The purpose of the mashout is to a) Denature the enzymes and lock in the sugar profile & B) make the mash runnier.
point a) would make me think that the 10 minutes is to ensure all the enzymes are killed & to ensure all of it has reached that temp so there aren't any dense colder spots left over.
no its not - not in BIAB anyway.
mashout in BIAB is about 2 things. Efficiency and starch conversion.
Starch Conversion - BIAB is not the ideal mash environment - people can bleat and moan all they like, but it isn't. BIAB also misses the vorlauff step that a normal mash/lauter tun has and finally BIAB radically disturbs the mash bed during the lauter process.
All of which means, that there is a reasonable chance of there being some unconverted starch and that any unconverted starch in the mash (and if you mention an idodine test at this point, you dont actually know how they work) is far more likely to end up in the boil kettle with a BIAB brew than it would be with a mash/lauter tun brew.
So - you intensify your mash and make sure you convert all your starch. The way you do it, is by increasing the temperature of the mash beyond the gelatinisation point of even the really resistant starches, by stirring to access the physically trapped and isolated starches and by doing this gradually over a period of time to give the remaining enzymes (which will be working overtime in the increased temperature environment) time to convert the starches as they are gelatinised.
Take 10 minutes or so to constantly stir your mash as you raise it to a temperature of 76-78� and something along the lines of all your starches will be liberated, just in time for the last gasp of the dying enzymes to convert them to sugars.
This doesn't require any rest period at the "mashout" temperature - there is no point to a rest. There is a point to constant stirring and a gradual raise in temperature. So any mashout you might do that doesn't involve both these things is more or less pointless and I would just skip it.
Efficiency - well, your mash is more intensive, you convert more starch to more sugar, and your lauter is at a "normal" lauter temperature.... it all works just a little better than if you dont do it and I would expect a bump in efficiency (measured in the kettle) of say 3-5% by doing a mash out vs just. pulling the bag at the end of a 60 min mash. Less return for effort if you normally mash for 90min, but still a little.
Do you "need" to do it?? Hell no! But its something that I think constitutes good brewing practise and will make it
more likely that you will end up with fault free beer that resembles the beer you were trying to make.
60 min mash
Raise gradually with constant stirring to 76-78�
Pull Bag
Adds maybe 5 mins to your brew day and in return you get a modest efficiency bump and a tick in a box for consistent quality brewing practise.
TB