Wort Boiling & Hop Character
The bitterness in beer is developed by adding hops to boiling wort. Likewise, boiling hops adds hop flavor and aroma. The bitterness keeps the beer from being too sweet and the flavor and aroma of hops add a special “spice” to the beer. The amount of bitterness in beer depends primarily on how much hops are added to the kettle and how long they are boiled. The hop flavor and aroma of a beer can be altered by adding hops at different times during the boil. Hop flavor and aroma can also be added to beer outside of the boil, for example, when a hop jack is used or the beer is dry hopped.
Since the boil is where most of the “action” happens with regards to hopping, we’ll start by reviewing what happens in the boil in a section I call . . .
What happens during the boil?
A lot happens during the boil, even though brewers don’t do much during this period. Let’s take a trip through the boil, 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, the volume shrinkage is approximately equal to two (12 oz./355 mL) beers.
Evaporation of Water: When wort boils, water evaporates from it. One consequence is that, the wort volume will shrink during the boil. (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: Evap. Rate = 1-(V60/V0) where V60 is your volume after boiling the wort for 60 minutes and V0 is the volume of wort before the boil (i.e. it’s been boiled for 0 minutes)
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.
Wort Color: 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.
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.
Wort pH: 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.
Isomerization of alpha acids: 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. The conversion of alpha acids to iso-alpha acids is relatively slow and never reaches completion. In the first few minutes of the boil, only a couple percent of the total alpha acids in the hops is converted to iso-alpha acids. After an hour, only 25-30% of the alpha acids are converted and boiling beyond an hour brings diminishing returns in terms of how much iso-alpha acid is converted over time. Most brewers boil their bittering hops for an hour, or sometimes as long as 90 minutes.
The “thickness” of the wort also affects the rate of conversion of alpha acids to iso-alpha acids. In higher gravity worts, fewer alpha acids are converted compared to in lower gravity worts. Most homebrew recipes calculators take this into account when calculating the estimated IBUs from a recipe. This is especially a concern in methods of extract brewing that involve boiling a concentrated wort.
“Traditional” Hopping
Hop growers often divide hop varieties into bittering hops and aroma hops. Bittering hops are varieties that have high levels of alpha acids, over 8% or so and up to 18% in some of the newest high-alpha types. Like all hops, they also contain oils and other compounds that result in hop aroma and flavor, but bittering hops are primarily used as a source of alpha acids. Aroma hops have lower alpha levels, some as low as only a few percent, but have a mixture of oils that are prized for their aroma properties. Some hops are called dual purpose hops because they can be used either as bittering hops or as aroma hops.
Most brewers add bittering hops at or near the beginning of the boil. The hops are then boiled for 60-90 minutes, until the rate of alpha acid isomerization slows (in other words, when boiling the hops longer would result in only small increases in bitterness). Sixty to ninety minutes is also the amount of time the wort is boiled to achieve the right amount of evaporation and hit the correct OG for a beer. During the first few minutes of this boiling time, most of the volatile oils in the hops are blown off.
If more hop flavor and aroma is desired than would be delivered by the single dose of bittering hops, aroma hops are added late in the boil. Usually, hops may be added with 20 to 10 minutes left in the boil if more flavor is desired. Hops added in the last 10 minutes of the boil – and especially those added during the last few minutes or right at knockout, when the heat is shut off – contribute hops aroma, from the volatile oils they contain. Adding hops late in the boil, and thus boiling them for only a short period of time, ensures that the hop oils are not all evaporated away.
Brewers should note that although hop varieties may be described bittering or aroma hops and kettle additons may be referred to as bittering, flavor or aroma ddtions, all hops contribute to bitterness, flavor and aroma to some degree. A beer brewed with a single addition of bittering hops will still retain a small amount of hop flavor abd aroma. Likewise, an addition of hops late in the boil will contribute a small amount of bitterness along with the flavor and aroma contribution.
“Traditional” hopping – with bittering hops added early, then flavor aroma hops added near the end – is widely practiced because it gives the biggest “bang for your buck” and results in the fewest problems in the brewhouse. If you pick a nice, high-alpha variety to add for your bittering hops and then throw in some aroma hops at the end, you’ll need fewer hops overall to reach your target bitterness. This saves money in terms of how much you pay for the hops and, for commercial brewers, how much wort is lost due to being absorbed into the hops. Also, with less hop “gunk” in your kettle, getting the wort from the kettle to your fermenter can be easier. And finally, alpha acids and the essential oils that convey flavor and aroma aren’t the only compounds in hops. As with most plant material, they also contain tannins. Methods of hopping that involve using more hops to obtain less bitterness run the risk of being overly grassy or – in extreme circumstances – excessively tannic or vegetal.
Multiple Addition or Continuous Hopping
Some beers, especially IPAs and other hoppy brews, are made with numerous hop additions, many in the middle of the boil. The Dogfish Head 60-, 90- and 120- Minute IPAs carry this approach to the extreme and add hops continually throughout the boil. Proponents of adding hops throughout the boil claim that this method gives a hop character you can’t achieve with a “bookend” approach of adding hops early and late, but none in the middle. Critics of the practice say it is a waste of hops as “middle hops” contribute little flavor or aroma, but aren’t boiled long enough to extract much bitterness either. They also claim that, because you are using more hops to achieve the same level of bitterness and aroma, beers brewed with middle additions could be more likely to show a grassy or vegetal character due to the increase amount of plant material added to the kettle. Homebrewers interested in IPA will most likely try both approaches and decide for themselves which is more suited to their tastes.
Hop Bursting
One homebrewers have experimented with adding all of their hops at or near the end of the boil. Some homebrewers refer to the practice as hop bursting. Typically, to overcome the decrease in hop utilization, a larger amount of hops is added than would be usual. The short boil time results in a beer with an enormous “burst” of hop flavor and aroma.
Whirlpool Hopping
In many commercial breweries, hot wort is pumped to a settling tank after the boil. The wort enters the tank at an angle to the side of the vessel, causing the wort to rotate. For this reason, this vessel is called the whirlpool. In the whirlpool, solid matter suspended in the wort collects in the center of the rotation and falls to the bottom of the tank as the spinning slows. This leaves a little cone of debris in the middle of the tank. The hot wort – now with less debris suspended in it – is then pumped to the chiller. In some craft breweries, hops are added at this stage. In some cases, enough hops are added that a significant percentage of the IBUs of the beer are obtained in the whirlpool. Since most homebrewersdo not employ a separate whirlpool tank, this method is not used much in homebrewing. Some homebrew recipes, such as the ACME IPA recipe on page XX, use this method by having the homebrewer stir stir his wort in a circular manner after knockout.
Hopback
A hopback (otherwise known as a hop-back, hop back or hop jack) is a device that strains spent, boiled hops out of wort. That’s where the name comes from, in fact: It’s something that’s supposed to hold the hops back. At the same time, a hopback also helps filter undesirable coagulated protein (“hot break”) from the boil. Most brewers would argue that a hopback is more than just a filter device: The term also has come to imply that the gizmo uses an addition of fresh, unboiled hops as a filter medium and as a way of imparting additional hop character to the wort. Technical brewing sources, by the way, use the term “hopback” for a device that is outside and separate from the brewkettle.
A commercial hopback is usually designed like a mash tun and has an inlet, where the wort (still containing kettle hops and coagulated proteins) enters. It features some kind of screen or false bottom, where the fresh hops are added, and an outlet.
First Wort Hopping
In first-wort hopping, some of the bittering hops are added to the kettle along with the very first runoff from the mash. This differs from the usual homebrew practice of adding hops after the wort has begun boiling (and perhaps after the first signs of a hot break are visible). In a blind taste test cited in “Principles of Brewing Science” by George Fix (Brewer’s Publications, 1999), beer drinkers showed a preference for pilsners brewed using first-wort hopping over pilsners brewed the traditional way. Specifically, the subjects in this test – originally published in a German magazine called Brauwelt – felt that first-wort hopped beers have a more rounded bitterness and a more elegant hop nose. Although the studies of first-wort hopping have been confined to pilsners, the same would presumably hold true for other lagers.
The procedure for first-wort hopping is simple. Take a portion of your bittering hops (the hops you would normally add early in the boil) and add them to the first wort that is run off from the mash. Typically between 1/4 to 1/2 of the bittering hops are used for first-wort hopping, although some brewers have used more. Using less than 1/4 will probably not make much of a difference in your finished beer. You may want to decrease the amount of hops slightly, since they will be boiled longer. Consult a hop utilization chart to see if the added boiling time will make a difference.
Practical Considerations
Once you have formulated the size and timing of your hop charges, the actual boiling of the hops is fairly easy, but there are a few practical considerations. In order for alpha acids to be isomerized efficiently, the wort must be hot and in motion. The churning action of the boil helps extract bitterness from the hops as well as the heat. Anything you do to decrease the amount of motion of the hops decreases your hop utilization. One way this may happen in a homebrewery is when homebrewers use a hop bag. A nylon or muslin bag works well to hold the hops and makes it easier to remove them from the kettle after the boil, where they won’t interfere with racking the wort to the fermenter. However, the bags also constrain the flow of wort past the hops required. If you do bag your hops, make sure to leave at least times the volume in the bag as the hops occupy. (In other words, don’t cinch up the ack tight around the hops, give them room to expand and room for hot wort to flow by them.
Hops that cling to the side of kettle will also decrease your hop utilization. Take your brewing spoon or paddle and knock these back into the kettle. Avoid boilovers, as any hops carried out the kettle by the boilover will no longer contribute any bitterness.
