BACTOFUGATION & BACTOTHERM:-
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 discbowl 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 73 oC 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 ensure safety of consumption.
Centrifugation is a technique for removing bacteria & somatic cells from milk by centrifugal force. It is often reffered to as bactofugation because the commercial equipment manufactured by Tetra-Pak Processing Systems is marketed under the trade name Bactofuge.
In a bactofuge a centrifugal force of 9000 g is used to remove bacteria and especially the spores farmed by specific bacteria strains, from milk.
WHAT IS BACTOFUGATION ?
Bactofugation is a process in which a specially designed centrifuge called a Bactofuge is used to separate micro-organisms from milk.
The bactofuge is a high speed centrifuge that operates at a force of 980000 kPa and 54° C.
It may remove up to 99 % of the bacteria in milk but under practical conditions usually removes 80 – 90 %. It usually removes 95 % of the spores in the milk. (Tetra Pak)
Bacteria, especially heat resistant spores, have a significantly higher density than the milk. A Bactofuge is therefore a particularly efficient means of ridding milk of bacteria spores.
It is a fast process, typically taking less than 1 s for passage of the milk through centrifuge. Separation of bacteria is based on differences in the specific gravity (SG) of milk and bacterial cells.
Since milk has SG of 1.028-1.038 g /ml bacterial spores have SG 1.30-1.32 g/ml it is reasonably efficient in removing bacterial spores from milk. However, normal vegetative bacterial cells have much lower SGs (1.07-1.12 g/ml) and are more difficult to remove from milk by centrifugation.
Spores are quite small, for the most part 1 to 1.5 μm, but the density difference with plasma is larger than that of bacteria, and at separation temperatures of 60 to 65°C, a substantial proportion can be removed, generally 90 to 95%. By using two bactofuges in series, a reduction by over 99% can generally be attained.
Since these spores are also resistant to heat treatment, the bactofuge makes a useful complement to thermisation, pasteurization and sterilization.
The Bactofugation can be done on the raw stream or on the skim line in conjunction with high temperature treatment of the cream.
Bactofugation efficiency is stated as a percentage reduction of the incoming level of bacteria and spores.
Generally, the efficiency ranges between 98-99.5% for anaerobic spores. Depending on the desired reduction, this can be performed in one step or two.
TYPE OF BACTOFUGE
Two types hermetic centrifuge are currently used:
One is like a normal clarifier, with either continuous or intermittent discharge of sludge. The sludge makes up less than 0.2% of the milk.
The other type is more like a cream separator, in that it has two outlets at the top, one for the cleaned milk and the other for the portion (about 3%) that contains the particles including spores. It also has an enhanced content of casein micelles.
Generally, the spore-rich liquid is sterilized and returned to the cleaned milk. If this is done with the sludge, it should first be diluted by adding milk. Heat treatment is generally for a few seconds at 130°C by steam infusion. Cooling is achieved by the immediate admixture of cleaned milk. Afterwards, the milk is low-pasteurized.
Two types of modern Bactofuge:
1.The one-phase Bactofuge
The one-phase Bactofuge has only one outlet at the top of the bowl for the bacteria-reduced milk. The bactofugate is collected in the sludge space of the bowl and discharged at preset intervals.
2.The two-phase Bactofuge
It has two outlets at the top:
One for continuous discharge of bacteria concentrate (bactofugate) via a special top disc, and another for the bacteria-reduced phase.
The amount of bactofugate from the two-phase Bactofuge is about 3% of the feed, while the corresponding amount from the one-phase Bactofuge can be as low as 0.15% of the feed. Bactofugate always has a higher dry matter content than the milk from which it originates.
This is because some of the larger casein micelles are separated out together with the bacteria and spores.
Higher bactofugation temperature increases the amount of protein in the bactofugate.
Optimal bactofugation temperature is 55 - 60°C.
Originally the Bactofuge was developed to improve the keeping quality of market milk. At the present time bactofugation is also used to improve the bacteriological quality of milk intended for other products like cheese, milk powder and whey for baby food.
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.
The Tetra Centri Bactofuge is used to decrease the bacteria and spores content of milk prior to heat treatment in order to improve the thermal impact.
Bactofuge units are traditionally incorporated in the pre-treatment of cheese milk, where typically butter acid spores (Anaerobic spores) are removed.
Bactofuge units are also used to enhance the quality of powders, consumption milk and cream where typically aerobic spores (e.g. Bacillus Cereus) are removed.
Operating Principle of the bowl
Transport Liquid return
Discharge, bacterially clarified product
Centrifugal pump, transport liquid
Centrifugal pump, bacterially clarified product
Hydro soft Feed
Solids holding space
Solids ejection ports
Closing water chamber
Operating water duct
The separator equipped with a self cleaning disk type bowl. A hydraulically movable sliding pistions opens & closes the bowl.
The product enters the distributor smoothly through the hydro soft feed where it is accelerated to the angular velocity of the bowl & led into the disk stack.
The disk stack consists of a larger number of conical disks situated on top of one another.
The product to be bacterially clarified enters the bowl through feed (2) & is separated in disk stack (6) into a light & heavy liquid phase.
The light, bacterially clarified phase flows to the centre of the bowl & the heavy phase with a high bacteria concentration flows to the bowl periphery.
The bacterially clarified phase is conveyed foam free & under pressure to discharge(3) by centrifugal pump(5).
A transport liquid is conveyed via the separating disk (16) under pressure & without foam to recycle outlet (1) by means of centrifugal pump(4).
From here the transport liquid returns to feed (2).
The task of the transport liquid is to convey even the samallest bacteria at the edge of the centrifugal field in the bowl to solids holding space(8).
The separated bacteria & solids collect in solids holding space(8) & are instantaneously ejected through gap(9).
Solids ejection is indicated by a control unit.
Feed & discharge is through a closed line system.
Depending on the application, the operating water connections are equipped with hand or automatic valves. Before operating the separator, check the functionally & safety relevant setting. Define the maintenance & cleaning procedures as a functions of the operating conditions.
Safety without compromising on nutritive value
Thus better product
ADVANTAGES OVER PASTEURISATION
Pasteurization inactivates pathogens but never able to destroy them .
High heat resistant harmful spores may continue to remain in pasteurised milk if not removed through bactofugation
Harmful spores like clostridium may cause health hazard if not removed physiaclly
Bactofugation delivers better shelflife of milk over pasteurization
Bactofugation also delivers better taste in milk.
ADVANTAGES OVER STERILISATION
Do not effect the nutritive value of milk which gets effected by sterilization.
Do not effect the flavour of milk which gets effected by high heat treatment of sterilization
It should be noted that bactofugation is by no means equivalent to sterilization: the product still contains heat-resistant bacteria and a small number of spores.
Bactofugation removes a portion of the milk, casein and leucocytes, as well as the
bacteria, which may amount to 1 – 2 % of the total quantity of the milk.
The bactofugate (sludge) may contain 12 – 16 % dry matter.
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 60-75 o C followed by pasteurization employing the HTST process.
The bactofugate is deaerated in a vacuum chamber and sterilized at 130-140o C for 3-4 sec using steam injection, and finally mixed with chilled bactofuged milk for further processing.
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Walstra, P., Geurts, T.J., Noomen, A., Jellema, A. and vanBoekel, M.A.J.S. (1999).
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Tetra Pack catalogue
Operating manual of bactofuge installed recently at AMUL DAIRY,ANAND.