It's weird that this very common method of mashing gets very little attention nowadays. Despite being a pretty standard process for over a hundred years.
In principle, it's a simple form of a step mash. It started with a simple infusion, which was allowed to stand for 20 or 30 minutes. Then a smaller quantity of hotter water was introduced to the mash from below via the underlet. The internal rakes were then given a couple of spins to mix the hotter water through the mash. And left to stand for two hours or so.
It seems that some brewers also used steam in addition to hot water to raise the temperature of the mash.
The time for application of the underlet must depend upon the malt itself, but in practical working I believe that 20 minutes is a sufficiently long stand after the malt has all been mashed in, unless the malt should happen to be low in diastase, when the time may well be extended to half an hour. There is, in my experience, no harm at all in bringing the second heat up to quite a high temperature— that is, 155 deg. to 158 deg.; indeed, in several breweries the second temperature is brought up to 159 deg., or even 160 deg., though in that case it is usual to extend the preliminary stand from 20 to 30 minutes. It is in such a case necessary either to use a very large quantity of underlet liquor at a very high temperature, or to supplement the underlet with free steam blown in under the plates. This second process answers perfectly well if the steam inlets are sufficiently numerous to allow of a fairly uniform heating of the mash; but in cases where one steam jet only is provided, there is a danger that one portion of the mash may be almost boiling whilst the other is only slightly heated. I strongly advocate the use of hot underlets, and the raising of a mash heat some 8 deg. or 10 deg. after 20 to 30 minutes’ stand, for by this means a wort low in albuminoids, a strong yeast, and a beer carrying a tough viscous head, brightening very rapidly and not prone to fret, are secured.
The Brewers' Journal vol. 38 1902, November 15th 1902, page 668.
Here's an example of an underlet mash from my favourite, Barclay Perkins. Who were very keen users of this technique.
| Barclay Perkins X Ale mashing scheme 27th October 1909 | |||||
| operation | barrels water | strike heat | tap heat | time stood (minutes) | OG wort |
| mash 1 | 140 | 160º F | 30 | ||
| underlet | 26 | 175º F | 150º F | 90 | 1082 |
| sparge 1 | 36 | 168º F | |||
| underlet | 63 | 168º F | 156º F | 30 | 1029 |
| sparge 2 | 190 | 165º F | 150º F | 1010 | |
| Source: | |||||
| Barclay Perkins brewing record held at the London Metropolitan Archives, document number ACC/2305/1/601. | |||||
Note that they used the longer pre-underlet rest of 30 minutes and a shorter post-underlet rest of just 90 minutes.
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