New Developments In Food Applications

3.1. Dairy applications

Ultrafiltration of milk and whey for the enrichment of proteins and the use of reverse osmosis for concentration are established processes in the dairy industry. The introduction of crossflow microfiltration using ceramic membranes at constant transmembrane pressures applying the UTP mode mentioned earlier, has opened new and exciting possibilities, and during the last few years, membrane separation processes for the fractionation of milk proteins have been developed [11].

Examples of interesting products are defatted whey protein concentrates, lactoferrin, lactoperoxidase, (i-lactoglobulin, a-lactalbumin and k-glycomacro-peptide, opening up new worldwide markets with potentially high added value. The same approach has been investigated for the fractionation of caseins. Micellar casein-enriched milks obtained through the use of microfiltration have provided opportunities for cheesemakers to improve the efficiency of their equipment. Native phosphocaseinate, also separated by microfiltration, will easily compete with traditional products, (i-casein, separated by microfiltration, in addition to expanding the manufacture of cheese varieties through modification of the a/Pcasein ratio in cheesemilk, is the required substrate for producing numerous peptides thought to have physiological activities [11],

3.1.1. Fat removal from whey

The production of whey protein concentrates (WPC) with a protein content of 35-80% of the total solids using ultrafiltration has been a commercial process for many years [12]. WPC's with high protein contents have a substantial amount of fat which leads to decreased functional properties and shorter storage time. Part of the fat is valuable phospholipids. Residual fat can be separated as proposed by Maubois et al. [13] by exploiting the ability of the phospholipids to aggregate through calcium binding when subjected to a moderate heat treatment (55 °C, 8 min). The resulting precipitate, as well as non-aggregated fat, is removed by microfiltration through a 0.2 jim membrane. The defatted whey leads to higher fluxes in UF compared to untreated whey, but UF membranes with smaller cut-offs than for untreated whey are required to obtain the same whey protein retention [14]. Whey fat aggregation is optimized by using ultrafiltered whey retentate instead of normal whey, adjusting and maintaining MF reteníate pH at 7.5, and finally applying the heat treatment described above. This has also led to major improvements in MF performance such as flux rate and protein recovery. Defatted whey is also a very good raw material for the production of lactoferrin and lactoperoxidase through ion-exchange chromatography.

Purified (J-lactoglobulin and a-lactalbumin can be prepared from the defatted WPC due to the fact that at low pH and moderate heat treatment (55 °C) a-lactalbumin polymerizes reversibly entrapping most of the other whey proteins except p-lactoglobulin. By using microfiltration (0.2 pm) (J-lactoglobulin can be fractionated from the other proteins. Further purification of p-lactoglobulin is carried out by ultrafiltration coupled with electrodialysis or diafiltration [14]. Purification of a-lactalbumin from the MF retentate is accomplished by solubilization at a neutral pH and then ultrafiltration through a 50,000 dalton membrane. Further work is required to improve the purity.

Alfa-Laval [7] has developed a process for the production of defatted, functional WPC containing 85% protein from sweet whey in which the whey is first ultrafiltrated (volume reduction factor 2.5-4) in order to reduce the whey volume and thus the membrane area of the microfiltration plant. In the MF plant, 90% of the feed is removed as permeate. The retentate contains nearly all the fat and denatured proteins from the whey and at least 99.5% of the bacteria and spores. A minimum constant capacity of 30-45 l/m2h dining 20 hours for UF-treated raw whey using 0.1 pm membranes and uniform transmembrane pressure microfiltration is possible, at a volume concentration factor of 4 in the UF plant. The MF retentate is then further concentrated in a second UF plant giving a final protein content of 85% and a fat content of less than 0.4% in the dried product [7], When MF is performed on raw whey at a concentration factor (CF) of 10, the overall flux is 90-100 l/m2h, while the MF flux of the UF retentate (CF 4) is about 40 l/m2h. meaning that the overall MF flux is about 4x40=160 l/m2h. This means that the overall gain in the MF flux is due to the removal of the free fatty acids and the phospholipoproteins which are normally the main source of irreversible fouling material in the MF ceramic membranes.

3.1.2. Removal of bacteria from milk

In the Bactocatch process developed by Alfa-Laval [6] for the extension of the shelf-life of pasteurized milk or for the improvement of the quality of cheese milk, the principle of uniform transmembrane pressure is used during microfiltration of skim milk through 1.4 jim ceramic membranes. In this way, the bacterial content of the permeate is reduced to less then 0.5% of the original value, while the retentate contains nearly all of the bacteria as well as spores. The retentate is then mixed with cream and heat treated at 130 °C for four seconds before remixing with the permeate. Finally, the mixture is pasteurized. The bacteriological quality of the milk is improved significantly. Fluxes of about 600 l/m2h during up to six hours are reported [6], Several commercial plants have been installed worldwide, a few of them in Sweden.

In spite of the decrease in rennetability of MF/HTT cheesemilk compared with low pasteurized milk, several cheese varieties were produced successfully without nitrate addition. From the view point of spore reduction, the MF/HTT technique is reported to be more efficient than one-stage bactofugation [15],

3.1.3. Fractionation of caseins

Ceramic membranes with a pore size of 0.2 pm allow a specific concentration of micellar casein in skim milk. The resulting permeate has a composition close to that of sweet whey, but it does not contain caseinomacropeptide, phospholipoproteins or bacteria. Its content of high molecular weight whey proteins, such as immunoglobulins and bovine serum albumin, may be different from that of a normal whey. The retentate is an enriched solution of native calcium phosphocase-inate. Depending on the concentration, it can be utilized as a casein standardized cheesemilk. If diafiltration is performed during the MF step, purified phosphocaseinate is obtained (up to 90% protein of total solids content) [11], Microfiltration using 0.2 pm membranes also allows for the separation of (3-casein when this component is solubilized from the casein micelles giving process streams suitable for modifying the p-casein/as-casein ratio in cheese milks and consequently the texture and flavour of the cheeses [16]. The main interest in (3-casein is related to the presence of peptides with biological activities in its sequence. The recovery of small amounts of valuable components or an alteration in composition of dairy liquids is regarded to be major challenges possible by membrane technology.

3.1.4. Ultrafiltration used in cheesemaking

Intensive work has been, and is being done, on ultrafiltration for the manufacture of fresh, soft, semihard and hard cheeses. Examples are given in Table 1.

Table 1.

Different types of cheese processed by ultrafiltration. Data from [17]

Cheese type Structure UF concentration UF and

Closed Open Partial Total evaporation



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