Beans
Chapter three

The chemistry of a stored seed

A seed has two jobs that pull against each other: pack enough energy and nitrogen to build a seedling, and stop anything else from eating it in the meantime. Almost every distinctive thing about beans — the cooking time, the flatulence, the iron that does not absorb, the four ways they can poison you — falls out of that conflict.

Where the mass is

Proximate composition

% dry matter
3.1 — Storage carbohydrate

Starch, and the third of it you do not digest

Bean starch is 30–40% amylose, against about 25% in cereal starch. Amylose is the linear chain — glucose joined α-1,4, no branches — and it behaves completely differently from the bushy amylopectin around it.

Linear chains can line up. When a cooked bean cools, amylose molecules re-associate into crystalline double helices that no human amylase can reach. That is retrograded resistant starch, type 3, and it means a cooled bean delivers measurably less glucose than the same bean eaten hot. Reheating only partly reverses it.

The rest of the resistant fraction is type 1: starch that is physically walled in. A bean cell has a thick pectin-rich wall that survives cooking intact, so granules inside it are never fully exposed. Grind the bean into flour and you destroy that protection — bean flour has a far higher glycaemic index than whole beans.

Gelatinisation begins near 65 °C and is complete by about 75 °C, but only where there is enough free water. In a thick, sugary or salty pot the granules gelatinise late and unevenly — which is half the reason baked beans take six hours.

Glycaemic index

glucose = 100
Pulses occupy the bottom of the scale almost exclusively. The gap between kidney beans at 24 and white bread at 75 is the largest single-food difference in the common GI tables.
3.2 — The raffinose family

One missing enzyme

Humans make no α-galactosidase in the small intestine. Every galactose attached by an α-1,6 bond therefore arrives in the colon intact, where roughly 1014 bacteria are extremely pleased to see it.

Build the chain

Oligosaccharide content

g / 100 g dry seed
Stachyose dominates in most beans; fava and grass pea are unusual in carrying more verbascose, the five-sugar member. Values are literature midpoints — cultivar and growing season move them by a third either way.

What actually happens

  1. Small intestine — the α-1,6 galactose bonds pass untouched. No human enzyme cleaves them.
  2. Caecum — Bacteroides, Bifidobacterium and clostridia hydrolyse them with their own α-galactosidases and ferment the sugars.
  3. Gases — hydrogen and carbon dioxide, plus methane in the 30–50% of people who carry Methanobrevibacter smithii. None of the three has any smell.
  4. The smell — comes from sulphur: hydrogen sulphide and methanethiol, produced from sulphur amino acids, at concentrations around a thousandth of the gas volume.
  5. The payoff — the same fermentation yields acetate, propionate and butyrate. Butyrate is the preferred fuel of the colonocyte, and the reason these oligosaccharides are classed as prebiotics rather than merely as a nuisance.
Adaptation is real. Controlled trials that put people on a daily half-cup of beans find reported flatulence falls back toward baseline within two to three weeks as the microbiome shifts.
3.3 — Storage protein

Phaseolin and its relatives

7S

Phaseolin

A vicilin-type trimeric glycoprotein, roughly 150 kDa, built from ~47–50 kDa subunits. It is 35–50% of all protein in a common bean and it is packed into protein storage vacuoles as a dense crystalloid. Its electrophoretic type — S, T, C, B or H — is the classic marker that tells you whether a landrace came from the Mesoamerican or the Andean gene pool.

11S

Legumin

The hexameric ~360 kDa globulin that dominates peas and soybeans, where it is called glycinin. Minor in Phaseolus. It carries more sulphur amino acids than vicilin does, which is a large part of why soy protein scores higher than bean protein.

Albumins

The defensive fraction

Water-soluble, only 10–15% of the total, and disproportionately trouble: lectins, α-amylase inhibitors, and the Kunitz and Bowman-Birk protease inhibitors. These are the proteins cooking has to destroy. Most denature between 80 and 100 °C; the Bowman-Birk inhibitor, with seven disulphide bridges holding an 8 kDa knot together, is the stubborn one.

Protein and fat, cooked, per 100 g

grams
3.4 — Pigment

Why black bean water turns blue

The seed coat of a black bean is about 2 mg of anthocyanin per gram — roughly 56% delphinidin-3-O-glucoside, 26% petunidin-3-O-glucoside, 18% malvidin-3-O-glucoside. Anthocyanins are pH indicators, and a good one.

Delphinidin-3-O-glucoside against pH

pH 3.0

pH 3.0
This is why a splash of vinegar keeps black beans looking black instead of grey-blue, why cooking them in a cast-iron pot turns them muddy — iron chelates the pigment — and why adding bicarbonate of soda to speed cooking will turn the pot an alarming teal.
3.5 — Antinutrients

The things that reduce what you get

These are not poisons. They are chemistry that lowers the nutritional return — and processing removes most of it.

Phytic acid — myo-inositol hexakisphosphate

C₆H₁₈O₂₄P₆ · 660.04
Six phosphate groups on one six-carbon ring. At gut pH they carry a large negative charge and chelate Fe²⁺, Zn²⁺ and Ca²⁺ into complexes that are not absorbed — which is why the iron figure on a bean label overstates what you actually take up, often by a factor of three or more. It is also the seed's entire phosphorus reserve, released by its own phytase on germination.

How much survives processing

Approximate reductions from a raw baseline. Sprouting and fermentation work because they activate the seed's own phytase; boiling barely touches phytate because it is heat-stable, but it destroys lectins outright.

Vitamin C eaten with the meal counteracts phytate directly: ascorbate keeps iron in the more soluble Fe²⁺ state and forms a chelate the gut can absorb. A squeeze of lime over beans is not a garnish.
Tannins and condensed proanthocyanidins
Concentrated almost entirely in the seed coat, and roughly proportional to how dark it is: a white navy bean runs near 0.08 g/100 g, a red kidney bean around 1.6. Tannins bind dietary protein and digestive enzymes alike, cutting protein digestibility by a few percent, and they inhibit non-haem iron absorption strongly. They are also most of the antioxidant capacity people cite when recommending dark beans — the same binding chemistry, valued differently.
Protease inhibitors
The Kunitz inhibitor (~21 kDa, one reactive site, mainly anti-trypsin) and the Bowman-Birk inhibitor (~8 kDa, two independent sites, anti-trypsin and anti-chymotrypsin). Both reduce protein digestion and force the pancreas to oversecrete. Kunitz denatures readily; Bowman-Birk is held together by seven disulphide bridges and survives a surprising amount of heat, which is why properly boiled soy still retains a few percent of its raw activity. Bowman-Birk inhibitor has also been through human cancer-prevention trials on the strength of the same protease inhibition.
Saponins
Triterpene glycosides at 0.1–0.5% of dry weight. They are surfactants — they are the reason bean cooking water foams, and the reason chickpea liquid whips into meringue. In the gut they form complexes with cholesterol and bile acids, and at ordinary dietary levels the net effect is mildly cholesterol-lowering rather than harmful.
α-Amylase inhibitor
A lectin-like protein in Phaseolus vulgaris that blocks pancreatic α-amylase and so slows starch digestion. Sold as a "carb blocker" extract, and it genuinely does inhibit the enzyme in vitro; the measured effect on human starch absorption is much smaller than the marketing, and it is destroyed by proper cooking anyway.
3.6 — Chemical defence

Four that will actually hurt you

Each has a specific molecule, a specific mechanism and a specific dose. None is a reason to avoid beans; three of the four are entirely handled by cooking them properly.

Phytohaemagglutinin

Raw kidney bean poisoning

PHA is a tetrameric glycoprotein, about 126 kDa, that binds glycoproteins on the intestinal epithelium. Raw red kidney beans carry 20,000–70,000 haemagglutinating units; a properly boiled bean carries under 400. As few as four or five raw beans have caused violent vomiting and diarrhoea within one to three hours.

  • Onset1–3 h
  • Destroyed by10 min at 100 °C
  • Made worse by80 °C slow cooking
Holding beans at 80 °C — the temperature inside a slow cooker on Low — can raise PHA activity up to fivefold above the raw bean. Boil dried kidney beans hard for ten minutes before they go anywhere near a slow cooker.
Vicine / convicine

Favism

Gut bacteria hydrolyse the glucosides in fava beans to divicine and isouramil, which redox-cycle and consume reduced glutathione in red blood cells. A cell with normal glucose-6-phosphate dehydrogenase regenerates it. A G6PD-deficient cell cannot, and haemolyses.

  • AffectedG6PD-deficient only
  • Prevalence~400 M worldwide
  • Onset5–24 h

G6PD deficiency is commonest around the Mediterranean, in the Middle East and in parts of Africa and South Asia — the same map as historical malaria, because the deficiency is protective against it. Cooking reduces vicine somewhat but does not remove the risk; low-vicine cultivars exist and are the real solution.

β-ODAP

Lathyrism

Grass pea contains β-N-oxalyl-L-α,β-diaminopropionic acid, a non-protein amino acid that acts as an AMPA-receptor agonist and is excitotoxic to upper motor neurons. Eaten as an occasional food it is harmless. Eaten as the dominant staple for three months or more — which happens when every other crop has failed — it causes an irreversible spastic paralysis of the legs.

  • Content0.1–2.5% dry wt
  • Risk threshold~3 months as staple
  • ReversibleNo

The cruelty of it is structural: grass pea yields when nothing else will, so it is eaten most exactly when it is most dangerous. Low-ODAP lines below 0.1% now exist, and steeping in hot water removes much of what remains.

Quinolizidine alkaloids

Bitter lupin

Lupanine and sparteine reach 4% of dry weight in bitter landraces — enough to cause anticholinergic poisoning: blurred vision, dry mouth, tachycardia, confusion. Traditional debittering is days of repeated soaking in changed or running water, and cases still occur when someone shortens it.

  • Bitter typesup to 4%
  • Sweet cultivars< 0.02%
  • AlsoEU declared allergen

Separately from the alkaloids, lupin protein cross-reacts with peanut: a substantial minority of peanut-allergic people react to lupin flour, which is why it must be declared on labels across the EU.

Canavanine against arginine

jack bean, up to 3% dry weight
A single oxygen where a carbon should be. Canavanine is close enough to arginine that arginyl-tRNA synthetase loads it by mistake, so the ribosome builds it straight into protein — but the guanidinooxy group is far less basic than a guanidino group, so the finished protein has the wrong charge distribution and often does not fold or function. It is a defence that works by sabotaging the herbivore's own translation machinery.