Why does starch boil over?
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Starches are polymers.
That is, they are molecules consisting of large chains of repeated smaller molecules that have been chemically bonded together.
In starches the basic subunit is glucose sugar.
Starch is a bit of an umbrella term for a range of natural polymers in foods that can have different sizes and structures. They can be long single chains, or may have some branched ends. They can also have other kinds of sugars attached at the ends like fructose or galactose. It is not a pure substance however they are all based upon long chains of glucose.
In starches their solubility in water varies based on the length of the glucose chains. In general starches are insoluble or nearly insoluble in cold or room temperature water. Those with short chains that are soluble in cold water are usually called “maltose.”
However they become increasingly more soluble as the temperature of water increases.
This does two things to a mixture of starch and water when it’s heated.
Individual starch molecules are surface active agents or “surfactants.” This means such molecules can reduce the surface tension of water or other liquids by interfering with the attraction forces between individual water molecules.
This makes bubbles more stable and much less likely to pop.
Soap is another surfactant, which explains why soaps cause foamy lather to form.
The second effect is increasing the viscosity of the solution, meaning it’s resistance to flowing. Because starch chains are much larger than water molecules they cannot flow as easily and cause the liquid to resist *shearing* forces. (This is why a Small amount of some food that contains starch like flour or corn starch is often used to thicken soups.)
This slows down the flow of bubbles in the mixture from the bottom of the pan to the surface. This can causes the to pile up like a traffic jam. This also slows the formation of a thin film between individual bubbles and with the air above the mixture. Bubbles can’t break or merge together until the film between them reaches a certain critical thinness.
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