Gelling Properties of Gelatinв€“Xanthan Gum Systems with High

Journal of Food Engineering 118 (2013) 289–295

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Journal of Food Engineering

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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 abstract

Article history: The effects of moisture content, xanthan gum (XG) addition and glucose syrup (GS):sucrose ratio on elas- Received 19 December 2012 tic (G0) and viscous (G00) moduli during in situ gelation and on large deformation rheological properties of Received in revised form 20 April 2013 cured gels were investigated. An increase in both moduli of the samples with XG addition indicates net- Accepted 22 April 2013 work structure being strengthened. All gel samples exhibited distinct fracture. An increase in GS:sucrose Available online 1 May 2013 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- Keywords: rich phases to form stronger connection within the network structure. Textural characteristics of samples Gelatin analyzed using a texture map, indicated that increasing GS:sucrose ratio rendered the sample texture Xanthan gum SAOS more rubbery when the samples contained XG. We also related factors affecting the gelling mechanisms Uniaxial compression in terms of Tg measured by different techniques including DMA and modulated DSC. Fracture stress Ó 2013 Elsevier Ltd. All rights reserved. Fracture strain Texture map Glass transition temperature

1. Introduction temperature (Tg) of a mixture with sucrose at any level, whereas a high DE glucose syrup may decrease Tg of a mixture with sucrose. Gelatin-polysaccharide mixtures have been widely investigated At high levels of glucose syrup, sucrose crystallization is effectively in terms of gel formation and gel structure, texture and stability for prevented (Hartel, 2001). Addition of sugar to a gel system gener- many food and pharmaceutical applications (Fonkwe et al., 2003; ally results in higher apparent gel strength and higher setting/ Kasapis and Al-Marhoobi, 2005; Kasapis, 2008; Sharma et al., melting temperatures up to levels of about 60%, due to hydrogen 2011). In gummy candies, gelatin is the ideal ingredient to meet bonding between water and hydrocolloid which leads to an in- requirements for the process and texture due to forming rubbery crease in total modulus. The brittleness of gels is reduced when networks with brilliant clarity (Kasapis and Al-Marhoobi, 2000). the sugar level increases (de Vries, 2004). There are studies for gelatin to modify the characteristics of gela- An understanding of structure–function relations of individual tin-based products, such as increasing melting point to avoid fast components in protein–polysaccharide mixed systems have been surface melting, reducing elasticity in some cases and broadening of particular interest for creating use of functional ingredients in choices for consumers (Tilly et al., 2002). Xanthan gum is a micro- foods (Sharma et al., 2011). The structure–function relations of bial heteropolysaccharide produced by fermentation with a con- polymers have been examined both based on their chemical and siderable practical value due to its high shear thinning behavior architectural aspects (Cowie, 1991). The tangible factors for the despite the high viscosity at rest. The behavior of high shear thin- chemical level information and the architectural aspects are melt- ning means ﬂow easily whereas the high viscosity of xanthan gum ing temperature (Tm), modulus, and (Tg), which are commonly used solutions at low shear rates accounts for their ability to provide to characterize the polymer. The central focus of a polymer science long-term stability to colloidal systems (Sworn, 2000). In confec- approach to the studies of structure–function relationships in food tionary products, glucose syrup and sucrose are used as co-solutes. systems is the insights obtained by the fundamental similarities The glucose syrup with a low dextrose equivalent (DE) contains between synthetic amorphous polymers and glass-forming food more long-chain polysaccharides, which increases glass transition 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). ⇑ Corresponding author. Tel.: +1 608 262 1019; fax: +1 608 262 1228. Small amplitude oscillatory shear (SAOS) measurements E-mail address: [email protected] (S. Gunasekaran). are commonly used to study linear viscoelasticity of foods

(Gunasekaran and Ak, 2003). The elastic (G0) and viscous (G00) 2. Materials and methods moduli can be obtained from SAOS measurements. For an amorphous polymer, Tm can be determined from moduli vs. temperature 2.1. Materials plot (Slade and Levine, 1996). For biopolymer gels, the point G0 00 takes over G is called Tm in a cooling curve (Kasapis et al., 2002). Pigskin gelatin (Type B) and laboratory-grade sucrose were pur- Strength is usually expressed in stress units, and its magnitude chased from EM Science (USA) and Fischer Chemicals (USA), is determined by the stress at the point of failure. In order to eval- respectively. GS was provided by Cargill, IA, USA (Lot number uate the response of materials to applied forces, it is better to use C007138). The dextrose equivalent (DE) of glucose syrup was stress–strain relations rather than force–deformation relations. In 43.4, the total solid content was 80.53%. The water content of the food rheology, the Hencky strain (or true strain, natural strain) GS was considered in calculating the composition of samples. (eH) is most commonly used when large deformations involved. The food grade XG (Lot number 3D0724A) was provided by CP Kel- Calzada and Peleg (1978) recommended using eH when the com- co U.S. Inc., Chicago, IL (USA). pression is 20% or greater. Hencky strain is a better measure of strain than engineering strain since it is additive and deformations are referenced to the current specimen height rather than to the 2.2. Sample preparation initial specimen height (Gunasekaran and Ak, 2003). Although food texture is a sensory property, and therefore must be related to a Several gelatin-XG systems were prepared. For each, the human response, it is often measured in terms of mechanical prop- required amount of gelatin and XG were dissolved separately in erties that are used in engineering. It appears that a major failure in deionized water to prepare 10% solution at 75 °C and 600 rpm understanding food texture has been due to the lack of successful for 20 min and 4% solution at 60 °C and 425 rpm for 2 h, respec- combination of these two approaches (Stanley, 1987). Stress and tively. The required amount of sucrose was mixed with 1/3 part strain conditions at breaking point are important due to their rela- of water in a temperature-controlled kettle by hand. Then GS, tion to sensory texture (Hamann, 1983). A texture map is a plot of gelatin, and XG solution were added into the sucrose solution. two measured mechanical properties for example, fracture stress The mixture was stirred by hand at about 90 °C for 30–60 min vs. fracture strain of a product manufactured or tested at varying depending on the desired level of total solids, which was checked conditions (composition, pH, age, etc.) (Gunasekaran and Ak, by a refractometer (Atago N-3E, Japan). Total solids content of the 2003). gels, which were cured overnight in a refrigerator at 0 °C, were Dynamic mechanical analysis (DMA) can be simply described as determined using the AOAC method (AOAC, 1990); the moisture applying an oscillating force to a sample and analyzing the mate- contents were calculated by subtracting total solids content from rial’s response to that force (Menard, 1999). For a viscoelastic 100. The compositions of all samples tested are presented in material, Young’s modulus can be expressed in terms of elastic Table 1. (E0) and viscous (E00) components. The ratio of E00/E0 is loss tangent, The gelatin-XG systems were investigated at two moisture con- often denoted as tan d (Kalichevsky et al., 1993). Tg from DMA is tents (20% and 25%), three gelatin:XG ratios (5:0, 9:1, and 4:1) and obtained at the maximum value of tan d (Kasapis and Al-Marhoobi, three GS:sucrose ratios (<1, 1 and >1) at each moisture content. For

2000). Tg can also be determined from the reversing heat ﬂow sig- each sample, two batches were prepared and tested. nal in the modulated temperature differential scanning calorime- Freshly prepared solutions were poured into chlorinated polyvi- try (MTDSC) (Royall et al., 1998). nyl chloride tube (18-mm inner diameter and 68-mm long) molds. The objectives of this study were to: (1) analyze the effects of The inside surface of molds was coated with vegetable oil to pre- moisture content, xanthan gum (XG) addition, and glucose syrup vent the gel from sticking. The ends of the molds were closed with (GS):sucrose ratio on elastic and viscous moduli of gelatin gels rubber stoppers. The tubes were placed vertically in a refrigerator with high levels of co-solutes during in situ gelation and on large at 0 °C for overnight. Prior to measurement, the gels were carefully deformation rheological properties of cured gels and (2) relate gel- removed from the molds and cut into cylindrical disk specimens. ling mechanisms and affecting factors in terms of Tg measured by Average aspect ratio (height/diameter) of the specimens was different techniques including DMA and MTDSC. 0.60 ± 0.01 (Ould Eleya and Gunasekaran, 2002).

Table 1 Composition of samples tested.

Sample number Moisture content (%) Gelatin (%) XGa (%) GSb (%) Sucrose (%) Gelatin:XG ratio GS:sucrose ratio 1 25 5 – 40 30 5:0 1.33:1 2 4.5 0.5 40 30 9:1 1.33:1 3 4 1 40 30 4:1 1.33:1 4 5 – 35 35 5:0 1:1 5 4.5 0.5 35 35 9:1 1:1 6 4 1 35 35 4:1 1:1 7 5 – 30 40 5:0 0.75:1 8 4.5 0.5 30 40 9:1 0.75:1 9 4 1 30 40 4:1 0.75:1 10 20 5 – 45 30 5:0 1.5:1 11 4.5 0.5 45 30 9:1 1.5:1 12 4 1 45 30 4:1 1.5:1 13 5 – 37.5 37.5 5:0 1:1 14 4.5 0.5 37.5 37.5 9:1 1:1 15 4 1 37.5 37.5 4:1 1:1 16 5 – 35 40 5:0 0.88:1 17 4.5 0.5 35 40 9:1 0.88:1 18 4 1 35 40 4:1 0.88:1

a XG: xanthan gum. b GS: glucose syrup. F. Altay, S. Gunasekaran / Journal of Food Engineering 118 (2013) 289–295 291

2.3. SAOS measurements measurements. The mean and standard deviation were determined using Excel (Microsoft) and factorial ANOVA (analysis of variance) The small amplitude oscillatory shear (SAOS) technique was was used for the signiﬁcance of the differences at p = 0.0001, using used to monitor in situ gelation of samples. Freshly prepared sam- a commercial statistical software (SAS 9.1 Windows Version ples were loaded onto a controlled-stress dynamic rheometer 5.1.2600, SAS Institute Inc., Cary, NC, USA). (Bohlin CVOR, Malvern Inc., Southampton, MA) equipped with a

40-mm parallel-plate geometry (1 mm gap). Measurements were 2.5. Tg measurements performed as described by Altay and Gunasekaran (2012) at a fre- quency of 1 rad/s and 1% strain. Samples were loaded at 60 °C and Tg measurements of samples were determined using a DMA and cooled down to 15 °C at a scan rate of 1 °C/min. G0 and G00 moduli a MTDSC following procedures we have previously published. The were determined at 25 °C. Mineral oil was used to cover the ex- aged gel samples were loaded to the DMA at 0 °C and cooled to posed edges between the parallel plates to minimize moisture loss. 60 °C at a scan rate of 1 °C/min. Tg was obtained at the maximum For each batch one measurement was made, and two batches were value of tan d (the ratio of viscous modulus to elastic modulus). The tested for each sample. calorimetric Tg was determined from the midpoint temperature on the reversing heat ﬂow during the second heating cycle from 2.4. Uniaxial compression testing 70 °Cto40°Cat1°C/min (Altay and Gunasekaran, 2013).

Mechanical properties (stress–strain relationships) of the gels 3. Results and discussion were determined at room temperature by uniaxial compression from 0% to 80% relative deformation at a compression rate of 3.1. SAOS measurements 1 mm/s in a Synergie 200 (MTS Systems Corp., Eden Prairie, MN, USA) universal testing machine (Li et al., 2006) equipped with Moduli (G0 and G00) values listed in Table 2 shows that for all 1-kN load cell (parallel-plate conﬁguration, diameter of the upper samples G0 > G00 indicating that they all had an elastic character at plate was 14 cm). For compression test, the true stress (rt) (Pa) 25 °C(Table 2). In the cooling curves of samples with gelatin, G00 can be deﬁned as (Ould Eleya and Gunasekaran, 2002): dominates G0 in the viscous region (Altay and Gunasekaran, 0 00 1 ðDh=h Þ 2012). As temperature decreases, G begins to surpass G at a cross- r ¼ F o ð1Þ T pr2 over point called melting point (Tm), and the system enters the rubbery region. The Tm values of samples with gelatin are higher for where r is the radius of the specimen (m), F is the applied force to samples containing 25% moisture (34.5–35 °C) than 20% moisture the specimen (N), ho and h are the specimen heights (m) before content (30–32 °C). Gelatin gels having more stabilized junction and after deformation, respectively, and Dh (h–ho) is the deforma- points by interchain hydrogen bonds probably melt at higher tem- tion (m). Similarly, true strain (or Hencky strain) (eH) (–) can be cal- peratures (Altay, 2006) due to fact that the pyrolidine-rich regions culated for uniaxial compression in terms of deformation rate (or of gelatin chains acts as a nucleation sites for the formation of crosshead speed), Vz (m/s) as below (Gunasekaran and Ak, 2003): potential junction zones stabilized by interchain hydrogen bonds which are temperature sensitive and responsible for the gel melt- hðtÞ ho Dh ho V zt eH ¼ ln ¼ ln ¼ ln ð2Þ ing by breaking (Ledward, 2000). Kasapis and Al-Marhoobi (2005) h h h o o o reported that the onset of network formation is a function of poly- where t is time (s). Since ho P h in uniaxial compression, eH, will mer concentration and it was below 30 °C for similar gelatin con- 0 00 have a negative value. Most often eH is multiplied with 1 to make taining systems. G and G of samples with XG, though converge the results positive for convenience (Gunasekaran and Ak, 2003). at about 40 °C, did not intersect over the entire temperature range

Five specimens for each sample were tested in uniaxial compression studied making it difﬁcult to identify Tm for those samples.

Table 2 Total solids content (TSC), moduli (G0,G00), fracture stress and fracture strain values of samplesa.

Sample numberb TSC (%) G0 (kPa)c* G00 (kPa)c* Fracture stress (kPa)d Fracture strain (–)d 1 74.68 ± 0.77 0.80 ± 0.00b 0.10 ± 0.01a 75.48 ± 2.08f 0.84 ± 0.03a 2 74.03 ± 0.27 0.82 ± 0.20b 0.35 ± 0.03b 59.43 ± 3.04d 1.21 ± 0.05bc 3 74.93 ± 0.58 0.83 ± 0.15b 0.45 ± 0.14c 29.79 ± 2.10b 1.87 ± 0.07e 4 75.41 ± 1.37 0.68 ± 0.06a 0.15 ± 0.04a 65.25 ± 4.50e 1.15 ± 0.06b 5 75.67 ± 1.40 0.92 ± 0.07b 0.39 ± 0.04b 22.76 ± 1.81a 1.71 ± 0.05d 6 75.71 ± 0.92 1.04 ± 0.27b 0.73 ± 0.02c 59.70 ± 3.47d 1.17 ± 0.08bc 7 75.93 ± 0.70 0.76 ± 0.09a 0.10 ± 0.00a 51.03 ± 2.19c 1.27 ± 0.05c 8 76.29 ± 0.19 0.86 ± 0.05b 0.39 ± 0.02b 58.06 ± 1.89d 1.20 ± 0.04bc 9 75.30 ± 0.54 1.06 ± 0.01b 0.65 ± 0.08c 88.74 ± 4.95g 0.78 ± 0.04a 10 81.47 ± 0.12 0.48 ± 0.02c 0.41 ± 0.04de 51.33 ± 1.35l 1.24 ± 0.03g 11 79.16 ± 1.08 1.41 ± 0.55fg 1.21 ± 0.40h 33.69 ± 2.52j 1.79 ± 0.09j 12 80.03 ± 0.03 1.18 ± 0.11def 0.95 ± 0.01g 27.33 ± 2.16i 1.80 ± 0.10j 13 82.18 ± 0.68 0.56 ± 0.01c 0.46 ± 0.03e 23.31 ± 1.37h 1.77 ± 0.18j 14 81.14 ± 0.12 1.09 ± 0.10de 0.83 ± 0.13fg 37.76 ± 1.57k 1.51 ± 0.11i 15 80.90 ± 0.01 0.96 ± 0.07d 0.70 ± 0.05f 63.59 ± 3.12m 1.14 ± 0.03g 16 81.54 ± 0.10 0.46 ± 0.01c 0.27 ± 0.01d 47.81 ± 2.34l 1.37 ± 0.04h 17 81.82 ± 0.44 1.25 ± 0.17efg 0.99 ± 0.22g 87.10 ± 1.56m 0.94 ± 0.02f 18 81.47 ± 1.01 1.44 ± 0.09g 1.21 ± 0.01h 62.14 ± 1.30m 1.18 ± 0.08g

a Values reported are mean ± standard deviation. b See Table 1 for compositions of different samples. c Values at 25 °C from cooling curves. d Means ± SD (n = 3); values within samples 1–9 and 10–18 followed by the same letter in column are not signiﬁcantly different (p < 0.01). * Means ± SD (n = 2); values within samples 1–9 and 10–18 followed by the same letter in column are not signiﬁcantly different (p < 0.01). 292 F. Altay, S. Gunasekaran / Journal of Food Engineering 118 (2013) 289–295

Furthermore, since G0 > G00 at 60 °C, the gelatin-XG system had an 100 elastic character even at that high temperature (data not shown) Fracture point (Altay, 2006). 75 0 00 The most important factors affecting G and G at 25 °C were Yield point gelatin:XG ratio and moisture content together with gelatin:XG ra- 50 tio (F < 0.01). For G00 only moisture content was also an affecting parameter. At both moisture content G0 and G00 significantly increased with XG. According to the Table 2, G0 decreased whereas 25 True Stress (kPa) G00 increased as moisture content decreased from 25% to 20% for samples without XG. Gelatin with nucleation sites for the inter- 0 0 0.5 1 1.5 2 2.5 chain hydrogen bonds (Ledward, 2000) may result that the system had more interchain hydrogen bonds in the abundance of water Hencky strain leading higher G0. The system had more viscous characteristics at Fig. 1. True stress-Hencky strain plots of 5% gelatin + 40% glucose syrup + 30% lower moisture content, meaning more flexible networks. For the sucrose (sample 1) (s), 4.5% gelatin + 0.5% xanthan gum + 40% glucose syrup + 30% samples containing 0.5% XG, both moduli increased as moisture sucrose (sample 2) (4) and 4% gelatin + 1% xanthan gum + 40% glucose syrup + 30% content decreased from 25% to 20%. This was also true for samples sucrose (sample 3) ( ). containing 1% XG with GS:sucrose ratio >1 and <1. When GS:sucrose ratio was 1, both moduli of samples containing 1% XG decreased with moisture content. 11, 14 and 17) as GS:sucrose ratio decreased. Fracture stress of At 25% moisture content, G0 and G00 increased with XG, indicat- samples containing 1% XG at 25% moisture content (samples 3, 6 ing stronger network formation. The same results obtained for and 9) increased with decreasing GS:sucrose ratio. Fracture stress samples with GS:sucrose ratio = 1 at 20% moisture content. For of samples at 20% moisture content (samples 12, 15 and 18) first other samples at 20% moisture content both moduli first increased increased then kept relatively constant as decreasing GS:sucrose and then decreased with XG content. ratio. The patterns for fracture strain were opposite of fracture The effect of GS:sucrose ratio on G0 and G00 of samples containing strain for all samples. Increasing GS:sucrose ratio in the presence only gelatin and samples containing XG were different and depen- of XG provides more heterogeneity to network structure due to fact dent on both moisture content and gelatin:XG ratio. G0 and G00 of that GS contains a range of saccharide polymers. The numbers of samples containing only gelatin at 20% moisture content (samples flexible crosslinks in these polymers probably increase as the 10, 13 and 16) first increased then decreased as GS:sucrose ratio structure becomes less ordered, more heterogenic. This heteroge- decreased. Samples containing XG at 25% moisture had similar pat- neity may reduce the fracture stress of the network for the samples terns. At 20% moisture content, G0 and G00 of samples with XG first containing 1% XG. These results may be attributed to phase separa- decreased and then increased as GS:sucrose ratio decreased. In tion between sugar-rich and polymer-rich phases with fewer inter- other words, at limited water content, increasing XG increased connections in the structure (Li et al., 2006) due to the reduced both moduli as GS:sucrose ratio decreased. When more water availability of water molecules. Sworn and Kasapis (1998) reported was available, both moduli increased as GS:sucrose ratio decreased similar results for gellan gels with high levels of co-solutes. Gellan in the presence of XG. It may be attributed that XG leads to stron- gels at 60% co-solute were less hard (lower yield stress) and less ger network formation with less co-solutes. The increasing amount brittle (higher yield strain) than gels at 30% co-solute. The reduc- of co-solutes may promote phase separation between polymers tion in modulus at 60% co-solute with the increase in the yield and co-solutes, which was supported by uniaxial compression re- strain is indicative of a more flexible (entropic), less aggregated sults presented in the next section. polymer network. Accordingly, the increase in fracture stress and the decrease in fracture strain with moisture content may result 3.2. Uniaxial compression tests in more-flexible (entropic), less-aggregated polymer network. Based on the values of fracture stress and fracture strain in Stress–strain relationships of samples 1, 2 and 3 determined by Table 2, texture maps were plotted for the gel samples (Fig. 2). uniaxial compression are depicted in Fig. 1. Stress–strain relation- The texture map can be divided into different regions to represent ships of all other samples were similar. All gels fractured or col- various material textures. Dividing a fracture stress vs. fracture lapsed before 80% deformation was reached. strain plot of surimi into four quadrants, Hamann and MacDonald From Fig. 1, it can be seen that samples had yield points before (1992) classified the products in Quadrant 1 (on lower left), fracture. At the yield point, the specimen is permanently deformed Quadrant 2 (lower right), Quadrant 3 (top right), and Quadrant 4 even if the load is reduced to zero (Gunasekaran and Ak, 2003). The (top left) as ‘‘mushy,’’ ‘‘rubbery,’’ ‘‘tough,’’ and ‘‘brittle’’, respectively. stress showed concave upward shape with strain until the yield At 20% moisture samples with only gelatin exhibited relatively point, which is typical of a compressive material (Peleg, 1987). rubbery texture compared to samples which showed tough-brittle After the yield point, the stress approximately linearly increased texture at 25% moisture (Fig. 2a). At 25% moisture, the texture of until a peak value, after which it decreased rapidly corresponding gels become more brittle with GS:sucrose ratio. As GS:sucrose ratio to the point where the gel was seen to fracture. The values of frac- increased, the gels become more rubbery from brittle texture at ture stress and fracture strain are listed in Table 2. both XG contents, probably due to increasing flexible crosslinking According to the statistical analysis, the most important factors of co-solutes (Fig. 2b). The similar trend was observed for samples affecting fracture stress and fracture strain were GS:sucrose ratio with 1% XG with 25% moisture content (Fig. 2c). The texture of the and moisture (F < 0.0001) (Altay, 2006). Fracture stress of samples gels become rubbery with GS:sucrose ratio. containing only gelatin at 25% moisture content (samples 1, 4 and

7) decreased with decreasing GS:sucrose ratio. At 20% moisture 3.3. Tg measurements content fracture stress of samples containing only gelatin (samples

10, 13 and 16) ﬁrst decreased then increased with decreasing Our published data on Tg of samples obtained from both DMA GS:sucrose ratio. Fracture stress of samples containing 0.5% XG at and modulated DSC measurements indicate that the XG addition

25% moisture content (samples 2, 5 and 8) first decreased and then decreases mechanical Tg significantly (F < 0.01) (Altay and increased, while it increased at 20% moisture content (for samples Gunasekaran, 2013). The XG addition accelerated the vitrification F. Altay, S. Gunasekaran / Journal of Food Engineering 118 (2013) 289–295 293

0 00 100 Table 3. In situ gelation of samples were characterized by G , G and a Brittle 20% mositure Tough Tm determined by rheological measurements. Fracture stress, frac- 25% moisture 75 1.33:1 ture strain, results from texture maps, mechanical and calorimetric 1:1 Tg were taken into account for aged samples. The effects of mois- 0.75:1 ture content were given for samples with only gelatin due to the 50 1.5:1V 0.88:1 fact that the effects can be seen more clearly. As a general trend, moisture content increased G0, T , fracture stress and mechanical 1:1 m 25 00 Tg, whereas G and fracture strain decreased for samples containing only gelatin (Table 3). It is interesting to note that both moduli re- Mushy Rubbery Fracture Stress (kPa) 0 corded during gelation showed similar behavior with fracture 0 0.5 1 1.5 2 stress and strain of aged gels regarding their relations to moisture Fracture Strain content. Increasing Tm indicated more interchain hydrogen bonds within gelatin molecules in the abundance of water (Altay and 100 b Brittle 0.5% xanthan Tough Gunasekaran, 2012). Water is considered as a plasticizer, meaning 1% xanthan its presence decreases Tg due to increasing free volume (Williams 75 et al., 1955) and reduces the G0 of the system (Tolstoguzov, 0.88:1 2000). The mechanical Tg values of samples with only gelatin in- 50 creased with moisture content (Table 3), as opposed to the ex- 1:1 pected plasticizer effect of water. The calculated free volumes of 25 the samples were higher with moisture content (Altay and Gun- 1.5:1 asekaran, 2012). This may indicate the possible effect of the total Mushy Rubbery Fracture Stress (kPa) 0 amount of GS and sucrose in the mixture which decreases with 0 0.5 1 1.5 2 increasing moisture at the same gelatin concentration in this study Fracture Strain (Table 1). A low DE GS (DE from 20 to 42) contains more long-chain polysaccharides, which increases Tg of a mixture with sucrose at 100 any level (Hartel, 2001). Besides, an increase in the junction zone c Brittle 0.5% xanthan Tough 0.75:1 1% xanthan amount and size (i.e., in the crosslinking density) decreases the dis- 1.33:1 75 tance between the crosslinks and the length of chain segments moved independently, which increases chain stiffness and Tg 1:1 50 (Tolstoguzov, 2000). The increase in mechanical Tg may be attrib- 0.75:1 uted to the increase in the interchain hydrogen bonds with gelatin 1.33:1 molecules in the abundance of water (as in case of T ) and GS and 25 m 1:1 sucrose presence which probably lead to the increase in the junction zone amount. The texture of gelatin gels at 25% moisture Fracture Stress (kPa) Mushy Rubbery 0 content became more brittle comparing to samples at 20% 0 0.5 1 1.5 2 moisture. As indicated before samples with high moisture content Fracture Strain had less sugar at the same gelatin concentration (Table 1). The Fig. 2. Texture map of the gels at different GS:sucrose ratios. (a) containing only brittleness of gels increases when sugar level decreases (de Vries, gelatin, (b) XG at 20% moisture content, (c) XG at 25% moisture content. Dashed 2004). This may be the reason for more brittle texture for the lines separate the graphic into four quadrants which represent brittle (top left), samples with only gelatin. tough (top right), mushy (lower left) and rubbery (lower right) textures. The XG addition increased both moduli (Table 3), indicating

stronger gel network. Samples containing XG did not exhibit Tm. The mechanical Tg decreased with XG addition due to network for- process, even though it decreased the glass transition region and mation. The lower Tg makes confectionery products thermally the onset of glassy state (Altay, 2006; Altay and Gunasekaran, unstable during handling and storage (Altay and Gunasekaran, 2012). As a general tendency, calorimetric Tg values were lower 2012). than mechanical Tg values. According to the statistical analysis, From uniaxial compression measurements, texture maps of the most important factor affecting calorimetric Tg was GS:sucrose aged gels were obtained to relate instrumental data with sensory ratio (F < 0.01). Tg values increased with increasing GS:sucrose characteristics. Results showed that as the most important factor ratio, which contains saccharide polymers. These polymers may GS:sucrose ratio affected gel samples differently depending on have more crosslinked network, leading to higher Tg. Furthermore, their moisture and XG contents. The gel samples containing XG Tg was dependent on all two-interactions of three effects. For at both moisture contents had a tendency to show rubbery texture instance, the combined effect of moisture content and GS:sucrose with increasing GS:sucrose ratio (Fig. 2b and c). At any given mois- ratio on Tg was significant (F < 0.01). At 25% moisture content, Tg ture content, increasing GS:sucrose ratio means increasing GS values significantly increased for samples with GS:sucrose amount as decreasing sucrose amount. The increasing GS amount ratio >1. Probably, in the presence of water, with the increasing may have resulted in gels becoming more rubbery. The most GS:sucrose ratio, crosslinking in the molecule increased, so Tg also important factor affecting calorimetric Tg was GS:sucrose ratio increased (Altay and Gunasekaran, 2013). and it significantly increased with GS:sucrose ratio. The calorimetry provides information about the mobility of the sugar molecules

4. Discussion (Aubuchon et al., 1998), indicating higher Tg means less mobile sugar molecules in the network probably due to more crosslinking.

In this study, in situ gelation and cured gel structures of gelatin Tolstoguzov (2000) correlated Tg and mechanical properties of a containing XG at high levels of co-solutes were discussed using gel by discussing gel memory concept. He argued that the effect of rheometry, uniaxial compression, DMA and DSC techniques. In or- gel memory originates from the glassy state of structural elements der to present a complete picture, all parameters and their affect- of the gel network. Formation of junction zones, aggregation and ing factors that were measured by different techniques are given in gelation of macromolecules require mutual adjustment and mobil- 294 F. Altay, S. Gunasekaran / Journal of Food Engineering 118 (2013) 289–295

Table 3 Affecting factors on properties of in situ gelation and aged gels of samples containing gelatin-XG with high levels of co-solutes.

In situ gelation Aged gels

0 00 Parameters G at 25 °C G at 25 °C Tm Fracture stress at Fracture strain at Texture at 25 °C Mechanical Tg Calorimetric Tg room temperature room temperature Affecting factors Moisture contenta More brittle –

20% ? 25% XG addition n/ab ––– –

0% ? 1% GS:sucrose ratio – – – – – More rubbery –

<1 ? 1<

–: No effect or an effect depending on interactions of other factors. : Increase. : Decrease.

a Effect of moisture content on samples without XG. b n/a: No Tm for samples with XG.

ity of chain segments. These processes of food structure formation Acknowledgement can take place at temperatures above Tg and result in an increased Tg and possible vitrification, which lead to a great increase in local Filiz Altay thanks the Council of Higher Education of Turkey concentration of macromolecules and their physical crosslinking (YOK) for the scholarship during her PhD study in the USA. (Tolstoguzov, 2000). Accordingly, increasing GS:sucrose ratio, leading to less mobility with increasing crosslinks during gelation which occurs above Tg, may result in more rubbery texture with References higher Tg. Altay, F., 2006. Rheological and Calorimetric Evaluations of Gelatin–Xanthan Gum Systems with High Levels of Co-Solutes. PhD Thesis, University of Wisconsin- Madison, Madison, Wisconsin. 5. Conclusions Altay, F., Gunasekaran, S., 2012. Rheological evaluation of gelatin–xanthan gum system with high levels of co-solutes in the rubber-to-glass transition region. In situ gelation of gelatin gels containing XG at high levels of co- Food Hydrocolloids 28, 141–150. Altay, F., Gunasekaran, S., 2013. Mechanical spectra and calorimetric evaluation of solutes revealed that accelerated process of vitrification. Tg de- gelatin–xanthan gum systems with high levels of co-solutes in the glassy state. creased with XG addition, which makes confectionery products Food Hydrocolloids 30, 531–540. thermally unstable during handling and storage. On the other AOAC, 1990. Moisture in sugars (925.45B), . 15th ed.. In: Helrich, K. (Ed.), Official Methods of Analysis of the Association of Official Analytical Chemists 15th ed., hand, samples with only gelatin collapse faster as temperature in- vol. 2 Association of Official Analytical Chemists, Inc., USA, p. 1011. creases due to their coefficients of thermal expansion (Altay and Aubuchon, S.R., Thomas, L.C., Theuerl, W., Renner, H., 1998. Investigations of the Gunasekaran, 2012). sub-ambient transitions in frozen sucrose by modulated differential scanning Ò Increasing GS:sucrose syrup ratio and decreasing moisture con- calorimetry (MDSC ). Journal of Thermal Analysis 52, 53–64. Calzada, J.F., Peleg, M., 1978. Mechanical interpretation of compressive stress–strain tent resulted in more-flexible (entropic), less-aggregated polymer relationship in solid foods. Journal of Food Science 57, 640–646. networks. Reduced availability of water molecules also lead to Cowie, J.M.G., 1991. The amorphous state. In: Polymers: Chemistry and Physics of phase separation between sugar-rich and polymer-rich phases Modern Materials, second ed. Nelson Thornes Ltd., UK, pp. 247–273. de Vries, J., 2004. Hydrocolloid gelling agents and their applications. In: Williams, with fewer interconnections in the structure. The decrease in frac- P.A., Phillips, G.O. (Eds.), Gums and Stabilisers for the Food Industry 12. The ture stress and the increase in fracture strain with decreasing Royal Society of Chemistry, Cambridge, UK, pp. 23–31. moisture content result in more-flexible (entropic), less-aggre- Fonkwe, L.G., Narsimhan, G., Cha, A.S., 2003. Characterization of gelation time and texture of gelatin and gelatin-polysaccharide mixed gels. Food Hydrocolloids gated polymer network, which are capable of extensive stretching 17, 871–883. before breaking (relaxing), thus enduring higher applied forces be- Gunasekaran, S., Ak, M.M., 2003. Fundamental rheological methods. In: Cheese fore network fracture. Co-solutes did not totally exclude them- Rheology and Texture. CRC Press, USA, pp. 31–112. Hamann, D.D., 1983. Structural failure in solid foods. In: Peleg, M., Bagley, E.B. selves from the gelatin-XG phase, but together with the water (Eds.), Physical Properties of Foods. The Avi Publishing Company, Inc., Westport, molecules formed a solvent of inferior quality that inhibits exten- Connecticut, pp. 351–383. sive polymer aggregation. Hamann, D.D., MacDonald, G.A., 1992. Rheology and texture properties of surimi and surimi-based foods. In: Lanier, T.C., Lee, C.M. (Eds.), Surimi Technology. The temperature stability of the system decreases due to the Marcel Dekker Inc., New York, pp. 429–500. lowering of Tg of samples with XG addition based on DMA mea- Hartel, R.W., 2001. Solution characteristics and glass transition. In: Crystallization surements. However, XG accelerates the vitrification process, in Foods. Aspen Publishers, Inc., USA, pp. 91–144. which is desirable for candy manufacturing. Kalichevsky, M.T., Blanshard, J.M.V., Marsh, R.D.L., 1993. Applications of mechanical spectroscopy to the study of glassy biopolymers and related systems. In: Thus, textural properties of candy products can be altered by Blanshard, J.M.V., Lillford, P.J. (Eds.), The Glassy State in Foods. Nottingham adjusting sugar and/or moisture contents in the formulation. The University Press, Loughborough, pp. 133–156. XG addition into gelatin with co-solutes at high concentration Kasapis, S., 2008. Recent advances and future challenges in the explanation and exploitation of the network glass transition of high sugar/biopolymer mixtures. can be used to produce different textural and/or rheological and Critical Reviews in Food Science and Nutrition 48, 185–203. mechanical properties in different food systems. In doing so, the Kasapis, S., 2012. Relation between the structure of matrices and their mechanical structure–function relations in food polymer containing systems relaxation mechanisms during the glass transition of biomaterials: a review. Food Hydrocolloids 26, 464–472. should be studied more thoroughly, so that predictive models Kasapis, S., Al-Marhoobi, I.M.A., 2000. Glass transitions in high sugar/biopolymer can be developed. mixtures-some recent developments. In: Williams, P.A., Philips, G.O. (Eds.), F. Altay, S. Gunasekaran / Journal of Food Engineering 118 (2013) 289–295 295

Gums and Stabilisers for the Food Industry, vol. 10. The Royal Society of Royall, P.G., Craig, D.Q.M., Doherty, C., 1998. Characterisation of the glass transition Chemistry, UK, pp. 303–313. of an amorphous drug using modulated DSC. Pharmaceutical Research 15 (7), Kasapis, S., Al-Marhoobi, I.M.A., 2005. Bridging the divide between the high- and 1117–1121. low-solid analyses in the gelatin/j-carrageenan mixture. Biomacromolecules 6, Sharma, D., George, P., Button, P.D., May, B.K., Kasapis, S., 2011. Thermomechanical 14–23. study of the phase behavior of agarose/gelatin mixtures in the presence of Kasapis, S., Abeysekara, R., Atkin, N., Deszczynski, M., Mitchell, J.R., 2002. Tangible glucose syrup as co-solute. Food Chemistry 127, 1784–1791. evidence of the transformation from enthalpic to entropic gellan networks at Slade, L., Levine, H., 1996. Mono- and disaccharides: selected physicochemical and high levels of co-solute. Carbohydrate Polymers 50 (3), 259–262. functional aspects. In: Eliasson, A. (Ed.), Carbohydrates in Food. Marcel Dekker, Ledward, D.A., 2000. Gelatin. In: Phillips, G.O., Williams, P.A. (Eds.), Handbook of Inc., USA, pp. 41–157. Hydrocolloids. Woodhead Publishing Limited, England, pp. 67–86. Stanley, D.W., 1987. Food texture and microstructure. In: Moskowitz, H.R. (Ed.), Levine, H., Slade, L., 1990. Cryostabilization technology: thermoanalytical Food Texture Instrumental and Sensory Measurement. Marcel Dekker, Inc., New evaluation of food ingredients and systems. In: Harwalkar, V.R., Ma, C.Y. York, pp. 35–64. (Eds.), Thermal Analysis of Foods. Elsevier Science Publishers Ltd., England, pp. Sworn, G., 2000. Xanthan gum. In: Phillips, G.O., Williams, P.A. (Eds.), Handbook of 221–305. Hydrocolloids. Woodhead Publishing Limited, England, pp. 103–115. Li, J., Ould Eleya, M.M., Gunasekaran, S., 2006. Gelation of whey protein and xanthan Sworn, G., Kasapis, S., 1998. Effect of conformation and molecular weight of co- mixture: effect of heating rate on rheological properties. Food Hydrocolloids 20, solute on the mechanical properties of gellan gum gels. Food Hydrocolloids 12, 678–686. 283–290. Menard, K.P., 1999. Dynamic Mechanical Analysis. CRC Press, USA. Tilly, G., Aubree, E., Houdoux, A., 2002. Hydrocolloids to replace gelatin. Food Ould Eleya, M., Gunasekaran, S., 2002. Gelling properties of egg white produced Ingredients and Analysis International 24 (10), 12–14. using a conventional and a low-shear reverse osmosis process. Journal of Food Tolstoguzov, V.B., 2000. The importance of glassy biopolymer components in food. Science 67 (2), 725–729. Nahrung 2, 76–84. Peleg, M., 1987. The basics of solid foods rheology. In: Moskowitz, H.R. (Ed.), Food Williams, M.L., Landel, R.F., Ferry, J.D., 1955. The temperature dependence of Texture Instrumental and Sensory Measurement. Marcel Dekker, Inc., New relaxation mechanisms in amorphous polymers and other glass-forming York, pp. 1–33. liquids. Journal of American Chemistry Society 77 (14), 3701–3707.