Does the percentage attenuation of the yeast, directly match the fermentability of the wort, ie, If i am after a one fifth gravity beer, then that has attenuated by 80% . so is the spirit indication and ash content taken into consideration for the final gravity result? As I see that yeasts can vary widely in there purported attenuation. from low 65% to high 80%. So, how do I match the fermentability of the wort with a particular attentive yeast strain, in order to get a one fifth gravity beer? Is there a chart for this? And if I were to use malt that had a percentage fermentability of 64%, with a yeast with a 70% attenuation, would I result in a one fifth gravity beer?

  • What are you trying to communicate with "one fifth gravity"? Are you looking at the 80% attenuation and figuring that the beer after fermentation will be the remaining 20% (1/5)? I.e. one fifth of the original gravity?
    – Frazbro
    Sep 6, 2018 at 22:42
  • Yes,thats it, So how come the yeasts ability expressed also in percentage, doesnt match up?
    – Custodian
    Sep 7, 2018 at 23:56
  • Because the yeast attenuation relates to what the yeast is able to eat, and the wort fermentability related to how much of it edible. Think if it like this: you go to a banquet where most of the food is real, but some is fake (say 20% is fake). That's the fermentability. You, however, have a limited appetite, so you can't finish all of the real food, no matter how hard you try. The amount to of food you can eat is the yeast's attenuation.
    – Frazbro
    Sep 7, 2018 at 23:59
  • I see ,so on an average DME , if I use 04 yeast , then I will be looking at a final gravity of circa 33% of the OG. ?
    – Custodian
    Sep 8, 2018 at 0:35
  • Can you show your working on that?
    – Frazbro
    Sep 8, 2018 at 0:38

2 Answers 2


First of all, there is nothing like a one fifth gravity beer. 1/5 = 0.2, which means nothing in terms of fermentation.

Your question must be answered in three parts, the first part being what are you measuring, how it is done, and what the results say, and the second part is about the work the yeast does, and the third about fermentability.

The hydrometer is your basic tool, which measures the density of a sugary water solution. When you measure 1.000, then you have pure water. When you measure in a solution with sugar then this number increases.

Before fermenting (after the boil), you measure the original gravity (OG). After the fermentation you measure the final gravity (FG). From this, the apparent attenuation is derived by the formula (OG - FG) / (OG - 1).

This is called the apparent attenuation. The reason for this is that the solution now also contains alcohol in addition to water and sugar. This has as a consequence that the liquid density (alcohol + water) is now less than that of water, and your hydrometer actually sinks lower.

So, to answer already part of your question: "spirit indication" is not taken into consideration, nor are other solids in solution.

Now for the second part. Wort is actually a very complex solution of all kinds of different sugars, in different kinds of concentrations depending upon the different malts used.

The attenuation of yeasts is influenced by two parameters: the number of different sugars that yeast can ferment and their flocculation, the rate of dropping to the bottom of the fermentation vessel.

The attenuation given by the producer of the yeast mostly indicates if yeasts will consume more differentiated sugars, and are, as a result, capable of attaining a higher attenuation.

However, this is also influenced by the flocculation. When the yeasts drop out of the beer, there will be less fermentation as a result. If it would be possible to regularly rouse the yeast, then fermentation would go on (in England they used to do this, certain English yeast strains drop out rather fast, and the brewers would then roll the casks where the beer was kept to get better attenuation).

You can influence the fermentability of your wort by playing with the mash temperatures, but there are no hard and fast rules.

There are two enzymes in the mash, alfa-amylase and beta-amylase, which convert starches to complex and simple sugars. Alfa-amylase works best on starches, but its optimal temperature is around 70° C. Beta-amylase works best in converting complex sugars to simple sugars, but it works best at 64° C. These enzymes don't last in the mash, because they are denatured by the temperature of the mash. This takes its time of course, but the higher the temperature, the faster they are denatured.

What does this mean for fermentability? A rule of thumb is that mashing for a long time at 64° C will create a very fermentable wort, while mashing at higher temperatures will decrease the fermentability.

However, it's not possible to put figures on this, i.e. if you mash at this temperature, you will get this kind of fermentability is not something that can be put into tables.

My experience is that most yeasts do ferment wort in the ranges given by the producer.

So, if the only thing you have is a particularly attenuating yeast, then mash at higher temperature, e.g. 67° C or 68° C, and see what happens to your FG.

  • I am afraid, that after reading your answer, I shall have to ask the question in a different way.
    – Custodian
    Sep 5, 2018 at 16:40
  • A one fifth gravity beer is one which has a final gravity that is 20% of the start gravity. Hope that helps.
    – Custodian
    Sep 8, 2018 at 9:09

Everything Chthon said is sound, but to put it a little more concisely, fermentability and attenuation are two unrelated things.

Wort fermentablility refers to what proportion of the wort consists of fermentable sugars.

Attenuation refers to what percentage of available fermentable sugars a yeast strain will typically ferment.

The two things are both important factors in determining final gravity, but they do not actually affect one another.

Does that help you at all?

  • i already know this.
    – Custodian
    Sep 7, 2018 at 23:39
  • Then you should know the answer to your original question "Does the percentage attenuation of the yeast, directly match the fermentability of the wort?" The answer is no, because the two things are entirely unrelated.
    – Frazbro
    Sep 7, 2018 at 23:47
  • Tell me why.....
    – Custodian
    Sep 7, 2018 at 23:57
  • I did tell you why, in the answer above. I've also just given you an analogy in the comments of your OP
    – Frazbro
    Sep 8, 2018 at 0:01
  • I mean, why they should at least not match up?
    – Custodian
    Sep 8, 2018 at 9:34

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