Thanks for that.
I'm a bit confused about the difference between dextrins and starch, something to do with how the bonds are formed. Way beyond my chemistry abilities.
So dextrins are sweet tasting and not fermentable?
Do distillers attempt to reduce dextrins, whereas beer makers favour them?
Starch is made up of very long chains of glucose molecules bonded together. There are two types of starch: amylose - which is linear chains, and amylopectin - which is branched chains. The branch points in amylopectin are a different type of bond than the linear chain bonds, and the branched bonds cannot be broken (technically hydrolyzed) by the alpha and beta amylase enzymes that are active in the mash. In order to make starch fermentable, it has to by hydrolyzed into one, two (maltose), and for some yeasts three (maltotriose) glucose unit molecules. But since the branched parts of the amylopectin cannot by hydrolyzed by alpha or beta amylase enzymes, the pieces that are left after everything that can be hydrolyzed has been hydrolyzed, are called limit dextrins. Dextrins are not fermentable, and they do taste sweet.
Distillers don't really care that much about the taste of the wash prior to distillation (as long as it doesn't have bad tasting components that are volatile, and can make it thru the process to the end product.) But, beer brewers do care about the taste since they don't plan to distill it. The dextrins in beer give it a sweet taste, so they add hops (or something else) to balance out the residual sweetness.
There are enzymes which can hydrolyze the branch bonds in dextrins, and thus convert the dextrins into fermentable sugars. The most well known of these enzymes is amyloglucosidase (aka glucoamylase, or just gluco.) If you use amyloglucosidase in beer, you will end up with a very dry, light bodied beer, which is usually considered undesirable. However, when distilling, the dextrins do not vaporize during distillation, so don't make it into the final product anyway. So, distillers might choose to add amyloglucosidase to their mash or fermenter in order to get more fermentable sugar (and no dextrin), and therefore more alcohol from a given amount of grain. This improves their overall efficiency.
Brew on