AT408178B - METHOD FOR TREATING RAW MILK BY HOMOGENIZATION AND MICROFILTRATION - Google Patents

Description

       

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   The present invention relates to a process for the treatment of raw milk by homogenization and microfiltration and to a process for the treatment of raw milk for the production of treated milk which has a lower bacterial content than raw milk, the milk being separated into a fat fraction with a minimum fat content of 10% and a skimmed milk fraction is separated, the skimmed milk fraction is passed through a microfilter which has an average pore size which is sufficient to reduce the bacterial content of the milk flowing through to provide a filtrate which has a lower bacterial content than the original - skimmed milk fraction and to provide a concentrate which has a higher bacterial content than the original skimmed milk fraction,

     the bacterial content of the fat fraction is reduced separately and then the skimmed milk fraction after microfiltration and the fat fraction with reduced bacterial content are combined, and a method for producing milk with a fat content of 2%, wherein a skimmed milk fraction is passed through a microfilter which has an average pore size which is sufficient to reduce the bacterial content of the milk flowing through in order to provide a filtrate which has a lower bacterial content than the original skim milk fraction and to produce a concentrate which has a higher bacterial content than the original skimmed milk fraction, the bacterial content of a cream fraction with a minimum fat content of 10% is reduced,

   and then the skimmed milk fraction after the microfiltration and the cream fraction are combined with the reduced bacterial content.



   The well-known pasteurization process for killing bacteria in milk has been used for many decades. Unfortunately, the higher temperatures required in the pasteurization process affect the taste of the milk. Furthermore, the pasteurization process does not eliminate all undesirable bacteria even at such high temperatures, which leads to a short shelf life of most milk products.
The predominant bacteria in conventionally processed milk of relatively advanced age are often of the Bacillus cereus type, since they survive the pasteurization process and thrive at low temperatures and thus promote the deterioration of the milk. There is a general need for a method for reducing the bacterial content in Milk, both whole and skimmed milk,

   to increase the shelf life of the product and improve its taste by eliminating the pasteurization process.



   Of great economic importance is the need to eliminate the very expensive and labor-intensive distribution process that is currently required to bring the milk into the hands of the consumer. It is currently necessary for every dairy to process the raw milk by homogenizing and other steps, to fill the milk in containers for distribution to the consumer and to transport it under refrigerated conditions. This requires every dairy to acquire and maintain a substantial fleet of refrigerated trucks to transport the processed milk to the point of distribution to the consumer.



   By creating a sterile or almost sterile milk product, it would be possible to eliminate the need to transport the milk under such cooled conditions. Unfortunately, the pasteurization process only creates milk with a reduced number of bacteria and not a sterile product.



   It would also be possible if a sterile milk product could be produced that
Eliminate need to store milk at the place of distribution under refrigerated conditions. Eliminating the need for large refrigerated compartments, as in typical shops, would also be of tremendous economic benefit.



   Even if today's pasteurization process is used, it is sometimes of particular importance to obtain milk with a reduced bacterial content before pasteurization, e.g. For example, a single batch of raw milk can be contaminated in such a way that mere pasteurization does not even lead to an adequate shelf life according to the current standard.



   For some applications, it is also valuable if it is possible to have a treated one
To create milk in which the bacterial content has been greatly reduced, e.g. B. to about 1/100 of the original value. For the production of cheese, it is particularly important to create a milk with a relatively low bacterial content, since incorrect bacterial cultures can destroy the cheese.



   For use in cheese production, it is normally not expedient to simply subject the milk to a sufficient degree of heat treatment, since such heat treatment results in a lower cheese yield and can also have a negative effect on the coagulation time.

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   Additives are normally used to reduce the problem, but in some cases it would be desirable to avoid using such additives.



   Various methods for producing milk with a lower bacterial content through the use of filtration are known in the art, but none has been widely accepted. The methods of the prior art generally either create low flow rates, so that the method becomes uneconomical on a large scale, or on the other hand influences the quality of the milk and thus makes the product unacceptable for the consumer.



   Conventional filtration media have been tried for the production of milk with a reduced bacterial content. A method is known in which the entire milk is divided into filtrate and concentration fractions by microfiltration. The filtrate that passes through the pores of the filter (the size of the pores can range from 0.1 µm to 10 m) consists of milk with a substantially reduced fat content and the concentrate, which is the fraction that passes through the surface - The surface of the filter is made of cream, since not only bacteria but also fat globules are essentially retained by the filter.



   The published Swedish patent application SE 67 15081 A discloses a method for sterilizing milk, in which fat is first separated from the skimmed milk. The fat fraction is then sterilized using heat and the skimmed milk fraction is sterilized using bacterial filtration (no filter pore size is specified). Finally, the sterilized fat and skim milk fractions are mixed together again to give a sterile milk product. In order to sterilize the skimmed milk fraction in this way by means of bacterial filtration, the pore size in the filter must be so small that no bacteria can pass through, which results in low throughput rates and an undesirable retention of fat globules and the protein content of the milk.



   US Pat. No. 5,064,674 relates to a process for the production of hypoallergenic milk by ultrafiltration processes which use membranes which allow molecules with a molecular weight of less than or equal to approximately 5 kDa to pass through. The secreted components that are collected by the membrane contain milk proteins, viable or non-viable bacteria, bacterial protein antigen and milk fat. The filtrate collected from the ultrafiltration process is therefore not only free from bacteria and bacterial protein antigen but also from fat and milk protein, which makes the product unusable for use as milk itself.



   It is therefore clear that the pores of bacterial filters that are used according to the prior art and which filters are effective for sterilized milk will not only remove the bacteria, but also fat balls and at least some of the proteins. Such a filter is quickly blocked by the collected material; this causes the flow rate through the filter to drop rapidly and the filter often has to be cleaned or replaced. The high cost of such an inefficient process is generally prohibitive. Furthermore, the quality of the milk is negatively affected because the filter retains fat globules and proteins.



   From the previous discussion, it is evident that there is a constant need for an improved milk filtration processing method that creates a sterile or more or less sterile product that has an improved shelf life and does not adversely affect milk quality.



   Attempts have therefore been made to use cross-flow or tangential flow filtration devices for the treatment of milk, such devices being known from the prior art.



   Several types of filter devices have been described which enable such a tangential or cross flow filtration to be carried out. Perhaps the oldest such known device is formed by a tube made of porous material which is fastened in a second tube, the suspension to be filtered flows under pressure at high speed through the annular space between the two tubes and the filtrate inside the porous tube flows.

   Improved devices with a similar structure use two concentric cylinders, the inner cylinder being formed by a microporous membrane and the liquid being subjected to a forced, helical flow around the inner cylinder
Other cross flow devices include a number of filter elements that are in the form of

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 Plates or disks are arranged one above the other and microporous membranes are arranged on their two surfaces, for. B. around a filtrate collecting tube, the suspension to be filtered flowing between the disks in a helical path one after the other.



   Many other variations for cross flow filtration systems have been developed. For example, US 5 009 781 A relates to a cross-flow filtration device with a filtrate network that includes a number of longitudinal filtrate chambers and one or more filtrate channels that pass through the chambers. US 5 035 799 A relates to a cross flow filter device having filter sheet compositions arranged in parallel in the filter tank with input under pressure to produce turbulent cross flow of the fluid over the media.



   US Pat. No. 5,015,397 A relates to a cross-flow filtration device and a method which includes a tube consisting of a helical wedge wire mesh. The contaminated liquid supplied enters at one end and as it flows through the pipe, it is enriched with contaminants, while clarified liquid passes through the pipe wall.



  US 5 047 154 A relates to a method and a device for increasing the flow rate of flow-through filtration systems. US 4 569 759 A relates to a tangential filtration device and a system which contains such a device.



   Cross flow filtration is significantly different from flow filtration because the liquid supply is introduced parallel to the filter surface and the filtration takes place in a direction perpendicular to the direction of the supply flow. Since the direction of the feed flow is tangential to the membrane surface, the accumulation of filtered solids on the filter medium is generally reduced by the shear effect of the flow in cross-flow filtration systems. Cross-flow filtration therefore offers the possibility of a quasi-steady state function with an almost constant flow if the throughput pressure differential is kept constant. Unfortunately, this theoretical possibility has not yet been achieved in practice. Therefore, the problem of decreasing filtration flow has disturbed conventional cross-flow filtration systems.

   The majority of the suspended solids were held on the wall of the tube and quickly formed a dynamic membrane (also filter cake or sludge layer). The dynamic membrane is largely responsible for the filtration, which takes place in the following.



   Those particles that initially penetrated the wall matrix are then captured last because the pore structure is irregular and intricate in nature. As micro-filtration continues, the presence of the dynamic membrane prevents the penetration of additional small particles into the wall matrix. The formation of the dynamic membrane together with the possible clogging of the pore structure of the tube by trapped particles results in a decrease in the filtration flow. In conventional systems, this decrease is almost exponential to the filtration time.



   Flow filtration of milk has been attempted but generally not accepted due to the problems discussed above. US Pat. No. 5,028,436 relates to a process for separating the dissolved and undissolved components of milk, using a microporous membrane with a pore size in the range from 0.1 to 2 m, soft membrane with an aqueous solution, dispersion or emulsion of lipids or Peptides has been pretreated. The milk is separated by passing it through the pretreated membrane. In the method of the patent, a first filtration step is carried out using a microporous membrane in a tangential flow mode. A clear filtrate and a thickly flowing concentrate are obtained.

   The filtrate contains all salts, lactose, amino acids, oligopeptides and polypeptides of low molecular weight in pure, undenatured form and the concentrate contains practically all of the casein and fat components of the milk. Thus the filtrate cannot be regarded as "milk" since the Fat substances have all been removed from it.



   US 4,876,100 A relates to a cross flow filtration process for the production of milk with a lower bacterial content. Raw milk is separated by centrifugal separation into a fraction consisting of cream and into a fraction consisting of skimmed milk. The skimmed milk fraction is passed into a microfilter in which parts of the fat globules, proteins and bacteria are separated. The microfilter gives a filtrate, which consists of skimmed milk with a lower fat protein and bacterial content, and a concentrate, which has an increased fat protein and bacterial content. The concentrate is then sterilized.

   The filtration method according to US 4,876,100 A thus reduces the fat and protein as well as the bacterial level.

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 content of the filtrate, the properties being changed compared to those of the original skim milk.



   Obviously, the use of cross flow filtration has not provided an acceptable method for reducing bacterial contamination in milk to date.



   GB 1 451 747 A also describes a method in which a skimmed milk fraction is separated from a cream fraction using a filter.



   From GB 2 128 464 A a method for the treatment of milk by homogenization and microfiltration is known. As a mandatory treatment step between homogenization and microfiltration, an enzymatic coagulation is provided which leads to a liquid which is subsequently subjected to microfiltration. The liquid obtained can be used as a feed additive or as a nutrient.



   A means to overcome some of the problems associated with classic cross-flow filtration technology has been found and is known as dynamic microfiltration. The dynamic filtration process eliminates the disadvantage in classic cross-flow technology, since the liquid to be filtered is not simply guided tangentially over the membrane surface. The membrane surface or a solid body close to the membrane surface is moved in such a way that the fluid in the intermediate layer between the rotor and the stator is subjected to a shearing action. The shear "scrubs" the membrane surface, which keeps it relatively clean from particulate material, and prevents a filter cake from forming on the membrane surface.

   The particulate material that would otherwise accumulate on the membrane surface remains in suspension and is finally removed in a second stream, generally referred to as a concentrate stream.



   Dynamic microfiltration systems can take various forms, e.g.



  US 5 037 562 A, 3 997 447 A and 4 956 102 A relate to dynamic microfiltration disk systems.



   Cylindrical, dynamic microfilter devices are used e.g. As taught in U.S. 4,956,102 A, 4,900,440 A, 4,427,552 A, 4,093,552 A, 4,066,554 A and 3,797,662 A. All patents cited in the present application are incorporated by reference herein.



   No one has ever used dynamic microfiltration in milk processing and the use of cross flow filtration in milk has been limited and has principally been used to separate milk into components based on fat content.



   It has now surprisingly been found that dynamic microfiltration of milk can be carried out successfully using the method of the present invention, so that the problems of the prior art, such as deterioration in milk quality, early filter clogging and inadequate bacterial removal, do not occur.



   According to the invention, in the method for treating raw milk by homogenization and microfiltration, the milk is subjected to dynamic microfiltration within five minutes after homogenization by passing the milk through a microfilter with an average pore size that is sufficient to contain the bacteria of the milk which has passed through is reduced sufficiently to provide, as the filtrate, a treated milk which has a lower bacterial content than the original raw milk and a concentrate with a higher bacterial content than the original raw milk.

   By performing the homogenization step first, the particle size of the fat globules and other large suspended components of the milk is significantly reduced, which enables the microfiltration of the milk without significant removal or entrainment of fat or other components.



   Milk is a mixture of fat, protein particles and water. Homogenization is a method of reducing the emulsion particle size to allow passage through a suitably sized, microporous membrane to retain the bacteria contained therein, without the unwanted removal of fat and protein content in the milk.



   After homogenization, the milk is filtered using dynamic microfiltration. The invention thus provides an improved method for producing milk with a lower bacterial content without the need for pasteurization. That part of the milk fraction which is retained by the microfilter (the concentrated fraction) can be recycled or removed as part of the inflow or in others Procedures are used.

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   The milk obtained has a very low bacterial content, such as 103 bacteria per milliliter, or less, and contains more organoleptic components than can be found in pasteurized milk with the same bacterial content.



   The milk which can be obtained as a result of the process according to the invention is generally of better shelf life than the milk which is obtained as a result of conventional pasteurization. A significant bacterial residue remains in the milk after pasteurization, since milk naturally contains certain bacteria which also survive the pasteurization process. Therefore, pasteurized milk still needs to be refrigerated to limit bacterial growth and spoilage.



   Unfortunately, some of the bacteria found in raw milk are both heat resistant (bacteria that survive pasteurization) and psychotropic (bacteria that grow below 15C at low temperatures), such as the Bacillus cereus. The presence of heat-resistant and psychotropic bacteria in packaged milk products is very disadvantageous since their rapid growth results in the milk spoiling even under chilled conditions.



   The present invention allows sterile milk to be produced that can be stored even at room temperature for long periods of time, such as 30 days or more. The sterile milk of the present invention can be characterized by the absence of bacteria in general and in particular the absence of bacteria and pathogens such as the following:
Heat-resistant bacteria:
Micrococus M. luteus. M. roseus
Streptococcus S. pneumoniae. S. lactis, S faecalis
Lactobacillus L. delbrueckii, L. lactis, L. helveticus,
L. casei, L. trichodes
Staphylococus S. aureus, S. epidermidis
Bacillus B. cereus. S. subtilis, B. macerans,
B. stearothermophilus
Clostridium C. butvrium.

   C.pasteurianum,
C botulinum, C. perfringens, C. tetani
Psychotropic bacteria:
Pseudomonas P. aeruginosa, P. fluorescens,
P. pseudomallei, P mailei
Archnomobacter
Alcaligenes
Acientobacter A liqnieressii. A. equirli
 EMI5.1
 Bacillus B. cereus. B. subtilis, B. macerans,
B stearothermophilus coli-like bacteria:
 EMI5.2
 



     Klebsiella pneumoniae
Various :
Listeria L. monocvtoqenes
Thus, the milk made by the method of the present invention can meet and exceed the requirement of pasteurized milk of grade A, which requires that the milk have a bacterial occupancy of 30,000 per ml and a coli-like bacteria number of 10 per ml, as by standard methodology determined, does not exceed.



   According to the invention, the filtration is carried out using a turntable microfilter, and the milk can be homogenized simultaneously with the filtration step.



   The dynamic microfiltration is preferably carried out with an effective surface speed of the microfilter of 3 m / sec to 50 m / sec.



   In particular, the microfiltration is carried out at a milk temperature in the range from 15 to 60 ° C. and particularly preferably in less than 30 seconds after the homogenization step.

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   According to the invention, the dynamic microfiltration is carried out with a shear rate of 10,000 sec-1 to 400,000 sec-1.



   Furthermore, a microfilter is used which has a pore size of 0.01 to 5.0 µm, preferably 0.1 to 1 m and particularly preferably 0.2 to 0.5 m.



   According to the invention, part of the concentrate is recycled. The filtration can be carried out using a cylindrical, dynamic microfilter.



   The microfiltration is preferably carried out at a milk temperature in the range from 15 to 60 ° C. and part of the concentrate is returned.



   In another embodiment, the present invention provides a method of treating raw milk to produce treated milk that has a lower bacterial content than raw milk, wherein the milk is separated into a fat fraction with a minimum fat content of 10% and a skim milk fraction, the skim milk fraction is passed through a microfilter that has an average pore size that is sufficient to reduce the bacterial content of the milk flowing through to deliver a filtrate that has a lower bacterial content than the original skim milk fraction and to provide a concentrate that has a higher bacterial content has as the original skim milk fraction,

   the bacterial content of the fat fraction is reduced separately and then the skim milk fraction after the microfiltration and the fat fraction with reduced bacterial content are combined, the skim milk fraction being homogenized within 5 minutes before the microfiltration and the microfiltration being carried out as dynamic microfiltration.



   The bacterial content of the fat fraction is preferably reduced by dynamic microfiltration.



   Alternatively, the bacterial content of the fat fraction can be reduced by pasteurization.



   In a further embodiment, the present invention provides a method for producing milk with a fat content of 2%, wherein a skimmed milk fraction is passed through a microfilter which has an average pore size which is sufficient to reduce the bacterial content of the milk flowing through in order to reduce the filtrate supply which has a lower bacterial content than the original skim milk fraction and to provide a concentrate which has a higher bacterial content than the original skim milk fraction, the bacterial content of a cream fraction with a minimum fat content of 10% is reduced, and then the skimmed milk fraction after microfiltration and the cream fraction are combined with the reduced bacterial content,

   whereby the skimmed milk fraction is homogenized within 5 min before the microfiltration and the microfiltration is carried out as dynamic microfiltration.



   Preferred embodiments of the present invention are described below, reference being made to the accompanying drawing, which is a graph showing the particle size in the milk after homogenization.



   The raw material is fresh untreated raw milk like from a pet e.g. one
Cow The method of the present invention can also be applied to processed milk, e.g. Milk that has already been pasteurized, but not all advantages can be realized, such as the production of milk with improved organoleptic properties when compared to milk that has not been pasteurized.



   The raw milk to be treated can first be passed through a heat exchanger to bring it to a certain temperature and, if desired, then passed through a centrifugal separator to remove all or part of the cream fraction in a conventional manner.



   The raw milk is homogenized and advantageously passed through a dynamic microfilter, a filtrate fraction and a concentration fraction being obtained. The pores of
Microfilters are sized to retain at least some of the bacteria. The filtrate, which is that portion of the milk fraction that passes through the retentive surface of the microfilter, consists of milk with no or low bacterial content (relative to the milk prior to microfiltration) with essentially no change in fat and protein content. The filtrate fraction can then be used directly to make other products, such as dry milk, or packaged without further treatment.



   The filtrate fraction is more advantageous than the milk obtained from conventional ones for many reasons
Pasteurization will contain more organoleptic ingredients than milk

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 pasteurized, which makes them tastier and more popular from the point of view of the consumer. Furthermore, the milk obtained according to the invention has a better shelf life since bacteria, such as the psychrophilic bacteria, in particular the Bacillus cereus, can also be completely removed from the process of the present invention, which is impossible when using conventional pasteurization.



   The concentrate fraction, which is the portion of the milk fraction that is retained and recovered by the restrained membrane surface of the microfilter, consists of milk with an increased bacterial content (relative to the milk before it was fed to the microfiltration) and essentially with unchanged fat globules and protein content. The concentrate fraction can then be removed or used in other processes.



   The filtrate may contain some bacteria, but the lower the bacterial content, the better the shelf life of the product. Full sterilization is desirable, but the initial growth rate of a small remaining concentration of bacteria is generally low enough to still achieve a very prolonged shelf life for the resulting milk product.



   The shelf life of milk made in accordance with the method of this invention is significantly increased over that for conventionally pasteurized milk, since in particular the concentration of Bacillus cereus bacteria is greatly reduced.



   Because the milk of the present invention can be made sterile, whereas the milk obtained by using conventional pasteurization techniques cannot be really sterile, the milk can have an extremely long shelf life under cooled or room temperature conditions, especially when the milk is filled into a container under aseptic conditions. One way to do this is by using the mold fill sealing technique, which is well known in the packaging industry. This technique is often used for the packaging of sterile solutions or the like, such as for the pharmaceutical industry.

   The milk made in accordance with the present invention can be packaged using the form fill seal technique and this milk can be stored for an extremely long time, even at room temperature.



   The exact process or machinery used to accomplish the filling is not critical. The following description is given only as an example and as an explanation of how such a mold filling sealing technique can be used
Homogenization is described below. The milk fraction is preferably first heated or cooled to a suitable temperature for the homogenization after the centrifugal separation, if used, and before the homogenization. Then the milk is passed into a homogenizer, where the fat emulsion size is reduced to a size sufficient to allow passage through the membrane. The size of all suspended particles is preferably smaller than one micrometer. It is important that the milk is filtered relatively soon after homogenization.

   Preferably the filtration is carried out in less than 5 minutes, preferably less than 2 minutes and most preferably less than 30 seconds.



   Again, the important factor is not the pre-filtration retention time, but rather the factor that the filtration occurs prior to any substantial agglomeration of the beads, thereby forming a substantial number of particles larger than 1 µm .



   Homogenization of lean or whole milk before filtration in a cylindrical, dynamic microfiltration unit is absolutely necessary in order to cleanly emulsify and suspend the fat components and other components of the milk, and to reduce the size sufficiently and thus achieve a clean filtration. A rotating disk filter, however, develops a significant shear rate directly on the surface of the rotating disk. Therefore, a certain degree of homogenization of the milk can be achieved essentially directly in the filtration. Such self-emulsification of the milk through the action of the dynamic microfilter enables skim milk to be processed through a rotating disc filter without the need for a separate homogenizer.



   In fact, the rotating disc device works to both homogenize the milk and filter it at the same time, which is not achieved in a rotating cylinder filter unit. A rotary disk filter can produce a shear rate of about 200,000 sec-, whereas one

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 rotating cylinder unit generates a shear rate of only 10,000 sec-1. Although the shear force in a rotary disc filter unit is considerable, in most cases it is not sufficient to homogenize milk sufficiently.



   According to the present invention, the filtration is carried out as a dynamic filtration, i.e. that is, the filtration medium itself is kept in constant motion so that the effective flow rate of the milk through the medium is extremely high. The particular physical shape of the dynamic membrane is not critical. Thus, the membrane medium z. B. have the shape of disks or cylinders. In general, the dynamic microfilter contains a cylindrical or disk-shaped membrane element which rotates within an outer impermeable cylinder. In a cylindrical, dynamic microfilter, when the liquid to be filtered is introduced into the gap between the stator and the rotating membrane, the moment of the rotating membrane is given to the liquid.

   The liquid closer to the inner cylinder experiences a higher centrifugal force than the liquid closer to the outer cylinder. Under certain conditions, this phenomenon creates a flow pattern known as Taylor swirls, which phenomenon prevents the development of a substantial residue on the membrane surface.



   The dynamic filtration process therefore uses the formation of Taylor swirls to keep the surface of the membrane free of possible residues, the components of which thus remain in the liquid to be filtered. The process separates the liquid into a filtrate (the portion of the liquid that passes through the membrane) and a concentrate (the fraction that contains the suspended particles that would normally have deposited on the surface of the membrane and would have blocked it). In this way, a high flow rate through the membrane can be obtained for a long period of time. The amount of feed and concentrate must be controlled in such a way that a stable liquid flow is created.

   Even with a low flow rate of the concentrate, it is possible to obtain a stable flow of the liquid to the surface of the membrane.



   The dynamic microfiltration allows an effective surface speed of 3 m / sec to 50 m / sec usually, in particular from 5 m / sec to 30 m / sec, preferably from 8 m / sec to 20 m / sec
In order to achieve the desired surface speeds, it is necessary to rotate a representative filtration medium in the form of a cylinder with a diameter of approximately 63.5 mm at approximately 1000 to approximately 6000 revolutions per minute (rpm), a rotation of approximately 5000 Rpm is typical.



   If a dynamic disc filtration device is used, a typical disc filtration medium will be from 50 mm to 1.2 m in diameter. Such discs can e.g. B. at speeds of 1000 rpm to 8000 rpm, typically from 3000 rpm to 6000 rpm, depending on the design of the individual dynamic microfilter that is used. The shear rate of such disc filters is preferably 100,000 sec-1 to 400,000 sec-'. Among the preferred disc filters are those of the type disclosed in US 5,679,249, filed Dec. 24, 1991, the description of which is incorporated herein by reference.



   The microfilter pores are sized to retain the bacteria present in the milk while still maintaining an acceptable flow rate through the microfilter.



  Appropriate membranes contain hydrophilic microporous membranes with good flow properties, low pore size scatter and constant bacterial removal performance for the important bacteria. The pore size dimension of the microfilter membrane should be from 0.01 to 5.0 µm as by the methods of the prior art, the tests known as the "bubble point" (ASTM F316-86) and the KL method (US 4,340,479 A) are known. The pore size dimension will preferably be between 0.1 and 1 µm. It is best to use filters with a pore size measurement of 0.2 to 0.5 m. Such microporous filters are well known and easily available.



   The preferred microporous membranes that can be used in accordance with the present invention include those sold by Pall Corporation under the brand names Ultipor N66 Fluorodyne and Posidyne (registered trademarks); those of Cuno Corporation, which are available under the brand name Zetapor, and those which are sold by Millipore under the brand name Durapore (registered trademark)

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 sold.



   Ultimately, the bacteria should be concentrated in a stream that is less than 5% of the inflow and more than 95% of the solids and proteins that should normally be found in milk pass through the membrane for a long period of time.



   The dynamic microfilter can work in one step without the need to recycle the concentrate. If desired, the concentrate can be reintroduced into the feed. If a cylindrical, dynamic microfilter is used, it can be operated with different ratios of filtrate flow to total feed flow (concentration factors).



  However, the cylindrical, dynamic microfilter is preferably operated with a filtrate flow rate of over 90%, in particular about 95%, and most preferably over 98%, in order to predominantly produce the filtrate with the very low bacterial content as the desired product.



   Finally, if a dynamic microfilter with turntables is used, it can be operated with different ratios of filtrate flow to total inflow. However, the dynamic rotary disc microfilter can operate with a filtrate inflow ratio over a wide range. Choosing a large ratio simply lowers throughput, while working at a low ratio results in higher throughput. It is believed that working at a ratio of about 40% is advantageous for maintaining a stable flow rate through the filter, although other ratios can be used.



   The filtration of freshly homogenized milk can be carried out warm at about 40 to 60 C, which is about or just above the crystallization temperature of about 40 C of the higher melting components of milk fat. This is below the temperature used in conventional thermal pasteurization. Alternatively, with a certain decrease in the flow rate, the milk can be filtered at much lower temperatures, such as from 15 to 35 C, in particular from 20 to 25 C.



   After microfiltration, the concentrate can be disposed of in any acceptable manner, subjected to further procedures, or used directly.



   The method according to the invention can be used advantageously if the desired end product is either whole milk, normal milk or skimmed milk.



   The flow rate through a bacteria-retentive membrane of milk with a reduced fat content is normally higher than the flow rate of milk with a high fat content. In certain situations, it is economically advantageous to produce milk with a higher fat content and with 2% fat in the milk by combining a filtered skim milk with a filtered fat fraction. This fat fraction can be a cream fraction with a minimum fat content of around 10%.



   The filtration of the cream fraction can e.g. B. by the method of US 5 395 531 A or by using a bacteria-retaining filter cartridge with a blind end. Filtration can be carried out in an industrially acceptable manner by heating the fat composition to a point where it is in a liquid state and can easily be filtered through a microporous membrane. The preheated fat can be homogenized before filtration. Alternatively, the fat composition can be pasteurized to reduce its bacterial content or a combination of pasteurization and microfiltration can be used.



   Furthermore, if the aim of the method is to obtain protein concentrates, such as from the milk of a transgenic animal, such as a transgenic cow, dynamic microfiltration is carried out in order to obtain a high concentration of the concentrate, with a microporous membrane having a pore size dimension of about 0.2 m or less is used
Suitable devices for carrying out the method according to the invention can be constructed by connecting conventional devices which contain centrifugal separators, microfilters, sterilization units, heat exchangers and pumps.

   Those skilled in the art will readily be able to provide valves for flow and pressure control and other necessary support devices to make the device usable and to carry out other conventional modifications to such devices as are required for special cases.



   The following examples will describe certain implementations further, but are not intended to limit the scope of the invention, which is defined in the claims
The main process steps carried out can e.g. B. be the following

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Process step A: temperature setting of the milk
Unless stated otherwise, the milk used in the following examples is pasteurized milk as obtained from a retail outlet. The temperature of the milk was adjusted to an appropriate process temperature before filtration. The preferred working temperature (40 to 60 C) was used because the majority of the fats in milk are not in crystallized form at such temperatures. A jacketed 35 l fermentation kettle (Type 3000 from Chemap A.G.) served as the working kettle.

   The kettle was filled with the milk and the contents were heated to about 50 C, unless otherwise stated, using a hot water jacket. To improve heat transfer, the milk was stirred while heating.



   When the milk had reached the desired process temperature, the milk was pumped into a homogenizer at a pumping rate of about 1 l per minute.



   Process step B: homogenization of the milk
After entering the homogenizer (Model 15 MR from APV Gaulin Inc.), the milk was subjected to a two-stage homogenization process, the first stage of which took place at approximately 17.5 Mpa and the second stage at approximately 3.5 Mpa. Normal procedures, such as starting and working, were followed as described in the APV Gaulin work manual for this unit. After homogenization, the milk was normally fed into an intermediate buffer tank, which was jacketed and kept at the desired process temperature.



  The tank acted as a liquid buffer between the outlet of the homogenizer and the supply to the filter. Whenever desired, the homogenized milk could be returned through the homogenizer to maintain a constant volume in the buffer tank.



   Process step C: introduction of bacteria into the milk supply stream
In some experiments, artificial inoculation of the milk flow with bacteria was used to demonstrate the very high titer reduction that is possible with the present invention. Bacterial vaccination was added to the feed stream via a measuring pump between the work tank and the homogenizer. The inoculation flow rate was maintained so that a desired bacterial concentration level of about 106 bacteria per milliliter of milk was achieved. Since the bacteria were placed in front of the homogenizer (i.e. process step C was carried out before process step B), the bacteria in the working liquid were mixed well before entering the filtration device.

   E.coli strain ATCC 15224 was used in most of the examples
Another method of inoculating milk with bacteria would be to add bacteria directly into the boiler at the desired concentration. Such a method is not preferred because it exposes the bacteria to a long residence time in temperatures that are higher than the ambient temperature. That could be undesirable growth or excessive killing of the
 EMI10.1
 stuff.



   Process step D: bacterial test
Mesophilic bacteria: The bacterial concentration was determined by serial dilution of the samples and by carrying out the corresponding dilutions through sterile 0.2 m membranes and culturing on Mueller-Hinton agar for 24 hours at 32 ° C. These procedures are described in detail in a publication entitled "Manual of Clinical Microbiology, 2nd Edition, 1974, ASM, Washington DC".



   Listeria monocytogenes ATCC 43256 was the pathogen tested. The population levels in the samples were determined by the methods described by Agello et al. method used (Agello G., Hayes P. and Feeley J., Abstract of the Annual Meeting, 1986, ASM, Washington D.C., page 5).



   Process step E: cleaning process
Sanitation and sterilization were performed prior to each experiment using 0.1 normal sodium hydroxide. In the sterilization process, the membrane and all associated devices are rinsed first with water and then with 0.1 normal sodium

  <Desc / Clms Page number 11>

 Treated hydroxide at 50 C for about half an hour. The caustic was then neutralized using phosphoric acid. This neutralized solution was then used to flush the system until all parts were neutral. Filtration tests were carried out immediately after this procedure. All device and membrane elements were sanitized using the sterilization process after each test was completed.



   Process step F: integrity test
Each membrane element was tested for intactness before the bacteria started. A flow test as described in NM 900a, "The Pall Ultipor membrane filter guide," copyright 1980, available from Pall Corporation, was used for the integrity test.



   Description of the filtration device
1. The cylindrical dynamic microfilter
The cylindrical dynamic microfilter (cylindrical DMF) used for this test was a BDF-01 manufactured by Sulzer Brothers Limited, Winterthur, Switzerland. The device is described by Rebsamen et al. (Dynamic Microfiltration and Ultrafiltration in Biotechnology, Rebsamen E. and Zeigler H., Proceedings of the World Filtration Congress IV, 1986, (Ostend B)). See also US 4,066,554 A and 4,093,552 A, which are incorporated by reference.



   2 Description of the membrane filter elements
The membrane filter elements normally used in these experiments are various grades of nylon membranes, Ultipor N66 and Posidyne (registered trademarks), commercially available from Pall Corporation, Glen Cove, NY. The pore sizes used were 0.2, 0.30, 0.45 and 0.65 m. The membrane elements had a surface area of 0.04 m2.



   3. The dynamic microfilter in disk format
The disk format consists of a carrier disk for a membrane with a diameter of 152.4 mm, the carrier disk being mounted on a hollow shaft and contained in a leak-thick housing with the required liquid inlet and outlet connections. The carrier disk has a device for leak-tight holding of the membrane sheets on its surface and contains drainage spaces in order to guide the filtrate flow through the membrane and the disk and to the outside through the shaft. The effective membrane area was 0.014 m2 and rotation rates up to 4500 rpm were available.



   Any of the dynamic disk microfiltration units previously discussed can be used in the practice of the present invention. Reference is also made to US 5 679 249 A, filed on Dec. 24, 1991, which contains a description of another dynamic disk format microfiltration device that can be used in the practice of the present invention.



   4. Description of the membrane filter elements
The membrane filter elements were of the same membrane quality as described in the section under the cylindrical DMF. Normally the membranes were circular, flat,
 EMI11.1
 in which dynamic microfilters were assembled, the filtrate chamber was sealed from the feed using O-rings. The membrane filter elements had a surface area of 0.014 m2.



   Method step G1: Operation of the cylindrical dynamic microfilter
Before filtration, a filter element, as described in the section under filter compositions, was assembled in the cylindrical dynamic microfilter (DMF). The sanitary measures and the sterilization were carried out using the methods shown under process step E. After monitoring the start-up method shown in process step G2, the milk to be filtered was pumped from the buffer tank into the cylindrical DMF via a positive displacement pump.

   The amount of concentrate was controlled by a second pump or a safety valve attached to the concentrate outlet. The temperatures and flow rates of the filtrate and concentrate feed and the feed pressure increased

  <Desc / Clms Page number 12>

 measured several times during the course of the experiment, typically at intervals of 10 minutes. Standard working conditions of the cylindrical DMF were a rotation speed of 5000 rpm, a filtrate feed ratio of more than 95% and a feed pressure of about 1.3 to 2.0 bar. All experiments with this device were carried out at constant feed flow rates.



   Method step G2: start of the dynamic filter
Before the milk was placed in the dynamic filter, warm deionized 0.2 m filtered water was passed through the system to start up the device involved. The rotational speed of the dynamic filter was brought up to working speed as water flowed through the system. When the system reached equilibrium, the milk supply was opened. The milk replaced the water in the system and filtration began.



    Method step H: the disk-shaped dynamic microfilter
A filler element of the disc-shaped DMF, as described in the section under filter compositions, was inserted into the disc-shaped DMF. Sanitary measures and sterilization were carried out using the method set out in process step E. After monitoring the start-up method set out in process step G2, the milk to be filtered was pumped from the buffer tank into the disk-shaped DMF. The amount of concentrate and the feed pressure were controlled by a valve located at the concentrate outlet.

   The temperature and flow rates of the filtrate and concentrate feed and the feed pressure were measured at various times during the course of the experiment, typically at 10 minute intervals. A feed rate of approximately 960 ml / min was maintained for all experiments. The filtrate flow values recorded are those obtained when the flow stabilized in the filtration unit.



   EXAMPLES
EXAMPLE 1
Skimmed milk at room temperature was pumped at a rate of 600 ml / min into a cylindrical DMF equipped with a 0.45 m Ultipor N membrane (registered mark).



  The working conditions in the DMF were obtained as described in process step G1 and are summarized in Table 1. The feed pressure began to rise rapidly a few minutes after the start of the test, indicating the clogging of the microporous membrane.



   EXAMPLE 2
Skim milk was heated to 50 C in accordance with process step A and then homogenized in accordance with process step B. The homogenized milk was then stored in an intermediate buffer tank for about 4 hours, the temperature of the milk being kept at about 50 ° C for this period. After this four hour delay, the milk was pumped into a cylindrical DMF equipped with a 0.45 (on the Ultipor N66 membrane) at a feed rate of about 600 ml / min. The preferred conditions of the DMF procedure as outlined in Method G1 The supply pressure began to rise rapidly after only a few minutes of operation, indicating that the microporous membrane was clogged, and the test had to be stopped.



   EXAMPLE 3
Skimmed milk, which was heated to 50 ° C. in accordance with process step A and was homogenized by process step b, was pumped into a cylindrical DMF equipped with a 0.45 m Ultipor N66 membrane within no more than 5 minutes after the homogenization Preferred conditions of the DMF procedure, as shown in process step G1, were observed. A stable filtrate flow of 1080 l / h / m2 was achieved while the milk supply was being pumped out. No increase in inflow pressure was found in the course of the experiment.



   When all the milk had been processed, the feed was switched to non-homogenized skimmed milk

  <Desc / Clms Page number 13>

 switched at 50 C without disturbing the system work process. Within a few minutes, the milk filtrate flow decreased rapidly and the system pressure rose, indicating that the membrane had clogged. This example clearly shows the need to homogenize the milk to achieve a sensible flow through a microfiltration membrane.



   Examples 1 to 3 show that it is necessary to subject the milk to sufficient shear (in this case by homogenization) prior to filtration to reduce the emulsion particle size of the milk enough to allow passage through the microporous membrane and thus one to achieve clean filtration. Example 2 in particular shows that the particle size distribution returns to larger sizes in a short time after homogenization. Therefore, the homogenization for a clean filtration must take place within a short time before the filtration, such as within less than 5 minutes or preferably shorter intervals.



   EXAMPLE 4
Skim milk was preheated through process section A and pumped into a disk-shaped DMF equipped with a 0.45 m Ultipor N66 membrane. The procedures as shown in step H were used. A steady flow of the filtrate was quickly established and maintained for about 100 minutes until the milk supply was used up.



   The working conditions of the disk-shaped DMF produce a calculated shear rate of approximately 200,000 sec-1 in the surface gap between the rotating disk and the membrane. This shear is in the range of the shear rates that were generated by the homogenizer under the conditions of process step B.



   This example demonstrates that the required shear can be achieved in one step before filtration, i.e. H. without the need for a separate homogenizer. The example clearly showed that the membrane did not become unusable due to the solids in the milk and that the shear generated by the rotation of the disk of approximately 200,000 sec-1 was sufficient to reduce the particle size in skim milk and thus the passage through to allow a microfilter membrane and thus to achieve a clean filtration.



   Table 1 summarizes the results of Examples 1 to 4; the data show that a steady state of filtrate flow through the membrane is achieved if the milk is subjected to sufficient shear within a short time before filtration.



   TABLE 1
 EMI13.1
 
 <tb> example <SEP> filtration <SEP> feed- <SEP> shear <SEP> in <SEP> homogeneous <SEP> delay <SEP> pore <SEP> river
 <tb> art <SEP> temp <SEP> C <SEP> filter <SEP> sec-1 <SEP> sation <SEP> after <SEP> the <SEP> size
 <tb> homogenization <SEP> m <SEP> 1 / h / m2
 <tb>
 <tb> 1 <SEP> cylinder <SEP> 25 <SEP> 10 <SEP> 000 <SEP> no <SEP> no <SEP> 0.45 <SEP> 0
 <tb> 2 <SEP> cylinder <SEP> 50 <SEP> 10 <SEP> 000 <SEP> yes <SEP> 4 <SEP> hours <SEP> 0.45 <SEP> 0
 <tb> 3 <SEP> cylinder <SEP> 50 <SEP> 10 <SEP> 000 <SEP> yes <SEP> 5 <SEP> minutes <SEP> 0.45 <SEP> 1080
 <tb> 4 <SEP> disc <SEP> 50 <SEP> 200 <SEP> 000 <SEP> no- <SEP> 0.45 <SEP> 1600
 <tb>
 
EXAMPLE 5
In order to determine the relationship between particle size and the time after homogenization, the skimmed milk was heated according to process step A and using the process,

   which was set out in step B, homogenized. The particle size distribution as a function of the time after homogenization was determined. Particle size distribution was measured using an Integrated Micro-Optical Liquid Volumetric Sensor (IMOLV-.2), available from Particie Measurement Systems, Colorado. This laser particle counter is designed to measure particle size distributions in the range from 0.1 to 5.0 (im.



   The milk samples were diluted 1,300,000 and then subjected to analysis as performed by the IMOLV device work manual. For dilution of the milk samples, 0.04 m filtered, 18 mega-ohm DI water with a particle count of

  <Desc / Clms Page number 14>

 less than 50 / ml used.



   The figure shows the results of the particle analyzes. A diagram of the number of particles in relation to the number of particles after 5 seconds as a function of the particle size is shown in the figure. The figure clearly shows that as the time period after homogenization increases, the number of larger particles increases. As the number of smaller particles decreases by the same amount over this period of time, it is evident that the smaller particles agglomerate over time to form larger particles.



   EXAMPLES 6 TO 9
Membranes with different pore sizes and bacterial retention properties were tested on the cylindrical DMF to determine the size of the continuous flow of filtrate from milk that can be achieved. The general procedures used for Examples 6 through 9 are listed below.



   1.The desired membrane filter element was assembled in the cylindrical DMF.



   2. An integrity test, as shown in process step F, was carried out. The
Membrane filter element was discarded if it failed the test.



   3. The device was made hygienic according to process step E.



   4. The milk to be filtered was preheated using the procedure outlined in step A.



   5. The milk was homogenized according to process step B.
6. The starting method as it was shown in process step G2 was carried out.



   7. The milk was fed from the buffer tank to the cylindrical DMF at a desired flow rate.



   8. The working parameters were set using the guidelines in process step G1.



   9. Corresponding measurements were made.



   Typically, the cylindrical DMF was operated at 5000 rpm corresponding to a shear rate of approximately 10,000 sec-1 in the filter. The feed temperature was 50 C and the feed pressure varied from 1.3 to 2.0 bar. The filtrate to feed ratio was maintained above 95% for each of these examples. The flow recorded in Table 2 is the steady state of filtration flow typically achieved 15 minutes after the start of filtration. The total time of the experiment varied in each case, whereas the volume of the milk to be filtered was constantly 30 l.



   EXAMPLE 6
A 0.2 m Ultipor N66 membrane was used for this example. A feed rate of 250 ml / min was used to maintain a steady filtrate state flow of 330 l / h / m2.



  Filtration continued for 130 minutes with no apparent drop in the filtrate flow rate, after which time no milk was left in the kettle.



   EXAMPLE 7
A 0.30 µm Ultipor N membrane was used for this example. A feed rate of approximately 550 ml / min was used to achieve a steady state flow of 775 l / h / m 2 for approximately 60 minutes after which the experiment was terminated.



   EXAMPLE 8
A 0.45 m Ultipor N66 membrane was used for this example. A feed rate of 740 ml / min was used to achieve a steady state flow of 1080 1 / h / m2. Filtration was continued for 40 min with no apparent drop in flow rate after which the milk supply had been depleted and the experiment ended.



   EXAMPLE 9
A 0.65 in Ultipor N66 membrane was used for this example. A feed rate of 1100 ml / min was used to achieve a steady state flow of 1680 1 / h / m2. Filtration was continued for about 30 minutes after which the milk supply was exhausted and the expe

  <Desc / Clms Page number 15>

 nment has ended.



   Examples 6 to 9 are summarized in Table 2. The data show that steady filtrate flows can be achieved using the filtration method of this invention using different classes of bacteria-retaining membranes. The table shows that membranes with smaller pores and thus increased bacterial retention can be used at the expense of the filtrate flow rate in the process according to the invention.



   Table 2
Milk flow when using different membranes in the cylindrical DMF
 EMI15.1
 
 <tb> example <SEP> liquid <SEP> membrane <SEP> pore <SEP> rpm <SEP> feed <SEP> feed <SEP> filtrate / <SEP> Experi- <SEP> river
 <tb>
 <tb> size <SEP> temp- <SEP> print <SEP> feed <SEP> mentation
 <tb>
 <tb>
 <tb> ratur <SEP> behaves <SEP> time
 <tb>
 <tb>
 <tb> No. <SEP> (um) <SEP> (C) <SEP> (bar) <SEP> nis <SEP> (min) <SEP> (1 / h / m2)
 <tb>
 <tb>
 <tb>
 <tb>
 <tb> 6 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.20 <SEP> 5000 <SEP> 50 <SEP> 2.0 <SEP> 0.97 <SEP> 130 <SEP> 330
 <tb>
 <tb>
 <tb> 7 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.30 <SEP> 5000 <SEP> 50 <SEP> 1.6 <SEP> 0.97 <SEP> 60 <SEP> 775
 <tb>
 <tb>
 <tb>
 <tb> 8 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.45 <SEP> 5000 <SEP> 50 <SEP> 1.5 <SEP> 0,

  97 <SEP> 40 <SEP> 1080
 <tb>
 <tb>
 <tb> 9 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.65 <SEP> 5000 <SEP> 50 <SEP> 1.3 <SEP> 0.97 <SEP> 30 <SEP> 1680
 <tb>
 
EXAMPLE 10
A 0.2! Am Posidyne membrane with a positive surface charge was used for this example. The membrane used has quaternary ammonium groups on its pore surfaces and has a high absorptive capacity for biological material.



   A feed rate of 260 ml / min was used to achieve a steady state flow of 360 1 / h / m2. The filtrate flow was of the same strength as that achieved with an uncharged membrane as described in Example 6. The filtration continued for about 120 minutes with no apparent drop in the filtrate flow rate, after which time there was no milk in the kettle. A ratio of filtrate to feed of over 97% was obtained during the experiment. Further data from the experiment can be found in Table 3.



   It was expected that a large amount of proteins from the milk would bind to the membrane surface and eventually clog them. This example showed that a membrane that normally shows protein affinity works well under dynamic mode.



   EXAMPLE 11
A feed rate of 740 ml / min of whole milk was used and a filtrate flow of 1130 l / h / m2 was achieved. Further data can be found in Table 3. Filtration was continued for about 40 minutes, after which the milk supply was exhausted and the experiment ended.



   This example shows that whole milk can be filtered using the method according to the invention. The observed difference in the filtrate flow between whole milk and skimmed milk (as in example 9) seems to be primarily due to the different viscosity.

   The ratio of the filtrate flow that was achieved for whole milk and for skimmed milk is approximately equal to the ratio of the viscosity of whole milk to skimmed milk

  <Desc / Clms Page number 16>

 Table 3
 EMI16.1
 
 <tb> example <SEP> liquid <SEP> membrane <SEP> pore <SEP> rpm <SEP> feed <SEP> feed <SEP> filtrate / <SEP> Experi- <SEP> river
 <tb>
 <tb>
 <tb> size <SEP> temp- <SEP> print <SEP> feed <SEP> mentation
 <tb>
 <tb>
 <tb> ratur <SEP> behaves <SEP> time
 <tb>
 <tb>
 <tb>
 <tb> No.

    <SEP> (um) <SEP> (C) <SEP> (bar) <SEP> nis <SEP> (min) <SEP> (l / h / m2)
 <tb>
 <tb>
 <tb>
 <tb>
 <tb>
 <tb>
 <tb> 10 <SEP> skimmed milk <SEP> Posidyne <SEP> 0.20 <SEP> 5000 <SEP> 50 <SEP> 2.0 <SEP> 0.97 <SEP> 120 <SEP> 360
 <tb>
 <tb>
 <tb>
 <tb> 11 <SEP> whole milk <SEP> Ultipor <SEP> N66 <SEP> 0.65 <SEP> 5000 <SEP> 50 <SEP> 1.4 <SEP> 0.93 <SEP> 40 <SEP> 1130
 <tb>
 
EXAMPLES 12 TO 16
The examples for determining the flow of filtrate through membranes retaining various bacteria were repeated using the disk-shaped dynamic microfilter.



  The general process steps for Examples 12 to 16 are described below.



   The general conditions described apply to each example, unless stated otherwise.



   
    

Claims (18)

1. The desired membrane filter element was assembled in the disc-shaped DMF. 2. An integrity test was carried out, as shown in process step F. The Membrane filter element was discarded if it failed the test 3. The device was made hygienic according to process step E.    4. The milk to be filtered was preheated by the procedure outlined in step A.    5. The milk was homogenized according to process step B. 6. The general starting method, as shown in process step G2, was carried out.    7. The milk was disc-shaped from the buffer tank at a desired flow rate DMF fed.    8. Appropriate measurements were made.    Typically, the disk-shaped DMF was kept at 3500 rpm, corresponding to a calculated shear rate of approximately 200,000 sec-1. The feed temperature was 50 C and the feed pressure was kept at about 0.2 bar. Milk was pumped into the filter at a rate of 960 ml / min to maintain a high cross flow rate across the membrane. The ratio of filtrate to feed was set separately for the membrane pore size, the feed temperature and for the rotation speed. The unfiltered portion of the feed was returned to the boiler. The flow recorded in the table below is the steady state flow that was typically achieved as a filtrate through the membrane half an hour after the start of the filtration.    EXAMPLE 12 A 0.2 m Ultipor N66 membrane was used for this example. A constant filtrate flow of 850 l / h / m2 was achieved.    EXAMPLE 13 A 0.45) in the Ultipor N66 membrane was used for this example. A constant filtrate flow of 1600 1 / h / m2 was achieved.    EXAMPLE 14 A 0.45 m Posidyne membrane was used for this example. A constant filtrate flow of 1600 1 / h / m2 was achieved.    The data shown in Table 4 summarize Examples 12-14. The data show that when using the filtration method according to the invention, different classes of bacteria-retaining membranes can be achieved when using a disk-shaped DMF, a stable filtrate flow. The table shows that the membranes have smaller pores  <Desc / Clms Page number 17>  and thus increased bacterial retention (titre reduction) can be used in the present invention at the expense of the filtrate flow rate.    Table 4  EMI17.1   <tb> Example <SEP> liquid <SEP> membrane <SEP> pore- <SEP> rpm <SEP> feed- <SEP> feed <SEP> filtrate / <SEP> Experi- <SEP> filtrate <tb> <tb> <tb> size <SEP> tempe- <SEP> pressure <SEP> feed- <SEP> menting- <SEP> flow <tb> <tb> <tb> <tb> temperature <SEP> behaves- <SEP> time <SEP> <tb> <tb> <tb> <tb> No.    <SEP> (um) <SEP> (C) <SEP> (bar) <SEP> nis <SEP> (min) <SEP> (l / h / m2) <tb> <tb> <tb> <tb> <tb> <tb> <tb> <tb> 12 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.20 <SEP> 3500 <SEP> 50 <SEP> 0.2 <SEP> 0.22 <SEP> 137 <SEP> 850 <tb> <tb> <tb> <tb> <tb> 13 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.45 <SEP> 3500 <SEP> 50 <SEP> 0.2 <SEP> 0.37 <SEP> 80 <SEP> 1600 <tb> <tb> <tb> <tb> <tb> 14 <SEP> skimmed milk <SEP> Posidyne <SEP> 0.45 <SEP> 3500 <SEP> 50 <SEP> 0.2 <SEP> 0.37 <SEP> 80 <SEP> 1600 <tb>   EXAMPLE 15 Skimmed milk at a temperature of 18 C and homogenized by process step B was pumped into a disc-shaped DMF equipped with a 0.45 (on the Ultipor Nee membrane).    The filtration was carried out at a feed rate of 860 ml / min, at which a steady flow of filtrate state of about 860 l / h / m2 was achieved through the membrane. A temperature of 25 C was measured for the filtered milk. The other conditions for this example are given in Table 5.    This example demonstrates that chilled skimmed milk at about 80 C can be treated by a bacteria-holding membrane by the method of the present invention. The reduced filtrate flow at this reduced temperature is believed to reflect the higher viscosity of milk at this temperature compared to higher temperatures.    EXAMPLE 16 A disc-shaped DMF was equipped with a 0.45 m Ultipor N66 membrane. Whole milk was fed into the disc-shaped DMF at a rate of 900 ml / min and a steady flow of filtrate through the membrane of about 850 l / h / m2 was achieved. The experiment was carried out without recirculating the unfiltered portions of the feed stream.    This example shows that whole milk can be filtered by the method of the present invention using a disc-shaped DMF. Skimmed milk produces an almost constant filtrate state flow of approximately 1600 1 / h / m2 under essentially identical conditions. The observed difference in the filtrate flow between skimmed and whole milk corresponds approximately to the difference in the liquid viscosity.    EXAMPLE 17 Using the methods described above, filtration was performed on the disc-shaped DMF while maintaining a high filtrate to feed ratio. A 0.45 m Ultipor N66 membrane was used in this experiment. The skim milk supply was kept at 115 ml / mm and a rotation speed of 2100 rpm was used. A filtrate flow of 460 1 / h / m2 was achieved.    Table 5  EMI17.2   <tb> Example <SEP> liquid <SEP> membrane <SEP> pore- <SEP> rpm <SEP> feed- <SEP> feed <SEP> filtrate / <SEP> Experi- <SEP> filtrate <tb> <tb> <tb> large <SEP> tempe- <SEP> pressure <SEP> feed- <SEP> mentation- <SEP> flow <tb> <tb> <tb> temperature <SEP> behaves- <SEP> time <tb> <tb> <tb> No.    <SEP> (m) <SEP> (C) <SEP> (bar) <SEP> nis <SEP> (min) <SEP> (l / h / m2) <tb> <tb> <tb> <tb> <tb> <tb> 15 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.45 <SEP> 3500 <SEP> 18 <SEP> 0.2 <SEP> 0.23 <SEP> 130 <SEP> 860 <tb>    <Desc / Clms Page number 18>    EMI18.1   <tb> Example <SEP> liquid <SEP> membrane <SEP> pore- <SEP> rpm <SEP> feed- <SEP> feed <SEP> filtrate / <SEP> Experi- <SEP> filtrate <tb> <tb> <tb> size <SEP> tempe- <SEP> pressure <SEP> feed- <SEP> menting- <SEP> flow <tb> <tb> <tb> temperature <SEP> behaves- <SEP> time <tb> <tb> <tb> <tb> No.    <SEP> (um) <SEP> (C) <SEP> (bar) <SEP> nis <SEP> (min) <SEP> (l / h / m2) <tb> <tb> <tb> <tb> <tb> <tb> <tb> <tb> 16 <SEP> whole milk <SEP> Ultipor <SEP> N66 <SEP> 0.45 <SEP> 3500 <SEP> 50 <SEP> 0.2 <SEP> 0.23 <SEP> 90 <SEP> 850 <tb> <tb> <tb> <tb> <tb> 17 <SEP> skimmed milk <SEP> Ultipor <SEP> N66 <SEP> 0.45 <SEP> 2100 <SEP> 50 <SEP> 0.5 <SEP> 0.92 <SEP> 50 <SEP> 460 <tb>   EXAMPLE 18 In order to demonstrate the method of operation on a large scale, an experiment was carried out with a large amount (500 l) of high, unpasteurized skimmed milk. The milk was preheated to 50 C by passing it through a plate heat exchanger. It was then homogenized in accordance with process step B and pumped into a cylindrical DMF equipped with a 0.65 m membrane. Typically, the dynamic microfilter was kept at 500 rpm for this example.    The feed pressure varied from 1.3 to 1.5 bar at a feed rate of approximately 1300 ml / min. The filtrate to feed ratio was maintained above 95%. A steady flow of filtrate state of around 1680 l / h / m2 was achieved. There was no decrease in the flow of filtered milk, nor was there any increase in feed pressure during the six hour operation required to process the 500 l. This example shows that it is possible to use the filtration method of this invention for extended periods of time.    EXAMPLES 19 AND 20 These examples were carried out to determine that no fractionation of the components in the milk takes place during the filtration process of the invention. In these examples, samples of the filtrate and concentrate feed were analyzed at several times during the filtration to determine protein concentrations by the Kjeldahl method and total solids by evaporation.    EXAMPLE 19 Feed, filtrate and concentrate samples were taken at various times during the experiment described in Example 18 and examined for total solids in each stream. The data in Table 6 show that there is no significant depletion of the total solids from the filtrate when using a 0.65 m membrane.    EXAMPLE 20 Feed, filtrate and concentrate samples were obtained at various times during the execution of Example 13 and examined for total solids and proteins in each stream. The data are shown in Table 6. There was also no significant depletion of solids or proteins from the filtrate milk when using 0.45 μm membrane.    Table 6  EMI18.2   <tb> Filtration- <SEP> Membrane- <SEP> Proteins <SEP>% <SEP> Total Solids <SEP>% <tb>    EMI18.3    EMI18.4   <tb> (um) <SEP> filtrate <SEP> feed <SEP> concentrate <SEP> filtrate <SEP> feed <SEP> concentrate <tb> <tb> Cylinder <SEP> 0.65 <SEP> - <SEP> - <SEP> - <SEP> 9.14 <SEP> 9.17 <SEP> 9.35 <tb> <tb> Example <SEP> 19 <tb> <tb> Slide <SEP> 0.45 <SEP> 3.38 <SEP> 3.15 <SEP> 3.35 <SEP> 8.64 <SEP> 8.70 <SEP> 8.85 <tb>    <Desc / Clms Page number 19>   EXAMPLES 21 TO 27 Examples 21-27 were performed to demonstrate the ability of the present invention to remove bacteria from milk. The general working procedure was kept the same as that for the experiments in samples 6 to 18, except that bacteria were added to the working stream by process step C.    The bacteria E.coli, which are generally found in milk, were used for these experiments for vaccination unless otherwise mentioned. Feed, filtrate and bacterial concentrate samples were taken at several times during filtration using sterile techniques. These samples were tested for bacteria using the method outlined in process step D and the results are recorded in Table 7.    As shown in the table, the present invention is able to achieve a dramatic reduction in the bacterial content of milk. The high removal rate for E. coli is directly transferable to the high removal of Bacillus cereus bacteria, which cannot be completely removed when using conventional pasteurization. E. coli are known to have a rod-shaped structure with dimensions of approximately 1.1 to 1.5 m to 2 to 6 μm, with bacilli bacteria such as Bacillus cereus having similar dimensions and also rod-shaped structures Have dimensions from about 1.0 to 1.2 µm to 3 to 5! -Im.    Thus, the ability to remove E. coli, as shown in Table 7, means that the method is capable of removing the very undesirable Bacillus cereus bacteria, resulting in a milk that has a very long shelf life even at room temperature.    EXAMPLES 21, 22 AND 23 Examples 6, 8 and 9 were repeated except for the fact that E. coli were introduced into the working stream through process step C. Samples of the feed, filtrate and concentrate were taken for bacterial analysis. The titer reduction data are shown in Table 7.    EXAMPLE 24 Example 13 was repeated except for the fact that bacteria were introduced into the feed stream through process step C and the bacterial concentrate was not returned to the boiler. A steady milk flow of around 1600 1 / h / m2 was achieved. The microbiological data are shown in Table 7.    The filtered milk contained only a very low level of 7 to 10 bacteria per ml milk, which was dramatically lower than the 106 / ml supply level. The titer reduction in this case was greater than 10. As a comparison during conventional pasteurization of milk, a titer reduction of only about 102 to 103 is achieved.    EXAMPLE 25 The experimental conditions and procedures of Example 12 were repeated in this example except that E coli was added to the feed stream through process step C and the concentrate was not returned to the work tank. A steady milk flow of around 850 1 / h / m2 was achieved. Samples of the filtrate and concentrate inflow were taken for bacterial analysis. The data in Table 7 show a titer reduction of greater than 106. Sterile milk was produced because no bacteria could be found in the filtered milk.    This example shows the possibility of the method according to the invention of essentially completely removing bacteria from milk by using a disc-shaped DMF filter and an appropriately selected membrane. Sterile milk can thus be produced.    EXAMPLE 26 Unpasteunized raw milk contains a wide variety of organisms including coli-like organisms such as E. coli and pathogens such as Listeria and Campylobacteria and Bacillus cereus bacteria. In this example, no external bacterial vaccination was carried out on the raw milk, rather the milk was tested for its own or "natural" bacteria.    Samples of the feed, filtrate and concentrate were used for bacterial analysis  <Desc / Clms Page number 20>  taken from the implementation of Example 18 and analyzed for natural bacteria using Method D.    Only 14 bacteria per ml were found in the filtrate. The feed had 2500 bacteria per ml and the concentrate had 2 x 104 bacteria per ml. Furthermore, no psychrophilic bacteria were registered in the filtrate. Psychrophilic bacteria are those that grow at cold temperatures and cause chilled milk to spoil.    Table 7 summarizes Examples 21 to 26. The data show that increased titre reduction was achieved both in the cylinder and in the disk mode at the expense of the filtrate flow. The table also shows that it is possible to obtain a sterile milk filtrate if the correct membrane is selected.    Table 7  EMI20.1   <tb> At- <SEP> pore size <SEP> membrane type <SEP> through- <SEP> supply <SEP> working- <SEP> supply- <SEP>% bacteria- <SEP> titer- <tb> <tb> <tb> game <SEP> editing <SEP> temp. <SEP> print <SEP> bacteria- <SEP> removal <SEP> reduc- <tb> <tb> <tb> Flow <SEP> concentration- <SEP> <tb> <tb> <tb> <tb> No.    <SEP> <SEP> m <SEP> 1 / h / n2 <SEP> C <SEP> bar <SEP> tion / ml <SEP> TR <tb>    EMI20.2    EMI20.3   <tb> (E.coli) <tb> <tb> <tb> <tb> 22 <SEP> 0.45 <SEP> cylinder <SEP> Ultipor <SEP> N66 <SEP> 1080 <SEP> 50 <SEP> 1.5 <SEP> 106 <SEP> 99.992 <SEP> 4x105 <tb> <tb> <tb> <tb> <tb> <tb> (E.coli) <tb> <tb> <tb> <tb> <tb> 23 <SEP> 0.55 <SEP> cylinder <SEP> Ultipor <SEP> N66 <SEP> 1680 <SEP> 50 <SEP> 1.3 <SEP> 106 <SEP> 99.96 <SEP> 4x102 <tb> <tb> <tb> <tb> <tb> <tb> (E.coli) <tb> <tb> <tb> <tb> <tb> 24 <SEP> 0.45 <SEP> disc <SEP> Ultipor <SEP> N66 <SEP> 1600 <SEP> 50 <SEP> 0.2 <SEP> 106 <SEP> 99.9995 <SEP> 8x105 <tb> <tb> <tb> <tb> <tb> <tb> (E.coli) <tb> <tb> <tb> <tb> <tb> 25 <SEP> 0.2 <SEP> disk <SEP> Ultipor <SEP> N66 <SEP> 850 <SEP> 50 <SEP> 0.2 <SEP> 106 <SEP> 100.00 <SEP> # 195 <tb> <tb> <tb> <tb> <tb> <tb> (E.coli)    <tb> <tb> <tb> <tb> <tb> 26 <SEP> 0.65 <SEP> cylinder <SEP> Ultipor <SEP> N66 <SEP> 1680 <SEP> 50 <SEP> 1.3 <SEP> 103 <SEP> 99.44 <SEP> 2x102 <tb> <tb> <tb> <tb> <tb> <tb> of course <tb>   EXAMPLE 27 Aside from the bacteria (E. coli) that have been tested for titer reduction, pathogenic organisms, such as Listeria in milk, are of real practical interest in the dairy industry. These pathogens pose a more serious challenge than the coli-like organisms (E. coli), which were also tested in the dynamic filter. The tests were carried out in accordance with the process described in process step D in order to see whether the membrane filter elements used would remove these pathogens efficiently. This test was carried out on an "off-line" test set-up and on the dynamic filter.    The data listed in Table 8 clearly show that a 0.45m Ultipor N66 membrane with a specific bubble point (ASTM F316-86) provides absolute removal of Listeria.  <Desc / Clms Page number 21>      Table 8 Reductions in titer of pathogenic organisms found in milk using Pall Corporation membranes  EMI21.1   <tb> membrane <SEP> bubbles <SEP> total <SEP> filtrate <SEP> titer reduction <tb> <tb> dot <SEP> of <SEP> Listeria <SEP> bacteria <tb> <tb> bacteria / ml <SEP> per <SEP> ml <SEP> TR <tb>    EMI21.2    EMI21.3   <tb> 0.45 <SEP> m <SEP> Ultipor <SEP> N66 <SEP> 154 <SEP> x <SEP> 10 <SEP> 3 <SEP> Pa <SEP> 6.6 <SEP> x <SEP > 107 <SEP> 1 <SEP> 4.7 <SEP> x <SEP> 106 <tb> 0.65 <SEP> um, <SEP> Ultipor <SEP> N66 <SEP> 115, <SEP> 5 <SEP> x <SEP> 10 <SEP> 3 <SEP> Pa <SEP> 7.10 <SEP> x <SEP> 107 <SEP> 7.5 <SEP> x <SEP> 103 <SEP> 6.8 <SEP> x <SEP> 102 <tb>   EXAMPLE 28 Filtered skimmed milk made by the method of Example 16 was collected in a hygienic container.    Commercially available cream was heated to 65 ° C and filtered through a 0.2m Ultipor N66 filtration cartridge available from Pall Corporation, East Hills, NY with a minimum titre reduction of 106 with E. coli bacteria. The filtered cream is essentially free of bacteria and was collected in a hygienic container.    The filtered skimmed milk and the filtered cream were then mixed and homogenized to obtain a milk with 2% fat with a lower bacterial content.    PATENT CLAIMS: 1 Process for the treatment of raw milk by homogenization and microfiltration, characterized in that the milk is subjected to dynamic microfiltration within five minutes after the homogenization by passing the milk through a microfilter with an average pore size which it is sufficient to reduce the bacterial content of the milk that passes through sufficiently to use a treated filtrate Milk that has a lower bacterial content than the original raw milk and a To deliver concentrate with a higher bacterial content than the original raw milk.   2. The method according to claim 1, characterized in that the filtration is carried out using a turntable microfilter.  3. The method according to claim 2, characterized in that the homogenization of the milk takes place simultaneously with the filtration step.  4. The method according to claim 1, characterized in that the dynamic microfiltration is carried out with an effective surface speed of the microfilter of 3 m / sec to 50 m / sec.  5. The method according to claim 2, characterized in that the microfiltration at a Milk temperature in the range of 15 to 60 C is carried out.  6. The method according to claim 1, characterized in that the filtration in less than 30 seconds after the homogenization step is carried out.  7. The method according to claim 2, characterized in that the dynamic microfiltration is carried out with a shear rate of 10,000 sec-1 to 400,000 sec-1 8. The method according to claim 1, characterized in that a microfilter is used which has a pore size of 0.01 to 5.0 m 9. The method according to claim 1, characterized in that a microfilter is used which has a pore size of 0.1 to 1) in.  10. The method according to claim 1, characterized in that a microfilter is used which has a pore size of 0.2 to 0.5 m 11. The method according to claim 2, characterized in that a part of the concentrate back  <Desc / Clms Page number 22>  to be led. 12. The method according to claim 1, characterized in that the filtration is carried out using a cylindrical, dynamic microfilter. 13. The method according to claim 12, characterized in that the microfiltration at a Milk temperature in the range of 15 to 60 C is carried out. 14. The method according to claim 13, characterized in that part of the concentrate is recycled. 15. Process for treating raw milk to produce treated milk that has a lower bacterial content than raw milk, the milk being separated into a fat fraction with a minimum fat content of 10% and a skim milk fraction, the skim milk fraction being passed through a microfilter , which has an average pore size, which is enough to reduce the bacterial content of the milk flowing through To provide filtrate, which has a lower bacterial content than the original one Skimmed milk fraction and to provide a concentrate which has a higher bacterial content than the original skimmed milk fraction, the bacterial content of the fat fraction is reduced separately and then the skimmed milk fraction after microfiltration and the fat fraction with reduced bacterial content are combined,  characterized in that the skimmed milk fraction is homogenized within 5 min before the microfiltration and the microfiltration is carried out as dynamic microfiltration. 16. The method according to claim 15, characterized in that the bacterial content of the fat Fraction is reduced by dynamic microfiltration. 17. The method according to claim 15, characterized in that the bacterial content of the fat Fraction is reduced by pasteurization. 18. A method of producing milk with a fat content of 2%, wherein a skim milk fraction is passed through a microfilter having an average pore size sufficient to reduce the bacterial content of the milk flowing through to provide a filtrate which is lower Bacteria content than the original skimmed milk fraction and to provide a concentrate that has a higher bacterial content than the original skimmed milk fraction, the bacteria content of a cream fraction is reduced with a minimum fat content of 10%, and then the skimmed milk fraction after the Microfiltration and the cream fraction are combined with the reduced bacterial content, characterized in that  that the skimmed milk fraction within 5 min before the Microfiltration is homogenized and the microfiltration is carried out as dynamic microfiltration.  TO THIS 1 SHEET OF DRAWINGS