Why should one boil water over medium-high heat for pasta?

Question

I have encountered a pasta recipe with this as the first step:

Bring a large pot of water to boil over medium-high heat.

What is the purpose of using medium-high heat? Isn't it faster and more efficient to use high heat to boil water?

While there can be situations where medium-high is optimal (MeltedPez described some below) it doesn't have to have a purpose in a recipe. Recipe authors tend to choose their recipe steps based on a "that's how I always do it" principle. There are those who engineer their recipes into some kind of optimum, but they are rare. — rumtscho, Jan 9, 2017 at 11:18

MeltedPez · Accepted Answer · 2017-01-09 01:30:45Z

A few lines of thought:

Pasta water boiling over happens to the best of us. Either from not paying attention, using too little water, or using too much water. Using the medium-high heat versus max heat helps mitigate boil overs from happening.
On my own stove, when the burner is at its highest setting, much of the heat ends up being directed up the sides of the pot, rather than on the bottom of the bot. The gas flame becomes so wide underneath the pot, that much of the heat is lost to the air, rather than being focused on the pot. This makes the medium-high setting more efficient than high. Obviously this all depends on the pot and the burner, but even my larger pots have this happen to them.
If you were shooting for another temperature, like 200F (93C), instead of boiling it would be hard to hit that number on high heat without going over. Even if you have a thermometer in the pot and turned the burner off immediately after hitting 200F, the water would continue to rise in temperature as the residual heat carries over. Ideally, as the water gets hotter and closer to 200F, you would want to slowly back off the heat until getting to the point that will allow you to hold the water at exactly 200F. I think it's the same idea for boiling water. It's okay to use high heat, but you want to back off the burner as the water gets closer until you end up on medium-high: a good happy-medium for keeping water boiling versus having hotspots on the bottom of the pot.

Water won't boil (under normal circumstances) at a temperature lower than 212 F. As noted in another answer here, pasta water definitely needs to be at a full boil so that the water is evenly heated (soft boils are only at 212 near the burner and not at the surface) so I'm not sure how your third point applies. — Catija, Jan 9, 2017 at 2:38
@Catija that other answer is wrong. You can simmer pasta perfectly well at less than 212 F, I have done it frequently. Now simmering is not boiling, but it wouldn't be the first time that a recipe writer mixes up the terms, especially nowadays when user-submitted recipes abound online. OK, "bring water to boil" is unlikely to also include simmering in this exact recipe, but MeltedPez's argument is valid in principle, it wouldn't be that surprising to come over a recipe which aims at simmering the pasta. — rumtscho, Jan 9, 2017 at 11:12
"Hot Spots" at the bottom of the pan would be the metal of the pan itself. Though the water itself will not go noticeably above 212F/100C, the pan itself will. This happens at medium high as well, just not a high. Not really an issue, unless say the pasta or anything else being cooked in the water happens to stick or sink, then it is going to be prone to scorching and sticking. Lower heat temps do not eliminate this, but do reduce the chances. Normally though I would want the heat high enough to maintain boil when the temperature shock happens by adding items. — dlb, Jan 9, 2017 at 13:44

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