Your question is a great reminder that color is not a
reliable indicator of doneness in ground beef. A ground
beef patty or meatloaf cooked to the required temperature
of 160 F (71 C) is safe. Yet under certain conditions
it may still be pink in color.
You're right, this may be caused by the presence of other
foods - particularly those containing nitrites. Nitrites
may come from cured products such as bacon or ham, which
are sometimes included in meatloaf recipes. And nitrates,
one of the simplest forms of nitrogen, occur naturally
in water and in vegetables such as carrots, celery, broccoli,
cauliflower, cabbage, turnips, spinach and parsley. During
cooking they convert to nitrites, which prevent beef from
turning brown - even when it is fully cooked.
There are other factors that may come into play as well.
Small amounts of carbon monoxide can be emitted from inefficient
burning of gas flames in gas grills or ovens. This too
can combine with the myoglobin in meat, causing it to
retain its pink color (though usually just on the surface)
even when well cooked.
Your dilemma may even rest with the meat itself. Its ultimate
color may be affected by the type of beef, its pH and
the part of the carcass it comes from.
Despite variations in color, a thermometer reading of
160 F (71C) is your best assurance that the meat is sufficiently