I have my own guess. Then I thought, why not just look at see what Jeffery says. And here it is:
This is prepared from a malt wort evaporated in a vacuum to give a syrup of about 80% solids. (Malt extracts are also obtainable as powders, but these are not usually used in brewing.) Non-diastatic malt extracts, or those with low diastase, are suitable for adding in the copper, and enable a larger brew to be made on occasions when perhaps the existing mash tun capacity is inadequate. However, apart from this, their regular use is of value and many brewers always include a proportion in their beers. They are of use in assisting head retention and there are special grades made in such a way as to have a high proportion of those protein products which assist in head retention. Diastatic malt extracts are also used in the mash tun if the malts are deficient in diastase, or if a high proportion of flakes is to be used. As flakes do not contribute head-retaining materials, their use tends to reduce the foam-retaining qualities of the beer; the use in the copper of the high nitrogen malt extracts, already referred to, will therefore counteract this tendency.
"Brewing Theory and Practice" by E. J. Jeffery, 1956, page 167.
Basically there were three uses of malt extract: to extend the brew length; to aid head retention; and to provide extra diastase.
I’ve just done a little survey of brewing records to check on the usage of malt extract. Of the fourteen breweries whose records I have from 1945 – 1949, seven used malt extract. As far as I can see, none employed it to extend their brew length. Four used it in the mash tun and three in the copper.
Here’s a summary:
|Malt extract usage|
|Brewery||where used||% malt extract||% unmalted grain||name||where listed|
|Barclay Perkins||XX, LS occasionally||1 - 2%||8 - 10%||malt extract||with the sugars|
|Shepherd Neame||all||1%||0%||EDME||with the sugars|
|Strong||all except Stout||5%||0%||Diamalt||with the malts|
|Flowers||all||2.5 - 6%||0%||ME||with the sugars|
|Lees||none||4 - 6%|
|Ushers, Trowbridge||Pale ales||2%||none in beers with malt extract||Diamalt|
|Robert Younger||all||1.50%||15%||malt extract||with the malts|
|Maclay||none||10 - 15%|
|Fuller||none||7 - 12%|
Jeffery claims that malt extract was used in grists with a high percentage of unmalted grains, yet only one of the brewers using it in the mash used any unmalted grains at all. The one exception was Robert Younger, where the proportion of unmalted grain was high at 15%.
Why did Ushers only include malt extract in their Pale Ale grists? I’d say head retention, but it was a diastatic malt extract, so that implies it was to aid mashing. Even though there was no unmalted grain.
Jeffery specifically mentions that diastatic malt syrup could be used to help avoid a stuck mash:
“The introduction of a highly diastatic malt syrup to the mash tun would be of considerable assistance in cases of a set mash owing to the presence of unconverted starch. But since the quantities of malt and sugar to be used in the brew have already been entered in the Excise book, some difficulty would be experienced in obtaining leave to add any further quantities, particularly in the middle of a brew.”
"Brewing Theory and Practice" by E. J. Jeffery, 1956, page 266.
If there hadn’t been the excise restriction, brewers might have waited until there were problems with the mash before adding malt extract. But as that was complicated, just throwing some in every time would have been a simpler solution. A sort of insurance against a stuck mash.
Those adding malt extract in the copper must have been doing it for head retention. The quantities involved are too small for it to be to extend the brew length.
I hope that’s answered your questions about malt extract. It sort of has mine.