Project Description

What's the Deal with Beerstone?

Beerstone can form on any surface that comes into contact with beer and wort (unfermented beer) and has been a problem for brewers as long as beer has been produced^1,2. The most problematic locations for its formation are heat exchangers, fermentation vessels, aging tanks, kegs, and beer dispense lines. Beerstone is comprised of a combination of precipitated calcium oxalate and entrapped beer polypeptides³. Oxalate enters the brewing process from the cereal grains and hops used to make beer⁴. Oxalate is present in the form of aqueous oxalic acid which is a corrosive, highly oxidized compound that has strong chelating activity⁵. These oxalate ions are soluble in both wort and beer, allowing them to combine with calcium ions to form calcium oxalate⁴. Calcium ions enter the brewing process through the water, grains and water- correction salts⁴. Calcium oxalate precipitates out of solution upon formation, and is one of the most insoluble metallo-organic compounds with a low solubility⁴. In a geographic region with high calcium levels in the drinking water, such as Guelph, Ontario, this can lead to up to 165g of calcium oxalate building up in a single 1000L batch of beer¹. This precipitation can happen either during the brewing or bottling processes, or after the beer has been bottled depending on when the calcium oxalate formation occurs⁴. The point during the process at which calcium oxalate forms is dependent on temperature, time, pH and ion concentration⁴. If calcium oxalate forms after filtration, or if all calcium oxalate crystals are not filtered out of the beer, haze and sediment may form⁴. In addition, calcium oxalate crystals can cause over-foaming during filling or when a pressurized container such as a can or bottle is opened⁴.

Deposits of beerstone provide protection and nutrients for bacteria to grow due to the porous surface of the beerstone, into which nutrient-providing proteins often become entrapped⁶. This allows for unwanted microbial growth and the formation of biofilms upon the beerstone, for example, species of the genera Pectinatus and Megasphaera. These microorganisms cause beer spoilage, products with reduced shelf-life, off-flavours and sour tastes, rendering the beer unsuitable for sale or consumption thus resulting in financial loss to the brewer7. Lactobacillus species in beer, in particular, causes high turbidity which manifests as a hazy appearance in the liquid. It also causes a high level of diacetyl in the beer, resulting in an unwanted ‘buttery’ flavour7. With the removal of beerstone, growth of these microbial contaminants will be prohibited, improving brew quality and reducing downstream processing.

Beerstone is difficult to remove for several reasons. Calcium oxalate is extremely insoluble in both hot and cold water, meaning that the use of harsh chemicals is currently required for effective methods of removing beerstone. These involve caustic or other harsh cleaners which are dangerous to work with due to the potential for exposure burns of the eyes and skin, and corrosive damage to surfaces2, 3, 8. The use of these cleaning agents require long and frequent pauses in production which lower the efficacy of the brewing process. Additionally, the equipment required to utilize these caustic chemicals and the chemical disposal requirements are costly and potentially environmentally damaging.

Oxalobacter formigenes and the Breakdown of Oxalate

Oxalobacter formigenes is an anaerobic, Gram- negative bacteria native to the human gut microbiota5. O. formigenes is a safe (biosafety hazard level 1) organism that relies solely on oxalate as its source of energy, as well as its main source of carbon5, 9. Metabolism of oxalate is accomplished by two enzymes, Formyl Coenzyme A Transferase (FRC) and Oxalyl-Coenzyme A Decarboxylase (OXC)5, 10. In the first reaction step, a Coenzyme A (CoA) is transferred to oxalate by the FRC enzyme10. It functions by forming a ternary complex between the substrates and the enzyme, resulting in an oxalyl-CoA complex5, 10. The OXC enzyme then catalyzes a reductive reaction in which formyl-CoA and CO2 are produced5, 10. The FRC enzyme then cycles the CoA back to a new oxalate molecule to start the process again5, 10. Thiamine pyrophosphate (TPP), Mg2+, acetate, and CoA are required for the reaction to take place5, 10. The reaction mechanism for the breakdown of calcium oxalate by FRC and OXC can be shown by the equations in Figure 110:

References

Rose, D. This is a test (2017). Sci. Awesome. 28-29