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====The Freshening Power of the Hop (Hop Creep)====
Also known as "dry hop creep", it was first discovered in 1893 by Brown and Morris that dry hopping increases the ABV of beers and dries them out. It was proposed that the likely cause is the release of glycolytic enzymes that break down starches into sugars that which viable yeast can then ferment. Brewers normally aim to control the final alcohol percentage in a beer through brewhouse operations rather than postfermentation dilutions with lower/higher alcohol beers or water. This approach to brewing is called "brewing to final gravity." Due to the need to have a predictable ABV for government regulatory reasons, unexpected fermentation is, therefore, a concern for many breweries <ref name="Kirkendall_2018">[https://www.tandfonline.com/doi/abs/10.1080/03610470.2018.1469081?journalCode=ujbc20 The Freshening Power of Centennial Hops. Jacob A. Kirkendall, Carter A. Mitchell & Lucas R. Chadwick. 2018. DOI: https://doi.org/10.1080/03610470.2018.1469081.]</ref>.
Historically, there have been two studies published on the phenomenon of hops releasing glycolytic enzymes that break down starches during dry hopping: [http://barclayperkins.blogspot.com/2018/03/why-dry-hop.html Brown and Morris (1893)] and [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1941.tb06070.x Janicki et al. (1941)]. More recently, several researchers and brewers have rediscovered revisited this phenomenon. Brown and Morris (1893) discovered that hops could break down maltodextrin, but failed to extract identify the enzymes from the hop plant material and hypothesized (probably incorrectly) that tannins were inhibiting the enzymes. Janicki et al. (1941) came to similar conclusions regarding the enzymes and tannin inhibitors, and they also concluded that the enzyme activity was independent of hop variety, geography, age, storage conditions, pH values between 4.1 and 4.8, and that one or more additional unknown factors were at play <ref name="Kirkendall_2018" />.
More recent studies have shown that there is a difference in this enzymatic power between different hop varieties. [https://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-2257-01.aspx Cibaka et al. (2017)] reported an increase in ABV when dry hopping with Amarillo and Sorachi Ace hops, but not when dry hopping with Citra or Hallertau Blanc. Interestingly, they also found that Mosaic hops resulted in the opposite effect and it was hypothesized (probably incorrectly) that Mosaic hops might release some sort of unidentified molecule that inhibits yeast fermentation/growth or viability <ref name="Kirkendall_2018" />.
Kirkendall et al. (2018) found that hop varieties also have a varying ability to ferment dextrins. They reported the following ABV increases when dry hopped in a pale ale at one pound per barrel: Centennial hops (+0.27%), Citra (+0.12%), Simcoe (+0.33%), Cascade (+0.49%) and Amarillo (+0.49%). Prolonged contact with Centennial hops (42 days) increased the ABV even more so and resulted in a nearly 1% ABV increase in ABV. Rousing the hops into suspension hastened the increase in ABV compared to samples that were left still. From their results, it appears as though contact with hops during dry hopping continues the breakdown of starches and dextrins into fermentable sugars. They also concluded that dry hopping at a temperature that is too cold for the yeast strain in the beer to ferment resulted in no change in ABV. They compared the enzymatic activity of Centennial hops that were stored at -20°C versus room temperature storage and found that there was no significant difference, indicating that the unidentified enzymes are relatively stable <ref name="Kirkendall_2018" />.
Kirkpatrick and Shellhammer (2018) found that the enzymes responsible for the conversion of dextrins into sugars include amyloglucosidase (removes glucose from non-reducing ends of α-1,4 and branching α-1,6 linkages, with a preference for α-1,4 linkages and longer chain oligosaccharides), α-amylase (hydrolyzes randomly along glucopolysaccharides to produce maltose, maltotriose, maltopentaose, and maltohexaose products from amylose as well as maltose, glucose, and branched dextrins from amylopectin), β-amylase (saccharifiying enzyme, cleaving maltose in small amounts from nonreducing ends of glucopolysaccharides, and to a minor extent, maltotriose), and limit dextrinase (debranches limit dextrins at α-1,6 linkages, producing linear α-1,4 chains which can further be degraded by the combined action of amylases). They were able to successfully extract them from Cascade pellet hops using commercially available assays (enzyme specific para-nitrophenyl blocked oligosaccharide substrates). The amount of α and β-amylase found in Cascade hops was well below that of malted barley, but within the range reported in other plant leaves. These enzymes are denatured by high temperatures, and as such would be denatured when boiling hops. They reported a similar increase in ABV of 1.3% after 40 days when dry hopping a beer with Cascade hops (and a decrease of 1.9°P) at a rate of 10 g/L. They also found that the hops contained glucose and a small amount of fructose, which accounted for a sugar increase of 0.02−0.03 °P per gram of hops. More studies on whether or not the amount of dry hopping has a large effect needs to be done, and whether or not warmer temperatures speed up the enzymatic breakdown of dextrins, and the authors hypothesized that the rate of dextrin break down could be slowed by dry hopping at lower temperatures <ref name="Kirkpatrick_2018">[https://pubs.acs.org/doi/pdf/10.1021/acs.jafc.8b03563 Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops (Humulus lupulus). Kaylyn R. Kirkpatrick and Thomas H. Shellhammer. 2018. DOI: DOI: 10.1021/acs.jafc.8b03563.]</ref>.