Direct answer to OP- having the water boiling first will cook potatoes faster, or anything else for that matter. This strictly refers to the time the spuds or whatever are in the water. It doesn’t count time waiting for the water to heat as cooking time, which is included in the starting them in cold water method.
To all the people who say not to cook starchy things in water that’s boiling already- what do you do with pasta?
They have a strong taste when boiled and mashed like potatoes. I like to plop mashed rutabagas on mashed potatoes since the taters are rather bland. They complement each other.
What does overcooking meat have to do with cooking vegetables fully? I didn’t say OVERCOOK the vegetables, I said cook until soft, that is, soft enough to eat. I often blanch vegetables in boiling water, but they’re still crunchy when blanched. I either use them cold, like in salads, or fully-cook them with another method later.
Pasta is thin; potatoes are thick. I suspect you’d have the same problem with pasta cooking the outside to mushy while the inside was still raw if you had 1/4" or larger squares of pasta.
I’m probably just looking for more precision in the statement than it ever intended. From much experience of cooking potatoes, I don’t disagree with anything you said.
As has been mentioned, heat transfer and mass of materials is key. Perhaps more science people will weigh in before this becomes a cooking thread, but, bearing that in mind, size and composition must be addressed.
The “Canadian Method” for poaching fish you might see floating around (heh). It’s given as "1 minute per inch of fish, viewed from top to bottom.
Common errors are that “poaching” temperature (hence total enveloping heat) is significantly lower than 212°. The water should give a lazy “plop” every now and then, or even lower temp, so it just shimmers (what the French, being French, call “smiling”.
But heat transfer follows a square law: a fish estimated at x thick gets y temperature, but a 3-x-inch thick fish doesn’t get 3-y temp but 9-y, ie it’s by square and not addition.
Now, a related mass (physics)/cooking point: with a thin filet of non-massy fish–cod rather than shark, say–you don’t do the bring up to boil method for any reason. The heat cooks the filet lickety split, and waiting longer will over cook the fish terribly.
Then there is the matter of what that mass contains. The most important (I am leaving aside baking or sugar work or Maillard reaction and basically everything that has to with heat and food–ie, cooking :)). Fish slime, as well as fish flesh and bone, are protein-rich. Too quickly heating proteins will “denature” them, untangling them, as it were. You wind up with white glop on and around your fish.
Before leaving fish, the “Canadian Method” is recognized trash, unless you like dry over cooked fish.
8 min per inch and check, bearing in mind the exponential requirement.
About protein untangling, again, heat over time is a factor. Plunging a chicken into boiling water or pouring boiling water over stew meat is sure fire death to the food. Ditto baking/roasting for tough (collagen rich) meats: the collagen must relax, not lock down and stiffen within the food.
As to greens, boil em if you like em that way, but chlorophyll is a tender beast: the colors will start to go mushy as the flesh does. For par-boiling (blanching), the short plunge into maximum heat–boiling–water does as much cooking as possible without losing color. And, since the food is still close to boiling when you take it out, to save or “fix” the colors (sometimes called “shocking” them) is to immediately put them under cold water and shut off the heat.
Finally, as to pasta (leaving aside the particular aspects of starch behavior), to maximize the efficiency and speed, you want to have as much heat, not temperature. So, lots and lots of water boiling, not a small pot boiling. I forget what the ratio for max water volume is before it becomes immaterial. It’s also worth it, if you really want to pick up time and be sure of quickest/highest heat transfer, is to re-cover the pot after you’ve tossed in the pasta, so the steam pressure increases the total heat in the pot and the boiling starts again in less time.
(Again, there is this issue with starch and heat: after being heated, the strands of gluten will crumple right up with each other: just-put-in strands of pasta are relatively cooler, and without stirring they’ll glom on to each other. Which, BTW, is why cold cereal leftovers are like little limpets to clean, as is dryer flour down a sink or on a board. The answer in cleaning: use cold water.
Sucks to wait longer for pot to boil, though. Also, low water volume means less room for the exuded starch, and the pasta gets all slimy when it doesn’t have enough liquid to diffuse in.
For beautifully clear presentation of the chemistry and physics of cooking nothing surpasses Harold McGee’s On Food and Cooking.
blanching is not boiling.
blanching is a prep procedure
boiling is a cooking procedure