According to a method such as this, fat-containing raw milk is directed into a vessel whe- rein the cream fraction is separated from the raw milk under the effect of gravity.

The invention also relates to the apparatus according to the preamble of Claim 5 for the treatment of raw milk.

By the separation of raw milk, fat is separated from the milk, whereby there are obtained a low-fat ("fat-free") milk and a fat-containing cream and/or cream-milk fraction. The separa- tion is complicated by the foaming of raw milk, which foaming is due to the proteins pre- sent in the milk. It is known that the preservability of milk and non-fermented milk products is relatively short, and thus efforts have been made to improve, for example by pasteuriza- tion, the preservability of milk and cream products obtained from separation.

It is an object of the present invention to eliminate the disadvantages associated with the state of the art and to provide an entirely novel solution for the treatment of raw milk. It is a particular object of the invention to provide a method and apparatus for the separation of raw milk, whereby at the same time the preservability of the separated milk and fat frac- tions can be improved.

The invention is based on the idea that an inert gas, such as nitrogen gas, is fed into the raw milk in order to promote separation. The gas is introduced into the milk in the form of small gas bubbles, these bubbles increasing the uplift of the fat particles emulsified in the milk and speeding up the separation of the fat. At the same time, oxygen can be effectively removed from the milk and the milk products.

In practice, the apparatus used comprises a container having a raw-milk inlet nozzle and means for removing the milk fractions obtained from the separation. According to the in- vention, in the lower portion of the apparatus there are further arranged means for intro- ducing and bubbling gas into the raw milk fed into the container.

More specifically, the method according to the invention is mainly characterized by what is stated in the characterizing part of Claim 1.

The apparatus according to the invention is characterized by what is stated in the charac- terizing part of Claim 5.

The invention provides considerable advantages. Thus, the invention provides a gentle separation of cream (fat) from the raw milk. Oxygen is at the same time removed from the raw milk, whereby the preservability of the raw milk and the cream being separated can be improved considerably. The invention can be used in the dairy industry for removing oxygen, increasing uplift that promotes the separation of cream from the raw milk. The treatment is gentle and, thus, it is well suited for a raw material that contains conjugated linolic acids (CLA). These are in general concentrated in the membranes of phospholipids.

By the treatment, the breaking of these can be prevented. In addition, the invention can be applied to the preparation (aeration) of vinegar and to smoking of solutions.

By the option according to the invention it is possible to replace the conventional centrifu- gal separation of fat. It is, however, also possible to combine the process according to the invention with conventional separation (i. e. the separation of particles by means of centri- fugal force), in which case the separation according to the invention and centrifugal sepa- ration are carried out successively. Centrifugal separation may also be implemented befo- re the separation process according to the invention or, most suitably, after it.

The invention will be discussed below more closely with the help of a detailed description, with reference to the accompanying drawing. Figure 1 a of the drawing depicts a cross- sectional side elevation of the structure in principle of the apparatus according to the in- vention. Figures 1b and 1c depict plan views of the alternative structural solutions of the inert-gas pipe systems.

In the process according to the invention, raw milk, by which is meant in general cow milk or goat milk from which significant amounts of fat have not been removed, is treated. The raw milk is treated by allowing spherules of fat emulsified in the aqueous phase to separa- te from the aqueous phase under the effect of gravity. Most simply this is carried out by allowing the milk to stand. It is, however, possible to increase the effect of separation by gravity through the use of centrifugal force (conventional separation technique).

According to the invention, the separation of fat spherules is promoted by bubbling an inert gas in small bubbles into the milk, the size of the bubbles being typically approx. 0.1 - 10 mm. The inert gas used can comprise nitrogen or noble gases, such as argon. It is also possible to use carbon dioxide, which is in this case to be regarded as inert, since it does not as such react with the components of milk, or even air from which at least most (at least 80 %, most suitably at least 90 %) of the oxygen has been removed.

The inert gas is introduced pressurized into the milk ; typically the pressure is approx. 110 - 300 kPa. Since there prevails a reduced pressure in the container (see below), it is also possible to allow this reduced pressure to suck into the milk inert gas from the source of gas. In particular, the proportion of oxygen in the gas fed into the raw milk should be less than 5 % by volume, most suitably less than 2 % by volume.

Since the bubbling of gas into raw milk may cause foaming of the milk, it is preferable to carry out the bubbling at reduced pressure. Typically the separation according to the in- vention is carried out at an absolute pressure of approx. 10-90 kPa. By means of the reduced pressure the removal of oxygen from the milk and milk products is reinforced and the nitrogen gas can be recovered. The reduced pressure is most suitably generated by combining the gas space above the milk to a source of reduced pressure, by means of which the pressure in the gas space is reduced. The gas space can be connected to the source of reduced pressure by means of a hose or pipe, which most suitably has a back- pressure valve.

It is preferable to carry out the separation of the milk to be treated in a container, typically a pressure vessel or a corresponding closed container, at the bottom of which there is installed a gas pipe system that is shaped into a circle, coil or grid, so that a maximally even flow of gas through the raw milk is attained. The pipes may be, for example, of sinter or an otherwise densely perforated material, through which gas flows as small bubbles evenly over the entire surface area of the bottom. The gas (e. g. nitrogen) is fed into the pipe system in a controlled manner, i. e. at a steady pressure and flow. It is also possible to feed the inert gas directly into the milk being introduced into the separation container.

The method according to the invention can be implemented as a batch or semi-batch pro- cess or as a continuous process. Typically it is operated as a batch process, wherein the milk quantity to be treated is fed into a closed separation container, vacuum is generated in the container, and an inert gas is bubbled into the milk. The bubbling is continued until

the raw milk has become segregated into a surface layer in which the fat is concentrated and a bottom layer in which the fat content of the milk is at maximum approx. 3 % by weight, typically 1-3 % by weight. Usually during the process the fat content of the milk is lowered to approx. 0.1-2 % by weight. In general there forms in the milk a fat concent- ration gradient, wherein immediately under the surface there is a cream layer containing a large amount (20-40 % by weight) of fat, and below this a cream-milk layer containing slightly less fat (5-10 % by weight).

A typical treatment time varies according to the amount of milk treated, but generally it is approx. 1 min-24 h. When the amount of milk is 100-1000 liters, the treatment time is approx. 1-10 hours.

The fat-containing cream layer on the surface of the milk is recovered. The cream and cream-milk layers can be withdrawn from the container from the top or be removed by decanting, and the fat-free milk layer is run out of the container from the bottom. The pro- ducts recovered are directed to further treatment.

Figure 1a depicts one embodiment of the apparatus according to the invention. The appa- ratus comprises a container 1, which is, for example, cylindrical or otherwise circular in its horizontal cross section, in order that no inner corners complicating the separation should form in the container. The container is closed at both ends 2,3 in order to form a pressure vessel. The pressure vessel stands on support members (legs) 4-6 on top of a base.

It is thus possible to generate reduced pressure inside the container in order to reduce foam formation and to promote the separation of cream. The bottom 3 of the container 1 is equipped with an inlet nozzle 7 for the raw milk to be treated, in which nozzle a mechanical filter 8 can be mounted for the removal of any solid impurities possibly present in the milk batch to be treated. The wall 9 of the container is provided with insulation in order to ensure isothermal activity. The insulation may also be"active", by which is meant that pipes for the transport of a cooling agent can be arranged in the container wall.

Through the wall of the upper end 2 of the container there passes a pipe 10, the opening of which inside the container is at a desired height from the container bottom. The pipe can be, for example, a telescopic pipe, in which case it is easy to set the position of the pipe end in the vertical direction. The cream/cream-milk fractions separated from the raw milk are removed via the pipe 10.

At the upper end 2 of the container there is further a suction pipe 11 connected to a sour- ce of reduced pressure, not shown, the suction pipe having a back-pressure valve, as well as a washing-system nozzle, indicated by reference numeral 12, for washing the con- tainer. A hose for feeding milk into the container can be connected to the inlet nozzle 7 in accordance with the figure.

As is evident from the general description above, according to the invention there is sepa- rated from the raw milk a fat-containing emulsion (a cream or cream-milk fraction) by fee- ding into the raw milk an inert gas in the form of small gas bubbles. The gas-feeding pipe system 13 can be arranged at the bottom of the container in the manner depicted in Figu- re 1 b or 1 c, either as a grid or as a circle or spiral. In addition, the invention can be applied in the preparation of vinegar (aeration), the"dissolving"of gas mixtures in liquids, and the "seasoning"of liquids with smoke substances.

It is essential that the gas be fed as evenly as possible over the entire cross-section of the container. The pipes in the gas-feeding pipe system are most suitably perforated, in which case the inert gas flows in the form of small bubbles out from the pipes into the surroun- ding milk. The pipes must be of a food-compatible material, plastic or metal.

Alternatively, the inert gas may be fed into the milk inlet nozzle, in which case it mixes effectively with the milk being introduced into the container and is released from it under the effect of the reduced pressure prevailing in the container.

The apparatus according to the figure operates according to the batch principle, raw milk being fed into the container 1 via the feed port 7. An inert gas is mixed with the milk, the gas giving the fat particles emulsified in the raw milk further uplift and promoting their se- paration. Fat rises to the surface of the raw milk and thus forms, at least typically, several cream and cream-milk fractions, the fat concentration of the cream fraction being approx.

25-30 % by weight and that of the cream-milk fraction respectively approx. 8-10 % by weight. The concentration of fat in the remaining milk is approx. 1.1-1.3 %. The cream and cream-milk fractions can be separated by withdrawing them out of the container by means of a telescopic outlet pipe. Via the reduced pressure pipe a moderate vacuum is maintained in the container in order to inhibit the expansion of the foam layer. Typically, the reduced pressure is approx. 10-90 kPa (absolute pressure). During separation the milk is maintained cold ; preferably its temperature is approx. 1-10 °C, most suitably ap- prox. 2-6 °C.

The cream and cream-milk fractions removed from the container are directed for further treatment. Their preservability properties are very good, since the inert gas fed into the milk separation has removed air from these fractions, whereby the oxidation of the fat and the breaking down of the proteins are inhibited. Most suitably the separated fractions are kept in an inert atmosphere even after their removal from the container.

The milk separation process can be controlled in different ways. In small-scale operation, inspection windows from which the efficacy of the separation can be observed can be installed in the container wall. For the needs of large-scale industry, the container can be equipped with sensors based on the specific gravity of liquid or with sensors based on electrical conductivity, by means of which the densities and fat concentrations of the frac- tions rising to the surface can be monitored.

After the separation treatment, the fat-containing fractions are separated by means of the outlet pipe and are directed to further treatment, which can be carried out in a conven- tional manner. Thus the cream and cream-milk fractions can be used for the production of milk products such as cheeses, fresh cheeses, fat spreads, yogurt, etc. The remaining milk, which contains fat in an amount of only one per cent or one and a half per cent (weight/weight), is removed from the container via the milk feed port. The milk can be af- ter-treated by, for example, pasteurization.