The dextrins you get from Carapils/Carafoam can't be broken down by alpha/beta amylase.
Why not? I don't understand why, in a single infusion mash of, say, the low 150s°F, alpha amylase wouldn't break down long-chain unfermentable sugars which would subsequently be broken down further by the remaining beta amylase before it becomes too denatured.
I think there may be some misinformation and homebrew lore out there regarding Cara-pils. I wonder if some of it stems from steeping it in extract brewing. In any case, I think it's more complex of a topic than is typically discussed. It seems like a lot of times homebrewers want things to be black and white, when there is really a lot of gray area and conditional outcomes.
It's been a while since I've seen a malt analysis for Cara-pils and I can't say I've really studied the science behind Cara-pils, but here are two things that made me question the standard information out there about using it:
-Increased Foam Stability:
Does using Cara-pils really increase the end content in the beer of the proteins that make up foam (primarily Protein Z and LTP1)? All I can say is that I've brewed quite a few batches of the same type of beer (Helles) using various amounts of Cara-pils (from none to 12%) with all other things being fairly equal and noticed no difference in foam formation/stability. In fact, I did notice that when I reduced or eliminated a protein-related rest, the foam formation/stability was significantly increased, regardless of the amount of Cara-pils used.
-Decreased wort fermentability:
Coinciding with the above, I noticed no difference in wort fermentability regardless of the amount of Cara-pils used. To increase my suspicion, I noticed that Horst Dornbusch's book about brewing Helles has recipes that call for up to 20% Cara-pils. Then another brewer on here (can't recall the name) brewed Helles with ~20% Cara-pils and it attenuated just fine.
I realize that none of this is hard evidence, but it is worth considering in my mind. So, I'm leaning towards thinking that Cara-pils is conceptually just like any other crystal malt. I.e., it does add dextrins, but they are broken down in the mash to the point that the alpha/beta amylase can break them down before becoming denatured too much. It may take a significant amount of Cara-pils and a higher mash temp to result in a noticeably higher dextrin content in the wort.