E: Food Engineering & Physical Properties JFS E: Food Engineering and Physical Properties Slow Molecular Mobility in the Amorphous Solid State of Fructose: Fragility and Aging NAT´ ALIA T. CORREIA,HERM´ INIO P. DIOGO, AND JOAQUIM J. MOURA RAMOS ABSTRACT: The molecular mobility of β -D-fructose was studied by thermally stimulated depolarization currents (TSDC) in the amorphous solid state. The amorphous solid samples were prepared in such a way that the tautomeric mixture was near the equilibrium composition. A broad secondary relaxation was observed, that merges, at high temperatures, with the alpha relaxation. The alpha relaxation temperature provided by the TSDC technique is T g = 13 ◦ C (at 4 K min −1 ). The fragility index calculated from TSDC data is m = 34, significantly lower when compared with the values reported in the literature obtained from Dielectric Relaxation Spectroscopy (DRS). The physical sig- nificance of the fragility obtained by the 2 dielectric techniques is discussed. The influence of physical aging on the secondary relaxation in amorphous fructose was analyzed as the glass structurally relaxes. A complex behavior was observed such that the faster components (lower temperature) of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes (higher temper- ature) show a significant dependence on aging and correspond to the genuine Johari–Goldstein β -relaxation. Keywords: aging, amorphous state, carbohydrate, fragility, glass transition, glassy state, secondary relaxations Introduction C arbohydrates are of universal importance playing a key role in the molecular logic of life (Lee and Lee 1995; Dwek and Butters 2002). The use of such compounds in biopreservation (Fox 1995; Crowe 2002), particularly in the cryopreservation of cells, has re- vived research on this important class of biomolecules. Their low toxicity (Kuleshova and others 1999) and stabilizer ability confer them a major role in food science (Levine 2002) and pharmaceutical industry (Werz and Seeberger 2005) by assur- ing processing, storage, and handling stability. Moreover, carbo- hydrates are recently considered as green raw materials for the chemical industry (Lichtenthaler and Peters 2004). Among the low molecular weight carbohydrates, fructose is being used as sweetener for replacing glucose since, contrary to glucose, it is found to be innocuous for diabetes patients (Oppen- heimer and others 2008). Furthermore, its effectiveness to reduce undesirable effects caused by freezing (for example, dehydration of cells and denaturation of globular proteins) is only exceeded by tre- halose and maltose (Green and Angell 1989). Both crystalline and amorphous states are important in the field of food and pharmaceutical technology. The amorphous form of drugs and excipients is of pharmaceutical interest since it has been reported to improve solubility, accelerate dissolution, and bioavail- ability and promote therapeutic activity (Yu 2001; Roberts and Debenedetti 2002). There is a general idea that, as long as the temperature remains below T g , some undesirable chemical reactions that lead to deteri- MS 20090290 Submitted 4/2/2009, Accepted 7/31/2009. Author Correia is with REQUIMTE, Depart. de Qu´ ımica, Faculdade de Ciˆ encias e Tecnolo- gia da Univ. Nova de Lisboa, 2829-516 Caparica, Portugal. Author Diogo is with CQE – Centro de Qu´ ımica Estrutural, Complexo I, IST, TULis- bon, 1049-001 Lisboa, Portugal. Author Moura Ramos is with CQFM – Centro de Qu´ ımica-F´ ısica Molecular and IN − Inst. of Nanoscience and Nanotechnology, Insti. Superior T´ ecnico, Univ. T´ ecnica de Lisboa, 1049- 001 Lisboa, Portugal. Direct inquiries to author Moura Ramos (E-mail: mouraramos@ist.utl.pt) oration may not proceed. However, evidence does exist that even below T g there are amorphous materials that still exhibit signifi- cant molecular mobility (Andronis and Zografi 1997). In the glassy state, the system is in a nonequilibrium state, with respect to both crystalline and “equilibrium” glassy state (Andronis and Zografi 1997). As a consequence, it will evolve at a rate that depends on the temperature and the thermal history of the glass; the evolu- tion that occurs below the glass transition temperature is referred to as structural relaxation or physical aging (Wungtanagorn and Schmidt 2001). Thus, to prevent the molecular processes respon- sible for destabilization of an amorphous substance over the nor- mal life-time of a pharmaceutical or food product, we need to know the time scales of molecular motion under a variety of conditions (Hancock and others 1995). It is also important to understand the factors that control crystallization, which is often desirable during processing or storage of food (Walstra 2003) or that can occur below T g , from the solid amorphous state (Andronis and Zografi 1997). Therefore, the purpose of this study was to characterize the time scales for molecular mobility of the monosaccharide fructose in the supercooled and glassy states. While most studies of sugar mobil- ity are carried out in aqueous solutions of these substances (Green and Angell 1989; Magazu and others 1998; Mehl 1998; Feeney and others 2001; Fuchs and Kaatze 2002; Shinyashiki and others 2008), studies of molecular mobility in the pure substances by relaxation techniques are scarce (Tombari and others 2001; Kaminski and oth- ers 2006). D-fructose is a monosaccharide with 6 carbon atoms that presents 5 tautomeric structures, 1 linear similar to that of sorbitol and 4 cyclic, that are shown in Figure 1. In the crystalline form, monosaccharides exist only in the pyranose form. The trans- formation of one sugar isomer to another is a solvent-mediated process and thus cannot occur once the monosaccharide is in the gas phase (Finch 1999). However, in the melt this transition is possible, and the equilibrium liquid is a mixture of different con- formers. Crystalline β -D-fructose is an hexose consisting entirely of β -D-fructopyranose, but in its molten state other conformers may form (Tombari and others 2001). Fructose in the liquid state E526 JOURNAL OF FOOD SCIENCE—Vol. 74, Nr. 9, 2009 C  2009 Institute of Food Technologists R  doi: 10.1111/j.1750-3841.2009.01363.x Further reproduction without permission is prohibited