Reader Glynis has early tomatoes and says she destroyed a few trying to oven roast them on an aluminum sheet pan. Her question: why does contact with aluminum ruin the taste of tomatoes? Love that one, Glynis! Aluminum is interesting stuff in that on the one hand it’s the most abundant metal in the Earth’s crust, but on the other it is found nowhere in nature in its pure, elemental state. The reason: because aluminum is extremely reactive.
Its reactivity is easily witnessed in the kitchen, say, when aluminum foil is used as a cover for a dish of leftovers. If the contents of the dish are wet enough to cause condensation on the foil, the result is a dark layer of aluminum oxide. However that’s child’s play compared to what happens to aluminum foil when it comes into contact with other metals like stainless steel, silver or iron in the presence of moisture. In those cases you get an electrolytic reaction that creates actual pits or even holes in the foil.
Similar reactions happen when aluminum comes into contact with very salty, acidic or alkaline foods. All of these — including cooked eggs which contain hydrogen sulfide — quickly penetrate the thin layer of aluminum oxide that normally covers an aluminum pan as a result of its exposure to air. What results are various aluminums salts which are harmless to eat but taste terrible.
Many manufacturers get around this problem by anodizing their aluminum utensils and pans. That is, by immersing them in acidic baths and passing electric currents through them. The process creates a much thicker aluminum oxide layer that prevents reactive molecules from reaching the pure aluminum beneath. The problem there is that all anodizing — depending on the process and the relative strengths of the acids and voltages — is not equal. Different processes create different thicknesses of oxide, and thus different degrees of protection.
My sheet pans, for example, get heavy use. They get deeply scratched and are regularly scrubbed with abrasives. All of that wears away at the oxide layer, making them more reactive with food. I therefore never take a chance with them when it comes to salty or acidic ingredients like tomatoes. I use baking parchment every time, and I suggest that you do too, Glynis! Better luck next time!