When you're supposed to boil and then let simmer, what does the boiling do?

There is a good discussion here

Basically it is so you can bring the heat up quickly. Once you have the food up to cooking temperature, you can lower the heat and simmer it. Simmering really is just a very gentle boil. A high boil could potentially make food tough or interfere with the melding of the flavors.

I would think it was to kill nasties that might survive a simmer.

When it is adding something to boiling water, like pasta, when you add the pata there is a temporary decrease in temperature, because the pasta is room temperature, so if the water was only at a simmer, it would barely have a bubble when you add the pasta.

For some foods it probably makes a difference chemically on how it cooks, but even if it doesn’t it certainly would reduce cookig time. Simmer helps food not scald on the bottom of the pan, and is easier to monitor, also helps dense or thicker foods cook through properly. So, it might be less practical to boil foods, which would seem to take the least amount of time, but a boil at the start, when there is a lot of liquid in what you are cooking may not harm the food in any way, and speed up cooking time.

If we go back to the pasta example, boiling constantly doesn’t work because the starch will boil over. Recomended cooking times are started at a boil, then simmer. But, there is an old lasagna trick to just sit the noodles in hot tap water for 20 minutes before layering into the lasagna pan.

When bringing food to a boil it quickly cooks the food and brings it to a temperature for cooking. Afterwards the simmer lets the remaining uncooked portion fully cook at a slower rate.

@world_hello No, because it was in the recipe for French Onion Soup, which has no nasties.

Plus, to kill “nasties” in water most experts say to boil for at least five minutes.

Like many of the previous answers, Boil and Simmer are close synonyms. With some minor fluctuations for dissolved ingredients, water boils at 100 C (212 F)—it does not get any hotter if it boils faster, just evaporates more quickly. “Bring to a boil and then simmer” is like saying, “Accelerate on the highway’s on ramp to 55 mph and then set your cruise control to 55 mph.” “Bring to a boil and then simmer for 15 min” is way different than “put your stove on low and wait 3 hours until it begins to simmer”... your recipe would not turn out very good. =]