Boiling and Cooling
The "Hot Break"
A foam will start to rise and form a smooth surface. This is good. If the foam suddenly billows over the side, this is a boil-over (Bad). If it looks like it is going to boil over, either lower the heat or spray the surface with water from a spray bottle. The foam is caused by proteins in the wort that coagulate due to the rolling action of the boil. The wort will continue to foam until the protein clumps get heavy enough to sink back into the pot. You will see particles floating around in the wort. It may look like Egg Drop Soup. This is called the Hot break and may take 5-20 minutes to occur, depending on the amount of protein in your extract. Often the first hop addition triggers a great deal of foaming, especially if hop pellets are used. I recommend waiting until the Hot break occurs before doing your first Hop addition and timing the hour. The extra boiling time won't hurt.
Covering the pot with the lid can help with heat retention and help you achieve your boil, but it can also lead to trouble. Murphy's Law has its own brewing corollary: "If it can boil over, it will boil over." Covering the pot and turning your back on it is the quickest way to achieve a boilover. If you cover the pot, watch it like a hawk.
Once you achieve a boil, only partially cover the pot, if at all. Why? Because in wort there are sulfur compounds that evolve and boil off. If they aren't removed during the boil, the can form dimethyl sulfide which contributes a cooked cabbage or corn-like flavor to the beer. If the cover is left on the pot, or left on such that the condensate from the lid can drip back in, then these flavors will have a much greater chance of showing up in the finished beer.
Did you ever wonder where Murphy's Law came from? Well back at work there was a photocopy of a short article from one of the aerospace trade journals on the wall of my friend's cubicle. It went something like this:
Captain Murphy was part of an engineering team out at Edward's Air Force Base in California. Their team was investigating the effects of high gravity de-accelerations on jet pilots back in the 1950's. One of their tests involved strapping a test pilot into a rocket chair equipped with strain gages and other sensors to help them quantify the effects of high G stopping. The responsibility for the placement of the various sensors was Capt. Murphy's. Well, the test was run (subjecting the pilot to something like 100 G's of deceleration) and he got pretty banged up.
Only after it was over did the team realize that of all the possible combinations of placing those sensors, Murphy had done it in the one configuration that resulted in useless data. They would have to run the test again. Upon realizing this, Murphy stated, "If there are two or more ways of doing something, and one of them can result in catastrophe, someone will do it that way." Upon hearing this the team leader said, "That's Murphy's Law." The next day at the test de-briefing the team leader shortened it to the now famous, "If anything can go wrong, it will." Murphy still likes his version better.