AU618575B2 - Process and device for investigating and controlling changes of state of a liquid or gelatinized medium by differential thermal analysis - Google Patents

Description

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PCTI

ORG ANIS OPI DATE 11/08/89 APPLN. I D 29'404 89 DEMANDE INTERNATIONALE PUBLI A0JP DATE 07/09/89 PCT MUW R PCI/FR89/00022 (51) Classification internationale des brevets 4 1) r$m PURL;-ion ern iale: WO 89/ 06794 C N25/02, 27/18 Al 4) Died blic n n ationale: 27 juillet 1989 (27.07.89)1 (21) Numniro de la demnande internationale: PCT/FR89/00022 (22) Date de dip6t international: 24 janvier 1989 (24.01.89) Numniro de la deniande prioritaire: 88/00803 (32) Date de priorit6: (33) Pays de prioriti: 25janvier 1988 (25.01.88) 283, rue de Charenton, F-75012 Paris (FR).

(74) Mandataire: PHELIP, Bruno; Cabinet HarA Ph~Iip, 21, rue de la Rochefoucauld, F-75009 Paris (FR).

(81) Etats disignis: AT (brevet europC'n), AU, BE (brevet europ~en), CH (brevet europeeri), DE (brevet europ~en), DK, Fl, FR (brevet eurap~en), GB (brevet europ~en), IT (brevet europ~en), JP, LU (brevet europ~en), NL (brevet europ~en), SE (brevet europ~en), us.

Publiie A vec rapport de recherche internonionole.

A vant l'expiration du d~Iai pr~~w pour la modification des revendications, sera repub~ie si de tel/es mnodifications sont re~'ues.

(7 1) DWposant (pour tous les Etats d~sign~s sauf US): I NSTI- TUT NATIONAL DE LA RECHERCHE AGRO- INOMIQUE (INRA) [FR/FR]; 147, rue de I'Universit F-75007 Paris (FR).

(72) Inventeurs; et lnvenc:irs/D6pos~nts (US seulernent) NOEL, Yolande [FR/FR]; 11, a116 de Ia Fontinette, F-94370 Sucy-en- Brie BELLON, Jean-Luc [FR/FR]; 26, rue Julien-Perrin, F-92 160 Antony HERRY, Jean-Marie [FR/FR]; 94, Grande Rue, F-92290 Arpajon (FR).

CERF, Olivier [FR/FR]; 314, rue Saint-Jacques, F- 75005 Paris PAIN, Jean-Pierre (FR/FR]; 36, rue Campion, F-60880 Le Meux ANTONINI, G&rard [FR/FR]; (54) Title: PROCESS AND DEVICE FOR INVESTIGATING AND CONTROLLING CHANGES OF STATE OF A LIQUID OR GELATINIZED MEDIUM BY DIFFERENTIAL THERMAL ANALYSIS (54)Titre: PROCEDE D'ETUDE ET DE CONTROLE DES CHANGEMENTS D'ETAT D'UN MILIEU LIQUIDE OU GELIFIE PAR MEEURE DIFFERENTIELLE DES CARACTERISTIQUES THERMIQUES DUDIT MILIEU ET DISPOSITIF POUR LA MISE EN CEUVRE DE. CE PROCEDE (57) Abstract In the prooess disclosed, two probes, preferably with platium resistors, are used. Energy is supplied to the medium through one probe. The temperature of both probes is controlled in order to differentiate between the effects due to the change of state phenomenon in the strict sense and the effect due to a change of temperature. The signal is processed by a microcomputer. Examples concerning solutions of gelatin and cold-renneted milk are given.

(57) Abr~gi Proc6d d'&tude et de contr~le des changements d'6tat d'un milieu Ii, quide ou g6lifi6, par mesure diff6rentielle des caract6ristiques thermiques dudit milieu. On utilise deux sondes, de prf6rence A r~sistance de platine.

Par l'urie d'elle, on apporte de 1'6nergie au milieu. On contr~le Ia temp~rature ur les deux sondei, pour discriminer les effets d~is au ph6nom~ne de changement d'6tat strictosensu de ce qui est dO A une variation de temp6rature, Le traitement du signal est r~alis6 A l'aide d'un microordinateur. Des exemples concernant des solutions de g~latine et du lait empr6sur6 A froid scrnt donn~s.

I i l- i -Xfm PROCESS FOR INVESTIGATING AND CONTROLLING CHANGES OF STATE OF A LIQUID OR GELLED MEDIUM BY DIFFERENTIAL MEASUREMENT OF THE THERMAL CHARACTERISTICS OF SAID MEDIUM AND DEVICE FOR IMPLEMENTING THIS PROCESS The present invention relates to a process for investigating and controlling changes of state in a liquid or gelled medium, for example the gelling of a liquid fluid or the liquefaction of a gel, by differential measurement of the thermal characteristics of said medium.

The invention also focuses on a device for implementing this process. This invention applies more particularly to the food industries.

A process for tile application of the hot wire anemometry principle to follow milk coagulation has already been described in EP-A-0-144,443. This method makes it possible more generally to follow a liquid-gel transition under isothermal <ci conditions. In fact, energy is introduced into the initially liquid medium in the form of heat by means of a platinum wire, after which a measurement of the temperature of this wire is carried out. In a liquid medium, natural convection produces S 25 an equilibrium of the heat transfers between the wire and the product so that the wire temperature is constant, slightly Shigher than that of the surrounding medium. If the medium coagulates or gels, the change in the structure of this medium is accompanied by change in the thermal regime with a changeover from natural convection to conduction. This is reflected in an increase in the temperature of the platinum wire, which corresponds to a change in the heat transfer co-efficient in 2 the medium, since the heat conductivity of the latter remains constant. In the food industry, firstly, the isothermal state is never perfectly achieved and, secondly, several cases of gelling or coagulation are associated with a temperature change, for example the gelling of gelatin, of polysaccharides and more generally the manufacture of sauces and jams, as also is the coagulation on heating of milk to which rennet has been added when cold, etc. In the abovementioned cases the perturbations introduced by the temperature change do not allow the sensor proposed in the abovementioned patent to displa; the coagulation phenomenon.

An object of the present invention is to take into account the temperature changes in the medium which is capable of being coagulated or gelled in order to control and to investigate the changes of state of this medium.

Another object of the present invention is to employ these characteristics to obtain information relating solely to the coagulation phenomena in the strict meaning of the term.

The subject of the present invention is therefore a process for investigating and controlling the phenomena of galling of a liquid fluid or of liquefaction of a gel by differential measurement of the thermal chatacteristics of said medium, wherein the measurement of the temperature of the medium is carried out by means of a first probe delivering a first signal indicating the temperature of this medium, while contributing energy in the form of heat to said medium by means of a second probe which is sufficiently far away from the first probe not to perturb the latter through the tempera- '9

A

'1 3 ture changes, this second probe delivering a signal orresponding to its temperature, after which the signals emitted by the probes are processed, after aiipification and correction, in an electrical data processing unit, using the medium temperature measurement signal to corect the signal supplied by the second heat quantity probe, so as to make the data contained in the signal relate to the coagulation phenomena in the strict meaning of the term.

The invention also extends to a device for sensing the thermal characteristics of a liquid fluid in the course of gelling, more particularly intended for making use of the abovementioned process, this device being characterized by the coupling of a first probe ensuring the measurement of the temperature of the liquid fluid and of a second probe intended 15 to contribute energy in the form of heat to the medium, these two probes delivering signals which are transferred, after amplification and correction, to an electrical Cata processing unit.

According to the present invention, platinum resistance probes are advantageously employed.

Other advantages and characteristics of the present invention will become apparent from the forms and examples of embodiment which are described in what follows, with reference to the attached diagrams, in which Figure 1 is a diagrammatic view of the sensor device according to the present invention, Figure 2 shos the 8 (temperature probe) and HQP (1) (heat quantity probe) curves illustrating the gel- 1~

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4 ling kinetics of a solution of gelatin at a concentration of 1%, Figure 3 shows the 08 (temperature probe) and HQP (2) (heat quantity probe) curves illustrating the gelling kinetics of a solution of gelatin at a concentration of Figure 4 shows the 80 (temperature probe) and HQP (3) (heat quantity probe) curves illustrating the gelling kinetics of a solution of gelatin at a concentration of Figure 5 shows the 04 (temperature probe) and HQP (4) (heat quantity probe) curves illustrating the coagulation kinetics of milk to which rennet has been added when cold, Figure 6 shows the H, (Hori type probe), HQP (heat quantity pruob.) and 05 (temperature probe) curves illustrating the coagulation kinetics of other milk to which rennet has been added when cold.

The sensor device shown in Figure 1 comprises two 100-ohm 20 (at 0°C) platinum resistance pr-bes 1, 2 packaged in a glass ampule exhibiting, for example, a diameter of 2 mm for a length of 12 mm. The first probe 1, called a temperature probe, ensures the measurement of the temperature of the medium, while the second probe 2, called a heat quantity probe, contributes energy in the form of heat to the medium and delivers a signal which shows its own temperature. The dimensions of the heat quantity probe 2 are chosen as a function of the product being investigated in order to conform to i i

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n i i a quantity of heat supplied per unit surface area which is sufficiently low to endow the sensor with a quality of fineness or discreteness. Such probes can be readily obtained commercially.

A platinum probe packaged in this way provides a mobility and especially a higher sensitivity than the temperature sensors of the thermocouple type which are generally employed.

The compromise between fineness and sensitivity is produced by packaging of the heatirT platinum probe described above.

Heating of the platinum wire of the heat quantity probe 2 is ensured by a Joule effect from a calibrated source of current G 2 of a constant intensity, for example of 35 mA, which gives a power of 0.2 watts for a flux density equal to 1600 W/m 2 This current muut be sufficiently stable and, moreover, independent of any resistance change to allow this resistance to be measured. Thus, the current sources employed G, and G 2 employing a high-gain amplifier and an FET-MOS transistor with excellent voltage insulation, ensure a current stability of 0.1 mA for a 10% change in the load resistance. The signal originating from the heat quantity probe 2 is then amplified in stage A 2 before being combined, in analog or digital fash- Sion, in a correcting stage L with the signal originating from the temperature probe i. The correctig stage L corrects the signal emitted by the probe 2 taking into account the signal emitted by the probe 1 to deliver information which indicates strictly the state of change of the medium.

The measurement of the Qemperature of the heat quantity Lprobe 2 corresponds' to the measurement of change in the re- 4"E A "P0 t

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11~- r 6 sistance of the platinum wire which is known from the change in the voltage at the terminals of this wire. Thus, in the case of a constant current I, this gives

V

2 RZ x I2, T being the temperature of the probe 2.

Before any change in the structure of the product is made, the heat quantity probe delivers a non-zero signal Vo, which is a function of the product temperature 0. In an isothermal state, when 0 is constant, Vo is a constant and only the difference

V

2 Vo is significant of change in the structure. Vo is then considered as an offset voltage which is written V,.

Nhen the change in structure of the product being investigated is a corollary of a temperature change, the offset voltage Vd is no longer constant but is a function of the product temperature 8. The temperature probe 1 which is supplied by the generator Gi with a current of 1 mA intensity delivers an analog voltage Vi proportional to the tempera- Sture 0. After suitable calibration, the offset voltage to be dedcuted from the signal supplied by the heat quantity probe 2 can be calculated at the L stage, after amplification in A 2 either by an analog method with an electronic adder, or by a digital method with a microcomputer or the like. For example, with a calibration in water between 10 and 80'C, using a signal gating chart endowed with an electronic adder, the offset voltage Vd can be related to V, by an equation of the type aX+b. The signal V delivered by the heat quantity probe 2, reduced by the offset voltage Vd is then amplified in the F stage.

v^4 The sensitivity of a platinum resistance probe is in the -C.Uw 2 7 region of 0.4 mV/°C. With I (G 2 35 mA, a sensitivity of 14 mV/°C is obtained. The sensitivity of a thermocouple is Av/c.

Thus, when compared with a sensor of the thermocouple type, the ratio of the sensitivities is 350 in favour of the platinum resistance probe, with the result that the circuit with the platinum resistance probe ensures, with a single amplification stage F with a gain of 100, a relatively noiseless and highly sensitive signal. In addition, a single lowpass filter with a Cutoff frequency below 1 Hz is sufficient.

The following examples of application are giv3n purely by way of illustration and do not in any way limit the scope of the present invention.

EXAMPLE 1 Gellinq The work is done on three solutions of gelatin.

A specified quantity of gelatin powder has been dissolved in distilled water and then heated in a mnicrowave oven until completely dissolved. The solution obtained is divided into two beakers, each of 50 ml. The latter are placed in a water bath, the temperature probe is immersed in the first beaker, the heat quantity probe in a second beaker. In this way the heat quantity probe does not perturb the measurement of the temperature of the medium. The signals delivered by the two probes are recorded continuously. Figures 2, 3 and 4 show the results obtained with solutions of gelatin at three different concentrations, 5% and 10% respectively. These solutions are subjected to a temperature decrease from 65 0 C to followed by an increase by the same amount.

'1 i '41 8 While the temperature is decreasing, a slight drift of the signal delivered by the heat quanitiy probe is noted, a drift due to the use of an analog method for calculating Vd. It should be noted that the use of a digital method would allow this drift to be eliminated. Then, when the gelling temperature is reached, at g, g 2 g 3 respectively, the slope of the signal reverses. The magnitude of this slope and the gelling temperature are a function of the concentration of gelatin in the solution. Lastly, while the temperature rises, a large increase in the signal appears, which can be associated with the phenomenon of gel liquefaction, for example at 12 and 13 respectively in Figures 2, 3 and 4, this phenomenon being the reverse of gelling.

EXAMPLE 2 Coagulation of milk to which rennet has been added when cold It is known that enzymatic hydrolysis takes place norma.ly in milk to which rennet has been added when cold. On the other hand, formation of the protein network which constitutes the coagulation in the proper sense does not take place, the milk to which rennet has been added thus remainiig liquid. It is possible, however, to obtain the formation of the protein network by increasing the temperature after a certain time of keeping cold. These phenomena have made it possible to develop a cheesemaking technology process called the Stenne-Hutin process (1965).

The sensor device of the present invention makes it possible to follow the progress of the phenomena. In the example illustrated in Figure 5 the milk used is reconstituted 9 from a skimmed milk powder in a proportion of 100 g/liter, to which 1 mmol/kg of CaCld has been added, the pH of rennet addition being 6.63. The coagulating agent is then added in the form of a solution prepared from rennet powder, namely mg/kg of milk, that is 222 Ag/kg of active chymosin. A coagulation of this milk is normally obtained in 15 minutes at In the present case, the milk to which rennet has been added is kept at 10°C for approximately 1 hour, after which it is subjected to a temperature increase from 10 to 30°C by means of a water bath, which requires 8 minutes. Throughout the period when the milk to which rennet has been added is kept at 10°C the heat quantity probe shows no signal change.

When the temperature set point of the water bath is changed, the signal oscillates in a disordered manner around a mean value close to the preceding value, thus reflecting the natural convection phenomena. When the temperature approaches the signal of the heat quantity probe increases abruptly before reaching a plateau. It is then possible to associate the time of the visual detection of the formation of the network, that is, the cheesemaker's setting time substantially at the time of the point of inflection of the curve.

n Figure 6 shows the same type of kinetics by comparing the results obtained in the curves 05 and HQP 5 with the sensor device according to the invention and in the curve H obtained with a probe of the type described by Hori in EP-A- 0,144,443.

As soon as the temperature set point change begins, curve H -r---hows that the signal of the Hori-type probe increases abruptly. It should be noted that this change, whose total amplitude is greater than 20 volts, does not allow the coagulation time to be detected, in contrast to the signal of the heat quantity probe incorporated in the sensor device of the present invention.

Thus, the problem of controlling the change and the coagulation of foodstuff products is found to be solved according to the present invention by virtue of the coupling of the two probes incorporated in the sensor device which makes it possible, in particular, to supply an output signal indicating any change in a medium which is reflected in a change in the heat transfer coefficient and consequently in convection within this medium.

It is clear that the present invention is not limited in any way to the forms and embodiments described above, but that it includes all the modifications and alternative versions within the reach of a specialist. Similarly, the references to the drawings are given merely by way of explanation and without limiting the present invention in any way. Furthermore, it should be stated that the reference signs inserted after the technical characteristics referred to in the claims are solely intended tqj facilitate understanding of the latter and do not Slimit their scope in any way.

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Claims (11)

1. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. Process for investigating and controlling changes of state in a liquid or gelled gelled medium, for example the gelling of a liquid fluid or the liquefaction of a gel, by differential measurement of the thermal characteristics of said medium, comprising: measuring the temperature of the medium by means of a first probe ds.eiing a first signal indicating the temperature of the medium, contributing energy to the medium in the form of heat by means of a second probe which delivers a signal indicating the temperature of said second probe, said second probe being spaced sufficiently far from said first probe so as not to affect said first p, be, after amplification, combining the abovementioned signals in a correcting stage which corrects the signal delivered by the second probe taking into account the signal ,delivered by the first probe to deliver infor'iation indicating strictly the change of state of the medium. 9

2. Process as claimed in Claim 1, wherein the signal delivered by the correcting :stage is amplified. 9 9

3. Process as claimed in either of Claims 1 and 2, wherein the correction in the 9 correctng stage is carried out numerically by employing electronic calculation means. o

4. Process as claimed in either of Claims 1 and 2, wherein the correction in the correcting stage is carried out digitally with the aid of a microcomputer.

Process as claimed in any one of Claims 1 to 4, wherein said first and second probes are 100-ohm platinum resistance probes.

6. Process as claimed in any one of the preceding Claims, wherein said first and second probes are fed with a low current.

7. Device for implementing the process as claimed in any on, of the preceding Claims, comprising a first probe for measuring the temperature of the medium and delivering a signal indicating said temperature, a second probe for contributing energy to ,Ll'i e medium in the form of heat and deliv'rlng a signal indicating its own temperature, i 12 means for amplifying the abovementioned signals and means for correcting the signal delivered by the second probe by means of the signal delivered by the first probe, said correcting means generating a signal in response to the signals delivered by said first and second probes.

8. Device as claimed in Claim 7, wherein it additionally comprises means for amplifying the signal generated by said correcting means.

9. Device as claimed in either of Claims 7 and 8, wherein said first and second probes are platinum resistance probes.

Process for investigating and controlling changes of state in a liquid or gelled medium, substantially as hereinbefore described with reference to the accompanying drawings. $*44

11. Device for investigating and controlling changes of state in a liquid or gelled medium, substantially as hereinbefore described with reference to the accompanying drawings. 9 4 DATED this 1st day of October, 1991. INSTITUT NATIONAL DE LA RECHERCHE AGRONOMIQUE (INRA) r o WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA