Solution to puzzle 5: The red menace
This "puzzle" is based on a real story that I heard form an amazing food chemist, Irwin Taub, who died a few years ago. Irwin was one of those people you could ask any "food question" and get an hour long lecture on the subtleties of biophysics.
Beef is red because of the oxygen carrier myoglobin that is present in the muscle tissue. Chicken has < 0.05% of this protein, but beef has 0.5-2%, hense its intense purple color, the color of Fe(II) in the haeme. When beef is exposed to air, myoglobin binds to oxygen from the air and becomes red. That is why red meet is red. When beef is cooked, Fe(II) in the oxygen form of myoglobin is thermally oxidized to Fe(III), and the iron binds to a water molecule that replaces the distal histidine as it comes off when the protein melts and denatures from heat. This Fe(III) complex absorbs light differently, and it gives cooked beef its familiar brown color, the color of metmyoglobin. The latter is extremely stable and under normal conditions the haeme stays in this form forever. However, irradiation of cooked red meat by gamma-rays reverses this oxidation, as hydrated electrons generated in the meat by the ionizing radiation reduce Fe(III) in the metmyoglobin back to Fe(II). This reaction is so efficient that even short radiation treatment suffices to restore the red color. Irradiated well-done steak tastes exactly like it should, but it looks like a piece of raw meat. If you irradiate this steak further, the color becomes even more garish, as the porphyrin ring of the haeme breaks down and a green (!) pigment is generated. When the US Army started to irradiate its food rations in the early 1970s this peculiar effect was not recognized (the rations were first packed, then irradiated) and their first delivery almost resulted in a mutiny, Battleship Potemkin-style. Irwin was given the task of finding a way of preventing the color change. He spent on this problem 15 years, and -- having run through hundreds of various additives -- gave up on it. As far as I know, this problem still has not been solved. The army ended up coloring irradiated meat using dyes, feeding the animals antioxidants right before the slaughter, and cooking sterilized meat. Keeping the familiar brown color turned out to be an intractable chemical problem.
Irwin had dozens of stories like that. I wish I had written them down, but I was young and believed that people around me would live forever.
Beef is red because of the oxygen carrier myoglobin that is present in the muscle tissue. Chicken has < 0.05% of this protein, but beef has 0.5-2%, hense its intense purple color, the color of Fe(II) in the haeme. When beef is exposed to air, myoglobin binds to oxygen from the air and becomes red. That is why red meet is red. When beef is cooked, Fe(II) in the oxygen form of myoglobin is thermally oxidized to Fe(III), and the iron binds to a water molecule that replaces the distal histidine as it comes off when the protein melts and denatures from heat. This Fe(III) complex absorbs light differently, and it gives cooked beef its familiar brown color, the color of metmyoglobin. The latter is extremely stable and under normal conditions the haeme stays in this form forever. However, irradiation of cooked red meat by gamma-rays reverses this oxidation, as hydrated electrons generated in the meat by the ionizing radiation reduce Fe(III) in the metmyoglobin back to Fe(II). This reaction is so efficient that even short radiation treatment suffices to restore the red color. Irradiated well-done steak tastes exactly like it should, but it looks like a piece of raw meat. If you irradiate this steak further, the color becomes even more garish, as the porphyrin ring of the haeme breaks down and a green (!) pigment is generated. When the US Army started to irradiate its food rations in the early 1970s this peculiar effect was not recognized (the rations were first packed, then irradiated) and their first delivery almost resulted in a mutiny, Battleship Potemkin-style. Irwin was given the task of finding a way of preventing the color change. He spent on this problem 15 years, and -- having run through hundreds of various additives -- gave up on it. As far as I know, this problem still has not been solved. The army ended up coloring irradiated meat using dyes, feeding the animals antioxidants right before the slaughter, and cooking sterilized meat. Keeping the familiar brown color turned out to be an intractable chemical problem.
Irwin had dozens of stories like that. I wish I had written them down, but I was young and believed that people around me would live forever.