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It's been far too long since I wrote anything about sugar. Brewing sugar. I'm still getting a grasp of the subject. There's so much more to it than just 1, 2, 3.

The array of sugars available to early 20th century brewers is baffling. And each had its own specific use. First there were the ever-popular invert sugars, mostly used in the copper. Straight glucose was also used in the copper. Occasionally, even plain old sucrose was used. Then there were proprietory sugars, blends of various other sugars. Caramel mixed with invert sugars was popular for priming dark beers and colouring them at the same time.

Dextrin-maltose is a sugar I've not paid too much attention to until now. Though I've just realised that it often makes an appearance in brewing records. It's just called DM, which is why it had escaped my attention. Ah, the value of background reading. It'd explaining many of the puzzling entries in brewing logs.

Dextrin-maltose seems to have had a very specific use. See what the experts say:

"Glucose. (It has already been noted that the sugars dextrose and levulose are more often referred to now as 'glucose' and 'fructose' respectively. Commercial glucose, while consisting mainly of dextrose (or glucose) contains other substances and the term 'glucose' as used for the pure sugar should not be confused with the term as used for the commercial product.)

Commercial glucose is a sugar made by the action of dilute acid upon commercial starches derived from rice, maize, sago or potatoes. The process of preparation is as follows:

The starch is dispersed in hot water containing 1 to 3% of acid. The solution is heated, usually under pressure, until the reaction is complete. The starch is progressively hydrolysed into dextrin, maltose and finally glucose, although these reactions overlap and at any time all three products may be present, their relative proportions depending upon the stage to which the action is allowed to proceed. It the action is stopped as soon as all the starch has been destroyed (when the cooled liquid ceases to give a blue colour with iodine) the product will consist of a mixture of dextrin and maltose with very little dextrose. If this product, after neutralizing the acid, is concentrated to a syrup containing 75-80% solids, the result is known as dextrin-maltose. It is largely unfermentable and is not very sweet. It can be used as a priming to produce fullness and body in beer without increasing the fermentable matter too much. As such it is often a constituent of priming sugars for pale ales, particularly to be used in hot weather.

If the hydrolysis is allowed to proceed to completion, the resulting product on neutralizing and concentrating to give about 80% solids will solidify and this is commercial glucose. Glucose is mainly used as a copper sugar to increase the fermentable matter in the wort. It imparts a somewhat dry flavour."
"Brewing Theory and Practice" by E.J. Jeffery, 1956, pages 164 - 165
The characteristics of dextrin-maltose were: not very sweet and not very fermentable. At first glance you'd wonder what use it could be put to. But those features turn out to be perfect for one thing: priming Pale Ale. And in particular low-gravity Pale Ales.

As this entry from The Brewer's Journal problem page confirms:

"North Essex,—We are anxious to get as lasting condition as possible on our small cask trade, and we think of using dextrin maltose sugars in both copper, also as priming. We have hitherto primed with cane candy, but condition is rapid and soon goes off on tap. We also use No. 2 invert in copper. We now suggest using dextrin maltose both in copper and cask. We realise this sugar is not so sweet, but we think the dextrin will give us a more permanent fulness and lasting beady condition while cask is on tap. We shall appreciate your criticism and advice. We suggest priming with D.M. at rate of quart per barrel at rack and sending out casks ten days later to secure a steady condition before cask goes out to customer.

Your suggestions are in accord with common practice. Dextrin maltose and other sugars of similar composition are very useful as primings with low-gravity beers, especially light bitters, with which fulness rather than sweetness is required. It does give more lasting condition than the more fermentable sugars, which are more suitable when the beer is sent out at racking for rapid consumption. It is advisable to ferment the light bitter fully before racking."
"The Brewers' Journal 1940" page 664. (Published August 21st, 1940.)
You see the problem with straight sugar? It fermented too quickly and readily in hot weather and casks went off too quickly. Something that doesn't ferment as easily and isn't particularly sweet (you wouldn't want you Pale Ale too sweet, would you?) is the perfect priming.