Vol. III · Water Plate 10 of 11hard water, calcium, magnesium, beans, pectin, limescale, tea, baking sodaMT

Hard and Soft Water.

The same recipe for beans can take an hour in one town and two in the next. The difference is the calcium in the tap.

Water picks up dissolved calcium and magnesium as it seeps through rock; water rich in them is called hard. In the kitchen hardness does three things. It leaves limescale in kettles and pans as the dissolved carbonate comes out of solution on heating. It changes the taste and clarity of tea and coffee. And it slows the softening of beans, pulses and some vegetables, because calcium ions cross-link the pectin in their cell walls (V.4, XX.2), holding the cells together.

Baking soda works the other way: by making the water slightly alkaline it speeds pectin breakdown, so a pinch in the soaking or cooking water shortens bean cooking — at some cost to flavour and vitamins if overdone. The workshop cooks dried beans in waters of different hardness.

Below: minutes to soften dried, soaked beans in simmering water against the water's hardness, with and without a pinch of baking soda (a teaching model built on the course's pectin-breakdown rate), and the limescale a year of boiling a litre a day would deposit.

What you will learn
Workshop
A teaching model: bean-softening rate is the course's pectin-breakdown rate at 98 °C, slowed by calcium in proportion to hardness and roughly doubled by a pinch of baking soda. Limescale: calcium carbonate left by boiling one litre a day for a year, taking hardness as mg/L calcium carbonate.

Calcium holds cells together

In this model, beans that soften in about 51 min in soft water take 2 h 15 min in hard water of 300 mg/L — the calcium binds the pectin that glues plant cells together, and the pectin must break down before the beans soften. The shape of the effect is well established, though real beans vary a great deal with variety and age; the numbers here are the model's, not a measurement of any bean. Old beans, stored for a year or more, are slow in any water.

Remedies: cook beans in filtered or soft water, add a pinch of baking soda (about a quarter teaspoon per litre) to the soaking or cooking water, and add salt — which, despite the old warning, does not toughen beans and in brined soaks helps them cook evenly (XX.8). Acidic ingredients such as tomatoes or vinegar do toughen them, so add those once the beans are tender. Limescale is just calcium carbonate: vinegar or citric acid dissolves it.

At the stove

Soak two handfuls of the same dried beans overnight, one in plain tap water, one in water with a quarter teaspoon of baking soda per litre. Cook both the same way and test every ten minutes. Note which softens first, and taste both.

Hardness is often given in degrees in Europe: one German degree is 10 mg of calcium oxide per litre (about 17.8 mg/L as calcium carbonate). London water is hard (around 250–300 mg/L); much of Scotland's is very soft.

Terms

Hard water
Water rich in dissolved calcium and magnesium.
Limescale
Calcium carbonate deposited when hard water is heated.
Pectin cross-linking
Calcium ions binding pectin chains together, firming plant tissue.
Baking soda
Sodium bicarbonate; alkaline in water, it speeds pectin breakdown.
Calcium in hard water firms pectin and slows beans; a pinch of baking soda speeds them.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.