Determination of glass transition temperatures during cooling and heating of low-moisture amorphous sugar mixtures M.A. Ruiz-Cabrera a,⇑ , S.J. Schmidt b a Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava No. 6, Zona Universitaria, C.P. 78210 San Luis Potosí, S.L.P., Mexico b Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 367 Bevier Hall, 905 South Goodwin Avenue, Urbana, IL 61801, United States article info Article history: Received 28 July 2014 Received in revised form 23 August 2014 Accepted 26 August 2014 Available online 6 September 2014 Keywords: DSC Glass transition temperature Amorphous form Sugar mixtures Predicted Tg curve abstract Glass transition temperatures (Tg) of amorphous sugar samples, prepared from 0% to 14% (wb) moisture content, were determined during both cooling and heating using differential scanning calorimetry (DSC). The DSC protocol used involved heating, cooling, and reheating the sugar samples, where Tg was obtained during both cooling and reheating steps. The Gordon-Taylor equation modeled the water plasticization effect, where the Tg of anhydrous solids and constant (k) values were determined simultaneously. No significant difference was found between the cooling and heating glass transition temperature values, with both values significantly influenced by sugar composition (p < 0.05). No significant effect of sugar composition was observed for the k values, resulting in discrepancies displayed by the Gordon-Taylor model for solid mass fractions below 0.84. Thus, k values should be estimated from measured Tg data at low solid mass fractions rather than extrapolated from high solid mass fractions, for prediction of Tg curve at high moisture contents. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The glass transition is a state transition between the glassy and rubbery states (or reverse) of a substance as it experiences changes in temperature and/or moisture. The temperature at which this transition occurs is well known as the glass transition temperature or simply Tg (Rhaman, 2010; Roos, 2010; Sablani et al., 2010). In recent years, the study of the relationship between Tg and water mass fraction (x w ) or solid mass fraction (x s ) has received consider- able attention and is useful for construction of simplified stability/ mobility diagrams or supplemented state diagrams of several sugar-rich products, such as dried fruit and fruit juice powders (Rahman, 2006; Jaya and Das, 2009; Rhaman, 2010; Roos, 2010; Sablani et al., 2010; Buera et al., 2011). For instance, undesirable physical changes in foods occurring during storage and distribution, such as collapse, re-crystallization, stickiness, caking due to the glass transition phenomena can be predicted using the simplified water activity/temperature state diagrams (Tg vs a w ) or simplified water content/temperature state diagrams (Tg vs x w )(Thomsen et al., 2005; Moraga et al., 2006; Rahman, 2006; Kasapis, 2006; Jaya and Das, 2009). On the other hand, a supplemented state diagram is a map of the different states of a food as a function of temperature over the entire solid mass fraction scale. Supplemented state diagrams are very helpful for developing food formulations, processing strategies, or storage procedures to optimize the stability of foods, which contain freez- able or unfreezable water (Schmidt, 2004; Rhaman, 2010; Sablani et al., 2010). In this context, state diagrams concerning dried fruit or fruit juice powders, have been reported in the literature for selected fruits such as raspberry, blueberries, strawberries, kiwifruit, grape, tomato, mango, pineapple, apple, persimmon, camu–camu, acai, borojó (Sá and Sereno, 1994; Welti-Chanes et al., 1999; Khalloufi et al., 2000; Bai et al., 2001; Sobral et al., 2001; Telis and Sobral, 2001, 2002; Moraga et al., 2004, 2006; Silva et al., 2006; Goula et al., 2008; Wang et al., 2008; Syamaladevi et al., 2009; Jaya and Das, 2009; Tonon et al., 2009; Mosquera et al., 2011). Differential scanning calorimetry (DCS) is the most extensively used technique for Tg analysis of sugar solutions and sugar-rich foods. In general, samples containing freezable and unfreezable water are cooled from room temperature to a temperature well below zero (100 °C) and then heated to a selected temperature, dependent upon the composition and water content of the food. Scan rates of 10 °C/min during heating are commonly used for Tg determination (Schmidt, 2004; Rhaman, 2010; Sablani et al., 2010). Freeze-dried or spray-dried samples are reconstituted by adding appropriate amounts of water or equilibrated over http://dx.doi.org/10.1016/j.jfoodeng.2014.08.023 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +52(444) 8262300 ext 6548. E-mail addresses: mruiz@uaslp.mx (M.A. Ruiz-Cabrera), sjs@illinois.edu (S.J. Schmidt). Journal of Food Engineering 146 (2015) 36–43 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng