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8. Ideas de cierre o conclusiones
compromise between being able to handle the highly viscous solutions and being able to
measure the lower viscosity solutions. It is well known that the viscosity of protein
solutions increases with decreases in temperature. The solutions were placed in the
Haake VT500 rheometer set to 1 5°C and left to equilibrate for 1 0 minutes. The NV and
MVl geometry concentric cylinders were used depending on the viscosity of the
solution. Following equilibration the solution was subjected to increases in shear rate
from 1 0 to 1 000s·1 at steps of 20s·1 over a period of 3 minutes immediately followed by
a corresponding decrease in shear rate.
Chapter 3. Rheology of Commercial Powder
3 . 1 .5.2 Rheological measurements using the Bohlin VOR Rheometer
5 5
The temperature at which shear sweep experiments were performed on the Bohlin varied from 1 5°C to 65°C. To prevent drying out a thin layer of paraffin oil was placed on top of the solutions. A temperature equilibration time of 1 0 minutes was used. The shear sweep experiments were performed over the shear rate range 1 . 1 6 to 1 1 6s·1 • The C 1 4 and C25 geometry cup and bob sets in conjunction with a 4g and O.3g torsion bar were used for the viscometry tests depending on the viscosity of the solutions. The shear rate was set to increase logarithmically so that the flow curve at low shear rates would be more accurately described, enabling the existence of a yield stress to be more readily identified. Each sweep took 8.4 1 minutes with measurements taken every 25s.
3.2 Effect of method of reconstitution on solubility
The main structural elements of a solution that can affect the rheology and depend on the method of reconstitution are: the presence of air bubbles (covered briefly in sections 3 . 1 .3 and 3.3) and, the degree of solubility of the powder, covered in this section. The problem of air incorporation in a solution generally occurs in solutions of high viscosity and causes the solution to possess an even higher viscosity. The presence of insoluble particles in a solution leads to a higher viscosity, compared to a solution which does not contain insoluble particles. The higher viscosity, caused by the presence of insoluble particles, may lead to air incorporation. Therefore the effect of the method of reconstitution on solubilisation was investigated first. The main variables thought to affect solubilisation of MPC85 are: temperature (Pierre et al., 1 992) and, the flow velocity of water during reconstitution (Schuck et al., 1 994).
The effect of reconstitution temperature on the solubility of commercial MPC85 was investigated over the temperature range 20 to 60°C following the method outlined in section 3 . 1 .4. The results of these experiments, shown in Figure 3- 1 , clearly show that the solubility of MPC85 decreases as the reconstitution temperature decreases below
50°C.
An
attempt to model the relationship between reconstitution temperature andsolubility with the Arrhenius Equation, and Power Law models did not arrive at an adequate solution (R2, s of 0.76 and 0.85 respectively). Log plots of the data did not
show linearity. From this analysis the relationship can only be described qualitatively: solubility increases rapidly with temperature and asymptotes to almost 1 00% solubility at about 50°C. The effect of temperature of reconstitution on the solubility of MPC85 reported here is comparable to that of Pierre et al., ( 1 992) who found that increasing the reconstitution water temperature from 24°C to 50°C increased the solubility of native micellar casein from 74.0% to 97.2%.
Schuck et al., ( 1 994) concluded that increasing the flow velocity of water during
reconstitution of MPC85 facilitated solubility. These authors found that the solubility increased when the period of agitation was extended to 900s or if agitation is increased to 1 0000 rpm. However, when this method was applied to the concentrations required for viscosity measurements in this work (up to 1 7.5% protein) excessive foaming occurred. It was thought that homogenisation would provide the increase in flow velocity of water necessary to facilitate solubility without excessive foaming. It is recognised though, that homogenisation is a complex process involving effects such as intense shear and cavitation as well as increasing the flow velocity of water.
To investigate the effect of homogenisation on solubility, the solution reconstituted at 20°C was homogenised at 1 50 bar and its solubility determined. This homogenisation step increased the solubility of the solution from 58.85% to 88.75%.
Chapter 3. Rheology of Commercial Powder 1 1 0 1 00 90 � � 80 :J (5 Cl) 70 60 50 1 0 20 30 40 50 Reconstitution temperature [0C] 57 60 70
Figure 3 - 1 The effect of reconstitution temperature on the solubility of commercial MPC85 in MilliQ water.
3.3 Effect of solubility on apparent viscosity
Having established the effect of the reconstitution temperature on the solubility of MPC85 the question arose of how sensitive rheological measurements of reconstituted MPC85 solutions are to insoluble solids. To answer this question a series of 20% w/w MPC85 solutions were prepared following the procedure outlined in section 3 . 1 .3 with the following exceptions. The temperature at which each solution was mixed was set at either 20, 30, 40, 50, or 60°C. A sixth solution was prepared with reconstitution water set to a temperature of 20°C. Following mixing this solution was homogenised at 1 50 bar in a single stage homogeniser. The rheology of each of these solutions was examined by a shear sweep experiment at 1 5°C conducted on the Haake VT500 rheometer. The details of these rheology experiments are described in sections 3 . 1 .5 and 3 . 1 .5. 1 .
The solutions showed no signs of air incorporation: no air bubbles could be seen on either the cup or the bob surface at the end of the experiment once the solutions were carefully poured out of the cup; and the flow curves showed no signs of hysteresis. Typically the flow curve of a solution with entrained air bubbles will show a much higher viscosity on the downward sweep (in extreme cases the solution may show no decrease in viscosity with decreasing shear) than the upward sweep and both flow curves show considerable "noise".
The flow curves of the reconstituted MPC85 solutions are shown in Figure 3-2. The scale of the apparent viscosity axis of this figure is logarithmic to show more clearly the differences between all of the flow curves on one graph. By inspection one can see that: the pseudoplastic nature of the solutions, indicated by the decrease in apparent viscosity with increasing shear rate, increased with the degree of insoluble matter; and that the apparent viscosity of the solutions also increased with the degree of insoluble matter. It should be noted that the solubility and apparent viscosity data were obtained from solutions comprised of 3.5% protein and 20% total solids respectively. It is likely that the solubility of MPC85 at 20% total solids is slightly different than at 3.5% protein due to differences in ionic strength. However, despite the difference in protein
Chapter 3. Rheology of Commercial Powder