Gelling properties of gelatin–xanthan gum systems with high levels of co-solutes 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 Wisconsin-Madison, Department of Biological Systems Engineering, 460 Henry Mall, Madison, WI 53706, USA article info Article history: Received 19 December 2012 Received in revised form 20 April 2013 Accepted 22 April 2013 Available online 1 May 2013 Keywords: Gelatin Xanthan gum SAOS Uniaxial compression Fracture stress Fracture strain Texture map Glass transition temperature abstract The effects of moisture content, xanthan gum (XG) addition and glucose syrup (GS):sucrose ratio on elas- tic (G 0 ) and viscous (G 00 ) moduli during in situ gelation and on large deformation rheological properties of cured gels were investigated. An increase in both moduli of the samples with XG addition indicates net- work structure being strengthened. All gel samples exhibited distinct fracture. An increase in GS:sucrose ratio led to a decrease in fracture stress and an increase in fracture strain, implying more flexible polymer network. Decreasing moisture content may lead to phase separation between sugar-rich and polymer- rich phases to form stronger connection within the network structure. Textural characteristics of samples analyzed using a texture map, indicated that increasing GS:sucrose ratio rendered the sample texture more rubbery when the samples contained XG. We also related factors affecting the gelling mechanisms in terms of T g measured by different techniques including DMA and modulated DSC. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Gelatin-polysaccharide mixtures have been widely investigated in terms of gel formation and gel structure, texture and stability for many food and pharmaceutical applications (Fonkwe et al., 2003; Kasapis and Al-Marhoobi, 2005; Kasapis, 2008; Sharma et al., 2011). In gummy candies, gelatin is the ideal ingredient to meet requirements for the process and texture due to forming rubbery networks with brilliant clarity (Kasapis and Al-Marhoobi, 2000). There are studies for gelatin to modify the characteristics of gela- tin-based products, such as increasing melting point to avoid fast surface melting, reducing elasticity in some cases and broadening choices for consumers (Tilly et al., 2002). Xanthan gum is a micro- bial heteropolysaccharide produced by fermentation with a con- siderable practical value due to its high shear thinning behavior despite the high viscosity at rest. The behavior of high shear thin- ning means flow easily whereas the high viscosity of xanthan gum solutions at low shear rates accounts for their ability to provide long-term stability to colloidal systems (Sworn, 2000). In confec- tionary products, glucose syrup and sucrose are used as co-solutes. The glucose syrup with a low dextrose equivalent (DE) contains more long-chain polysaccharides, which increases glass transition temperature (T g ) of a mixture with sucrose at any level, whereas a high DE glucose syrup may decrease T g of a mixture with sucrose. At high levels of glucose syrup, sucrose crystallization is effectively prevented (Hartel, 2001). Addition of sugar to a gel system gener- ally results in higher apparent gel strength and higher setting/ melting temperatures up to levels of about 60%, due to hydrogen bonding between water and hydrocolloid which leads to an in- crease in total modulus. The brittleness of gels is reduced when the sugar level increases (de Vries, 2004). An understanding of structure–function relations of individual components in protein–polysaccharide mixed systems have been of particular interest for creating use of functional ingredients in foods (Sharma et al., 2011). The structure–function relations of polymers have been examined both based on their chemical and architectural aspects (Cowie, 1991). The tangible factors for the chemical level information and the architectural aspects are melt- ing temperature (T m ), modulus, and (T g ), which are commonly used to characterize the polymer. The central focus of a polymer science approach to the studies of structure–function relationships in food systems is the insights obtained by the fundamental similarities between synthetic amorphous polymers and glass-forming food materials with regard to their thermal and thermomechanical properties (Levine and Slade, 1990). The some applications of this approach to biomaterials were summarized by Kasapis (2012). Small amplitude oscillatory shear (SAOS) measurements are commonly used to study linear viscoelasticity of foods 0260-8774/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2013.04.018 ⇑ Corresponding author. Tel.: +1 608 262 1019; fax: +1 608 262 1228. E-mail address: guna@wisc.edu (S. Gunasekaran). Journal of Food Engineering 118 (2013) 289–295 Contents lists available at SciVerse ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng