Understanding the structure, composition and behaviour of what is being cooked can guide how to cook it. So, here is Potato Science 101.
Potatoes are related to tomato, chilli and tobacco – members of the deadly nightshade family, and at least potato and tomato were once considered poisonous. Which potatoes sort of are – they contain toxic alkaloids (solanine; chaconine). At normal levels these alkaloids contribute flavour, at higher levels they are bitter and dangerous. The occasional small, green tomato-like fruits of the potato are poisonous.
Greening indicates high alkaloid levels in and below the skin. Such potatoes should be peeled thickly or discarded. Potatoes should not be purchased in this condition. Greening is accelerated by light.
They are best stored between 7-10C. Much higher and they sprout and decay. Colder, and they begin to convert their starches into sugars, giving a brown core.
Potatoes contain starch, sugar, fibre, minerals and pectin, and no fat. They are wet, 70-80% water (not much less than milk at 87%). They are a good source of vitamin C (and were once used to treat scurvy). A potato contains about the same calories as an apple. All the ‘goodness’ is not in the skin – even though potato skin contains some additional nutrients, the amount of skin on a potato is so small compared to its interior that this additional ‘goodness’ is insignificant.
What is in a potato that matters when cooking? Mostly it’s the starch and pectin.
Potato contains two forms of starch – amylose and amylopectin. Starch is a poly-saccharide, that is it’s a long chain of sugar molecules (glucose). In amylose, these chains are linear and about 1,000 sugar molecules long. In amylopectin, the chains are clumped and branching, and contain about 10 times the number of molecules as amylose.
The linear nature of amylose means it can nestle up against itself and bind, and it is primarily a gelation agent. The branched structure of amylopectin can’t do this, but its complex structure interferes with water flow, making it a thickening agent.
In a potato, these starches are locked together in starch granules, which adhere to the inner walls of the potato cells. The potato cell-walls themselves are made up of fibre and strengthened by pectin, and pectin binds the potato cells to each other (which, with enough cells, makes a potato).
The starch granules in the cells have a roughly alternating concentric structure of amylose and amylopectin. The two forms of starch bind strongly to each other, allowing no room for water, giving the granules a sort of dry concentric crystalline structure. The potato contains a lot of water, but it’s not found in the granules.
When heated, the starches in the granules become agitated and separate and are now free to bind with water in the cell (and with each other). They swell. Significantly. Ultimately they fill the cell itself. This is good for eating, because the granules are now a form of gel but are still contained within the potato cell.
With continued cooking, they can swell so much they break the cell wall and escape. What’s more they break up themselves releasing their starches. This is bad. We now have wallpaper glue. It’s a fine line…
A property of the gelled granules (before they burst) is that if you stop the cooking and allow the potato to cool, the amylose, amylopectin and water re-arrange themselves and form a stable structure. It is actually more stable than the original dry granule (the process is called retrogradation). So, if heating is now resumed, it is possible to heat the potato to even higher temperatures without the risk of the granules continuing to hydrate, swell and burst.