What are the effects of having a super rapid crazy boil vs. a very slow, slightly rolling boil, and the different stages in between? If you're boiling 6 gallons on an electric stove or trying to do the same thing on a propane burner, the level of your boil is different. What effects will this have on the final product?
I have half a blog post in my head about the six -ations of the boil. Here's a sample.
Also listen to the thermal load episode of Brew Strong for a lot of good information.
In general, shoot for an 8 — 12% evaporation rate.
The most obvious one. The more water you drive off, the more concentrated your wort will turn out. This has the effect of increasing sweetness, hop flavor and bitterness. If your recipe is formulated for a particular finish volume, you may miss flavor targets. It would have to be a pretty extreme difference.
A non-obvious, but important one. S-Methylmethionine (SMM) forms during the boil, which gives way to DMS, a cooked corn off-flavor. DMS has a low taste threshold, meaning just a little is perceptible in the finished product. Luckily for us, SMM easily volatilizes in the boil and is driven out. (This is why it's a bad idea to boil with the lid on.) A weak boil will not eliminate as much DMS precursor, possibly spoiling your beer.
This is a physical process - the act of particles bumping into each other in the boil makes them stick together. Good hot and cold break partly rely on turning over the volume of work. Only a good rolling boil can achieve good coagulation. The final product will be hazier and more prone to spoiling over long-term storage.
Responsible for many complex malty flavors, this -ation requires heat. Stronger heat needed to rapidly boil will increase the reactions. I doubt the difference between 5% and 20% evaporation rate (a easy measure of boil intensity) over a typical 60 minute boil will be very noticable.
(Technically, a Maillard reaction, but that doesn't rhyme. Caramelization involves oxygen, not present in the boil.)
The other two
Boil intensity won't affect sterilization or isomerization much.
One more: energy
More energy is required to heat the boil, duh. May be a factor if you use propane. No effect on the beer.
Evaporation rate is the biggest effect. You'll have to compensate for more water loss with a real aggressive boil.
The next would be slightly faster darkening of the wort as more heat is being applied to the bottom of the pot making it hotter.
For the most part the rate of hop utilzation stays the same as boiling is boiling from a temperature and chemistry standpoint.
I suppose the more vigourous boil would stir the wort more. This may cause more of the hot break to actually breakdown and increase the dissolved protein content in the wort. Similar to the effect Lambic brewers get with super long boils (approaching 6 hours). But at the homebrew level, I would venture to guess that possibility is slim.