Can raw grains produce fermentable sugar without using malted barley or another malted grain? Yes. While malt greatly facilitates the process, unmalted barley, rye too, can produce a fermentable mash.
See Edward Skeates White, a 19th century authority on malts and malting, here, at 46-47 (The Maltster’s Guide, 1860).
Numerous books confirm this including Brewing With Raw Grain: A Practical Guide (1883) by Thomas Lovibond, well-known brewing scientist of the same era. See the table at p. 73 where he states he made a mash from 100% raw barley (“barley 100”). He gives the respective yields of barley 100 and mashes of mixed malted and unmalted grains. It is no surprise the 100% unmalted version is lowest in yield, but a wort is produced and alcohol results.
See the extracts below (pp 133-134) from Horace Kephart’s Our Southern Highlanders, an early (1913) ethnographic study of the mountain people of the Appalachians, viz the mash for mountain whiskey. If malt was available they used it with the raw grains. If it wasn’t, e.g., due to the “blockade” or British embargo during the Revolutionary War, they made a whiskey mash anyway.*
Raw grains have enzyme, b-amylaze, in small amounts but this can convert polymer starches to maltose.
White explains why it generally isn’t done in brewing: rawness of taste, instability 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 which palliate the disadvantages of raw grain, but he clearly opts for a mixture approach and indeed that is the basis of large-scale brewing today of adjunct lager.
Whether one approves of mixed mashes is a question of taste. I generally plump for all-malt based on some 40 years experience tasting beer. However, adjunct used in small amount can produce excellent beer, too.
*Re-reading the Kephart account, perhaps really the mountaineers were making their own malt as there was a sprouting, drying, and grinding of moistened corn. Perhap corn is different, but the other authorities mentioned are clear that raw barley, for example, can produce a fermentable mash. Raw corn must be cooked, too, to hydrolize the starches. Nonetheless the account is valid IMO as showing an artisan practice – no very sophisticated knowledge or equipment is needed to mash without commercial malt. It may be noted too he explains fermentation can be achieved without adding yeast. Last year I devoted numerous posts to this aspect of “wild” fermentation.