I know some people who refuse to drink in large coffee chains as they say that the coffee they produce tastes "burnt".
Is it actually possible to burn coffee while brewing, or is the flavour they're complaining about coming from something else?
Burn? I've heard that some large chains "burn" their beans while roasting :-) But not while brewing.
A major coffee chain, seen on almost every other corner in some places, is known for their very dark, "burnt" roast. Many people like this sharp taste and associate it with caffeine and their morning pick-me up.
Other people find this "burnt" roast to be overwhelming and consider this heavy roast to be destructive when using high-quality beans. In any case the burning happens at roast - not when brewing.
I've had coffee that tasted burnt in a lot of cheaper restaurants and from common coffeemakers in homes. It's uniformly been stuff that's sat on a warmer for a long time (like, an hour or more) after brewing. Keeping coffee on heat after brewing "cooks" it down, just as keeping a soup or sauce on a simmer reduces it. However, it has the opposite effect on coffee than on sauces or soups; instead of making it richer, it destroys the more delicate flavor compounds and makes it taste burnt. Even reheating coffee in the microwave can make it taste more like an ashtray than coffee.
It depends on the method of brewing. Brewing methods where grounds are exposed to a heat source directly (Turkish, for example) could allow the grounds to be heated to the point that they started to roast/cook/burn (again). Most methods of brewing, however, only have the grounds exposed to boiling or near boiling water which is ~210F and less than would be necessary to start cooking/roasting/burning the coffee beans again.
Many of these other answers actually talk about burning or cooking down stored coffee after it has been brewed.
As to your question about large chains, my person belief is that the burnt taste most people experience is a branding issue. Lightly roasted or medium roasted coffee will taste more like the bean it came from, or its origin. The darker you roast, the more the origin flavors go away and are replaced by the roasting flavors of the bean. As it so happens, the origin flavors of roasted coffee fade very quickly (sometimes in as short as a week). However, the roasting flavors seem to hang around a bit longer. Large chains dark roast their coffee because it makes it easier to consistently produce close to the same taste and the flavor of those dark roasted bean will hang around a bit longer (less shipping and storage concerns).
Yes, coffee can be burned when brewing. When an empty pot sits on the heating element in a restaurant coffee machine it can get very hot. When the first stream of a new brew cycle hits the bottom of the pot, boils off very quickly, producing a horrible smell and taste that infuses the entire pot.
In addition to the water boiling off, the various organic compounds in the coffee are subjected to temperatures well above the boiling point of water for a brief time, until the volume of new coffee cools the bottom of the pot enough to stop the boiling. Knowledgable staff know to leave a little clear water in the bottom of the pot to prevent the temperature from getting too high before the next pot is brewed.
In addition to all the great answers (most of the burnt flavor is typical for dark, e.g. italian roast) I want mention something I encountered during my time as a bartender:
When having a portafilter (professional espresso machine, as used for cappuccino etc) it sometimes happens that you prepare (e.g. attach the handle with coffee to the machine) and then something comes up and you don't start the extraction process right away. What happens then is that the hot machine heats up the ground coffee. Which in turn results in a burnt taste of the coffee if left unattended for long enough. My educated guess is that (due to the fact that ground coffee has a very large surface / very small particles) the coffee is burnt/ roasted even further at low temp.
Also note that the water is only 94°C but the metal surroundings do not have to be this "cold". As it should be able to maintain the water temperature even if you brew a lot of coffee.
The word bitter is being used to describe two different things. There can be bitterness in a coffee from over extraction which causes tannin to be present in the brew. This can happen with light roast coffees which are over extracted as well as dark roast coffees. Burnt coffee is also described as bitter coffee when it is really a burnt flavour. There is much confusion about this nomenclature.
I think I know what you are talking about, the question is if a steam brewer can burn the ground during brewing due to the higher temperature vs. a pump operated brewer here is a link that might help