From the Grainfather FAQ site:

Calculating estimated Final Gravity

When calculating estimated final gravity three variables come into play; Yeast attenuation, Mash temperature and Ingredients that add increase the density of the wort but are either partially or not fermentable by the yeast, like lactose. These points are calculated separately and added to the OG and FG at the end.

Yeast attenuation is the percentage of sugars in the wort the yeast typically consumes. Most yeasts have a range of attenuations for the Grainfather Recipe creator before yeast is added to the recipe it takes a default of 75% attenuation. When yeast is added the recipe creator takes the middle of the range obtained from the yeast lab as the value used to calculate the estimated FG. If multiple yeasts are used the recipe creator uses an average of the yeast’s attenuation. If we have an OG of 1.032 and using yeast with an average attenuation of 75% then:

FG = OG - (OG -1) x Attenuation = 1.032 -(1.032-1) x 0.75 = 1.008

Since mashing grains at different temperatures promotes the activity of either alpha or beta amylase making more either fermentable or less fermentable sugars. the Grainfather recipe creator uses a ‘Brewers window’ which is between 62.5 and 72.50C. We then have a centre point in this range occurring at 67.50C any mash temp that is conducted in this range is then subjected to an adjustment factor based on if it is above or below 67.50C (below 67.5more fermentable, above 67.5 less fermentable).

Attenuation = Attenuation - 0.0225 x (lowest Mash Temperature - 67.5)

Based on the estimated FG will change significantly between 62 and 63 due to the edge of the brewer window being 62.5. The brewer's window is set at 62.5 due to the coefficient(+/- 50C) applied to the middle value of 67.5. The results of the different studies done by Michael Lewis and Tom young 2002 and Palmers (Lewis & Young,, 2002) (Palmer, 2017) work led us to use 62.5 as this was the midpoint in the edge between the two studies, this is also the same edge most other brewing software use.

As for the ‘time at different mash temperatures’ affecting fermentability. The calculations get complicated very quickly with multiple mash steps, so we try to keep it as simple as possible. with the use of highly modified malt and high proportions of high diastatic powered malts that allow for very quick conversion of starches, we use the mash step with the lowest temp in the range of 62.5˚C to 72.5˚C when calculating the FG. If a recipe has it's main mash step (Beta-amylase) just outside the range at 62˚C and, it's secondary mash step (Alpha-amylase) in the range at 72˚C. Then for this example, the calculation is run using the secondary mash step temp. However, by changing the first mash step to 63˚C then puts the primary step in range and the calculation for FG would be run off the primary mash step.

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