Let’s start this off by saying that boiling your beer’s wort prior to fermentation is not absolutely necessary. While it is optional, there are several advantages to this process including: Sanitation purposes, sugar concentration, hop utilization, flavor development, and dimethyl sulfide (DMS) mitigation. Let’s take a spin through these then look at some best practices.
As Louis Pasteur found, we don’t need to bring our pre-beer wort up to a boil to kill off almost all the microorganisms that would have survived the mash. But some mold spores are the exception to this rule. So boiling our wort before fermentation ensures we are starting with a clean slate, nearly free from competing microorganisms.
Many modern recipes will call for 60- to 90-minute long boils. Homebrewers typically lose anywhere from 10–25% of their volume in that time when brewing a standard 5-gallon (19-L) batch. That means that the sugar concentration of the wort will go up by about 10–25% as well. The higher the sugar concentration in the wort, generally the higher the final alcohol %, or ABV. Long boils (2+ hours) are often called for with very high ABV beers where an all-grain brewer is looking to get as much sugar from the mash tun as possible, then concentrate it down.
As brewers of the modern hazy or juicy IPA can attest to, you don’t have to boil hops to gain bitterness from them, but you do need to use A LOT to gain a similar level of bitterness as boiling them. Boiling hops imparts a bitterness that is a hallmark of most modern beer styles. Thanks to the isomerization process (a fancy term for saying a structural shift) of a compound called alpha acids found in the hops, iso-alpha acids are formed in the boil and in general are considered many times over more bitter than their non-isomerized cousin.
This isomerization conversion is dependent on several factors, but length of the boil, wort pH, and temperature are the main ones. The longer the hops are boiled, the more iso-alpha acids are formed until a saturation point is reached. Wort pH has an effect, with lower pH (more acidic) meaning less isomerization. Also, the higher the temperature, the faster the conversion happens. This is important when you note that boil temperature is dependent upon a brewer’s elevation.
There are chemical and physical changes going on in brewer’s wort other than hop isomerization that will have an effect on the beer. Many brewers will know the term Maillard reaction as one that occurs between an amino acid (protein building blocks) and a sugar of some sort. Because there are several different kinds of amino acids and sugars to act as potential actors in this reaction that have different results, we generally just refer to these new chemicals as Maillard products. These reactions are extremely important throughout the culinary world and most folks refer to them as the browning of a food; bread crust, for example, is loaded with Maillard reaction products. In brewing, these reactions occur most notably in the malting process, but also in our brew kettle during the boiling process.
While Maillard reaction products may sound good when brewing a brown porter or oatmeal stout, you may not want them when brewing a Czech Pilsner. It turns out that pH of the wort greatly affects the rate at which Maillard reactions occur during the boil. Lower pH wort stymies the reactions from occurring. If you notice your beer is darker colored than expected, your wort pH during the boil is one possible culprit and an acid addition to the brew kettle may remedy it.
Finally we get to a big one: The potential beer flaw known as DMS. It can present itself as creamed corn, cabbage/sauerkraut, or even asparagus in character. It comes from a compound found in malts (S-methyl methionine or SMM), which is brought into solution during the mash and can be converted to DMS during the boil or by yeast. SMM and DMS can be driven off during the boil. The goal is to reduce both and vigorous boils will help that happen. SMM is found in the highest concentration in less kilned malts, so often brewers will extend their boil times to 75–90 minutes when using a malt like Pilsner to help drive down the higher SMM and DMS concentrations.
First of all I highly recommend you pick up an anti-foaming agent from your homebrew shop (or Gas-X from a pharmacy). It can save you a mess from dreaded boilovers. Once you’ve achieved a stable boil, keep it at a nice rolling boil. It shouldn’t appear as a volcanic eruption or with convective currents barely perceptible. Apply just enough power to have it rolling around on itself. Also note the volume in your kettle just before and just after the boil to figure out what % water boiled off. Having an idea of this is extremely helpful to determine how much pre-boil wort you’ll need dependent upon the length of your boil. For example, my most recent batch of homebrew was 12 gallons (45 L) and I lost roughly 1.2 gallons (4.5 L) to vapor, which means I had a 10% boil-off rate . . . which is generally my goal. Try to hone yours.