Relation between a Transition in Molecular Mobility and Collapse Phenomena in Glucose-Water Systems Ivon J. van den Dries, ² Nicolaas A. M. Besseling, ‡ Dagmar van Dusschoten, § Marcus A. Hemminga, § and Erik van der Linden* ,² Department of Food Technology and Nutritional Science, Food Science Group, Wageningen UniVersity, P.O. Box 8129, 6700 EV Wageningen, The Netherlands, Department of Physical and Colloid Chemistry, Wageningen UniVersity, P.O. Box 8038, 6700 EK Wageningen, The Netherlands, and Department of Molecular Physics, Wageningen UniVersity, P.O. Box 8128, 6700 ET Wageningen, The Netherlands ReceiVed: January 6, 2000; In Final Form: June 8, 2000 In concentrated glucose glasses with water contents between 10 and 30 wt %, an indication for a transition in mobility of the sugar protons above the glass transition temperature is observed, using proton magnetic resonance techniques. The first transition in mobility is positioned at the glass transition temperature while the second is about 20-30 °C higher. The temperature of this second transition is found to depend on water content, resulting in a new line in the state diagram of glucose-water mixtures. In freeze-concentrated glucose glasses we find two similar transitions. We interpret the second transition as the so-called crossover temperature, where the dynamics changes from solid-like to liquid-like. In freeze-concentrated glasses an increase in the amount of ice melting per degree is observed just above the temperature of the second transition. We propose that both in concentrated and freeze-concentrated glucose glasses, this second transition relates to so-called collapse phenomena in glasses. Introduction Sugars in foods exist as crystals, in an amorphous state or in solution. The amorphous state of sugars can be formed during cooling from a melt or by rapid removal of the solute from the sugar solution by means of dehydration or the formation of ice. In these processes oversaturated sugar solutions are produced and if the sugar does not crystallize, an amorphous sugar matrix or so-called sugar glass is formed. 1 Bringing the temperature of the sugar glass above its so-called glass transition temperature, the sugar matrix shows macroscopically visible changes in physical properties compared to below the glass transition temperature (T g ). These changes are generally attributed to a reduction in viscosity upon increasing temperature above the glass transition temperature, such that flow on a practical time scale occurs. It is found that the time span that is required for the visible change in physical properties to occur depends on the temperature. In fact, this time span decreases with increasing temperature, above T g . It is also found that this time span exhibits an abrupt decrease at a specific temperature. This temperature is usually referred to as the collapse temperature. 2,3 For sugar glasses, collapse temperatures and the concomitant collapse phenomena occur in general 15-25 degrees above T g . 1 Examples of collapse phenomena include (1) the stickiness and caking of powders; (2) plating of particles on amorphous granules; and (3) structural collapse of freeze-dried materials. 4 The onset of collapse phenomena is still under debate. For example, collapse phenomena in glasses that are formed due to removal of water from the sugar solution by means of ice formation (i.e., freeze-concentrated glasses) have been related to the so-called onset of ice melting transition which can be observed using differential scanning calorimetry (DSC). 5,6 This transition is observed in a DSC curve as a shoulder on the ice melting peak. It was suggested 6 that collapse phenomena in other types of glasses, such as glasses obtained by drying, have the same origin as this onset in ice melting transition in freeze- concentrated systems. In view of our interest in the onset of the above-described collapse phenomena we studied two types of glucose glasses from a molecular mobility point of view. One type of glass is the so-called freeze-concentrated glass, and the other type is obtained by cooling from the melt (i.e., concentrated glass). Specifically, we determined the molecular mobility of sugar and water using a proton nuclear magnetic resonance ( 1 H NMR) technique. 7,8 In the freeze-concentrated glucose glasses we also used this technique to determine the glucose concentration in the solution phase during ice melting. In both types of glasses we observe a transition in molecular mobility at a temperature situated above the glass transition temperature and we propose that this transition in molecular mobility is related to the origin of collapse in both types of glucose glasses. Materials and Methods Preparation of Samples. Sugar glasses were prepared in two ways. (1) Concentrated glucose-water glasses were prepared by mixing the appropriate amounts of sugar and water, melting the mixture, and quickly cooling it below the glass transition temperature. Concentrated glasses prepared this way were compared with concentrated glasses prepared by freeze-drying and because no differences were found we used only glasses * Author to whom correspondence should be addressed at the Department of Food Technology and Nutritional Science, Food Science Group, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands. Telephone: +31 317 485417 or +31 317 485515. Fax: +31 317 483669. E-mail: erik.vanderlinden@phys.fdsci.wau.nl. ² Department of Food Technology and Nutritional Science, Food Science Group. ‡ Department of Physical and Colloid Chemistry. § Department of Molecular Physics. 9260 J. Phys. Chem. B 2000, 104, 9260-9266 10.1021/jp000074x CCC: $19.00 © 2000 American Chemical Society Published on Web 09/07/2000