|MadSci Network: Physics|
By pastry, I assume you mean the dough before it is baked? If you mean the pastry after it is cooked, then it is really more like a solid than a liquid, and so the concept of viscosity no longer applies. Instead you will want to characterize the tensile strength, modulus of elasticity, etc. Viscosity measures how adjacent layers of flowing liquid resist relative longitudinal motion. It is a measurement of the shear friction, in essence. Dough can be considered a liquid since it will flow in layers. Granted, it is a very viscous liquid, and its viscosity varies sharply as a function of how wet it is, what other ingredients are present, how much it is stirred, etc. It is important to recognize a difference, however. In normal liquids like gasoline, water, etc, the viscosity is created by what are essentially Van der Walls (VDW) forces. That is, the molecules resist motion primarily because they experience weak bonds forming and breaking as they flow past. In dough, however, there are VDW forces as well as long polymers entangling with one another. This completely alters the theoretical framework for calculating viscosity since it involves knot theory, and is incredibly complex (since doughs have many types of polymers!) So how is it done these days? Empirically. The food industry is actually very interested in the viscosity of various doughs because they require this knowledge to design robotic food processing plants. Consider the automatic generation of bread loaves. The dough may be automatically squirted through a nozzle into a pan, and then robotically moved into an oven for an exact period of time, etc. Therefore, there are a number of statistics that the industry pays close attention to including the viscosity. They measure viscosity using a device called a farinograph which essentially stirs a rod through the dough and measures how much force is required to stir the rod. If you wanted, you could build one using an old fashion hand-held drill, and a spring to measure the moment you are applying to the handle of the drill. Or you could buy one from Brabender (http://www.brabender.com). The viscosity from a Brabender farinograph is measured in BU: Brabender Units! The following links may be helpful: An explanation of some of the stats involved in measuring dough, as well as the viscosity using a farinograph: http://www.cooknaturally.com/DetailLotInfo.html A detailed explanation of mixture viscosity prediction including Grunberg’s equations. It is applied to alcohols – which do not have polymer tangling: Canosa J, Rodriguez A, Tojo J., Predition of Dynamic Viscosity of Mixtures With Alcohols, Chemical Engineering Department, University of Vigo, 36200 Vigo, Spain. E-mail: email@example.com http://www.deq.uem.br/biblioteca/deq/Anais/CobeqXIII/pdf/877.pdf Md Zaidul I. Abd Karim A., Manan D., Ariffin A., Nik Norulaini N., Mohd Omar A. A farinograph study on the viscoelastic properties of sago/wheat flour dough systems. Journal of the Science of Food and Agriculture, Volume 84, Number 7, May 2004, pp. 616-622(7), Publisher: John Wiley & Sons, Ltd. The Faironograph Handbook 3rd edition (a book on the subject!) http://www.aaccnet.org/bookstoretitles/50376.asp Hope this helps! Zack
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