying heat to protein causes microscopic changes in the chemical makeup. This process, called denaturization, causes the chemical bonds to break. When additional heat is applied, the bonds recombine and change color. When beef is cooked to a high internal temperature, the heat causes the bonds to break down and recombine, which turns the muscle fibers from red to a whitish color. Further cooking changes the whitish color to brown and, if cooked too long, to burnt black. Less heat leaves the bonds unbroken which gives rare meat its red or pink hue.
Sausage is sold in bulk and in links. For beef sausage, the meat is ground and mixed with salt and spices. These spices can subtly affect the color of the cooked meat. When cooked, bulk sausage turns from pink to tan to brown, depending on how long it is heated. Because links are enclosed in opaque casings that don't undergo a dramatic color change when cooked, the finished sausage can look slightly gray.
The Air in There
The exposure to oxygen also affects meat color. The longer meat is exposed, the more the oxygen molecules affect the chemical makeup. This process, called oxidation, causes meat to turn from red to grayish-brown. Because sausage meat comes into contact with air during the grinding and packaging process, some of the red pigment may be gone before you buy it. When the meat has already lost its pink pigment, the cooked sausage looks more gray than brown. As it begins to spoil, the proteins in the meat change from brown to green.