Heat applied to a boil

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Hamsterbite

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Just out of curiosity, assuming we're at sea level, what effect does increasing applied heat to a boil have?

I'm trying to get a handle on my brew process and have a better understanding of what people mean when they say to boil vigorously, or boil moderately, etc...to me, boiling is boiling. 212F is 212F.

For instance, lets say my gas burner valve can be opened on a scale of 1 to 10. Typically, I crank it up to 7 or 8 to get a 6 gallon pot to boil from ambient. Once it reaches a full rolling boil, I can back off the valve to 3 or 4 to maintain the boil. Besides wasting money on propane, What would be the effect on the wort if I kept the valve wide open throughout the boil? Would I get increased evaporation? Would I get increased carmelization? Is it irrelevant since the wort cannot be raised beyond 212F without pressurizing it?
 
It's my understanding that a more vigorous boil due to higher heat causes more Maillard reactions, darkening the color of the wort, and adding flavors.
It's not that the water is hotter than 212F, its that more energy is input into the system, causing more reactions.
Higher heat = more evaporation, more vigorous boil , more Maillard reactions.
 
I second what Arc said, energy input can make a large difference.

Also, the more you boil off the higher your ABV, since you are concentrating the sugars who like salt do not boil off. I mash extra to boil off about a fifth of my oatmeal stout in order to run up the ABV and thicken it up a bit.

As far as taste goes, I find with my stouts that a little caramelizing on the bottom of the kettle is a good thing, but not with my German whites. (by little I mean more like a cooked on residue, not a crust) Depending on what you are brewing I may pull off a gallon or so and boil harder to see what effect it would have on the finished product.
 
McRedimus said:
Also, the more you boil off the higher your ABV, since you are concentrating the sugars who like salt do not boil off.
Click to expand...

Not really. You could get the same result with 5 gallons of 1.070 wort as you would taking 6 gallons of 1.060 wort and boiling off a gallon. It's all about your efficiency, not how much you boil off.
 
Theoretically the only result you will see in increasing the propane flow to the burner is increased consumption of propane (your burner/kettle combination is probably more efficient at moderate heat than it is at high unless it is skirted to trap the flame) and increased evaporation rate. If you boil vigorously to the point where you must add makeup water after the boil you will get a different beer than if you boil less vigorously for a longer period of time and don't need makeup water. The vigorous boil is good for promoting break formation, for hops extraction etc. Also if you wind up with more concentrated wort the temperature will rise above 212 and you can get more Maillard products. If you apply lots of heat then local conditions can get set up at the bottom of the kettle where it is 'dry' (i.e. covered with steam rather than wort) for part of the time and the temperature there can go up thus increasing caramelization. What will happen with your particular setup for a particular beer can probably only be determined by experiment.
 
ajdelange said:
Theoretically the only result you will see in increasing the propane flow to the burner is increased consumption of propane (your burner/kettle combination is probably more efficient at moderate heat than it is at high unless it is skirted to trap the flame) and increased evaporation rate. If you boil vigorously to the point where you must add makeup water after the boil you will get a different beer than if you boil less vigorously for a longer period of time and don't need makeup water. The vigorous boil is good for promoting break formation, for hops extraction etc. Also if you wind up with more concentrated wort the temperature will rise above 212 and you can get more Maillard products. If you apply lots of heat then local conditions can get set up at the bottom of the kettle where it is 'dry' (i.e. covered with steam rather than wort) for part of the time and the temperature there can go up thus increasing caramelization. What will happen with your particular setup for a particular beer can probably only be determined by experiment.
Click to expand...

Ahah. So this "dry" border zone you mention...is that what is known as a film boil?
 
Increased heat will lead to a larger boil-off. That’s why I suggest keeping track of your boil-off rate. It should be between 10 and 15% per hour. In this range you have a sufficient vigorous boil to scrub DMS but the intensity is not too high to scorch the wort.

To avoid scorching you also want to make sure that the flame heats a large area of the pot or that the pot’s bottom distributes heat well.

Kai
 
Kaiser said:
Increased heat will lead to a larger boil-off. That’s why I suggest keeping track of your boil-off rate. It should be between 10 and 15% per hour. In this range you have a sufficient vigorous boil to scrub DMS but the intensity is not too high to scorch the wort.

To avoid scorching you also want to make sure that the flame heats a large area of the pot or that the pot’s bottom distributes heat well.

Kai
Click to expand...

Thanks Kai.

By the way, I grew up in your neck of the woods and actually lived in Pepperell for a short time. I still crave for a C&S cheese-steak now and again...I was sad to see they were gone when I visited friends a few years ago.
 
I'm sure it does and lots of other strange stuff goes on in the kettle too. For example consider pressure. A foot of wort exerts a pressure of half a pound per square inch on the bottom of the kettle (1/30th of an atmosphere). This is enough to raise the boiling point there by almost 1 °C.
 
Following the wiki trail, it looks like Critical Heat Flux (CHF) is part of the phenomenon I'm interested in. http://en.wikipedia.org/wiki/Critical_heat_flux

Interesting stuff. This may explain why my brew pot sometimes gets a caramel stain on the bottom in the shape of my burner. This is apparently happening in the absense of direct contact with the water. Instinct told me that a kettle wouldn't scorch that was in contact with water, so it was a bit of a mystery. Turns out, the reason it scorches, is that the water is seperated by a film of steam when input temps reaches a certain (condition variable) point.
It may also explain how sometimes just as a boil starts to roll, it can fall back down and cycle despite the heat source remaining constant. Once that film develops, heat transfer is reduced. I can certainly see why having a wide pattern burner is advantageous.
 

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