What Causes Stained Aluminum Frames?

If you’ve seen mom or dad cooking rice in an aluminium pressure cooker, you might notice they put a slice of lemon in the cooker. Aluminum, like other metals, is susceptible to corrosion, oxygen in the air, as well as airborne dust and dirt. Wonder why they do that? What would happen if the lemon wasn’t there?

Let’s Understand Aluminium
Aluminium is an interesting metal. It is easy to beat into shape (malleable), so engineers
like it a lot for building things. It’s also very light in weight because it isn’t dense. And it doesn’t react very easily, so things made from aluminium don’t corrode. That makes it quite the opposite of iron or steel, which are heavy and rust easily.

This makes it a good material to make aeroplanes and space shuttles. But it also makes it a good material to make kitchen utensils. An aluminium cooker won’t leak aluminium ions when boiling rice in it (whereas a steel* cooker will). But over time, the acids present in food can cause some corrosion.

Why it blackens
There are two kinds of aluminium cooking vessels. The cheap ones are made of metallic
aluminium. ‘Anodised’ aluminium vessels have a protective coating (see below), but they’re more expensive.

Rice and dal and most other foods are generally acidic in content. During cooking, the acid
in the food helps the oxidation of aluminium to insoluble oxides. These settle on the base of the vessel, forming a dull grey layer. This layer is not smooth, so it can trap dirt, caramel (from some burnt sugar in the food) and other stuff, forming a dark stain which is difficult to clean.

What we can do about it
You can beat it by putting a slice of lemon (or a spoonful of vinegar) in the cooker while
cooking. There are acetate, oxalate(such as Magnesium Oxalate) and citrate ions present in the lemon, which react with the aluminium oxides, forming compounds that dissolve in water. You can also use potassium hydrogen tartarate (cream of tartar) to remove stains. The same principle works for hard water.

Or you can buy a cooker made of ‘anodised’ aluminium. ‘Anodising’ is a method by which the surface of the aluminium is oxidised using an electric current. A thin film of aluminium oxide forms on the surface. This thin film is smooth, and clings very tightly to the vessel. The advantage is that it is easier to clean, and prevents further corrosion of the aluminium by the acids in food.