Fermentability and Summary

One of (if not the) most important objective of mashing is to obtain the required sugar spectrum. The sugar spectrum is key to fermentability.

Most yeasts used in commercial brewing can only ferment mono, di and trisaccharides. Earlier, we said that a typical wort is around 75% fermentable, so 75% of the wort by weight will be these sugars. To determine how fermentable a wort is, we undertake a wort attenuation limit (WAL) test (also called a limit attenuation).

The word attenuate means to make thin or weak, and in brewing, we use it to describe the reduction in the gravity or extract in the wort by yeast.

In a WAL test, wort is added to a flask with 7.5 g/100 mL of compressed yeast. The flask containing the wort and yeast is then kept warm and shaken to enable the yeast to consume all the sugar in the wort. This accelerates fermentation so we can get an idea of the fermentability of the wort in the lab before it has fermented in the brewery. The test is complete when the gravity of the wort ceases to fall. The gravity at this point is called the attenuation limit.

The gravity before and after the WAL test is measured, and from this, we can calculate the percentage of apparent attenuation.

begin mathsize 14px style text % Apparent Attenuation =  end text fraction numerator left parenthesis OG minus PG right parenthesis over denominator OG end fraction cross times space 100 end style

OG = original gravity of wort added to flask

PG = attenuation limit test gravity

We use the word apparent because the yeast converts most of the sugar to ethanol. The gravity of ethanol is lower than that of water, so the drop in gravity is actually greater than it would be if solely caused by the reduction in the concentration of sugars. We can apply a factor to roughly account for the difference in gravity caused by the increase in ethanol. Once we have done this, the figure is called the percentage real attenuation and is the percentage fermentability of the wort. Here’s the calculation.

begin mathsize 14px style text % Real Attenuation =Apparent Attenuation × 0.819 end text end style

So, for our typical 75% fermentable wort, it would look like this:

begin mathsize 14px style text % Apparent Attenuation = end text fraction numerator left parenthesis 12.5 degree straight P minus 1.05 degree straight P right parenthesis over denominator 12.5 degree straight P end fraction space cross times space 100 equals 91.6 percent sign end style

begin mathsize 14px style text % Real Attenuation = 91.6 × 0.819=75% end text end style

We can also calculate the fermentability of our worts fermented in the brewery using the OG and PG in the same way. The WAL is almost always lower than the PG achieved in the brewery, so this figure will be a lower percentage.

Regularly undertaking WAL tests on wort produced in mashing highlights changes in the level of fermentability. We need to maintain a consistent WAL and hence level of fermentability in the wort for each brand produced. If our WAL test shows there is a change in fermentability, we need to adjust our mashing parameters to redress the change.

As with most things in brewing, the key to successful mashing is consistency.

Outcomes of mashing -  summary

We can bring our discussion of the mashing process to a close by summarising the effects of mashing on downstream processes and beer quality. Depending on how well we have done our job in mashing these effects can be negative or positive. The table below provides a brief summary.

Table: Summary of the downstream effects of mashing

Process stage

Positive

Negative

Wort separation

Maintaining correct particle size and achieving adequate breakdown of β-glucans and pentosans allows effective wort separation. Correct pH at end of mashing helps minimise wort viscosity and extraction of polyphenols.  

Too much undegraded β-glucan/pentosan or gelling through high agitator speeds and damage to husk particles can give poor/slow wort separation.

Wort boiling

Correct pH at end of mashing helps minimise colour pick up and helps achieve the target pH for the end of the boil.  

Excessive protein breakdown causes excessive colour and flavour pick up. Inadequate protein degradation increases trub formation, reducing hop utilisation.

Wort clarification

Ensuring adequate  breakdown of β-glucans and pentosans helps effective wort clarification in hop backs and whirlpools.

Inadequate protein degradation increases trub volume making clarification more difficult.

Fermentation

Correct sugar spectrum gives consistent fermentations in the target timeframe.

Inadequate production of FAN or provision of other nutrients leads to poor/slow/inconsistent fermentations and inadequately attenuated beer.

Filtration

Adequate protein. β-glucans and pentosan breakdown allows effective beer filtration.

Inadequate protein. β-glucans and pentosan breakdown gives slow beer filtration and reduces the volume of beer required to block filters.

Beer foam  

The correct level of protein breakdown ensures sufficient foam stabilising proteins and polypeptides are available to provide a stable beer foam. Maintaining correct pH provides improved foam stability.

Excessive lipid extraction and protein/polyphenol breakdown causes reduction in foam stability.

Beer flavour

Achieving a consistent sugar spectrum helps ensure consistent beer palate and aroma.

Poor fermentations due to low FAN lead to off flavour formation and inconsistent attenuation. Excessive breakdown of protein and polypeptides is thought to lead to reduced body in beer. High levels of FAN can lead to excessive levels of Maillard reaction derived flavour compounds.  

Beer colloidal stability

Sufficient breakdown of haze forming proteins improves colloidal stability.

Extraction of haze forming polyphenols reduces colloidal stability. Inadequate conversion of starch or breakdown of pentosans and β-glucans also causes hazes.

Beer flavour stability

Minimising LOX activity and oxygen pick up improves flavour stability. Correct pH in mashing improves flavour stability.

Extraction of lipids reduces flavour stability. Excessive FAN production can lead to formation of Strecker aldehydes in the boil which can lead to stale flavour. Higher thermal loads in mashing reduce flavour stability.

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