Figure 4. A possible cycle in the wine and juice industry with integrated membrane processes. (Reproduced with the permission from Reference [28]).

In the beer industry, several full-scale installations exist for the production of low-alcohol beer by tight RO membranes, followed by water dilution of the retentate. Apart from the cost, the loss of aroma compounds in the permeate is an important factor. Generally, cellulose acetate membranes (ethanol retention about zero) are used in order to remove sufficient amounts of ethanol [29]. Such membranes have a poorer retention of aroma compounds than polyamide membranes, but the ethanol retention of polyamide RO membranes is as high as about 70% and they are thus not suited for the production of low-alcohol products. The aroma loss problem can, to some extent, be overcome by changing the brewing process, i.e., producing a more "aromatic" beer.

In beer production, almost all the yeast and a significant proportion of the micro-organisms are removed in the traditional filtration step, which includes kiselguhr filtration [30]. Also tank bottoms, which contain some beer, can be further processed in order to recover the beer. Much research has been devoted to the application of microfiltration for beer clarification and for beer recovery from tank bottoms. Another potential application is the replacement of the final pasteurization by microfiltration. The thermal load is avoided using crossflow microfiltration, and the beer clarity is also improved, since particles passing the traditional filter can be removed at the same time [30].

Crossflow microfiltration has a significant potential in brewing. The impact would be substantial if many current processes could be combined into a single stage, e.g., rough beer clarification, tank bottom recovery and sterilization. The use of this technology for wort separation provides an exciting opportunity to bring continuous processing into the brewery and to match further developments in continuous fermentation [31]. Much work has been done using ceramic membranes and high crossflow velocities. Fouling problems have occurred, leading to decreased fluxes and changes in separation properties, which result in the risk of losing some colour, bitter flavour components and foam stabilization proteins.

Using the "BACKSHOCK" process described earlier for membranes with a so-called reversed asymmetric structure (the feeds in contact with the larger pores of the membranes, while the flow-determining pores are located at the interface of the membrane and the permeate), very stable fluxes over 200 l/m2h have been reported when microfiltering beer. In this study, performed with polymeric membranes, very low transmembrane pressures (less than 0.01 MPa) and very low crossflow velocities (less than 0.5 m/s) were used [8].

In a few years, crossflow microfiltration will most probably replace traditional methods to a quite large extent in the beer industry.

3.4. Non-aqueous systems

The use of membrane technology for the separation of solutes from nonaqueous solutions is a new field. As the solvent resistance of membranes is being improved, it is of increasing interest to apply ultrafiltration and microfiltration in the process for refining of edible oils.

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