3.2. Cross-flow filtrations
By using the newly modified method for the determination of the mean specific resistance during
cross-flow filtrations of milk a study was carried out, which allows an evaluation of the mean specific
fouling resistance as a function of pressure and colloidal interactions between casein micelles in the
deposit. Due to the extremely fast flux decline, when milk was used as a filtration fluid, milk was diluted
by protein free milk serum (10 kDa UF-permeate).
The speed of the deposit formation could thus be reduced. As shown in fig. 2, a dilution did not lead
to a significant change in deposit build up and thus resulted in an equivalent specific fouling resistance. A
dilution of 1:5 (milk: UF-permeate) gave the required speed reduction for gaining a sufficient amount of
data points (>>10) for the ln-linearization (4). Hence, this concentration was used for all experiments to
keep the casein concentration as close to the native concentration as possible. Fig. 2 shows, that an
increase in the pressure drop through the deposit layer increased the mean specific resistance for the
filtration of milk at native pH 6.8 as well as for a lower pH 5.9. During filtrations at reduced pH, the
pressure dependency was significantly higher. For a better visualization, trend lines based on e-functions
are used in the diagram.
For cross-flow filtrations a pressure increase not solely leads to a compression of the deposit, but also
to an altered composition of the deposit layer. Steps towards higher transmembrane pressures resulted in
increased initial flux values (J0). Thus the particle size cut-off shifted to larger particle diameters due to
the increased convective transport. Increased transmembrane pressures, therefore, resulted in deposits of
higher polydispersity. This should result in a reduced specific fouling resistance for a constant porosity
with regard to equation (3). Since small micelles can move through cavities in the deposit, certain cavities
can be filled up by small casein micelles. As a consequence the porosity decreases and the mean specific
resistance increases [11].
The porosity impacts equation (3) reciprocal by the fourth power. Hence, pressure dependency of the
mean specific resistance during cross-flow filtrations is very likely due to a change in the polydispersity
of the casein micelle deposit. This effect must be enhanced by a compaction of the micelle deposit as
similarly found during model dead-end filtrations.
An acidification of milk and, thus, reduction of the repulsion between casein micelles must lead to an
increase in compressibility similar to the results for the dead-end filtration of pure casein solution.
Additionally, a reduced repulsion between casein micelles facilitates their deposition. Thus, the
polydispersity of the deposition increases due to the higher probability of deposition and the deposit
becomes more compact for higher transmembrane pressures.