Bactofugation

Bactofugation

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 specific gravity is higher than that of milk.

Bactofugation normally separates the milk into a fraction which is more

or less free from bacteria and a concentrate (bactofugate) which contains

both spores and bacteria in general and amounts up to 3 % of the feed to

the Bactofuge.

Bactofugation of milk is always a part of milk pretreatment. In applications

where quality milk for cheese and powder production is the objective,

the Bactofuge is installed in series with the centifugal separator, either

downstream or upstream of it.

When the quality of the surplus cream produced by direct in-line fat

standardisation is an important consideration, the Bactofuge should be

installed upstream of the separator. By doing so the cream quality will be

improved as the load of spores of aerobic sporeformers such as Bacillus

cereus will be reduced.

The same temperature is often chosen for bactofugation as for separation,

i.e. 55 – 65°C or typically 60 – 63°C.

There are two types of Bactofuge:

• The two-phase Bactofuge

• The one-phase Bactofuge

The two-phase Bactofuge has two outlets at the top:

– one for continuous discharge of the heavy phase (bactofugate) via a

special top disc, and

– one for the bacteria-reduced phase.

The one-phase Bactofuge has only one outlet at the top of the bowl, for

bacteria-reduced milk; the bactofugate is collected in the sludge space of

the bowl and discharged at preset intervals through ports in the bowl body.

These two types make it possible to choose various combinations of

equipment to optimise the bacteriological status of milk used for both

cheesemaking and other purposes.

In this context it may also be mentioned that whey, if intended for production

of whey protein concentrate as an ingredient in infant formulae,

should be bactofuged after recovery of fines and fat.

Process alternatives

There are about ten possible ways to configure a bactofugation plant; three

examples are given here:

1 Two-phase Bactofuge with continuous discharge of bactofugate

2 One-phase Bactofuge with intermittent discharge of bactofugate

3 Double bactofugation, with two one-phase Bactofuges in series.

1. Two-phase Bactofuge with continuous discharge of bactofugate

This concept, shown in figure 14.3, works under airtight conditions and

produces a continuous flow of air-free bacteria concentrate (bactofugate) as

the heavy phase. This phase, comprising up to 3 % of the feed flow (adjusted

by an external lobe-rotor pump with variable speed control) is often

sterilised and remixed with the main flow. The steriliser is of infusion type,

and a sterilisation temperature of approx. 130°C for a few seconds is sufficient

to inactivate spores of Clostridia micro-organisms. The hot bactofugate

leaving the steriliser is mixed with about half the volume of the bactofuged

milk to lower the temperature before it is reintroduced into the rest of

the bactofuged flow. Following mixing, the milk is routed to the pasteuriser

to be pasteurised at 72°C for 15 seconds, followed by regenerative and

final cooling to renneting temperature.

The Bactofuge with continuous discharge of bactofugate is used in applications

where

– remixing of sterilised bactofugate is possible,

– there is an alternative use for the bactofugate in a product where the

heat treatment is strong enough to inactivate the micro-organisms.

Nominal hourly capacities are 15 000 l and 25 000 l (two sizes of centrifuge),

which empirically achieve at least 98% reduction of anaerobic spores.

2. One-phase Bactofuge with intermittent discharge of bactofugate

To achieve the same reduction effect as mentioned above, nominal capacities

of 15 000 l/h and 25 000 l/h are likewise recommended. The bactofugate

from a one-phase Bactofuge is discharged intermittently through ports

in the bowl body at preset intervals of 15 – 20 minutes, which means that

the bactofugate will be rather concentrated and thus also low in volume,

0.15 – 0.2% of the feed. When the bactofugate is to be re-introduced into

the cheese milk, it must be sterilised. This is illustrated in figure 14.4, which

also shows that before being pumped to the infusion steriliser the concentrate is diluted with bactofuged milk, some 1.8 % of the feed, to obtain a

sufficient volume for proper sterilisation. Start and stop of the discharge

pump (6) is linked to the operation mode of the discharge system of the

Bactofuge.

As it leaves the steriliser the hot bactofugate is cooled by admixture of

bactofugated milk, about 50 % of the basic feed.

Where legislation does not permit reuse of the bactofugate, it can be

discharged to the drain or collected in a tank for products to be sent to a

destruction plant.

3. Double bactofugation with two one-phase Bactofuges in series.

Bactofuging milk once is not always sufficient, particularly at high spore

loads in the milk. With double bactofugation, reduction of Clostridia spores

reaches more than 99%. Figure 14.5 illustrates a plant with two one-phase

Bactofuges in series serving one sterilising unit.

What was said above about treatment of the bactofugate applies here

too.

Double bactofugation is sufficient in most cases to produce cheese

without addition of bacteria-inhibiting chemicals. During periods when very

high loads of spore-formers are expected, small amounts of chemicals, 2.5

– 5 g per 100 l of milk, may however be used for safety if legally allowed.

Without any mechanical means of reducing spores it is normal to add

some 15 – 20 g of sodium nitrate per 100 l of milk to inhibit their growth,

but with single bactofugation and a high load of spores in milk, 2.5 – 5 g per

100 l of milk will prevent the remaining spores from growing.

Microfiltration

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 skimmilk 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. The principle of microfiltration is discussed in Chapter 6.4,

Membrane filters.

In practice, membranes of a pore size of 0.8 to 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.

Figure 14.6 shows a milk treatment plant with microfiltration. The microfiltration

plant is provided with two loops working in parallel. Each loop can

handle up to 5 000 l/h of skimmilk, which means that this plant has a

throughput capacity of approximately 10 000 l/h. Capacity can thus be

increased by adding loops.

The raw milk entering the plant is preheated to a suitable separation

temperature, typically about 60 – 63°C, at which it is separated into skimmilk

and cream. A preset amount of cream, enough to obtain the desired fat.