Fermentability of crystal malts

[Simonh82]

I've often heard it said that crystal malts produce largely un-fermentable sugars. This is what gives the beer body and residual sweetness.

I understand the process of creating crystal malts. Essentially mashing within the grain husk as part of the malting/kilning process.

What I would like to know is why these sugars don't break down during the proper mash in to more fermentable forms? Surely the enzymes in the mash will act on these long chain sugars in the same way they act on the sugars and starches in the base malt. What makes these sugars different?

 

[Oginme]

When making crystal or caramel malts, the green malted grains are kilned or roasted at temperatures which activate the enzymes which will break down the starches into sugars before raising the temperature to dry out and 'caramelize' the sugars. Since the water within the grain kernels is limited during the enzyme activity, only some of the starch is converted to sugar. Some of the starch which is not degraded to simple sugars are partially degraded to longer chain dextrins. While some of these dextrin chains are long enough for alpha and beta enzymes to degrade further during mashing, most are too short or contain branched sections and are not long enough for the enzymes to affect.

 

[Simonh82]

When making crystal or caramel malts, the green malted grains are kilned or roasted at temperatures which activate the enzymes which will break down the starches into sugars before raising the temperature to dry out and 'caramelize' the sugars. Since the water within the grain kernels is limited during the enzyme activity, only some of the starch is converted to sugar. Some of the starch which is not degraded to simple sugars are partially degraded to longer chain dextrins. While some of these dextrin chains are long enough for alpha and beta enzymes to degrade further during mashing, most are too short or contain branched sections and are not long enough for the enzymes to affect.

So the alpha and beta amylase only work on starches and long chain dextrins, they don't work on the shorter chain surgars which might have been created as part of the malting process. That makes sense now.

I thought that beta-amylase would snip away at anything larger than a maltose molecule unless it was a branch section.