When a potato cooks, the starches contained in each cell are released as the cell walls break down. These starches absorb the potato’s internal moisture and swell and soften. These two processes are what transform a raw, hard potato into a cooked, softer potato fit for mashing.
If you cut your potatoes up before boiling them, the starches absorb the internal potato moisture as well as the water in which they’re being boiled. If they boil for too long, they absorb too much water and your mashed potatoes will become gummy. The difference between perfectly cooked potato pieces and soggy pieces can sometimes be as little as a minute or two.
An easy workaround is to boil potatoes whole. They’ll take a bit longer to cook but you can leave them in the hot water after boiling without undesirable effects, keeping them warm until it’s time to mash them. I start my potatoes boiling as soon as I begin cooking and mash them immediately before dinner.
If they need more moisture, you can add a bit of hot potato boiling water or another liquid. This way, you have more control of their moisture content.
Great question. Osmosis will be minimal when the total concentration of solutes in the potato’s cells matches the total concentration of solutes outside of the potato’s cells.
The problem here is twofold. First, the solute concentration in the potato is quite high as it’s low moisture and is comprised of starches, sugars, minerals, and other cell contents, so you need a ton of salt in the water to match the total concentration. Second, even if the solute concentrations are equal, making your boiling water an isotonic solution, you still have to deal with simple diffusion. The cooking process is blowing up cell walls, so some starch is going to migrate out while salt moves in to take its place. By mass, you don’t need to add much salt to a potato to make it unpalatable. You’d probably be alright with a whole potato but cut up potato pieces would end up really salty.
Oh it hadn’t occured to me that not only salt causes osmosis. Thank you for the explanation!
Seems like I’m going to boil the potatoes whole in the future, thanks again :)