IDM | Membrane filtration
Milk protein fractionation by
means of microfiltration – Part 2
How to characterize polymeric microfiltration membranes – a new
approach using a stirred test cell
24 · 9 2017 | international-dairy.com
Authors: Martin Hartinger1 (left), Hans-Jürgen Heidebrecht1 (middle),
Felicitas Arndt2, Hermann Nirschl2, Ulrich Kulozik1 (right)
1) Chair of Food and Bioprocess Engineering, Technical University of Munich
2) Institute for Mechanical Process Engineering and Mechanics (MVM),
Karlsruhe Institute of Technology
Polymeric spiral-wound membranes (SWM) are the most frequently
used membranes in the dairy industry because of their relatively low
price compared to ceramic membranes. They are mostly applied to
concentrate complex media like protein solutions to increase dry
matter before spray drying. In microfiltration (MF) applications, however,
the filtration task is to selectively fractionate proteins of different sizes.
Up to now users of microfiltration membranes decide on the best membrane
specification for their purpose mostly based on the nominal pore size
(nps). The nps is defined as the pore size most frequently present in the
membrane. Nevertheless, comparability of different membranes is poor
due to non-standardized measurement methods to assess the pore size.
Furthermore, the pore size is not a distinct value, but is distributed around a
mean value in each membrane specifically (Ulbricht et al., 2007), depending
on variations in membrane manufacturing conditions. It is common experience
that the prediction of the filtration properties only by means of nps is
barely possible or meaningful. This is why the objective of this work was to
develop a standardized protocol to better characterize polymeric MF membranes
using the example of milk protein fractionation.
Pore size distribution measurement
by means of capillary flow porometry
A pore size measurement using capillary flow porometry (CFP) was conducted
for two different membranes made from polyvinylidene fluoride
(specified in the data sheets as nps = 0.1 μm and nps = 0.3 μm, respectively).
The results are shown in Fig. 1.
Both membranes exhibit pores substantially larger than the nominal pore
size (largest pore 3.9 μm and 1.9 μm, respectively). Surprisingly, the membrane
with the larger specified pore size showed smaller pore sizes. This is
possibly due to the usage of different measurement methods by the manufacturer
for qualifying the membranes. The large pore size distribution
was found in seven different commercial membranes. In addition, other
studies showed the same phenomenon (Ulbricht et al., 2007; Wang et al.,
2008). The conclusion is that it is not possible to determine a membrane´s
characteristics based on non-standardized methods.
Characterization of polymeric
membranes by a stirred test cell
This is why the next step was to investigate the influence of different
pore sizes on filtration performance of polymeric membranes. This was
done by means of a stirred test cell (transmembrane pressure ΔpTM = 0.5
bar; test volume V = 50 ml; temperature ϑ = 20 °C, stirring rate N = 700
Fig. 1: Pore size distribution of two different microfiltration membranes
(membrane 1: nps = 0.1 μm; membrane 2: nps = 0.3 μm).