Vol. XX · Vegetables, Fruit and Legumes Plate 8 of 12dried beans, lentils, chickpeas, kidney beans, soaking, quick soak, salt, hard water, acid, baking soda, lectin, phytohaemagglutinin, flatulence, pressure cooking, 豆MS

Dried Beans and Lentils.

Raw red kidney beans soaked and slow-cooked at 80 °C can be more toxic than raw. Boiled hard for ten minutes first, they are perfectly safe.

A dried bean is a seed at about 10 % water, with a tough coat and a store of starch and protein. To cook evenly it must first take up roughly its own weight of water, which enters mainly through a small pore near the scar (hilum) and moves through the seed by diffusion. Soaking does this in advance: about eight to twelve hours in cold water, much faster in warm water; a "quick soak" — boil two minutes, stand an hour — does much the same. Lentils and split peas are small and thin, need no soaking, and cook in 20–40 minutes.

The cooking water matters (XX.2). Salt in the soak (about 1 %) swaps sodium for calcium and magnesium in the skins and softens them; it does not toughen beans — an old myth. Hard water does toughen them, and acid (tomato, vinegar, wine) or sugar added early can keep them hard for hours; a pinch of baking soda softens them faster. And beans carry defences: red kidney beans contain phytohaemagglutinin, a lectin that a few raw or undercooked beans can turn into hours of vomiting. A ten-minute hard boil destroys it; a slow cooker at 80 °C does not, and can make it worse (XII.9). The workshop soaks chickpeas.

Below: the water taken up by dried beans soaked at the temperature you choose (a saturating teaching curve, about an hour to half at 20 °C, faster when warm by an activation energy of 35 kJ/mol), and teaching factors for how the cooking water changes the cooking time.

What you will learn
Workshop
Water uptake: U = Umax × t ÷ (t + t½), Umax 100 % of the dry weight, t½ = 1 h at 20 °C and shortened by an Arrhenius factor with 35 kJ/mol (teaching values in the published range for beans). Cooking-time factors: teaching values in the measured directions, relative to soaked beans in soft water.

Soak, salt, boil hard, then simmer

With these teaching values beans take up nine-tenths of their water in about 9 hours at 20 °C, 21 hours in the fridge and 2.4 hours at 50 °C — which is why an overnight soak works, and a warm or quick soak works the same day. Soak beans with plenty of water (they double in weight) and about 1 % salt; soaking in the fridge in summer keeps them from starting to ferment. Old beans, stored for years, cook slowly whatever you do: buy from shops with turnover.

Method: drain and rinse soaked beans; cover with fresh water by several centimetres; bring to a boil and boil hard for ten minutes (for kidney beans this step is essential); skim; then simmer gently until creamy inside, 45–90 minutes for most beans, longer for chickpeas. Add salt any time; add tomatoes, wine and vinegar only once the beans are tender. A pressure cooker (III.4) cuts the time to about a third. Soaking and long cooking also reduce the oligosaccharides that ferment in the gut; discarding the soaking water helps a little.

At the stove

Weigh 100 g of dried chickpeas, soak them overnight, drain and weigh again. Then cook half in soft water and half with a teaspoon of tomato paste from the start, and compare.

In 1976 an outbreak of illness in Britain was traced to undercooked red kidney beans in a salad, and in the 1980s to beans cooked in slow cookers; the advice to boil kidney beans hard for ten minutes first comes from those investigations.

Terms

Quick soak
Boil dried beans two minutes, then stand covered for an hour.
Phytohaemagglutinin
A toxic lectin in raw red kidney beans, destroyed by a hard boil.
Hilum
The scar on a bean where water mainly enters.
Oligosaccharides
Sugars in beans that gut bacteria ferment.
Dried beans must take up their weight in water; soaking does it in hours, faster when warm. Salt softens the skins; hard water and early acid keep them hard. Boil kidney beans hard for ten minutes.

Sources

  1. U.S. FDA, Bad Bug Book, 2nd ed. (2012): Bacillus cereus emetic toxin is heat-stable and is formed in starchy foods held warm; Staphylococcus aureus enterotoxin survives cooking; scombrotoxin (histamine) forms in temperature-abused fish and is not destroyed by cooking; phytohaemagglutinin in raw red kidney beans — a few raw beans can cause illness; boiling 10 minutes destroys it, while heating to only about 80 °C can increase toxicity.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.