From it's placement in the brewing record, I assumed it was a type of sugar, guessing that the G stood for glucose. How wrong I was.
Someone helpfully posted a link to the 1909 Brewer's Journal with a reference to B.P.G. Calling it "Beane’s Patent Gist" I realised the last word was misspelt, because it vaguely rang a bell. So I started digging around a bit. It soon became clear that it wasn't a type of sugar.
So, what was it?
I found this mention of in the newspaper archive:
The proceedings of the departmental committee appointed by the Treasury to consider the question of beer materials, so far as they have gone, have just been made known. At the first day’s inquiry, the Earl of Pembroke and Montgomery presiding, Mr. John Steele, Chief Inspector of Inland Revenue and Excise, was examined. He stated that his principal work was to advise the Board of Inland Revenue with regard to regulations in connection with the beer duty. Each brewer had to take out a yearly licence to brew, at a cdst of one sovereign. In brewing 42 lb. weight of malt or corn of any description and 28 lb. of sugar were deemed the equivalents of a bushel of malt, and a brewer was expected to produce eighteen gallons of beer at a gravity of 55 deg. from each bushel. The bushel might be grain or it might be sugar ; 421b. of the one or 28 lb. of the other was fixed by law so as to have the charge upon the worts instead of the materials. It was difficult for them to say that the use of a certain material was a practice of adulteration if the brewer obeyed the proper regulations in using it. In reply to questions, the witness said that the brewer had full liberty to use any kind of saccharine material — having the benefit of what was called the "free mash tun." No action would be taken against the use of hop substitutes that were not injurious to health. The Board would not interfere with the use of quassia, but it would with the use of cocculus indicus, because the latter was poisonous. It was used in beer many years ago. The materials used in brewing included malt, gelatinized rice, flaked rice, maize flour, torrefied malt, malt flour, Dutton's malt flour, Beane’s patent grist, patent rice malt, gelatinized maize, desiccated rice, maizone, cerealine, rizine, patent flaked maize, rice shells, and Sheppherd’s corn malt. Besides these ingredients there were black malt sugar, dextrinous caramel, glucosine caramel, caramelized dextro-maltose, viscosiline, liquorice, malto-dextrine, ground sago, and varieties of sugars glucoses, and saccharums under fancy trade names. The Government officers had power to enter public-houses and take samples. There was no restriction as to the quantity of salt put into beer."
St James's Gazette - Thursday 22 October 1896, page 15.
Note that it's listed along with various types of unmalted grains. OK, Probably some sort of prepared grain. But which grain?
Stopes also list it along with a variety of adjuncts:
"Other malts are now known to commerce. It is extremely difficult to procure information conceming the manufacture and nature of some of them, and that they are classed here with malt at all will doubtless be objected to by some.
Of these mention may be made of short-malt germless maize, short-malt rice, germless-maize, Beane’s patent grist, granulated grain, zealine, cerealine, grits, hominy, &c."
"Malt and Malting" by Stopes, F, W, Lyon, London, 1885, page 167.
Unfortunately, he gives no further details as to its composition. I need to dig further.