WO2002069724A1

WO2002069724A1 - Method of pretreating milk in microfiltration

Info

Publication number: WO2002069724A1
Application number: PCT/SE2002/000398
Authority: WO
Inventors: Stefan HEDSTRÖM; Harald Hermansson; Jeanette Lindau
Original assignee: Tetra Laval Holdings and Finance SA
Current assignee: Tetra Laval Holdings and Finance SA
Priority date: 2001-03-08
Filing date: 2002-03-07
Publication date: 2002-09-12
Anticipated expiration: 2003-09-08
Also published as: SE0100801D0; SE518901C2; SE0100801L; AR032969A1

Abstract

The invention relates to a method of pretreating milk before the milk is to be microfiltered. The method includes the method steps that raw milk (1) is heated and separated into a cream fraction (5) and a skimmed milk fraction (4). A milk fraction (11) which constitutes at least the skimmed milk fraction (4) is caused, at a temperature of approx. 50-55°C, to stay for at least two minutes in an open vessel (12) before the milk fraction (11) is led further to the microfiltration.

Description

METHOD OF PRETREATING MILK IN MICROFILTRATION

TECHNICAL FIELD

The present invention relates to a method of pretreating milk in microfiltration, comprising the method steps that raw milk is heated and separated into a cream fraction and a skimmed milk fraction, and that a milk fraction, constituting at least the skimmed milk fraction, is led at a temperature of approx. 50-

55°C to microfiltration.

BACKGROUND ART

The microfiltration of milk, with a view to obtaining a consumer milk possessing superior shelf life and a flavour which is reminiscent of pasteurised milk is an increasingly employed method. Previously, the heat treatment of milk has been the only alternative. Depending on treatment temperature in combination with the length of time the milk is treated at this temperature, a sterile milk will be obtained or alternatively milk of extended shelf life.

Sterile milk or commercially sterile milk may be defined as a product which is free of micro-organisms which can grow under the prevailing conditions. A sterile milk, packed under aseptic conditions in aseptic packages, enjoys the advantage of being able to be distributed and stored at room temperature for a lengthy period of time. Heat treatment with UHT (Ultra High Temperature) is the commonest method of producing sterile milk. The milk is normally treated at 135-150°C for 4-15 seconds. Through the heat treatment, the micro-organisms in the milk are exterminated. However, the drawback in UHT treated milk is that the elevated temperature imparts to the milk a cooked (boiled) flavour.

The microfiltration of milk in order to produce a sterile consumer milk is, for example, described in Swedish Patent Specification SE 9702359-2. In the described method, skimmed milk is caused to pass through a microfilter. The microfilter may, for example, consist of a membrane filter of ceramics. In the microfilter, the skimmed milk is divided up into a permeate flow which has a considerably reduced content of micro-organisms, as well as a retentate flow which contains the greater part of bacteria and spores. By selecting a microfilter with suitable pore size in combination with a gentle heat treatment, a sterile end product will be obtained. The described method moreover enjoys the advantage that the retentate flow from the microfiltration is not used in the end product. By causing the retentate flow to pass through additional microfilters, there will finally be obtained a retentate which only constitutes a minor part of the original skimmed milk quantity. This residual quantity is not used in production.

The microfiltered skimmed milk can, by the admixture of a certain quantity of heat treated cream, obtain a desired fat content.

These prior art methods have all employed the same pretreatment before the microfiltration. The raw milk coming into the dairy was heated to a suitable separation temperature and separated into a skimmed milk phase and a cream phase.

The heated skimmed milk or alternatively standardised milk was thereafter led straight on to microfiltration.

However, it has proved that microfiltration immediately after separation/standardisation does not give sufficiently long operating times. Already after a few hours, the filter may be blocked, so-called fouling, whereafter it is necessary to interrupt production in order to clean the microfilters.

OBJECT OF THE INVENTION

One object of the present invention is to realise a pretreatment method of the milk which substantially extends operating time for the microfiltration.

SOLUTION

This and other objects have been attained according to the present invention in that the method described by way of introduction has been given the characterising feature that the heated milk fraction is allowed to stay for at least two minutes in an open vessel before the microfiltration.

Preferred embodiments of the present invention have further been given the characterising features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings, in which: Fig. 1 is a flow diagram for which the pretreatment method according to the present invention may be employed; and

Fig. 2 shows diagrams of different pretreatment methods.

DESCRIPTION OF PREFERRED EMBODIMENT

The flow diagram in Fig. 1 shows a process for producing sterile consumer milk with the aid of microfiltration, the process further including the method steps of stabilisation and enzyme deactivation. The ghosted line area in the Drawing encompasses the pretreatment method according to the present invention. When raw milk 1 enters into the dairy, it is normally at a temperature of 4-

6°C. Many times, the raw milk 1 is stored at this temperature in large buffer tanks before being treated for the production of sterile consumer milk. The cooled raw milk 1 enters into the process and is normally heated directly to 50-60°C. This heating may, for example, take place in some form of heat exchanger 2, such as, for example, a^{^}plate heat exchanger.

The heated milk is led further to a separator 3. A conventional separator 3 requires that the milk be at a temperature of 50-60°C in order to attain an optimum separation of the raw milk into a skimmed milk phase 4 and a cream phase 5. Alternatively, cold separation can be employed with the aid of a special cold separator. Cold separation takes place at 4-6°C and in the event that this method step is chosen, the milk must be heated after the separation.

The cream phase 5 from the separator 3 is led to a standardisation unit 6 in which it may be possible to add a certain quantity of fat to the skimmed milk phase 4 before the microfiltration. Extra cream 7 may be added if necessary to the standardisation unit 6 and any possible surplus cream 8 may be taken out from the unit 6 for other use.

The cream phase 5 or parts of it is heated treated in a conventional UHT plant 9. The UHT plant 9 normally includes a heat exchanger, for example a plate heat exchanger or a tube heat exchanger, in which the cream phase is heated to 135-150°C during a period of time of 1-15 seconds, whereafter a sterile cream phase 10 is obtained. Later in the process, the sterile cream phase 10 can be added to the skimmed milk phase 4 as is apparent from the flow diagram.

A minor part of the cream may possibly be added to the skimmed milk phase 4 from the separator 3 for the formation of a milk phase 11. Possibly, the milk phase 11 consists only of the skimmed milk phase 4. The milk phase 11 should be at a temperature of 50-55°C in order that an optimum microfiltration can be carried out.

However, the heated milk phase 11 in accordance with the inventive concept as herein disclosed should stay in an open vessel 12 for at least two minutes before the microfiltration takes place. Preferably, the milk should stay in the open vessel 12 for at least five minutes. The open vessel 12 may consist of an open tank or a tank which is only partly filled. Alternatively, the open vessel 12 may consist of some form of de-aerator. A simple form of de-aerator is a container which has an intermediate partition which does not reach right up to the upper wall of the container. The container is only partly filled, but the liquid surface should be located above the upper edge of the intermediate partition. By introducing the milk phase 11 in the bottom of the one section of the container, the milk is forced up towards the open surface of the container and further down to an outlet located in the bottom of the second section of the container. Such a de-aerator gives a guaranteed stay time of at least two minutes, at the same time as the air may be discharged into the atmosphere.

While the milk phase 11 stays in the open vessel 12, the air which cannot be dissolved in the milk will be discharged into the atmosphere. The raw milk 1 contains an amount of dissolved air and when the milk is heated up to 50-55°C before the microfiltration, the surplus of air must be given the opportunity to escape into the atmosphere, since heated milk cannot dissolve as much air as cooled milk. If the air is not given the opportunity to escape from the milk phase 11 before the microfiltration, this will negatively affect the microfiltration result.

The stay time also affords the opportunity for stabilisation of chemical reactions which take place in the milk on heating, int. al. reduced solubility of calcium phosphate. The solubility of calcium phosphate is reduced with increased temperature. On the heating of the milk phase 11, a part of the previously dissolved calcium phosphate will partly become insoluble and associate with the casein micelles of the milk. Microfiltration of an unstabilised milk phase 11 may give rise to shortened operating time because of fouling on microfiltration.

After the milk phase 11 has stayed in the open vessel 12 for at least two minutes, and preferably at least five minutes, the milk phase 11 is led to a microfilter 13. The filter 13 is preferably manufactured from ceramics, but it may also be made of glass, polymers or the like. In the microfilter 13, the milk phase 11 is divided into two flows - a retentate flow 14 and a permeate flow 15 - where the permeate flow constitutes the larger proportion.

The microfilter 13 has an effective pore size of 0.5 μm, which implies that the filter 13 separates off all spores normally occurring in the milk. Alternatively, the filter 13 has an effective pore size of 0.3 μm, which implies that the microfilter 13 separates off the major proportion of thermoresistant bacteria and all spores. The permeate 15 which passes through the microfilter 13 thus has a substantially reduced content of micro-organisms.

The retentate 14 may be led to an additional microfilter, or alternatively be recycled back to the input side of the microfilter 13. The intention is to reduce the quantity of retentate 14, since the retentate 14 is not used in the process. The retentate 14 not being employed, an extremely pure milk product will be obtained.

The permeate 15 is led further to a heat treatment 16 which, for example, may be carried out in a heat exchanger such as a plate heat exchanger. The permeate 15 is heat treated at 90-105°C, if the microfilter 13 has an effective pore size of 0.5 μm, and at 72-98°C if the filter 13 has an effective pore size of 0.3 μm. This gives a sterile milk product, i.e. a product free of micro-organisms which may grow under the prevailing conditions.

The process together with which the method according to the present invention may be employed can also include stabilisation 18 of the product as well as deactivation 19 of enzymes. This may be put into effect in one context, in that the sterile permeate 17 is kept at a predetermined temperature during a given time. Alternatively, the stabilisation 18 may take place at one specific temperature and deactivation 19 of enzymes at another. A milk product which should be able to be stored at room temperature must be stable and not contain active enzymes. Otherwise, the finished milk product may be subjected to a certain degradation during storage, which may result in changes in flavour and/or consistency, often accompanied by a deterioration in the aroma.

With a view to obtaining a standardised milk product at a predetermined fat content, it is possible to admix into the permeate 15, 17, a part of the sterile cream phase 10. The flow diagram in Fig. 1 shows how the sterile cream phase 10 may be admixed at different points in time in the process, such as after the microfiltration 13, after the heat treatment 16, after the stabilisation 18 or after the enzyme deactivation 19. Through the above-described process, there will be obtained a sterile and stable milk product. This product may thereafter possibly be homogenised 20 in order to prevent cream setting in the finished product, whereafter the product is packed in aseptic consumer packages in an aseptic filling machine 21. The pretreatment method according to the present invention is illustrated in

Fig. 1 in that the method steps included in the method are defined by means of a ghosted line area. The method steps that are included are consequently the raw milk 1 which enters into the plant and is heated 2, whereafter the heated raw milk 1 is separated 3 into a cream phase 5 and a skimmed milk phase 4. A milk phase 11 which at least includes the skimmed milk phase 4 and which is at a temperature of 50-55°C is thereafter to stay for at least two minutes, and preferably at least five minutes, in an open vessel 12 before the milk phase 11 is led further to the microfilter 13.

This pretreatment method has proved to be necessary in order to achieve an efficient n icrofiltration during a reasonable production time. In order that a microfilter 13 of the type that can be employed for this process is to function efficiently, the increase of the transmembrane pressure TMP per unit of time of the filter 13 must not be too great. The transmembrane pressure TMP is defined as (Pι+P₂)/2 - P₃, where Pj is the pressure which is measured in the retentate flow on the entry into the microfilter 13, P₂ is the pressure which is measured in the retentate flow 14 at the outlet from the filter 13 and P₃ is the pressure which is measured in the permeate flow 15.

Fig. 2 shows diagrams for microfiltration with different pretreatment methods. All microfiltrations in the trials have been carried out with a filter 13 having an effective pore size of 0.5 μm and where the milk phase 11 in to the filter 13 is heated to 50-55°C. The one axis in the diagram designates the transmembrane pressure TMP in the microfilter 13 expressed in bar, and the other axis defines operating time expressed in hours.

The two trials 22 and 23 which are illustrated with hollow symbols have employed a conventional pretreatment which does not include the method step that the milk phase 11 was caused to stay for at least two minutes in an open vessel 12. The two trials 24, 25 which are symbolised by solid symbols were carried out with a pretreatment method according to the present invention which comprised a stay time of at least two minutes and preferably at least five minutes in an open vessel 12. In order to be able to function efficiently, the filter 13 which is employed in the trials must display a transmembrane pressure TMP whose increase does not exceed 100 millibar/hour, preferably 30-40 millibar/hour and a desired operating time of at least ten hours. In the two trials 22, 23 which employed a conventional pretreatment method, the transmembrane pressure TMP increases by an average of almost 200 millibar/hour in an accelerating cycle. Already after 3.5-4 hours, the filter 13 has "blocked up", for which reason production must interrupted, the filter 13 cleaned and the plant sterilised and restarted. An operational downtime of this type takes approx. 4 hours to remedy. On the other hand, in the trials 24, 25 where a pretreatment method according to the present invention was employed and the milk phase 11 was caused to stay in an open vessel 12 for at least two minutes and preferably at least five minutes, there is an operating time exceeding ten hours.

As will have been apparent from the foregoing description, the present invention realises a method for the pretreatment of milk which is to be microfiltered which gives considerably longer operating times for a microfiltration plant than corresponding prior art pretreatment methods.

Claims

WHAT IS CLAIMED IS:

1. A method of pretreating milk in microfiltration, comprising. the method steps that raw milk (1) is heated and separated into a cream fraction (5) and a skimmed milk fraction (4), and that a milk fraction (11), constituting at least the skimmed milk fraction (4), is led at a temperature of approx. 50-55°C to microfiltration, characterised in that the heated milk fraction (11) is caused to stay for at least two minutes in an open vessel (12) before the microfiltration.

2. The method as claimed in Claim 1, characterised in that the stay time in the open vessel (12) is at least five minutes.

3. The method as claimed in Claims 1 and 2, characterised in that the open vessel (12) consists of a de-aerator.

4. The method as claimed in Claims 1 and 2, characterised in that the open vessel (12) consists of a container with an intermediate partition, the container having an open liquid surface located above the upper edge of the intermediate partition, and that the container has an inlet in the bottom of the one section of the container and an outlet in the bottom of the other section of the container.

5. The method as claimed in any of the preceding Claims, characterised in that the microfiltration takes place in a membrane filter (13), said membrane filter (13) having an effective pore size of <0.5 μm.

6. The method as claimed in any of the preceding Claims, characterised in that the microfiltration takes place in a membrane filter (13), said membrane filter (13) having an effective pore size of <0.3 μm.