The development of electro-membrane filtration for the isolation of bioactive peptides: the effect of membrane selection and operating parameters on the transport rate

The ability to produce functional food ingredients from natural sources becomes increasingly attractive to the food industry. Antimicrobial (bioactive) ingredients, like peptides and proteins, can be isolated from hydrolysates with membrane filtration and/or chromatography. Electro-membrane filtration (EMF) is an alternative for the isolation of these usually strongly charged components. It is believed to be more selective than membrane filtration and less costly than chromatography. The isolation of bioactive peptides from a hydrolysate of αs2-casein, a protein originating from milk, was studied as a model separation for the development of EMF. This separation can be used as an example application for the isolation of other charged components from complex feedstocks in sseveral industries. After 4 h EMF the product consisted for 100% of proven or anticipated charged bioactive components. Diffusion and convection were negligible in relation to electrophoretic transport, since only charged components were recovered in the permeate product. The most important peptide (26% on total protein, starting from 7.5% in the feed) was αs2casein f(183–207), a very potent peptide against Gram positive and Gram negative microorganisms. The transport rate of αs2-casein f(183–207) was reduced strongly when a polysulphone membrane with a molecular weight cut-off below 20 kDa was used. The amount of αs2-casein f(183–207) transported increased practically linearly with the concentration and the applied potential difference. The use of desalinated feeds to further increase the electrical field strength in the feed compartment resulted in higher transport rates, but this increase was lower than expected probably due to the lower electrophoretic mobility. An average transport rate of 2.5 and 4 g/m2.h at maximum was achieved during 4 h EMF using GR60PP (25 kDa) and GR41PP (100 kDa) membranes respectively.

The feasibility of EMF for the isolation of bioactive peptides from an αs2-CN hydrolysate is proven. EMF of this hydrolysate can produce a permeate consisting for 26% of the potent bioactive peptide αs2-CN f(183–207), starting from 7.5% in the feed. All the other components in the product are also proven or anticipated antimicrobial peptides. For the isolation of αs2-CN f(183–207), membranes with a MWCO in excess of 10 kDa (3 times the molecular weight of the peptide) should be used to avoid a strong reduction of the transport rate of the target peptide, probably as a result of increased friction in the pores of the membrane. For a desalinated hydrolysate solution during 4 h EMF operation average transport rates of 2.5 g/m2.h and 4 g/m2.h at maximum are obtained for GR60PP (25 kDa) and GR40PP (100 kDa), respectively. Reduction of the hydrolysate concentration results in a practically linear reduction of the amount of αs2-CN f(183–207) recovered in the permeate product. The use of a feed with higher conductivity, a non-desalinated hydrolysate solution, results in a slight reduction of the amount of αs2-CN f(183–207) transported to the permeate. However, this reduction is smaller than anticipated on the basis of the difference in the electrical field strength in the feed compartment. The low conductivity of the desalinated feed at the start of the experiments probably leads to retardation of the transport due to a reduced availability of counter-ions. An increase of the overall potential difference from 40 to 60 V leads to a practically linear increase in the amount of αs2-CN f(183–207) transported to the permeate. A further increase of the achieved transport rates therefore seems possible by increasing the hydrolysate concentration and the potential difference. The effect of the feedstock conductivity on the transport rate can be improved by optimizing the product of the electrical field strength in the feed compartment and the electrophoretic mobility. The effect of process parameters and the MWCO of the membrane on the separation selectivity and energy consumption should be further investigated.