Debacterization of Cheese Milk by Microfiltration
Together with bactofugation, microfiltration (MF) is currently the only technique that is available at the industrial level for debacterizing milk without any heating treatments. Besides the production of consumption milk with extended shelf life, microfiltration can be applied to cheese milk for the production of dairy products with improved microbiological characteristics. The MF debacterization process involves the use of a membrane of 1.4 mm pore diameter for filtering skim milk: retention of bacteria is over 99.0% (decimal reduction 2.1-3.1, depending on bacteria morphology and volume), and casein transmission is around 99% (Daufin et al., 2001). Fat is previously removed by centrifugation, and is treated in a heat exchanger (90°-95°C, 2-3 minutes) before being readded to MF milk. The most-known commercial process is available under the name Bactocatch. It is operated at 50°C, at high cross-flow velocities, typically 6-8 m/s, and involves the use of ceramic membranes (Saboya & Maubois, 2000).
As a matter of fact, microfiltration makes possible cheese making with almost sterile milk: due to strong retention of Listeria and Salmonella, the process can be used for producing safe cheese from raw milk. From the hygienic point of view, MF cheeses are as safe as those made from pasteurized milk, but some further valuable features has to be considered. First of all, the contemporary elimination of the nonpathogenic microflora, if supported by highly hygienic cheese manufacturing practices, can be the basis for the production of minimally treated products and extension of shelf life of fresh cheeses. Second, removing of all spores allows the production of long-ripened cheeses without the need of additives for preventing late blowing. Finally, elimination of the somatic cells, which are totally retained by the MF membrane, eliminates a series of enzymes that can lead to abnormal evolution of some biochemical events during cheese storage (Law et al., 1995). It has to be pinpointed that MF cheeses cannot be fully considered raw milk cheeses, due to application of a strong heat treatment to the milk fat fraction. Moreover, the process is not compatible with the production protocols of European PDO cheeses, which are based on the preservation of endogenous microflora.