Lesson 6 - Wort Boiling Final SAQ

*** With reference to specific flavour compounds, how does wort boiling effect beer flavour?

Response:

A good place to start this question is to consider what beer would taste like if wort was not boiled. Lack of sterilisation would lead to the more heat tolerant microbes derived from the raw materials metabolising the wort. This would lead to a range of off flavours including DMS (sweetcorn), diacetyl (butterscotch), butyric (vomit) and lactic acid (sour taste). Boiling ensures that we destroy microbes and reduce the risk of these off-flavours being present. Boiling also destroys malt enzymes. Mashing off at about 78°C helps denature enzymes but more heat stable ones can survive. Also limit dextrinase is found in free and bound form in the mash. The bound form is able to survive mashing off temperatures and can be converted into its free form during fermentation. The continued operation of malt enzymes would result in minimal residual carbohydrate in the beer so potentially a very thin tasting beer. Boiling prevents this, leaving the sugar spectrum for the wort as it was at the end of mashing.

Thankfully, as well as preventing the beer from tasting appalling, boiling helps fermentation transform sticky and grainy wort into crisp, fresh and fruity beer. We can now discuss the changes in flavour that result directly from the boil.

Removal of undesirable volatiles

Wort is characterised as smelling grainy and of course worty. This character is due to a number of different flavour compounds but the least desirable in beer include these aldehydes; 3-methylbutanal, isobutyraldehyde, 2-methylbutanal, 2-methylpropanal and 3-methylthiopropionaldehyde. Some of these are Strecker aldehydes and are also formed during boiling by Strecker degradation of amino acids as a result of the Maillard reaction. They are all more volatile than water so their concentrations are substantially reduced during wort boiling. Another volatile that is both produced and removed by effective wort boiling is DMS. DMS Is formed from its precursor SMM and should be removed if cooked sweetcorn is not an essential requirement of the beer’s flavour profile.

Development of flavours by non-enzymatic browning

Non-enzymatic browning takes two forms, the Maillard reaction and caramelisation. Both have complex reaction mechanisms and the types of flavour compounds they produce are similar and in some cases the same. The Maillard reaction contributes the bulk of the flavour forming furans, furanones, thiopenes and pyrazines. The flavour these compounds provide ranges from sweet caramel to roast coffee. Common examples include maltol, 2-(or 5)-ethyl-5-(or 2)-methyl-4-hydroxy-3(2H)-furanone, 2,5-dimethyl-4-hydroxy-3(2H)-furanone isomaltol and methyl pryrazine. Caramelisation produces similar compounds although the contribution of the boil may not be sufficient for them to be above their flavour threshold in beer. As well as directly contributing flavour compounds, non-enzymic browning provides precursors that are converted into flavour compounds during fermentation. The furanones above are examples of this. The Maillard reaction also produces reductones that could help slow staling reactions as beer ages. There is conflicting scientific evidence on their effectiveness. Not all the flavour produced by non-enzymatic browning in the boil is desirable. The furans furfural and 5-methylfurfural have nutty and bready aromas and are associated with stale beer flavour.

Hop flavour extraction

Bitterness and aroma are extracted from hops during the boil. Heat is required to isomerise the α-acids, converting them into the more bitter and soluble iso-α-acids. Aroma compounds are also extracted during boiling, although boiling is not necessary for them to be dissolved. Boiling also drives off the more volatile hop aroma compounds. Therefore, late hop aroma and flavour resulting from essential oil components like linalool and myrcene can be said to be despite wort boiling rather than as a result of it. Hop polyphenols are also dissolved during wort boiling. These are said to contribute bitterness, astringency and contribute to mouthfeel. They can also help reduce the rate of staling by complexing with and removing the metal ions that might catalyse oxidation of flavour compounds.

Increasing thermal load

The boil adds to the thermal load on the beer. Higher thermal loads have been linked to more rapid staling in beer so are likely to contribute to the production of stale beer flavour compounds. Some brewers also feel that thermal load reduces the quality of fresh beer flavour.

Concentration of the wort

By evaporating water, the boil raises the wort’s gravity. If the wort is not diluted before fermentation, this increase in gravity may lead to an increase of esters. This is especially the case if the wort gravity is already fairly high.

Trub formation

If trub is not formed and removed, fermentation problems can result as yeast cells are coated with solid material. Unhappy yeast produce unpleasant flavours such as H₂S (rotten eggs) and higher sulphur compounds (drains and rotting meat). Unpleasant organic acids like butyric (vomit) and caprylic (goats) may also result. Fermentation problems can also mean the wort is not attenuated sufficiently resulting in oversweet flavour. Also, in problematic fermentations the pH drop is not sufficient, leading to a less crisp taste. Trub formation is also important for removing lipids which are linked to stale beer flavour and polyphenols which can lead to astringency. So, the overall effect of trub formation is clean tasting beer with fewer off-flavours.