Wort boiling is a critical control point in brewing. In the past before taking this brewing course I wasn't fully aware of how important it really is. So much so, that I'll write an entire blog post about it. I’m concentrating on evaporation and protein removal for the most part, as I think these are the least understood part of the process.
First, what is a full boil? It seems to be accepted that a boil of at least 60 minutes is required for the following actions to sufficiently occur. I know some brewers only do 45 minute boils and I would recommend against this, as it can affect flavour in the final beer as I will describe in detail. As well, the wort must actually be fully boiling a "rolling boil" with two-phase nucleate boiling where steam bubbles pass through the wort.
A summary of the achievements of wort boiling:
Requirement: Dependant Conditions
Wort sterilization: Time & Temperature
Isomerisation of hops: Time & Temperature
Protein denaturation & enzyme inactivation: Time & Temperature
Protein coagulation: Turbulence, boil vigour & time
Formation of colour & flavour components: Time & Temperature
Removal of unwanted volatiles: Time, Temperature & Evaporation
Formation of reducing agents: Time & Temperature
Wort concentration by evaporation: Time, Temperature & Evaporation
Adapted from reference J. Andrews, Brewer Distiller Int. 2008 (ref 2)
A lot of brewers measure the amount of boil by the amount of evaporation that has occurred. This is a fairly easy metric, as one just needs to measure the volume of the kettle before and after the boil. However, evaporating water into steam is very energetically expensive. We cannot, however, drive off unwanted volatile components without a certain amount of evaporation. What we want to evaporate is volatile off-flavour components such as DMS and aldehydes.
Dimethyl sulphide (DMS), an off flavour component that forms during the boil from S-methylmethionine (SMM), is quite volatile at wort boiling temperatures. Given enough time, say 60 minutes of boiling, DMS should be formed and driven off enough to bring it below the detectable flavour threshold. During boiling we're basically distilling the DMS out, as it is more volatile than water. However, you must evaporate some water in order to do this. The amount of water needed to evaporate is dependent on the concentration of DMS that you need to get rid of. The more DMS in the wort, the more water you need to evaporate. There is a minimum amount of water that you need to get rid of certain concentration of DMS, regardless of what type of wort boiling system you are using [ref 1].
In the “perfect boiling system”, the boil would run at the optimal liquid-vapour equilibrium to have as little water evaporated possible for the maximal volatile removal. This would save energy. Boiling systems do not run at this optimal point, of course… but imagine the energy savings over a year if you could reduce your evaporation by a few percentage points? It would be very significant [see ref 2 for a good read on this].
Luckily, most malts stored properly and mashed with proper procedures don't create too much DMS to require extra boil lengths. The potential for DMS formation occurs in the malting process, and is generally higher for lager malts. Breweries these days are operating on evaporation rates of 4-8%, which should be enough to drive off 'normal' amounts of DMS. If you have problems with DMS in the brewery, you might need a slightly longer boil (careful, though - microbial infection can lead to DMS problems downstream too).
Wort concentration – what of this then? It would seem that if it were possible to boil wort with less evaporation, you could save a lot of money. You could re-calculate your mash recipe to have less-dilute wort being passed to the kettle. But don’t mistake what I’m saying here as an excuse to turn down your steam jacket and let the wort simmer. To get extremely low evaporation rates with good volatile removal plus hot break formation, you need a more expensive boiling system or even a wort stripping system. For your little kettle, you need as vigorous a boil as possible, and I’ll tell you why…
One of the most important reactions that occurs in the wort boil is the formation of "hot break". Hot break is the coagulation of proteins, formation of protein-polyphenol complexes, and reaction with hop compounds to create larger particles that will sediment out in the whirlpool at the end of the boil. These reactions occur at higher rates at higher temperatures and more agitation. This is why your wort boil must be full and rolling... the more vigorous and turbulent the boil, the more of these compounds form over time. This process maximizes around two hours.
Protein/nitrogenous compounds are required for fermentation, but there must be a balance. Too few and the fermentation will become stuck, too many and several changes in flavour will be noted. In particular, in the presence of excessive protein content can lead to higher levels of esters and higher (fusel) alcohols. This occurs when the yeast metabolizes the excess amino acids to form higher alcohols (eg propanol, butanol), which then can be further converted to various esters (eg ethyl acetate, isoamyl acetate). Higher alcohols give the beer a slight warming feeling (and seem to give me a bit of a headache), and esters are known for their fruity/floral contributions to beer. This can be good if this is what you want in the beer style you are making, but if your Czech pilsner tastes like a fruity English ale you might want to check your boil vigour.
As well, removal of protein-polyphenol complexes is important for colloidal stability over time (known as chill haze). Improperly stabilized beer can find reduced shelf-life as it can go cloudy when stored cold due to these complexes. Most of this should happen while the beer is in cold conditioning. While some will form in hot-break, the key here is that we are removing lots of protein from the equation. Less protein means less possibility to complex with polyphenols in the chilled beer.
Stability, Colour, Flavour
In a full boil, any microbe interested in living in beer will be killed. Several organisms can actually survive the mashing process, so the wort must be sterilized before fermentation.
Usually when mashing the temperature is raised at the end of the mash to 75-78 C in order to make the wort less viscous and stop the amylase enzymes from working. However, some enzymes are not fully denatured until the boil. As well, the many chemical reactions that occur during the boil will lower the pH. Maillard reaction products form between the wort sugars and amino acids when heated in the boil. These reactions contribute to flavours as well as darken the wort slightly.
A lot of attention is given to the isomerisation of hop alpha-acids in wort, and you can find much better information than my little blurb here.
Alpha-acid oils in hops need to be heated to isomerise, which causes them to be soluble in wort and creates the bitter flavour we love in beer. In a ‘typical’ boil, one cannot expect to get much more than 35% by weight iso-alpha-acid extraction from even pellet hops over 90 minutes. Often, this can be less depending on wort concentration, pH, and wort boiling temperature (ambient air pressure can change wort boiling temp quite significantly, eg due to altitude).
You will note that this is a time/temperature reaction… there seems to me to be a prevailing idea in the homebrewing community that as soon as you stop the boil the hop isomerisation stops. Not so… these will continue as long as the wort is hot, however at a lower reaction rate depending on the resting temp.
Hops also contain a great deal of volatile flavour and aroma compounds. The longer the hops are boiled for, of course, the more of these components are boiled off. Thus, adding the hops at the end of the boil will result in more of these volatiles being present. Remember, of course, that just because the wort may not be currently boiling does not mean volatiles are not being driven off. If the wort is hot, the volatiles will still be vaporizing slowly.
---References & Further Reading---
 Sommer, K. & Hertel, M. Engineering fundamentals of the wort boiling process. 31st Congress of the EBC, Venice.
 Andrews, J. Evaporating the Myths. Brewer & Distiller International (vol 4, #3, March 2008)
Bamforth, C.W. Wort composition and beer quality. In: Brewing Yeast Fermentation Performance. Ed: K. Smart. Blackwell Science, 2003.
Barnes, Z.C. Brewing Process Control, In: Handbook of Brewing, 2nd Ed. Eds: FG Priest & GG Stewart. Taylor & Francis, 2006.
O’Rourke, T. The funcation of wort boiling. The Brewer International, Feb 2002.
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