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Rheological Evaluation of Gelatin-Xanthan Gum System With Food Hydrocolloids 28 (2012) 141e150 Contents lists available at SciVerse ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd Rheological evaluation of gelatinexanthan gum system with high levels of co-solutes in the rubber-to-glass transition region Filiz Altay a, Sundaram Gunasekaran b,* a Istanbul Technical University, Faculty of Chemical and Metallurgical, Department of Food Engineering, Maslak, Istanbul 34469, Turkey b University of WisconsineMadison, Department of Biological Systems Engineering, 460 Henry Mall, Madison, WI 53706, USA article info abstract Article history: Effects of moisture content, xanthan gum (XG) addition, and glucose syrup (GS):sucrose ratio on the Received 20 October 2011 gelation of gelatin-XG systems with high levels of co-solutes were investigated in the rubbery and the Accepted 8 December 2011 glass transition regions. Frequency sweep tests were performed between 0.1 and 100 rad and the storage (G0) and loss (G00) moduli of the system were measured in the temperature range of 60 to À15 C. The Keywords: onset of glass transition region increased with decreasing moisture content. The timeetemperature Gelatin superposition yielded master curves of G0 and G00 as a function of timescale of measurement. G00 and Xanthan gum 00 G were superimposed with the horizontal shift factor aT, which was temperature dependent according Tg e e WLF equation to the Williams Landel Ferry (WLF) equation. Glass transition temperature (Tg) of the samples were Free volume determined by dynamic mechanical analysis (DMA) from the peak of tan d. Tg decreased with XG addition. The energy of vitrification of samples with XG increased compared to samples containing only gelatin. Relaxation spectra of the samples were calculated from rheological measurements using the first and second approximations. The Rouse theory was more closely followed with the second approximation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction constructing master curves of mechanical spectra, spanning many decades of frequency. Isothermal data obtained by frequency Dynamic mechanical measurements are widely used to probe sweeps at different temperatures are shifted along the frequency structureeproperty relationships in amorphous synthetic polymers axis and overlaid to obtain a master curve at an arbitrarily chosen during vitrification. The synthetic polymer approach, in which the reference temperature. TTS can express the effects of time and idea of molecular mobility governing the kinetics of phase/state temperature on viscoelastic properties separately by enlarging the transitions and chemical reactions is applied, has been extended to effective time or frequency scale available for experimental food biopolymers (Kasapis, Al-Marhoob, & Deszczynski, Mitchell, & measurements. The superposition of curves from frequency sweeps Abeysekara, 2003; Kasapis, Al-Marhoobi, & Sworn, 2001; Levine & at constant temperature intervals yields the shift factor (aT), which Slade, 1988). This approach has been applied extensively together indicates how much the time scale of measurement shifts with with free volume theory to high-concentration mixtures of sugars temperature (Ferry, 1980). The underlying basis of TTS is the and biopolymers (Deszczynski, Kasapis, MacNaughton, & Mitchell, equivalence between time (or frequency) and temperature as they 2003; Kasapis, Al-Marhoobi, & Giannouli, 1999; Kasapis, Des- affect molecular processes that influence the viscoelastic behavior brieres, Al-Marhoobi, & Rinaudo, 2002; Kasapis et al., 2001; Kasapis of polymeric materials and glass-forming small molecules (Slade & & Sworn, 2000) and it has been reported that small addition of Levine, 1993). The criteria for the applicability of TTS are as follows polysaccharides to sugar-containing systems accelerate their vitri- (Ferry, Fitzgerald, Johnson, & Grandine, 1951): (a) shapes of adja- fication (Kasapis et al., 2001). cent curves should match exactly, (b) the same values of aT must The synthetic polymer approach includes the application of the superpose all the viscoelastic functions, and (c) the temperature principle of timeetemperature superposition (TTS), which is also dependence of aT must have a reasonable form consistent with known as the method of reduced variables. TTS has been used for experience. For the last criterion, Williams, Landel, and Ferry (1955) proposed an empirical relationship known as the Williams-Landel- Ferry (WLF) equation. * Corresponding author. Tel.: þ1 608 262 1019; fax: þ1 608 262 1228. The glass transition is relevant to the behavior of food materials E-mail address: [email protected] (S. Gunasekaran). for several reasons. For both polymers and low molecular weight 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.12.007 142 F. Altay, S. Gunasekaran / Food Hydrocolloids 28 (2012) 141e150 glasses, there is a large change in material properties while going 2. Materials and methods through the glass transition. Functional behavior of the material is affected by the dramatic slowing of diffusive processes as a material 2.1. Materials is cooled towards Tg. This slowing will affect stability to crystalli- zation and time-dependent processes associated with crystalliza- Pigskin gelatin (Type B) and laboratory grade sucrose were tion, drying/rehydration and spoilage, when the rate-limiting step purchased from EM Science and Fischer Chemicals, respectively. is the rate of diffusion encountered between substrates or enzyme Glucose syrup, with dextrose equivalent of 43.4 and total solids of and substrate (Noel, Ring, & Whittam, 1993). In addition, it has been 80.5%, was obtained from Cargill, IA, USA (lot number C007138). presumed that at temperatures below Tg amorphous sugars in foods The water content of GS was taken into account in calculating the are stable. Food products are subject to changes in moisture content sample composition. Food-grade XG (lot number 3D0724A) was and temperature during processing and storage, both of which obtained from CP Kelco U.S. Inc., Chicago, IL, USA. decrease stability of amorphous compounds in their rubbery state by increasing temperature difference (T À T )(Roos & Karel, 1993). g 2.2. Sample preparation A glass forms when a typical liquid, a state with a disordered molecular structure, is cooled to a temperature generally w100 C Several gelatin-XG systems were prepared. For each, the below its equilibrium crystalline melting temperature (T )or m required amount of gelatin and XG were dissolved separately in freezing point, at a cooling rate sufficiently high to avoid crystalli- deionized water to prepare 10% solution at 75 C and 600 rpm for zation of the liquid. This solidification process, known as vitrifica- 20 min and 4% solution at 60 C and 425 rpm for 2 h, respectively. tion, results in immobilization of the disordered structure of the The required amount of sucrose was mixed with 1/3 part of water in liquid such that the resulting glassy solid is spatially homogeneous, a temperature-controlled kettle. Then GS, gelatin, and XG solution but without any long-range lattice order, and is incapable of were added into the sucrose solution. The mixture was stirred at exhibiting any long-range, cooperative relaxation behavior (e.g., about 90 C for 30e60 min depending on the desired level of total translational mobility) on a practical time scale. The most impor- solids, which was checked by a refractometer (Atago N-3E, Japan). tant distinction between dimensionally extended (a) relaxations, Total solids content of the gels, which were cured overnight in which give rise to the glass transition as translational motions a refrigerator at 0 C, were determined using the AOAC method become restricted at T , and small-scale (b and g) relaxations, for g (AOAC, 1990); the moisture contents were calculated by subtracting which small-scale rotational motions do not become restricted as T total solids content from one hundred. The compositions of all falls below T , is the cooperative nature of a relaxations (Slade & g samples tested are presented in Table 1. Levine, 1993). The gelatin-XG systems were investigated at two moisture Fitting the master curves to WLF equation enables predicting contents (20 and 25%), three gelatin:XG ratios (5:0, 9:1, and 4:1) the T . It has been proposed that the rheological T is a point g g and three GS:sucrose ratios (<1, 1 and >1) at each moisture between the T region and the glassy state. The T can signify the g g content. For each sample, two batches were prepared and tested. transformation from free-volume phenomena of the polymeric backbone in the Tg region to an energetic barrier to motions in the glassy state involving stretching and bending of chemical bonds 2.3. Rheological measurements (Kasapis et al., 2001). Free volume can be defined as holes of the order of molecular (monomeric) dimensions or smaller voids 2.3.1. SAOS associated with packing irregularities. Many properties of liquids, The small amplitude oscillatory shear (SAOS) technique was whether polymeric or not, can be attributed to the presence of used to determine the dependence of viscoelastic behavior on a substantial proportion of free volume. The thermal expansion temperature and time. Freshly prepared samples were loaded onto coefficient of liquids represents the creation of additional free a controlled-stress dynamic rheometer (Bohlin CVOR, Malvern Inc., volume with rising temperature. At high temperatures, where local Southampton, MA) equipped
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