Difference between starches and sugars in finished beer?

When it comes to unfermentables in our finished beer, is there a difference between sugars that are not fermentable by brewers yeast and starches that have not been converted into sugars in the mash? Is one more susceptible to to wild yeast and bacteria than the other? Are there other considerations like flavor?



(Posted this question at the end of a thread in the General Beer Discussion Forum. Hoping someone here has answers.)

The main difference, in my understanding, is size. Starches are large polysaccharides, where as dextrins (and other unfermentables) are small polysaccharides >3 monosaccharide/disaccharide units, which have been broken down by amylase enzymes and are unfermentable. Both bacteria and wild yeast have the enzymes necessary to break large polysaccharides into more managable units to be used as carbon sources, and they can already utilize the sugars that are unfermentable by S. cerevisiae. Bacteria and wild yeast will have no problems using either as a carbon source.

In terms of other considerations, I have read that leaving unconverted starches in beer will leave a stubborn haze, different than chill haze or particles in suspension. The larger polysaccharide starch molecuels will significantly contribute to viscosity and sweetness, similar to using a high-temp mash.

I'm interested in what others have to say about this.

Starch is not something you want in your finished beer. Yeast, at least the ones that produce good tasting beer, do not eat starch. Only unwanted micro-organisms in the brew can metabolize it. If they do you are going to run into unwanted side effects like overcarbonation/gushing bottles, sour beer, bad tasting beer, etc. At best your beer is going to have a starch chill haze which will affect the way it looks and run the risk that the really bad side effects can still take place down the line.

Unfermentable long chain sugars should be in the beer for a reason, like color and flavor. The issue of infection is a separate one and you should always follow good procedures of cleaning & sanitizing.