Fouling by proteins during micro filtration a literature review
Applications of Membrane Processing
UF to concentrate milk, whey proteins, clarify juices, wines and process antibiotics CFMF for separation of individual proteins, casein standardisation of cheese milk De-fatting and clarification of whey in the production of high quality whey protein concentrate (WPC) Cell recovery from fermentation broths and sterile filtration of heat labile protein solutions etc.
Membrane fouling is a major problem retarding further application of CFMF
Previous studies are pertinent to fouling of polymeric UF and MF membranes in stirred/unstirred batch cells using constant pressure operation
Fouling results in a decline in flux with time and change in selectivity of the membrane. The increase in retention is an advantage in UF applications but is a disadvantage in some MF applications that require high protein transmission.
CFMF and Ceramic membranes
Cross-flow devices are generally preferred due to reduction in concentration polarisation and surface fouling compared to dead-end devices.
There is growing interest in the use of ceramic membranes for MF. There are three major advantages of using ceramic membranes: 1) they are resistant to chemical cleaning; 2) they can be steam sterilized and backflushed; and 3) they have long membrane life.
Constant flux vs TMP operation
It is suggested in the literature that MF operation at constant flux is better than operation at constant pressure because the former avoids high permeate fluxes in the first few minutes. Very few studies have been conducted in cross-flow mode under constant permeate flux conditions using ceramic membranes.
Studies indicated that fouling of MF membranes is influenced by 1) membrane properties 2) operating conditions and 3) properties of feed material.
Severe membrane fouling has been reported in microfiltration experiments with bovine serum albumin (BSA) even when the pore size was much larger than the protein (Bowen & Hughes, 1990; Franken et al., 1990; Kelly & Zydney, 1994, 1995; Tracey & Davis, 1994; Jonsson et al., 1996; Mueller & Davis, 1996; Herrero et al., 1997 etc). The reason for the severe fouling appears to be the presence of protein aggregates in the feed. However, Hlavacek and Bouchet (1993) showed that prefiltered BSA solutions can still foul the membrane.
Kelly and Zydney (1995) showed that initial fouling was caused by the convective deposition of protein aggregates on to the membrane surface.
Jonsson et al. (1997 have proposed two consequent steps in fouling, surface blocking and cake formation during BSA filtration.
In almost all these cases, fouling occurs predominantly on the membrane surface by protein aggregates.
On the other hand, Franken et al. (1990), Bowen and Gan (1991, 1992) and Jonsson et al. (1992a) have suggested that shear within the membrane pores causes the protein to deposit.
Tracey & Davis (1994) and Mueller & Davis (1996) during fouling studies using BSA on 0.2 µm MF membrane hypothesised that protein molecules or aggregates deposit at the pore walls or mouths.
Bowen and Gan (1993) and Marshall et al. (1997) have suggested that fouling during MF of protein solutions is most likely due to the interaction of protein with the pore geometry at the pore entrance. There is considerable debate whether fouling is initiated on the membrane surface or within the pores or at the pore entrance.
Although the above studies gave some understanding of the possible mechanisms of protein fouling, there are still conflicting views about the underlying principles that govern protein fouling during MF.
The increase in retention is an advantage in UF applications but is a disadvantage in some MF applications that require high protein transmission.
- Proteins can be separated in whey (WPC)
- Milk can be concentrated prior to cheese
- making at the farm level
- Apple juice and wine can be clarified
- Waste treatment and product recovery in
- edible oil, fat, potato, and fish processing
- Fermentation broths can be clarified and separated
- Whole egg and egg white ultra filtration as a pre
- concentration prior to spray drying
Microfiltration (MF) membrane fouling has been a major problem in the process industry.
Even though the pore sizes in MF membranes are generally over an order of magnitude larger than the characteristic size of the protein, there is considerable experimental evidence that severe fouling occurs and proteins play a critical role in MF fouling.
A number of fouling mechanisms may arise depending upon operational variables, feed and membrane properties.
Proteins are complex molecules and a greater understanding of their conformation, stability and interactions in different membrane environments and under conditions of shear is crucial to understand and control fouling in these processes.
The literature review reveals that protein adsorption is the first step in the fouling process, although its effect is small on MF membranes.
Pore fouling is usually the second step. Pore fouling appears to be dominated by pore plugging most probably at the pore entrance by aggregates that are present in the feed or those produced during processing.
Protein to pore size ratio seems to be an important factor determining this step.
Surface layer formation or accumulation of protein aggregates on the membrane surface as a third step appears to follow once the MF membrane pores are completely plugged or covered by protein deposition.
The surface layer could be in the form of a gel layer, if the protein size is much bigger than the pore size and if there are protein-protein interactions. The situation is exacerbated under high flux conditions apparently due to concentration-induced effects