MODELIZACIÓN Y SIMULACIÓN PARA EL VEHÍCULO REMUS 100

If an increase in the degree of Ca-dependent colloidal interactions between κ-casein hairy layers is at least partly responsible for the formation of observed covalent bonds between proteins under conditions of heat treatment, then it is imperative that a clear understanding of how UF favors these interactions is needed. This cannot take place unless the basic principles of the UF process are first discussed.

UF describes a variety of membrane filtration processes in which hydrostatic pressure forces a fluid against a semipermeable membrane. Solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass

through the membrane into the permeate stream. In bovine milk, the general separation of components is illustrated by Figure 7. The efficiency of a membrane process is determined by both the membrane’s selectivity, governed by its reflection, and the permeate flux. The rejection (R^*) of a particular solute x is described by the equation (Figure 8).

Figure 7: Flow of the components removed from ultrafiltered milk, from Anonymous (2001)

(⁾ *+_, -+/^.)0

+_,  1  +_. +_,/⁾

Figure 8: Description of rejection R* of a solute x, where qw = flux

(kg•m^-2 •s^-1) of a solvent through the membrane, and qx = flux (kg•m^-2 •s^-1) of solute x, c* = concentration (kg solute / kg water) of x at pressure side of membrane

In ideal filtration situations, R* = 1 for all retained components, and R* = 0 for all components which permeate through the membrane. However, most components will have R* values between 1 and 0. Several factors in general are known to affect R*, including the type of membrane utilized, molecular weight of the solute, viscosity of the solute, concentration factor, and transmembrane pressure. With milk in particular, pH and temperature, which not only dictate solution viscosity but also alter the interactions between minerals and proteins, have an effect on both rejection and permeate flux, which is important in gauging the efficiency and speed of the UF process.

The permeate flux is described as the quantity q of liquid which passes through the membrane per unit time and surface area, and is defined by the equation (Figure 9):

+  12

34  5

Figure 9: Description of the permeate flux of a membrane, where q = total permeate discharge, B = permeability coefficient of the membrane, h = effective thickness of the membrane, 4p = transmembrane pressure, and η = viscosity of the permeating liquid

There are several additional factors which effect the permeate flux during skim milk UF. For instance, protein molecules typically adsorb onto the membrane surface over the course of the UF process and reduce the effective pore width, reducing flux. In addition, at high flux values, a gel layer is compressed across the membrane surface which enhances membrane selectivity. High levels of calcium phosphate in retentate promote gel formation, and thus removal of calcium (for example by electrodialysis) is known to increase flux rates.

At typical operating pressures of 20 psi to 80 psi, the turbulent flow of the membrane surface prevents build up of particles and excessive membrane fouling (Pabby et al., 2009). UF membranes contain pores in the range of 0.1 to 0.001 µm. The typical UF membrane utilized by the dairy industry is a pressurized system containing a spiral wound module with negatively charged polyethersulfone membrane with a nominal molecular weight cut-off of 10 kDa. UF applications in the dairy industry include, but are not limited to, production pre-concentrated milk for cheese making, production of milk protein and whey concentrates and isolates, and acquisition of lactose and mineral rich permeates. It was estimated that over 300,000 square meters of membrane were installed in the dairy industry worldwide as of 1998 (Cheryan, 1998).

2.4.3.1. The Effect of UF and DF on Casein Micelle Size Distributions The effect of UF on casein micelle structure is a topic of emerging research.

Along with the previously discussed work of Erdem (2006), who observed the increasing hydrophobicity of the concentrated milk system during UF using the ANS hydrophobic probe, a number of authors have contributed observations to the area of casein micelle size changes as a result of UF. In highly concentrated UF milk retentate, caseins are allowed to interact over short distances, and this has shown to influence the size distribution of micelles. During UF, the distance between casein micelles was shown to decrease from approximately 120 nm to approximately 10 nm (Walstra and Jenness, 1984). Unfortunately, data relating casein micelle size to the UF process is not consistent across all studies. Some studies reported that casein micelle size appeared to be

Karlsson et al. (2007) reported no observable changes in casein micelle size when UF retentate was observed under TEM, using three different preparation techniques, though it was must be noted that any TEM preparation (and virtually any microscopy preparation) may itself interfere with sample size. Singh (2007), utilizing electron microscopy, reported an increase in micelle size during the course of UF. It was also noted that micellar swelling occurred during DF, an increase in nonmicellar material was observed, and that the non-micellar material appeared to link together intact micelles, which is consistent with the findings of Srilaorkul et al. (1991). Meanwhile, Erdem (2006) observed decreases in micelle size as a result of UF. The most recent study was performed by Martin et al. (2010), who reported that casein micelles were not altered during the manufacture of MPC. This conclusion was reached by applying photon correlation spectroscopy (PCS) using the Cumulant method, also described in Martin et al (2007), to raw skim milks, UF/DF retentates, and concentrated UF/DF retentates. It was observed that differences in micelle size between these materials were less than 3 nm, and that this difference was not significant relative to the accuracy of the measurements.

In addition, it was found that MPC reconstituted in water at 60 °C exhibited micelle sizes in the range of 210 nm to 197 nm within 30 minutes after reconstitution, to 197 nm to 195 nm after 1 h reconstitution, which is similar to size observed in skim milk in the same study and other studies of skim milk casein micelle size (Griffin and Anderson, 1983, Holt et al., 1973).

These results show that, in the determination of submicron particle sizes, PCS methods and other light-scattering techniques may be advantageous over microscopy methods because they do not require a sample preparation step that may alter sample

properties, and future work would benefit from the application of PCS to the study of ultrafiltered skim milk for this reason. The results of Martin et al. (2010), obtained by PCS methods, were consistent with those obtained through TEM by Karlsson et al.

(2007) using three different sample preparation steps. Both studies were also consistent with (McKenna, 2000), who utilized electron microscopy and reported no change in casein micelle size distribution under conditions of UF. This body of work is also in agreement with the work of Montero (2010), who utilized dynamic light scattering (DLS) to observe the size of casein micelles at conditions up to 5X UF and noted no statistically significant difference in the size of casein micelles. Despite the earlier microscopy-based work of (Walstra and Jenness, 1984) and Singh (2007), the general agreement of PCS, TEM, and DLS methods indicates that casein micelle size is probably not affected to a significant degree as a result of UF. It is clear from the above research that microscopy-based methods alone, with the possible exception of TEM, cannot be used to draw accurate conclusions about the effect of UF on casein micelle size.