Gelatinized (flaked or torrefied) Grains
Flaked grains, such as flaked barley, flaked rice, flaked wheat, and flaked maize, are adjuncts that can be added to beer to increase alcohol content with very little flavor input. Why use flaked grains, instead of things such as rice from the grocery store or corn from the Co-Op? As the grains develop, the carbohydrates are stored within the cell walls in vacuoles (or storage vessels). These cell walls within the grains have to be broken open in order for the enzymes to access the carbohydrates. Highly modified malted barleys have already been held in a temperature range that allows the β-glucanase enzymes to break down these cell walls and help to make the carbohydrates accessible for the amylase enzymes. Non-gelatinized grains, on the other hand, have not had these cell walls broken open and therefore the enzymes can not reach the carbohydrates in order to convert them into fermentable sugars.
Flaked grains have been moistened and passed through heated rollers to obtain a sort of “cell wall” splitting action that opens the cell walls up and allows for conversion of starches. Non gelatinized grains have the carbohydrates sealed away where they can not be accessed for conversion.
This explains the lack of effectiveness of non flaked grains, but are there any problems using them? Whole grains, such as corn, still have the husk intact as well as the bran and the germ also still being present. The husk will add tannins and astringency to the beer, whereas the bran and germ will affect head retention and can cause premature staling because of the possibility of the oils becoming rancid.
In the end, it is much easier and more reliable to go ahead and use the pre-gelatinized grains for your beer, than it is to try to work with adjuncts that may not be converted at all, or if converted, could potentially stale your beer, affect head retention, or create a chill haze due to tannins.