Inside the Matrix….Beta Glucan and Arabinoxylan
𝛃-glucan is a glucose polymer that is found in the cell walls of the endosperm. 𝛃-glucan along with another non-starch carbohydrate called arabinoxylan (or pentosan) comprise more than 70% of the molecular weight of these cells.
High 𝛃-glucan levels in finished malt can lead to a laundry list of headaches for brewers, so it lands squarely on the shoulders of the maltster to manage the degradation of these polymers. In truth, we are simply setting the stage for a variety of enzymes to go to work breaking down these matrices.
Steeping the grain to approximately 45% moisture with adequate fresh air delivery is the key first step. This hydration activates the aleurone layer to synthesize hydrolytic enzymes such as 𝛂-amylase, 𝛃-glucanase, and pentosanases which are released into the endosperm. Cell wall degradation quickly follows and 𝛃-glucan is solubilized. This process continues over the four day germination period. Maltsters focus on delivering large amounts of humidified air and maintaining cool temperatures in the grain bed to support this enzymatic digestion. The final kilning step stabilizes the enzyme package, making them available for use in the mash.
The success (or failure) of the maltster’s efforts in this department are reflected in the 𝛃-glucan levels concentration reported in the Certificate of Analysis. These levels are reported in milligrams per liter. This methodology primarily captures the longer chain polymers that represent the high molecular weight portion that contributes to filtration issues and stuck mashes in the brew house.
In the United States, most brewers look for this concentration to be below 100 mg/L. However, European brewers accept much higher levels, sometimes exceeding 250 mg/L. This discrepancy could be a result of the need for rapid filtration among the larger breweries in the United States.
Arabinoxylan levels are not reported in the CoA, but are present in all cereal grains but found in highest concentrations in rye. These carbohydrates play a significant role in determining viscosity and filterability rates for brewers. Published research has documented levels exceeding 300 mg/L from samples that registered ~50 mg/L 𝛃-glucan. In other words, that stuck mash could be a result high molecular weight arabinoxylans and not 𝛃-glucan.
So what can we do as maltsters to drive degradation of arabinoxylan? The answer is not much. Early enzymatic activity within the endosperm is focused on stripping sections of pentosan away which allows access to the 𝛃-glucans beneath. Current research has documented the presence of a xylanase inhibitor that develops midway through the germination stage which reduces further degradation. Some have suggested that the introduction of exogenous enzymes would aid in this function.
Just something to consider the next time you are dealing with a slow runoff or stuck mash!