Dynamics of water in strawberry and red onion as studied by dielectric spectroscopy H. Jansson, C. Huldt, R. Bergman, and J. Swenson Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden (Received 24 September 2003; revised manuscript received 23 March 2004; published 7 January 2005) We have investigated the microscopic dynamics of strawberry and red onion by means of broadband dielec- tric spectroscopy. In contrast to most of the previous experiments on carbohydrate-rich biological materials, which have mainly considered the more global dynamics of the “biological matrix,” we are here focusing on the microscopic dynamics of mainly the associated water. The results for both strawberry and red onion show that the imaginary part of the permittivity contains one conductivity term and a clear dielectric loss peak, which was found to be similar to the strongest relaxation process of water in carbohydrate solutions. The temperature dependence of the relaxation process was analyzed for different water content. The relaxation process slows down, and its temperature dependence becomes more non-Arrhenius, with decreasing water content. The reason for this is most likely that, on average, the water molecules interact more strongly with carbohydrates and other biological materials at low water content, and the dynamical properties of this biological matrix changes substantially with increasing temperature (from an almost rigid matrix where the water is basically unable to perform long-range diffusion due to confinement effects, to a dynamic matrix with no static con- finement effects), which also changes (i.e., reduces) the activation energy of the relaxation process with increasing temperature (i.e., causes a non-Arrhenius temperature dependence). This further changes the con- ductivity from mainly polarization effects at low temperatures, due to hindered ionic motions, to long-range diffusivity at T  250 K. Thus, around this temperature ions in the carbohydrate solution no longer get stuck in confined cavities, since the motion of the biological matrix “opens up” the cavities and the ions are then able to perform long-range migration. DOI: 10.1103/PhysRevE.71.011901 PACS number(s): 83.80.Lz, 77.22.Gm I. INTRODUCTION Water is the foundation of life. It is the medium for bio- molecular movements and biological reactions. Thus, in or- der to understand biological processes, it is essential to elu- cidate how geometrical confinement and interactions with surfaces and other molecules affect the structure and dynam- ics of water. Also carbohydrates and their derivatives are widely distributed in living organisms, where they have both structural and metabolic roles [1]. The simplest carbohy- drates are small monomeric molecules, the monosaccharides; examples of such sugars are glucose and fructose. Monosac- charides can be linked to each other to form oligosaccha- rides, which consist of a few monosaccharide units, or polysaccharides, which are polymers of monosaccharides. When two monosaccharide molecules are linked to each other a disaccharide is formed. Sucrose, which is built up of one glucose molecule and one fructose molecule, is an ex- ample of a disaccharide. Glucose, fructose, and sucrose are the major carbohydrates in most fruits and vegetables, such as the strawberry and red onion here studied [2,3]. The presence of biological processes that are promoted by water is the reason why cooling and drying are important methods for food storage [4]. The behavior of frozen and/or dried biological materials differs from that of fresh materials and research in this behavior is crucial for the possibility to optimize the stability of the treated food. Several studies [5–8] have been performed on the dynamical behavior of freeze-dried fruits and vegetables. However, most of these studies concentrate on the whole material and very few spe- cifically on the dynamics of its interfacial carbohydrate solution. Experiments on carbohydrate-rich food, such as veg- etables and fruit, reveal a drastic increase in glass transition temperature T g with decreasing water content [5], suggesting a strong concentration dependence of the dynamics of their carbohydrate solutions. In this study, the microscopic relax- ational behavior of strawberry and red onion was studied by means of dielectric spectroscopy. Due to the high dielectric constant of water we mainly probe how the water dynamics is affected by the concentration of carbohydrates in the solu- tion [9]. We observe one clear dielectric loss peak, which therefore is interpreted as a relaxation process of the super- cooled water in the carbohydrate solution. The relaxation time  of this process has a temperature dependence which follows the Vogel-Fulcher-Tammann (VFT)[10–12] behav- ior,  =  0 exp  DT 0 T - T 0  . 1 D is a material-dependent constant describing the degree of non-Arrhenius behavior, T 0 is the temperature where  goes to infinity, and  0 is the relaxation time extrapolated to infi- nite temperature, which usually corresponds to quasilattice and molecular vibrations of the order of 10 -14 s. Here one should note that this kind of temperature dependence is gen- erally observed also for the viscosity of supercooled liquids. When  of a main - relaxation process has reached a value of 100 s, the supercooled liquid behaves like a solid and is called a glass. So called strong glass formers, in An- gell’s strong-fragile classification scheme [13,14], have an -relaxation time with a nearly Arrhenius temperature de- PHYSICAL REVIEW E 71, 011901 (2005) 1539-3755/2005/71(1)/011901(7)/$23.00 ©2005 The American Physical Society 011901-1