How Long should you boil your wort?

How long should I boil my wortI enjoy wort boiling, however sadly for me my wife does not.  It is the smell of the wort boiling stage that I love so much, the worty hoppiness is just a truly fantastic aroma.  However, for exactly the reason I love wort boiling, my wife hates it.  The brewing equivalent of the Marmite effect perhaps!  She is always asking me why I have to boil the wort for so long, could I not reduce the boiling time a little bit, if not for her sakes then do it for the environment.  After all wort boiling is an exceptionally energy intensive part of the home brewing process.  This gives rise to a question that I often get asked.  How long should you boil your wort?  For me the answer lies in why we have to boil our wort in the first place.

Why do we boil wort?

Wort boiling is an energy intensive stage of the brewing process and certainly for commercial brewers this represents a large cost to the business.  So surely if we could do without wort boiling it would have been got rid of a long time ago.  However, there are a number of critical reactions that occur in the wort that would not happen without the heat energy introduced during wort boiling.  There are essentially 8 key reasons that we boil our wort and they are, in no particular order:

  1. Stop any enzyme activity that remains from mashing
  2. Precipitate protein and polyphenols to prevent haze in final beer
  3. Colour formation
  4. Water evaporation
  5. Formation of the bitter iso-alpha acids from the non-bitter hop alpha acids
  6. Drive off unwanted flavour compounds
  7. Sterilise wort prior to pitching with yeast
  8. Reduce wort pH and formation of reducing compounds

It is perhaps worth discussing each of these stages in a little bit more detail as it can help us answer the question of how long should boil your wort.

When we separate wort from spent grains there will be some residual enzyme activity present in the wort.  This is especially true if the sparge water that we use is not very hot.  Typically the temperature of sparge water is 75ºC or greater but during wort run-off, unless you have a mash tun that can be heated, the first strong worts will have been run-off at the temperature of mashing which is not hot enough to denature the amylolytic enzymes that have been present during mash conversion.  Even with the addition of sparge water at a temperature of 75ºC the overall temperature of the wort will not have been raised to a high enough temperature to ensure that all of the starch degrading enzymes, especially alpha amylase, are destroyed.  Therefore by boiling our wort we are effectively raising the temperature of the wort to a level at which the mashing enzymes denature and are therefore no longer active.

Proteins, over time, will complex with polyphenols to produce a haze in your beer.  This haze will often develop when the beer is being stored creating an unsightly haze in the glass.  Although there is nothing wrong with the haze itself, and no harm will be caused from drinking it, for many of us a hazy beer doesn’t look very appetizing.  It is therefore seen as a sign of poor brewing technique.  Boiling the wort causes the proteins to denature and interact with polyphenols forming larger clumps which will precipitate quickly as the wort is cooled.  We can speed the process up by adding fining agents such as copper finings.  Copper finings, such as Irish Moss, are mainly composed of Carageenan, which is a strongly sulphated polygalactose molecule.  The presence of the sulphate group on the polygalactose molecule makes it highly negatively charged.  At the pH of wort, proteins are strongly positively charged, and therefore interact electro-statically with the carageenan forming large flocs which quickly sediment out of the cooling wort.  Without the energy introduced during boiling the proteins in wort do not denature, are less likely to interact with polyphenols and so don’t readily coagulate into large clumps so are more likely to remain in the wort and therefore in the beer.

Colour is an important aspect of the beer that we brew.  The majority of colour is derived from the malt that we use but boiling is an important stage in terms of colour development.  Colour in beer is created through a number of chemical reactions.  For example there is some caramelization of sugar in the wort, this is especially true where the heating surface comes into direct contact with wort.  For example if you have an electric wort boiler with a bar type heating element there will be a certain amount of caramelization at the surface of the heating element.  However, the most important reaction that occurs, which gives rise to both colour and flavour, is the Maillard Reaction.  The Maillard reaction occurs when peptides and amino acids interact with simple sugars to create what are known as melanoidin products.  As well as being highly coloured melanoidins have a characteristic flavour which is important in beer.  The critical thing that is required for the Maillard reaction to occur is the application of heat during wort boiling.  As I have mentioned most of the colour in our beer comes from the malt that we use.  Therefore if you are brewing a highly coloured ale or stout the colour contribution from wort boiling is fairly minimal.  However, if you are trying to brew a very low coloured pale ale or lager then the duration of your wort boil can influence, quite dramatically, the colour of your wort and should therefore be an important consideration when deciding how long you boil your wort.

Wort is largely water and if you boil it then some of this water will evaporate.  Therefore brewers, to a greater or lesser extent, use wort boiling to concentrate up their wort and thus fine tune the starting gravity of their wort.  The typical rate of evaporation during wort boiling is 6 to 8%.  I have to admit that I do not set my boil to achieve a certain evaporation rate and wort boiling should not be seen as an opportunity to increase the gravity of your wort to make up for inefficient mashing.  In my opinion evaporation is a by product of boiling and should not be the aim.  Of course you can reduce the evaporation rate by putting a lid on your boiler but be especially careful if you choose to do this as you run the risk of having a boil over.

There are certain flavours present in wort that are considered to be off flavours if they survive into the finished beer in high concentrations.  The main example is Dimethylsulphide (DMS) which is a highly volatile flavour compound that gives rise to a sweetcorn flavour in beer.  In some beers this flavour is considered acceptable.  For example the distinctive flavour of a lager style beer is partly attributable to DMS.  However, even in lager too much DMS is considered unacceptable.  The formation of DMS is through a number of chemical reactions but it begins in malt as S-methylmethionine (SMM).  SMM is broken down during malt kilning into DMS and the active precursor dimethylsulphoxide (DMSO).  This process also occurs during wort boiling.  Interestingly DMSO can break down into DMS.  Therefore to remove DMS from wort a vigorous boil is required to convert both SMM and DMSO into DMS.  The DMS, due to its volatility, is then driven off during wort boiling.  If you want to maintain DMS in your wort this can be achieved by placing a lid on your wort boiler but again please be very careful if you choose to do this as you run the risk of encountering a boil over.

It is essential to ensure that your wort is sterile before you pitch your yeast.  Many bacteria which, if they are not destroyed, can quickly spoil the wort.  It is therefore a primary function of wort boiling to remove any potential microbiological contamination and ensure your wort is sterile.

During boiling the reaction of calcium ions with phosphates and polypeptides to form insoluble compounds with the release of hydrogen ions leads to the acidification of wort.  For example the pH of unboiled wort is around 5.8 whereas boiled wort is 5.5.  The impact of a lower wort pH helps to improve protein coagulation, improves beer flavour, encourages yeast growth and inhibits the growth of many other spoilage micro-organisms.  However, as with anything in life there are some disadvantages to a lower wort pH and that is poorer hop utilisation and a reduction in colour formation.

Wort boiling aids in the production of substances which can provide a protecting anti-oxidant effect in wort and beer.  These substances, sometimes known as reductones and which include the melanoidins, react with oxygen in wort thus protecting the wort and beer from flavour damage due to the presence of oxygen.

Last but by no means least we come to the formation of the bitter iso-alpha acids.  As we all know the bitter flavour that is essential to the character of beer is derived from the alpha acids in hops.  However, alpha acids are insoluble in cold wort and do not have a bitter flavour.  It is only when the chemical structure of the alpha acids is altered, or they become isomerised, that they become significantly more soluble and acquire that distinctive bitter flavour.  The isomerisation of the alpha acids requires energy and this is supplied during wort boiling in the form of heat.  Research has been done to show that the actual process of isomerisation is relatively rapid with over 90% of the wort bitterness being produced in the first 30 minutes of wort boiling.  Full extraction generally occurs within 60 – 70 minutes.

As we have mentioned earlier in this article wort boiling is a very energy intensive stage of the brewing process irrespective of whether we use gas or electricity to heat.  As home brewers we should not ignore this cost.  So how long should you boil your wort?  As you can see from this article, there a large number of important reactions that occur during wort boiling.  Many of the reactions that we have discussed do not require a long time at boiling temperature to achieve the desired effect.  For example the enzymes present in wort are rapidly denatured at boiling point so it is unnecessary to boil your wort for hours.  For me the most important reaction during wort boiling is the extraction and isomerisation of the hop alpha acids and research has shown that most of this occurs within 60 – 70 minutes.  So how long should you boil your wort?  There is therefore a good argument to suggest that boiling for longer than 60 minutes has no beneficial impact on your wort and therefore on your beer.  However, there is a caveat and that is the boil must be a vigorous rolling boil.  If you do not achieve this during your wort boiling then you may want to extend your boil to 90 minutes.  But if you do manage a good vigorous rolling boil you may like to look at fixing your boil at 60 minutes.  By doing so you may find you have a more harmonious life with your partner and save yourself a penny or too.

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