What acidic food does to aluminium — and why steel does not react

Ask a halwai why the old aluminium patila got replaced and you will usually hear the same two things: it started tasting of metal, and it wore thin near the top of the food line. Both have the same cause.

Acid and aluminium

Aluminium sits high on the reactivity series. In the presence of an acid — citric from lemon and tomato, tartaric from tamarind, lactic from curd — the surface oxide layer is dissolved faster than it re-forms, exposing bare metal that then reacts in turn.

The practical results in a working kitchen:

• Taste transfer. A faint metallic edge that is most obvious in mild, light dishes.
• Discolouration. White gravies, kheer and milk-based preparations turn slightly grey.
• Pitting. Small shallow craters, usually worst at the waterline where the food, the metal and the air all meet.
• Thinning. Over years, the vessel measurably loses wall thickness.

None of this is dramatic on any one day. It is cumulative, which is exactly why it tends to be noticed late.

Why stainless steel behaves differently

The chromium oxide layer on stainless steel is far more stable than aluminium's oxide. It does not dissolve in the weak organic acids found in food, and where it is physically scratched it re-forms almost immediately from the oxygen in the air.

So an acidic preparation held in a steel vessel for hours does what it is supposed to do — reduce and concentrate — without any exchange with the metal.

The test that convinces most people is simple: cook the same tamarind preparation in both, side by side, and taste them against each other the next morning.

Where steel does have a limit

We would rather tell you this than have you discover it. Steel is not infinitely inert — it has one genuine vulnerability, and it is undissolved salt sitting on a hot surface.

Dry salt crystals in contact with steel can locally break down the passive layer and cause pitting. This is entirely avoidable:

• Add salt to liquid that is already moving, not to a dry hot pan.
• Do not store salted water or brine in a vessel overnight.
• Rinse and dry after heavily salted preparations.

Do that and the surface stays intact. There is more on this in our article on salt pitting.

What this means for a commercial kitchen

If your menu is built on tomato, tamarind, curd or vinegar — which is to say, if you cook Indian food — the vessel choice is not a matter of preference. Anything held for more than a few minutes should be in steel.

That is the reasoning behind the whole of our range, from the jointless patila that holds the day's dal to the lagan used for slow-cooked meat.