Function fermentableAttenuationFraction

Apparent attenuation of one fermentable, as a fraction of its own extract.

• An explicit fermentabilityPercent always wins.
• sugar ferments out completely: 1.0.
• non-fermentable (lactose, maltodextrin) does not ferment at all: 0.0.
• Everything else attenuates at whatever the yeast manages.

This is why a recipe with 10 % dextrose finishes drier than the yeast's published attenuation alone would suggest, and a milk stout finishes sweeter.