Gelation kinetics of an imitation-mortadella emulsion during heat treatment determined by oscillatory rheometry

Journal of Food Engineering 95 (2009) 677–683

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Gelation kinetics of an imitation-mortadella emulsion during heat treatment determined by oscillatory rheometry J.J. Ferris a, A.J. Sandoval a,*, J.A. Barreiro a,1, J.J. Sánchez b, A.J. Müller b a b

Universidad Simón Bolívar, Departamento de Tecnología de Procesos Biológicos y Bioquímicos, Apartado 89000, Caracas 1080-A, Venezuela Universidad Simón Bolívar, Departamento de Ciencia de los Materiales, Apartado 89000, Caracas 1080-A, Venezuela

a r t i c l e

i n f o

Article history: Received 19 August 2008 Received in revised form 20 June 2009 Accepted 22 June 2009 Available online 25 June 2009 Keywords: Gelation Kinetics Mortadella Oscillatory rheometry

a b s t r a c t The gelation kinetics of an imitation-mortadella emulsion during thermal treatment under both, isothermal and non-isothermal conditions was followed by means of oscillatory rheometry. A first order-kinetic process was found to better describe the variation of the storage modulus G0 (at 1 Hz and strain amplitude of 0.8%) during the two types of thermal treatments. An Arrhenius-type model was able to describe the effect of temperature during gelation of the emulsion under non-isothermal conditions. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Meat emulsion can be considered as a mixture composed of muscular tissue, fat particles, water, spices, and solubilized proteins jointed together by different attraction forces (Varnam and Sutherland, 1995). In such a system, fat is the disperse phase in a water continuous phase where the solubilized meat proteins act as emulsifiers. According to Foegeding (1988), proteins are the main constituents forming structure in meat products, affecting their texture. Myosin, one of such proteins, is one of the major factors in thermal aggregation needed for structure formation, especially in finely ground processed meat products (Egelandsdal et al., 1986). On the other hand, other non-meat components normally found in meat emulsions, such as starch, soy protein isolate, egg albumen and other complex carbohydrates (i.e., j-carrageenan), also affect product properties during cooking. Mortadella-type sausage products are mainly composed of meat components, animal skin, fat, protein and complex carbohydrate products. Minor components are condiments, spices, and other ingredients approved by state regulations. Processing of these products involves preparation of an emulsion by finely cutting meat components followed by mixing with the remaining ingredients, casing, and thermal (heating/cooling) treatment. Recommended thermal processing is generally limited to the time required for

* Corresponding author. Tel.: +58 212 9063976; fax: +58 212 9063971. E-mail address: [email protected] (A.J. Sandoval). 1 Presently with Dr. J.A. Barreiro and Assocs. Professor (R) Universidad Simón Bolívar. 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.06.035

the geometrical center of the sausage to reach a temperature range of 72–75 °C (Bou et al., 2004; Carballo et al., 1996). Quality of sausages depends on several factors such as: raw materials, good manufacturing practices, presence of additives (i.e., salt, nitrates), pH, processing conditions and microbial load, among others. Within the quality attributes of meat sausage affected by cooking during processing (i.e., color, texture, taste, juiciness, etc.), texture is one of the more relevant. During cooking, meat emulsion is transformed into a gel-like product exhibiting viscoelastic behavior whose characteristics will depend on the processing conditions and composition. Viscoelastic properties of meat emulsion have been considered important quality factors determining product acceptability from the consumer point of view (Siripurapu et al., 1987). The study of such properties during and after cooking has been normally carried out by uniaxial compression experiments, although dynamic testing has been previously suggested (Correia and Mittal, 1991; Li and Yeh, 2003). Bruno and Moresi (2004) used dynamic methods to distinguish among different categories of high-quality bologna sausages using an oscillatory shear dynamic technique. Dynamic rheometry has also been used to characterize the thermorheological behavior of Alaska pollock and Pacific whiting surimi, by using small amplitude oscillatory shear (Yoon et al., 2004). Data for kinetic studies can be obtained from different experimental conditions. In the traditional experiments under isothermal conditions, the sample is quickly heated up to an established temperature and kept at this temperature for the required processing time and the reaction evolution at different periods recorded. After the isothermal period, the reaction is quenched by rapidly cooling

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the sample. Although these experiments are time-consuming and in materials with low thermal conductivity the thermal lag can be an issue, they still remain a useful tool to follow changes upon thermal treatment. Another kind of experiments involves studying changes on a quality parameter while subjecting the sample to an increasingly linear varying temperature. This approach has been used by several authors to measure in situ changes in acid-catalyzed hydrolysis of sucrose (Rhim et al., 1989a), color changes of grape juice (Rhim et al., 1989b), and more recently in evaluating the thermorheological behavior of surimi during gelation at different moisture contents (Yoon et al., 2004). Undoubtedly, structure fixation and therefore, the final textural properties of these products, are affected by the contribution of the heating and cooling treatments. In the same way, the heat treatment of mortadella type meat emulsion is carried out to pasteurize the product in order to achieve microbiological stability. Both purposes must be met satisfactorily. The knowledge of the gelation kinetics as well as the pasteurization requirements are needed in order to obtain a product of the best possible quality. Consequently, the aim of this work was to study the kinetics of thermal gelation of a mortadella-type meat emulsion using both, isothermal and non-isothermal approaches.

2. Materials and methods 2.1. Raw material Meat emulsion for the bologna-type sausage was prepared by mixing 52.8% of chicken meat, and pork/chicken skin in an UFR chopper–mixer, along with 3.13% wheat flour, 1.04% soy protein isolate, curing agents (1.67% NaCl, and 0.02% sodium nitrite), additives (1.88% natural color, 1.15% sucrose, 0.31% sodium polyphosphate, 0.31% egg albumin powder, 0.08% sodium erythorbate, 0.42% j-carrageenan), seasoning (0.27% bologna seasoning, 0.21% garlic, 0.1% coriander, 0.07% rosemary and 0.07% onion), and water/ice (36.5%). The meat emulsion was prepared in a local meat processing facility according to their manufacturing procedures. The emulsion temperature was kept below 10 °C to prevent heat induced gelation during processing. Uncooked meat emulsion was kept frozen at 18 °C until its utilization. 2.2. Physical and chemical characteristics of the imitation-mortadella emulsion To evaluate meat emulsion characteristics, the following analyses were performed: water activity (aw) at ambient temperature employing a psychrometric equipment (Aqualab, model CX-2. Decagon Devices, Inc., Pullman, WA, USA), and pH at room temperature with a pH-meter (Coleman, Model 39, Maywood, IL, USA) in a filtered sample extract previously dissolved in hot water (Covenin 1315, 1979). Also, proximal composition analysis was determined by standard procedures from AOAC (1990), as follows: moisture content (No. 950.46), protein content (Kjeldahl Method No. 928.08) with a conversion factor of 6.25, fat content through a gravimetric method ISO 1443 using Weibull pretreatment and subsequent Soxhlet solvent extraction (Covenin 1219, 2000), total ashes (Method No. 950.153). To complete the proximal composition, carbohydrate content was obtained by difference. Three determinations were carried out in each analysis. 2.3. Rheometry studies Gelation kinetic studies of the imitation-mortadella emulsion were carried out by oscillatory rheometry, using a Rheometrics Sci-

entific strain-rate controlled rheometer (Model RDA II, Rheometric Scientific Inc., Piscataway, NJ, USA), provided with data acquisition software RSI Orchestrator V6.4.4 and equipped with a 25 mm parallel plate geometry with a smooth surface. All the experiments were done using a parallel plate testing geometry assembly, by spreading the sample over all the surface of the lower plate, to have a thickness of 2.00 mm of the emulsion. The testing assembly was modified to reduce sample dehydration during the experiments at high temperature, according to the design presented by Brouillet-Fourmann et al. (2003). A sectional view of the assembly is shown in Fig. 1. Additionally, a saturated atmosphere was achieved by placing distilled water in the lower annular space inside the cup, in order to prevent sample dehydration. Glycerol was used as a trap to seal the gap between the upper movable plate and the external cup. The response measured in the equipment was not significantly affected by this seal, as shown by preliminary experiments. 2.4. Linear viscoelastic range determination The linear viscoelastic range of uncooked meat emulsion samples was determined in the oscillatory rheometer in quintuplicate at 28 °C. A dynamic strain sweep from an initial strain of 0.1% to a final one of 10%, at a frequency of 1 Hz was applied. These experiments were carried out at room temperature in order to assure that the further cooking experiments were within the viscoelastic linear region. The storage modulus (G0 ) and the loss modulus (G00 ) were obtained from these tests. The linear viscoelastic limit was empirically defined as the strain for which the storage modulus (G0 ) corresponded to 95% of the maximum storage modulus (G0max ) obtained during the sweep tests (Yoon et al., 2004). 2.5. Frequency sweep tests Frequency sweep tests were done from 0.1 Hz to 10 Hz at the critical strain value for the uncooked meat emulsion previously determined to establish the linear viscoelastic range. From these tests, the storage modulus (G0 ) and the loss modulus (G00 ) were obtained as a function of frequency. 2.6. Cooking kinetic experiments The kinetic studies of the cooking process were done inside the rheometer using linear temperature increments, and also outside the rheometer by cooking the samples in a water bath. In both cases, the G0 value was taken as an indicator parameter of the cooking kinetics, since a more elastic material is obtained as the cook-

Upper plate

Glycerol

Sample

Distilled water

Lower plate Fig. 1. Sectional view of the testing geometry for the sampling loading in rheological experiments at high temperatures.

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ing mechanism proceeds. Rheometry experiments were done as described above. 2.6.1. Studies inside the rheometer Linear temperature increments from 30 to 95 °C were carried out at two heating rates (1 and 10 °C/min), at a frequency 1 Hz, using a strain corresponding to the limit of linear viscoelasticy previously determined. Each experiment was done in quintuplicate. The non-isothermal kinetic data analysis was applied according to the procedure outlined by other researchers (Rhim et al., 1989a; Yoon et al., 2004). The rate of reaction can be evaluated through the following equation:

dC n ¼ kC dt

ð1Þ

where C is concentration, t is time, k is the reaction rate constant, and n the order of reaction. The effect of temperature was considered through the wellknown Arrhenius equation with time dependent temperature:

  Ea k ¼ k0 exp  RTðtÞ

ð2Þ 3. Results and discussion

where k0 is pre-exponential factor, Ea the activation energy (J/mol), R the universal gas constant (8.314 J/mol K), and T the absolute temperature (K). Under such conditions, and by combining Eqs. (1) and (2), the temperature dependent kinetics can be expressed as in Eq. (3):

Z

C

C0

dC ¼ k0 Cn

Z 0

t

  Ea dt exp  RTðtÞ

ð3Þ

When non-isothermal treatments are carried out by using a constant linear heating rate (A), from an initial temperature (T0), the temperature profile can be written as:

T ¼ T 0 þ At

ð4Þ

By differentiating Eq. (4), combining it with Eqs. (1) and (2), and providing the linear heating rate with time it yields (Rhim et al., 1989a; Yoon et al., 2004):

  1 dC Ea ¼ lnðk0 Þ  ln  n RT C dt

ð5Þ

The kinetic parameters in Eq. (5), k0 and Ea, can then be estimated from an Arrhenius-type plot of Eq. (5). 2.6.2. Studies outside the rheometer in a water bath Cooking experiments were carried out in an isothermal water bath (Forma Scientific circulating bath, ±0.05 °C) at 60, 70 and 80 °C, using hermetically sealed containers assimilated to infinite slab geometry (2 mm thickness, 100 mm wide and 120 mm length), heated from both sides. The containers consisted in two aluminum flat slabs (0.8 mm thick) with a rubber seal between them, in the periphery, secured by bolts and nuts. High vacuum grease (Dow Corning) was used over the rubber seal to provide a hermetic seal. Due to the small product thickness and the high thermal conductivity of aluminum, heating of the confined sample could take place very fast. Therefore, the lag time to reach the processing temperature was negligible. In the same way, boundary effects were also negligible due to the dimensions involved. The uncooked meat emulsion was processed in these containers for: 70, 140, 210, 280 and 350 s at 60 °C; 50, 100, 150, 200 and 250 s at 70 °C; and 18, 36, 54, 72 and 90 s at 80 °C. Two containers were processed for each time-temperature combination. After the processing time the cooking process was quenched by introducing the assembly in an ice-water bath (T  0 °C). After cooling, three disk samples from each container were cut into 25 mm disks using

3.1. Physical and chemical characteristics The results of the proximate analysis (expressed as g/100 g), of the cooked imitation-mortadella emulsion were (±standard deviation): moisture = 61.3 ± 0.2; protein = 12.6 ± 0.2; fat = 19.4 ± 0.6; ash = 2.80 ± 0.1; carbohydrates = 3.90 ± 0.9. Other physico-chemical characteristics were pH 6.50 ± 0.0; aw (26.3 °C) = 0.982 ± 0.003. 3.2. Viscoelastic evaluation of the uncooked meat emulsion The linear viscoelastic region was assessed by performing strain sweep tests. Fig. 2 shows average results from five strain sweep tests at low frequency (f = 1 Hz) obtained for the raw emulsion at room temperature (28 °C). As shown in Fig. 2 the storage modulus (G0 ) was greater than the loss modulus (G00 ) during the test, which indicates a predominant elastic behavior of the sample for the whole range of deformation experimented (0.1–10%). From Fig. 2, two regions can be identified, a linear region where G0 is greater than G00 and both remain almost constants (region I), and a non-lin-

10000 region II

region I

G´, G´´ (Pa)



a stainless steel sharp-edged round die. A total of six samples resulted for each time-temperature combination. The samples were covered on both sides with wax paper and kept in plastic bags to prevent dehydration until subjected to dynamic rheometry. The evolution of the storage modulus in the treated samples was followed by means of dynamic oscillatory frequency sweep tests at room temperature (28 °C), using a parallel plate geometry. In order to prevent the samples from slipping away, 25 mm diameter sand paper disks (Buehler Ltd., Lake Bluff, IL, USA) were stuck on both plate surfaces. Subsequently, the zero position of the upper plate was adjusted. The disk samples were placed over the lower plate of the geometry used in the experiment. Once each sample disk was confined between the parallel plates, frequency sweep tests (f = 1  100 rad/s) were performed at a constant strain amplitude of 0.8% within the linear viscoelastic region. From these tests, the storage modulus (G0 ) and the loss modulus (G00 ) were obtained for each sample as a function of frequency. The cooking kinetics was expressed by the storage modulus (G0 ) at a frequency of 1 Hz (6.28 rad/s). Eqs. (1) and (2) were employed to perform the kinetic analysis to the data collected by this approach.

1000

100 0.1

γlin

1

10

γ (%) Fig. 2. Strain sweep test for the imitation-mortadella emulsion at room temperature and at a frequency of 1 Hz (G0 : filled markers y G00 : open markers).

J.J. Ferris et al. / Journal of Food Engineering 95 (2009) 677–683

ear region where both moduli decreased with increasing deformation (region II). The linear viscoelastic limit was then defined from Fig. 2 at around 0.9%, this being the maximum strain (clin) for which the uncooked emulsion behaves as a linear viscoelastic material (dotted line in Fig. 2). Since this strain limit is below 5%, and a flowing tendency at high deformation values was observed, the uncooked meat emulsion can be referred as a weak gel (Ross-Murphy, 1995). In order to assure that the material response would always be within the linear viscoelastic region, a maximal strain value of 0.8% was chosen for further analysis. Values varying from 0.5% (at 1 Hz) in Bologna sausage (Bruno and Moresi, 2004) to 6% in heat gelation of beef myofibrils proteins (Samejima et al., 1985) have been reported. The evaluation of the viscoelastic properties of the uncooked meat emulsion with time was performed by frequency sweep tests. Fig. 3 shows average results from five frequency sweep tests carried out at room temperature and at a strain value of 0.8%. As can be seen in this figure, the curves showed a parallelism in their mechanical spectrum throughout the frequency range tested in this work (0.1–10 Hz), with G0 about one order of magnitude greater than G00 . Similar results have been obtained for cooked Bologna sausage by Bruno and Moresi (2004). Although gel classification from frequency sweep tests (as in the strain sweep tests) is controversial, due to the frequency dependence of G0 and G00 moduli (Ross-Murphy, 1995), plots of the complex modulus (G*) resulted in a slight dependence of frequency (not shown here) as indicated by Stading et al. (1995) for weak gels. Therefore, the uncooked meat emulsion employed in this work can be considered a weak gel from the frequency sweep test point of view. A frequency of 1 Hz was chosen in this work for further studies, as it is a common in polymer gelation studies, which make easier comparisons with earlier works. 3.3. Gelation of the imitation-mortadella emulsion under nonisothermal conditions

meat proteins. It has been widely recognized that myosin is the main component involved in the meat emulsion gelation mechanism (Samejima et al., 1981; Tornberg, 2005). Changes in G0 during heating of the meat emulsion from 30 °C to 95 °C at 1 and 10 °C/ min are presented in Fig. 4a and b, respectively. The shape of both curves was similar but that corresponding to 10 °C/min was shifted towards higher temperatures and also showed greater dispersion, due to the higher thermal lag occurring at that heating rate. Curves depicted in these figures show a typical behavior of thermosetting gelation in food systems similar to that considered in this work; such as corn starch (Brouillet-Fourmann et al., 2003), Alaska pollock and Pacific whiting surimi gels (Yoon et al., 2004), an in porcine myofibrillar protein (Westphalen et al., 2005). The initial decrease in G0 values shown in Fig. 4a and b could be related to the breaking of hydrogen bonds as temperature increases. It has been pointed out that unfolding of myosin, occurring around 56 °C (Tornberg, 2005), could also lead to an increase in fluidity of the semi-gel structure, disrupting some interactions between native proteins (i.e., highly folded), which would lead to a decrease in G0 values (Yoon et al., 2004). As can be seen in Fig. 4a and b, the gel starting points (as evidenced by the sudden increases in G0 ) were 55 °C and 60 °C when heating rates of 1 and 10 °C/min were employed, respectively. Further increases in G0 with temperature (after the point were gelation starts) have been attributed to a network formation by

a

4000

During meat emulsion cooking by heating, a great number of reactions take place, in which several mechanisms are involved (i.e., protein gelation, starch gelatinization, interactions among emulsion constituents, etc.). However in this type of product, observed macroscopic changes due to thermal processing (heating/ cooling) may be mainly related to the effect of temperature on the main constituents of the emulsion, i.e., meat, and particularly

3000

2000

1000

20

30

40

50 60 70 Temperature (ºC)

80

90

100

30

40

50 60 70 Temperature (ºC)

80

90

100

b

G' (Pa)

10000

G´ , G´´ (Pa)

7000 6000 5000

G' (Pa)

680

1000 1000

20

100 0.1

1 Frequency (Hz)

10

Fig. 3. Frequency sweep test for the imitation-mortadella emulsion at room temperature and strain amplitude 0.8% (G0 : filled markers y G00 : open markers).

Fig. 4. (a) Changes of the elastic modulus (G0 ) with temperature during gelation of the imitation-mortadella emulsion by effect of heating at 1 °C/min at a frequency of 1 Hz and 0.8% strain. (b) Changes of the elastic modulus (G0 ) with temperature during gelation of the imitation-mortadella emulsion by effect of heating at 10 °C/ min at a frequency of 1 Hz and 0.8% strain.

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10000

G' (Pa)

aggregation and entanglement of unfolded protein molecules through nonspecific hydrophobic and sulfhydryl–disulfide interactions, due to their more reactive conditions towards neighboring molecules (McSwiney et al., 1994; Aguilera and Rojas, 1997). Changes in the muscle protein conformation have been observed to occur within different temperature ranges such as: 54–58 °C for myosin, 65–67 °C for collagen and sarcoplasmic proteins, 75.6–78.4 for titin, and 80–83 °C for actin (Markowski et al., 2004; Tornberg, 2005). Additionally, contribution from the connective tissue proteins in increasing G0 during heating (Fig. 4a and b) must also be acknowledged, as collagen fibers contraction can occur up to a fourth part of their original length under heating at 60– 70 °C if they are unrestricted (Tornberg, 2005), as it would be expected in a high moisture content environment such as the meat emulsion considered here. At temperatures between 60 and 70 °C, the connective tissues and the muscle fibers are longitudinally contracted, and such a contraction increases with temperatures (Siripurapu et al., 1987). It is also worth mentioning the possible contribution, although probably in lesser degree, due to the proportion of these components present in the formulation, of the other ingredients different from meat proteins. Wheat starch gelatinization for example, a well-known phenomenon occurring in excess of water, is expected to increase rigidity of the gel in a temperature range of 50–70 °C for wheat starch (Rolee and Le Meste, 1999). Towards the end of heating stage (around 90 °C), a slight decrease in G0 can be observed in Fig. 4a and b, which may be attributed to a connective tissue and collagen softening at this stage, as observed by Siripurapu et al. (1987) in meat emulsion products. According to these authors, collagen can be dissolved and transformed into gelatin if fibers are not stabilized through heat-stable intermolecular bonds. However, meat fibers hardening may also occur during this late heating, and the occurrence of one or the other would depend on processing conditions upon heating. The behavior observed in Fig. 4a and b may also be related to melting of the remaining crystalline structure after starch gelatinization took place due to the extra energy supplied by the heating, causing a decrease in G0 value (Rolee and Le Meste, 1999). Besides the gelation mechanism discussed above, many of the changes found in this study could be described by the interaction of two or more components. According to some authors, egg albumen could negatively affect the gelation mechanism, however, others stated that this component could contribute to the formation of a stronger protein matrix (Carballo et al., 1995). On the other hand, mixtures of wheat starch and soy protein, could also affect the macroscopic changes of meat emulsions during heating, due to the highly hydrophilic behavior of soy protein, which could interrupt the formation of starch–protein complexes (Ryan and Brewer, 2005). Likewise, interactions between starch and lipids have been evidenced through a variety of rheological behaviors ranging from viscous weak gels to elastic strong gels (Eliasson and Kim, 1995). Regarding the effect of fat, Carballo et al. (1996) found that a reduction in this component resulted in an increase of the moisture content of the product, which led to a decrease in the protein concentration available for forming the gel resulting in a reduction of product firmness. Average minimum and maximum values of G0 obtained for the meat emulsion considered in this work at the two heating rates, at a frequency of 1 Hz, were statistically different from one another (p < 0.05). For a heating rate of 1 °C/min, these values were 930 ± 70 Pa and 5500 ± 450 Pa, respectively. Corresponding values for the 10 °C/min heating rate were 750 ± 80 Pa and 4500 ± 400 Pa, respectively. The maximum G0 value in both heating rates was reached at temperatures around 85 °C. The higher values of G0 at the lower heating rate could be due to the greater time availability for the aggregation mechanism, which has also

60 °C 70 °C 80 °C

1000 0

50

100

150 200 250 Processing time (s)

300

350

Fig. 5. Changes of the average elastic modulus (G0 ), measured at a frequency of 1 Hz and 0.8% strain with processing time for the imitation-mortadella emulsion heated in a thermal bath at different temperatures (shown in the graph). Dash lines represent the first order-kinetic fits at each temperature.

been reported in the literature for protein gels (Stading et al., 1995). 3.4. Gelation of the imitation-mortadella emulsion under isothermal conditions Samples treated in a water bath at temperatures of 60, 70, and 80 °C for different times were subjected to frequency sweep tests at room temperature and at a strain amplitude of 0.8%. Fig. 5 shows the variation of the elastic modulus (G0 ), registered at a frequency of 1 Hz and strain 0.8%, with processing times for samples previously heated in a thermal bath at temperatures of 60, 70 and 80 °C. Results shown in Fig. 5 indicate an exponential growth with a relatively high initial slope and a tendency to stabilize towards higher processing time at each temperature. An increase in G0 values with increasing temperature can also be observed in Fig. 5. Maximum values for G0 (G0max ) read from Fig. 5 at 60, 70, and 80 °C were 5700 ± 900 Pa, 7600 ± 2000 Pa, and 11500 ± 200 Pa, respectively. As expected these values are higher than those obtained in the dynamic studies inside the rheometer at their respective temperatures, as there was a further cooling phase after thermal treatment, which is closer to the real processing of this type of products. Tang et al. (1994) reported comparable values of G0 under similar conditions (heating at 80 °C followed by cooling at 20 °C) for whey protein concentrates, which were between 3700 and 12,800 Pa. Another work in whey protein concentrates has also shown values around 12,000 Pa after heating at 90 °C and cooling at 10 °C (Aguilera and Rojas, 1997). It is believed that the higher values of G0 obtained when previously heated samples are cooled, is due to the formation of hydrogen bonds between aggregates during cooling. Other explanations have been related to the partial regeneration of the denaturated proteins and fat solidification upon cooling (Aguilera and Rojas, 1997; Foegeding, 1988; Tang et al., 1994). On the other hand, minor ingredients contributing to the meat emulsion structural properties, such as carrageenan, may also be involved in the macroscopic behavior observed in these experiments. 3.5. Kinetic parameters during cooking 3.5.1. Non-isothermal kinetic study Due to the greater dispersion of the data obtained from kinetic experiments under linear temperature increments at a heating rate

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J.J. Ferris et al. / Journal of Food Engineering 95 (2009) 677–683 Table 1 First order-kinetic parameters for G0 changes (measured at 1 Hz and 0.8% strain), in an imitation-mortadella emulsion after heating in a thermal bath at different temperatures.

-1.5

Y = ln [(1/G')(dG'/dt)]

-2.0 -2.5 -3.0

Temperature (°C)

Reaction rate constant k (min1)

Stabilized G0 value G01 (Pa)

R2

60 70 80

0.649 0.811 0.828

5530 7420 16580

0.930 0.959 0.981

-3.5 -4.0 -4.5 -5.0

2.80

2.85

2.90 2.95 3 -1 1/T (x10 K )

3.00

3.05

Fig. 6. Arrhenius-type graph for the changes in G0 during gelation of the imitationmortadella emulsion heated in a linear temperature increment protocol of 1 °C/min and at a frequency of 1 Hz and 0.8% strain (Y = ln [(1/G0 ) (dG0 /dt)]).

of 10 °C/min, only the results obtained at a heating rate of 1 °C/min were considered for kinetic parameter determinations. Results from changes in G0 during heating of the meat emulsion at 1 °C/ min at a frequency of 1 Hz, in the range where important changes occurred (Fig. 4a), i.e., 60–90 °C, were adjusted to Eq. (5). In so doing an exponent of n = 1 better described those changes. A first order reaction (i.e., n = 1) has been previously reported in non-isothermal experiments with linear temperature increments in the acid-catalyzed hydrolysis of sucrose (Rhim et al., 1989a) and the kinetics of color changes of grape juice (Rhim et al., 1989b). For the gelation of surimi under non-isothermal conditions an exponent of n = 2 was reported by Yoon et al. (2004). Fig. 6 shows the Arrhenius-type graph representing changes in G0 (average of five runs) during gelation of the meat emulsion. A significant linear regression (a = 0.05) was obtained with R2 = 0.990. The activation energy was calculated from the slope and the pre-exponential factor from the intercept, according to Eq. (5). The following values were obtained: Ea = 130 ± 10 kJ/mol, ln k0 = 48.7 ± 3.7 s1; for the gelation of the meat emulsion heated at 1 °C/min and a frequency of 1 Hz, for the temperature range of 60–90 °C. Yoon et al. (2004) found that the activation energy for thermal gelation of surimi increased with moisture content of the gel, and reported a variation of 172.8–232.9 kJ/mol, for moisture contents between 80% and 95%, respectively. The activation energy obtained in this work was closed to the lower value of the range obtained for these researchers, bearing in mind that the moisture content of the imitation-mortadella emulsion was 61.3%. Other values of activation energy varying between 118 and 179 kJ/mol have been found in gelation of xanthan and carob mixtures by Yoon and Gunasekaran (2000). Wagner and Añon (1985) reported Ea values between 228 and 379 kJ/mol for myosin denaturation, while a range of 247–515 kJ/mol was obtained for the heat gelation of beef myofibril proteins (Samejima et al., 1985). On the other hand, activation energies of 133 and 76 kJ/mol for temperatures of 60–75 °C and 75–100 °C, respectively, have been reported for wheat starch gelatinization (Turhan and Gunasekaran, 2002). According to the values found in the literature, the activation energy values reported in this work were in the range of those reported previously for gelation mechanisms. The average value obtained for ln k0, on the other hand, was also within those reported in the literature, which vary from 27.9 to 55.8 s1 (Wagner and Añon, 1985; Rhim et al., 1989a,b).

3.5.2. Isothermal kinetic study Due to the exponential variation of G0 with processing time in the thermal bath at each considered temperature (Fig. 5), kinetic data were adjusted to a first order reaction, as previously found in the literature for gelation of polymeric food systems (Beveridge et al., 1984; Wu et al., 1991; Aguilera and Rojas, 1997). For kinetic modeling an equilibrium G0 has to be defined (Wu et al., 1991). A storage modulus stabilization value (G01 ), obtained from the regression model, was chosen as pseudo-equilibrium storage modulus, since it was experimentally observed that G0 showed very little changes towards the end of the experiment. Additionally, an initial value at the beginning of the gelation mechanism (G00 ) was also chosen as the uncooked meat emulsion exhibited a defined value at time zero. The G0 value was estimated from the integrated form of Eq. (2), making the following substitutions:

C¼



1

G0 G01



  G0 C 0 ¼ 1  00 G1

ð6Þ ð7Þ

After making the appropriate changes, the kinetics of G0 at constant temperature was expressed as:

G0 ¼ G00 þ ðG01  G00 Þð1  expðktÞÞ

ð8Þ

An initial value at the beginning of the gelation mechanism (G00 ) of 1000 Pa was assumed. The selection of this value was done considering the results obtained from the viscoelastic domain determination, as well as from the minimum G0 values observed during the non-isothermal kinetic studies (i.e., where gelation began). A non-linear regression statistical analysis of the experimental data was performed, using StatisticaTM version 6.0 (StatSoft, Inc., Tulsa, OK). Values obtained for k and G01 , at each considered temperature are presented in Table 1. Dash lines in Fig. 5 represent the fitting at each temperature. According to van Boekel (1996) in complex systems, such as foodstuff, the reaction order concept is merely empiric, i.e., it is a mathematical tool that allows the experimental data to be described, without giving a mechanistic insight into the reaction. Nonetheless, in the case considered here, if changes observed in G0 at macroscopic level may be associated to the creation of interactions by heating/cooling effect at molecular level, then it is plausible that with each formed bond the amount of available sites for the new bonds formations decreased. Consequently, the rate of reaction would depend on how much the reaction has proceeded, which is a typical first order reaction, described by the parameters presented in Table 1. 4. Conclusions Although, establishing a mechanism for the gelation process of the meat emulsion considered in this work is a difficult task due to the complexity of its composition, results presented here are useful to describe the macroscopic behavior of the imitation-mortadella emulsion during heat treatment. By means of oscillatory rheometry, it was possible to determine the kinetics of thermal gelation

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of an imitation-mortadella emulsion by using both, isothermal and non-isothermal treatment. Non-isothermal experiments carried out at 1 °C/min and 10 °C/min indicated an initial decrease on the storage modulus, with a further increase at temperatures greater than around 55 °C and 60 °C, respectively; and a slight final decrease at around 90 °C. The form of both curves was explained in terms of the different referenced literature changes occurring in meat emulsions, such as the imitation-mortadella considered here. An exponential growth was obtained for the samples previously treated in a thermal bath under isothermal conditions. A first order-kinetic equation was found to better describe the variation of the storage modulus G0 (at 1 Hz and strain amplitude of 0.8%) during the two types of thermal treatments. Under non-isothermal conditions, an Arrhenius-type model was able to describe the effect of temperature during gelation of the imitation-mortadella emulsion.

References Aguilera, J.M., Rojas, G.V., 1997. Determination of kinetics of gelation of whey protein and cassava starch by oscillatory rheometry. Food Research International 30 (5), 349–357. AOAC, 1990. Official Methods of Analysis of the Official Analytical Chemists. Washington, DC. Beveridge, T., Jones, L., Tung, M.A., 1984. Progel and gel formation and reversibility of gelation of whey, soybean, and albumen protein gels. Journal of Agriculture and Food Chemistry 32 (2), 307–313. Bou, L., Ascanio, N., Hernández, P., 2004. Diseño de un plan de análisis de peligros y puntos críticos de control (HACCP) para el aseguramiento de la inocuidad de la mortadela elaborada por una empresa de productos cárnicos. Archivos Latinoamericanos de Nutrición 54 (1), 72–79. Brouillet-Fourmann, S., Carrot, C., Mingnard, N., 2003. Gelatinization and gelation of corn starch followed by dynamic mechanical spectroscopy analysis. Rheologica Acta 42 (1–2), 110–117. Bruno, M., Moresi, M., 2004. Viscoelastic properties of Bologna sausage by dynamic methods. Journal of Food Engineering 63 (3), 291–298. Carballo, J., Barreto, G., Jiménez-Colmenero, F., 1995. Starch and egg white influence on properties of bologna sausage as related to fat content. Journal of Food Science 60 (4), 673–677. Carballo, J., Fernández, P., Barreto, G., Solas, M.T., Jiménez-Colmenero, F., 1996. Characteristics of high- and low-fat Bologna sausages as affected by internal cooking temperature and chilling storage. Journal of the Science of Food and Agriculture 72 (1), 40–48. Correia, L.R., Mittal, G.S., 1991. Gelation kinetics of meat emulsions containing various fillers during cooking. Journal of Food Process Engineering 14 (1), 35– 49. Covenin 1315, 1979. Alimentos. Determinación del pH (Acidez iónica). Fondonorma, Caracas, Venezuela. Covenin 1219, 2000. Carne y productos cárnicos. Determinación de grasa total. Fondonorma, Caracas, Venezuela. Egelandsdal, B., Fretheim, K., Harbitz, O., 1986. Dynamic rheological measurements on heat-induced myosin gels: an evaluation of the method’s suitability for the filamentous gels. Journal of the Science of Food and Agriculture 37 (9), 944–945. Eliasson, A.C., Kim, H.R., 1995. A dynamic rheological method to study the interaction between starch and lipids. Journal of Rheology 39 (6), 1519–1534.

683

Foegeding, E.A., 1988. Thermally induced changes in muscle proteins. Food Technology 42 (6), 58–64. Li, J.Y., Yeh, A.I., 2003. Effects of starch properties on rheological characteristics of starch/meat complexes. Journal of Food Engineering 57 (3), 287–294. Markowski, M., Bialobrzewski, I., Cierach, M., Paulo, A., 2004. Determination of thermal diffusivity of Lyoner type sausages during water bath cooking and cooling. Journal of Food Engineering 65 (4), 591–598. McSwiney, M., Singh, H., Campanella, O.H., 1994. Thermal aggregation and gelation of bovine b-lactoglobulin. Food Hydrocolloids 8 (5), 441–453. Rhim, J.W., Nunes, R.V., Jones, V.A., Swartzel, K.R., 1989a. Determination of kinetic parameters using linearly increasing temperature. Journal of Food Science 54 (2), 446–450. Rhim, J.W., Nunes, R.V., Jones, V.A., Swartzel, K.R., 1989b. Kinetics of color change of grape juice generated using linearly increasing temperature. Journal of Food Science 54 (3), 776–777. Rolee, A, Le Meste, M., 1999. Effect of moisture content on thermomechanical behavior of concentrated wheat starch–water preparations. Cereal Chemistry 76 (3), 452–458. Ross-Murphy, S.B., 1995. Structure-property relationships in food biopolymer gels and solutions. Journal of Rheology 39 (6), 1451–1463. Ryan, K.J., Brewer, M.S., 2005. Purification and identification of interactin components in a wheat starch–soy protein system. Food Chemistry 89 (1), 109–124. Samejima, K., Ishiroshi, M., Yasui, T., 1981. Relative roles of the head and tail portions of the molecule in heat-induced gelation of myosin. Journal of Food Science 46 (5), 1412–1418. Samejima, K., Egelensdal, B., Fretheim, K., 1985. Heat gelation properties and protein extractability of beef myofibrils. Journal of Food Science 50 (6), 1540– 1555. Siripurapu, S.C.B., Mittal, G.S., Blaisdell, J.L., 1987. Textural and viscoelastic characteristics of meat emulsion products during cooking. Lebensmittel Wissenschaft und Technologie 20 (1), 68–73. Stading, M., Langton, M., Hermansson, A.M., 1995. Small and large deformation studies of protein gels. Journal of Rheology 39 (6), 1445–1450. Tang, Q., McCarthy, O.J., Munro, P.A., 1994. Oscillatory rheological comparison of the gelling characteristics of egg white, whey protein concentrates, whey protein isolate, and b-lactoglobulin. Journal of Agriculture and Food Chemistry 42 (10), 2126–2130. Tornberg, E., 2005. Effects of heat on meat proteins – implications on structure and quality of meat products. Meat Science 70 (3), 493–508. Turhan, M., Gunasekaran, S., 2002. Kinetics of in situ and in vitro gelatinization of hard and soft wheat starches during cooking in water. Journal of Food Engineering 52 (1), 1–7. Van Boekel, M.A.J.S., 1996. Kinetic modelling of sugar reactions in heated milk-like systems. Netherlands Milk Dairy Journal 50 (2), 245–266. Varnam, A.H., Sutherland, J.P., 1995. Meat and Meat Products. Chapman & Hall, Great Britain. p. 417. Wagner, J.R., Añon, M.C., 1985. Denaturation kinetics of myofibrillar proteins in bovine muscle. Journal of Food Science 50 (6), 1547–1563. Westphalen, A.D., Briggs, J.L., Lonergan, S.M., 2005. Influence of pH on rheological properties of porcine myofibrillar protein during heat induced gelation. Meat Science 70 (2), 293–299. Wu, J.Q., Hamann, D.D., Foegeding, E.A., 1991. Myosin gelation kinetic study based on rheological measurements. Journal of Agricultural and Food Chemistry 39 (2), 229–236. Yoon, W.B., Gunasekaran, S., 2000. Evaluation of structure development during gelation of xanthan and carob mixture. In: Yoon, W.B., Gunasekaran, S., Park, J.W., 2004. Journal of Food Science, 69 (7), 338–343. Yoon, W.B., Gunasekaran, S., Park, J.W., 2004. Characterization of thermorheological behavior of Alaska Pollok and Pacific Whiting surimi. Journal of Food Science 69 (7), 338–343.