Expert: Walter Hintz - 3/31/2008

Question
Hi:

Why is old blood brown? Is brown blood oxygenated or deoxygenated? When fresh, deoxygenated blood is dark bluish-red while oxygenated fresh blood is bright red. Does the color of stale blood vary according the amount of oxygen in it or is it always brown?

Also, what color does fresh human hemoglobin turn when heated? Does it turn brown? I've heard that the color does not change because the iron continues to maintain the same oxidation state. If that is the case, then why does red meat turn brown when boiled? Why does pink meat turn white when boiled? If the hemoglobin maintains redness even when boiled [due to the heat-tolerant oxidation state of iron], then why does the iron in myoglobin [the hemoglobin in muscles and meat] lose its redness when heated? Iron is still iron regardless of what protein it is attached to.


Thanks,

Green

Answer
Hi green
There is a multitude of questions here so I start with the basics.  Hemoglobin is red in color.  In oxygenated blood (arterial blood) is is bright red and in venous blood it is darker, almost a bluish, which has led to the myth that blood is blue until it is exposed to air.  When hemoglobin breaks down one of the by-products is bilirubin which is a yellow-brown pigment.  The amount of O2 is not a factor in color changes in old blood.
With regard to the changes in blood during cooking I am afraid that I do not have any answers.