Bacteria and spore removal

In certain cheese types, especially semi-hard varieties, microfiltration or bactofugation techniques are used on the cheese milk to remove micro-organisms such as bacteria and spores.

It has been known for a long time that a membrane filter with a pore size of approximatly 0,2 micron can filter bacteria from a water solution. In microfiltration of milk, the problem is that most of the fat globules and some of the proteins are as large as, or larger than, the bacteria. This results in the filter fouling very quickly when membranes of such a small pore size are chosen.

It is thus the skim milk phase that passes through the filter, while the cream needed for standardisation of the fat content is sterilised, typically together with the bacteria concentrate obtained by simultaneous microfiltration.

In practice, membranes with a pore size of 1,4 micron are chosen to lower the concentration of protein. In addition, the protein forms a dynamic membrane that contributes to the retention of micro-organisms.

The microfiltration concept includes an indirect sterilisation unit for combined sterilisation of an adequate volume of cream for fat standardisation and of retentate from the filtration unit.

Bactofugation is a process in which a specially designed hermetic centrifuge, the Bactofuge®, is used to separate bacteria, and especially the spores formed by specific bacteria strains, from milk. Bactofugation has proved to be an efficient way of reducing the number of spores in milk, since their density is higher than that of milk. Bactofugation normally separates the milk into a fraction that is more or less free from bacteria, and a concentrate (bactofugate), which contains both spores and bacteria in general and amounts to up to 3 % of the feed to the Bactofuge.