title demo

23 January 2021 | New Scientist | 51

The back pages

SH

UT

TE

RS

TO

CK

/ES

IN^

DE

NIZ

Sam Wong is social media
editor and self-appointed
chief gourmand at
New Scientist. Follow
him @samwong1

These articles are

posted each week at

newscientist.com/maker

What you need
Beans
Tahini
Garlic
Lemon juice
Chilli flakes
Salt
Water
Red onion
Olive oil
Parsley

IF YOU are taking part in Veganuary

or just cutting down on meat,

beans are a valuable source of

protein for anyone on a plant-

based diet. They owe their high

protein content to a symbiosis

with Rhizobium bacteria, which

the plants foster inside nodules

in their roots. These bacteria can

absorb nitrogen from the air and

convert it into ammonia, which

the plants can use to make

protein-building amino acids.

Beans, peas and lentils  – and

peanuts – are the seeds of legume

plants. They have a tough coat

made up mostly of cell-wall

carbohydrates. Inside the seed

are two embryonic leaves called

cotyledons, which store starch

and protein to nourish a seedling.

Some beans, such as kidney beans,

contain phytohaemagglutinin,

a toxin that causes red blood

cells to clump, but it is destroyed

with proper cooking.

Tinned beans are convenient,

but their flavour is a poor relation

to that of dried beans cooked at

home. You can cook them without

pre-soaking, but they cook more

quickly and evenly after being

soaked for at least 12 hours. The

low permeability of the seed coat

means they absorb water slowly,

letting it in initially through

the hilum or “eye” of a bean.

After 12 hours, beans have

typically doubled in weight.

Once a bean is hydrated, its

structural polymers are sensitive

to heat and molecular changes

can soften it: unfolding of

proteins, gelatinisation of starch

and solubilisation of pectin.

Beans can provide a source of protein if you want to cut back on

meat. But you need to know how to cook them, says Sam Wong

Science of cooking

Getting the best out of beans

In the seed coat and the

cotyledons, cells are held together

tightly by pectin molecules, with

calcium ions forming strong cross

links between the pectin. The

pectin molecules must break apart

and dissolve for the beans to soften.

Hard water with a lot of calcium

or magnesium in can reinforce

the bean cell walls, increasing the

time needed to cook them. Many

people say you shouldn’t add salt

when you soak and cook beans,

but I found that beans soaked

and cooked in salted (2 teaspoons

per litre) water softened faster

and tasted better than beans

soaked in unsalted water. The

sodium ions may weaken the

bonds between pectin molecules.

You can cut the cooking time

further by adding a teaspoon of

bicarbonate of soda per litre to the

salted water. This makes the water

alkaline, helping to weaken pectin

as well as unfolding proteins.

Acids, on the other hand, inhibit

softening, which is why beans can

be simmered for hours in tomato

sauce without falling apart. It is

best to cook beans alone before

adding them to soups if you aren’t

planning a lengthy cooking time.

The Turkish salad piyaz

(pictured) is a delicious way to

enjoy any bean. Mix tahini, garlic,

lemon juice, chilli flakes and salt,

adding water to make a smooth

sauce. Use this to dress cold, cooked

beans along with finely chopped

red onion. Drizzle with olive oil

and scatter over some parsley. ❚

Feedback

Shorts that tickle and

utile resistance: the

week in weird p56

Tom Gauld for 

New Scientist

A cartoonist’s take

on the world p55

Almost the last word

What is the smallest

animal with eyes?

Readers respond p54

Puzzles

Try our crossword,

quick quiz and

logic puzzle p52

Twisteddoodles

for New Scientist

Picturing the lighter

side of life p56

Science of cooking
appears every four weeks

Next week
Science of gardening