3. Discussion3.1. Dead-end filtrati

3. Discussion
3.1. Dead-end filtrations
Fig. 1 (a) shows the mean specific resistance of the casein micelle deposit layer at pH 6.8, 6.2 and 5.9
as a function of transmembrane pressure. An increase in transmembrane pressure results in higher specific
resistances. At low transmembrane pressures (ΔpTM < 0.5 bar) the pressure dependency (slope of the
curve) was highest and identical for pH 6.8 and 5.9. At ΔpTM > 0.5 bar the specific resistance at pH 5.9
showed a stronger pressure dependency than the resistance at the native pH of milk.
From the specific resistance the mean porosity of the deposit layer can be calculated using equation (3).
Results are depicted in fig. 1 (b). The porosity values were ~ 0.63 at the lowest pressure (0.08 bar) for
both pH values. An increase in transmembrane pressure leaded to a compaction of the deposit layer,
resulting in a mean porosity of 0.30 for pH 5.9 and 0.35 for pH 6.8 at ΔpTM = 4 bar.
Decreasing the pH from 6.8 to 5.9 leaded to a 50% reduction of the repulsive interaction energy
between casein micelles. But repulsive energies are still measurable [2]. Interestingly, the reduction of the
repulsive interaction energy does not change the porosity of the deposit layer below a transmembrane
pressure of 0.5 bar in spite of the strong pressure dependency of the porosity at low values, see fig. 1 (b).
These results show that casein micelle deposits are highly compressible and that a minimal pressure
exists below which the particle separation distance is not significantly reduced when micelle repulsion is
decreased. Above the critical pressure a reduction of micelle repulsion increases the compressibility of the
deposit layer.