Gelation of whey protein and xanthan mixture: Effect of heating rate on rheological properties

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1 Food Hydrocolloids 2 (26) Gelation of whey protein and xanthan mixture: Effect of heating rate on rheological properties J. Li, M.M. Ould Eleya, S. Gunasekaran* Food and Bioprocess Engineering Laboratory, University of Wisconsin-Madison, 46 Henry Mall, Madison, WI 376, USA Received 6 July 2 Abstract The effects of heating rate and xanthan addition on the gelation of a % w/w whey protein solution at ph 7 and in. M phosphate buffer were studied using small-amplitude oscillatory shear (SAOS) rheological measurements and uniaxial compression tests. WPI solutions were heated from 2 to 9 8C at five heating rates (.,,, and 2 8C/min). Gelation temperature of WPI decreased with decreasing of heating rates and with xanthan addition. Under uniaxial compression, the WPI gels prepared with no more than.2% w/w xanthan exhibited distinct fracture point and were tougher (i.e. higher fracture stress and fracture strain) than the gels prepared with no less than.% w/w xanthan. In general, the fracture strain of WPI gels increased with heating rate, though not significantly, at all xanthan contents investigated. However, the fracture stress of WPI gels, generally, decreased with heating rate when xanthan content was.2% and increased with heating rate when xanthan content was. and %. q 2 Elsevier Ltd. All rights reserved. Keywords: Whey protein; Xanthan mixture; Heating rate. Introduction Whey proteins (WP) are widely used in the food industry for their well-known functional and nutritional properties. The gelation of WP, one of their most important functional properties, has been studied extensively (Mulvihill & Kinsella, 987; Stading & Hermansson, 99; Hines & Foegeding, 993). It is known that the gelation of WP is affected by many factors, such as concentration, temperature, ph, ionic strength, heating rate, and presence of specific ions (Mulvihill & Kinsella, 987). These factors also have important effects on the rheological properties of the final product. In many food systems, presence of polysaccharides plays an important role in the structure and stabilization of proteins (Dickinson, 993). Therefore, mixed systems of * Corresponding author. Tel.: C ; fax: C address: guna@wisc.edu (S. Gunasekaran). Present address: Institut Supérieur d Enseignement Technologique (ISET-Rosso), B.P. 42 Nouakchott, Mauritania. 268-X/$ - see front matter q 2 Elsevier Ltd. All rights reserved. doi:.6/j.foodhyd.2.7. proteins and polysaccharides are often used in the food industry to manufacture products with varying textures. Interactions between proteins and polysaccharides can be either attractive or repulsive depending on the nature of the interacting biopolymers and the medium conditions. Such interactions often lead to the formation of a protein polysaccharide complex or thermodynamic incompatibility, respectively (Tolstoguzov, 997). The latter generally leads to phase separation when the overall polymer concentration exceeds a certain concentration called the phase separation threshold. Thermodynamic incompatibility, which arises from the low entropy of mixing upon blending two polymers, usually occurs under conditions that promote protein self-association (i.e. around protein pi) and/or when the two types of polymers show varying affinity towards the solvent (Picullel & Lindmann, 992). The effects of anionic polysaccharides such as xanthan on WP gelation and gel properties have been previously investigated under various conditions, such as ph (Sanchez, Schmitt, Babak, & Hardy, 997a,b; Turgeon & Beaulieu, 2), temperature (Walkenstrom, Panighetti, Windhab, & Hermansson, 998), fluid shear (Walkenstrom et al., 998), and presence of cations (Turgeon & Beaulieu, 2; Garawany, Korolczuk, & Salam, 22). It was found that

2 J. Li et al. / Food Hydrocolloids 2 (26) xanthan and WP form coacervate complexes, below protein pi, that could be used in the fabrication of fat substitutes (Laneuville, Paquin, & Turgeon, 2; Sanchez & Paquin, 997). Above protein pi, native whey protein and xanthan are compatible and, therefore, form miscible solutions. However, phase separation is observed when pre-denatured proteins are mixed with xanthan, presumably due to repulsive electrostatic forces between the predominately negatively charged WP and the anionic xanthan and to steric exclusion and protein self-association that occur upon protein unfolding (Bryant & McClements, 2; Euston, Finnigan, & Hirst, 22). When protein solutions are heated (usually above 6 8C), protein denaturation and aggregation take place followed by gelation when protein concentration is above the gel critical concentration (Ferry, 948; Aguilera, 99). When mixed solutions of proteins and polysaccharides are heated, a competition between gelation and phase separation processes takes place. Once the gelation occurs, the basic structure of the gel is established, and phase separation is therefore retarded (Clark, 99; Owens & Jones, 998; Ould Eleya & Turgeon, 2). If the competition between phase separation and gelation processes is controlled, it could result in the formation of an infinite number of stable phase distributions with interesting structural and mechanical properties. One way of controlling these two competing processes is to retard or accelerate the gelation kinetics through modification of variables that have an effect on protein gelation such as heating rate. If the heating rate is varied, for instance, there will be varied levels of competition between establishment of gel structure and phase separation which will lead to creation of a variety of gel structures with different rheological and structural properties. Heating rate is also an important factor in controlling structure and functional properties of WP. The effect of heating rate on gelation of different proteins have been reported: b-lactoglobulin (Stading & Hermansson, 99; 993), myosin (Foegeding, Allen, & Dayton, 986), fibrinogen (Foegeding et al., 986), albumin (Foegeding et al., 986), meat batter (Barbut & Mittal, 99), and muscle protein (Camou, Sebranek, & Olson, 989). These authors found that the gelation temperature and gel strength are different under different heating rates. However, the effect of heating rate on the gelation and rheological properties of polysaccharide-protein mixture has not been reported. At higher heating rates, when heating over a given temperature range, the heating duration is shorter. Because of this the protein will denature to different extents which will invariably affect the gelation kinetics and eventual mechanical properties of the gel. The objectives of this study were to: () investigate the gelation of WPI and WPI xanthan mixed solutions under different heating rates and (2) characterize small and large deformation rheological properties of the WPI and WPI xanthan gels. 2. Materials and methods 2.. Materials WPI (9 92% protein,.% lactose,..% fat, 2 3% ash, 4.% moisture) was obtained from Davisco Foods International Inc. (Eden Prairie, MN). Xanthan (M w O,,) was purchased from Sigma-Aldrich Co. (St Louis, MO). Sodium phosphate monobasic and sodium phosphate dibasic were manufactured by Mallinckrodt Inc. (Paris, KY) Preparation of Stock WPI and xanthan solutions Sodium phosphate monobasic and sodium phosphate dibasic were dissolved in deionized water to prepare phosphate buffer solutions (ph 7,. ionic strength). Stock solutions of WPI (3% w/w) were prepared at room temperature (2 8C) by dispersing WPI powder into phosphate buffer solution and stirring for 2 h. Stock solutions of xanthan (.2,.4, and 2% w/w) were prepared by dissolving xanthan powder in phosphate buffer solution and stirring for 2 h. Stirring was not too vigorous in order to minimize air incorporation into the solutions. After stirring, WPI and xanthan stock solutions were centrifuged ( rpm, 3 min) (Centrifuge 4C, Brinkmann Instruments Inc., NY) to eliminate air bubbles and insoluble substances. Supernatants were collected and stored at 4 8C to ensure full hydration of the biopolymers Preparation of WPI and WPI xanthan mixed solutions Stock solution of WPI (3% w/w) was diluted in phosphate buffer solution at : (w/w) ratio for 3 min at 2 8C to prepare % w/w WPI solutions. Stock solutions of xanthan (.2,.4, and 2% w/w) were mixed with WPI stock solution at : (w/w) ratio for 3 min at 2 8C to prepare WPI xanthan mixed solutions. The solutions were kept at ph 7. After that, the WPI and WPI xanthan mixed solutions were degassed for 3 min by gentle stirring under vacuum Rheological measurement of WPI and WPI xanthan mixture Small amplitude oscillatory shear (SAOS) tests were performed using 2-mm diameter parallel plates in a dynamic rheometer (CVO-R, Bohlin Instrument Inc., E. Brunswick, NJ). The gap between parallel plates was set to mm. Silicone oil was applied around the sample edge to prevent moisture loss during the experiment. Temperature sweep tests (from 2 to 9 8C, Hz, at different heating rates.,,,, 2 8C/min) were conducted to record in situ rheological properties during heat-induced gelation. To ensure linear viscoelastic measurements, the applied strain for each measurement

3 68 J. Li et al. / Food Hydrocolloids 2 (26) was limited to.% using the target strain function of the rheometer. Frequency sweep tests ( K2 2 Hz) were conducted to investigate the rheological behavior of gels at isothermal condition after the sample was cooled down to 2 8C. The mechanical spectra of the gels were recorded. All the tests in this section were replicated three times. 2.. Preparation of WPI and WPI xanthan heat-induced gels WPI and WPI xanthan mixed solutions were poured into 7-mm inner diameter (66-mm long aluminum tube molds. The inside surface of the molds was coated with silicon oil to prevent the gel from sticking. The ends of the molds were closed with rubber stoppers. Heat-induced gels were obtained by placing the molds into the heating chamber of a controlled heating instrument designed and fabricated in our laboratory (Li, 23). The controlled heating instrument houses a 22-mm inner diameter (-mm long aluminum cylinder mounted vertically. This cylinder was surrounded by heating tape. This tube serves as the heating chamber when the annulus between the tube and the gel mold is filled with water. The power supply to the heating tape is controlled by a temperature controller unit (CN 44, Omega Engineering, Inc., Stamford, CT) by sensing its temperature with a type-t thermocouple. The heating medium temperature and the gel solution temperature are also monitored using thermocouples connected outside and inside of the mold, respectively. The temperature data are recorded by a computer data acquisition/switch unit (3497A, Hewlett Packard, Palo Alto, CA). Heating of the WPI and WPI xanthan solutions were conducted from 2 to 9 8C at different heating rates of.,,,, and 8C/min. The heating rate of 2 8C/min was not attainable due to the limitation of the instrument. Upon completion of the heat treatment, gel molds were taken out of the heating chamber and cooled immediately in water (G 8C) for 2 h and left overnight in a refrigerator (4G 8C). After storage, aluminum tubes were equilibrated again at 2 8C for 2 h. The gels were carefully removed from their molds and cut into 7-mm diameter (.-mm long cylindrical specimens. Care was exercised to ensure that the ends of the test specimens were flat and parallel for accurate determination of mechanical properties (Gunasekaran & Ak, 22) Uniaxial compression properties of WPI and WPI xanthan thermally induced gels Mechanical properties (stress strain relationships) of the gels were determined at 2 8C by uniaxial compression from to 8% relative deformation at a compression rate of mm/s in a Synergie 2 (MTS Systems Corp., Eden Prairie, MN) universal testing machine equipped with -N load cell. The test platen specimen interfaces were lubricated with silicone oil in order to minimize shear stress losses by friction at the interface of plate and gel (Ak & Gunasekaran, 992). For compression test, the true stress (s t ) can be defined, as s t Z FðH KDHÞ A o H Z sðk3þ where, F is recorded force; H is initial sample height; A o is original sample cross-sectional area; DH is change in sample height; 3 is engineering or Cauchy strain; and s is engineering stress. Similarly, the true strain (or Hencky strain) (3 H ) can be defined as: 3 H Z ln H KDH Z lnðk3þ H The use of s t and 3 H (instead of s and 3) compensate for change in cross-sectional area during compression. Calzada and Peleg (976) recommended using 3 H when the compression is 2% or greater. Five samples were tested for each treatment. The experimental data were analyzed by one-way ANOVA (analysis of variance) test for the significant differences at PZ.. 3. Results and discussion 3.. Gelation of WPI and WPI xanthan solutions The change in storage modulus (G ) of WPI and WPI xanthan solutions during temperature sweep from 2 to 9 8C at.,,,, and 2 8C/min are presented in Fig.. Magnitude of G of WPI and WPI xanthan systems exhibited a strong inverse dependence on heating rate. This is due to lower extent of protein denaturation at higher heating rates (due to shorter heating duration) and thus exposing fewer interaction sites for bond formation. Addition of xanthan to WPI resulted in an increase in G and at all heating rates. A small but noticeable decrease in G was observed before gelation of WPI.% w/w xanthan and WPI % w/w xanthan solutions at the heating rate of. 8C/min but not at, and 8C/min. This decrease in G may not be related to the order disorder conformational transition of xanthan. According to Milas and Rinaudo (986), the transition temperature of xanthan should be no less than 8C in. M ionic strength. Such a decrease in G prior to gelation temperature (T gel ) has been previously observed for meat batter xanthan mixture (Foegeding & Ramsey, 987) and for myofibrillar protein polysaccharide mixture (Xiong & Blanchard, 993).

4 J. Li et al. / Food Hydrocolloids 2 (26) Effect of heating rate and xanthan addition on T gel The T gel was obtained by extrapolating the rapidly rising G values to zero as shown in the insert of Fig. a. The T gel values determined at different heating rates are listed in Table. For both WPI and WPI xanthan solutions, initial structure development started at a lower temperature when the heating rate was lower. Thus, T gel increased with increasing heating rate. This kind of relationship is in agreement with previously published results on egg white (Donovan, Mapes, Davis, & Garibaldi, 97), vicilin, and ovalbumin. (Arntfield & Murray, 992). At slower heating rate there is more time for protein denaturation and aggregation; thus, gelation occurs at a lower temperature than it would at a faster heating rate (Arntfield & Murray, 992; Foegeding et al., 986). In the presence of xanthan, T gel of WPI decreased. At heating rates of and 2 8C/min, there was no increase in G of % w/w WPI solution when the temperature reached 9 8C. However, when.% w/w xanthan was present, % w/w WPI solution gelled at 8C/min heating. It is quite obvious that the presence of xanthan in the system accelerated the aggregation of WPI. The increase in aggregation rate may be due to increased protein concentration in the protein-rich phase as a result of phase separation between protein aggregates and xanthan molecules. (a) Storage modulus, G' (kpa) G' (Pa) Gel point Gel point (b) Storage modulus, G' (kpa) G' (Pa) Fig.. Temperature sweep of % WPI solution (ph 7., ionic strength. M) at different heating rates (.,,, 8C/min). The figure inset indicates the determination of gel point. (a) WPI, (b) WPI C.% xanthan, (c) WPI C.% xanthan and (d) WPI C% xanthan.

5 682 J. Li et al. / Food Hydrocolloids 2 (26) (c) Storage modulus, G' (kpa) G' (Pa) (d) Storage modulus, G' (kpa) G' (Pa) Fig. (continued) 3.3. Frequency sweep of WPI and WPI xanthan gels Frequency dependence of WPI and WPI xanthan gels after cooling (down to 2 8C) showed an almost linear relationship over a range of frequencies, especially for gels formed at low heating rate, as shown in Fig. 2. The magnitude of equilibrium storage modulus was dependent on the heating rate. G depends on the strand characteristics, number of elastically active network strands, number of network chains per junction zone, etc. (Stading, Langton, & Hermansson, 993). b-lactoglobulin gels formed at slow heating rates (. and 8C/min) reportedly consist of many fused particles and are stiffer than the strands formed at fast heating rates ( and 8C/min) (Stading et al., 993). Table Gelation temperature (8C) of WPI and WPI xanthan at different heating rates Heating rate (8C/min) % WPI % WPI.% xanthan % WPI.% xanthan % WPI % xanthan. 68.3G2.2 aa 8.2G.9 ba 8.G2.7 ba 9.2G2. ba 8.4G3.4 ab 73.G3.7 bb 7.8G2.3 ba 74.7G3. bb 83.4G.8 ab 77.6G2. bb 83.2G3. cb 82.G3.4 cc * 84.3G3.8 ac 84.7G2.4 ab 8.2G2. ac 2 * * * * Different letters indicate significant (P!.) differences, lower case letters between xanthan contents and capital letters between heating rates. *G increase was not observed.

6 J. Li et al. / Food Hydrocolloids 2 (26) (a).e+6 (b).e+6 Storage modulus, G' (Pa).E+.E+4.E+3.E+2.E+. 2.E+.E-2.E-.E+.E+.E+2 Frequency, f (Hz) Storage modulus, G' (Pa).E+.E+4.E+3.E+2.E+. 2.E+.E-2.E-.E+.E+.E+2 Frequency, f (Hz) (c).e+6 (d).e+6 Storage modulus, G' (Pa).E+.E+4.E+3.E+2.E+. 2.E+.E-2.E-.E+.E+.E+2 Frequency, f (Hz) Storage modulus, G' (Pa).E+.E+4.E+3.E+2.E+. 2.E+.E-2.E-.E+.E+.E+2 Frequency, f (Hz) Fig. 2. Frequency sweep of % WPI xanthan gels (ph 7., ionic strength. M) formed at different heating rates (.,,,, 2 8C/min). The experiments were conducted at 2 8C. (a) %WPI, (b) %WPI.%xanthan, (c) %WPI.%xanthan and (d) %WPI %xanthan. The straight-line portion of the mechanical spectra of the gels became wider when the heating rate was slower. At low frequency, the stress is applied over a long time scale, some of the bonds in the gel have a chance to relax, so these bonds do not contribute to G, and the applied stress is within the linear viscoelastic region of the gel. When the test frequency exceeds a certain value, the stress is applied over a very short time scale, and there is little time for bonds to relax, comparatively little energy is dissipated and these bonds contribute to G,soG increases sharply (Ferry, 98). At the fast heating rate of 2 8C/min and in the cases of WPI, WPI.% w/w % xanthan, WPI.% w/w xanthan, the linear region became narrow. This phenomenon may indicate that WPI did not gel well at 2 8C/min heating rate and even after cooling when the xanthan component was below.% w/w. When % w/w WPI or % w/w WPI xanthan solutions were heated up from 2 to 9 8C at fast heating rate (such as 2 8C/min), the end products we obtained were smooth and pasty, typical of a concentrated, weakly interacting protein aggregate dispersions. This also was substantiated by the analysis of phase angle data. Phase angle remained relatively high (w28) over the. Hz frequency range, confirming that no real gel was formed Uniaxial compression properties of WPI and WPI xanthan heat-induced gels Stress strain relationships determined by uniaxial compression are depicted in Fig. 3. All the gels fractured or collapsed before 8% deformation was reached. In the cases of WPI gel, and WPI.% w/w xanthan and WPI.2% w/w xanthan mixed gels, the stress increased approximately linearly with strain until a peak value, after which it decreased rapidly corresponding to the point where the gel was seen to fracture. This is a characteristic of brittle materials (Gunasekaran & Ak, 22). When xanthan concentration exceeds.% w/w, the mixed gels exhibit maximum stress and undergo significant plastic deformation before fracture. This is a typical characteristic of ductile material (Gunasekaran & Ak, 22). However, the ductile material should have a higher fracture strain compared to brittle material, and the fracture strains of the mixed gels we tested were significantly lower than those of brittle gels. In the cases of WPI.% w/w and WPI % w/w xanthan gels, it was very difficult to distinguish the fracture points on the compression curves. So the peak stress (and corresponding strain) of these gels was used as fracture stress (and strain) values in comparing with fracture stress (and strain) of other gels. 3.. Fracture stress heating rate relationship The values of fracture stress of gels made at different heating rates are listed in Table 2. The fracture stress of WPI gels increased with decreasing of heating rate, more drastically heating rates less than 8C/min. This indicated that heat-induced WPI gels formed at ph 7 and slow heating rate are more rigid and do not readily dissipate strain energy. Fracture stress values of WPI.% w/w xanthan gels were fairly similar to those of corresponding WPI gels with

7 684 J. Li et al. / Food Hydrocolloids 2 (26) (a) 4 True stress (kpa) Hencky strain (b) 4 True Stress (kpa) Hencky strain (c) 4 True Stress (kpa) Hencky strain. (d) 8 True Stress (kpa) Hencky strain. Fig. 3. True stress-hencky strain of % WPI xanthan gel (ph 7., ionic strength. M) as a function of heating rate. The gels were formed at different heating rates (.,,,, 8C/min). The compression experiments were conducted at 2 8C. (a) % WPI, (b) %WPI.%xanthan, (c) %WPI.%xanthan and (d) %WPI %xanthan. similar trend of increasing fracture stress with decreasing of heating rate. Nonetheless, at. 8C/min heating rate the fracture stress of WPI.% xanthan gel was higher than that of the WPI gel. When xanthan content was increased to.2% w/w, the fracture stress of the gel formed at. 8C/min heating rate was lower than that of the corresponding WPI gel. But in the cases of and 8C/min heating rate, the fracture stress of WPI.2% w/w xanthan gels were higher than those of WPI and WPI.% w/w xanthan gels. These indicate that both heating rate and xanthan content influence the mechanical properties of the WPI gel. Sanchez et al. (997a,b) studied the effect of xanthan concentration on the rheology of WPI xanthan mixed gels (ph 7, 9 8C, 3 min), and they found that there were synergistic interactions between xanthan and WP when xanthan content was below certain level (such as.% w/w). With. and % w/w xanthan, fracture stress vs. heating rate profiles were different than those of WPI, WPI.% w/w xanthan and WPI.2% w/w xanthan gels. Basically, the fracture stress of WPI.% w/w xanthan and WPI % w/w xanthan gels decreased with decreasing of heating rate. This situation was quite contrary to those of WPI gels with %.2% w/w xanthan content. For example, the gels formed at the heating rate of. 8C/min were very weak and soft; it was very difficult to distinguish the fracture point on the stress strain curve. This may indicate that adverse effect of high xanthan content (O.%) on Table 2 Fracture stress and fracture strain of WPI and WPI xanthan gels as a function of heating rate Heating rate (8C/min) Xanthan content (%) Fracture stress (kpa) Fracture strain aa ba 3.39 ca 4.2 da 2.63 ea.9 aa.94 aa.84 ba.7 ca.28 da 29. ab 3.48 ab 2.4 ab 9.82 bb 2.69 ba. ab.3 ab.2 ab.6 ba. bb 2. ac 22.6 ac 2.2 ab.94 bb 2.26 bb. ab.8 bb.96 ac.8 ca.6 cb 7.9 ac 9.6 ac bb 2.63 cb 4.2 cb.3 ab.8 ab.94 ac.67 ba.9 bb.7 ac 8.2 ac 2.88 ab 3.63 ab 6.2 ab.28 ab.22 ab.27 ad.84 ba.64 bb Different letters indicate significant difference, lower case letters between xanthan content and capital letters between heating rate.

8 J. Li et al. / Food Hydrocolloids 2 (26) the firmness of WPI gels especially when the heating rate is low (. or 8C/min) Fracture strain heating rate relationship As shown in Table 2, the fracture strain vs. heating rate trends were similar for all gels, i.e. the fracture strain increased with heating rate, though, generally, the differences were statistically not significant. When the fracture strain was high, the fracture terminated at the specimen ends. Of particular note is the substantial decrease in the gel fracture strain when the xanthan content was R.% w/w. The aspect ratio (length-to-diameter ratio) of the gel samples we used in uniaxial compression was.6 which is less than.7 recommended in the literature (Peleg, 977; Chu & Peleg, 986). This may have had a little effect on the stress and strain data collected. When the xanthan concentration was increased to or more than.% w/w, the polysaccharide behavior determined the mechanical properties of the mixed gel. When the WPI.% w/w xanthan or WPI % w/w xanthan were heated at low heating rate (. and 8C/min), WPI and xanthan become incompatible and phase separation circumvented gel formation. Therefore, phase-separated gels were formed. Bryant & McClements (2) studied the influence of xanthan on phase separation, they found that phase separation occurred in the presence of salt between xanthan and heat-denatured WPI with xanthan rich regions embedded in WPI gel. On the other hand, when the WPI.% w/w xanthan or WPI % w/w xanthan were heated at high heating rate (R. 8C/min), gelation may circumvent phase separation and uniform gels were formed. Sanchez et al. (997a,b) proposed that there were synergistic WPI xanthan interactions, which lead to higher rigidity, if ph R7 and low xanthan concentration (. or.% w/w). They also proposed that antagonism between WPI and xanthan occurred at high xanthan concentration, and the mixed gels were weakened and less deformable. The adverse effect of high xanthan concentration on the mechanical properties of food protein xanthan mixed gels has been previously observed for gels of meat protein xanthan mixture (Foegeding, Bowland, & Hardin, 99; Foegeding & Ramsey, 987; Xiong & Blanchard, 993) and WPI xanthan gels (Sanchez et al., 997a,b) if ph was equaled to or higher than six. Sanchez et al. (997a,b) reported that when the WPI gel is heated to and held at 9 8C for 3 min or when the heating is discontinued after gel temperature reached 9 8C, the gel became more rigid in the presence of low xanthan content and softer when the xanthan content exceeded a certain concentration. Our data supports such an effect of added xanthan on the mechanical properties of WPI gels the WPI.% xanthan or WPI % xanthan gels were softer and fractured more readily compared to gels made with.2% w/w xanthan. 4. Conclusions The T gel of WPI decreased with heating rate. The presence of xanthan lowered the T gel of WPI, although increasing the xanthan content had no significant impact on the T gel. The WPI gels and WPI xanthan mixed gels formed at slower heating rates were stronger as determined from their higher G and their relatively frequency-independent G profiles compared to the gels formed at higher heating rates. Fracture stress of WPI gels increased with decreasing heating rates. In the presence of. and.2% w/w xanthan, there were no significant differences in the mechanical properties between WPI gels and WPI xanthan mixed gels. However, when the xanthan content was.% w/w or greater, the mechanical properties of WPI gel changed dramatically. Compared to the WPI gels and WPI.% and WPI.2% w/w xanthan mixed gels the fracture stress and fracture strain of WPI.% and WPI.% xanthan mixed gels were significantly lower, especially at lower heating rates. It could be due to phase separation between WPI and xanthan during heating as the phase separation could render WPI gels weaker with fewer interconnections in the structure. References Aguilera, J. M. (99). Gelation of whey protein. Food Technology,, Ak, M. M., & Gunasekaran, S. (992). Stress-strain curve analysis of cheddar cheese under uniaxial compression. Journal of Food Science, 7, Arntfield, S. D., & Murray, E. D. (992). Heating rate affects thermal properties and network formation for vicilin and ovalbumin at various ph values. Journal of Food Science, 7, Barbut, S., & Mittal, G. S. (99). Effect of heating rate on meat batter stability, texture and gelation. Journal of Food Science,, Bryant, C. M., & McClements, D. J. (2). Influence of xanthan gum on physical characteristics of heat-denatured whey protein solutions and gels. Food Hydrocolloids, 4, Calzada, J. F., & Peleg, M. (978). Mechanic interpretation of stress-strain relationship in solid foods. 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