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Anthony Bourdain's Les Halles cookbook says that a stock should NEVER be boiled, why is this? Does the higher heat extract bitterness or something else undesirable?
It seems a little strange because the bones used to make the stock were previously roasted in a hot oven.
I've only made stock once, well I guess it was a broth (see this post) but would like to improve on my technique.
Well there's a few things.
First, higher temperatures mean more volatile aroma and flavour compounds will be released, leaving a flatter-tasting stock.
Second, boiling means more motion within the liquid, which makes it harder to skim off the protein scum which forms on the surface while a stock is cooking. In addition, some of the scum will just become reincorporated into the stock via a process similar to emulsion.
Third, boiling induces faster breakdown of proteins and other particulate matter, leaving you with a cloudier end product. This is of course of very little concern for stocks which will end up incorporated into (opaque) soups or sauces.
Of the three, only the first is of any real concern to home cooks. The other two issues aren't a huge deal at the restaurant level (modulo need for clarity in consomme etc), particularly if the restaurant uses Superbags or algae filtration for clarifying stocks.
For regular stocks, the main difference is aesthetic: a boiled stock will be cloudy because broken down protein and fat are emulsified into the stock. Once emulsified, you won't be able to easily degrease the stock.
The length of the boil and the temperature (in a pressure cooker for instance) will affect how much fat is emulsified and this can impact the flavour.
As this Salon article by Francis Lam states:
ARGH! I let it come to a boil / don’t have a fine mesh strainer!
OK. Alain Ducasse would probably deem your stock unfit to water his weeds with, but honestly, it’s fine. What happens is that a boil will emulsify some of the fat into the stock, making it less than crystal clear, and some will say it dulls the flavor. A less-than-utterly perfect strain will leave some errant bits of protein or whatever to sink to the bottom. I think if you can taste the difference, no one is good enough a cook for you anyway. So don’t stress. Also, some classic Asian versions actually call for the liquid to be boiled, specifically to get that fat and protein emulsified, resulting in thick, rich, milky-looking stock. And they are fantastic.
I've seen occasional reports online that boiling it will turn the stock bitter, but these don't seem to be substantiated by any food science I'm aware of. It's vastly more likely that the bitter flavour is because the stock was cooked for too long, which can be particular problem for vegetable stocks. It's also the case that an unsalted stock will taste bland and unappetising and can have bitter notes that salt camouflages. And of course it won't resemble commercial stock cubes at all. Stock is meant as a versatile base and needs to be seasoned in the final recipe.
You know, I experimented with this a long time ago and these are my conclusions:
Stews where you want to eat the meat, should NOT boil, because you ruin the cells before the tough stuff can gelatinise
people translated this to stock, but you wont eat that meat (if any), so this is nonsense
volatiles escaping when boiling in stead of simmering? difference of five degrees or so? Yes, because of the bubbles and the mechanics of the movement, but I never tasted any difference really
cloudy because of mechanic emulsification: yes. But cool it down, get rid of the fat, and filter with paper. You will have a pretty clear stock.
Why a clear stock anyway? Beats me. Visuals I suppose.
Makes boiling and getting muddy stock a different taste? mmmm, if you really boil them, yes, (see emulsion). If you de-fat and filter? None.
So: a perfect clear broth is a status symbol, like a gold watch: it takes effort to get it, but it is pretty useless really. It shows that the cook made an effort, but taste difference is pretty much zero. Boil your stock if that is easier for you. Use it unfiltered and with the fat still in it, makes great greasy sauces. Cool it down, and scoop that fat off if you want to be a bit more fancy. Filter it if you want to impress even more. Use the egg white method if you want near-perfection. But dont worry about boiling or not and checking for hours and hours.
Mrs Beeton said "A boiled stew is a spoiled stew". Keep it under a boil, so a couple of bubbles come up now and them.
If it bubbles away, some meats go tough, some flavour is lost and steam causes condensation in the bathroom upstairs. But eggs boil nicely. Put the lid on, you will trap the heat and all the water wont evaporate. Turn it down a bit. Good to steam puddings.
Full rolling boil is only for pasta and jam.
If you hold your hand above a pot of boiling stock, you'll discover another reason not to boil. It seems that the fats and oils from the stock get into the steam and then travel wherever the steam travels. Boil 8 or 10 batches of stock and you'll find your kitchen covered in a thin layer of grease if you don't have a ventilation system.
Simmering your stock reduces steam, which reduces the amount of fats and oils that escape from the pot.
