Technical Notes on Fermentability

Wort fermentability can be a confusing topic. In a simple sense, base malts, toasted specialty malts, and unmalted adjuncts contribute long chains of sugar molecules (i.e., starches). The enzymes contributed by the base malt clip chains of sugar molecules of various lengths from the starches. Shorter chains are fermentable sugars, slightly longer ones are unfermentable dextrins.

The saccharification rest temperature is the simplest variable to adjust to alter the percentage of carbohydrates in the wort that will be short enough for brewer’s yeast to ferment. This is because the enzyme (alpha amylase) that works most effectively at the upper end of the standard 140-160°F range produces both sugars and dextrins, while the enzyme (beta amylase) that works best at the lower end of the range produces maltose, which is easily fermentable by brewer’s yeast. At lower temperatures especially, allowing more time for the beta amylase to work also boosts fermentability (if you only mash for 10 minutes at 142°F, the result will not be a very dry beer). Not much controversy there I hope.

But, what other aspects of the recipe/process influence fermentability? I’m not talking about the fermentation, which is a completely separate, but equally important topic. Today I'm focusing on the carbohydrate profile of the wort. Maybe the water-to-grain ratio of the mash? How about the percentages of various base malts, specialty malts, and unmalted grains? Or a long boil with all that kettle caramelization? Over the last few weeks I bumped into three experiments/studies which address these questions.

The most common secondary factor that I've heard influences fermentability is the water-to-grain ratio of the mash. It is often said that thicker mashes yield less fermentable worts. I’d never paid too much attention to this rule, but it is worth knowing (especially if you are considering switching to thin brew-in-a-bag mashes) that it isn’t accurate. Kai Troester found that: “Contrary to common believe no attenuation difference was seen between a thick mash (2.57 l/kg or 1.21 qt/lb) and a thin mash (5 l/kg or 2.37 qt/lb).” An assertion he also supports with several references to technical brewing texts. His white paper summarizes this, and a number of other interesting experiments he has done on factors influencing fermentability and efficiency as well (e.g., calcium, pH etc.).

Many homebrewers think of crystal/caramel malts as adding only or mostly unfermentable sugars. While this makes sense when steeping them for an extract beer, I’d always wondered how dextrins added to an enzymatic mash would survive when much larger starches contributed by base malts do not. Over on HomeBrewTalk, Nilo posted results from experiments he did showing that crystal malts (especially paler varieties) are not a great way to decrease wort fermentability. Steeping crystal malt alone resulted in a wort that was only 40-50% fermentable with S-04. However, a mash with equal parts of pale 2-row and crystal lowered the fermentability (compared to 100% base malt) by only about 3% for C10, 11% for C40, and 13% for C120 (significantly higher attenuation than would be expected by averaging the attenuation of the tests with crystal and 2-row alone). His results suggest that using a more reasonable 15% crystal malt would only result in a reduction of the attenuation by 1% for C10, 3% for C40, and 4% for C120. Not insignificant, but only an addition of .0005-.002 to the final gravity for a beer that starts at 1.050.

According to another of Kai’s experiments, in the same paper referenced above, lowering the enzymatic content of the mash by using a base malt with a low diastatic power, like dark Munich, can reduce fermentability compared to a paler malt. It may be that some of the difference in Nilo’s crystal malt test could be accounted for by this, making his results even less significant under real world recipe conditions. Kai theorized that a similar affect could be achieved by adding a large percentage of unmalted grain, although it is unclear at exactly what average diastatic power of the grist wort fermentability is reduced, considering he achieved similar results from both Pilsner and Munich malts. This seems to dispute the basis that some brewers have for adding corn or rice to the mash to boost fermentability. While these adjuncts will dilute the malt flavor, they do not have the same effect as adding highly fermentable sugar to the boil. At best the amylase enzymes contributed by the base malt will produce a similar sugar profile from adjunct starches as the starches from the malt itself.

In the final study, Ankita Mishra examined how the fermentability of wort is altered through non-enzymatic processes in the boil (among many other changes to the wort). During the boil sugars and amino acids in the wort interact to produce melanoidins, in a process called the Maillard reactions. According to the study this can significantly, although only slightly, reduce the fermentability of the wort (the difference of extending a 30 minute boil to 120 minute was an increase of only .001 to the FG). This would also suggest that the melanoidins produced by either malting or decoction mashing could reduce fermentability, possibly explaining some of the differences Kai witnessed when mashing dark Munich compared to Pilsner malt. Caramelization happens primarily at much higher temperatures than those achieved during a standard wort boil, especially close to the target pH of 5.0.

You can look at the methods for manipulating fermentabilty either as ways to preserve body and some sweetness in a clean beer, or as ways to preserve carbohydrates for the slower working wild yeast and bacteria in a mixed fermentation. Brettanomyces, Pediococcus, and some strains of Lactobacillus are capable of producing enzymes which break apart carbohydrate chains too long for brewer’s yeast to ferment. My understanding is that melanoidins are not fermentable by any of the microbes found in sour beer, but I’d be interested if anyone knows otherwise!

Hopefully this brief literature review helps when designing a recipe, or altering your process.