I cooked some poultry stuffing and left it in a bowl in the fridge overnight, covered with aluminium foil. In the morning there were holes in the foil where it had touched the stuffing, which was stained black under each hole. Uncooked stuffing does not produce this effect, and it makes no difference whether the stuffing is cooked inside the bird or separately. What is going on here, and are the black stains poisonous?
鈥� Without its submicroscopic insoluble skin of oxide, aluminium cookware would catch fire easily.
Normally, breaks in the oxide skin of aluminium heal instantly when the exposed metal reacts with say, air or water. But if, for example, mercury or certain alkalis or acids dissolve this skin, the exposed underlying metal reacts vigorously. So, while aluminium cookware and foil are safe and useful in the kitchen, it is important to keep them away from strong salt solutions or caustic soda, for example, and also from wet food when it is not actually cooking.
Advertisement
Wet, fatty materials such as cooked lard form fat-soluble detergents that penetrate microscopic chinks in the oxide layer, exclude air that otherwise would reseal the skin, and corrode pinholes into the metal. If floating fat has coated the metal, even cold chicken soup can eat through a thick aluminium pot overnight.
鈥淚f floating fat has coated the metal, even cold chicken soup can eat through a thick aluminium pot overnight鈥�
The black stain is mainly from small amounts of iron in the aluminium. It is not deadly, but it is better not to eat food contaminated with high levels of metals, which also spoil the taste. For wrapping cooked fatty or acidic food for more than short periods, plastic film is much better.
Jon Richfield, Somerset West, South Africa