MARKET MILK

Module 6. Common dairy operations

Lesson 21
BACTOFUGATION

21.1 Introduction

Bactofugation is the process of removal of microorganisms from milk using centrifugal force. It is a special form of separation of microorganisms, mainly spore formers (Bacilli/Clostridia) to enable milk to be sterilized at lower temperature-time combinations. Most of the microorganisms are inactivated by pasteurization. However, the highly heat resistant spores survive pasteurization. They can lead to significant quality defects in hard cheese, semi-hard cheese or long-life products due to proteolysis, lipolysis and gas formation. Therefore, bactofugation is mainly used in the manufacture of these products. The objectives of bactofugation are as follows:

• To improve hygienic quality of milk

• To avoid heat resistant bacteria without resorting to excessive heating

• To ensure exceptionally high degree of bacteriological purity in milk.

It removes bacteria, both living and dead, from treated substances whereas traditional heat treatment kills bacteria and leaves them in food. The microorganisms involved in causing milk spoilage, reducing the quality of powder and butyric fermentation thereby causing late blowing of cheese, are mostly spore formers. Bactofugation is important in foodstuffs infected with bacteria containing thermostable endotoxins.

21.2 Bactofuge

Bactofuge are special nozzle clarifying separator with high separation precision that can remove microorganisms from milk based on their density difference (skim milk – 1.036; bacteria – 1.07 – 1.13 g/cm³).

21.2.1 Design

Separated micro-organisms are concentrated on the periphery of the bowl and the small portion of such discharged milk, highly enriched with the bacteria (bactofugate) is thrown outside via nozzles, and then collected in the sludge discharge pipe. The bactofugate which still corresponds to 2 -3 % of the treated milk may be sterilized separately and added back to the bactofuged milk to avoid the losses of milk solids (Fig. 21.1).

Fig. 21.1 Hermatic bactofuge

21.3 Bactotherm Process

Clarified and standardized milk is pumped into a plate heat exchanger, where it is heated to a temperature of 60-75°C prior to being fed to the bactofuge. The centrifugal acceleration is increased to 10,000g. The slurry of bacteria (bactofugate) is discharged continuously through nozzles due to their greater specific weight. It is about 3% of the feed by volume and represents a reduction in total bacteria by approximately 50-60%. The bactofugate stream is UHT processed (130 -140°C for 3-4 seconds) and remixed in the normal stream. This time- temperature profile is sufficient to inactivate all spores. The sterilized bactofugate is re-chilled in the plate heat exchanger and can be added back to the de-aerated milk or discharged separately for other suitable applications (Fig 21.2). Continuous recycling of the sterilized bacterial concentrate into the milk avoids losses of products. There are many other applications of the Bactotherm process such as butter oil separation, high fat cream, and other non-dairy applications. For each product, the geometry of the bowl could be altered to suit the product.

Advantages

• Use of bactofuged cheese milk prevents swelling in certain cheeses which may be caused by heat resistant butyric acid bacteria. Removal of bacteria without pasteurization enables cheddar cheese production with more typical cheese flavor.

• In powders, it reduces the count of microbes and allows significant removal of heat resistant bacteria.

• The severity of heat treatment can be reduced in sterilized milks.

• In cream, defects caused by heat resistant Bacillus cereus can be avoided.

Fig. 21.2 Flow sheet of the bactotherm process