Other Centrifugal Processes of Milk

The principle of differential movement of heavy and light components of milk subjected to a centrifugal force has been utilized in a few applications other than clarification and cream separation. These include bactofugation and clarifixation.

 

i. Bactofugation

We may recall clarification of milk wherein it was stated that the centrifugal removal of heavy dirt particles etc. results in elimination of a part of bacteria as well. Such bacteria removal is made more effective using a special high-speed disc-bowl centrifuge called ‘bactofuge’. This process known as ‘bactofugation’ is particularly applicable to removal of bacterial spores from milk, which are not only difficult to inactivate by heat treatment but also heavier (or denser) than vegetative cells.

The bactofuge is kind of high-speed (up to 20,000 rpm) clarifier provided with discharge nozzles in the bowl wall. The centrifugal force generated in it is upto 10,000 g (g = gravitational acceleration). The bacteria in milk being bactofuged are collected in the form of ‘bactofugate’ in the sludge space. The bactofugate is approx. 3% of the feed volume and contains primarily bacterial spores and milk proteins. Anaerobic spores are removed to an extent of 98-99%. A double-bactofuge treatment at 73oC yields more than 99.9% reduction in bacterial spore count of milk. However, since bactofugation does not effectively eliminate all microorganisms,pathogens in particular, the milk must ordinarily be pasteurized so as to ensuresafety of consumption.

The main application of this expensive process is in the field of cheese making where removal of anaerobic (clostridial) spores from milk is useful in avoiding the problem known as ‘late blowing’ in hard and semi-hard cheeses. Bactofugation has also been employed to extend the shelf life of milk under refrigeration.

In order to destroy the bacteria contained in the bactofugate and improve the economy of the process by utilizing the milk protein in it, a process called ‘Bactotherm’ has been evolved. Clarified and standardized milk is bactofuged at 60-75oC followed by pasteurization employing the HTST process. The bactofugate is deaerated in a vacuum chamber and sterilized at 130-140oC for 3-4 sec using steam injection, and finally mixed with chilled bactofuged milk for further processing.

 

ii. Clarifixation

A clarifixator is a machine working on the same principle as that of the centrifugal separator but has an additional provision to effect reduction of the size of fat globules in the cream fraction before it is remixed with the outgoing skim milk. The resulting milk, sometimes called ‘stabilized’ milk has a reduced tendency to creaming upon undisturbed storage because of small-size fat globules.

The break-down of fat globules is brought about by the peripheral spikes or protrusions on the ‘paring disc’ provided in the cream passage at the top of the centrifuge. A paring disc is a fixed circular structure acting as a stationary centripetal pump. The cream separated from milk strikes the protruding obstacles before entering the paring disc. The fat globules thus experiencing an intense turbulence or shearing action are broken down to a smaller size (less than 2 mm). The cream is then mixed with the incoming milk to be recycled through the bowl. The fat globules of sufficiently reduced size will not get re-separated when the cream passes through the bowl discs again and will therefore, exit the separator through the skim milk outlet which thus discharges ‘homogenized’ whole milk. However,because of its lower effectiveness as compared to pressure homogenization,clarifixation has not been used to any significant extent in the dairy industry.