Shear
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During boiling and chilling, shear must be minimized to prevent disruption of the protein flocs leading to increased trub settling time. The shearing of wort is largely due to the use of centrifugal pumps for recirculation in the kettle through a wort chiller and/or to create a whirlpool.[1]
Pumping hot wort gently is necessary to avoid a reduction in beer quality (49). For mash transfer, pipe design should minimize the length, extent, and number of bends, with velocities lower than 1.5 m/s (10). Mashes should be transferred at low speeds with widethroat pumps that are operated at the optimum region of their performance curves to minimize shearing within the pump casing (10).[2]
Viscosity, the measurement of this internal friction in a fluid, can be easily understood when movement between two layers of fluid is considered. The greater the friction (i.e., the higher the viscosity), the greater the amount of force required to cause movement between the adjacent layers. Such a force per unit of surface area required for movement is termed "shear stress". The velocity gradient between the two layers as they move by one another is termed "shear rate". The viscosity of the fluid is equal to the shear stress per shear rate. According to Newton, the viscosity at a given temperature is independent of the shear rate. Liquids, such as water and beer, behaving in this way are referred to as “Newtonian fluids”. However, many fluids, including β-glucan solutions at high concentrations, do not follow Newton’s assumptions and are designated "non-Newtonian fluids". The viscosity of a non-Newtonian fluid is called "apparent viscosity", because it is shear dependent. If a fluid displays a decreased apparent viscosity with an increasing shear rate (i.e., shear thinning), it is known as a "pseudoplastic fluid".[2]
Shearing enhances β-glucan precipitation.[2] Therefore precipitation of β-glucans in beer is accelerated by centrifugal pumping because of high shear rates.
Stirring must be carefully monitored during the mashing process, because the speed of stirring is directly related to the amount of shear stress transferred to the mash. A low shear rate, and a slower stirring speed, improves the quality of the finished beer. If the mash is stirred too violently, the high shear stress on the mash can result in lower enzymatic activity, smaller protein fragments, and fracturing of the grist that decreases the particle size (which may result in a stuck mash during the next step in the process.)[3]
Shearing has a substantial effect on both apparent viscosity and filtration performance of beer.[2] Also, the haze level can be enhanced by turbulent shearing.
- Narziss, L. (1993). Beta-glucan and filterability. Brauwelt Int. 11:435-442.
- Patelakis, S., Speers, R. A., Paulson, A., and Stewart, R. J. (1999). O8. Effect of β-glucan on the rheological and filtration properties of beer. (Abstr.) Am. Soc. Brew. Chem. Newsl. 59(2):16.
- How control flow rate Blichmann Riptide, per LODO teachings?
- https://www.themodernbrewhouse.com//forum/viewtopic.php?f=2&p=63122#p63122
References[edit]
- ↑ Golston AM. The impact of barley lipids on the brewing process and final beer quality: A mini-review. Tech Q Master Brew Assoc Am. 2021;58(1):43–51.
- ↑ a b c d Jin YL, Speers RA, Paulson AT, Stewart RJ. Barley β-glucans and their degradation during malting and brewing. Tech Q Master Brew Assoc Am. 2004;41(3):231–240.
- ↑ Mosher M, Trantham K. Brewing Science: A Multidisciplinary Approach. 2nd ed. Springer; 2021.