Can raw grains produce fermentable sugar without use of malted barley or another malted grain? Yes they can. While malt greatly facilitates the process, unmalted barley, or rye, can produce a fermentable mash.
See the discussion by Edward Skeates White, a nineteenth century authority on malts and malting, here, pp. 46-47 (The Maltster’s Guide, 1860).
Numerous books are in agreement, see e.g., Brewing With Raw Grain: A Practical Guide (1883) by Thomas Lovibond, a well-known brewing scientist of the same era. In his table at p. 73 he states he made a mash from 100% raw barley (“barley 100”). He gives the respective yields of this barley 100 as against various mashes that combine malted and unmalted grain. It is no surprise that the 100% unmalted version gives the lowest yield, but a wort is still produced and hence alcohol can be made.
Raw grains such as barley have an enzyme, b-amylaze, in small amounts but enough to convert polymer starches to maltose.
See also the extracts below (pp 133-134) from Horace Kephart’s Our Southern Highlanders, an early (1913) ethnographic study of the mountain Appalachians, addressing the mash for a mountain whiskey. He states if malt was available it was used with raw grains. If it wasn’t, due say to the “blockade” (British embargo during the Revolutionary War), a whiskey mash could still be made.
Perhaps, in this last case, the mountaineers were really making a corn malt as a sprouting, drying, and grinding of moistened corn are mentioned. Corn is probably different to raw barley in regard to the potential for self-germination. Raw corn must be heated to a high temperature, for example (cooked), to hydrolize the starches. Nonetheless the account is of interest as showing an artisan practice: no very sophisticated knowledge or equipment were needed to mash without sourced barley malt. It may be noted, too, that he explains fermentation can be achieved without adding yeast. Last year I devoted numerous posts to this aspect of “wild” fermentation.
White explains why this generally isn’t done in brewing: rawness of taste and instability (likelihood to sour or putrefy) of the wort. Stewart & Thomson make a similar point (see pp 15-16) in their 1849 text on brewing and distilling. Lovibond claims in his book to offer methods that reduce the disadvantages of raw grain, but he clearly opts for a mixture (malted and unmalted) grain. Indeed, this is the basis of mass market brewing today for adjunct lager.