EP0550607A1

EP0550607A1 - Method for detecting status changes of a liquid or gelled medium and sensor device for implementing such method

Info

Publication number: EP0550607A1
Application number: EP19910917570
Authority: EP
Inventors: Jean-Luc Bellon; Alain Dulac; Olivier Cerf
Original assignee: Institut National de la Recherche Agronomique INRA
Current assignee: Institut National de la Recherche Agronomique INRA
Priority date: 1990-09-28
Filing date: 1991-09-24
Publication date: 1993-07-14
Also published as: AU8647091A; FR2667400A1; WO1992006369A1; FR2667400B1

Abstract

L'invention est relative à une méthode de détection des changements d'état d'un milieu liquide ou gélifié selon laquelle on mesure le déphasage entre un courant sinusoïdal basse fréquence redressé alimentant une sonde de résistance placée dans le milieu et le signal de sortie de ladite sonde pour délivrer une information indicative du changement d'état du milieu. L'invention est également relative à un dispositif de détection pour la mise en oeuvre de cette méthode.The invention relates to a method of detecting changes in the state of a liquid or gelled medium according to which the phase difference between a rectified low frequency sinusoidal current supplying a resistance probe placed in the medium and the output signal of the medium is measured. said probe for delivering information indicative of the change in state of the medium. The invention also relates to a detection device for implementing this method.

Description

Method for detecting changes of state of a liquid or gelled medium and sensor device for implementing this method

The present invention relates to a method for detecting changes in the state of a liquid or gelled medium, for example the phenomena of gelling of a liquid fluid or of liquefaction of a gel. The invention also relates to a device for detecting changes in the state of a liquid fluid in the process of gelling or liquefaction. This invention is particularly applicable to the food industries.

We know that an objective of the dairy industry is to optimize the time of shelling, that is to say the moment when the coagulum, obtained from a mixture of milk and coagulating agent, is separated on the one hand, the curd and on the other hand, the whey. This optimization can be obtained in particular by characterizing the coagulating agents from the same milk (model milk).

Berridge method may be used (standard method OG March 20, 1981) that are placed in three different tubes an identical mixture of milk and coagulant, these tubes being arranged in a water bath at 30 ^° C and driven by a rotational movement of 3 rpm. We then observe, for each tube, the moment when flakes appear on the walls and we thus calculate an average time which is called visible flocculation time (Tfv).

This method is based on the visual acuity of the experimenter and manufacturers want an automatic device.

Thus, in document EP-A-01 4443, a process has been described for applying the principle of hot wire anemometry to monitor the coagulation of milk. This method allows more generally to follow a liquid-gel transition under isothermal conditions. In fact, energy is supplied to the initially liquid medium in the form of heat by means of a platinum wire which is supplied by a direct and constant current, after which the temperature of this wire is measured. In the liquid medium, natural convection leads to a balance of heat transfers between the wire and the product so that the temperature of the wire is stable, but higher than that of the liquid. If the medium coagulates or gels, the change in structure of this medium is accompanied by a change in thermal regime with passage from natural convection to conduction. This results in an increase in the temperature of the platinum wire, which corresponds to a modification of the heat transfer coefficient in the medium since the thermal conductivity of the latter remains constant.

The method and the device described in document EP-A-0144443 detect changes in the state of the medium, by measuring the variations in the temperature of the probe wire. This measurement is made from that of the voltage across the wire, this wire being supplied by a constant direct current. The moment when the change of state of the medium occurs is associated with the instant (Ti 9) corresponding to the inflection point of the temperature curve as a function of time. An experiment was carried out using the method of

Berridge described above, a probe similar to that described in document EP-0144443 being placed in one of the three tubes and having the same rotational movement as the tube so as to eliminate any relative movement between the probe and the tube. This probe being supplied by a direct current, the temperature response curve presents an inflection point. It has been noted that the moment (Ti θ) at which the inflection point intervenes corresponds to the visible flocculation time (Tfv) determined from the observation of the other two tubes.

However, we wish to simplify the device by no longer rotating either the probe or the tube. In this case, we see that the visible flocculation time (Tfv) is located before the instant (Ti &) associated with the inflection point of the temperature curve. It then appears necessary, in this case, to have other information making it possible to determine, in a more precise manner, the visible flocculation time, always by means of an automatic device. The method for detecting changes of state according to the invention uses the highlighting of exploitable information on the setting time from the measurement of the phase shift of the response of a probe supplied by a low frequency sinusoidal current. It appears in fact that the moment when the change of state of the medium takes place corresponds substantially to the instant (TiR) associated with the point of inflection of the phase shift or delay curve (R).

Thus, the invention relates to a method for detecting changes in the state of a liquid or gelled medium according to which the phase difference between a rectified low frequency sinusoidal current supplying a resistance probe placed in the medium and the output signal of the medium is measured. said probe for delivering information indicative of the change in state of the medium. The invention also relates to a device for detecting changes in the state of a liquid or gelled medium comprising a probe supplied, by means of a generator, by a rectified low frequency sinusoidal current and a measurement unit of the voltage connected on the one hand, to the terminals of the probe and on the other hand, to an electronic data processing unit.

According to the present invention, a platinum wire probe is advantageously used.

The tests carried out with a probe placed in a tube, the probe and the tube being fixed, made it possible to deduce that the visible flocculation time (Tfv) was after the instant associated with the point of inflection of the phase shift curve ( R).

It therefore appeared advantageous to associate the method for detecting changes of state according to the invention with the method described in document EP-A-0144443 for automatically determining two values lying on either side of the visible flocculation time (Tfv) normally detected by a human experimenter and thus obtaining a good approximation of its value.

The invention therefore also relates to a method for detecting changes in the state of a liquid or gelled medium according to which energy is supplied in the form of heat to the medium by means of a first probe, powered by a current continuous and constant, and which delivers a signal indicative of the temperature of this first probe, this signal providing first information indicative of the change in state of the medium, characterized in that the phase difference between a rectified low-frequency sinusoidal current is measured, supplying a second probe and the output signal of said second probe to deliver a second piece of information indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs. Thus, the invention also relates to a device for detecting changes in the state of a liquid or gelled medium comprising a first probe supplied, via a first generator, by a direct and constant current, and a first voltage measurement unit connected on the one hand, to the terminals of the first probe and on the other hand, to an electro-computer processing unit, characterized in that it further comprises a second probe supplied by intermediary of a second generator, by a rectified low frequency alternating current and a second voltage measurement unit connected on the one hand, to the terminals of said second probe and on the other hand, to said electro-computer processing unit .

It is advantageous to supply the first probe, not with a direct and constant current, but with a high frequency alternating current. This provides filtering discriminates the signal delivered by the first probe and ensures greater accuracy in temperature measurement. This further simplifies the operations performed by the processing unit. Thus, the invention also relates to a method of detecting changes in the state of a liquid or gelled medium according to which energy is supplied in the form of heat by means of a first probe which delivers a signal indicative of the temperature of this first probe, this signal providing first information indicative of the change in state of the medium, characterized in that said first probe is supplied by a high frequency alternating current and in that the phase difference between a sinusoidal current is measured rectified low frequency supplying a second probe and the output signal of said second probe to deliver a second piece of information indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs.

The invention also relates to a device for detecting changes in the state of a liquid or gelled medium comprising a first probe supplied with current via a first generator, and a first connected voltage measurement unit on the one hand, at the terminals of the first probe and on the other hand, at the terminals of an electronic data processing unit, characterized in that said first generator delivers a high frequency alternating current and in that said device comprises plus a second probe supplied, via a second generator, by a rectified low frequency alternating current and a second voltage measurement unit connected on the one hand, to the terminals of said second probe and on the other hand, to said electronic data processing unit.

However, in the field of food industries on the one hand, the isothermal condition is never perfectly achieved and on the other hand, several cases of gelling or coagulation are associated with a temperature variation, for example the gelling of gelatin, polysaccharides, and more generally the manufacture of sauces and jams, as well as the hot coagulation of cold renneted milk and the like . In the cases described above, the disturbances brought about by the temperature variation do not allow the device described in document EP-0144443 to demonstrate the coagulation phenomenon. This is why patent application FR-2626371 discloses a method of studying and controlling the phenomena of gelling of a liquid fluid or of liquefaction of a gel, the purpose of which is to take into account the variations in temperature of the medium susceptible to be coagulated or gelled and to use these characteristics to obtain information solely relating to coagulation phenomena in the strict sense.

According to this method, the temperature of the medium is measured by means of a first probe delivering a first signal, while providing said medium with energy in the form of heat by means of a second probe sufficiently distant from the first probe so as not to disturb the latter by variations in temperature, this second probe delivering a signal corresponding to its temperature, after which the signals emitted by the probes are processed after amplification and correction in an electronic data processing unit using the signal for measuring the temperature of the medium to correct the signal supplied by the second thermal quantity probe, so that the information contained in the signal relates to coagulation phenomena strictly speaking.

The device more particularly intended for the implementation of the above method is characterized by the association of a first probe ensuring the measurement of the temperature of the liquid fluid and a second probe intended to supply energy in the form of heat between. These two probes are supplied by DC and constant currents of low value and deliver signals which are transferred, after amplification and correction, to an electronic data processing unit. The invention therefore also relates to a method for detecting changes in the state of a liquid or gelled medium according to which:

the temperature of the medium is measured by means of a first probe, supplied with constant direct current, and delivering a first signal indicative of the temperature of this medium,

energy is supplied in the form of heat to the medium by means of a second probe, supplied by a direct and constant current and which delivers a second signal indicative of the temperature of this second probe,

- the two aforementioned signals are combined in a corrector stage which corrects the signal delivered by the second probe by taking into account the signal delivered by the first probe to deliver a first piece of information, strictly indicative of the change in state of the medium, characterized in that :

-measuring the phase shift between a rectified low frequency sinusoidal current, supplying a third probe and the output signal of said third probe, to deliver a third signal, - combining said first and third signals in a corrector stage to deliver second information strictly indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs. The invention therefore also relates to a device for detecting changes in the state of a liquid or gelled medium comprising:

- a first probe supplied, via a first generator, by a direct and constant current and a first unit for measuring the voltage connected to a on the one hand, at the terminals of the first probe and on the other hand, at an electronic data processing unit and

a second probe supplied, via a second generator, by a direct and constant current and a second voltage measurement unit connected on the one hand, to the terminals of the second probe and on the other hand, to said electronic data processing unit, characterized in that it further comprises a third probe supplied, by means of a third generator, by a rectified low frequency alternating current and a third unit for measuring the connected voltage d on the one hand, at the terminals of said third probe and on the other hand, at said electronic data processing unit, said processing unit respectively combining the signals coming from the second and third probes with that delivered by the first probe in a correcting stage , to determine the moment of change of state of the medium.

As indicated above, it is advantageous to supply the second probe, which delivers a signal indicative of its temperature, not by a constant direct current, but by a high frequency alternating current.

Thus, the invention also relates to a method for detecting changes in the state of a liquid or gelled medium according to which: - the temperature of the medium is measured by means of a first probe, supplied with constant direct current, and delivering a first signal indicative of the temperature of this medium,

energy is supplied in the form of heat to the medium by means of a second probe, which delivers a second signal indicative of the temperature of this second probe,

- the two aforementioned signals are combined in a corrector stage which corrects the signal delivered by the second probe, taking into account the signal delivered by the first probe to deliver first information, strictly indicative of the change of state of the medium, characterized in that said second probe is supplied by a high frequency alternating current and in that:

the phase shift between a rectified low frequency sinusoidal current, supplying a third probe and the output signal of said third probe, is measured to deliver a third signal,

- Said first and third signals are combined in a correction stage to deliver a second item of information strictly indicative of the change in state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs.

The invention also relates to a device for detecting changes in the state of a liquid or gelled medium comprising:

a first probe supplied, via a first generator, by a direct and constant current and a first voltage measurement unit connected on the one hand, to the terminals of the first probe and on the other hand, to an electronic data processing unit and a second probe supplied with current via a second generator and a second voltage measurement unit connected on the one hand, to the terminals of the second probe and on the other hand, to said electronic data processing unit, characterized in that said second generator delivers a high frequency alternating current and in that said device further comprises a third probe supplied, via a third generator, with alternating current .jse rectified frequency and a third voltage measurement unit connected on the one hand, to the terminals of said third probe and on the other hand, to said processing unit electro-computer, said processing unit, respectively, combining the signals from the second and third sensors with that delivered by the first probe, in a correction stage, to determine the moment of change of state of the medium.

Other advantages and characteristics of the present invention will appear on reading the nonlimiting exemplary embodiments described in the following, with reference to the appended drawings in which:

Figure 1 is a schematic view of a first detection device according to the present invention.

FIG. 2 represents, for a half-period, the curve (I) of an example of sinusoidal current supplying the probe according to the invention as well as the temperature curve (^), corresponding to the response signal of the probe.

FIG. 3 represents the curve (R) of the delay of the response signal of the probe supplied by a sinusoidal current of frequency 25 Hz as a function of time as well as the curve (& _max ) of maximum temperature as a function of time, corresponding to this response signal.

Figure 4 represents the curve (R) of the delay of the response signal of a probe fed by a sinusoidal current of frequency of 50 mHz as a function of time as well as the maximum temperature curve ⁽ - ^ ' _max ) and ⁿ function of the time, corresponding to this response signal.

FIG. 5 represents the curve (R) of delay of the response signal of a probe supplied by a sinusoidal current of frequency of 100 mHz as a function of time as well as the maximum temperature curve (^ " _max ) ^{as a} function of time, corresponding to this response signal.

Figure 6 shows a schematic view of a second detection device according to the invention. Figure 7 shows a schematic view of a third detection device according to the invention.

FIG. 8 represents a curve of evolution of the maximum temperature as a function of the dry extract for a frequency of 100 mHz. Figure 9 represents the evolution curve as a function of time of the decimal logarithm of the reduced temperature of a probe fed by a step current.

FIG. 10 represents a curve of evolution of the dry extract as a function of the temperature of a probe supplied with a direct and constant current.

The elements common to the different figures will be designated by the same references.

The sensor device shown in Figure 1 comprises a platinum resistance probe 1 of 100 ohms at 0 ° C and 138.5 ohms at 100 ° C, packaged in a glass bulb, for example having a length of 20 mm, and a 2 mm diameter, this probe having a heating length of 14 mm. The platinum lead of the probe is supplied with sinusoidal current rectified from a current source 2. The thermal probe is therefore supplied with a sinusoidal intensity of the form i (t) = 1 + I _maχ | sin (tlN (t) | with I _maχ = 50 mA (1)

This value of l _max corresponds to an effective intensity I ₀ of 35 mA. The probe supply current therefore comprises a succession of positive half-waves.

The response curves illustrated in Figures 2 to 5 were obtained using the products and materials which will be described. In a 10 ml stainless steel beaker with a diameter of 20 mm, 10 ml of a model milk were placed and

1 ml of a model enzymatic solution, diluted to a hundredth.

The model milk is reconstituted from a milk powder at the rate of 100 g / kg, supplemented with 10 ml of calcium chloride (stock solution of 0.1 mol / 1) and 1 ml of thiomersal (stock solution of 100 g / 1) and using a temperature of

45 ° to dilute the powder.

The model rennet is in the form of a solution prepared from powdered rennet, the composition of which is as follows: 4438 mg of active chy osine and 63 mg of bovine pepsin per kg. A 50 mg concentration of rennet powder for 1 kg of milk, i.e. 0.22 mg of active chymosin. The solution comprises 0.05 mol / l sodium acetate buffer (pH approximately 5.5-5.6) to which 0.1% bovine serum albumin is added. The final concentration is 5% (m / v) of rennet. A thermal probe is placed in the beaker and it is then supplied by a sinusoidal current corresponding to equation (1) above, with N = 50 mHz, and generated by generator 2. For half a period, this signal is illustrated by curve I of FIG. 2. The voltage across the terminals of the wire is measured by a measurement unit 3, at the output of which is connected an electro-computer processing unit 4. This delivers the response signal from the probe, illustrated by the curve of this same Figure 2. It can be seen that this response curve, temperature curve, has a maximum value (φ _maχ ) and a delay (R) with respect to to the sinusoidal supply current.

Using the same products and the same materials, three other tests were carried out by supplying the probe with a sinusoidal current corresponding to equation (1) above, the frequency N taking successively the values 25 mHz, 50 mHz and finally 100 mHz, taking, for each period, the maximum temperature (θ " _max ) of the probe and the delay (R) between the temperature of the probe and the sinusoidal supply current. Via unit 4 of computer processing, the curves shown in Figures 3, 4 and 5 were then obtained. Figures 4 and 5 show that, for frequencies equal to or greater than 50 mHz, the delay (R) between the temperature of the probe and the intensity does not change during coagulation.

Figure 3 shows that, for a frequency of 25 mHz, the delay signal increases suddenly before reaching a plateau. It is then possible to associate the moment when the medium changes state at the moment (TiR) of the inflection point of the delay curve of the response (R). With regard to the maximum temperature curves shown in Figures 3 to 5, the analysis of descriptive parameters of the evolution of & _maχ over time shows that these curves are identical regardless of the frequency.

We can thus refer to Table 1 which gives the mean values as well as the parameter variation coefficients , A, P _j û), characteristics of the temperature curves obtained for the three frequencies, T ^ being the time associated with the inflection point of the maximum temperature curve (Q- _max ), At the amplitude between the low level (liquid and high level (gel) of the maximum temperature curve (φ _max ) and Pi © being the slope at the point of inflection of the curve (6> - _max ).

TABLE 1

The values grouped in this table show that the inflection point is not influenced by the frequency and that the same is true for the amplitude. The high coefficient of variation obtained for the parameter Pi - is due to the excessive input time (between and 10 and 40 seconds depending on the frequencies), which influences the precision.

Thus, whatever the frequency used, the maximum temperature curve corresponds to the temperature curve which would be obtained with a probe supplied by a constant direct current.

Supplying a probe with a low frequency sinusoidal current provides information on the coagulation moment, which is associated with the moment (TiR) corresponding to the inflection point of the delay curve (R).

Figure 3 shows that the instant (TiR) linked to the inflection point of the delay curve (R) is before that (Ti ^) of the inflection point of the maximum temperature curve ( _max ) • This shows that the visible flocculation time (Tfv) is between the instant (TiR) associated with the point of inflection of the delay curve (R) and the instant (Ti σ) associated with the point of inflection of the curve of temperature.

The determination of the optimal frequency is subject to two constraints: having a relatively short input time and therefore a relatively high frequency as well as a relatively large phase shift to easily determine the inflection point of the delay curve (R) and therefore a relatively low frequency. The various measurements carried out show that this optimal frequency is between 10 and 50 mHz. It then appears interesting to associate the detection method which has just been described with that of document EP-01444443 which has just been recalled. This second method according to the invention will be described with reference to Figure 6.

FIG. 6 represents a second device for detecting changes of state according to the invention comprising a first probe 5, with platinum resistance, supplied by a direct and constant current, for example of 35 mA, via a first generator 6. The voltage at the terminals of this first probe 5 is measured by a measurement unit 7 the output of which is connected to a unit 4 of electronic data processing. This establishes the response curve of the probe supplied with direct current, temperature curve as a function of time and can determine the moment (Ti &) when the inflection point of the temperature curve occurs. This second device further comprises a second probe 1, with platinum resistance, supplied by a rectified sinusoidal current corresponding to equation (1), for example with I _max = 50 mA, via a second generator 2 The voltage at the terminals of this second probe 1 is measured, as in the first device according to the invention, by a measurement unit 3, the output of which is connected to the computer processing unit 4. As before, unit 4 establishes the curve (R) of the delay of the response signal from the second probe supplied with alternating current, as well as the curve (φ max- ¹ ° ^th maximum temperature, corresponding to this response signal. L unit 4 can then determine the instant (TiR) corresponding to the point of inflection of the delay curve. It can finally, according to the values of Ti Ç ^* st TiR, give an improved value of the visible flocculation time (Tfv ) and thus precise information on the moment when the change of state of the environment occurs.

The unit 4 can also determine, from the curve (& max- ¹ * ^th maximum temperature, a value of the instant corresponding to the inflection point of the curve φ- _max "As we saw previously, this information is not directly usable but can be used concurrently with that obtained by analyzing the temperature curve (Θ ~) of the first probe 5 supplied with direct current.

It is also possible to envisage supplying the first probe 5 with a high frequency alternating current. This current can for example be chosen so that the effective intensity is 35 A. This is very advantageous since a discriminatory filtering is thus obtained ensuring greater precision in the measurement of the temperature of the wire. This also makes it possible to simplify the processing of the signal delivered by this first probe 5, processing carried out by the unit 4. The device according to the corresponding invention is similar to the second device described with reference to Figure 6, the main difference concerns the first generator 6 which must be able to deliver high frequency alternating current.

A frequency higher than 200 mHz is preferably chosen. '

As explained above, it is sometimes necessary to overcome the disturbances brought about by the variation of the temperature of the medium, in particular when the isothermal condition is not achieved or in the case of gelling or coagulation associated with a variation in temperature. This is why the invention relates to a third method for detecting changes in state which will be described with reference to FIG. 7.

The third detection device represented in FIG. 7 comprises, in addition to the second detection device according to the invention, another probe 10 supplied, by means of another generator, by a constant direct current, for example of 1 my. This probe consists of a platinum resistance probe. The voltage across this probe is measured by a measurement unit 9, the output of which is connected to the computer processing unit 4.

Thus, unit 4 determines:

the response signal from the first probe 10 which is supplied by a direct current, this signal being indicative of the temperature of the medium,

- the response signal from the second probe 5, which is supplied by a direct current, this signal being indicative of the temperature of this probe, and - the phase shift signal between the rectified low frequency sinusoidal current, supplying a third probe and the signal output from said probe.

The unit combines these three signals in a correction stage, which corrects the response signals of the second and third probes 5 and 1, taking into account the signal delivered by the first probe to deliver an improved value of Tfv which is strictly indicative of the change of state of the environment.

As before, it is possible to envisage supplying the probe supplied with direct current and which delivers a signal indicative of the temperature of this probe, that is to say here the second probe 5, by a high frequency alternating current. This brings about the advantages which have been described in the context of the second device. The device according to the invention is again in this case, similar to the third device described with reference to Figure 7, the difference between these two devices concerns the generator 6 which must be able to deliver a high frequency alternating current. This frequency is preferably greater than 200 mHz.

The third device which has just been described with reference to FIG. 7, is of particular interest for the determination of the dry extract of a milk.

We know that the dry extract of milk is on average 12.5%. Dairy manufacturers wish to have an automatic device which allows them to precisely determine the dry extract of any milk to detect "abnormal" milks, that is to say whose extract is either less at the average value of 12.5% (wet milk), or higher than this (in the case of infected milk, for example). Cheese makers also wish to standardize milks so as to obtain a constant percentage of fat content dry extract of a given type of cheese. The determination of the dry milk extract then makes it possible to adjust it as a function of the desired final result in the cheese.

The device described with reference to FIG. 7 makes it possible to obtain four information on the dry extract of the milk tested by carrying out four different calibrations from skimmed milk by proceeding as follows. The probe 10 is supplied by a constant current of 1 mA and delivers a signal giving the temperature U _M of the medium.

The probe 1 is supplied with a low frequency alternating current. Unit 4 determines the delay curve (R) and the maximum temperature curve (© " _max ) and takes account of the signal - _M delivered by the probe 10 to deliver information strictly indicative of the composition of the medium. A calibration is performed between the dry extract and the maximum temperature Max 'corrected by the temperature of the medium θ ^* _M , for a determined frequency An example of calibration is shown in Figure 8 for a frequency of

100 mHz. Calibration can be performed for other frequencies.

It is also possible to carry out a calibration between the dry extract and the delay curve (R) for a low frequency.

The probe 5 is supplied with a direct current of a value of 1 mA during a given time interval, the current then suddenly passing to a value of 35 mA. The temperature of the probe is designated by &^" F when it is supplied with a current of 35 mA.

d F-

With reference to Figure 9, the evolution of the decimal logarithm of the reduced temperature ("R) shows four distinct parts.

The first portion of the curve corresponds to the first part of the test where the probe is supplied with a current of 1 mA. Q- is worth on average CT M _/ (y- R is very close to 1, and has a zero decimal logarithm. supplying the probe with an intensity of 35 A, (actual start of the probe), the heat flow supplied by the Joule effect (Rtl ² ) dissipates first in the probe (platinum wire and cylindrical body glass) causing it to heat up. This dissipation is due to the fact that the probe has a certain specific heat, it does not yet exchange with the surrounding medium. This phenomenon, which corresponds to the second portion of the curve, is less than 13 s. A milk-probe gradient then appears which has the effect of stimulating a conductive exchange between the latter two. The total flow is divided between the probe and the milk. As the temperature of the probe increases, the proportion of flow supplied to the milk increases, which has the effect of slowing down the temperature rise of the probe. This results in a sudden change in slope, which corresponds to the start of the third portion of the curve. Thus, the film of milk near the probe undergoes an increase in its temperature therefore a decrease in its density and it is subjected to an upward force. A free convection regime then appears. This is the end of the third portion of the curve. Finally, the temperature of the probe stabilizes around F and R oscillates around a low value. It is the fourth and last portion of the curve, where the exchange of heat by free convection is stationary. The slope of the third portion of the curve can be related to the conductivity of the surrounding medium. There is a correlation between thermal conductivity and dry extract. In this regard, we can refer to the article by AKTONINI et al. entitled "Probe for in situ measurement of thermal conductivity in concentrated solid / liquid suspensions" published in Revue Générale de Thermique n ° 279, March 1985, pp. 247 to 251. It then suffices to calibrate the values of the slopes with the dry extracts of the products.

When the probe 5 is supplied with a continuous and constant current of 35 mA in stationary phase, it is It is also possible to carry out a calibration between the temperature signal (Δθ ') delivered by the unit 4, which has been corrected by the signal φ * _M delivered by the probe 10, and the value of the dry extract. Figure 10 illustrates an example of calibration from a sample containing 0 to 300g / l of skim milk powder.

It is therefore possible, using the device described with reference to Figure 7, to obtain a value of the dry extract from four values obtained by calibrations carried out differently.

The reference signs inserted after the technical characteristics set out in the claims, have the sole purpose of facilitating the understanding of the latter and can in no way have the effect of limiting the invention to the particular embodiments which have just been described .

Claims

1. Method for detecting changes in the state of a liquid or gelled medium, according to which the phase difference between a rectified low frequency sinusoidal current supplying a resistance probe placed in the medium and the output signal of said probe is measured to deliver a information indicative of the change of state of the environment.

2. Detection method according to claim 1, characterized in that the frequency of the sinusoidal current supplying the probe (1) is between 10 and 50 mHz.

3. Device for detecting changes in the state of a liquid or gelled medium comprising a probe (1) supplied, by means of a generator (2), by a rectified low frequency sinusoidal current and a unit (3) for measuring the voltage connected on the one hand, to the terminals of the probe (1) and on the other hand, to a unit (4) of electronic data processing.

4. Detection device according to claim 3, characterized in that the probe (1) is a platinum resistance probe.

5. Detection device according to one of claims 3 or 4, characterized in that the frequency of the sinusoidal current generated by the generator (2) is between 10 and 50 mHz.

6. Method for detecting changes in the state of a liquid or gelled medium according to which energy is supplied in the form of heat by means of a first probe, supplied by a direct and constant current and which delivers an indicative signal of the temperature of this first probe, this signal providing first information indicative of the change in state of the medium, characterized in that the phase difference between a rectified low-frequency sinusoidal current supplying a second probe (1) and the signal of output of said second probe (1) to deliver a second information indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs.

7. Method for detecting changes of state of a liquid or gelled medium according to which energy is supplied in the form of heat by means of a first probe which delivers a signal indicative of the temperature of this first probe, this signal providing first information indicative of the change in state of the medium, characterized in that said first probe is supplied by a high frequency alternating current and in that the phase shift between a rectified low frequency sinusoidal current supplying a second probe is measured ( 1) and the output signal of the second probe (1) for delivering second information indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs.

8. Detection method according to claim 7, characterized in that the frequency of the sinusoidal current supplying the first probe (5) is greater than 200 mHz.

9. Detection method according to one of claims 6 to 8, characterized in that the frequency of the sinusoidal current supplying the second probe (1) is between 10 and 50 mHz.

10. Device for detecting changes in the state of a liquid or gelled medium comprising a first probe (5) supplied, via a first generator, by a direct and constant current and a first unit for measuring the voltage connected on the one hand, to the terminals of the first probe and on the other hand, to the terminals of an electronic data processing unit, characterized in that it further comprises a second probe (1) supplied by intermediary of a second generator (2), by a rectified low frequency alternating current and a second unit (3) of measurement of the voltage connected on the one hand, to the terminals of said second probe (1) and on the other hand, to said electro-computer processing unit (4).

11. Device for detecting changes in the state of a liquid or gelled medium comprising a first probe (5) supplied with current via a first generator, and a first voltage measurement unit connected to a on the one hand, at the terminals of the first probe and on the other hand, at the terminals of a computer processing unit, characterized in that said first generator (6) delivers a high frequency alternating current and in that said device further comprises a second probe (1) supplied, via a second generator (2), by a rectified low-frequency alternating current and a second unit (3) for measuring the voltage connected on the one hand, to the terminals of said second probe (1) and on the other hand, to said electro-computer processing unit (4).

12. Detection device according to claim 11, characterized in that the first generator (6) delivers a current of frequency greater than 200 mHz.

13. Detection device according to one of claims 10 to 12, characterized in that the two probes (1, 5) are platinum resistance probes.

14. Detection device according to one of claims 10 to 13, characterized in that said second generator (2) delivers a current of frequency between 10 and 50 mHz.

15. Method for detecting changes in the state of a liquid or gelled medium, according to which:

- energy is supplied in the form of heat to the medium by means of a second probe, powered by a current continuous and constant and which delivers a second signal indicative of the temperature of this second probe,

the phase shift between a rectified low frequency sinusoidal current, supplying a third probe (1) and the output signal of said probe, is measured to deliver a third signal,

16. Method for detecting changes in the state of a liquid or gelled medium, according to which:

- the two aforementioned signals are combined in a corrector stage which corrects the signal delivered by the second probe taking into account the signal delivered by the first probe to defrost a first piece of information, strictly indicative of the change in state of the medium, characterized in that said second probe (5) is supplied by a high frequency alternating current and in that:

- the phase shift between a rectified low frequency sinusoidal current is measured, supplying a third probe (1) and the output signal of said third probe, to deliver a third signal,

- Said first and third signals are combined in a corrective stage to deliver a second item of information strictly indicative of the change of state of the medium, these two pieces of information being used to determine the moment when the change of state of the medium occurs.

17. Detection method according to claim 16, characterized in that the frequency of the current supplied to said second probe (5) is greater than 200 mHz.

18. Detection method according to one of claims 15 to 17, characterized in that the frequency of the supply current of said third probe (1) is between 10 and 50 mHz.

19. Device for detecting changes in the state of a liquid or gelled medium comprising:

a first probe supplied, via a first generator, by a direct and constant current and a first voltage measurement unit connected on the one hand, to the terminals of the first probe and on the other hand, to an electronic data processing unit and

a second probe supplied, via a second generator, by a direct and constant current and a second voltage measurement unit connected on the one hand, to the terminals of the second probe and on the other hand, to said electronic data processing unit, characterized in that it further comprises a third probe (1) supplied, by means of a third generator (2), by a rectified low frequency alternating current and a third unit ( 3) for measuring the voltage connected on the one hand, to the terminals of said third probe (1) and on the other hand, to said electro-computer processing unit (4), said processing unit (4) respectively combining the signals from the second and third probes (5, 1) with that delivered by the first probe (10), in a stage corrector, to determine the moment of change of state of the medium.

20. Device for detecting changes in the state of a liquid or gelled medium comprising: - a first probe supplied, via a first generator, by a direct and constant current and a first voltage measurement unit connected on the one hand, to the terminals of the first probe and on the other hand, to an electronic data processing unit, and - a second probe supplied with current by means of a second generator and a second measurement unit of the voltage connected on the one hand, to the terminals of the second probe and on the other hand, to said electronic data processing unit, characterized in that said second generator (6) delivers a high frequency alternating current and in that said device further comprises a third probe (1) supplied, via a third generator (2), by a rectified low frequency alternating current and a third unit (3) for measuring the voltage connected on the one hand, to the terminals of said third probe (1) and on the other hand, to said electro¬ computer processing unit (4), said processing unit (4) respectively combining the signals from the second and third probe (5, 1) with that delivered by the first probe (10), in a corrector stage, to determine the moment of change of state of the medium.

21. Detection device according to claim 20, characterized in that the second generator (6) delivers a current of frequency greater than 200 mHz.

22. Device according to one of claims 19 to 21, characterized in that the probes (10, 5, 1) are platinum resistance probes.

23. Device according to one of claims 19 to 22, characterized in that the frequency of the alternating current delivered by the third generator (3) is between 10 and 50 mHz.

24. Application according to the method according to one of claims 15 to 18 and of the device according to one of claims 19 to 23 to the determination of the dry extract of a milk before renneting.