Why mash then top up?

[Underground Joe]

Today I was a bit a 'tard and totally messed up my mash temperature resulting in having to quickly chuck in 12l of extra water to get the temperature right. This left me with just 5.6l to put in at top up and a nagging question. Why do mash then top up, rather than just mash with the top up volume included?

[markpeace]

The enzymes in the malt are more or less active depending on the thickness of the mash

[liquidman]

Yea but are they really??

I mash with my full pre boil volume in one go (BIAB)

works for me...

[Underground Joe]

I was slightly under on gravity but slightly over on volume collected. That combined with new mash tun and boiler makes it impossible to say whether the enzymes were affected or not.

[gregorach]

The usual reasons to top up at the end of the mash are to raise the grain bed temperature enough to stop further conversion, and to improve lautering efficiency.

[simple one]

Short answer. Efficiency.

Draining and topping up will leave a less saturated solution.
Also higher efficiency through hitting the correct pH... more water = higher pH. Yes you could acidify, but then your final beer will have a pretty low pH.

Although mashing is pretty forgiving, and most methods work