Common sorrel

Sorrel is the acid note of the spring basket. Its sourness is oxalic acid, roughly 300–500 mg per 100 g fresh leaf, and it behaves much more like a seasoning than a vegetable — a small handful stirred in at the end of a sauce does what a squeeze of lemon cannot. The whole craft with sorrel is keeping that acidity from turning dull.

Oxalic acid, heat and the colour cliff

Raw sorrel is bright green and squarely sour. The moment it meets a hot pan it collapses, releases water and turns a khaki brown within about forty seconds. That colour change is not oxidation damage you can prevent by working faster — it is the chlorophyll being stripped of its magnesium by the acid the leaf itself is releasing. Salt and a short, very fast cook are the only tools that slow it.

The practical rule is to cook sorrel for under a minute and to add it last. In soup, blend the base smooth, then drop the shredded leaf in off the heat and let it stand two minutes with the lid on. You keep the sharpness, the colour stays a flat olive-green rather than brown, and the leaf does not break down into slime.

How much is a reasonable amount

Oxalic acid binds with calcium and, at high intakes over long periods, is a genuine concern for anyone with a history of kidney stones or on a low-calcium diet. The realistic picture: a 30 g serving of raw sorrel two or three times a week is a normal, enjoyable amount and not a risk for most people. A daily 200 g bowl of sorrel soup through the whole of spring is the pattern worth avoiding.

Cooking lowers the load somewhat but does not remove it, because oxalic acid is water-soluble and mostly leaves with the cooking water — which is why the cooking liquid from sorrel should not be reduced into the dish. Pairing sorrel with a dairy element is not just a flavour habit: the calcium in cream or cheese binds the oxalate in the gut before it is absorbed.

Keeping sorrel past May

By mid-May the flower spike lifts, the leaves double in fibre and the oxalate rises; the crop is over for the year. Two methods stretch it. The first is a straight sorrel purée: blanch 30 seconds, ice-shock, squeeze dry, blend with 10 % of its weight in butter and freeze flat in 1 cm sheets — it keeps nine months and drops straight into a sauce.

The second is sorrel salt, which is the old French trick. Layer 100 g of shredded leaf with 30 g of coarse salt in a jar, leave it three days, then blend and dry at 40 °C. What you get is a dull green powder that keeps a full year and gives a clean sour edge to fish, eggs and potatoes.