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Science Lesson: Science of what we do

How dry hopping can lead to diacetyl

As the craft brewing industry pushes IPA hop additions to the limit, hop creep has presented a serious challenge: Brewers are forced to either wait for heavily dry-hopped beers to pass a diacetyl test, or take the risk that their super-fresh beers turn into diacetyl timebombs. In this science lesson we explain how diacetyl formation works.

Let’s take a second to break down the science behind diacetyl formation, and how it can happen during hop creep. The first thing to know is that wort contains many different types of sugars. Some are fermentable by yeast and some are non-fermentable. Importantly, some of the non-fermentable sugars can be converted into fermentable sugar by certain amylase enzymes that are present in hops. So, when you add a big dry hop charge after primary fermentation, the hop amylases go to work on the non-fermentable sugars resulting in more fermentable sugars and a secondary fermentation. During this secondary fermentation, ordinary yeast will make α-acetolactate, which is the immediate precursor to diacetyl. If the yeast is healthy and given enough time, it will consume the α-acetolactate. But if the α-acetolactate remains at the time of packaging, it will turn your beer into a ticking diacetyl timebomb.

Figure 1. Fermentable sugars are transformed by yeast into a metabolite called pyruvate, in a biosynthetic process called glycolysis. Pyruvate is a metabolite that is central to many other biosynthetic processes, like ethanol production and amino acid biosynthesis. In amino acid biosynthesis, pyruvate is transformed to α-acetolactate, which is a chemical intermediate in those biosynthetic pathways. When excess α-acetolactate accumulates during fermentation, it is secreted into the fermenting beer and slowly forms diacetyl. In some cases, yeast can consume the diacetyl provided a sufficiently long diacetyl rest. But in other cases, especially with heavily dry hopped beer, the diacetyl rest may require a weeks-long rest, after which point the yeast is no longer capable of consuming the diacetyl, and much of the brewery-fresh hop character has been lost.

To find out how yeast can help with diacetyl, check out how yeast can prevent diacetyl formation