Introduction High pressure treatment is a novel technology in food processing with various advantages. Cheftel (1992) gives an overview. At present, potential applications in science and engineering are discussed in various re- spects. For a better understanding of the processes occurring under pressure and the effects of pressure on cellular and molecular structures there is a need for data on thermophysical properties of food ingredients under pressure. So far, there is a lack of knowledge, because suitable in-situ measurement techniques are missing. An important thermophysical property is the viscosity, because it provides or supplements informa- tion on pressure-induced structural changes, on fluid dynamic and diffusion processes. There are some data in the field of petrochemistry – see, e.g. Kuss and Golly (1972); for lubricating oils see, e.g. Galvin et al. (1973) or Larsson and Jonsson (1997) and for poly- mers see Mackley and Spitteler (1996) or Briscoe et al. (1999). However there is a lack of data concerning the pressure dependence of the viscosity of fluid food in- gredients and aqueous solutions. Only few data are available, i.e. sodium chloride solutions (Horne and Johnson 1967), urea solutions (Sawamura et al. 1997) and water-ethanol mixtures (Matsuo 1994; Harlow 1967). No data have been found in the literature by the authors for sugar solutions. The knowledge of these viscositiesisimportantastheyarethebasisforrealfood systems, e.g. milk or fruit juices. In addition to the lack of data, there is no theory to describe the pressure dependence of the viscosity of aqueous solutions. So far, only some theories exist for the pressure dependence of the viscosity of pure liquids, which molecules are spherical and have simple inter- molecular interactions; see, e.g. Frisch et al. (1940) and VanWijkandSeeder(1937).However,thesetheoriesare not able to predict the viscosity of unknown liquids in compressed state. Therefore the present work provides new measure- ment data on the pressure dependence of the viscosity ORIGINAL CONTRIBUTION Rheol Acta (2002) 41: 369–374 DOI 10.1007/s00397-002-0238-y Petra Fo¨rst Franz Werner Antonio Delgado On the pressure dependence of the viscosity of aqueous sugar solutions Received: 13 August 2001 Accepted: 5 February 2002 Published online: 16 April 2002 Ó Springer-Verlag 2002 P. Fo¨ rst (&) ® F. Werner ® A. Delgado Lehrstuhl fu¨r Fluidmechanik und Prozessautomation, Weihenstephaner Steig 23, D-85354 Freising E-mail: petra.foerst@wzw.tum.de Present address: P. Fo¨ rst Lehrstuhl fu¨r Lebensmittelverfahrenstech- nik und Molkereitechnologie, Weihenstephaner Berg 1, 85354 Freising Abstract New experimental data for the pressure dependence of the vis- cosity of aqueous solutions of dif- ferent sugars are presented. Measurements were carried out with a gravity driven high pressure visc- ometer with a maximum pressure of 700 MPa. The influence of both concentration and temperature on the pressure dependence of the vis- cosity is considered. Next to the data, a viscosity model based on a suspension model is introduced and it is shown that it is able to predict the viscosity for sugar solutions in a broad parameter range, including pressure. It is demonstrated that the relative viscosity for varying sugar mass fraction at constant pressure coincides with the pressure-depend- entrelativeviscosityforafixedsugar mass fraction, suggesting that there are no structural changes occurring under pressure. A brief interpretation of the viscosity model is given. Keywords High pressure viscosity ® Sucrose and glucose solutions ® Viscosity model