Attenuation ?

[ESPY]

A buddy of mine brewed a porter that finished around 1030 so we have been contemplating attenuation rates and there's something we're wondering: To truly get an accurate measure of attenuation and/or FG, shouldn't you calculate it based on the amount of fermentable sugars and not the total OG? Here's an example to illustrate my point:

Porter with OG of 1.072. Approx 14 gravity pts come from unfermentables. Let's suppose you get attenuation of 75%. Most software predict FG as 72-(.75x72) = 18, right? But you know that 14 of the 72 pts won't ferment. Shouldn't the calculation be FG = 72-[(72-14)x.75] = 28.5?

Alternately, you would calculate % attenuation from OG and FG as follows: (OG-FG) / (OG - pts from unfermentables). In by friend's porter example, (72-30) / (72-14) = 72.4%. I think Promash is calculating his attenuation as (72-30)/72 = 58%.

This doesn't seem like any huge revelation to subtract unfermentables out of the attenuation calculation, but we've never seen it done. Anyone know why this isn't done?

Thanks,
SP

 

[Kaiser]

This doesn't seem like any huge revelation to subtract unfermentables out of the attenuation calculation, but we've never seen it done. Anyone know why this isn't done?

It's not done b/c the amount of unfermentable extract is unknown unless you make a corret fast ferment test to find the limit of attenuation. If it would be common to do such a test, brewers would also through around the difference between limit of attenuation and actual attenuation as a parameter when analyzing beer.

It's similar to the different definitions for Efficiency between German and American brewres. German brewers like to express brewhouse efficiency as amount of extract gotten from the total weight of the grain. Americans express it as amount of extract gotten from the limit of extract they could have gotten from the grain (based on laborarory analysis of the grain).

Kai