Just about every stew/soup recipe I’ve tried includes the step “Raise the heat to [medium-]high and bring [the protostew] to a boil. Reduce heat and simmer.” What is the purpose behind bringing the stew to a brief boil? It doesn’t seem like such a brief boiling period would serve to reliably pasteurize the stew.
Sua
I figure it’s just to get it to boiling faster.
I believe the point is that it heats the liquid quickly, and lets you keep it at the slow simmmer for a more reliable period of time, rather than just letting it slowly heat and eventually achieve a simmer, during which time you had very little control over how long it was at what temperature.
Remember, at a simmer, the liquid is still the same temperature as at a full boil.
I would suspect that’s because it’s a good way to tell that all of the mixture is hot enough to be simmering well.
If you just put the mixture at a low heat and wait until tiny bubbles or steam appear, that might just tell you that the stuff at the bottom is hot, but other parts of the dish aren’t. Then it might stop ‘simmering’ for no particularly obvious reason
Getting up to a full boil, on the other hand, is more likely to distribute heat around the whole thing, at which point you can reduce to a good simmer.
That’s a culinary WAG though.
Wow, beaten to the punch twice! 
It seems to be a consistent method of ensuring that the stew is fully cooked in a reasonable amount of time.
WAG: The full boil establishes the convection currents which the simmer will maintain but which may take a long time to get started at only a simmer, thus distributing heat evenly throughout the matter-to-be-cooked in a more reliable fashion?
That makes sense. Thanks.
True (well, near as dammit). If you continue cooking at a full boil, the liquid contents of the pan won’t be appreciably hotter, but the metal base of the pan will be; this can cause more moisture to be driven off from the bottom layer of stew and it is thus much more likely to stick and burn.
Ah, no it isn’t. A simmer is defined as being between 180 and 210F as this very informative website shows.
That “very informative” website shows nothing except a bare definition that is not universally agreed upon. As is noted at Wikipedia (no, not definitive, but a good basic explanation), some chefs assert that temperatures as low as 180 F are simmering, but in general, water begins to simmer at 200 F, and I maintain that, for the average cook in a kitchen, your “simmering” water is probably at or very close to 100 C. It takes a great amount of precise control, usually absent with most average stoves and cookware, to get to where you notice the difference and can keep it that way indefinitely.
As a practical matter I once made a stew without first bringing it to a boil, just simmering for several hours. The veggies weren’t done, in particular the potatos.
I have checked my thermometer for accuracy and I need to have a rolling boil for it to be 212 F. At simmer it is significantly lower. I am sure the water at the bottom of the pot where the bubbloe are forming is a boiling, but the rest of the pot is not. Also, meat is much tougher when cooked at boiling than at simmer.
WHOOSH! 
I will swear that Cook’s Illustrated had a note on simmering a while back, but I don’t see it on their website. Frankly I am too lazy to dig through all my back copies to find it.
Getting back to the OP, I did find this on the Cook’s Illustrated website
“Simmering – cooking foods over moderate heat – is an important technique in making soups, stews, braises, sauces, and stocks. But why boil first? It boils down to two major issues: time (and energy) efficiency and food safety. If you bring a stew or braise up to a simmer over low heat, the total cooking time will be considerably longer. In one recent test here in the kitchen, an osso buco recipe required an extra hour when we failed to bring the liquid to a boil before turning down the heat. Starting from a boil also ensures that all of the ingredients (proteins and starches as well as the liquid) in the pot get up to a
safe temperature quickly and evenly. If you let foods come to a simmer very slowly, they are likely to spend more time in the so-called danger zone, between 40 and 140 degrees, which in certain foods promotes the growth of bacteria.”
Also from Cook’s on a boil vs a simmer
“The extra time required to “beef up” the canned broth in our Hearty Beef and Vegetable Soup is well worth it. Although it’s mostly a hands-off procedure, you do need to watch the pot. If the broth is allowed to boil, the fat and meat particles will get broken into smaller and smaller pieces that become suspended in the liquid. The resulting broth will be cloudy and greasy. For a clear, grease-free broth, make sure the broth simmers slowly. In this case, the notion that “a watched pot never boils” is a good one. For the clearest broth, use a wide spoon to skim the impurities that float to the surface in the first few minutes of cooking.”
This would tend to indicate that a simmer is a lower temp than a boil.
Slow, wet cooking will turn tough connective tissue into stock (and, when cooled down, gelatin.) That turns a cheap cut of meat melt-in-your-mouth tender. As Rick said, food safety is an issue. You need to rush it through the danger zone, then you can safely back down to a simmer, above 140 degrees F.
So what about when using a slow-cooker (crockpot)? I just received one (and a cookbook of recipes for it) for Christmas and I’ve used it a few times now, usually on the “Low” setting for 8 to 10 hours. How fast do slow-cookers get out of this ‘danger zone’? Are they designed to automatically bring things up to a high temp fairly quickly and then back down to a simmer?
For batches, of chicken stock, I set the stove so there is 1 bubble every few seconds and the temperature still reads between 99C and 100C. There is no temperature difference between simmering and boiling. The notion that simmering involves cooking at sub boiling temperatures is a widespread kitchen myth.
How are you measuring the temp? I dipped my instant read thermometer a few inches into a simmering pot of water and saw less that 212 F. I’ll check again tonite.
Depending on ambient air pressure (due to altitude or the weather) as well as additional ingredients, the boiling point just plain and simply not be 212/100 degrees. It may be lower (air pressure) or higher (salt, sugar, other ingredients). When I make beer wort at ~600 feet altitude, I have a good, full simmer at 170 degrees F. That actually surprises me, because I would expect it to be higher due to a higher boiling point.