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We all know those topics that pop up time and time again on any home brewing forum. One of those topics is under-attenuation. Why didn't my beer attenuate? someone asks, followed by a string of replies asking how much crystal malt was used, after which ensues the explanation that the addition of unfermentable crystal malt sugars is to blame. I want to investigate whether or not this reasoning is actually sound, and what the effects of using crystal malts are on the fermentability of the wort.As with any investigation, we don't know all the parameters and have to make assumptions. This is no exception, so let's start by taking it as fact that after the malting process, crystal malts contain a lot of unfermentable sugar. I've never seen a crystal malt specification sheet showing the breakdown of the different sugar molecules, but there seems to be evidence to support this, so let's roll with it. During the malting process, the grain essentially starts to germinate; the starch-converting enzymes start working away creating sugar. Before too much of the sugar is used by the seed for growth, the process is halted, typically by dry heat. This part of the process has the side effect of denaturing enzymes, notably the starch and sugar conversion enzymes (a- and B- amylase, limit dextrinase etc.). Now you have grain with high sugar content and no diastatic power. So it makes sense that if you steep crystal malt in the absence of starch/sugar-converting enzymes, then the resulting sweet liquor will contain a substantial amount of unfermentable sugar. Therefore, in extract plus specialty grains brewing the old theory seems reasonable.What happens if you bring other malts in to the equation? Most of the time in all-grain brewing (or partial mash brewing for that matter), the crystal malts are mashed in with all the other malts. Modern base malts are high in diastatic power, so there are plenty of enzymes to go around, and any low or non-diastatic malts used in relatively small amounts convert just fine. But we're supposed to believe that the enzymes from the base malts aren't going to work on the sugars from the crystal malts? That seems pretty suspect to me. Are the enzymes xenophobic or something? Is there some sort of magical force field around the sugars from the crystal malt?In my pursuit of knowledge I came across an experiment by fellow HBT-member nilo' that was aimed at getting to the bottom of my question. The experiment was documented in the author's blog Homebrewing Beer and discussed in detail in this HBT thread . In brief, Nilo made seven samples: three 100% crystal malt samples, each using a different crystal malt with a different colour rating between 10 and 120L; one 100% base malt sample; and three 50% base malt/50% crystal malt samples (the same crystal malts as the 100% samples). All samples were mashed or steeped at 155F (~68C) for 30 minutes. The original gravities (OGs) were recorded along with the apparent attenuations (AAs) as a function of time. All samples were fermented at 70F (~21C) with Fermentis Safale S-04 yeast.shows the measured OGs of the samples along with the OG you would expect for the 50/50 samples by taking the average of the straight base malt and corresponding straight crystal malt sample. Figure 1 suggests that the enzymes in the base malt do in fact work on the crystal malts. This is illustrated by the fact that in each case the yield increased when 50% base malt was used, but the increase was more than that expected when simply averaging the extracts from the crystal and base malt.shows the AAs of the samples and illustrates two things. First, it supports the idea that crystal malts contain substantial amounts of unfermentable sugar after the malting process, and that steeping in the absence of diastatic malts results in a not very fermentable wort. Second, it supports the idea that adding diastatic power makes the resulting wort more fermentable. While the AAs of the 100% crystal malt samples were quite low (40-50%), the samples with 50% base malt showed 67-75% AA compared to the 100% base malt sample which exhibited 80% AA.So yes, the crystal malt lowered the AA even when mashed with base malt. However, the presence of base malt had a large impact on the fermentability of extract obtained from crystal malt. Also, remember these were 50% crystal malt, which is unusually high for a beer grain bill. Using a linear interpolation between the 100% base malt (80% AA) and worst attenuating 50/50 sample (67% AA), I determined the following table of percentage crystal malt, AA, and corresponding FG from an OG of 1.050.So for a much more reasonable recipe using only 5-10% crystal malt, you would only expect the FG to be maybe a point higher compared to 100% base malt. However, there are so many other factors that affect the attenuation such as yeast health, pitching rate, nutrient content of the wort, etc., which most likely produce a larger change. This makes it very hard to realistically point the finger at crystal malt for causing major changes to the apparent attenuation in the context of all-grain brewing.So, do crystal malts reduce fermentability? Yes. However, as always, the real world results aren't so straight forward, and a proper understanding is required to make a sensible assessment of the situation.//www.pinterest.com/pin/create/extension/