Method and apparatus for determining the composition of fluids

A method and apparatus for analyzing a fluid to obtain the concentrations of N of its components, N being at least three. The method comprises the steps of bringing a sample of the fluid to a plurality of successive temperatures, measuring a combined total of at least N acoustical and electrical parameters of the sample at each temperature, and determining simultaneously from the measured acoustical and electrical parameters, using a system of N linear equations, the concentrations of the N components. The apparatus combines acoustic and electrical measuring elements in one unit.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for determining the content of fluid components, in particular of milk components, based on the generation of acoustical and electrical fields and the measurement of acoustical and electrical characteristics of the sample. More particularly the present invention can be used to determine the content of fat, protein, lactose, mineral salts and water and optionally additional components of milk, by the determination of the acoustical and electrical parameters of a milk sample at different temperatures.

There are many applications where it is desirable to determine various characteristics of a fluid, such as the concentration of material in a solution, suspension or emulsion. An example of such emulsion is milk. Milk contains globules of butterfat (2-6%), as well as 2-6% proteins (mostly casein), 4-5% milk sugar (lactose), and 0.5-1% mineral salts dispersed in an aqueous solution. The proteins, lactose and mineral salts, taken together, comprise the solid-not-fat (SNF) component. Water constitutes 80-90% of the whole milk [Bhatti, S. S. et al. Acustica, Vol. 62, p. 96-99, 1986]. The determination of the chemical components in milk and milk products is of considerable importance in the dairy industry, since the amount of such components, particularly the butterfat and protein contents thereof, is the usual basis for determining the products' price, food value and compliance with US state and federal laws and regulations.

Various attempts have been made to determine the content of milk components using different physical methods. U.S. Pat. No. 5,033,852 discloses an optical measuring device for measuring the fat content of milk. The optical measurement device comprises a light source, an optical condenser, a glass test tube provided for holding a set amount of the milk to be tested, a tubular housing for the milk test tube; a lens and a photosensitive element, and a digital display. The milk fat content is obtained by automatically measuring, using the photosensitive element, the diffusion depth of the light projected on a milk sample. The device provides quick, accurate and reliable measurement results. However the disclosed method is limited in its application to determining the fat content, and other milk component cannot be analyzed.

U.S. Pat. No. 5,983,709 describes a device and method for measuring, monitoring, and controlling the fat and protein (micellar associated protein) content of a milk sample. The invention is especially adaptable for the standardization of milk for cheese production. The device consists of a sampling inlet feeding a milk sample into a diluter. The diluter has the ability to select an appropriate diluent for dilution that renders the milk sample analyzable by a spectrophotometer. For fat determination, the milk sample is diluted with a detergent-chelating agent that breaks up micellar protein. Light absorbance readings on the spectrophotometer of chelator-detergent diluted milk samples are concentration-dependent only on the fat content. Dilution of the milk sample with water gives absorbance readings that are dependent on the concentration of fat plus micelle-associated proteins. The micelle-associated protein concentration can then be obtained by subtracting the fat concentration from the total. The method described involves many steps, protein and fat determination are carried out in different samples and the protein determination is indirect since it is based on the subtraction of the fat concentration of the sample from the fat-plus-protein concentration. Since the fat and fat-plus-protein contents are measured by light absorbance, only fat and micelle-associated proteins such as caseins can be determined, while other milk components such as salt and lactose cannot be analyzed.

U.S. Pat. No. 4,566,312 describes apparatus and processes for automatic determinations of fat contents of foods, such as diary products, e.g. milk, wherein automatic density and solids content determining apparatus are employed, together with a computer, to determine the fat content of food being tested. The density determining apparatus is preferably one that is electromagnetically excited to vibrate at its natural resonant frequency, so that the mass of the sample may be determined from the change in such frequency, in comparison with a control. The means for measuring the solids content include electromagnetic radiation (microwave energy), which is employed to drive off the volatile material (usually mostly water) in the sample, which is automatically weighed before and after such volatilization. The fat content is calculated from the data on the density and the solids content. By means of this invention the fat contents of diary products, as well as contents of certain comparable components of other materials can be rapidly and accurately determined from small samples of such materials, thereby facilitating rapid evaluations, production controls and standardization of such materials leading to an important saving of time and money. However the apparatus described is applicable only for fat determination, while other milk components cannot be analyzed.

U.S. Pat. No. 4,447,725 describes an improved electro-optical apparatus for measurement of fat, protein, lactose and solids in milk. The invention also discloses an improved method and apparatus for quantitatively measuring fat concentration in fat emulsions such as synthetic and natural dairy products, including specifically milk, which minimizes errors and inconsistencies due to variations in fat molecular weight or degree of lipolysis. The method is based on irradiation of a milk sample with energy in the infrared spectrum at a wavelength characteristic of different linkages, groups and bonds. The apparatus described cannot be used to measure milk samples (e.g. taken from a cow) on-line, and requires transferring samples to a laboratory, which lengthens the time of analysis.

It is widely recognized that there is a lack of suitable sensors for providing information about the physicochemical properties of foods, especially for the continuous monitoring of foods (milk, wine, juices and others) during processing. One of the major problems in developing analytical techniques for use in the food industry is the diversity and complexity (both compositional and structural) of liquid food products. Many traditional “wet-chemistry” techniques have limited application because they are destructive, time consuming, and labor intense. Consequently there has been a drive to develop new technologies, or to apply techniques currently used in other areas, for analysis of liquid food products.

Over the last decade there has been increasing interest in the use of ultrasound for characterizing food materials. In this technique, a high-frequency sound wave is propagated through the material being tested. Information about the properties of a material is then obtained by measuring the type and degree of interaction between the sound wave and the material. Ultrasound has major advantages over many other analytical methods because it is nondestructive, rapid, precise, relatively inexpensive, and can be applied to concentrated and optically opaque samples [McClements, D. J., Critical Reviews in Food Science and Nutrition, Vol. 37, p. 1-46, 1997].

As known in the art, e.g. as exemplified by the teachings of U.S. Pat. No. 3,040,562, the ultrasound velocity propagation in whole milk varies with the amount of fat present in the milk and with the sample temperature. U.S. Pat. No. 4,145,450 discloses a method and apparatus for controlling the butterfat content of a stream of milk on a continuous basis. The innovation of the method disclosed is the determination of sound velocity at two different temperatures, preferably 45° C. and 65° C., and the application of an equation system from which high concentrations of milk components can be determined.

U.S. Pat. No. 4,145,450 shows that the ultrasound velocity, U, in the milk is directly correlated with the percentage of butterfat (%F) and the percentage of solids-not-fat (%SNF) in the milk, with the relationship between U, %F and %SNF dependent upon the particular temperature of the milk. Consequently, by measuring ultrasound velocity at two temperatures, e.g. at 45° C. and 65° C., it is possible to evaluate the fat content in the milk. Thus, the following simultaneous linear equations could be solved:
U45=K1(%F)+K2(%SNF)+K3
U65=K4(%F)+K5(%SNF)+K6
where U45and U65are the ultrasound velocity at respectively 45° C. and 65° C., and K1-K6are parameters determined empirically by correlation with a separate analysis performed by known chemical methods, K1-K6hold substantially constant for butterfat levels ranging between 0 and 20%. For a wider range of fat content, second order equations are needed. The method described in U.S. Pat. No. 4,145,450 is limited in its application for analyzing the butterfat and solid-not-fat content of milk, while constituents of the solid-not-fat itself (i.e. protein, lactose and salts) and other milk components cannot be determined.

Since the quality and price of the milk product is determined according to the percent of fat, protein and somatic cells, previous methods that determine only %F and %SNF are not adequate.

There is thus a widely recognized need for, and it would be highly advantageous to have, an apparatus and method for the simultaneous, on-line determination, of the content of at least three components of a fluid, in particular milk, rapidly and accurately, using small samples of the fluid.

SUMMARY OF THE INVENTION

The present invention discloses an innovative method and apparatus for determining the content of fluid components, particularly milk or other liquid food products, based on the generation of acoustical and electrical fields in the sample.

Specifically, the present invention can be used to determine the content of fat, protein, sugar, salt, somatic cells, water, and optionally additional components by the determination of the acoustical and electrical parameters of the fluid sample at different temperatures.

More specifically, the present invention uses the combined measurement of acoustical parameters and their temperature derivatives (slopes) together with electrical conductivity of a fluid, for example milk, to obtain N independent parameters that can be used in a set of N equations, N being at least three, to evaluate at least three fluid component concentrations, which include, in the case of milk, SNF components. The combined use of acoustic and electrical measurements that yield the compositions of at least three fluid components is novel and non-obvious, and a key feature of the present invention.

According to the present invention, there is provided a method of analyzing a fluid, comprising the steps of: a. bringing a sample of the fluid to a plurality of successive temperatures; b. measuring N physical parameters of the sample, N being at least three, at each temperature; and c. determining simultaneously the concentrations of N components of the fluid, N being at least three, using the physical parameters, thereby obtaining a quantitative composition of the fluid.

According to the present invention, there is provided an apparatus for the simultaneous determination of the concentrations of N components of a fluid, N being at least three, comprising: a. a measuring unit including at least one acoustical and at least one electrical measurement cells; b. means for introducing a sample of the fluid into the measuring unit; c. thermostating means to bring the fluid sample to a series of temperatures; d. means for measuring acoustical and electrical parameters of the fluid sample; and e. data processing means for evaluating the concentrations of the N components from the measured acoustical and electrical parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses an innovative method and apparatus for determining the content of fluid components, particularly milk or other liquid food products, based on the generation of acoustical and electrical fields in the sample.

Specifically, the present invention can be used to determine the content of fat, protein, sugar, salt, somatic cells, water, and optionally additional components by the determination of the acoustical and the electrical parameters of the fluid sample at different temperatures.

More specifically, the present invention uses the combined measurement of acoustical parameters and their temperature derivatives (slopes) together with electrical conductivity of a fluid, for example milk, to obtain at least three independent parameters that can be used in a set of at least three linear equations, to evaluate at least three fluid component concentrations.

The principles and operation of the apparatus according to the present invention may be better understood with reference to the drawings and the accompanying description.

FIG. 1illustrates a preferred embodiment of the apparatus of the present invention. The apparatus includes a measuring chamber22used for measurements of acoustical parameters of a fluid sample, whose main elements are an acoustical cell10and an electrical cell24. Preferably, acoustical cell10is a cylindrical acoustical standing wave resonator, and electrical cell24is an electrical conductivity measurement tube with a metal ring electrode26. A particularly advantageous embodiment of cell10is similar to that employed in U.S. Pat. No. 5,533,402, which is incorporated by reference for all purposes set forth herein. U.S. Pat. No. 5,533,402 discloses a method and apparatus for analyzing liquids utilizing cylindrical acoustic standing waves, generally in the ultrasonic region. The invention facilitates measurements of acoustic parameters of a fluid such as sound velocity and attenuation. A central component of the invention is the cylindrical resonator used to contain fluid and generate cylindrical standing waves therein. The resonator may be a radially polarized cylindrical piezoelectric tube having self associated electrodes, or a non-piezoelectric tube acoustically coupled to a source of oscillation.

In the present invention, tubes10and24are most preferably placed in series, forming a single tube. Cell10includes a piezotransducer12having a transmitting piezotransducer12aand a receiving piezotransducer12b. Cell10is placed in a thermostating unit that includes a thermo sensor14attached to tube10, a thermal jacket16with a Peltier element18, and a heat sink20. The apparatus also includes an electronic unit28, and a driver32used for moving a piston34inside tube10for delivering and removing the sample of the measured liquid (i.e. supplying the sample for both acoustical and electrical measurements).

FIG. 2shows another embodiment of the apparatus of the present invention, which includes a second acoustical cell50with two piezotransducers, a transmitter52aand a receiver52b. Tube50is preferably placed in thermal jacket16symmetrically in relation to tube10. Tube50has an elastic plug56, a stopper58and a hermetic sealing cap60. Section A—A shows the apparatus in a different perspective.

FIG. 3shows a yet another, multichannel embodiment of the apparatus of the present invention. In this embodiment, resonator tubes10,50, and a third tube70are located in common in thermal jacket16and have identical design elements, namely: piezotransducers (transducers12a,52a, and72a, and receivers12b,52b, and72b; tubes10,54, and74; thermo sensors14,76and78; and pistons34,80, and82.

FIG. 4shows a detailed view of electronic unit28, which is used to control the apparatus. Unit23typically includes a phase detector100, one input of which is connected to receiver12b, and a second input of which is connected to the output of a voltage controlled controlled oscillator (VCO)102and to transducer12a. The output of phase detector100is connected to the input of an integrator104, which has its output connected to the input of oscillator102. In addition and optionally, unit28may include a pi-regulator106, which has an input108connected with thermosensor14, and has an output110connected to Peltier element18. Unit28may further include a conductivity-measuring unit112, connected to electrode26. A processor114, operationally connected to both the acoustic elements and the pi-regulator, is used to measure the operating frequency F of the standing wave of the acoustical resonator, and to provide temperature control.

FIG. 5shows another embodiment of electronic unit28that can be used in differential measurements. In this embodiment, a multiplexer120is used to control switching between piezotransducers12a,band52a,b.

FIG. 6shows a more detailed view of conductivity measuring unit112, which includes (in “b”) a clock signal generator150that is electrically connected to an input152of a voltage-current converter154. An output156of converter154is connected to electrode26(clamp Z in “a”) and to an amplitude detector158. The metal tube of acoustical cell10can serve as one electrode for the tube of electrical cell24, whereas the second circular electrode26of the electrical cell can be separated from the metal tube of acoustical cell10by a segment of non-conductive tube24.

FIG. 7shows an alternative embodiment of the conductivity-measuring unit112that utilizes two separate ring electrodes114and116, incorporated in tube24. Electrodes114and116are electrically connected to unit112by clamps Z and0respectively.

In a typical operating procedure, a sample of the fluid to be measured is delivered to tube10by moving piston34. The desired value of the temperature of the measured fluid is reached with the help of a thermostating unit with thermosensor14, pi-regulator106(FIGS.4,5) and Peltier element18. Ultrasonic standing waves are generated in the sample of the measured fluid with the help of the acoustical apparatus that includes acoustical resonator tube10, piezotransducers12a,b, phase detector100, integrator104and controlled oscillator102. The frequency F of the standing wave of the acoustic resonator is measured with the help of the processor114. A detailed description of the procedure by which one can obtain both ultrasound velocity and ultrasound attenuation from the measured frequency F is given for example in detail in U.S. Pat. No. 5,533,402. The temperature derivatives of these two ultrasound parameters can be obtained from either a continuous measurement (many data points) of ultrasound velocity and ultrasound attenuation as the temperature is changed in a given range, for example between 20 and 30° C., or just by using two values of each parameter at the end points of the temperature range. When the measurements of all parameters involved are done continuously as the temperature is changed, the procedure is referred to as “continuous scanning”. When the measurements are done after the temperature is changed from one value to another, then stabilized at that value, the temperature change is referred to as “step-wise”.

As mentioned above, the geometry of the acoustic resonator is preferably cylindrical, as described in U.S. Pat. No. 5,533,402. However, it may also include parallel piezotransducers or any other arrangement of transducers capable of generating standing acoustic waves in the fluid sample. Electrical conductivity measurements in the fluid are carried out simultaneously with the acoustic measurements. Methods of measuring electrical conductivity of fluids are well known in the art. In particular, the conductivity can be measured from the active component of the complex electric impedance of a fluid sample. However, the combination of acoustic and electrical measurements in a single apparatus, wherein part of the acoustical setup may serve for the electrical conductivity measurements, is novel and non-obvious.

While the method of the present invention describes in detail the combined use of at least two acoustical parameters and at least one electrical parameter (conductivity) to obtain the concentrations of at least three fluid components, it is obvious that the use of four such combined parameters (acoustical plus electrical) can yield concentrations of four components, the use of five combined parameters can yield five component concentrations, etc. While the present invention specifies four independent acoustical parameters and one electrical parameter to yield five component concentrations, additional parameters, for example the frequency dependence of acoustical velocity, acoustical attenuation, acoustical impedance and their temperature derivatives, are fully considered to be within the scope of the present invention. Thus, in general, the present invention uses the combined (in one apparatus) measurement of N physical parameters, N being at least three and including at least two acoustical parameters and at least one electrical parameter, to obtain the concentrations of N fluid components.

The multichannel apparatus embodied inFIG. 3provides simultaneous temperature control of several samples of the measured fluid, allowing shorter measurement times, since there is no need to spend time on changing successive temperatures and waiting for their equilibration.

Evaluation of Component Concentrations

The measurement data obtained by the apparatus described above are further processed to obtain required compositional parameters of the measured liquid sample, for example the concentration of components of milk. For example, for determining the five main components in a sample of milk (butterfat (F), protein (P), lactose (L), mineral salts (S), and water (W)) the following five simultaneous linear equations are solved by using for example a PC or similar computer system:
UT=K1(%F)+K2(%P)+K3(%L)+K4(%S)+K5(%W)
dU/dT=K6(%F)+K7(%P)+K8(%L)+K9(%S)+K10(%W)
αT=K11(%F)+K12(%P)+K113(%L)+K14(%S)+K15(W)
dα/dT=K16(%F)+K17(%P)+K18(%L)+K19(%S)+K20(%W)
C=K21(%F)+K22(%P)+K23(%L)+K24(%S)+K25(%W)

Where: UTis the sound velocity through the sample in the measuring cell when the sample is at a temperature T; dU/dT is the sound (ultrasonic) velocity slope (derivative) as a function of temperature; αTis the sound attenuation through the sample in the measuring cell when the sample is at T. dα/dT is the sound attenuation slope as a function of temperature; C is the electrical conductivity; and constants K1-K5, K6-K10, K11-K15, K16-K20and K21-K25are derived by empirical correlation with actual, separate analyses performed by known chemical methods. The set of five equations above is referred to herein as “data processing means”. Obviously, if N components have to be determined, N independent physical parameters need to be measured simultaneously. Thus, for 4 components (see Example below), the measurement of four parameters will suffice.

EXAMPLE

On-line Determination of %F and Percentages of SNF Components in Milk

The content of a milk sample taken from cow was determined on-line using the following procedure: The ultrasonic device was connected to the milking system. A milk sample was transmitted to the test cell using a piston. The milk sample was brought to 20° C. and 30° C. For each temperature ultrasound attenuation, ultrasound velocity, their temperature derivatives, as well as electrical conductivity were measured. The electronic signals were transmitted to a data processing unit, and the main milk components were computed. The milk components were also analyzed by IR-Fosselectric method (Foss Milko-scan-Milkotester MK III) and the results obtained by the two methods were compared. The results are summarized in Table I.

Table I: Milk sample analysis: comparison between the results obtained by the method of the present invention (NDT) and by the IR-Fosselectric method (“Ref”). Components analyzed: Fat, Protein, Lactose and Somatic Cells.

TABLE IMilk sample analysis: comparison between the results obtainedby the method of the present invention (NDT) and by the IR-Fosselectricmethod (“Ref”). Components analyzed: Fat, Protein, Lactose and Somatic Cells.SomaticSomaticFatFatdifferProteinProteindifferLactoseLactosedifferCellscells(NDT)(Ref)(%)(NDT)(Ref)(%)(NDT)(Ref)(%)(NDT)(Ref)3.823.850.783.273.231.244.84.663.001591084.534.621.954.114.130.484.995.163.29111943.082.973.702.782.80.714.844.693.2039172.892.952.033.443.491.4354.941.2186442.992.951.363.443.51.715.074.942.6368444.54.663.433.533.61.944.8553.00143623.253.363.273.153.191.254.884.910.61116873.843.752.402.972.950.684.854.693.41125914.274.42.953.53.62.784.874.972.011301274.214.322.553.343.442.914.8353.40146604.844.810.623.463.552.544.94.880.411203723.743.822.092.953.022.324.915.042.5899483.293.290.003.563.570.285.054.93.06751454.24.180.483.533.560.844.844.712.761481273.033.081.623.073.173.1555.020.40784114.14.181.913.633.681.364.884.831.041331753.042.982.012.992.912.754.594.521.5571010524.644.71.283.033.112.574.714.812.08101715.375.410.743.473.411.764.744.671.502032423.893.870.522.923.012.994.814.983.411391125.225.332.062.82.732.564.354.291.40330946R0.9960.9860.8940.847STD1.0410.9131.053DEV
The comparison shows that the results obtained with the method and apparatus of the present invention are accurate, and very close to results obtained with accepted chemical methods.