Wort Boiling
Homer Simpson is my favorite TV character. One of my favorite Homerisms comes when Homer realizes that he is no longer in touch with modern music and he vows to “get out of this rut . . . and back into the groove.” It’s easy to get in a rut while brewing. Many homebrewers continue to follow the same brewing procedures they learned when they started. It’s possible, however, that better information has filtered into the homebrewing community since you began. I’ve finally unlearned much of what I “knew” and have started making a few changes when I boil.
Homebrewers employ a variety of equipment to boil their worts, ranging from kitchen pots on the stovetop to modified kegs heated by propane burners. Most homebrew setups involve a “simple” kettle — one with no internal structures for heating or circulation — heated by an external heating source.
Concentrated vs. full wort boils
Most extract brewers perform concentrated wort boils. A “thick” wort is boiled then diluted with water to working strength prior to fermentation. The smaller the volume of wort boiled, the higher the specific gravity of that wort in the kettle. For example, 7 lbs. (3 kg) of liquid malt extract (LME) dissolved in 5 gallons (19 L) of water yields a specific gravity of 1.051. The same amount of LME in 3 gallons (11 L) yields a specific gravity of 1.086. To calculate your boiling gravity, multiply your target original gravity (OG) times your batch size divided by the amount of wort you are boiling. For example, if you are making a 5-gallon (19 L) batch of porter with a target OG of 1.060 and boiling 3 gallons (11 L) your boiling gravity is 60(5/3) = 100 (a specific gravity of 1.100).
In a full-wort boil, the entire wort is boiled at working strength. At the beginning of the boil, the volume of wort is greater than the batch size and more dilute. Boiling condenses and concentrates the wort to working volume and concentration. A five-gallon brewer will typically start with 5.5 gallons (21 L) of wort and boil it down to just over five gallons (19L). After cooling, they can transfer five gallons (19 L) of wort to the fermenter, leaving the trub and hop debris in the kettle. There are several advantages to this method.
Hop utilization increases in full wort boils compared to boiling a concentrated wort. In other words, you get more bitterness out of your hops as your wort gets thinner. Table 1 shows hop utilization vs. specific gravity.
A full wort boil also leads to less wort darkening. The more concentrated the sugars are in the boil, the more likely they are to react with each other or amino acids in the wort.
A full-wort boil also promotes better break formation. When heated, proteins, carbohydrates and tannins in the wort react and form what brewers call break material. Some of this break material appears as solids while the wort is boiling. This is called hot break (or hot trub). Other break material only becomes insoluble in cold wort and is called the cold break (or cold trub). If proteins or lipids don’t get formed into break, they can carry over into the finished beer and cause problems with chill haze. They also make the beer susceptible to bacterial growth.
The advantage of a concentrated wort boil is the convenience while the advantages of a full-wort boil relate to beer quality. Extract brewers should therefore seek to boil as much wort as the size of their brewpot, power of their stove and time constraints allow. Many extract recipes give specific amounts of liquid to boil the malt extract in. These recipes are meant to be quick and simple to brew and the recommended wort volume reflects this. You can — and should — boil larger volumes if you can.
Boil times
All brewers must decide how long to boil their wort. These days, most homebrew recipes call for a one-hour boil. However, an even longer boil may help improve beer clarity and stability.
The longer that wort is boiled, the more break material is formed. More break material removed from your wort will ultimately yield clearer beer. Also, your beer will more stable. So, if you have the time on brewing day, try extending your boil times and see if that makes a difference in your final beer. Note that you will need a larger initial volume if you are boiling for 90 minutes. For 5 gallons (19 L) of beer, you should start with 5.75 gallons (22 L) of wort compared to 5.5 gallons (21 L) for a 60 minute boil. You can add 0.25–0.33 gallons (1–1.2 L) to that to account for the hops and trub that settles to the bottom of the kettle.
It’s interesting to note that traditionally some of the lightest-colored beers got boiled the longest. See Table 2 for a list of recommended boil times for various beer styles.
For extract brewers, there is one further variable to consider. Many liquid malt extracts (LMEs) are already boiled. So, when making beer from LME, you can boil for at little as 15 minutes, just long enough to sterilize the wort. Beers made from dried malt extract (DME) still need to be boiled for at least 45 minutes.
What happens during the boil?
A lot happens during the boil, even though brewers don’t do much during this period. Let’s tour the boil and find out what’s going on and what, if anything, we can do.
Wort Expansion: Wort expands when heated. A five-gallon brewer is unlikely to notice this, but larger-
volume homebrewers may notice the volume shrinkage upon cooling. At 68 °F (20 °C), ale fermentation temperature, wort occupies about 4% less volume than it did at boiling (around 215 °F/102 °C for most worts). For a 5-gallon (19-L) batch, this amounts to just over two “lost” (12 oz./355 mL) beers.
Evaporation of Water: When wort boils, water evaporates from it. One consequence is that the wort volume will shrink. This shrinking more than counteracts the expansion due to heating, which stops once boiling starts and the temperature is no longer rising.
An easy way to determine the vigor of your boil is to measure the evaporation rate. To calculate this, measure your wort volume at the beginning of the boil and again one hour later. Your evaporation rate, given in percent per hour, is calculated as:
For example, let’s say you had 6 gallons (23 L) at the beginning of the boil (time 0) and 5 gallons (19 L) one hour later (time 60). Your evaporation rate would be 1-(5/6) = 0.1667, an evaporation rate of 16.67%. For most homebrews, a 10% evaporation rate per hour is a good wort vigor. Less than this and your hop extraction and break formation suffers. A greater evaporation rate can yield too much darkening.
Another consequence of evaporation is that the concentration of sugars will increase in the wort. You can estimate how the gravity of your wort will change by using the formula C1V1 = C2V2. In the equation, C1 is the concentration of wort at the beginning of the boil and V1 is the volume at the beginning of the boil and C2 is the unknown concentration of wort at the end of boil, when the wort will have a volume of V2.
Let’s say that you have 6 gallons (23 L) of wort at a specific gravity of 1.040 and plan to boil it down to 5 gallons (19 L). Substituting the numbers into the equation, we get 6(40) = 5(X), where X is our unknown specific gravity. (Notice that you only use the decimal portion of specific gravity — i.e. 1.040 becomes 40.) Solving for X, we get 6(40)/5 = 240/5 = 48. So our expected specific gravity would be 1.048.
This formula will, however, consistently overestimate your final gravity. Your early reading of specific gravity will be inflated by soluble proteins and other molecules in the wort. These will cause your hydrometer to float higher. Late in the boil, these substances will have precipitated out and will not affect the gravity. My estimate is usually off by three or four gravity points when I use this formula.
A third consequence of the evaporation of water is that color-bearing molecules will become more concentrated, darkening the wort.
Wort darkens for two reasons. Primarily, the wort gets darker because it is getting more concentrated and secondarily because chemical reactions are forming colored molecules from colorless precursors. The caramelization of sugars is one example of this type of reaction. Maillard reactions are another. Caramelization occurs when (colorless) sugars react with other sugars and form color-bearing polymers. Maillard reactions occur between sugars and amino acids.
If you want to differentiate between the effect of wort concentration and direct color development in your wort, try this experiment. Take a sample of wort immediately after the hot break then take a second sample at the end of your boil. You can compare the two to see the extent of wort darkening. To estimate how much of the darkening was due to color-developing reactions, dilute your final wort back to the concentration it was when you took the first sample. Comparing the early and late worts, corrected for loss of water, should show you how much wort color comes from Maillard reactions and sugar caramelization.
Don’t take this test too seriously, though. Other things that affect color are going on as well, including the effect of the precipitated break material. However, this is good, quick check for extract brewers whose beers are too red. You can check if the color is developing during the boil or if your extract was simply carrying too much color to begin with.
Evaporation of DMS: Other volatile chemicals, including DMS, are also evaporated during the boil. DMS is a molecule that leads to a cooked corn smell in the beer. Precursors to DMS are found in lightly kilned malts. A good, rolling boil — followed by fast wort cooling — will minimize DMS.
Chemical reactions: Wort is a complex mix of water and biochemical molecules, including carbohydrates, proteins, lipids and other molecules. When you heat this mixture, many chemical reactions occur. I’ve already mentioned two important reactions — those that form Maillard products and those that form break material.
The chemical reactions involving hops and their bittering compounds are obviously of interest to brewers. In the boil, alpha acids in hops are converted via heat to iso-alpha acids. Alpha acids are insoluble in wort and are not bitter. Iso-alpha acids, however, are both soluble and bitter. The amount of alpha acids converted to iso-alpha acids depends on how long the wort is boiled and the specific gravity of the wort. Most brewers boil their bittering hops for at least one hour. On average, a homebrewer will convert 25% of the alpha acids in their hops to iso-alpha hops in a one-hour boil.
In the boil, calcium ions in the water and phosphates derived from the grain react and drop out of solution. This results in a drop in pH. The wort should drop from a pH of 5.4–5.6 to a pH around 5.2. If your wort pH is too high, the resulting beer may taste dull and lifeless. Adding a small amount of calcium — about 1/4 tsp. gypsum or calcium chloride per 5 gallons (19 L) — can help the pH get to the right point.
Convection currents: Wort is not heated evenly. When temperature differences within a volume of liquid exist, convection currents result. In commercial kettles, the shape of the kettle — and the presence and placing of internal heating elements — are designed to induce currents in the kettle. Convection currents help mix the wort and help with break formation. Homebrewers don’t need to worry about convection currents. Stirring the wort a few times during the boil should ensure adequate mixing.
Cessation of biological activity: Boiling will kill bacteria and yeasts. Some bacteria and fungi can form spores and survive a boil, but there are no common wort or beer spoilers that do this. Boiling will also inactive the enzymes you utilized in the mash.
Kettle additions: The boil is also a time for kettle additions such as Irish moss, which helps clear break material, and yeast nutrients.
Conclusion
Pay attention to your boiling procedures and you’ll make better beer, which according to Homer is “the cause of — and solution to — all of life’s problems.”
