Mash out is a step that is really only applicable when fly (continuous) sparging. Fly sparging can take 45 minutes or more, and if the mash is within the temperature range where amylase enzymes are active, then the enzymes will continue to chop up the unfermentable dextrins into fermentable sugars. This could cause the wort to be more fermentable than desired for the intended beer style, and the beer could ferment to too low an FG (come out too dry.) In order to stop the continued enzymatic action, the temperature of the mash is raised (either by adding near boiling water, or applying heat to the MLT) to a range where the amylase enzymes will be denatured in about 10 minutes, thus stopping their action.
When BIAB'ing (or batch sparging), the wort containing the bulk of the sugar is in the BK within a few minutes of the end of the mash, and heating can start immediately, which will denature the enzymes just like a mash out. So, a separate mash out is not required (no matter what the recipe says.) Also, when BIAB'ing (or batch sparging), your mash should have a higher water to grain ratio (be thinner) than when fly sparging. The thinner mash will will have a lower wort viscosity, so heating to lower the viscosity prior to run off is not necessary.
Finally, if your mash conversion is complete, a mash out will have no effect on efficiency. However, if your conversion is incomplete, then a mash out is effectively just an extension to the mash time, with a rate boost as the higher temps will speed up enzymatic action (until the higher temps denature the enzymes and action stops.) If you find you are having low conversion efficiency, you are better off going to a finer grain crush and/or longer mash times in order to get more complete conversion.
Brew on
