Why Does Washing Quinoa Make Foam? The Plant Is Defending Itself
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A class of plant compounds that foam in water taste bitter, deter animals and were used as soap long before soap was manufactured. Rinsing removes them from food.
Why they foam
The molecules have two parts joined together, a water-attracting sugar portion and a water-repelling portion built from a fat-like or steroid-like core. That split structure makes them accumulate at the boundary between water and air exactly as detergent molecules do, lowering the surface tension and stabilising bubbles, which is why agitating a solution produces persistent foam. The same property lets them emulsify fats and lift dirt, which is the whole basis of their use as a cleaning agent, and the name derives from the Latin word for soap for that reason.
What they do for the plant
The compounds are a chemical defence rather than an accident:
- •Bitter taste, which deters mammals and birds from eating the seed
- •Toxicity to fish, by disrupting the membranes of their gills
- •Activity against fungi and bacteria attacking the plant
- •Deterrence of insects feeding on leaves and seeds
- •Concentration in the outer coat of seeds, where attack comes first
- •Production increasing when the plant is damaged
Where they are used deliberately
Human uses run from ancient to entirely modern. Soapwort, soapbark and soapnut have been used for washing textiles and hair for a very long time, and museum conservators still use soapwort for cleaning delicate historic fabric because it is far gentler than modern detergent. Several cultures used plants rich in these compounds to stun fish, crushing the material into a pool so the fish surfaced and could be collected, a practice now prohibited in most places. They are used as foaming agents in some drinks and in fire extinguishers. And certain of them are studied as adjuvants in vaccines, where they provoke a stronger immune response to the antigen.
The ones inside animals
A small number of animals produce closely related compounds, which is unexpected given the name and the plant association. Sea cucumbers and starfish generate them and use them defensively, releasing them into the water when attacked, where they disrupt the cell membranes of a predator exactly as the plant versions disrupt fish gills. That is a striking case of unrelated organisms arriving at the same chemistry for the same purpose. Some of those marine compounds are studied for medical activity, and the sea cucumber ones are potent enough that the animals can kill fish confined with them in a small volume of water.
Why quinoa has to be rinsed
The most familiar encounter is in the kitchen and the reason is straightforward. Quinoa seeds carry a heavy coating of these compounds in their outer layer, which protects them from birds in the field and which tastes strongly bitter and soapy. Rinsing under running water until the foam stops removes most of it, since the compounds dissolve readily, and commercial quinoa is frequently pre-washed or mechanically polished for that reason. The bitterness is why the crop needed no pesticide historically. Several varieties have been bred with lower levels, which improves the eating and makes the crop more attractive to birds in the field.
The takeaway
Molecules joining a sugar portion to a fat-like one gather at the boundary between water and air, which lowers surface tension and makes foam, and the same structure lifts dirt. Plants use them as a defence, since they are bitter, deter insects and disrupt fish gill membranes. Soapwort is still used by conservators on historic textiles, and quinoa is rinsed until the foam stops.