3.2. Estudio de las medidas de conciliación
3.2.2. Medidas adicionales de conciliación
The point of calcium addition during processing was identified as having a significant effect on the samples produced. The samples where calcium was added before the heating step (A and B), including the extended mixing time sample, were all observed to form a soft paste, as in the previous part of the experiment. The sample where the calcium was added midway through the heating step (C) was observed to form a cohesive cheese like mass. However, this sample did not incorporate all of the fat into the structure of the sample, with a pool of molten fat left in the bottom of the Brabender after processing. The sample produced when the calcium was added at the start of the kneading step (D) was observed to form a cohesive cheese-like mass. This product was softer than samples produced at lower calcium levels. However, unlike sample C, this sample had the fat incorporated into the structure.
4.3.7.1 Inline torque measurements
In order to investigate the role of calcium and heat on the structure of the model system during processing, an inline torque system was used. The inline torque measurements indicated that each of the samples had a distinct torque profile related to the development of its structure during processing. The torque required by the motor to maintain a constant mixing speed can be related to the viscosity of the system. The greater the torque required by the motor, the more viscous the sample.
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Figure 4.8: A comparative torque profile of model systems produced with calcium added at
points A ( ) and B ( ) [Temperature ].
As shown in Figure 4.8, initially both samples underwent a period of variable torque during which the components were first mixed together. Following this, sample A was observed to have a brief period where little change was evident in the torque profile. This was followed by a rapid increase in the viscosity of the system, indicated by the rise in the torque. The viscosity of the system was then found to drop off in an approximately linear fashion. The initial part of this decrease occurred independent of temperature, as it occurred prior to the temperature ramp. As the temperature was increased, the viscosity of the sample was found to continue to decrease. Once the temperature stabilised at 70°C, so did the viscosity of the system.
After the initial variation, sample B maintained a relatively stable viscosity until the calcium was added to the system. Following the addition of calcium, the viscosity was observed to increase. However the maximum torque was found to be lower for sample B
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than sample A. Sample B then mirrors sample A in the decrease in viscosity up to 70°C followed by a stabilisation when the system has reached a constant temperature.
The lower maximum torque in sample B may be the result of the action of calcium during the hydration of the protein powder. Sample A has the calcium added with the water at the beginning of processing. The calcium can cause large aggregates as the powder particles do not have sufficient time to break apart prior to the bridging effect of calcium. Whereas, in sample B, the protein powder is allowed to hydrate prior to the addition of calcium. This allows the powder can breakdown into micelles and the addition of the calcium forms smaller aggregates resulting in a lower viscosity.
The inline monitoring of the samples during processing indicated that the addition of calcium initially assists in the formation of structure. This is due to calcium assisting in the formation of protein-to-protein cross bridges (Ennis et al., 1998). However, as the concentration of calcium exceeds the critical calcium concentration, the system ceases to form a solid at high temperatures.
4.3.7.2 Processing effect on uniaxial compression
In order to determine the effect that the point of calcium addition had on the resulting texture of the model cheeses, uniaxial compression was carried out to study the hardness of the samples.
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Figure 4.9: Hardness of model systems with different points of calcium addition [Calcium:Protein = 2.8%]
A one way ANOVA was conducted based on the hardness results to identify whether there were any significant differences between the samples. This analysis produced a p value of 0.00, indicating that not all of the sample population means are the same.
Based on this finding a Tukey’s analysis was carried out on the data to identify where the differences occurred. This found that there were three distinct groups that were significantly different from one another. The first, where no significant difference was identified between the samples, included sample C, D and the samples with the extended mixing time. Although no difference was found between the measured hardness of these three samples, they were all identified as having different observable bulk structures. However, it is likely that differences in the hardness between these samples would be evident if the experiment was carried out at different temperatures. As the structures of the samples differ, the contribution that the fat imparts to the hardness would differ in
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the samples, especially at temperatures where the fat would be in a molten state. However, due to the time constraints of this study this work was not conducted.
The other two groups were sample A and sample B, each found to be significantly different from one another and the previous group. Sample B was identified as having the lowest hardness out of the three distinct groups while sample A had a hardness between the other two groups.
The potential mechanisms that could be responsible for this difference in hardness could be an ion migration effect, as mentioned earlier. Another possibility is that the additional mixing time results in a homogenisation effect. This would be the result of the extra mixing breaking down the size of the fat particles in the system. Smaller fat particles deform to a lesser extent than large pockets of fat (Rudan et al., 1998), therefore the hardness of the resulting system is likely to be higher.
4.3.7.3 Processing effects on dielectric profile
Dielectric spectroscopy was used to identify whether there were any differences in component mobility between the different model cheese samples. The samples produced with the calcium added at point C and D both formed a solid during processing, however, sample D formed a porous, foam-like structure and sample C was observed to have large pockets of fat dispersed within the structure. This made these two samples unsuitable for dielectric spectroscopy due to the large inhomogeneities in the structure interfering with the signal obtained from the sample.
The dielectric constant was analysed over a frequency range from 200 MHz to 1.3 GHz. The results are displayed below at a frequency of 1250 MHz. This frequency was chosen as the dielectric constant is dominated by dipolar rotation, which is the most dominant mechanism above a frequency of 1 GHz (Ryynänen, 1995).
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Figure 4.10: Change in the dielectric constant at 1250 MHz with temperature (Sample: A, B and Extended)
All three samples exhibited a trend where the dielectric constant increased with temperature. Sample B was found to have a much higher dielectric constant at each temperature than the other two samples analysed. A higher dielectric constant is indicative of a greater level of dipolar mobility, dominated by water in food systems (Datta et al., 2005; Kudra et al., 1992). As all of the samples have similar moisture contents, the difference in dielectric constant could be the result of free water within the system. This is due to the dielectric constant not only being sensitive to the moisture content, it can also be used to interpret the level of water mobility within a system (Tsoubeli, Davis, & Gordon, 1995).
The dielectric loss factor was also used to assess the three different systems. Full frequency sweeps were conducted on the samples and a frequency of 255 MHz was chosen to display the relationship between the dielectric loss factor and temperature. This
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frequency was chosen due to the dominant effect of ionic conduction at lower frequencies (Fagan et al., 2005).
Figure 4.11: Effect of temperature on the dielectric loss factor at 255 MHz (Sample: A, B and Extended)
The dielectric loss factor was found to increase with temperature in all three samples. As with the dielectric constant, the dielectric loss factor was found to be higher in sample B than the other two samples at each temperature studied. This indicates that the ions within sample B are more mobile than the other two samples. This suggests that there is a greater quantity of free ions present in the sample as the composition is identical to the other samples.
No significant difference was found to exist between the sample with the extended mixing time and sample A. This suggests that if migration of calcium is occurring during processing, the extended mixing time does not result in further migration than the standard mixing time of 10 minutes. The effect of the homogenisation due to additional mixing could result in the difference in the hardness in between system A and the
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extended mixing time. The additional mixing would likely result in the reduction in the size of the fat globules within the system, modifying the contribution that the fat imparts to the hardness of the cheese.