New Sugar Components of Hop Creep: Plant Secondary Metabolites and Starch

Abstract: Dry hopping in beer can lead to a highly variable and unintended secondary fermentation known as "hop creep," posing challenges for breweries and raising concerns about product quality and safety. While existing research has primarily focused on dextrin degrading enzymes, this study aims to explore previously overlooked sources of fermentable sugars within hops and their potential contribution to hop creep. This research identifies two hidden sugar sources.
The first hidden source of fermentable sugar is starch-laden stores that accumulate from natural photosynthesis during diurnal cycles.  Plants convert excess energy from photosynthesis into starch throughout the day, utilizing it as energy overnight. The amount of starch in the hop bract varies depending on time of harvest (as it continues to build throughout the day), leading to varying levels of starch in the pellet and subsequent fermentations. While starch is not immediately fermentable, the dextrin degrading enzymes present during hop creep can cleave these into simple sugars.   
The second hidden source of fermentable sugar resides within plant secondary metabolites (PSMs), which plants produce for various purposes, such as signaling and defense against herbivory. Brewers are aware of PSMs primarily because most aromas are PSMs. To stabilize these compounds within the plant while ensuring their rapid availability, plants can bond PSMs to sugars like glucose. These bonds are cleaved by promiscuous enzymes or acid hydrolysis (at fermentation pH levels), releasing the sugars for fermentation. Despite their importance, sugars bound to PSMs have received little attention and remain largely unexplored, with the quantities of PSMs bound to sugars unknown. Once released, these sugars can contribute to unintentional secondary fermentation through dry hopping.
Initial identification of starch-laden sugars involved hot water soak and iodine staining, revealing blue dyed stomatal cells indicative of starch. HPLC analysis confirmed starch levels of 1.5% in both pelletized and whole cone products, contributing to the total nonstructural carbohydrate content of 8.5% found. To quantify glycosidically bound glucose, 64 hop samples underwent MCW extraction and enzymatic treatment followed by analysis with Megazyme D-glucose and D-fructose assay kits. Results indicate a substantial increase in glucose levels following enzymatic treatment, with the mean glucose level in the enzyme addition treatment nearly tripling that of the free sugar treatment (6.7g/100g hops vs. 1.67g/100g hops).
This study provides new insights into hop-derived sugars and their potential role in hop creep, advancing our understanding of this phenomenon. By considering factors such as glycosidically bound and starch-laden sugars, this research offers a new perspective on the origins of hop creep, with implications for brewing practices and product outcomes.