Wash the inside of your pressure cooker in your normal manner to try to remove the burnt food residue. Do this with mild dishwashing liquid, a nylon scrubbing pad and hot water. Rinse the cooker well.
Pour 1 quart of water into the pressure cooker if stubborn burned food remains. Add 2 tsp. of lemon juice or white distilled vinegar to the water. A tbsp. of cream of tartar in the water is another option and is effective in loosening the debris while the water boils.
Replace the lid and lock it. Bring the solution to a boil for about a half hour. Lift the pressure cooker off the hot eye and put it on a cool one. Allow it to cool for about 2 hours.
Use a nylon scrubbing pad or a soapy piece of steel wool to remove the stuck food debris.
Dump the food residue in the garbage disposal and then wash, rinse and dry the aluminum pressure cooker in the normal manner.