Method and flow system for spectrometry and a cuvette for the flow system

The present invention relates to an on-line method and a flow system as well as a cuvette for carrying out IR spectrometry for analysis of liquid food products, possibly containing dissolved gases, in a process line in a liquid food product processing plant, especially a dairy processing milk and milk products. A liquid sample is extracted from the process line to a measuring branch, the sample is thermostated and passed to a measurement cuvette. The IR-absorbance spectrum is measured, e.g. in the MID-IR or NIR-range. In order to obtain an on-line monitoring of the process line the liquid food sample is extracted directly from the process line into the measurement branch, in which the pressure is maintaining at least as high as in the adjacent process line. The high pressure ensures that dissolved air will stay dissolved in the liquid food. Before each new sample the measurement branch and cuvette are flushed by high flow rates with a part of the new sample to clean the cuvette. The measurement cuvette has strong windows, preferably diamond windows to stand a high pressure and high flow rates. The on-line system is arranged to carry out eg. 120 measurements per hour.

TECHNICAL FIELD
 The present invention relates to a method and a flow system for carrying
 out spectrometry for analysis of a liquid food product, possibly
 containing dissolved gases, in a process line in a liquid food product
 processing plant, especially milk and milk products in a dairy, and
 comprising the following steps: 1) providing a liquid food sample from the
 process line to a measuring branch, 2) thermostating the liquid food
 sample, 3) passing the thermostated liquid food sample to a sample
 cuvette, 4) measuring at least part of the absorbance spectrum of the
 liquid food sample in the sample cuvette. The invention also relates to a
 measurement cuvette for the flow system. The present invention is
 specifically intended for IR-measurements, e.g. MID-IR and/or
 NIR-measurements for a determination of the quantities of specified
 components in the liquid food product.
 BACKGROUND ART
 A presently used method includes providing a milk sample from the process
 plant in an open sample container or cup from which dissolved gases may
 escape, passing part of the degassed milk sample from the container
 through a measurement branch into a measurement cuvette, performing the
 test and passing the tested milk sample to a waste outlet.
 The presently used test instrument includes generally a flow system, an IR
 spectrophotometer, and a computer comprising a PC with hard-disc, floppy
 disc drive, monitor and keyboard.
 Instead of the above mentioned method it would be preferable to perform the
 test on-line and in-line in the process plant, and preferably in such a
 way that it also would be possible to let the tested milk sample be
 returned into the process line, to avoid the waste.
 U.S. Pat. No. 5,137,738 discloses a system and a method for controlling the
 butterfat content of milk. The entire product stream is monitored
 continuously by the use of optical density sensors. The preferred sensors
 have stainless steel bodies and housings med Pyrex windows and mount
 directly on the product output lines and operate at full flow and
 pressure. U.S. Pat. No. 4,144,804 discloses a photoelectric monitoring
 system for continuously measuring the butterfat content of a sample of
 homogenized milk as the latter flows continuously through the processing
 system under the pressure of the homogenizer in the system. The known
 on-line sensors for milk products do not apply spectrometric analysis of
 the content. On-line spectrum measurement for determining a property of a
 product is known, e.g. as described in U.S. Pat. No. 5,452,232 for a
 hydrocarbon product. GB-A-2 104 681 describes an apparatus for the
 continuous investigation of chemical reactions by infrared absorption by
 use of an IR spectrophotometer having a through flow cell through which
 flows a continuous sample stream branched off from the reaction container.
 U.S. Pat. No. 4,910,403 describes a flow cell utilized on-line in the
 analysis of molten polymer. The flow cell includes diamond windows for
 passing mid to far infrared radiation through the molten polymer flowing
 through the flow cell.
 Testing milk in-line in a process plant in a dairy presents several
 problems. One problem is due to dissolved air, which is normally present
 in the raw milk. If air bubbles are released and enter the measuring
 cuvette, the measurement result will obviously not show a correct analysis
 of the milk product itself. A further problem is that milk includes
 several components and specifically the fat globules can give reason to
 failures. According to the known art the temperature of the milk sample
 should be raised to about 35-42.degree. C., and preferably homogenized to
 make reliable, reproducible measurements possible in an IR cuvette.
 Obviously, the milk in the process plant is generally kept at a low
 temperature to avoid the milk to be spoiled by some unwanted reactions,
 such as growing bacteria's.
 A further problem is that thin layers of milk tend to adhere to the
 IR-windows of the cuvette. The measurements may be seriously deteriorated
 due to such milk coatings. Therefore, an IR cuvette needs regular thorough
 cleaning.
 An in-line and on-line system must be able to measure reliably and normally
 without needing any regular calibrations and adjusting. Spectrometric
 measurements require the utmost stability of the components of the optical
 system. It is very important that the cuvette is extremely stable and not
 liable to suffer from wear. In fact the cuvette is highly exposed to wear
 as the liquid product and rinsing and cleaning solutions pass through the
 cuvette under high pressure and flow rates.
 It has therefore until now not been possible to perform the desired testing
 procedures on milk in an in-line system. To the applicants best knowledge
 there does not on the market exist any reliable apparatus able to perform
 accurate and reliable in-line determinations of the quantities of the
 components in raw milk or in a milk product, e.g. fat, protein, lactose,
 urea and casein.
 SUMMARY OF THE INVENTION
 According to the invention the method as defined in the preamble comprises
 extracting the liquid food sample directly from the process line into the
 measuring branch, providing and maintaining a pressure that is at least as
 high as the pressure in the said process line, and before each measurement
 flushing the cuvette with part of the recently (latest) extracted liquid
 food sample, further having provided that the cuvette having windows of a
 pressure and wear resistant material, being especially resistant to
 mechanical and chemical influences of the kind appearing in dairies. The
 flushing is performed under a pressure of from 100-200 bars, preferably
 from 110-150 bars across the cuvette, so the flushing rate will be high
 enough to ensure a thorough removal of the old sample including a cleaning
 of the cuvette.
 The high pressure ensures that dissolved air will stay dissolved in the
 liquid food. The pressure and wear resistant materials, preferably diamond
 windows, allow the cuvette to stand high pressures and high flow rates.
 Other window materials might break or move causing the cuvette to widen
 and to thereby causing the IR transmission loss through the cuvette to
 raise, thereby influencing the measurement result. Accordingly the
 pressure in the measurement branch should preferably be kept high, at
 least as high as the pressure in the process line at the location on which
 the sample is extracted, and during measurements the pressure shall be
 kept constant in the cuvette. Preferably, the pressure in the measuring
 branch exceeds the pressure in the process plant to ensure that dissolved
 air stays dissolved in the liquid food.
 The method is specifically fitted for liquid food products such as raw milk
 or processed milk and other dairy products.
 In one embodiment the measuring branch forms a closed system together with
 the process line. This means that the measuring branch only receives
 liquids flowing in the process conduit to which the measuring branch is
 connected.
 Preferably, a regular (e.g. daily) cleaning of the measuring branch
 including the cuvette is performed when the dairy plant is subjected to
 the regular cleaning process and/or by flushing the branch with the same
 cleaning solutions used for cleaning the dairy plant. In an advantageous
 embodiment of the present invention at least one of the cleaning or
 rinsing liquids of the dairy may be used in the measuring branch for an
 adjustment, such as a standardization based on characteristics in the
 measured spectrum or spectra of the cleaning or rinsing liquids,
 especially characteristics originating from the appearance of ions
 belonging to the group comprising NO.sub.3 -ions and PO.sub.4.sup.3-. (By
 "standardization" is meant an adjustment of the instrument, (e.g.
 performed in the instrument software) made in order to make a plurality of
 spectrum measuring instruments performing in the same way so that copies
 of the same calibration software can be used on all the instruments and
 whereby all instruments will provide the same result when measuring the
 same sample.)
 Preferably, the spectrometry is performed in the IR spectral range, e.g. in
 the MID-IR and/or the NIR spectral range, these ranges being specifically
 favourable for the analysis of milk.
 In an advantageous method the liquid food sample is thermostated while it
 is maintained inside the cylinder of a single stroke pump whereby a
 separate preheater possibly can be dispensed with. A homogenizer may be
 included in the measuring branch. A thorough homogenization is preferred
 in order to obtain a representative sample inside the very thin cuvette.
 If the measurement system is located at a position in a dairy where the
 milk product passing the sample intake always is adequately homogenized, a
 further homogenizing can be deleted, and the measurement branch may be set
 up without homogenizer. As it will appear clearly from the detailed
 description the method according to the invention allow about 120 on-line
 IR spectrometry measurements per hour.
 The present invention further provides a flow system for extraction of a
 sample stream from a liquid food processing plant such as a dairy
 processing milk and milk products, and for carrying out the method
 according to claim 1. According to the invention the flow system is
 directly connected to the liquid food processing plant, the flow system
 including a measurement cuvette for spectrometric measurements for
 determination of the quantities of the known components in a liquid food
 product in a liquid food processing plant, and the flow system comprises
 pump means and back pressure valve means to maintain a predetermined
 pressure inside the cuvette, said pressure being at least as high as the
 pressure in the process plant, and the cuvette having windows of a
 material, which is resistant to pressure and wear and especially to
 mechanical and chemical influences of the kind appearing in a diary.
 Preferably, the measurement cuvette has diamond windows, as diamond is an
 extremely wear and pressure resistant material.
 Preferably the liquid food sample is extracted from the process line by a
 pump providing a high flow rate in the measuring branch, said flow rate
 inside the cuvette at least for a short period exceeding 5 m/s, preferably
 exceeding 20 m/s and more preferably reaching about 25-30 m/s, to flush
 the measuring branch and specifically the cuvette to avoid the building up
 of a coating of liquid food on the IR-windows. In a preferred embodiment
 the pump means is a single stroke pump. The single stroke pump is
 specifically advantageous in that the suction stroke can be slow in order
 to avoid cavitation when sucking in a new sample, still allowing a fast
 pump stroke in order to provide the high flow rate of the sample when the
 cuvette is flushed with a new sample in order to remove all rests or
 remnants from the former sample.
 The present invention further provides a measurement cuvette for a flow
 system according to claim 7, the cuvette comprising: a first and a second
 steel member enclosing an IR measurement chamber between two IR windows,
 the first steel member having boreholes for inlet and outlet of a liquid
 flow to and from the IR measurement chamber, one of the steel members
 having an opening for arrangement of an optical detector, the other member
 having an opening provided for an IR-light beam coming from an IR-source,
 each opening being sealingly dosed by a diamond disc, forming one of the
 windows, and the two steel members with diamond windows being tightly
 secured to each other by fastening means. Thereby a stable, wear resistant
 cuvette is provided. This cuvette is well fitted for a flow system and
 measurement system wherein regular calibrations and adjustments can be
 dispensed with. spacer_being tightly secured to each other by fastening
 means. Thereby a stable, wear resistant cuvette is provided. This cuvette
 is well fitted for a flow system and measurement system wherein regular
 calibrations and adjustments can be dispensed with. The spacer defines the
 correct distance (path length) between the windows. The spacer supports
 and stabilizes the windows, when the chamber is flushed and filled with
 the liquid food. Preferably the opening in the spacer defining the light
 penetrating part of the windows has a diameter of less than 2.5 mm and
 preferably about 2 mm. It is preferred to have small windows because this
 will reduce the deflection when the chamber is exposed to high pressures.
 In an advantageous embodiment the spacer has a wedge form providing a
 change in width/height of spacer (equal to light path in chamber) of about
 3-10 .mu.m, e.g. about 6 .mu.m across the 2 mm opening. Such wedge form
 will reduce or eliminate the tendency to the occurrence of internal
 reflections of the light beam inside the chamber.
 Utility of the Invention
 The improved method and the measurement cell therefore are intended for use
 in liquid food processing plants and more specifically dairies. In the
 following description the term milk includes raw milk and food products
 derived from that, and may include other kinds of liquid food products.
 Advantages Obtained by the Invention:
 The measurement takes place at normal process line pressure or even higher.
 Therefore, dissolved gases will not be released and accordingly, no air
 bubbles are produced. The measuring system is chemical resistant to all
 substances which stainless steel is resistant to. (This feature is an
 advantage during a regular cleaning of the food product processing
 system). The diamond windows ensure that the mechanical dimensions of the
 optical system are not subject to changes due to mechanical wear. The
 measuring system can operate in a process environment for a long time
 without any need for attended operators to perform zero adjustment,
 re-calibration, separate cleaning or other kind of maintenance operation
 on the flow system and IR measurement equipment. The measurement is an
 on-line measurement, always available to maintain an optimal production.
 Regularly, during normal operation, the flow system is "cleaned" by the
 samples themselves. Preferably, in a preferred embodiment, the normal
 process line cleaning procedure includes the measuring branch, i.e. the
 above mentioned flow system. Further, in the preferred embodiments, no
 "foreign" substances are introduced, i.e. no special cleaning or
 calibration agents are needed besides the cleaning and rinsing agents
 generally used in dairies. Therefore, a possible option comprises the
 return of the sample to the process line after the measurement.

DETAILED DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION
 A preferred embodiment of a system for carrying out the method according to
 the invention comprises the following major components: A flow system 100,
 an optical MID-IR spectrometry system 200 and a control system 300 as
 indicated in FIG. 1.
 The flow system may comprise the following components as shown in the three
 examples FIG. 1-3.
 Sample intake means, comprising a tube 20 and a pump and valve means, e.g.
 a piston pump 40 having at least one one-way valve 21, 22 at the pump
 inlet and at least one one-way valve 23, 45 at the pump outlet. The sample
 intake means 20 is connected to a section of a process conduit 10 from
 which the samples are taken through a filter 15 and through a detachable
 connection e.g. mini clamps 13 comprising two flange parts and a gasket.
 Preferably all such connections are made according to the hygienic
 standards for food processing plants. The process conduit 10 is part of a
 food processing plant such as a dairy, which is not shown.
 Thermostatizing means 30, preferably comprising: a preheater or cooler,
 e.g., a coiled steel tube embedded in or wound around an electrically
 heated copper cylinder, providing e.g. from 1 to 5 ml, preferably 1.5 ml
 of heated milk or a heated copper cylinder having an inner volume of about
 15 ml and assigned temperature sensoring means (not shown) connected to
 control means 300 for controlling the preheater or cooler. The heating
 means 30 are designed to heat e.g. 1.5 ml milk from 1.degree. C. to a
 temperature about 40.degree. C.-50.degree. C. in about 25 seconds.
 A high pressure pump 40, (e.g. a LPA/MSC50h-pump as used in a FOSS ELECTRIC
 MILKOSCAN 50 or a single stroke pump providing a whole sample volume--e.g.
 1.5 ml in one single stroke) provides the high pressure (e.g. about
 400-500 bar). Typically at least a pressure of 200 bars is needed for
 homogenizing. Further the pump yield 40 will ensure a high flow rate
 through the IR-cuvette during a flushing period, so that the cuvette is
 cleaned by means of the flow rate of the milk, making further cleaning
 unnecessary for a number of hours. During the flushing period the pressure
 across the cuvette may reach 100-200 bars. To avoid degassing in the
 measurement period, the pressure of the measuring branch is maintained at
 at least the same pressure as the pressure at the location on which the
 sample is extracted from the process plant Preferably the pressure in the
 measuring branch exceeds the pressure in the process plant During a
 measurement the pressure is maintained at a substantially constant level
 by the use of a back pressure valve 88 as explained later.
 In the embodiment shown in FIG. 1 an in-line filter 35 provides a filtered
 milk passing through the measurement branch comprising the cuvette 70.
 Optionally a valve 45 (FIG. 1) allows the milk to bypass the filter, the
 milk running directly towards waste 90. The high flow rate of milk along
 the inside of the filter 35 will provide a cleaning of the filter 35 when
 the valve 45 is open. To this end the valve 45 can be controlled by the
 control means 300. Preferably, the valve 45 also act as a safety valve
 which is set to open if the pressure exceeds e.g. 400 bars.
 A homogenizer 50 (e.g. a S4000 as used in FOSS ELECTRIC MILKOSCAN 4000). A
 thorough homogenization of the liquid food product is needed in order to
 obtain a representative sample (a sample containing all components in the
 liquid food product) inside the very thin cuvette (typically having a
 width of 37-50 .mu.m). A further reason for including homogenization is
 that the scattering of the infra red light passing through the cuvette
 depends on the particle size of the liquid sample. Accordingly a uniform
 homogenization is essential in order to have reproducible measurement
 conditions. The pressure drop across the homogenizer is about 200 bars. In
 the embodiments shown in FIG. 2 and 3 a safety valve also functioning as a
 contra valve 51 follows immediately after the homogenizer.
 A further preheater or cooler 60, e.g. a coiled tube preferably wound on
 the periphery of a temperature stabilised IR cuvette, having an electrical
 resistor soldered to a copper body thermostatizing the milk sample to a
 predetermined temperature, e.g. to about 40.degree. C. and preferably to
 50.degree. C. before entering the cuvette, and preferably comprising
 assigned temperature sensoring means connected to the control means 300
 for controlling the temperature of the preheater or cooler. These controls
 and assignments are illustrated by phantom lines in FIG. 1.
 An IR cuvette 70, comprising a milk flow path and an IR light path crossing
 the milk flow path. The IR cuvette is part of an IR spectro-photo-meter
 allowing the analysis and/or quantitative determination of specific
 components of the milk in the JR cuvette. Preferably the IR cuvette
 structure indudes a bypass milk flow path 83 (FIG. 9) for the auxiliary
 milk, which is unable to pass through the very thin measurement chamber in
 the IR cuvette; A presently preferred embodiment of the cuvette 70 is
 shown in FIG. 6-8 and described in detail later in this specification.
 A back pressure valve 88 maintains a predetermined pressure at the cuvette
 70 during the measurement, typically 12 bars, and at least as high as the
 pressure in the process conduit 10. This pressure will ensure that
 dissolved gases remain dissolved in the milk in order to avoid air bubbles
 in the cuvette. Further, in stationary periods (i.e. no flow) the back
 pressure valve will ensure that the pressure do not exceed 12 bars A
 substantially constant pressure in the cuvette is necessary to obtain
 reliable and reproducible spectrometric measurements.
 An outlet 90 for waste and means (not shown) for collecting or evacuating
 the waste. Optionally the sample may be returned to the milk processing
 plant. This possibility is available because no foreign means or agents
 have been added to the sample.
 In a second embodiment shown in FIG. 2 the flow system comprises: a
 fast-loop, i.e. an U-formed tube 12 having an inlet 14 and an outlet 16
 arranged inside a process conduit 10 which is part of a process plant such
 as a dairy, which is not shown in the drawings. The is fast-loop tube 12
 is small compared to the process conduit 10. Typically the inner diameter
 of the fast loop is about 10 mm in a process conduit 10 having an inner
 diameter of about 70 mm. The drawing does not show the true dimensions.
 The inlet 14 opens towards the direction of the process flow in the conduit
 10, in such a way that a fraction of the process flow is diverted through
 the fast-loop. At the outlet 16 the process flow will generate suction,
 forcing the diverted flow to return into the process conduit 10.
 A sample intake is provided by a tiny flexible tube 20, having an inlet
 opening with filter communicating with the fast-loop through a detachable
 connection e.g. so-called mini clamps 19 comprising two flange parts and a
 gasket Preferably all such connections are made according to the hygienic
 standards for food processing plants. Similar to the system in FIG. 1 a
 pump 40 transfers the sample to a preheater 30, followed by a homogeniser
 50, a further preheater 60, an IR-cuvette 70 and a back pressure valve 88.
 The sample may be returned to the fast-loop through a further tiny tube
 91, e.g. a plastic tube or hose. Alternatively it may be released as
 waste.
 In a third preferred embodiment shown in FIG. 3 the flow system comprises a
 fast loop 12, connected to a process conduit 10. An integrated unit
 comprising a single stroke pump 40, surrounded by a heater, e.g. a heating
 coil, forming the first preheater 30, and two one way valves 21, 23
 providing that the single stroke pump during a suction stroke takes in
 milk from the fast loop, and during the pressure stroke sends the milk
 directly into the homogeniser 50. The homogeniser 50 is at the outlet
 provided with a valve 51 acting as a safety valve or pressure release
 valve, in order to release an extraordinary high pressure which otherwise
 might destroy the cuvette. A tiny tube 52 carries the homogenised milk to
 a further preheater 60, the cuvette 70 and the back pressure valve 88 in
 the same way as in the other embodiments. In a modification of the
 embodiments shown in FIG. 1-3 the one way valve 23 is deleted as the one
 way function is inherently incorporated in the preferred homogenizer 50.
 The Optical System
 The optical system 200 for measuring the IR absorption, preferably the
 MID-IR absorption, can be chosen between several known MID-IR
 spectrometric systems, and realised in several ways. Preferably a scanning
 interferometer, i.e. a FT-IR instrument is used, e.g. an IR-unit as used
 in FOSS ELECTRIC MILKOSCAN 120. However, the optical system may instead
 include a filter wheel, comprising a plurality of IR filters appropriate
 for the desired measurements, e.g. as used in a FOSS ELECTRIC MILKOSCAN 50
 or MILKOSCAN 102-104 and as described in GB-B-2 028 498, EP 0 012 492 and
 EP 0 629 290.
 A simplified diagram of a suitable optical arrangement appears from FIG. 1
 and FIG. 4. The Box 120 is an IR-source and scanning interferometer, 140
 is a detector, and 160 is a computer. Scanning interferometers can be
 realised in several ways, cf., e.g. "Fourier Transform Infrared
 Spectrometry", Peter R. Griffiths and James A de Haseth, John Wiley &
 Sons, 1986, and shall not be the topic of this application. The
 calculations for the determination of the quantities of the components in
 the milk are performed in the computer 160, and they are also well known
 to people in the art, e.g. as described in the above reference.
 The IR Cuvette
 In the following the IR cuvette is described in detail. The IR-cuvette is
 designed for an optical path length of e.g. 37-50 .mu.m. The IR-cuvette
 must be made of strong materials, which are resistant to wear and pressure
 and resistant to mechanical and chemical influences, e.g. materials as
 used for the process plant itself. Preferably the IR cuvette is made from
 a stainless steel and diamond. Accordingly, the IR cuvette can be cleaned
 by the same liquid means which are used for the milk processing plant
 anyway in the dairy. Milk processing plants in dairies are regularly, e.g.
 daily/every 24 hours, flushed with several cleaning liquids, e.g. strong
 bases and/or acids.
 The diamond and steel materials ensure that the cuvette will not be
 subjected to any noticeable wear.
 The physical dimensions and properties of the IR light path should be kept
 constant, preferably for the whole lifetime of the cuvette. Only the milk
 sample to be tested is changing. This is extremely important to the
 accuracy and reproducibility of the IR measurements.
 An enlarged view of a first example of an embodiment of a cuvette is shown
 is FIG. 5A and FIG. 5B. An alternative embodiment of the IR cuvette is
 shown as a second example in FIGS. 6A, 6B, and 7. FIG. 8 is a schematic
 top view which applies to both examples. Therefore the sections applied in
 FIGS. 5A, 5B are shown on the schematic FIG. 8 as well as the section C--C
 applied in FIG. 7. In both embodiments the cuvette comprises two steel
 members 71, 72, e.g. two circular discs. The first steel member 71 has a
 recess for mounting a diamond window 73, and two stepped boreholes 74
 (flow channels) for the milk flow. The second steel member 72 has a recess
 for mounting a second diamond window 76. The two steel members 71, 72 are
 tightly secured to each other by fastening means, e.g. four recessed
 screws (not shown) in threaded bores 79.
 As shown in the embodiment of FIG. 5A, 5B one of the steel members 71 may
 have a recessed central area in which a protruding portion 69 of the other
 steel member 72 can intrude or visa versa.
 As shown in FIG. 5A the cuvette is preferably connected in the flow system
 by tiny tubes 78 which are inserted in each borehole 74. For the sake of
 clarity only one tube 78 is shown.
 In the embodiment of FIG. 5A, 5B the first steel member 71 has an upper
 stepped opening 77 provided for an IR light beam 121 from the optical
 system 200 in FIG. 4, and the second steel member 72 has a lower stepped
 opening 75 provided for a detector 140 in the optical system. An
 alternative arrangement is shown in the embodiment of FIGS. 6A, 6B and 7.
 In FIGS. 6A, 6B and 7 the first steel member 71 has the stepped opening 75
 provided for the detector 140 in the optical system, and the second steel
 member 72 has the stepped opening 77 provided for an IR light beam 121
 from the optical system.
 When designing the cuvette enclosing the very thin measurement chamber
 between the two diamond windows it was found appropriate to design the
 cuvette so that the milk sample is let in through a first bore hole 74,
 which is made from the outer periphery of the cuvette, and a second bore
 hole 84, which is made from that side of the steel member, which will abut
 the second steel member when they are assembled. Therefore, the two bore
 holes form a mutual angle of about 100.degree.-140.degree., preferably
 120.degree.-125.degree.. Further it may be an advantage to let the bore
 hole 74 be inclined to form an angle .alpha. (not shown in the drawings)
 with the interface between the two steel members 71, 72, where .alpha. is
 between 0.degree. and 30.degree., preferably between 5.degree. and
 15.degree., and most preferably about 10.degree.. The adjacent bore (84 in
 FIG. 9) forms an angle .beta. (not shown in the drawings) with the
 interface between the two steel members 71, 72, where .beta. is between
 about 40.degree. and 85.degree., preferably between 55.degree. and
 80.degree., and most preferably between 60.degree. and 75.degree..
 Preferably, a thin spacer 81 is used to support the peripheral rims of the
 diamond windows thereby ensuring the desired height (equal to the optical
 path length, e.g. 37-50 .mu.m) of the inner measurement chamber 80 of the
 cuvette, when the two steel members are assembled. The spacer 81 can be a
 thin circular disc having a circular opening forming the measurement
 chamber 80, two slots 89, connecting the chamber 80 to the thin bore holes
 84, and a pair of guide holes 87' for guide pins 87, used to help
 positioning the spacer 81 when the cuvette is assembled. The bore holes
 85, 86 appearing in the sectional view of FIG. 7 are used for mounting the
 cuvette on holding means (not shown). The item 82 is an O-ring ensuring
 that no milk can intrude in the interface between the two cuvette parts.
 In the preferred embodiment of the cuvette structure the bore holes 74 and
 openings 75, 77 are stepped, having stepwise or gradually decreasing
 diameters, in order to accomodate the inlet and outlet of liquid and IR
 light to the small measurement chamber 80 in the center of the cuvette.
 Even in the enlarged scale view of FIGS. 5A, 5B, 6A, 6B and 7 the
 measurement chamber 80 containing the milk sample to be measured is hardly
 visible. The measurement chamber can be, e.g. about 2-3 mm in diameter and
 about 30-50 .mu.m in width (equal to the optical path length). The actual
 dimensions in a cuvette in an apparatus according to the invention may
 depend on the milk or food product to be measured and the parameters
 wanted.
 FIG. 9 shows a section through the centre of the cuvette further enlarged
 in a large scale view, in order to show the inner details of a presently
 preferred embodiment. Milk flowing through the measuring branch enters the
 cuvette 70 through the tiny tube 78 at the bore 74 (FIG. 5A) passing the
 adjacent bore 84 (FIG. 5A and FIG. 9), flows through the slot 89 into the
 measurement chamber 80 and exits from the cuvette through the opposite
 slot 89 and the bores 84, 74. Inside the cuvette this flow is forced into
 a thin (e.g. 30-50 pm thick) and relatively wide (e.g. 0.5-2 mm) flow
 passing the thin circular measurement chamber 80 between the diamond
 windows 73, 76. An optional bypass flow path 83 may be provided along the
 periphery of the windows 73, 76. If the track 83 is provided, preferably
 it can be restricted and possibly obstructed in order to ensure the
 measurement chamber is flushed thoroughly with the liquid to be measured.
 FIG. 10 shows the spacer 81 and the adjacent track 83 and inlet and outlet
 bores 84. However in the presently preferred embodiment the cuvette has no
 bypass track 83.
 However, the use of diamond windows may cause a serious problem. Due to the
 dielectic properties of diamond strong internal reflections of the
 IR-light beam may occur inside the cuvette. Such reflections will strongly
 influence the obtained spectrometric measurements and should preferably be
 avoided. Therefore, in an advantageous embodiment the spacer has a wedge
 form, i.e. having slightly inclined surfaces, providing a difference in
 light path of from 3 to 10 .mu.m, preferably from 4 to 8 .mu.m and most
 preferred about 6 .mu.m, across the window opening for the IR light,
 thereby providing a light path varying e.g. from about 34 to 40 .mu.m
 across the 2 mm opening. By this feature internal reflections may be
 eliminated or at least reduced. For sake of simplicity the wedge form is
 not illustrated in the drawings.
 The Control System
 As it will be well known to people in the art such an optical system is
 delicate and has to be kept very stable, in temperature, as well as
 humidity and protected against vibrations. Therefore a control system 300,
 preferably including a computer, e.g. a PC with adequate software,
 monitors and adjusts the temperature at the cuvette. Preferably, the
 humidity of the optical system 200 is monitored, and connected to an
 visual or audial alarm function to inform attendant personal that the
 system need attention, e.g. purging by a suitable gas in order to remove
 any humidity, as water vapour in the optical path will cause a
 deterioration of the measurements. Typically means for removing humidity
 can be included in the optical system. Further, the control system is
 connected to the pump 40 and valves 23, 45 for control of their operation.
 The above mentioned monitoring and controlling connections are indicated
 by dotted and dashed lines 301-305 in FIG. 1. The control system is not
 shown in details. It may be realized in several known ways. Preferably,
 the temperature control means comprise a sensor located at a copper member
 enclosing the cuvette and a heat resistor able to heat the copper member
 in order to temperature stabilise the cuvette.
 The Best Way for Carrying out the method according to the invention.
 When the pump 40 is of a type using several pump strokes to provide a
 sample the method according to the invention is carried out as follows:
 During a first step of a few seconds duration, e.g. two seconds, the pump
 40 is activated by the control means 300. A milk sample is extracted from
 the process conduit 10 through the fast loop 12 towards the measuring
 system by the pump. During a second step the milk sample (eg. 1.5 ml milk)
 is temperature stabilised (heated or cooled) by the means 30. The milk
 sample stays in the preheater 30 for a period of time, e.g. 25 sec's to
 attain a predetermined temperature, e.g. 50.degree. C. which is suitable
 for the homogenizer 50.
 During the following third step the pump starts working again, providing a
 high pressure pushing the heated milk sample through the homogenizer, in
 which the fat globules of the milk are crunched. From the homogenizer the
 milk sample flows through the further preheater 60 located upstream the IR
 cuvette to ensure the sample has the correct temperature for the
 measurement in the IR cuvette 70.
 In the embodiment shown in FIG. 2 the pump is active for about two seconds
 and preferably less to displace about 1.5 ml through a thin steel tube
 preferably having a diameter of less than 1 mm.sup..o slashed. e.g. 0.7
 mm, connected to the inlet of the cuvette and through the cuvette, through
 a further thin steel tube connected to the outlet of the cuvette, to the
 back pressure valve 88 and out of the measuring branch, either to a waste
 outlet or returned to the fast loop. The major part of the 1.5 ml is
 flushed through the cuvette with a high flow rate. Assuming a spacer
 thickness of 50 .mu.m, the sectional area of the cuvette inlet i.e. the
 slot 89 is 0.050 mm.times.0.7 mm=0.035 mm.sup.2. When the 1.5 ml are
 forced through the cuvette inlet in a short period of preferably less then
 2 seconds, the flow rate will exceed, e.g. 45000 mm/2sec=22.5 m/sec. in
 the inlet 89 to the chamber 80. Thereby, the incoming jet will flush the
 chamber cleaning IL By adapting. e.g. rounding the transition from the
 slots 89 to the chamber 80 a laminar flow may be promoted. In the opposite
 way a sharp transition as shown in FIG. 10 may promote a tubulent flow,
 which is believed to have a cleaning effect on the cuvette windows. The
 steel tube leading to the cuvette has an inner area of about 0.4 mm.sup.2
 providing a flow rate of about 1.9 m/s.
 When the pump stops, a fraction of the new preheated sample is present in
 the cuvette. The pressure inside the cuvette will stay at a constant
 level, of about 10-20 bars preferably 12 bar, due to the back pressure
 valve 88. Preferably, the following step is about 25 sec's long. In this
 step the sample is analysed by the IR-spectrometer. In a further step a
 new sample is extracted from the fast loop. Preferably, the new sample is
 heated during the following about 25 sec's long step while a portion of
 the already heated (preceding) sample is measured in the cuvette. And so
 on. Accordingly, a new sample can be introduced approximately every 30
 seconds, corresponding to about 120 measurements per hour.
 By the embodiment shown in FIG.3 the pump 40 is a single stroke pump
 surrounded by heating means, e.g. a coil. The method is carried out as
 follows: In a first step of about two sec's a sample is sucked into the
 pump cylinder by one suction stroke of the piston. Then in a second step
 the sample is heated inside the pump cylinder for about 25 secs. In a
 third preferably short step of about 0.5-1.0 sec. duration the piston
 stroke is reversed, pushing the new sample through the homogenizer and
 partly through the cuvette and the back pressure valve. Thereby the old
 sample is flushed away either into a waste outlet 90 or returned into the
 fast loop and the process conduit If the third step is performed in 0.5
 sec. the flushing rate through the cuvette will be about 10-30 m/s with a
 displaced milk volume of about 1.5 ml. This flushing rate will ensure a
 thorough cleaning of the cuvette windows between each measurement. The
 advantage of the third embodiment is that the suction stroke may be slow
 (to avoid cavitation) and the reverse pumping stroke can be fast, thereby
 providing a high flushing rate and a better cleaning of the cuvette. Also
 in this embodiment a measurement rate of about 120 samples per hour is
 possible.
 Due to the very stable and wear resistant properties of the cuvette
 calibrations and zero adjustments will normally only be necessary in the
 very first upstart of the equipment. However a routine check of the
 performance of the equipment may be carried out as follows. The
 measurement apparatus can be checked either
 1) by application of a known calibration sample. The known sample is
 entered in the measurement branch by separating the flexible hose from the
 sample inlet at the fast loop and by dipping the flexible hose into a cup
 (110 in FIG.: 2-3) filled with the known sample. A measuring cycle is
 executed and the result is noted and compared to the known sample data;
 and/or
 2) by taking out a sample from the process stream at a point close to the
 measuring branch, e.g. at the fast-loop, possibly by separating the
 flexible hose from the pump 40, and collecting a sample in a cup, (110 in
 FIG. 2-3), bringing the sample to a reference instrument, e.g. a MilkoScan
 50, 120 or system 4000 from Foss Electric A/S, and measuring the sample on
 the reference instrument.
 3) by taking out a number (at least one) samples from the flow of already
 measured samples leaving the waste outlet 90 in FIG. 1, bringing the
 sample to a reference instrument, e.g. a MilkoScan 50, 120 or system 4000
 from Foss Electric A/S, and measuring the sample on the reference
 instrument. In order be able to compare the measured waste milk results to
 the test results from the cuvette the last mentioned procedure has to be
 carried out in a period of stable production, wherein the samples do not
 vary noticeably.
 In a presently preferred embodiment shown in FIG. 11 a pressure transducer
 95 and optionally a flow meter 94 is included, preferably between the
 cuvette 70 and the back pressure valve 88. The pressure transducer
 provides a monitoring of the flow system. It will reveal any leakage in
 the flow system during the measurement During the measurement stable
 conditions are essential to ensure reproducibility of the measurements. A
 further advantage is that by measuring the pressure increase (and
 optionally the flow rate) during flushing the pressure transducer (and
 optionally the flow meter) provides means for indicating whether the
 flushing of the cuvette has been sufficient.
 Further the preferred embodiment includes an optional arrangement for the
 intake of a pilot sample in order to monitor the performance of the
 instrument. A pipette 96 can be arranged to aspirate a pilot sample from a
 cup (which is not shown). Such a pilot sample has a known specific
 composition, providing a specific measurement result. In case the actually
 obtained result differs from the expected values, this indicates that the
 instrument needs adjustment
 To ensure a thorough cleaning of the pilot sample pipette the pipette 96 is
 arranged to be placed in a holder 97 through which it is included in the
 flow path from a rinsing/cleaning liquid reservoir 100 to the pump 40 when
 it is not in use for aspiration of a pilot sample.
 The provision of the reservoirs 99, 100 for rinsing/cleaning liquid and
 zero liquid provides for the possibility to arrange a fast cleaning of the
 cuvette 70 whenever needed. In a presently preferred embodiment a regular
 cleaning every two hours is recommended. Preferably the liquids are
 preheated in heaters 98 to the same temperature as the milk samples before
 entering the measuring branch and the cuvette. Thereby the cleaning can be
 carried out fast as the heating step (about 25 sec) in the single stroke
 pump 40 can be dispensed with. I advantageous way the sample pipette 96 is
 applied for introducing the rinsing/cleaning liquid from the
 rinsing/cleaning liquid reservoir 100 and/or a zero liquid from a zero
 liquid reservoir 99. In FIG. 11 the intake is shown through a valve 101
 (e.g. a 3/2 way valve) located after (downstream) the sample heater/cooler
 30, but the intake may also be located before (upstream) the
 heater/cooler. Two 2/2 way valves 102, 103 are provided adjacent to the
 heaters 98 for control of the supply of zero liquid and rinsing/cleaning
 liquid. Also shown in FIG. 11 is a linear actuator 104 and a motor 105 for
 driving the single stroke pump 40, 106 indicate an in-line monitoring of
 the temperature in the process line 10. All components in FIG. 11 having
 reference numbers used in the previous FIGS. 1-3 are similar to and act
 the same way as already described with reference to said Figures.
 In all the described embodiments of the flow system according to the
 invention the flow system, i.e. the measuring branch, is capable to
 withstand standard cleaning procedures for milk plants. Slow build up of
 deposits on the IR windows of the cuvette is avoided by exposing the
 cuvette to the standard cleaning procedures of the plant. This may be a
 so-called "CIP" ("Clean In Place") procedure that uses heated acid and
 base solutions During cleaning the typical flow rate will be below 10 m/s
 in the conduits of the milk plant but much higher in the cuvette In an
 specifically advantageous embodiment of the present invention a number of
 the cleaning or rinsing liquids of the dairy are used as a calibration
 liquid for an adjustment or calibration of the optical part and
 specifically for an adjustment of the calculation and calibration software
 used for the calculation of the amounts of the tested components. A
 spectrum measured on a liquid containing. e.g. NO.sub.3 -ions and/or
 PO.sub.4.sup.3- -ions, may be used for an adjustment or calibration of the
 frequency/wavelength-axe.
 As it appears from the foregoing pages a new reliable measurement method,
 flow system and cuvette has been provided, allowing almost continuous
 measurements on-line and in-line in diaries with the capacity of about 120
 measurements pr. hour. It is obvious to people in the art that the
 different embodiments of the invention as described in the specification
 and shown in the drawings may be modified in several ways within the scope
 of the invention and accordingly the scope of protection is defined by the
 patent claims.