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Rheological properties of commercial mustards
Journal of Food Engineering 63 (2004) 209–217 www.elsevier.com/locate/jfoodeng
Rheological properties of commercial mustards Lesław Juszczak b
a,*
, Mariusz Witczak b, Teresa Fortuna a, Agnieszka Banys a
a Department of Analysis and Evaluation of Food Quality, University of Agriculture, Balicka 122 Street, 30-149 Krakow, Poland Department of Engineering and Machinery for Food Industry, University of Agriculture, Balicka 122 Street, 30-149 Krakow, Poland
Received 25 March 2003; accepted 24 July 2003
Abstract Seven commercial mustards, made by different manufacturers, were studied to examine their physicochemical and rheological properties and to establish relationships between those properties. Physicochemical analysis revealed distinct differences between the mustards in the dry matter and extract contents and smaller differences in the protein, fat and ash levels. Two of the investigated mustards did not satisfy the requirements of the relevant Polish standard regarding dry matter content. Rheological studies included determination of the following: (a) flow curves with a controlled shear rate or shear stress, (b) curves showing the time and temperature dependence of apparent viscosity, and (c) mechanical spectra. Apparent viscosity and complex viscosity were correlated using the generalised Cox–Merz rule. The rheological investigations confirmed that mustard is a pseudoplastic fluid exhibiting a yield stress, thixotropy and viscoelastic (with dominant elastic) properties. Statistical analysis of the results showed significant linear correlations between the dry matter, fat, protein and ash contents of mustards and some parameters of rheological models. 2003 Elsevier Ltd. All rights reserved. Keywords: Mustard; Rheology
1. Introduction Mustard, a pungent, spicy-tasting paste used as a condiment, is made from party deoiled mustard flour and/or mustard seeds, water, food acids, vinegar, salt, sugar and flavouring additives (PN-A-86964, 1997). The popularity of this condiment dates back to the Middle Ages when the manufacturers of Dijon in Central France started to produce mustard (Moutarde de Dijon) on a larger scale. Much earlier, already in ancient times, ground seeds of the mustard plant were used to season meat dishes. In physical terms, mustard is a suspension with solid particles smaller than 30 lm suspended in a continuous water phase (Gerhards & Schubert, 1993). When stored, mustard exhibits syneresis which lowers its quality and sensory acceptance. This phenomenon is particularly pronounced at a high (e.g. 40 C) storage temperature (Aguilar, Rizvi, Ramirez, & Inda, 1991b; Gerhards & Schubert, 1993). Gerhards and Schubert (1996) distin-
*
Corresponding author. Fax: +48-12-626-58-41. E-mail address: [email protected] (L. Juszczak).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.07.002
guished two independent kinds of syneresis: gravitational syneresis and syneresis by shrinking. Those disadvantageous phenomena may be reduced without using any food additives, simply by employing native mustard mucilage, which, however, requires a two-step manufacturing process (Gerhards & Schubert, 1996; Gerhards & Walker, 1997). To assess the quality of semi-solid food products, it is necessary to understand their rheological properties. It is known that mustard, a semi-solid food, exhibits a non-Newtonian, pseudoplastic flow with a distinct yield stress and thixotropy (Aguilar, Rizvi, Ramirez, & Inda, 1991a, 1991b; Bhattacharya, Vasudha, & Krishna Murthy, 1999; Gerhards & Schubert, 1993; Yoo, Rao, & Steffe, 1995). As shown by Aguilar et al. (1991a), the rheological properties of processed mustard are determined by the size of solid phase particles. Those properties can be created or modified by controlling the particle size distribution (milling degree) in the manufacturing process. The rheological properties of mustard are also influenced by its dry matter content, the presence of oil fraction and the addition of thickeners (Aguilar et al., 1991b; Bhattacharya et al., 1999; Gerhards & Schubert, 1993).
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The rheology of mustard has also been investigated by using back extrusion and imperfect squeezing flow methods (Brusewitz & Yu, 1996; Suwonsichon & Peleg, 1999). The latter method deserves special attention since it offers the opportunity to test samples without affecting their structure before measurement (e.g. stress caused by placing a sample in a gap between measuring cylinders can be avoided) and to investigate samples with suspended large particles of spices or mustard seeds. The present study was aimed to reveal differences in the rheological properties of Polish commercial mustards and to find statistically significant correlations between the rheological and the physicochemical properties of the products.
2. Materials Seven commercial mustards not containing mustard seeds, seed pieces or spices have been made in Poland by different manufactures. According to the composition information from the labels, all products have been manufactured without thickening agents. Samples of mustards were marked with symbols MP1 to MP7. In the period of analyses the samples were stored at room temperature and prior to measurement they were gently stirred for homogenisation.
The following curves were obtained: 1. Flow curves with a controlled shear rate (CSR) at a temperature of 25 ± 0.2 C; the shear rate was increased from 1 to 600 s1 during 10 min, remained constant at 600 s1 during 2 min, and was decreased from 600 to 1 s1 during 10 min. The experimental data were described by the Casson model with n ¼ 0:25, which is more suitable for pseudoplastic dispersion solutions exhibiting a yield stress than the model with the exponent 0.50 (Rao & Tattiyakul, 1999), and by the power law model. The area of the thixotropy hysteresis loop was also determined. 2. Flow curves with a controlled shear stress (CSS) in the range of 0–10 Pa, during 10 min at a temperature of 25 ± 0.2 C. The experimental data were described by the Herschel–Bulkley model. 3. Curves showing the time-dependent behaviour of shear stress in the time range of 0–60 min at a temperature of 25 ± 0.2 C with a constant shear rate of 100 s1 . The experimental data were described by the Weltman model (Rao, 1999). 4. Curves showing the temperature-dependent behaviour of apparent viscosity over the temperature range of 8–30 C. The experimental data were described by the Arrhenius equation (Rao, 1999). 3.3. Viscoelastic properties
3. Methods 3.1. Physicochemical properties The dry matter content of mustards was determined in accordance with the relevant Polish Standard (PN-90/ A-75101/03, 1990), extract content––by refractometry (PN-90/A-75101/02, 1990), protein content (N · 6.25)–– by Kjeldahl’s method, fat content––by Soxhlet’s method with petroleum ether as a solvent (after hydrolysing the sample with a 2M HCl solution), and ash content––according to PN-90/A-75101/08 (1990). A one-way analysis of variance and Duncan’s test (at p ¼ 0:05) were used to establish the significance of differences in the physicochemical properties between the mustard samples studied. 3.2. Rheological properties 3.2.1. Flow behaviour Determinations were conducted using a rotational rheometer Rheolab MC 1 (Physica Messtechnik, Germany) in a system of coaxial cylinders (bob diameter 25 mm, cup diameter 27.12 mm). A mustard sample was placed in a measuring element and then held in a thermostat during 5 min in order to achieve temperature equilibrium and stress relaxation.
Measurements were performed using an oscillatory rheometer RheoStress RS 150 (Haake, Germany) at a temperature of 25 ± 0.2 C in the cone–plate system (cone diameter 35 mm, angle 2, gap size 0.105 mm). Mechanical spectra were plotted over a frequency range of 0.1–10 Hz at a constant strain amplitude of 0.1% (within the range of linear viscoelasticity). The experimental data were described by the following equations: 0
G0 ¼ K 0 xn
00
G00 ¼ K 00 xn
jg j ¼ K xn1 where G0 is storage modulus, G00 ––loss modulus, jg j–– complex viscosity, and K, K 0 , K 00 , n, n0 and n00 are parameters determined experimentally. Correlations between the values of complex viscosity, jg j, and those of apparent viscosity, ga , were established by using the generalised Cox–Merz rule that is known to hold for heterogenous dispersions and commercial foods (Bistany & Kokini, 1983; Gunasekaran & Ak, 2000; Rao & Tattiyakul, 1999): a
jg jðxÞ ¼ C½ga ðc_ Þ jx¼c_ where C and a are parameters determined experimentally.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 250 Shear rate [Pa]
To find relationships between the physicochemical and the rheological parameters of mustards, the values of linear correlation coefficients were calculated and their significance was examined by Student’s t-test at p ¼ 0:05.
211
200 150 100 50 0
4. Results and discussion
0
100
200
300
400
500
600
Shear rate [s-1]
4.1. Physicochemical properties
MP1
4.2. Rheological properties 4.2.1. Flow behaviour Examples of flow curves with a CSR are shown in Fig. 1, and with a CSS––in Fig. 2. The parameters of the rheological models employed to describe the experimental data and the values of the thixotropy hysteresis loop area are given in Table 2. The models used differed in their fit to the data. The Casson model with the exponent 0.25 exhibited a better fit than the traditional equation with the exponent 0.5. The same result was
MP3
MP5
Fig. 1. Flow curves (CSR mode) of mustard.
200 Shear stress [S-1]
The results of the physicochemical analysis of mustard samples are shown in Table 1. The mustards had a dry matter content in the range of 16.82 g/100 g (sample MP7) to 29.25 g/100 g (sample MP3). Mustards MP6 and MP7 failed to satisfy the requirements of the Polish Standard (PN-A-86964, 1997), which provide that the dry matter content of a mustard should not be less than 20 g/100 g, and mustard MP4 had the amount of dry matter close to the minimum value specified by that standard. The results for extract followed the same pattern: the sample which contained the smallest amount of dry matter (MP7) had also the lowest extract content (Table 1). The protein content ranged from 3.91 g/100 g for sample MP1 to 6.46 g/100 g for sample MP3. Mustard MP3, again, had the highest fat and ash contents, while MP7 contained the smallest amount of fat and MP2––of ash. The differences between samples were slighter for fat and ash than for other investigated properties.
MP2
160 120 80 40 0 0
20
40 60 Shear stress [Pa] MP2
MP3
MP4
80
100
MP5
Fig. 2. Flow curves (CSS mode) of mustard.
achieved by Rao and Tattiyakul (1999) for heated dispersions of tapioka starch. The power law model turned out to be best fitted to the experimental data (R2 > 0:98) but that model does not take account of yield stress which is a very important rheological parameter. For the Herschel–Bulkley model the degree of fit (R2 > 0:94) was poorer than that obtained by Bhattacharya et al. (1999). Nonetheless, this model is recommended by some authors for describing the flow curves of mustard (Aguilar et al., 1991a; Bhattacharya et al., 1999; Gerhards & Schubert, 1993). Although all investigated mustards exhibited non-Newtonian, pseudoplastic flow with a yield stress and a thixotropy hysteresis loop (Figs. 1 and 2), which confirms earlier observations on the rheology of mustards (Aguilar et al., 1991a; Bhattacharya et al., 1999; Gerhards & Schubert, 1993), there were
Table 1 Physicochemical properties of mustards Sample
Dry matter [g/100 g]
Extract content [g/100 g]
Protein content [g/100 g]
Fat content [g/100 g]
Ash content [g/100 g]
MP1 MP2 MP3 MP4 MP5 MP6 MP7
24.08 25.24 29.25 19.93 22.36 18.93 16.82
23.62 20.93 23.82 15.55 20.13 13.23 11.16
3.91 4.35a 6.46 5.12 4.37a 4.66 4.34a
4.53a 4.47ab 5.74 4.78 4.37ab 4.30ab 4.27b
3.10 2.56 3.22 2.92a 2.76b 2.82ab 2.69b
Values followed by the same letter in the same column are not significantly different (p ¼ 0:05). * Mean value of two repetition. ** Mean value of three repetition.
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Table 2 Rheological parameters of models describing flow curves of mustards Sample
CSR mode Casson model
MP1 MP2 MP3 MP4 MP5 MP6 MP7
S [Pa/ (s cm3 )]
Power law model
s0C [Pa]
gC [Pa s]
R2
K [Pa sn ]
n
R2
15.05 31.19 31.07 18.75 22.13 27.69 23.02
0.0042 0.0036 0.0106 0.0030 0.0044 0.0057 0.0041
0.9879 0.9658 0.9587 0.9920 0.9837 0.9711 0.9796
17.30 33.85 35.91 20.88 24.90 31.59 26.65
0.28 0.24 0.29 0.26 0.26 0.26 0.26
0.9973 0.9945 0.9888 0.9931 0.9966 0.9929 0.9973
374 724 1845 520 463 411 475
CSS mode Herschel–Bulkley model s0HB [Pa]
K [Pa sn ]
n
R2
3.04 1.69 2.45 3.12 3.48 4.21 0.26
22.97 17.33 24.42 18.63 19.91 23.71 21.11
0.36 0.36 0.35 0.35 0.37 0.33 0.32
0.9763 0.9684 0.9941 0.9673 0.9545 0.9445 0.9876
s0 ––yield stress; gC ––Casson plastic viscosity; K––consistency coefficient; n––flow behaviour index; S––area of thixotropy hysteresis loop.
some differences between samples. Sample MP3 showed the greatest values of shear stresses in the given range of shear rates (Fig. 1), a high value of the Casson yield stress (together with MP2), markedly highest Casson’s plastic viscosity, and the highest consistency coefficient and flow behaviour index from the power law model. It also had the largest area of the thixotropy hysteresis loop (Table 2). The rheological parameters of this sample correlated well with its physicochemical features (Table 1). The other mustards (except MP2) exhibited smaller values of Casson’s yield stress and much lower values of Casson’s plastic viscosity (Table 2). The consistency coefficients of the samples were between 17.30 Pa sn (MP1) and 35.91 Pa sn (MP3). The flow behaviour indexes ranged from 0.24 (MP2) to 0.29 (MP3) with the values for samples MP4 to MP7 being equal (Table 2). According to Gerhards and Schubert (1993), beyond the yield stress, the flow curves become flatter with increasing shear rate, as is the case of the curves shown in the present study. This can be attributed to the breakdown of the inner structure of fluid which is formed through physical interactions between molecules. As the shear rate increases, those forces weaken and the molecules orientate themselves along the flow lines, which causes a drop in viscosity. Examples of flow curves obtained at a constant shear stress are shown in Fig. 2. Individual mustard samples achieved the maximum assumed shear stress at different values of shear rate. Sample MP3 exhibited the greatest resistance to the shear stress applied, which corresponds with the highest values of shear stresses obtained for this sample in the CRS mode (Fig. 1). As compared to the Casson model, using the model of Herschel–Bulkley for the description of CSS flow curves produced much lower values of yield stress: from 0.26 Pa (sample MP7) to 4.26 Pa (sample MP6). The value of yield stress, one of the parameters characterising the properties of semi-solid food products, is of importance to the technological process (e.g. pumping) as well as to the perception of the quality of a product by the consumers. According to Bhattacharya et al. (1999), experimental methods (e.g. stress relaxation
method, stress–strain plot) give more reliable results than the extrapolation of flow curves and the use of rheological equations. In the case of the latter, the application of the CSS mode is more sensitive and leads to more correct results (Bhattacharya et al., 1999). Due to the variety of test methods and rheological models, the results on the yield stress of mustard, cited in the literature, greatly differ and range from about 0.5–1 Pa (Bhattacharya et al., 1999) to 28–49 Pa (Gerhards & Schubert, 1993). Much higher values were obtained by Campanella and Peleg (1987) who tested mustard with a squeezing flow method. Also Yoo et al. (1995) who studied mustards by using a vane method found greater values of their yield stress: 83–104 Pa (static yield stress) and 53–70 Pa (dynamic yield stress); those values differed depending on which parameter was controlled during the experiment, shear rate or shear stress. According to the latter authors, determination of yield stress by a vane method is simpler, unequivocal and more sensitive in the CSR mode. The consistency coefficient had lower values for the Herschel–Bulkley model (CSS mode) than for the power law model (CSR mode); among all samples, mustard MP3 showed in both cases the highest value of that coefficient. The values of the flow behaviour index, which also are indicative of a considerable pseudoplasticity of mustard, were greater for the Herschel–Bulkley model than for the power law model. This was probably due to the different fit of the two models to the experimental data and to the fact that the power law model disregards yield stress. In addition, the two experiments used different ranges of shear rate: the maximum value of shear rate was 600 s1 for the CSR mode and about 190 s1 for the CSS mode. According to Suwonsichon and Peleg (1999), such low values of the flow behaviour index may also result from slippage at the walls of the cylinder, especially in the range of higher shear rates. The values of the consistency coefficient and flow behaviour indexes obtained in this study (Table 2) are similar to the data reported by such authors as Dickie and Kokini (1983) and Steffe (1996) but differ from the results of Hamza-Chaffai (1991), who recorded much
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217
4.2.2. Time-dependent behaviour Fig. 3 shows the shear stress vs. shear time curves obtained at a constant shear rate (100 s1 ) for chosen
180 Shear stress [Pa]
lower values of the consistency coefficient, and Gerhards and Schubert (1993), who obtained higher values of that coefficient, in the range 57–62 Pa sn . The values of the two parameters mentioned depend on the temperature of measurement, the range of shear rate or stress and the kind of sample. According to Aguilar et al. (1991a), one of the most important factors determining the rheological properties of mustard is the milling degree of mustard seeds which affects the size of solid particles in the final product. Gerhards and Schubert (1993) found that the stirring of a sample before measurement leads to a drop in the value of both yield stress and flow behaviour index but practically does not influence the value of the consistency index. The parameters in question are also affected by the time and conditions of the storage of samples (Aguilar et al., 1991a, 1991b; Gerhards & Schubert, 1993). As claimed by Bhattacharya et al. (1999), the water content of mustard has an effect on its rheological properties: the higher it is, the lower is the value of yield stress and the consistency coefficient and the greatest is the value of the flow behaviour index. All the tested mustards showed thixotropy. The intensity of this phenomenon may be measured by the area contained between the flow curves obtained with increasing and decreasing shear rate (Gerhards & Schubert, 1993). As can be seen from Table 2, the area of the thixotropy hysteresis loop was the largest for mustard MP3 and the smallest for MP1. Gerhards and Schubert (1993) also observed the presence of distinct thixotropy hysteresis whose magnitude depended on the way the sample was treated before measurement. The thixotropy phenomenon has a practical significance: a low viscosity of sol is advantageous when a product is mixed with another substances or removed from the container. On the other hand, when the structure of the suspension after mixing is being rebuilt too slowly, sedimentation of the suspended particles cannot be avoided.
213
160 140 120 100 80 60 40 0
10
20
30
40
50
MP3
MP4
MP5
MP6
Fig. 3. Time-dependent behaviour curves of mustard.
samples, and Table 3 lists the parameters of the Weltman model used to describe the experimental data. That model was found to be relatively well fitted (R2 > 0:97) to the data. The shear stress had the greatest values for mustard MP3 and the lowest for MP4 and MP1, which corresponds with the values of the parameter A of Weltman’s model (Table 3) that reflects the value of shear stress in the first second of the test. For sample MP3 the highest values of shear stresses correlate well with the highest values of physicochemical parameters (Table 1). MP3 had also the greatest value of the parameter B of Weltman’s model (coefficient of thixotropic breakdown), that in turn correlates with the largest area of the thixotropy hysteresis loop (Table 2). Other mustards, having much lower values of the coefficient B (Table 3), exhibited also considerably smaller areas of the thixotropy hysteresis loop (Table 2). A strong linear correlation (0.9853) was found between the two parameters mentioned. According to Aguilar et al. (1991a, 1991b), the values of the Weltman’s model parameters depend on the size distribution of solid particles suspended in the continuous phase. 4.2.3. Temperature-dependent behaviour Fig. 4 shows some examples of apparent viscosity vs. temperature curves. It can be seen that the course of the curves varies according to the mustard sample. The values of apparent viscosity in the studied temperature range were the highest for mustard MP6 and the lowest for MP2. The parameters of the Arrhenius model, which
Table 3 Rheological parameters of Weltman and Arrhenius models Sample
MP1 MP2 MP3 MP4 MP5 MP6 MP7
Weltman model
Arrhenius model
A [Pa]
B
R
g1 [Pa s] · 103
E [kJ/mol]
R2
85.63 122.10 203.05 85.81 101.38 129.03 118.50
4.53 6.05 13.56 5.19 3.98 5.35 5.39
0.9979 0.9794 0.9926 0.9925 0.9922 0.9873 0.9962
6.33 14.06 0.30 0.88 5.92 9.75 9.21
16.37 13.62 23.51 20.61 16.41 15.38 14.95
0.9864 0.9680 0.9883 0.9705 0.9845 0.9749 0.9909
2
60
Time [min]
A––value of stress at first sec. of test; B––coefficient of thixotropic breakdown; g1 ––frequency factor (material constant); E––activation energy.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 10000
9 Storage modulus [Pa] Loss modulus [Pa]
Apparent viscosity [Pa s]
214
8 7 6 5 4
1000
100
3 8
10
12
14
16
18 20 22 Temperature [°C]
MP2
MP3
MP4
24
26
28
1
30
1 Frequency [Hz] MP1
MP6
MP4
MP5
10 MP6
Fig. 5. Mechanical spectra of mustard. G0 ––empty markers, G00 ––filled markers.
Fig. 4. Temperature-dependent behaviour curves of mustard.
was used to describe the experimental data, are given in Table 3. The values of flow activation energy indicate that mustard samples had different sensitivity to the increase in temperature. According to Steffe (1996), a system is the more sensible to temperature changes, the higher is the value of flow activation energy. It is worth noticing that sample MP3 had the greatest flow activation energy and the lowest material constant (Table 3). At a temperature of 8 C it exhibited a high apparent viscosity which considerably decreased during heating, to about 3.5 Pa s (Fig. 4). Sample MP6, which also had a high apparent viscosity at 8 C, showed a markedly smaller drop in viscosity with increasing temperature. The smallest decrease in apparent viscosity was characteristic of sample MP2 having the lowest apparent viscosity at 8 C. This corresponded with the lowest value of flow activation energy and the highest value of material constant––the parameters of the Arrhenius equation.
samples in the frequency range of 0.1–10 Hz. In this range the values of the storage modulus (G0 ) were greater than those of loss modulus (G00 ) for all the mustards studied, the ratio G00 =G0 being about 0.22–0.28. This testifies to the dominance of elastic properties over viscous ones. A similar course of mechanical spectra, but at higher values of G0 and G00 , was observed by Gerhards and Schubert (1993). The spectra shown in Fig. 5 do not resemble those of strong gels: the storage modulus depends on frequency and the differences between the values of the two moduli are less than one order of magnitude. Among the samples studied, MP6 had the greatest values of G0 and G00 moduli and MP4 the smallest. The dependence of G0 and G00 on frequency (x) was described by the power law function which showed a good fit to the experimental data (R2 > 0:98). Even better fit (R2 > 0:99) was found for the power law function describing the relationship between complex viscosity and frequency. The parameters of the relevant equations are listed in Table 4. The values of parameters K 0 , K 00 and K correspond with the values of G0 , G00 and jg j, respectively, at x ¼ 1 rad/s. Samples MP6, MP3 and MP7 had the highest values (at a similar level), and sample MP4––the lowest values of those parameters (Table 4). The values of K 0 and K 00 , determined in this study, showed a slightly wider range than those established by Aguilar et al. (1991a, 1991b). According to the authors mentioned, the K 0 and K 00 values depend on the
4.2.4. Viscoelastic behaviour In order to determine the range of linear viscoelesticity, the curves of the function G ¼ f ðcÞ were plotted (curves are not shown). It was found that for strain ðcÞ higher than 0.1–0.15% (which corresponds with stress of about 2–3 Pa) the value of the complex modulus (G ) decreases with increasing strain. A similar maximum amplitude of strains for the range of linear viscoelasticity was established by Gerhards and Schubert (1993). Fig. 5 shows the mechanical spectra of selected mustard
Table 4 Parameters of power law functions describing storage and loss moduli and complex viscosity Sample
MP1 MP2 MP3 MP4 MP5 MP6 MP7
G0 ¼ K 0 xn0
G00 ¼ K 00 xn n 00
00
jg j ¼ K xn1
K 0 [Pa sn0 ]
n0
R2
K 00 [Pa s ]
n00
R2
K [Pa sn ]
n
R2
735.4 1031.5 1196.4 629.2 1027.9 1219.5 1191.7
0.170 0.168 0.154 0.166 0.169 0.168 0.161
0.9962 0.9949 0.9950 0.9987 0.9977 0.9990 0.9994
187.2 263.9 275.3 145.1 236.5 274.2 277.8
0.188 0.172 0.189 0.209 0.219 0.219 0.196
0.9960 0.9973 0.9813 0.9981 0.9933 0.9919 0.9936
758.3 1064.0 1226.7 645.2 1053.8 1248.9 1222.7
0.172 0.168 0.156 0.169 0.172 0.171 0.163
0.9999 0.9998 0.9999 1.0000 1.0000 1.0000 1.0000
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 10000 Apparent viscosity [Pa s] Complex viscosity [Pa s]
milling degree of mustard seeds in the production process. The samples exhibited similar values of parameters n0 (for G0 ) and n (for jg j): from 0.154 (MP3) to 0.170 (MP1) and from 0.156 (MP3) to 0.172 (MP1), respectively, while the parameter n00 (for G00 ) had markedly greater values (Table 4). This is in agreement with Aguilar et al. (1991a, 1991b) who also observed higher values of the exponent reflecting changes in the storage and loss moduli with frequency.
215
1000 100 10 1 0.1 1
1 10 Shear rate [s-1], Angular frequency [rad/s] MP1
5. Correlation between complex and apparent viscosity The complex and apparent viscosities of two mustard samples (MP1 and MP7) are presented in Fig. 6 as functions of frequency and shear rate (at x ¼ c_ ), respectively. The values of the viscosities were correlated using the generalised Cox–Merz rule. The values of the parameters of that relation are given in Table 5. It can be seen that the value of parameter C ranged between 15.85 (MP3) and 28.2 (MP7). It is worth mentioning that mustard MP3 had the highest dry matter content and showed the greatest values of other physicochemical parameters, while sample MP7 contained the smallest amounts of dry matter and extract (Table 1). The values of the parameter a ranged from 1.20 (MP1) to 1.295 (MP6) (Table 5), which indicates that in the studied cases the generalised Cox–Merz rule cannot be reduced to a one-parameter linear function jg jðxÞ ¼ Cga ðc_ Þjx¼c_ called the extended or modified Cox–Merz rule (Gunasekaran & Ak, 2000). While the original formula of Cox
MP7
Fig. 6. Complex (empty markers) and apparent (filled markers) viscosities of mustard.
Table 5 Parameters of generalised Cox–Merz relation Sample
C
a
R2
MP1 MP2 MP3 MP4 MP5 MP6 MP7
26.99 19.32 15.85 20.93 25.34 20.27 28.20
1.1200 1.2169 1.2685 1.1340 1.2144 1.2950 1.2233
1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000
and Merz (1958) directly describes the relationship between dynamic and apparent viscosity for homogenous systems, it is unsuitable for complex food systems (Bistany & Kokini, 1983, 1984; Gunasekaran & Ak, 2000; Rao & Tattiyakul, 1999). Yu and Gunasekaran (2001)
Table 6 Significant coefficients of linear correlation between physicochemical and rheological properties Dry matter [g/100 g] s0C [Pa] gC [Pa s] K [Pa sn ] n S [Pa/(s cm3 )] s0HB [Pa] K [Pa sn ] n A [Pa] B g1 [Pa s] E [kJ/mol] K [Pa rad/sn ] n K 0 [Pa rad/sn0 ] n0 00 K 00 [Pa rad/sn ] n00 C a *
Significant at p ¼ 0:05. Significant at p ¼ 0:1.
**
Extract content [g/100 g]
0.7603
Protein content [g/100 g]
Fat content [g/100 g]
0.8181
0.8135
0.8957
0.6846 0.9343
Ash content [g/100 g]
0.9523
0.7095 0.8039 0.6903
100
0.8104 0.9039 )0.6738 0.8788
0.7444 0.9237 )0.7106 0.9008
)0.7843
)0.7565
)0.8295
)0.7694
)0.7749
)0.6873
)0.7933 0.7970
216
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established that the original Cox–Merz rule correlates well the properties of honey which is a Newtoniam fluid, whereas the generalised (two-parameter) form of this relation is well suited to the description of cream cheese. They also found that the Cox–Merz equation in the reduced (one-parameter) form can be used for describing condensed milk, mayonnaise and ketchup. The same authors (Yu & Gunasekaran, 2001) observed that non-fat yoghurt and process and Mozarella cheeses do not follow the Cox–Merz rule. According to Bistany and Kokini (1984), for mustard the deviation from the original Cox–Merz relation, measured as a ratio jg jðxÞ=ga ðcÞj, is between 10 and 30. In the present study it was found that this ratio depends on the origin of a mustard sample and ranges from about 20 to 60.
6. Correlation between physicochemical and rheological properties Table 6 lists the values of statistically significant coefficients of linear correlation between the physicochemical and the rheological properties of mustard samples. The dry matter content correlated only with the thixotropy hysteresis loop area and the coefficient B of Weltman’s model. A relatively high coefficients of linear correlation were also found for the dry matter content and the Casson plastic viscosity as well as the flow behaviour index of Herschel–Bulkley’s model (0.6252), however, at the number of the degrees of freedom used in the study they were statistically nonsignificant. In contrast, Bhattacharya et al. (1999) found an increase in the values of yield stress, consistency coefficient and apparent viscosity of model mustard samples with decreasing water content. In the present study the extract content significantly correlated solely with the flow behaviour index of Herschel–Bulkley’s model. The ash content correlated only with some rheological parameters (Table 6) while the protein and fat contents significantly correlated with most of them.
7. Conclusion The commercial mustards studied differed in their physicochemical and rheological properties. Two of them did not comply with the provisions of the Polish Standard relative to dry matter content. The course of flow curves confirmed that mustards have a non-Newtonian, pseudoplastic (with a tendency to exhibit a yield stress) and thixotropic character. The mechanical spectra determined showed that elastic properties prevail over viscous ones in the whole investigated frequency range.
The generalised Cox–Merz rule indicated that the values of apparent viscosity obtained from flow curves correlate with the values of complex viscosity obtained from the dynamic test. Statistical analysis of the results showed that there are significant linear correlations between some physicochemical properties and some parameters describing rheological properties of mustards.
References Aguilar, C., Rizvi, S. S. H., Ramirez, J. F., & Inda, A. (1991a). Rheological behavior of processed mustard. I. Effect of milling treatment. Journal of Texture Studies, 22, 59–84. Aguilar, C., Rizvi, S. S. H., Ramirez, J. F., & Inda, A. (1991b). Rheological behavior of processed mustard. II. Storage effects. Journal of Texture Studies, 22, 85–103. Bhattacharya, S., Vasudha, N., & Krishna Murthy, K. S. (1999). Rheology of mustard paste: a controlled stress measurement. Journal of Food Engineering, 41, 187–191. Bistany, K. L., & Kokini, J. L. (1983). Dynamic viscoelastic properties of foods in texture control. Journal of Rheology, 27, 605–620. Bistany, K. L., & Kokini, J. L. (1984). Comparison of steady shear rheological properties and small amplitude dynamic viscoelastic properties of fluid materials. Journal of Texture Studies, 14, 113– 124. Brusewitz, G. H., & Yu, H. (1996). Back extrusion method for determining properties of mustard slurry. Journal of Food Engineering, 27, 259–265. Campanella, O. H., & Peleg, M. (1987). Determination of the yield stress of semi-liquid foods from squeezing flow data. Journal of Food Science, 52, 214–215, 217. Cox, W. P., & Merz, E. H. (1958). Correlation of dynamic and steady flow viscosities. Journal of Polymer Science, 28, 619–622. Dickie, A. M., & Kokini, J. L. (1983). An improved model for food thickness from non-Newtonian fluid mechanics in the mouth. Journal of Food Science, 48, 57–61, 65. Gerhards, Ch., & Schubert, H. (1993). Viscoelastic and shear thinning behavior of processed mustard. Rheology, 12, 256–263. Gerhards, Ch., & Schubert, H. (1996). Manufacture of mustard of reduced syneresis. Nahrung/Food, 40(6), 340–341. Gerhards, Ch., & Walker, F. (1997). Rheological properties of mustard mucilage isolated from raw and from processed mustard. Nahrung/ Food, 41(2), 96–100. Gunasekaran, S., & Ak, M. M. (2000). Dynamic oscillatory shear testing of foods––selected applications. Trends in Food Science & Technology, 11, 115–127. tude comparative de la rheologie de Hamza-Chaffai, A. (1991). E quatre produits alimentaires semi-solides. Sciences des Aliments, 11, 673–681. Polish Standard PN-A-86964. (1997). Musztarda. Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/02. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci ekstraktu ogolnego. Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/03. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci suchej masy metodaz wagowaz . Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/08. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci popiou ogolnego i jego alkalicznosci. Polski Komitet Normalizacyjny, Warszawa.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 Rao, M. A. (1999). Rheology of fluid and semisolid foods. Principles and applications (pp. 34, 49–52). Gaithersburg, Maryland: Aspen Publishers, Inc. Rao, M. A., & Tattiyakul, J. (1999). Granule size and rheological behavior of heated tapioca starch dispersions. Carbohydrate Polymers, 38, 123–132. Steffe, J. F. (1996). Rheological methods in food process engineering (pp. 33, 358–360). East Lansing: Freeman Press.
217
Suwonsichon, T., & Peleg, M. (1999). Imperfect squeezing flow viscometry of mustards with suspended particulates. Journal of Food Engineering, 39, 217–226. Yoo, B., Rao, M. A., & Steffe, J. F. (1995). Yield stress of food dispersions with the vane method at controlled shear rate and shear stress. Journal of Texture Studies, 26, 1–10. Yu, Ch., & Gunasekaran, S. (2001). Correlation of dynamic and steady flow viscosities of food materials. Applied Rheology, 11(3), 134–140.
Rheological properties of commercial mustards Lesław Juszczak b
a,*
, Mariusz Witczak b, Teresa Fortuna a, Agnieszka Banys a
a Department of Analysis and Evaluation of Food Quality, University of Agriculture, Balicka 122 Street, 30-149 Krakow, Poland Department of Engineering and Machinery for Food Industry, University of Agriculture, Balicka 122 Street, 30-149 Krakow, Poland
Received 25 March 2003; accepted 24 July 2003
Abstract Seven commercial mustards, made by different manufacturers, were studied to examine their physicochemical and rheological properties and to establish relationships between those properties. Physicochemical analysis revealed distinct differences between the mustards in the dry matter and extract contents and smaller differences in the protein, fat and ash levels. Two of the investigated mustards did not satisfy the requirements of the relevant Polish standard regarding dry matter content. Rheological studies included determination of the following: (a) flow curves with a controlled shear rate or shear stress, (b) curves showing the time and temperature dependence of apparent viscosity, and (c) mechanical spectra. Apparent viscosity and complex viscosity were correlated using the generalised Cox–Merz rule. The rheological investigations confirmed that mustard is a pseudoplastic fluid exhibiting a yield stress, thixotropy and viscoelastic (with dominant elastic) properties. Statistical analysis of the results showed significant linear correlations between the dry matter, fat, protein and ash contents of mustards and some parameters of rheological models. 2003 Elsevier Ltd. All rights reserved. Keywords: Mustard; Rheology
1. Introduction Mustard, a pungent, spicy-tasting paste used as a condiment, is made from party deoiled mustard flour and/or mustard seeds, water, food acids, vinegar, salt, sugar and flavouring additives (PN-A-86964, 1997). The popularity of this condiment dates back to the Middle Ages when the manufacturers of Dijon in Central France started to produce mustard (Moutarde de Dijon) on a larger scale. Much earlier, already in ancient times, ground seeds of the mustard plant were used to season meat dishes. In physical terms, mustard is a suspension with solid particles smaller than 30 lm suspended in a continuous water phase (Gerhards & Schubert, 1993). When stored, mustard exhibits syneresis which lowers its quality and sensory acceptance. This phenomenon is particularly pronounced at a high (e.g. 40 C) storage temperature (Aguilar, Rizvi, Ramirez, & Inda, 1991b; Gerhards & Schubert, 1993). Gerhards and Schubert (1996) distin-
*
Corresponding author. Fax: +48-12-626-58-41. E-mail address: [email protected] (L. Juszczak).
0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.07.002
guished two independent kinds of syneresis: gravitational syneresis and syneresis by shrinking. Those disadvantageous phenomena may be reduced without using any food additives, simply by employing native mustard mucilage, which, however, requires a two-step manufacturing process (Gerhards & Schubert, 1996; Gerhards & Walker, 1997). To assess the quality of semi-solid food products, it is necessary to understand their rheological properties. It is known that mustard, a semi-solid food, exhibits a non-Newtonian, pseudoplastic flow with a distinct yield stress and thixotropy (Aguilar, Rizvi, Ramirez, & Inda, 1991a, 1991b; Bhattacharya, Vasudha, & Krishna Murthy, 1999; Gerhards & Schubert, 1993; Yoo, Rao, & Steffe, 1995). As shown by Aguilar et al. (1991a), the rheological properties of processed mustard are determined by the size of solid phase particles. Those properties can be created or modified by controlling the particle size distribution (milling degree) in the manufacturing process. The rheological properties of mustard are also influenced by its dry matter content, the presence of oil fraction and the addition of thickeners (Aguilar et al., 1991b; Bhattacharya et al., 1999; Gerhards & Schubert, 1993).
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The rheology of mustard has also been investigated by using back extrusion and imperfect squeezing flow methods (Brusewitz & Yu, 1996; Suwonsichon & Peleg, 1999). The latter method deserves special attention since it offers the opportunity to test samples without affecting their structure before measurement (e.g. stress caused by placing a sample in a gap between measuring cylinders can be avoided) and to investigate samples with suspended large particles of spices or mustard seeds. The present study was aimed to reveal differences in the rheological properties of Polish commercial mustards and to find statistically significant correlations between the rheological and the physicochemical properties of the products.
2. Materials Seven commercial mustards not containing mustard seeds, seed pieces or spices have been made in Poland by different manufactures. According to the composition information from the labels, all products have been manufactured without thickening agents. Samples of mustards were marked with symbols MP1 to MP7. In the period of analyses the samples were stored at room temperature and prior to measurement they were gently stirred for homogenisation.
The following curves were obtained: 1. Flow curves with a controlled shear rate (CSR) at a temperature of 25 ± 0.2 C; the shear rate was increased from 1 to 600 s1 during 10 min, remained constant at 600 s1 during 2 min, and was decreased from 600 to 1 s1 during 10 min. The experimental data were described by the Casson model with n ¼ 0:25, which is more suitable for pseudoplastic dispersion solutions exhibiting a yield stress than the model with the exponent 0.50 (Rao & Tattiyakul, 1999), and by the power law model. The area of the thixotropy hysteresis loop was also determined. 2. Flow curves with a controlled shear stress (CSS) in the range of 0–10 Pa, during 10 min at a temperature of 25 ± 0.2 C. The experimental data were described by the Herschel–Bulkley model. 3. Curves showing the time-dependent behaviour of shear stress in the time range of 0–60 min at a temperature of 25 ± 0.2 C with a constant shear rate of 100 s1 . The experimental data were described by the Weltman model (Rao, 1999). 4. Curves showing the temperature-dependent behaviour of apparent viscosity over the temperature range of 8–30 C. The experimental data were described by the Arrhenius equation (Rao, 1999). 3.3. Viscoelastic properties
3. Methods 3.1. Physicochemical properties The dry matter content of mustards was determined in accordance with the relevant Polish Standard (PN-90/ A-75101/03, 1990), extract content––by refractometry (PN-90/A-75101/02, 1990), protein content (N · 6.25)–– by Kjeldahl’s method, fat content––by Soxhlet’s method with petroleum ether as a solvent (after hydrolysing the sample with a 2M HCl solution), and ash content––according to PN-90/A-75101/08 (1990). A one-way analysis of variance and Duncan’s test (at p ¼ 0:05) were used to establish the significance of differences in the physicochemical properties between the mustard samples studied. 3.2. Rheological properties 3.2.1. Flow behaviour Determinations were conducted using a rotational rheometer Rheolab MC 1 (Physica Messtechnik, Germany) in a system of coaxial cylinders (bob diameter 25 mm, cup diameter 27.12 mm). A mustard sample was placed in a measuring element and then held in a thermostat during 5 min in order to achieve temperature equilibrium and stress relaxation.
Measurements were performed using an oscillatory rheometer RheoStress RS 150 (Haake, Germany) at a temperature of 25 ± 0.2 C in the cone–plate system (cone diameter 35 mm, angle 2, gap size 0.105 mm). Mechanical spectra were plotted over a frequency range of 0.1–10 Hz at a constant strain amplitude of 0.1% (within the range of linear viscoelasticity). The experimental data were described by the following equations: 0
G0 ¼ K 0 xn
00
G00 ¼ K 00 xn
jg j ¼ K xn1 where G0 is storage modulus, G00 ––loss modulus, jg j–– complex viscosity, and K, K 0 , K 00 , n, n0 and n00 are parameters determined experimentally. Correlations between the values of complex viscosity, jg j, and those of apparent viscosity, ga , were established by using the generalised Cox–Merz rule that is known to hold for heterogenous dispersions and commercial foods (Bistany & Kokini, 1983; Gunasekaran & Ak, 2000; Rao & Tattiyakul, 1999): a
jg jðxÞ ¼ C½ga ðc_ Þ jx¼c_ where C and a are parameters determined experimentally.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 250 Shear rate [Pa]
To find relationships between the physicochemical and the rheological parameters of mustards, the values of linear correlation coefficients were calculated and their significance was examined by Student’s t-test at p ¼ 0:05.
211
200 150 100 50 0
4. Results and discussion
0
100
200
300
400
500
600
Shear rate [s-1]
4.1. Physicochemical properties
MP1
4.2. Rheological properties 4.2.1. Flow behaviour Examples of flow curves with a CSR are shown in Fig. 1, and with a CSS––in Fig. 2. The parameters of the rheological models employed to describe the experimental data and the values of the thixotropy hysteresis loop area are given in Table 2. The models used differed in their fit to the data. The Casson model with the exponent 0.25 exhibited a better fit than the traditional equation with the exponent 0.5. The same result was
MP3
MP5
Fig. 1. Flow curves (CSR mode) of mustard.
200 Shear stress [S-1]
The results of the physicochemical analysis of mustard samples are shown in Table 1. The mustards had a dry matter content in the range of 16.82 g/100 g (sample MP7) to 29.25 g/100 g (sample MP3). Mustards MP6 and MP7 failed to satisfy the requirements of the Polish Standard (PN-A-86964, 1997), which provide that the dry matter content of a mustard should not be less than 20 g/100 g, and mustard MP4 had the amount of dry matter close to the minimum value specified by that standard. The results for extract followed the same pattern: the sample which contained the smallest amount of dry matter (MP7) had also the lowest extract content (Table 1). The protein content ranged from 3.91 g/100 g for sample MP1 to 6.46 g/100 g for sample MP3. Mustard MP3, again, had the highest fat and ash contents, while MP7 contained the smallest amount of fat and MP2––of ash. The differences between samples were slighter for fat and ash than for other investigated properties.
MP2
160 120 80 40 0 0
20
40 60 Shear stress [Pa] MP2
MP3
MP4
80
100
MP5
Fig. 2. Flow curves (CSS mode) of mustard.
achieved by Rao and Tattiyakul (1999) for heated dispersions of tapioka starch. The power law model turned out to be best fitted to the experimental data (R2 > 0:98) but that model does not take account of yield stress which is a very important rheological parameter. For the Herschel–Bulkley model the degree of fit (R2 > 0:94) was poorer than that obtained by Bhattacharya et al. (1999). Nonetheless, this model is recommended by some authors for describing the flow curves of mustard (Aguilar et al., 1991a; Bhattacharya et al., 1999; Gerhards & Schubert, 1993). Although all investigated mustards exhibited non-Newtonian, pseudoplastic flow with a yield stress and a thixotropy hysteresis loop (Figs. 1 and 2), which confirms earlier observations on the rheology of mustards (Aguilar et al., 1991a; Bhattacharya et al., 1999; Gerhards & Schubert, 1993), there were
Table 1 Physicochemical properties of mustards Sample
Dry matter [g/100 g]
Extract content [g/100 g]
Protein content [g/100 g]
Fat content [g/100 g]
Ash content [g/100 g]
MP1 MP2 MP3 MP4 MP5 MP6 MP7
24.08 25.24 29.25 19.93 22.36 18.93 16.82
23.62 20.93 23.82 15.55 20.13 13.23 11.16
3.91 4.35a 6.46 5.12 4.37a 4.66 4.34a
4.53a 4.47ab 5.74 4.78 4.37ab 4.30ab 4.27b
3.10 2.56 3.22 2.92a 2.76b 2.82ab 2.69b
Values followed by the same letter in the same column are not significantly different (p ¼ 0:05). * Mean value of two repetition. ** Mean value of three repetition.
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Table 2 Rheological parameters of models describing flow curves of mustards Sample
CSR mode Casson model
MP1 MP2 MP3 MP4 MP5 MP6 MP7
S [Pa/ (s cm3 )]
Power law model
s0C [Pa]
gC [Pa s]
R2
K [Pa sn ]
n
R2
15.05 31.19 31.07 18.75 22.13 27.69 23.02
0.0042 0.0036 0.0106 0.0030 0.0044 0.0057 0.0041
0.9879 0.9658 0.9587 0.9920 0.9837 0.9711 0.9796
17.30 33.85 35.91 20.88 24.90 31.59 26.65
0.28 0.24 0.29 0.26 0.26 0.26 0.26
0.9973 0.9945 0.9888 0.9931 0.9966 0.9929 0.9973
374 724 1845 520 463 411 475
CSS mode Herschel–Bulkley model s0HB [Pa]
K [Pa sn ]
n
R2
3.04 1.69 2.45 3.12 3.48 4.21 0.26
22.97 17.33 24.42 18.63 19.91 23.71 21.11
0.36 0.36 0.35 0.35 0.37 0.33 0.32
0.9763 0.9684 0.9941 0.9673 0.9545 0.9445 0.9876
s0 ––yield stress; gC ––Casson plastic viscosity; K––consistency coefficient; n––flow behaviour index; S––area of thixotropy hysteresis loop.
some differences between samples. Sample MP3 showed the greatest values of shear stresses in the given range of shear rates (Fig. 1), a high value of the Casson yield stress (together with MP2), markedly highest Casson’s plastic viscosity, and the highest consistency coefficient and flow behaviour index from the power law model. It also had the largest area of the thixotropy hysteresis loop (Table 2). The rheological parameters of this sample correlated well with its physicochemical features (Table 1). The other mustards (except MP2) exhibited smaller values of Casson’s yield stress and much lower values of Casson’s plastic viscosity (Table 2). The consistency coefficients of the samples were between 17.30 Pa sn (MP1) and 35.91 Pa sn (MP3). The flow behaviour indexes ranged from 0.24 (MP2) to 0.29 (MP3) with the values for samples MP4 to MP7 being equal (Table 2). According to Gerhards and Schubert (1993), beyond the yield stress, the flow curves become flatter with increasing shear rate, as is the case of the curves shown in the present study. This can be attributed to the breakdown of the inner structure of fluid which is formed through physical interactions between molecules. As the shear rate increases, those forces weaken and the molecules orientate themselves along the flow lines, which causes a drop in viscosity. Examples of flow curves obtained at a constant shear stress are shown in Fig. 2. Individual mustard samples achieved the maximum assumed shear stress at different values of shear rate. Sample MP3 exhibited the greatest resistance to the shear stress applied, which corresponds with the highest values of shear stresses obtained for this sample in the CRS mode (Fig. 1). As compared to the Casson model, using the model of Herschel–Bulkley for the description of CSS flow curves produced much lower values of yield stress: from 0.26 Pa (sample MP7) to 4.26 Pa (sample MP6). The value of yield stress, one of the parameters characterising the properties of semi-solid food products, is of importance to the technological process (e.g. pumping) as well as to the perception of the quality of a product by the consumers. According to Bhattacharya et al. (1999), experimental methods (e.g. stress relaxation
method, stress–strain plot) give more reliable results than the extrapolation of flow curves and the use of rheological equations. In the case of the latter, the application of the CSS mode is more sensitive and leads to more correct results (Bhattacharya et al., 1999). Due to the variety of test methods and rheological models, the results on the yield stress of mustard, cited in the literature, greatly differ and range from about 0.5–1 Pa (Bhattacharya et al., 1999) to 28–49 Pa (Gerhards & Schubert, 1993). Much higher values were obtained by Campanella and Peleg (1987) who tested mustard with a squeezing flow method. Also Yoo et al. (1995) who studied mustards by using a vane method found greater values of their yield stress: 83–104 Pa (static yield stress) and 53–70 Pa (dynamic yield stress); those values differed depending on which parameter was controlled during the experiment, shear rate or shear stress. According to the latter authors, determination of yield stress by a vane method is simpler, unequivocal and more sensitive in the CSR mode. The consistency coefficient had lower values for the Herschel–Bulkley model (CSS mode) than for the power law model (CSR mode); among all samples, mustard MP3 showed in both cases the highest value of that coefficient. The values of the flow behaviour index, which also are indicative of a considerable pseudoplasticity of mustard, were greater for the Herschel–Bulkley model than for the power law model. This was probably due to the different fit of the two models to the experimental data and to the fact that the power law model disregards yield stress. In addition, the two experiments used different ranges of shear rate: the maximum value of shear rate was 600 s1 for the CSR mode and about 190 s1 for the CSS mode. According to Suwonsichon and Peleg (1999), such low values of the flow behaviour index may also result from slippage at the walls of the cylinder, especially in the range of higher shear rates. The values of the consistency coefficient and flow behaviour indexes obtained in this study (Table 2) are similar to the data reported by such authors as Dickie and Kokini (1983) and Steffe (1996) but differ from the results of Hamza-Chaffai (1991), who recorded much
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217
4.2.2. Time-dependent behaviour Fig. 3 shows the shear stress vs. shear time curves obtained at a constant shear rate (100 s1 ) for chosen
180 Shear stress [Pa]
lower values of the consistency coefficient, and Gerhards and Schubert (1993), who obtained higher values of that coefficient, in the range 57–62 Pa sn . The values of the two parameters mentioned depend on the temperature of measurement, the range of shear rate or stress and the kind of sample. According to Aguilar et al. (1991a), one of the most important factors determining the rheological properties of mustard is the milling degree of mustard seeds which affects the size of solid particles in the final product. Gerhards and Schubert (1993) found that the stirring of a sample before measurement leads to a drop in the value of both yield stress and flow behaviour index but practically does not influence the value of the consistency index. The parameters in question are also affected by the time and conditions of the storage of samples (Aguilar et al., 1991a, 1991b; Gerhards & Schubert, 1993). As claimed by Bhattacharya et al. (1999), the water content of mustard has an effect on its rheological properties: the higher it is, the lower is the value of yield stress and the consistency coefficient and the greatest is the value of the flow behaviour index. All the tested mustards showed thixotropy. The intensity of this phenomenon may be measured by the area contained between the flow curves obtained with increasing and decreasing shear rate (Gerhards & Schubert, 1993). As can be seen from Table 2, the area of the thixotropy hysteresis loop was the largest for mustard MP3 and the smallest for MP1. Gerhards and Schubert (1993) also observed the presence of distinct thixotropy hysteresis whose magnitude depended on the way the sample was treated before measurement. The thixotropy phenomenon has a practical significance: a low viscosity of sol is advantageous when a product is mixed with another substances or removed from the container. On the other hand, when the structure of the suspension after mixing is being rebuilt too slowly, sedimentation of the suspended particles cannot be avoided.
213
160 140 120 100 80 60 40 0
10
20
30
40
50
MP3
MP4
MP5
MP6
Fig. 3. Time-dependent behaviour curves of mustard.
samples, and Table 3 lists the parameters of the Weltman model used to describe the experimental data. That model was found to be relatively well fitted (R2 > 0:97) to the data. The shear stress had the greatest values for mustard MP3 and the lowest for MP4 and MP1, which corresponds with the values of the parameter A of Weltman’s model (Table 3) that reflects the value of shear stress in the first second of the test. For sample MP3 the highest values of shear stresses correlate well with the highest values of physicochemical parameters (Table 1). MP3 had also the greatest value of the parameter B of Weltman’s model (coefficient of thixotropic breakdown), that in turn correlates with the largest area of the thixotropy hysteresis loop (Table 2). Other mustards, having much lower values of the coefficient B (Table 3), exhibited also considerably smaller areas of the thixotropy hysteresis loop (Table 2). A strong linear correlation (0.9853) was found between the two parameters mentioned. According to Aguilar et al. (1991a, 1991b), the values of the Weltman’s model parameters depend on the size distribution of solid particles suspended in the continuous phase. 4.2.3. Temperature-dependent behaviour Fig. 4 shows some examples of apparent viscosity vs. temperature curves. It can be seen that the course of the curves varies according to the mustard sample. The values of apparent viscosity in the studied temperature range were the highest for mustard MP6 and the lowest for MP2. The parameters of the Arrhenius model, which
Table 3 Rheological parameters of Weltman and Arrhenius models Sample
MP1 MP2 MP3 MP4 MP5 MP6 MP7
Weltman model
Arrhenius model
A [Pa]
B
R
g1 [Pa s] · 103
E [kJ/mol]
R2
85.63 122.10 203.05 85.81 101.38 129.03 118.50
4.53 6.05 13.56 5.19 3.98 5.35 5.39
0.9979 0.9794 0.9926 0.9925 0.9922 0.9873 0.9962
6.33 14.06 0.30 0.88 5.92 9.75 9.21
16.37 13.62 23.51 20.61 16.41 15.38 14.95
0.9864 0.9680 0.9883 0.9705 0.9845 0.9749 0.9909
2
60
Time [min]
A––value of stress at first sec. of test; B––coefficient of thixotropic breakdown; g1 ––frequency factor (material constant); E––activation energy.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 10000
9 Storage modulus [Pa] Loss modulus [Pa]
Apparent viscosity [Pa s]
214
8 7 6 5 4
1000
100
3 8
10
12
14
16
18 20 22 Temperature [°C]
MP2
MP3
MP4
24
26
28
1
30
1 Frequency [Hz] MP1
MP6
MP4
MP5
10 MP6
Fig. 5. Mechanical spectra of mustard. G0 ––empty markers, G00 ––filled markers.
Fig. 4. Temperature-dependent behaviour curves of mustard.
was used to describe the experimental data, are given in Table 3. The values of flow activation energy indicate that mustard samples had different sensitivity to the increase in temperature. According to Steffe (1996), a system is the more sensible to temperature changes, the higher is the value of flow activation energy. It is worth noticing that sample MP3 had the greatest flow activation energy and the lowest material constant (Table 3). At a temperature of 8 C it exhibited a high apparent viscosity which considerably decreased during heating, to about 3.5 Pa s (Fig. 4). Sample MP6, which also had a high apparent viscosity at 8 C, showed a markedly smaller drop in viscosity with increasing temperature. The smallest decrease in apparent viscosity was characteristic of sample MP2 having the lowest apparent viscosity at 8 C. This corresponded with the lowest value of flow activation energy and the highest value of material constant––the parameters of the Arrhenius equation.
samples in the frequency range of 0.1–10 Hz. In this range the values of the storage modulus (G0 ) were greater than those of loss modulus (G00 ) for all the mustards studied, the ratio G00 =G0 being about 0.22–0.28. This testifies to the dominance of elastic properties over viscous ones. A similar course of mechanical spectra, but at higher values of G0 and G00 , was observed by Gerhards and Schubert (1993). The spectra shown in Fig. 5 do not resemble those of strong gels: the storage modulus depends on frequency and the differences between the values of the two moduli are less than one order of magnitude. Among the samples studied, MP6 had the greatest values of G0 and G00 moduli and MP4 the smallest. The dependence of G0 and G00 on frequency (x) was described by the power law function which showed a good fit to the experimental data (R2 > 0:98). Even better fit (R2 > 0:99) was found for the power law function describing the relationship between complex viscosity and frequency. The parameters of the relevant equations are listed in Table 4. The values of parameters K 0 , K 00 and K correspond with the values of G0 , G00 and jg j, respectively, at x ¼ 1 rad/s. Samples MP6, MP3 and MP7 had the highest values (at a similar level), and sample MP4––the lowest values of those parameters (Table 4). The values of K 0 and K 00 , determined in this study, showed a slightly wider range than those established by Aguilar et al. (1991a, 1991b). According to the authors mentioned, the K 0 and K 00 values depend on the
4.2.4. Viscoelastic behaviour In order to determine the range of linear viscoelesticity, the curves of the function G ¼ f ðcÞ were plotted (curves are not shown). It was found that for strain ðcÞ higher than 0.1–0.15% (which corresponds with stress of about 2–3 Pa) the value of the complex modulus (G ) decreases with increasing strain. A similar maximum amplitude of strains for the range of linear viscoelasticity was established by Gerhards and Schubert (1993). Fig. 5 shows the mechanical spectra of selected mustard
Table 4 Parameters of power law functions describing storage and loss moduli and complex viscosity Sample
MP1 MP2 MP3 MP4 MP5 MP6 MP7
G0 ¼ K 0 xn0
G00 ¼ K 00 xn n 00
00
jg j ¼ K xn1
K 0 [Pa sn0 ]
n0
R2
K 00 [Pa s ]
n00
R2
K [Pa sn ]
n
R2
735.4 1031.5 1196.4 629.2 1027.9 1219.5 1191.7
0.170 0.168 0.154 0.166 0.169 0.168 0.161
0.9962 0.9949 0.9950 0.9987 0.9977 0.9990 0.9994
187.2 263.9 275.3 145.1 236.5 274.2 277.8
0.188 0.172 0.189 0.209 0.219 0.219 0.196
0.9960 0.9973 0.9813 0.9981 0.9933 0.9919 0.9936
758.3 1064.0 1226.7 645.2 1053.8 1248.9 1222.7
0.172 0.168 0.156 0.169 0.172 0.171 0.163
0.9999 0.9998 0.9999 1.0000 1.0000 1.0000 1.0000
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 10000 Apparent viscosity [Pa s] Complex viscosity [Pa s]
milling degree of mustard seeds in the production process. The samples exhibited similar values of parameters n0 (for G0 ) and n (for jg j): from 0.154 (MP3) to 0.170 (MP1) and from 0.156 (MP3) to 0.172 (MP1), respectively, while the parameter n00 (for G00 ) had markedly greater values (Table 4). This is in agreement with Aguilar et al. (1991a, 1991b) who also observed higher values of the exponent reflecting changes in the storage and loss moduli with frequency.
215
1000 100 10 1 0.1 1
1 10 Shear rate [s-1], Angular frequency [rad/s] MP1
5. Correlation between complex and apparent viscosity The complex and apparent viscosities of two mustard samples (MP1 and MP7) are presented in Fig. 6 as functions of frequency and shear rate (at x ¼ c_ ), respectively. The values of the viscosities were correlated using the generalised Cox–Merz rule. The values of the parameters of that relation are given in Table 5. It can be seen that the value of parameter C ranged between 15.85 (MP3) and 28.2 (MP7). It is worth mentioning that mustard MP3 had the highest dry matter content and showed the greatest values of other physicochemical parameters, while sample MP7 contained the smallest amounts of dry matter and extract (Table 1). The values of the parameter a ranged from 1.20 (MP1) to 1.295 (MP6) (Table 5), which indicates that in the studied cases the generalised Cox–Merz rule cannot be reduced to a one-parameter linear function jg jðxÞ ¼ Cga ðc_ Þjx¼c_ called the extended or modified Cox–Merz rule (Gunasekaran & Ak, 2000). While the original formula of Cox
MP7
Fig. 6. Complex (empty markers) and apparent (filled markers) viscosities of mustard.
Table 5 Parameters of generalised Cox–Merz relation Sample
C
a
R2
MP1 MP2 MP3 MP4 MP5 MP6 MP7
26.99 19.32 15.85 20.93 25.34 20.27 28.20
1.1200 1.2169 1.2685 1.1340 1.2144 1.2950 1.2233
1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000
and Merz (1958) directly describes the relationship between dynamic and apparent viscosity for homogenous systems, it is unsuitable for complex food systems (Bistany & Kokini, 1983, 1984; Gunasekaran & Ak, 2000; Rao & Tattiyakul, 1999). Yu and Gunasekaran (2001)
Table 6 Significant coefficients of linear correlation between physicochemical and rheological properties Dry matter [g/100 g] s0C [Pa] gC [Pa s] K [Pa sn ] n S [Pa/(s cm3 )] s0HB [Pa] K [Pa sn ] n A [Pa] B g1 [Pa s] E [kJ/mol] K [Pa rad/sn ] n K 0 [Pa rad/sn0 ] n0 00 K 00 [Pa rad/sn ] n00 C a *
Significant at p ¼ 0:05. Significant at p ¼ 0:1.
**
Extract content [g/100 g]
0.7603
Protein content [g/100 g]
Fat content [g/100 g]
0.8181
0.8135
0.8957
0.6846 0.9343
Ash content [g/100 g]
0.9523
0.7095 0.8039 0.6903
100
0.8104 0.9039 )0.6738 0.8788
0.7444 0.9237 )0.7106 0.9008
)0.7843
)0.7565
)0.8295
)0.7694
)0.7749
)0.6873
)0.7933 0.7970
216
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established that the original Cox–Merz rule correlates well the properties of honey which is a Newtoniam fluid, whereas the generalised (two-parameter) form of this relation is well suited to the description of cream cheese. They also found that the Cox–Merz equation in the reduced (one-parameter) form can be used for describing condensed milk, mayonnaise and ketchup. The same authors (Yu & Gunasekaran, 2001) observed that non-fat yoghurt and process and Mozarella cheeses do not follow the Cox–Merz rule. According to Bistany and Kokini (1984), for mustard the deviation from the original Cox–Merz relation, measured as a ratio jg jðxÞ=ga ðcÞj, is between 10 and 30. In the present study it was found that this ratio depends on the origin of a mustard sample and ranges from about 20 to 60.
6. Correlation between physicochemical and rheological properties Table 6 lists the values of statistically significant coefficients of linear correlation between the physicochemical and the rheological properties of mustard samples. The dry matter content correlated only with the thixotropy hysteresis loop area and the coefficient B of Weltman’s model. A relatively high coefficients of linear correlation were also found for the dry matter content and the Casson plastic viscosity as well as the flow behaviour index of Herschel–Bulkley’s model (0.6252), however, at the number of the degrees of freedom used in the study they were statistically nonsignificant. In contrast, Bhattacharya et al. (1999) found an increase in the values of yield stress, consistency coefficient and apparent viscosity of model mustard samples with decreasing water content. In the present study the extract content significantly correlated solely with the flow behaviour index of Herschel–Bulkley’s model. The ash content correlated only with some rheological parameters (Table 6) while the protein and fat contents significantly correlated with most of them.
7. Conclusion The commercial mustards studied differed in their physicochemical and rheological properties. Two of them did not comply with the provisions of the Polish Standard relative to dry matter content. The course of flow curves confirmed that mustards have a non-Newtonian, pseudoplastic (with a tendency to exhibit a yield stress) and thixotropic character. The mechanical spectra determined showed that elastic properties prevail over viscous ones in the whole investigated frequency range.
The generalised Cox–Merz rule indicated that the values of apparent viscosity obtained from flow curves correlate with the values of complex viscosity obtained from the dynamic test. Statistical analysis of the results showed that there are significant linear correlations between some physicochemical properties and some parameters describing rheological properties of mustards.
References Aguilar, C., Rizvi, S. S. H., Ramirez, J. F., & Inda, A. (1991a). Rheological behavior of processed mustard. I. Effect of milling treatment. Journal of Texture Studies, 22, 59–84. Aguilar, C., Rizvi, S. S. H., Ramirez, J. F., & Inda, A. (1991b). Rheological behavior of processed mustard. II. Storage effects. Journal of Texture Studies, 22, 85–103. Bhattacharya, S., Vasudha, N., & Krishna Murthy, K. S. (1999). Rheology of mustard paste: a controlled stress measurement. Journal of Food Engineering, 41, 187–191. Bistany, K. L., & Kokini, J. L. (1983). Dynamic viscoelastic properties of foods in texture control. Journal of Rheology, 27, 605–620. Bistany, K. L., & Kokini, J. L. (1984). Comparison of steady shear rheological properties and small amplitude dynamic viscoelastic properties of fluid materials. Journal of Texture Studies, 14, 113– 124. Brusewitz, G. H., & Yu, H. (1996). Back extrusion method for determining properties of mustard slurry. Journal of Food Engineering, 27, 259–265. Campanella, O. H., & Peleg, M. (1987). Determination of the yield stress of semi-liquid foods from squeezing flow data. Journal of Food Science, 52, 214–215, 217. Cox, W. P., & Merz, E. H. (1958). Correlation of dynamic and steady flow viscosities. Journal of Polymer Science, 28, 619–622. Dickie, A. M., & Kokini, J. L. (1983). An improved model for food thickness from non-Newtonian fluid mechanics in the mouth. Journal of Food Science, 48, 57–61, 65. Gerhards, Ch., & Schubert, H. (1993). Viscoelastic and shear thinning behavior of processed mustard. Rheology, 12, 256–263. Gerhards, Ch., & Schubert, H. (1996). Manufacture of mustard of reduced syneresis. Nahrung/Food, 40(6), 340–341. Gerhards, Ch., & Walker, F. (1997). Rheological properties of mustard mucilage isolated from raw and from processed mustard. Nahrung/ Food, 41(2), 96–100. Gunasekaran, S., & Ak, M. M. (2000). Dynamic oscillatory shear testing of foods––selected applications. Trends in Food Science & Technology, 11, 115–127. tude comparative de la rheologie de Hamza-Chaffai, A. (1991). E quatre produits alimentaires semi-solides. Sciences des Aliments, 11, 673–681. Polish Standard PN-A-86964. (1997). Musztarda. Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/02. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci ekstraktu ogolnego. Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/03. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci suchej masy metodaz wagowaz . Polski Komitet Normalizacyjny, Warszawa. Polish Standard PN-90/A-75101/08. (1990). Przetwory owocowe i warzywne. Przygotowanie probek i metody badan fizykochemicznych. Oznaczanie zawartosci popiou ogolnego i jego alkalicznosci. Polski Komitet Normalizacyjny, Warszawa.
L. Juszczak et al. / Journal of Food Engineering 63 (2004) 209–217 Rao, M. A. (1999). Rheology of fluid and semisolid foods. Principles and applications (pp. 34, 49–52). Gaithersburg, Maryland: Aspen Publishers, Inc. Rao, M. A., & Tattiyakul, J. (1999). Granule size and rheological behavior of heated tapioca starch dispersions. Carbohydrate Polymers, 38, 123–132. Steffe, J. F. (1996). Rheological methods in food process engineering (pp. 33, 358–360). East Lansing: Freeman Press.
217
Suwonsichon, T., & Peleg, M. (1999). Imperfect squeezing flow viscometry of mustards with suspended particulates. Journal of Food Engineering, 39, 217–226. Yoo, B., Rao, M. A., & Steffe, J. F. (1995). Yield stress of food dispersions with the vane method at controlled shear rate and shear stress. Journal of Texture Studies, 26, 1–10. Yu, Ch., & Gunasekaran, S. (2001). Correlation of dynamic and steady flow viscosities of food materials. Applied Rheology, 11(3), 134–140.