Tomatoes, tomato paste and acidity

Why

Tomato paste is concentrated tomato flesh with plenty of sugars and acids; briefly fried in fat, some of the sugars brown and the raw, sharply sour taste turns full and rounded, but it must be stirred, because it scorches easily on a dry pan base. Long simmering, fat, cream or a little sugar soften a sharp sauce; sugar doesn't neutralise the acid, though, it only balances it on the palate. Baking soda really does neutralise acid, but in tests it made the sauce taste flat, and larger amounts foam and taste soapy, so a pinch at most. Acid firms up the cell walls of vegetables and pulses, so beans, chickpeas and potatoes soften more slowly in tomato sauce. Acidic food doesn't belong in uncoated aluminium: the German BfR advises against cooking or storing acidic and salty food, such as tomato purée, in uncoated aluminium pots, because aluminium migrates into it; with cast iron, manufacturers say long cooking of acidic food strips the protective seasoning.

How to do it

• Stir the paste into hot fat and fry it for a few minutes, stirring constantly, until it darkens and smells sweet; then add liquid at once.
• Simmer a sour sauce for longer and round it off with fat or cream; add sugar little by little and taste.
• Use baking soda only as an exception, a pinch per pot at most, and let it fizz out before tasting.
• Cook beans, chickpeas and potatoes until tender first, or allow longer cooking in the sauce – see the Cooking pulses card.
• Cook and store tomato sauces in stainless steel, enamel or glass, not in uncoated aluminium or new, lightly seasoned cast iron.

If it goes wrong

Paste that has scorched on the base won't lose that taste: pour the sauce into a clean pot without scraping and start again with a fresh batch. An overly sour sauce is saved by longer simmering, a knob of butter or a spoonful of cream. Dilute a sauce that tastes soapy after soda with canned tomatoes and leave the soda out next time.