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Why does boiling water always seem to release and become the colors of whatever vegetables I am boiling?

For example, beet root and red cabbage both vividly color hot water.

I'd assume it's something to do with collisions with water particles breaking down cell walls and releasing a pigment, but how would this work on a physical level?

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  • $\begingroup$ I believe this is a Chemistry question. $\endgroup$ – QuantumBrick Oct 17 '16 at 16:07
  • $\begingroup$ @QuantumBrick I'm very interested in analysing it from a physics perspective, specifically that of how the kinetic energies of the particles in the water succeed in breaking down the solid cell wall. $\endgroup$ – theonlygusti Oct 17 '16 at 16:10
  • $\begingroup$ cross post at: biology.stackexchange.com/questions/52533/… $\endgroup$ – setempler Oct 19 '16 at 7:22
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These dyes (NOT pigments$^*$) are highly water soluble. As you pointed out, boiling causes cell walls to rupture, releasing the cell plasma into the water. Boiling of the plasma fluid causes pressure build up, something the cell walls aren't designed to withstand.

You can also extract them with cold water by mashing up the raw material in a food blender or grinding it in a mortar and pestle. This has the same effect of rupturing the cell walls.

An interesting fact is that many of these dyes are pH sensitive: their colour is different in acid or alkaline conditions. Notably red cabbage extract changes from wine red to greenish blue when you add some sodium bicarbonate (baking powder) to it, because 'Bicar' is a (weak) alkali. Then adding a lot of vinegar (a weak acid) will change it back to its original colour.

$^*$ pigments are inorganic substances (many are refined minerals) and mostly insoluble in water.

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  • $\begingroup$ But how does the water actually cause the cell walls to rupture? $\endgroup$ – theonlygusti Oct 17 '16 at 16:17
  • $\begingroup$ Just edited it: it's due to pressure. $\endgroup$ – Gert Oct 17 '16 at 16:18

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