Note: Descriptions are shown in the official language in which they were submitted.
CA 0209~0~7 1997-12-16
PRODUCTION OF STERILE MILK
THROUGH DYNAMIC MICROFILTRATION
FIELD OF THE INVENTION
The present invention relates to a method for
producing milk, either whole or skim milk, with a lowered
bacterial content, the product of said method, and a method
for distributing milk to consumers.
BACKGROUND OF THE INVENTION
The well known pasteurization process to kill bacteria
in milk has been used for many decades. Unfortunately, the
higher temperatures needed in the pasteurization process
adversely affect the flavor of the milk. Further, even
with the use of such high temperatures, the pasteurization
process does not eliminate all undesirable bacteria,
leading to the short storage stability of most milk
products.
Bacillus cereus are often the predominant bacteria in
conventionally processed milk of relatively advanced age,
because they can survive the pasteurization process and
they thrive at cold temperatures, promoting the spoilage of
the milk. A general need exists for a method for reducing
the content of bacteria in milk, both whole and skim milk,
to enhance the storage stability of the product and to
improve its flavor by elimination of the pasteurization
process.
Of great economic importance, also, is the need to
eliminate the very expensive and laborious distribution
method that is now necessary to place milk in the hands of
the consumer. It is now necessary for every dairy,
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after processing of the raw milk through homogenization
and other steps, to fill the milk into containers for
distribution to the consumers and to transport that milk
under refrigerated conditions. This requires every dairy
to purchase and maintain a significant fleet of
refrigerated trucks to transport the processed milk to
the point of distribution to the consumer. By providing
a sterile or nearly sterile milk product, it would be
possible to eliminate the need for transportation of milk
under such refrigerated conditions. Unfortunately, the
pasteurization process only provides milk with a reduced
bacterial count, and not a sterile product.
Further, if a sterile milk product could be
produced, it would also be possible to eliminate the need
for storing the milk at the point of distribution under
refrigerated conditions. Elimination of the need for
large refrigerated compartments, as in the typical
grocery store, would also be of tremendous economic
benefit.
Even when the present day pasteurization process is
employed, in some instances it is of particular
importance to obtain milk with a lowered bacterial
content, prior to pasteurization. For example, a
particular batch of raw milk may be so contaminated that
mere pasteurization will not result in even adequate
storage life by today's standards.
For some applications, moreover, it is of value to
be able to provide treated milk in which the bacteria
content has been greatly reduced e.g., to about one
hundredth of the original value. It is especially
important to provide milk with a relatively low bacterial
content for the production of cheese, since incorrect
bacteria cultures can destroy the cheese. It is normally
not suitable to simply heat-treat milk to a sufficient
degree for use in cheese production, because such heat
treatment may give a lower yield of cheese and can also
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adversely influence the coagulation time.
Conventionally, additives are employed to reduce the
problem. In many instances, however, it would be
desirable to avoid the use of such additives.
Various methods for producing milk with a lowered
bacterial count through the use of filtration have been
known in the art, but none have found wide acceptance.
The prior art methods generally provide either poor flow
rates, rendering the method uneconomical on a large
scale, or adversely affect the quality of the milk,
making the product unacceptable to consumers.
Conventional filtration means for producing milk
with a lowered bacterial content have been attempted.
Swedish patent publication No. 380,422 discloses a method
in which whole milk is divided into filtrate and
concentrate fractions by microfiltration. The filtrate
that passes through the pores of the filter (the size of
the pores may range broadly from 0.1 micron - 10 micron)
consists of milk with substantially reduced fat content
and the concentrate, which is the fraction retained by
the surface of the filter, consists of cream, as not
only bacteria, but also fat globules are substantially
retained by the filter.
Swedish published patent application No. SE A 67
15081 discloses a method for sterilizing milk in which
fat is first separated from the skim milk. Next, the fat
fraction is sterilized by means of heat, and the skim
milk fraction is sterilized by means of bacteria
filtering (no filter pore sizes are given). Finally, the
sterilized fat and skim milk fractions are remixed to
yield a sterile milk product. In order to so sterilize
the skim milk fraction by means of bacteria filtering,
the pore size in the filter must be so small that no
bacteria can pass through it, resulting in poor
throughput rates and the undesired retention of fat
globules and protein content from the milk.
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U.S. Patent No. 5,064,674 relates to a method for
making hypoallergenic milk by ultrafiltration methods
employing membranes that will allow molecules having a
molecular weight of less than or equal to about 5kDa to
pass therethrough. The excluded components that are
trapped by the membrane include milk protein, viable or
non-viable bacteria, bacterial protein antigen, and milk
fat. The filtrate collected from the ultrafiltration
process therefor is free not only of bacteria and
bacterial protein antigen, but also fat and milk protein,
making the product unsuitable for use as milk, per se.
It is clear, then, that the pores of bacteria
filters used in the art, which filters are effective to
sterilize milk, also will remove not only the bacteria,
but also the fat globules, and at least some of the
proteins. Such a filter quickly becomes blocked by
trapped material; hence, the flow rate through the filter
rapidly declines and the filter must be frequently
cleaned or replaced. The high cost of such an
inefficient process is generally prohibitive. Further,
because the filter holds fat globules and proteins, the
quality of the milk is also adversely affected.
From the foregoing discussion, it is apparent that
there is a continuing need for an improved milk
filtration processing method that can provide a sterile,
or more nearly sterile product, that has an improved
storage life, and does not adversely affect milk quality.
Some attempts have heretofore been made to use
cross-flow, or tangential flow, filtration devices to
treat milk, such devices being known in the art.
Several types of filtering devices have been
described which enable such tangential or crossflow
filtration to be accomplished. Perhaps the oldest such
apparatus known, described in Soviet Pat. No. 142,626 to
Zhevnovatyi, A. I. in 1961, is formed by a tube of porous
material fixed inside a second tube, the suspension to be
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filtered passing under load at high velocity in the
annular space between the two tubes, the filtrate flowing
within the porous tube. Improved devices of similar
construction use two concentric cylinders, with the
internal cylinder being formed by a microporous membrane,
the liquid being subjected to a forced helicoidal flow
around such internal cylinder.
Other crossflow devices comprise a series of
filtering elements superposed in the form of plates or
disks, on the two faces of which microporous membranes
are arranged, for example, around a filtrate-collecting
tube, the suspension to be filtered passing between the
disks in a helicoidal path one after another.
Many other variations on the crossflow filtration
system have been developed. For example, U.S. Patent No.
5,009,781 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 which
transect the chambers. U.S. Patent No. 5,035,799 relates
to a crossflow filter assembly having filter leaf
assemblies arranged in parallel within the filter tank,
with pressurized input to create turbulent crossflow of
fluid over the media.
U.S. Patent No. 5,015,397 relates to a crossflow
filtration apparatus and process which includes a tube of
helically wound wedge wire. Contaminated influent enters
at one end and as it flows through the tube, it becomes
more concentrated with contaminants, while clarified
liquid permeates through the tube wall. U.S. Patent No.
5,047,154 relates to a method and apparatus for enhancing
the flux rate of crossflow filtration systems. U.S.
Patent No. 4,569,759 relates to a tangential fiItration
apparatus and a plant comprising such an apparatus.
Cross-flow filtration is substantially different
from through-flow filtration, in that the liquid feed is
introduced parallel to the filter surface, and filtration
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occurs in a direction perpendicular to the direction of
the feed flow. In cross-flow filtration systems,
generally, because the direction of the feed flow is
tangential to the membrane surface, accumulation of the
filtered solids on the filtering medium is reduced by the
shearing action of the flow. Cross-flow filtration thus
affords the possibility of a quasi-steady state operation
with a nearly constant flux when the driving pressure
differential is held constant. Unfortunately, this
theoretical possibility has not been achieved in
practice. Thus, the problem of declining filtration
fluxes has plagued conventional cross-flow filtration
systems. The majority of the suspended solids are
retained on the wall of the tube and quickly form a
dynamic membrane (also referred to as a "filter cake" or
"sludge layer"). The dynamic membrane is largely
responsible for the filtration which subsequently occurs.
Those particles initially entering into the wall
matrix ultimately become entrapped within it, because of
the irregular and tortuous nature of the pore structure.
As microfiltration proceeds, penetration of additional
small particles into the wall matrix is inhibited by the
presence of the dynamic membrane. The formation of the
dynamic membrane, together with the possible clogging of
the pore structure of the tube by entrapped particles,
results in a decline in the filtration flux. In
conventional systems, this decline is approximately
exponentially related to filtration time.
Crossflow filtration of milk has been attempted, but
has not been generally accepted because of the problems
discussed above. U.S. Patent No. 5,028,436 relates to a
process for separating the dissolved and undissolved
constituents of milk, using a microporous membrane with a
pore size in the range of 0.1 to 2 microns, which has
been pretreated with an aqueous solution, dispersion or
emulsion of lipids or peptides and the milk separated on
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the pretreated membrane. In the method of the patent a
first filtration step is employed 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 poly-peptides of low molecular weights in genuine,
non-denatured form and the concentrate contains
practically all casein and fatty components of the milk.
Thus, the filtrate cannot be considered to be "milk" as
the fatty substances have all been removed therefrom.
U.S. Patent No. 4,876,100 relates to a crossflow
filtration method for producing milk with a lowered
bacterial content. Raw milk is divided by centrifugal
separation into one fraction consisting of cream and
another fraction consisting of skim milk. The skim milk
fraction is directed into a microfilter in which part of
the fat globules, protein, and bacteria are separated.
From the microfilter there is obtained a filtrate which
consists of skim milk having a lowered fat, protein and
bacterial content, and a concentrate having an increased
fat, protein and bacterial content. The concentrate is
subsequently sterilized. Thus, the filtration method of
the '100 patent, besides reducing bacterial levels in the
filtrate, also reduces the fat and protein content of the
filtrate, altering its characteristics from that of the
original skim milk.
Clearly, the use of crossflow filtration, to date,
has not provided an acceptable method for reducing
bacterial contamination in milk.
One means to overcome some of the problems
associated with classical crossflow filtration
technology, known as dynamic microfiltration, has
emerged. The dynamic filtration process overcomes the
disadvantage in the classical crossflow technology
because the liquid to be filtered is not simply guided
tangentially over the membrane surface. The membrane
CA 0209~0~7 1997-12-16
surface or a solid body near the membrane surface is moved
such that the fluid at the interface between the rotor and
the stator is subjected to shearing action. The shearing
action tends to "scrub" the membrane surface, keeping it
relatively clear of particulate material, and preventing a
filter cake from forming on the membrane surface. The
particulate material that would otherwise collect on the
membrane surface remains suspended, and is ultimately
removed in the secondary stream, generally referred to as a
concentrate stream.
Dynamic microfiltration systems may take various
forms. For example, U.S. Patent Nos. 5,037,562, 3,997,447,
S,037,562, 3,997,447 and 4,956,102 relate to dynamic
microfiltration disc systems.
Cylindrical dynamic microfilters devices are taught in
U.S. patent nos. 4,956,102; 4,900,440; 4,427,552;
4,093,552; 4,066,554; and 3,797,662, as well as many
others.
No one has ever applied dynamic microfiltration to the
processing of milk, and the use of crossflow filtration of
milk has been limited, and principally used to fractionate
milk into components based upon fat content.
SUMMARY OF THE INVENTION
It has now been surprisingly discovered that dynamic
microfiltration of milk can be successfully accomplished,
without the prior art problems of degradation of milk
quality, premature filter plugging, and inadequate
bacterial removal, through the practice of the method of
the present invention.
In accordance with the present invention milk, either
whole or skim milk, is first homogenized and then subjected
to filtration. By performing the homogenization step
first, the particle size of the fat
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g
globules and other large, suspended components of the
milk is significantly reduced, allowing for
microfiltration of the milk without significant removal
and entrainment of the fat and other components.
Milk is an emulsion of fat and protein particles in
water. Homogenization provides for a method of reducing
the emulsion particle size to allow passage through an
appropriately sized microporous membrane, to retain
bacteria contained therein without unwanted removal of
the fat and protein content of the milk.
The milk, after homogenization, is filtered through
the use of dynamic microfiltration. The invention thus
provides an improved method for producing milk with a
lowered bacterial content, without the need for
pasteurization. That portion of the milk fraction that
is retained by the microfilter (the concentrate
fraction), may be recirculated as part of the feed, or
may be discarded or used in other processes.
Thus, in one aspect, the present invention provides
a method for treating raw milk to produce treated milk
having a lower bacterial content than the raw milk. The
method comprises homogenizing the milk and within about 5
minutes from the homogenization, subjecting the milk to
dynamic microfiltration by passing the milk through a
microfilter having an average pore size sufficient to
reduce the bacterial content of the milk flowing
therethrough, to yield a filtrate which has a lower
bacterial content than the initial raw milk and a
concentrate having a higher bacterial content than the
initial raw milk. The resulting milk has a very low
bacterial content, such as about 103 bacteria per
milliliter, or less, and retains more organoleptic
components than that found in pasteurized milk with the
same bacterial content.
The milk that may be obtained as a result of the
process of the present invention, in general, is more
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storage stable than milk obtained as a result of
conventional pasteurization. Significant residual
bacteria remains in milk, after pasteurization, because
milk naturally contains certain bacteria, which also
survive the pasteurization process. Thus, pasteurized
milk still must be refrigerated to reduce bacterial
growth and spoilage.
Unfortunately, some of the bacteria present in raw
milk are both thermoduric (bacteria that survive
pasteurization) and psychrotrophic (bacteria that thrive
at low temperatures, below 15~ C) such as Bacillus
cereus. The presence of thermoduric, psychrotrophic
bacteria in the packaged milk product is very
detrimental, as their rapid growth, even under
refrigerated conditions, results in the spoilage of the
milk.
The present invention is capable of producing
sterile milk, which may be stored even at room
temperature for extended periods of time, such as for 30
days or more. The sterile milk of the present invention
may be characterized by the absence of bacteria,
generally, and in particular by the absence of bacteria
and pathogens such as the following:
Thermoduric Bacteria
Micrococus M. luteus, M. roseus
Streptococcus S. pneumoniae, S. lactis,
S. faecalis
Lactobacillus L. de:brueckii, L. lactis,
L. he_veticus, L. casei,
L. tr_chodes
Staphylococus S. aureus, S. epidermidis
Bacillus B. cereus, B. subtilis,
B. macerans,
B. stearothermophilus
Clostridium C. butyrium, C. pasteurianum
C. botulinum, C. perfringens,
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11
C. tetani
Psychrotrophic Bacteria
Psuedomonas P. aeruqinosa, P. fluorescens,
P. pseudomallei, P. mallei
Archnomobacter
Alcaligenes
Acientobacter A. lignieressii, A. equirli
Flavobacterium F. aquatile, F. meniqosepticum
Bacillus B. cereus, B. subtilis,
B. macerans,
B. stearothermophilus
Coliform Bacteria
Enterobacter E. co i, Salmonella TYphi
Shige la Dysenteriae,
Klebs ella Pneumoniae
Miscellaneous
Listeria L. monocytogenes
Thus, the milk of the present invention will be
capable of meeting and typically exceeding the
requirement for Grade A pasteurized milk which requires
the milk not to exceed a bacterial plate count of 30,000
per milliliter, and a coliform count exceeding 10 per
milliliter, as determined by standard methodology.
In another embodiment the present invention provides
a method for treating raw milk to produce treated milk
having a lower bacterial content than the raw milk,
comprising (1) separating said milk into a fat fraction
with a minimum fat content of about 10% and a skim milk
fraction, (2) homogenizing the skim milk fraction and
within about 5 minutes from the homogenization,
subjecting the skim milk fraction to dynamic
microfiltration by passing the skim milk fraction through
a microfilter having an average pore size sufficient to
reduce the bacterial content of the milk flowing
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12
therethrough, to yield a filtrate which has a lower
bacterial content than the initial skim milk fraction and
a concentrate having a higher bacterial content than the
initial skim milk fraction, (3) separately reducing the
bacterial content of the fat fraction, and (4)
subsequently combining the skim milk fraction after
microfiltration and the fat fraction having the lowered
bacterial content.
In a further embodiment, the present invention
provides a method for making milk having a fat content of
about 2%, comprising (1) homogenizing a skim milk
fraction and within about 5 minutes from the
homogenization, (2) subjecting the skim milk fraction to
dynamic microfiltration by passing the skim milk fraction
through a microfilter having an average pore size
sufficient to reduce the bacterial content of the milk
flowing therethrough, to yield a filtrate which has a
lower bacterial content than the initial skim milk
fraction and a concentrate having a higher bacterial
content than the initial skim milk fraction, (3) reducing
the bacterial content of a cream fraction having a
minimum fat content of about 10%, and (4) subsequently
combining the skim milk fraction after microfiltration
and the cream fraction having the lowered bacterial
content.
The present invention also provides a method for
processing milk for consumption by a consumer, comprising
obtaining raw milk, homogenizing the milk and within
about 5 minutes from the homogenization, subjecting the
milk to dynamic cross-flow microfiltration by passing the
milk through a microfilter having an average pore
diameter sufficient to reduce the bacterial content of
the milk flowing therethrough, to yield a filtrate which
has a lower bacterial content than the initial raw milk,
packaging the milk into a container for use by the
consumer, and transporting the milk, without
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refrigeration, to a point for distribution to the
consumer.
Most generally, then, the present invention provides
a method for distributing milk for consumption by a
consumer, comprising obtaining raw milk, reducing the
bacterial content of the milk to a level of 103 bacteria
per milliliter or below, packaging the milk into a
container for use by the consumer, and transporting the
milk, without refrigeration, to a point for distribution
to the consumer. This eliminates the need for
refrigerated transportation and delivery vehicles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the equipment
employed in accordance with the method of the present
invention.
FIG. 2 is a plot of the particle sizes in milk after
homogenization.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The initial material is fresh, untreated raw milk,
as from a domestic animal, such as a cow. The method of
the present invention may also be applied to processed
milk, such as that already subjected to pasteurization,
but the full advantages will not 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 processed may first be directed
through a heat exchanger to adjust it to a suitable
temperature, and if desired, then passed through a
centrifugal separator, to remove all or a portion of the
cream fraction in a conventional manner.
As an overview, the raw milk is homogenized and
reasonably promptly passed through a dynamic microfilter,
yielding a filtrate fraction and a concentrate fraction.
The pores in the microfilter are sized to retain at least
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14
a portion of the bacteria. The filtrate, which is the
portion of the milk fraction that passes through the
retaining surface of the microfilter, consists of milk
with no, or a lowered, bacterial content (relative to the
milk before microfiltration), with essentially no change
in the fat and protein content. The filtrate fraction
may then be used directly to make other products, such as
powdered milk, or packaged without further treatment.
The filtrate fraction is more desirable than the
milk obtained by conventional pasteurization, for many
reasons. It retains more organoleptic components than
milk that has been pasteurized, making it more flavorful
and desirable from the standpoint of the consumer.
Further, the milk obtained in accordance with the present
invention has a much greater storage life, because
bacteria, such as the psychrophilic bacteria, especially
Bacillus cereus, can be completely removed by the present
invention, an impossibility using conventional
pasteurization.
The concentrate fraction, which is the portion of
the milk fraction that is retained by and recovered from
the retaining membrane surface of the microfilter,
consists of milk with an increased bacterial content
(relative to the milk feed prior to microfiltration) and
essentially no change in fat globular and protein
content. The concentrate fraction subsequently may be
discarded or used in other processes.
The filtrate may contain some bacteria, but the
lower the bacterial content, the more storage-stable the
product. Full sterilization is desirable, but the
initial growth rate of a small remaining concentration of
bacteria is usually low enough to still result in greatly
increased storage life of the resultant milk product.
The storage life of milk produced according to the
3S method of this invention is substantially increased over
that for conventionally-pasteurized milk because the
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concentration of Bacillus cereus bacteria, in particular,
is greatly reduced.
Because the milk of the present invention may be
rendered sterile, whereas milk obtained by use of
conventional pasteurization techniques cannot be truly
sterile, the milk may have extremely long storage life
under refrigerated or room temperature conditions,
especially if the milk is placed into a container under
aseptic conditions. One means to do this is by use of
the form-fill-seal techn;que that is now well known in
the packaging industry. This t~-hn;que is often used for
packaging sterile solutions and the like, as for the
pharmaceutical industry. The milk made in accordance
with the present invention may be packaged using the
form-fill-seal tPchnique, and such milk may exhibit
extremely long storage life, even at room temperature.
The exact method or machinery used to accomplish the
filling is not critical. As merely one example and
explanation as to how such a form-fill-seal technique can
be employed, the following description is provided.
Some vertical form, fill and seal machines use a
flat web of synthetic thermoplastic film which is unwound
from a roll and formed into a continuous tube, in a tube
forming section, by sealing the longitudinal edges of the
film together. In other machines the tube is extruded
from a resin melt at the time of use. The tube thus
formed is advanced to a filling station where it is
collapsed across a transverse cross-section, the position
of the cross-section being at a sealing device below the
filling station. A transverse heat seal is made, by the
sealing device, at the collapsed portion of the tube,
thus making an airtight seal across the tube. After
making the transverse seal, a quantity of material to be
packaged, e.g. liquid, is caused to enter the tube, at
the filling station, and fill the tube upwardly from the
aforementioned transverse seal. The tube is then caused
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16
to move downwardly a predetermined distance and sealed
and severed transversely at the second transverse
section.
One such vertical form, fill and seal machine of the
type described above is sold under the trade mark PREPAC,
and another is disclosed in U.S. Patent No. 5,038,550.
HOMOGENIZATION
The milk fraction is first preferably heated or
cooled, after centrifugal separation, if employed and
prior to homogenization, to a suitable temperature for
homogenization. The milk is then passed into a
homogenizer where the fat emulsion size is reduced to a
size sufficient to allow passage through the membrane.
Preferably, the size of all suspended particles is less
than about 1 micron. It is important that the milk,
after homogenization, be filtered relatively soon
thereafter. Preferably, the filtration will be in less
than about 5 minutes, preferably less than about 2
minutes, and most preferably in less than about 30
seconds.
Again, the important factor is not the holding time
prior to filtration, but rather the fact that filtration
occurs prior to any substantial agglomeration of
globules, forming a substantial number of particles
larger than about 1 micron.
Homogenization of skim or whole milk prior to
filtration in a cylindrical, dynamic microfiltration unit
is absolutely necessary in order to properly emulsify and
suspend the fat constituents and other components of the
milk and reduce the size sufficiently, and thus achieve
proper filtration. A rotating disc filter, however,
develops a significant shear rate immediately at the
surface of the rotating disc. Hence, some degree of
homogenization of the milk can occur essentially
instantaneously with filtration. Such "self" emulsifying
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.
of the milk by the action of the dynamic microfilter
allows skim milk to be processed by a rotating disc
filter without the need for a separate homogenizer.
In effect, the rotating disc environment acts to
both homogenize and concurrently filter the milk, which
is not accomplished in a rotating cylindrical filter
unit. A rotating disc filter may generate a shear rate
of about 200,000 sec~l, whereas a rotating cylindrical
unit may generate a shear rate of only 10,000 sec~l.
Although the shear force is substantial in a rotating
disc filter unit, it is not believed that it will be, in
most instances, sufficient to adequately homogenize whole
milk.
DYNAMIC FILTRATION
In the present invention the filtration is performed
as a dynamic filtration, that is the filtration medium
itself is kept in constant motion so that the effective
flow rate of milk across the medium is extremely high.
The particular physical form of the dynamic membrane is
not critical. Thus, the membrane medium may take the
form of discs or cylinders, for example. Such dynamic
microfiltration devices have been discussed previously
and are suitable in the practice of the present
invention. In general, the dynamic microfilter comprises
a cylindrical or disc membrane element that spins inside
an outer impermeable cylinder. In a cylindrical dynamic
microfilter, when the fluid to be filtered is introduced
into the gap between the stator and the rotating
membrane, momentum from the spinning membrane is imparted
to the fluid. The fluid near the inner cylinder
experiences a higher centrifugal force than the fluid
near the outer cylinder. This phenomena, under certain
conditions, generates a flow pattern that is known as
Taylor vortices, which phenomena prevents the development
of substantial residue on the membrane surface.
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18
The dynamic filtration process, then, takes
advantage of the generation of the Taylor vortices to
keep the surface of the membrane free of potential
residue, the constituents of which then remain suspended
in the fluid being filtered. The process then splits the
feed into a filtrate (the portion of fluid that permeates
through the membrane) and a concentrate (the fraction
that contains the suspended particles that normally would
have deposited on the surface of the membrane, plugging
the same). In such a manner, a high flux rate through
the membrane can be maintained for a long period of time.
The amount of feed and concentrate must be controlled in
a fashion that results in a stable fluid flow. Even with
low flow rates of concentrate, it is possible to maintain
a stable flow of fluid to the surface of the membrane.
The dynamic microfiltration allows for a wide range
of effective surface velocities for the filtration medium
relative to the milk feed. For example, an effective
surface velocity of from about 3 m/sec to about
50 m/sec is usable, especially from about 5 m/sec to
about 30 m/sec, most preferably from about 8 m/sec to
about 20 m/sec.
To achieve the desired surface velocities a
representative filtration medium in the form of a
cylinder with a diameter of about 2.5 inches will need to
be rotated at a rate of about 1,000 to about 6,000
revolutions per minute (rpm) with a rate of about 5,000
rpm being typical.
If a dynamic disc filtration device is employed, a
typical disc filtration medium will have dimensions of
about 2 inches to about 48 inches, in diameter. Such
discs may, for example, be rotated at speeds from about
1,000 rpm to about 8,000 rpm, typically from about 3,000
rpm to about 6,000 rpm, depending upon the design of the
particular dynamic microfilter that is employed.
Preferably the shear rates of such disc filters will be
CA 0209~0~7 1997-12-16
from about 100,000 sec~l to about 400,000 sec~l. Among the
preferred disc filters are those of the type disclosed in
copending Canadian Patent Application No. 2,126,672, filed
on December 22, 1992.
The microfilter pores are sized so as to retain the
bacteria that are present in the milk while still maintain-
ing an acceptable flow rate through the microfilter. Use-
ful membranes include hydrophilic microporous membranes
with good flow properties, narrow pore size distribution
o and consistent bacterial removal performance for the bac-
teria of interest. The pore size rating of the microfilter
membrane should be from about 0.01 to about 5.0 microns, as
determined by those methods known in the art, the tests
known as the "bubble point" (ASTM F316-86) and the KL
method (U.S. patent No. 4,340,479). Preferably, the pore
size rating will be from about 0.1 to about 1 micron. Most
preferably, filters are employed that have pore size
ratings from about 0.2 to about 0.5 micron. Such micro-
porous filters are well known and are readily available.
Preferred microporous membranes that may be used in
accordance with the present invention include those sold by
Pall Corporation under the trademarks Ultipor N66,
Fluorodyne~, and Posidyne~; those available from Cuno
Corporation under the trademark Zetapor, and those sold by
Millipore under the trademark Durapore~.
The cylindrical membrane elements of use in the pre-
sent invention include those that may be attached to a
support in a leak-tight manner, in accordance with methods
known in the art.
Ultimately, the bacteria should be concentrated into a
stream that is less than about 5~ of the feed and greater
than about 95% of the solids and proteins normally found in
the milk should pass through the membrane, for extended
periods of time.
-- 19 --
20950S7
The dynamic microfilter can be operated single pass
without the necessity of recycling the concentrate. If
desired the concentrate can be recycled to the feed.
When a cylindrical dynamic microfilter is employed, it
can be operated at various ratios of filtrate flow to the
total feed flow (concentration factors). However, the
cylindrical dynamic microfilter is advantageously
operated at filtrate to feed ratios of above 90%,
especially about 95%, and particularly above 98% in order
to produce predominately the very low bacteria content
filtrate as the desired product.
Similarly, when a rotary disc dynamic microfilter is
employed, it also can be operated at various ratios of
filtrate flow to the total feed flow. However, the
rotary disc dynamic microfilter may be operated at
filtrate to feed ratios over a wide range. Selecting a
high ratio will simply lower the throughout, whereas
operation at a low ratio will result in a higher
throughput. It is believed that operation at a ratio of
about 40% is advantageous in order to maintain a stable
flow rate through the filter, although other ratios may
be employed.
The filtration of the freshly homogenized milk may
be done warm at 40~C to 60~C, which is at or somewhat
above the crystallization temperature of about 40~C of
the higher melting components of milk fat. This is below
the temperatures employed in conventional thermal
pasteurization. Alternatively, with some degradation in
flow rate, the milk may be filtered at much lower
temperatures, such as from about 15~ to about 35~C,
particularly from about 20~ to about 25~ C.
CA 0209~0~7 1997-12-16
GENERAL
After microfiltration, the concentrate may be discard-
ed in any acceptable manner, subjected to further process-
ing, or used directly.
The method of this invention may be used to advantage
where the desired end product is either whole milk,
standardized milk, or skim milk.
The flux rates through a bacterially retentive mem-
brane, of milk with a lowered fat content, are normally
lo higher than the flux rates of milk with a high fat content.
In certain situations it is economically more advantageous
to produce a milk with a higher fat content, such as 2% fat
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 about 10%.
Filtration of the cream fraction can occur, for exam-
ple, by the method of copending Canadian application Serial
No. 2,084,578, filed December 4, 1992 or with the use of a
dead end bacterially retentive filtration cartridge. The
filtration can be carried out in an industrially accept-
able manner by heating the fat composition to a point where
it is in a liquid state and is easily filtered through a
microporous membrane. The preheated fat may be homogenized
prior to filtration. Alternatively, the fat composition
can be subjected to pasteurization to lower its bacterial
content, or a combination of pasteurization and microfil-
tration may be used.
Further, if the object of the process is to obtain
protein concentrates, as from the milk of a transgenic
animal such as a transgenic cow, the dynamic microfiltra-
tion is operated to achieve a high concentration of concen-
trate, using a microporous membrane with a pore size rating
of about 0.2 microns, or lower.
- 21 -
2095057
22
Suitable apparatus for carrying out the method of
the invention can be constructed by interconnecting
conventional equipment including 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 equipment to make such apparatus
operable, and to then make further conventional
modifications to such apparatus as needed in particular
cases.
All references discussed hereinabove are
incorporated herein by reference.
The examples below further illustrate particular
embodiments, but in no way are intended to limit the
scope of the invention, which is defined in the claims.
GENERAL PROCEDURE
For the examples, the general procedures employed
were as follows.
Method A: Temperature adjustment of milk
Unless otherwise indicated, the milk employed in the
following examples was commercial milk, obtained from a
retail outlet. The temperature of the milk was adjusted
to a suitable process temperature, prior to filtration.
The preferred operating temperature (40-60~ C) was used
because the majority of the fats in milk are not in a
crystallized form at such temperatures. A 35 liter
jacketed fermenter vessel (Type 3000 from Chemap A.G.)
served as the process vessel. The vessel was filled with
milk and the contents were heated to about 50~ C, unless
otherwise noted, via a hot water jacket. The milk was
stirred during the heating process to enhance heat
transfer.
Once the milk attained the desired process
temperature, the milk was pumped to a homogenizer at the
rate of about 1 liter/min.
209~057
23
Method B: Homoqenization of milk
Upon entering the homogenizer (Model 15 MR from APV
Gaulin, Inc.) the milk underwent a two-stage
homogenization process, the first of which was at about
2500 psi and the second at about 500 psi. Normal
procedures of startup and operation were followed as
outlined in the APV Gaulin operating manual for this
unit. Typically, after homogenization, the milk was
transferred to an intermediate surge tank which was
jacketed and maintained at the desired process
temperature. This tank functioned as a fluid buffer
between the homogenizer outlet and the feed to the
filter. Whenever desired, the homogenized milk could be
recycled through the homogenizer to maintain a constant
volume in the surge tank.
Method C: Introduction of bacteria
into the milk feed stream
In some experiments artificial seeding of the milk
stream with bacteria was used to demonstrate the very
high titer reduction possible with the present invention.
Bacteria inoculate was added to the feed stream via a
metering pump, between the process vessel and the
homogenizer. The inoculate flow rate was maintained such
that a desired concentration level of bacteria of about
106 bacteria per milliliter of milk was achieved. Since
the bacteria was introduced prior to the homogenizer, the
bacteria was well mixed in the process fluid prior to
entering the filtration device. Most of the examples
used E. coli strain ATCC 15224.
An alternate method of seeding the milk with
bacteria would be by the addition of the bacteria
directly into the process vessel at the desired
concentration. Such a method is not preferred because it
exposes the bacteria to long residence times in
temperatures greater than ambient. This could cause
20950~7
24
unwanted growth or excessive killing of the bacteria
before entering the filtration device, depending on the
strain being used.
Method D: Bacterial assay tests.
Mesophiles: Bacterial concentration was determined
by serial dilution of the samples and passing the
appropriate dilutions through sterile 0.2 micron
membranes and culturing on Mueller-Hinton Agar for 24
hours at 32~ C. These procedures are detailed in a
publication entitled "Manual of Clinical Microbiology,
2nd Edition, 1974, ASM, Washington, D.C."
Listeria monocytogenes ATCC 43256 was the pathogen
tested. The population levels in the samples were
determined by the method used by Agello et. al. (Agello,
G., Hayes, P. and Feeley, J. Abstracts of the Annual
Meeting, 1986, ASM, Washington DC, p5.)
Nethod E: Cleaning Procedure.
Sanitization and sterilization were conducted prior
to every experiment using 0.1 N sodium hydroxide. In the
sterilization process, the membrane and all of the
associated equipment were first flushed with water and
subsequently treated with 0.1 N sodium hydroxide at 50~ C
for approximately half hour. The caustic solution was
then neutralized using phosphoric acid. This neutralized
solution was then used to flush the system until all
portions were neutral. Filtration tests were conducted
immediately after this procedure. The entire equipment
and membrane elements were sanitized using the
sterilization procedure upon conclusion of each test.
Method F: Integrity Testing.
Each membrane element was tested for integrity prior
to bacterial challenge. A forward flow test as described
in publication NM 900a, 'The Pall Ultipor membrane filter
CA 0209~0~7 1997-12-16
guide', copyright 1980, available from Pall Corporation,
was used for the integrity test.
Description of filtration apparatus
1. The cylindrical dynamic microfilter
The cylindrical dynamic microfilter (cylindrical DMF)
used for these tests was a BDF-01 manufactured by Sulzer
Brothers Limited, Winterthur, Switzerland. The equipment
is described by Rebsamen et. al. tDynamic Microfiltration
0 and Ultrafiltration in Biotechnology, Rebsamen, E. and
Zeigler, H., Proceedings of the World Filtration Congress
IV, 1986, (Ostend, B)). See also, U.S. Patent Nos.
4,066,554 and 4,093,552.
2. Description of membrane filter elements
The membrane filter elements used in these experiments
typically were various grades of nylon membrane, Ultipor
N66~ and Posidyne~, co~mercially available from Pall
Corporation, Glen Cove, NY. The pore sizes used were 0.2,
0.30, 0.45 and 0.65 microns. The membrane elements had a
surface area of 0.04 m2.
3. The dynamic microfilter in disc format:
The disc format consists of a six inch diameter mem-
brane support disc mounted on a hollow shaft and contained
within a leak-tight housing, with required fluid inlet and
outlet connections. The support disc had provisions for
sealing membrane sheets to its face in a leak-tight manner
and contained drainage spaces to carry filtrate flow
through the membrane and disc, and out through the shaft.
Effective membrane area was 0.014 m2 and rotation rates up
to 4500 rpm were available.
Any of the dynamic disc microfiltration units
discussed previously may be employed in the practice of the
present invention. Reference is also made to U.S.
- 25 -
209~0~7
patent application no. 07/812,123, filed on December 24,1991, for description of another dynamic microfiltration
device in disc format that may be used in the practice of
the present invention.
4. Description of membrane filter elements
The membrane filter elements were the same grades of
membrane described in the section under the cylindrical
DMF. Typically, the membranes were circular flat sheet
"donuts," cut to fit the disc DMF. When assembled in the
dynamic microfilter, the filtrate chamber was sealed from
the feed with the use of o-rings. The membrane filter
elements had a surface area of 0.014 m2.
Method G1: Operation of the cylindrical dYnamic
microfilter
Prior to filtration, a filter element, as described
in the section under filter assemblies, was assembled in
the cylindrical dynamic microfilter (DMF). Sanitization
and sterilization was conducted using the procedure
outlined in method E. After observing the startup
procedures outlined in method G2, the milk to be filtered
was pumped from the surge tank into the cylindrical DMF
via a positive displacement pump. The amount of
concentrate was controlled by a second pump or pressure
relief valve attached on the concentrate port.
Temperatures and flow rates of the feed, filtrate and
concentrate, and the feed pressures were taken at various
times during the course of the experiment, typically, in
intervals of ten minutes. Standard operating conditions
of the cylindrical DMF were a rotation speed of 5000 rpm,
filtrate to feed ratio of greater than 95%, and a feed
pressure of about 1.3 - 2.0 bar. All examples with this
device were carried out at constant feed flow rates.
209~0~7
27
Method G2: StartuP of the dynamic filter
Prior to introducing the milk into the dynamic
filter, warm, deionized, 0.2 micron filtered water was
passed through the system to startup the associated
equipment. The rotational speed of the dynamic filters
was brought up to operational speed with water flowing
through the system. When the system had reached an
equilibrium, the flow of milk was turned on. The milk
displaced the water in the system and the filtration
commenced.
Method H: Operation of the disc dynamic microfilter
A disc DMF filler element described in the section
under filter assemblies was assembled in the disc DMF.
Sanitization and sterilization was conducted using the
procedure outlined in method E. After observing the
startup procedures outlined in method G2, the milk to be
filtered was pumped from the surge tank into the disc
DMF. The amount of concentrate and feed pressure was
controlled by a valve placed on the concentrate port.
Temperatures and flow rates of the feed, filtrate and
concentrate, and the feed pressures were measured at
various times during the course of the experiment,
typically, in intervals of ten minutes. A feed rate of
about 960 ml/min was maintained for all examples. The
filtrate fluxes reported are those acquired when the flow
had stabilized in the filtration unit.
2095 0~7
EXAMPLES
EXAMPLE ONE
Room temperature skim milk was pumped at the rate of
about 600 ml/min into a cylindrical DMF, equipped with a
0.45 micron Ultipor N~ membrane. The operating
conditions in the DMF were maintained as specified by
method Gl, and are summarized in Table I. The feed
pressure started to rise rapidly a few minutes after the
start of the test, indicating plugging of the microporous
membrane.
EXAMPLE TWO
Skim milk was heated to 50~C according to method A
and homogenized by method B. The homogenized milk was
then stored in the surge tank for about four hours while
the temperature of milk was maintained at about 50~C for
this duration. After this four hour lag period the milk
was pumped into a cylindrical DMF equipped with a 0.45
micron Ultipor N~ membrane, at a feed rate of about 600
ml/min. The preferred conditions of DMF operation as
outlined in method G1 were maintained. The feed pressure
started to rise rapidly after only a few minutes of
operation, indicating plugging of the microporous
membrane and the test had to be terminated.
EXAMPLE THREE
Skim milk, heated to 50~C according to method A and
homogenized by method B, was pumped into a cylindrical
DMF, equipped with a 0.45 micron Ultipor N~ membrane,
within no more than five minutes of homogenization. The
preferred conditions of DMF operation, as outlined in
method G1 were maintained. A stable filtrate flux of
1080 L/hr/m2 was obtained until the milk supply was
exhausted. No increases in feed pressure were observed
during the course of the experiment.
20950~7
29
When all the milk had been processed the feed was
switched to unhomogenized skim milk at 50~ C, without
disturbing system operation. Within a few minutes, milk
filtrate flux decreased rapidly and the system pressure
rose, indicating that plugging of the membrane had taken
place. This example clearly shows the need to homogenize
milk to achieve significant flow through a
microfiltration membrane.
Examples 1-3 show that it is necessary to impart
sufficient shear on milk (via homogenization in this
case), prior to filtration, to reduce the emulsion
particle size of the milk sufficiently to allow passage
through the microporous membrane and thus achieve proper
filtration. Example 2, in particular, indicates that the
particle size distribution reverts to larger sizes within
a short period of time after homogenization. Hence, for
proper filtration, the homogenization must be within some
short period of time prior to filtration, such as within
less than 5 minutes or preferably shorter intervals.
EXAMPLE FOUR
Skim milk was preheated by method A and pumped into
a disc DMF equipped with a 0.45 micron Ultipor N~
membrane. The procedures outlined in method H were
employed. A steady flux of filtrate was established
quickly and was maintained for about 100 minutes until
the milk supply was exhausted.
The disc DMF operating conditions produce a
calculated shear rate of about 200,000 sec~~ at the
interface gap between the rotating disc and the membrane.
This shear is in the range of shear rates generated by
- the homogenizer by the conditions in method B.
This example demonstrates that the required shear,
prior to filtration, can be achieved in one step, i.e.,
without the need for separate homogenization equipment.
20950~7
The example clearly demonstrated that the membrane was
not being fouled by the solids in milk and that the shear
generated by the rotation of the disc, about 200,000 sec~
1, was sufficient to reduce the particle size in skim milk
to allow passage through a microfilter membrane, and thus
achieve proper filtration.
Table 1 summarizes the results of examples one
through four; the data shows that a steady state filtrate
flux through the membrane is achieved when sufficient
shear is imparted to the milk within a short time prior
to filtration.
Table 1
Example 1FiltrationFeed Shear in llo.no~en-Lag after Pore size Flux
Mode Temp. Cfilter sec-lizationI l~"-o~er,i~dlionmicrons Llhr/m2
cylinder 25 10,000 N0 N0 0.45 0
- 15 2 cylinder 50 10,000 YES 4 hours 0.45 0
3 cylinder 50 10,000 YES 5 minutes 0.45 1080
4 disc 50 200,000 N0 - 0.45 1600
EXAMPLE FIVE
In order to determine the relationship between
particle size and time after homogenization, skim milk
was heated by method A and homogenized using the
procedure outlined in method B. The particle size
distribution with respect to time after homogenization
was determined. The particle size distribution was
measured using an Integrated Micro-Optical Liquid
Volumetric Sensor (IMOLV-.2), available from Particle
Measurement Systems, Colorado. This laser particle
counter is designed to measure particle size distribution
in the range of about 0.1 to 5.0 microns.
The milk samples were diluted 1:300,000 and then
subjected to analysis as specified by the operating
manual for the IMOLV device. 0.04 micron filtered, 18
mega-ohm DI water, with particle counts less than 50 per
milliliter was used for diluting the milk samples.
20950~7
31
Figure 2 shows the results of the particle analysis.
A plot of number of particles relative to the number of
particles at 5 seconds vs. particle size is shown in the
figure. Clearly, the figure demonstrates that as the
period of time after homogenization increases, the number
of larger particles increases. As the number of smaller
particles commensurately decrease over that period, it is
apparent the smaller particles agglomerate, with the
passage of time, to form the larger particles.
EXAMPLES SIX to NINE
Membranes of various pore sizes and bacterial
retentive properties were tested on the cylindrical DMF
to determine the magnitude of the steady filtrate flux of
milk which could be achieved. The general method used
for examples six through nine are given below.
1. The desired membrane filter element was
assembled in the cylindrical DMF.
2. An integrity test as outlined in method F was
conducted. The membrane filter element was rejected
if it did not pass the test.
3. The equipment was sanitized according to
method E.
4. Milk to be filtered was preheated by the
procedure outlined in method A.
5. Milk was homogenized according to method B.
6. The startup procedure outlined in method G2 was
conducted.
7. The milk was transferred from the surge tank to
the cylindrical DMF at a desired flow rate.
8. The operating parameters were set using the
guidelines in method G1.
9. Appropriate measurements were made.
Typically, the cylindrical DMF was operated at 5000
rpm, corresponding to a shear rate of about 10,000 sec~l
2095057
32
in the filter. The feed temperature was 50~ C and the
feed pressure varied from 1.3-2.0 bar. The filtrate to
feed ratio was maintained at over 95% for each of these
examples. The flux reported in table 2 is the steady
state filtrate flux obtained, typically, fifteen minutes
after start of filtration. The total time of the
experiment varied in each case since the volume of milk
filtered was a constant 30 liters.
EXAMPLE SIX
A 0.2 micron Ultipor N~0 membrane was used for this
example. A feed rate of 250 ml/min was used to obtain a
steady state filtrate flux of 330 L/hour/m2. The
filtration continued for about 130 minutes, at which time
there was no more milk in the process vessel, without
apparent decay in filtrate flux rates.
EXAMPLE SEVEN
A 0.30 micron Ultipor N~0 membrane was used for this
example. A feed rate of about 550 ml/min was used to
obtain a steady state flux of 775 L/hour/m2, for about 60
minutes, whereupon the experiment was terminated.
EXAMPLE EIGHT
A 0.45 micron Ultipor N~0 membrane was used for this
example. A feed rate of 740 ml/min was used to obtain a
steady state flux of 1080 L/hour/m2. The filtration
continued for about 40 minutes without apparent decay in
flux rates, whereupon the milk supply was exhausted and
the experiment was terminated.
EXAMPLE NINE
A 0.65 micron Ultipor N~0 membrane was used for this
example. A feed rate of 1100 ml/min was used to obtain a
steady state flux of 1680 L/hour/m2. The filtration
20950S7
continued for about 30 minutes, whereupon the milk supply
was exhausted and the experiment was terminated.
Examples 6-9 are summarized in table 2. The data
shows that by using the filtration process of this
invention, stable filtrate fluxes can be obtained using
various grades of bacterially retentive membranes. The
table shows that in the process of the present invention
membranes with smaller pores and hence, increased
bacterial retention, can be used at the expense of
filtrate flux rates.
Toble 2: Flux of milk usin~ various ....,...L.~ ,~s in the c~ ;c~l DMF
Ex~mple .YFluid 1~ L. ~Pore size RPM Feed F~dFiltrat~/ Expt. Flux
(micron~ T~mp Press Feed time (Llhrlm2)
(C) (Bar) ratio(mins.)
6 skim milk Ultipor N~ 0.20 5000 50 2.0 0.97 130 330
15 7 skim milk Ultipor N~ 0.30 5000 50 1.6 0.97 60 775
8 skim milk Ultipor N~ 0.45 5000 50 1.5 0.97 40 1080
9 skim milk Ultipor N~ 0.65 5000 50 1.3 0.97 30 1680
EXAMPLE TEN
A 0.2 micron Posidyne~ membrane with a positive
surface charge was used for this example. The membrane
used has its pore surfaces populated by quaternary
ammonium groups and has a high absorptive capacity for
biological material.
A feed rate of 260 ml/min was used to obtain a
steady state flux of 360 L/hour/m2. The filtrate flux was
in the same magnitude as obtained with uncharged membrane
as described in example six. The filtration continued
for about 120 minutes, at which time there was no more
milk in the process vessel, without apparent decay in
filtrate flux rates. A filtrate to feed ratio of above
97% was maintained throughout the experiment. Other
experimental conditions are given in table 3.
It was expected that a large amount of proteins from
milk would bind to the membrane surface ultimately
2095057
34
plugging it. This example showed that under a dynamic
mode a membrane that normally shows protein affinity
performed well.
EXAMPLE ELEVEN
A feed rate of 740 ml/min of whole milk was used,
and a stable filtrate flux of 1130 L/hour/m2 was obtained.
Other experimental conditions are given in Table 3. The
filtration continued for about 40 minutes, whereupon the
milk supply was exhausted and the experiment was
terminated.
This example shows that whole milk can be filtered
using the process of this invention. The observed
difference in filtrate fluxes between whole milk and skim
milk (as in example 9) appears primarily to be due to the
differences in their viscosities. The ratio of the
filtrate flux achieved for whole milk to skim milk is
approximately equal to the ratio of the viscosity of
whole milk to skim milk.
Tabie 3
Example # Fluid 1\1 ~IL.. ~a Pore size RPM Feed Feed Filtrate/ Expt. Flux
(micron) TempPress Feed time (L/hr/m2)
(C) (Bar) ratio(mins.)
skim milk POa;d~ 0.20 500050 2.0 0.97 120 360
11 whole milk Ultipor N~0 0.65 5000 50 1.4 0.93 40 1130
EXAMPLES TWELVE to SlX'l'h'hN
Examples for determining filtrate fluxes through
various bacterially retentive membranes were repeated
using the disc dynamic microfilter. The general
procedures for examples twelve through sixteen are
described below. The conditions described generally hold
for each example unless specifically stated otherwise.
1. The desired membrane filter element was
assembled in the disc DMF.
209~057
2. An integrity test as outlined in method F was
conducted. The membrane filter element was rejected
if it did not pass the test.
3. The equipment was sanitized according to
S method E.
4. Milk to be filtered was preheated by the
procedure outlined in method A.
5. Milk was homogenized according to method B.
6. The general startup procedure outlined in
method G2 was conducted.
7. The milk was transferred from the surge tank to
the disc DMF at a desired flow rate.
8. Appropriate measurements were made.
Typically the disc DMF was maintained at 3500 rpm,
corresponding to a calculated shear rate of about 200,000
sec~~. The feed temperature was 50~ C and the feed
pressure was maintained at about 0.2 bar. Milk was
pumped into the filter at the rate of 960 ml/min in order
to maintain a high crossflow velocity across the
membrane. The filtrate to feed ratio was adjusted
specially for the membrane pore size, feed temperature
and rotor rpm. The unfiltered portion of the feed was
recirculated into the process vessel. The flux reported
in the table below is the steady state flux obtained
through the membrane as filtrate, typically half an hour
after start of filtration.
EXAMPLE TWELVE
A 0.2 micron Ultipor N~ membrane was used for this
example. A steady state filtrate flux of 850 L/hour/m2
was obtained.
20950S7
EXAMPLE 'l'~lK'l'~N
A 0.45 micron Ultipor N~ membrane was used for this
example. A steady state flux of 1600 L/hourJ* was
obtained.
EXAMPLE FOURTEEN
A 0.45 micron Posidyne~ membrane was used for this
example. A steady state flux of 1600 L/hour/m2 was
obtained.
The data shown in table 4 summarizes examples 11-
13. The data shows that by using the filtration process
of this invention, stable filtrate fluxes can be obtained
using various grades of bacterially retentive membranes,
while using a disc DMF. The table shows that membranes
with smaller pores and hence, increased bacterial
retention (titer reduction) can be used in the present
invention at the expense of filtrate flux rates.
Tsble 4
Exsmple $ Fluid ~ ~1,. no Pore size RPMTemp Feed Filtrste/ Expt. Filtr~te
(micron) (C) Press Feed time Flux
(Bar) rstio (mins.) (Llhr/m2)
12 skim milk Ultipor N~ 0.20 3500S0 0.2 0.22 137 850
13 skim milk Ultipor N~",~ 0.45 350050 0.2 0.37 80 1600
14 skim milk Posidyno~ 0.45 3500 50 0.2 0.37 80 1600
EXAMPLE FIFTEEN
Skim milk at 18~ C and homogenized by method B was
pumped into a disc DMF equipped with a 0.45 micron
Ultipor N~ membrane. The filtration was performed at a
feed rate of 860 ml/min, at which a steady state filtrate
flux of about 860 L/hour/m2 through the membrane was
achieved. The filtered milk was measured at 25~ C.
Other conditions of this example are given in the table
5.
This example demonstrates that refrigerated skim
milk, at about 18~ C can be processed by the method of
209~0S7
37
the present invention through a bacterially retentive
membrane. The reduced filtrate fluxes at this reduced
temperature is believed to reflect the higher viscosity
of milk at this temperature, compared to higher
temperatures.
EXAMPLE SIXTEEN
A disc DMF was equipped with a 0.45 micron Ultipor
N~0 membrane. Whole milk was fed into the disc DMF at a
rate of 900 ml/min and a steady-state filtrate flux of
about 850 L/hour/m2 through the membrane was achieved.
The experiment was conducted without recycle of the
unfiltered portion of the feed stream.
This example demonstrates that whole milk can be
filtered by the method of the present invention, using a
disc DMF. Skim milk under essentially identical
conditions gave an approximate steady-state filtrate flux
of about 1600 L/hour/m2. The observed difference in
filtrate flows between skim and whole milk, correspond
approximately to the ratio differences in the fluid
viscosities.
EXAMPLE SEVENTEEN
Using the methods described previously a filtration
experiment was conducted on the disc DMF, while
maintaining a high filtrate to feed ratio. A 0.45 micron
Ultipor N~0 membrane was used in this experiment. Feed
of skim milk was maintained at 115 ml/min and a
rotational speed of 2100 rpm was used. A filtrate flux
of 460 L/hour/m2 was obtained.
20950~ 7
T~bl~ S
Example ~ Fluid 1~1 ,IL..<,,I.o Pore siz~ RPM T~mp Feed Filtrate/ Expt. Flltrate
(micron) (C)Press Feed time Flux
(Bar) ratio(mins.) (Llhr/m2)
skim milk Ultipor N~ 0.45 3500 18 0.2 0.23 130 860
16 whole milk Ultipor N~~ 0.45 3500 50 0.2 0.23 90 850
17 skim milk Ultipor Ne~~ 0.45 2100 50 0.5 0.92 50 460
EXAMPLE EI~l~N
To demonstrate extended operation, an experiment was
conducted with a large quantity (500 liters) of raw,
unpasteurized, skim milk. The milk was preheated to 50~
C by passing it through a plate heat exchanger. It was
then homogenized according to method B and then pumped
into a cylindrical DMF equipped with a 0.65 micron
membrane. Typically, the dynamic microfilter was
maintained at 5000 rpm for this example. The feed
pressure varied from 1.3-1.5 bar at a feed rate of about
1300 ml/min. The filtrate to feed ratio was maintained
at over 95%. A steady state filtrate flux of about 1680
L/hour/m2 was obtained. There was no decline in the flow
of filtered milk nor was there any increase in feed
pressure during the six hours of continuous operation
required to process the 500 liters.
This example shows that it is possible to use the
filtration process of this invention for extended periods
of time.
EXAMPLE NINETEEN
This example demonstrates the ability of the method
of the present invention to be used to filter milk, using
a disc DMF, for the purpose of recovering proteins from
the milk. Proteins in milk are generally in a size range
from about 0.02 to about 0.30 micron (D.G. Schmidt, P.
Walstra and W. Buchheim, Neth. Milk Dairy J. 27
(1973):128), making them susceptible to recovery in
accordance with the process of the present invention.
2095057
39
This is especially important for recovering biologically
significant proteins from transgenic animals, such as
transgenic cows, sheep, and the like that have been
genetically altered to stimulate the production of such
proteins, as in accordance with the techniques already
known in the art.
A disc DMF was used that was equipped with a 0.2
micron nylon filter. The filtration of the milk was
performed at a feed rate of 840 ml/min, at which a
steady-state permeate flux of about 850 L/hour/m2 through
the membrane was achieved, at a rotor speed of 3500 rpm,
with recycle of the retentate, the permeate being
discarded in the present method. The feed, permeate and
retentate were periodically sampled and then analyzed for
total protein in accordance with the Kjeldhal method. It
was determined that the protein content in the retentate
initially was the same as that in the feed, but increased
(4.9~ retentate, 3.1~ feed) with prolonged recirculation
of the retentate.
Use of a smaller pore size membrane should provide
even better concentration of protein into the concentrate
stream.
EXAMPLES TWENTY AND TWENTY-ONE
These examples were carried out to establish that no
fractionation of the components in the milk, takes place
during the filtration process of this invention. In
these examples, samples of the feed, filtrate and
concentrate were analyzed at various times during
filtration to establish protein concentrations by the
Kjedahl method and total solids by evaporation.
2095057
EXAMPLE TWENTY
Feed, filtrate and concentrate samples were taken at
various times during the trial described in example
eighteen and analyzed for total solids in each stream.
The data in table 6 shows that there is no significant
depletion of the total solids from the filtrate while
using a 0.65 micron membrane.
EXAMPLE TWENTY-ONE
Feed, filtrate and concentrate samples were acquired
at various times while executing example thirteen and
analyzed for total solids and proteins in each stream.
The data is shown in table 6. Once again there was no
significant depletion of solids or proteins from the
lS filtrate milk while using a 0.45 micron membrane.
Table 6
FiltrationMe ,-b,dne Proteins % Total Solids %
mode pore size
(micronsJ
Filtrate FeedConce"(-dl~ Filtrate FeedConcenlrale
Cylinder 0.65 - - - 9.14 9.179.35
Example 20
Disc 0.45 3.38 3.15 3.35 8.64 8.708.85
E~cample 21
EXPERIMENT TWENTY-TWO TO TWENTY-EIGHT
Experiments twenty-two through twenty-eight were
conducted to demonstrate the ability of the present
invention to remove bacteria from milk. The general
operating procedure was maintained the same as those for
the experiments in examples six through eighteen, except
that bacteria was added to the process stream by method
C. The bacteria E. coli, commonly found in milk was used
in these experiments for seeding, unless otherwise
stated. Samples of feed, filtrate and bacterial
concentrate were taken at various times during the
- 20950~7
41
filtration, using sterile techniques. These samples were
assayed for bacteria by using the procedure outlined in
method D and results are reported in table 7.
As shown in that table, the present invention is
capable of achieving dramatic reductions in the bacteria
content of milk. The high removal rate for E. coli is
directly translatable to a high removal of Bacillus
cereus bacteria, which cannot be completely removed using
conventional pasteurization. E. coli is known to have a
rod-like structure, with dimension of about 1.1 to 1.5 ~m
by 2 to 6 ~m, whereas Bacillus bacteria such as Bacillus
cereus have similar dimension, and also have rod-like
structures, with dimensions of about 1.0 to 1.2 ~m by 3
to 5 ~m. Thus, the ability to remove E. coli, as shown
in table 7, also means that the process is capable of
removing the very undesirable Bacillus cereus bacteria,
resulting in milk with a very long storage life, even at
room temperature.
EXAMPLE TWENTY-TW0, TWENTY-THREE and TWENTY-FOUR
Examples six, eight and nine were repeated except
that E. coli was introduced in the process stream by
method C. Samples of feed, filtrate and concentrate were
taken for bacterial analysis. The titer reduction data
is shown in the table 7.
EXAMPLE TWENTY-FIVE
Example thirteen was repeated, except that bacteria
was introduced into the feed stream by method C and the
bacterial concentrate was not recycled back into the
process vessel. A steady state milk flux of about 1600
L/hour/m2 was achieved. The microbiological data is shown
in table 7.
The filtered milk contained only very low levels of
7 to 10 bacteria per ml of milk, dramatically lower than
the feed levels of 106 per ml. The titer reduction in
20950S7
42
this case was greater than 105. As a comparison, during
conventional pasteurization of milk, titer reductions of
only about 1o2 to 103 are achieved.
EXAMPLE TWENTY-SIX
The experimental conditions and procedures of
example twelve were repeated in this experiment, except
that E. coli was added into the feed stream by method C
and the concentrate was not recycled into the process
vessel. A steady state milk flux of about 850 L/hour/m2
was achieved. Samples of feed, filtrate and concentrate
were taken for bacterial analysis. The data shown in the
table 7 shows a titer reduction of greater than 106.
Sterile milk was produced since no bacteria was detected
in the filtered milk.
This example demonstrates the ability of the method
of the present invention to essentially completely remove
bacteria from milk, using a disc DMF and an appropriately
chosen membrane. Thus sterile milk can be produced.
EXAMPLE TWENTY-SEVEN
Unpasteurized raw milk contains a wide variety of
organisms including coliforms like E. coli and pathogens
like listeria and campylobacteria, and Bacillus cereus
bacteria. In this example no external bacterial seeding
of the raw milk was done, rather the milk was tested for
the inherent or "native " bacteria.
Samples of feed, filtrate and concentrate were taken
for bacterial analysis, during the execution of
experiment eighteen and were analyzed for native bacteria
by method D.
Only 14 bacteria per ml were found in the filtrate.
The feed had 2500 bacteria per ml and the concentrate had
2x104 bacteria per ml. Further, no psychrophilic bacteria
were detected in the filtrate. Psychrophilic bacteria
2095057
43
are those that grow at cold temperatures and cause the
spoilage of refrigerated milk.
Table 7 summarizes experiments 22 through 27. The
data shows that in both the cylinder and disc modes
increased titer reduction is gained at the expense of
filtrate flux. The table also shows that by choosing the
correct membrane it is possible to get sterile milk
filtrate.
T~bb 7
0Ex~mpb Poro Sko Membr~noAv~. Fhx F~d Op~t~tb~ Fo~d Conc. 76 Bacteri~ rlter
J mkron t~po Um'/hrTemp ~CPro~uro B~r E ri .'- ' Remov~l Roduction Tp
22 0.2 cylb~ r UKipor No~ 330 50 2.0 10~ 99.9992% 105
~E. coli)
23 0.45 c~RrlderUKiporN~", 1080 50 1 5 loA 99 992% 4x105
~E. coli)
24 0.65 cylindorUKipor No~ 1680 50 1.3~E. coli) 99.96% 4x102
1525 0.45di c UKiporN~"s 1600 50 0.2 10~ 99.9995% 8x105 IE. coli)
26 0.2discUKiporN""~ 850 50 0.2 10~ 100.00% >10
~E. coli)
27 0.65 cylind~rUKiporN0" 1680 50 1.3(N~tivo) 2x10
EXAMPLE ~lw~N~l~Y-EIGHT
Apart from the bacteria (E. coli) tested for titer
reduction, there are pathogenic organisms in milk like
listeria, which are of real practical concern in the
dairy industry. These pathogens provide a more severe
challenge than the coliforms (E. coli) that have also
been tested in the dynamic filter. Tests were conducted
by the methods outlined in method D to see if the
membrane filter elements used would efficientIy remove
these pathogens. This test was conducted on an off-line
test rig and not on the dynamic filter.
The data shown in table 8 clearly shows that a 0.45
~m Ultipor N~ membrane with a specific bubble point
(ASTM F316-86) will provide absolute removal of listeria.
20950S7
44
Table 8: Titer reductions of pathogenic organism found in milk using Pall membranes
Membrane Bubble pointTotal Challenge Filtrate Titer
of Listeria bacteria/ml bacteria/mlReduction T,
0.45 llm, Ulitpor N66~ 24 psi 7.10 x107 0 >5x1o6
0.45~m, Ultipor N66~22 psi 6.6x107 1 4.7x106
50.65 Jlm, Ultipor N66~ 16.5 psi 7.10x107 7.5x10~ 6.8xlO2
EXAMPLE TWENTY-NINE
Filtered skim milk produced by the method of Example
Sixteen is collected in a sanitized container.
Commercially available cream, is heated to 65~C and
filtered through a 0.2 micron Ultipor N66~ filtration
cartridge, available from Pall Corporation, East Hills,
NY, with a minimum titer reduction of 106 with E-Coli
15 bacteria. The filtered cream is substantially depleted
of bacteria and is collected in a sanitized container.
The filtered skim milk and the filtered cream are
then mixed and homogenized to attain a 2% fat milk with
lowered bacterial content.
