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.
This is your problem. It isn’t that the potatoes absorbed too much water while cooking them, you left them to continue cooking in the hot water which is why they are getting overdone. You are trying to compensate with larger mass to make up for letting them soak after cooking, which is why they absorb too much water.
If you just pulled them out and made them when they were done they wouldn’t turn into soggy messes and you would have a lot more control over when they are done. If they are done early, they will retain their heat just fine in a covered dish.
The fact that you can overcook them in a couple of minutes when the total cook time cut is around 10-15 minutes depending on size is right in line with cooking pretty much anything. Cooking pasta for a couple minutes longer will change their consistency too!
Just to preface, I’m actively trying to not be the “well, ackshally” guy. I’m just really passionate about potatoes and potato education. Really just science education but potatoes are science, dammit.
The part you quoted, when in the context of the rest of the post, describes how boiling them whole specifically allows you to leave them in the pot without them getting soggy. You can leave them in there for an extended length of time as the amount of water they’ll absorb relative to their total volume is minimal due to the low surface area to volume ratio, plus the intact skin severely reduces the rate of water absorption.
Here’s where the well ackshally avoidance part is important. I have an MSc in botany, so I took a lot of plant physiology courses, plus I wrote a paper about this as a “fuck you” response to a stupid assignment by an asshole professor. The following is a crash course on potato science, no one is under no obligation to read it.
It’s not overcooking itself that causes soggy taters. When you cook a potato, the starch absorbs water and swells. When you cook a whole potato, the amount of water available is rather limited as potatoes are low moisture tubers, so the starch can only absorb as much water as is available in the tater itself. When boiled whole, the periderm (skin) maintains the structure of the potato, keeping it from breaking apart, and limits the amount of water that can be absorbed into the pith, the starchy interior. The latter is actually one of the primary natural functions of the skin. The total moisture content of a boiled whole potato is shockingly close to its raw state. Don’t believe me? Weigh a few whole taters using a decent scale, boil them for an hour, then weigh them again. It’ll be damn near the same. That’s one of the perks of cooking them my way - I can add more milk and butter without them getting soggy.
When you cut potatoes up, the starch can maximally swell as it has practically unlimited access to water, plus vastly more surface area, increasing the rate of water absorbtion. Since the skin no longer maintains the shape of the potato, small fissures appear as the structure of the pith begins to degrade. These further increase the surface area, consequently increasing water absorbtion even more, which further increases pith degradation. It’s this excessive water absorption by starch that leads to a gluey consistency and being cut up facilitates absorbtion through these mechanisms. This is also why wax potatoes are more resistant to falling apart than floury potatoes when boiled, as the starch content of a wax potato is much lower. Side note: I actually prefer a 1:1 ratio of wax to floury potatoes in my mash.
A way to test and verify is to boil up some spuds using your preferred method and mash them as normal, but add a fair bit more than normal of hot water, milk, or another primarily water based liquid, i.e., not cream or butter. You’ll end up with a gluey tater paste.
I love you for this.
Aww, thanks! It’s by far the most Lemmy thing I’ve ever done.
Wow, awesome. Thank you!
Do you know whether salting the water heavily affects the water absorption rate of cut potatoes? It should reduce it on cell level, but I don’t know if that is the case with starch
Unless you’re salting so heavily that the end product is inedible, it shouldn’t make a big difference. You’d need a pretty high salt concentration to make the water isotonic or hypertonic when compared to the potato.
Isn’t the osmosis minimized when the salt concentration of the water matches that of the potatoes, and thus it shouldn’t affect the saltiness of the end product? Assuming that the water is thrown away of course.
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 :)
Your explanation of why you make sous vide mashed potatoes because you can’t handle a 10 minute cooking time is hilarious, thanks.