Method for measuring a gel-point temperature

Abstract: There are stationarily provided in a quantity of fluid to be measured a heat generating element and a fluid temperature measuring element, factors such a fluid temperature ϑ ∞ , a average temperature ϑw of heat generating element, a surface temperature ϑs of heat generating element, a temperature difference between the said average temperature ϑw and the fluid temperature ϑ ∞ , a temperature difference between the said surface temperature ϑs and the fluid temperature ϑ ∞ , and a heat transfer coefficient α at the surface of heat generating element are continuously measured while the quantity of gelled or molten fluid is being heated or cooled, respectively, and a significant change in these factors is detected to obtain a gel-­ point temperature in non-destructive fashion.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for measuring gel-point temperatures of gelled and/or molten fluid such as foodstuffs and high molecular compounds, according to the preamble of claim 1. Industrially, appropriate control of the gel-point temperature of aqueous foodstuffs during manufacturing process is very useful to achieve quality improvement of the respective final products. Methods of gel-point temperature measurement are known as follows: (1) The subjective method in which an iron ball is placed on a mass of gel contained within a tube, then the mass of gel is heated while visually observing the iron ball which begins to sink into the mass of gel. A temperature at which the iron ball thus begins to sink into the mass of gel is measured as a gel-point temperature. (2) The method to determine a process of gelation, e.g., for aqueous solution of starch or the like, in which a change in quantity of transmitted light as a function of a temperature change is measured and a change of physical properties is related to a change of the light quantity ( Japanese Disclosure Gazette No. 1979-121190 ). Of the well known methods as have been mentioned above, the method utilizing the iron ball is relatively low in the measurement accuracy, since, in accordance with this method, the moment at which the iron ball begins to sink is visually observed by a human operator; the method based on the quantity of transmitted light is limited in its application because the object to be measured must be transparent. These methods of prior art accordingly have the common weak point that is impossible with these methods to measure gel-point temperatures of certain substances. An object of this invention is to provide a method by which a gel-point temperature of substantially any substance is measured with a high accuracy and in an objective manner. The present invention therefore uses a method for measuring a gel-point temperature comprising steps of stationarily providing in a quantity of fluid to be measured a heat generating element and a fluid temperature measuring element, measuring a temperature Θ ∞ of said fluid, and measuring an average temperature Θ w of said heat generating element. This method is disclosed in EP-A 0 150 111, where it is mentioned that the use of said method may be incorporated into automated processes for the manufacture of gel-like foods (e.g. cheese, yogurt, jelly) in which changes of kinematic viscosity are important. Parameters such as coagulation rate and the conversion from a liquid to a semisolid state can be evaluated with the lapse of time. The task of the present invention is to define precise measuring steps based on the known proposal of EP-A 0 150 111. The invention is characterized by the steps of the characterizing part of claim 1. The method of this invention provides effects as follows: A. Gel-point temperature measurement is also possible even when the fluid to be measured is accompanied with no significant endothermic reaction and/ or is opaque. B. Gel-point temperature measurements are possible selectively at various temperature changing rates and it is possible to calculate a gel-point temperature at the temperature changing rate or zero(i.e., a reference value representing a gelation characteristic of this fluid) from the measurements obtained at various temperature changing rates. C. Instead of relying upon visual observation by a human operator, a gel-point temperature can be derived from a viscosity change of fluid objectively by the apparatus itself and, therefore, with a high accuracy. D. Both the off-line measurement by fluid sampling and the in-line measurement during the manufacturing process are possible. E. The method according to this invention is free from structural destruction of the gel which has been inevitably caused by the mechanical measuring method of prior art. Therefore, the gel-point temperature measurement has been difficult due to such structural destruction. F. Nearly any size of the heat generating element is possible, so that even a small sample can be measured. G. The measurement is possible even under conditions of high temperature and high pressure. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects of the invention will be seen by reference to the description taken in connection with the accompanying drawings, in which: Fig. 1 is a sectional view showing an embodiment of the heat generating element used for the method of the invention; Fig. 2 is a conceptual view of the measuring method according to the invention; Fig. 3 is a graphic diagram showing a relationship of a differential value between the surface temperature ϑ s of said heat generating element stationarily provided in 1% aqueous solution of gelatin which is being cooled and the fluid temperature ϑ ∞ with the average temperature ϑ f of said stagnant fluid layer; and Fig. 4 is a graphic diagram showing a relationship of the cooling rate with the gel-point temperature. DETAILED DESCRIPTION OF THE INVENTION As the first step of the method according to the present invention, there are stationarily provided in a mass of molten or gelled fluid a heat generating element 1 adapted to generate heat and at the same time to measure a temperature of itself and a fluid temperature measuring element. Then, electric resistance values of the respective elements are continuously measured by utilizing the well-known four-terminal method, and thereby factors such as said average temperature ϑ w of heat generating element, said surface temperature ϑ s of heat generating element, the temperature difference between said average temperature ϑ w and said fluid temperature ϑ ∞ , the temperature difference between said surface temperature ϑ s and said fluid temperature ϑ ∞ , and a heat transfer coefficient α at the surface of said heat generating element so that any significant change in these values may be detected to determine a gel-point temperature. It should be understood that, for gelled fluid, the measurement may be performed while the mass of said fluid is being heated and, for molten fluid the measurement may be performed while the mass of said fluid is being cooled. The average temperature ϑ w of heat generating element is obtained by measuring the electric resistance value Rw of the element contained within the heat generating element 1 and then calculating a following equation: and Rw = Vw/iw (2) where R₀ through R₂ represent temperature coefficients of electric resistance. The fluid temperature ϑ ∞ is also calculated from the electric resistance value of the element contained within the thermoresistor 1b. It should be noted here that said thermoresistor may be of the construction identical to that of the heat generating element 1. As disclosed by the inventors in Japanese Disclosure Gazette No. 1988-132149, it is known that the average temperature ϑ w of heat generating element can be used to express the surface temperature ϑ s of heat generating element by a following equation: ϑ s = ϑ ∞ + k₁( ϑ w -ϑ ∞ ) k 2 (3) where k₁ and k₂ represent specific constant values of the heat generating element. Accordingly, the surface temperature ϑ s of heat generating element can be calculated from the average temperature ϑ w of heat generating element and the fluid temperature ϑ ∞ . Thus, it is possible to determine or to calculate the average temperature ϑ w of heat generating element, the surface temperature ϑ s of heat generating element, the temperature difference between said average temperature ϑ w of heat generating element and said fluid temperature ϑ ∞ and the temperature difference between said surface temperature ϑ s of heat generating element and said fluid temperature ϑ ∞ . The heat transfer coefficient α is given by the following equation: α = Q/S( ϑ s -ϑ ∞ ) (4) where Q represents a heat flux from the heat generating element, S represents a surface area of the heat generating element and Q = Rw(iw)² (5) The average temperature ϑ w of heat generating element, the surface temperature ϑ s of heat generating element, the temperature difference between said average temperature ϑ w of heat generating element and the fluid temperature ϑ ∞ , the temperature difference between said surface temperature ϑ s of heat generating element and said fluid temperature ϑ ∞ , and the heat transfer coefficient α are index values reflecting the kinematic viscosity of the fluid as described in the above-mentioned Japanese Disclosure Gazette No. 1988-132149. In view of a fact that gelation or melting of fluid is accompanied with a significant change in the viscosity thereof, said index values may be determined while the fluid temperature ϑ ∞ is gradually changed, and a point at which said index values significantly change can be detected to determine a gel-point temperature. In order to achieve a further accurate measurement of the gel-point temperature, the average temperature ϑ f of the stagnant fluid layer can be used instead of the fluid temperature ϑ ∞ in the above-mentioned procedure. Concerning the calculation of ϑ f , Sparrow proposed the model of an imaginary layer of stagnant fluid through which heat is transfered only by conduction (Sparrow, E.M. and Gregg, J.L. 1956; Trans. Amer. Soc. Mech. Engrs. 78: 1823-1829). According to this model, the temperature distribution over said stagnant fluid layer is expressed by and ϑ f is given as an integrated average temperature of said temperature distribution by A relationship in the equation 7 between d f and δ are respectively expressed by following equations: d f = d+2δ (8) where δ represents a thickness of said stagnant fluid layer. Thus the average temperature ϑ f of the stagnant fluid layer is calculated from said diameter d of the heat generating element, the thermal conductivity λ of the stagnant fluid, the fluid temperature ϑ ∞ , the surface temperature ϑ s of heat generating element and the heat transfer coefficient α . For aqueous gel, λ is practically constant and can be assumed as 0.6W/m k. EXAMPLE 1 Now the experiment procedures conducted by the inventors and the result thereof will be described below as embodiments of the present invention. In this Example, a change in the average temperature ϑ f of the stagnant fluid layer with respect to the temperature difference between the surface temperature ϑ s of heat generating element and the fluid temperature ϑ ∞ was measured while a molten sample was being cooled, and a significant change was detected to determine a gel-point temperature. Before a progress of the experiment is described in detail, a particular heat generating element 1 which was used in this example will be explained in reference with Fig.1. This heat generating element 1 consists of an element 2 comprising platinum wire which is 5cm long, exhibits a resistance value of approximately 5 Ω at a temperature of 0°C and is contained within a piece of ceramic pipe, and a piece of stainless pipe 3 having an outer diameter of 2mm and a length of 10cm which contains said element 2 fixed therein. As far as this specific embodiment of the heat generating element 1 is concerned, k₁ and k₂ in the above-mentioned equation 3 were 0.521 and 0.941, respectively. As shown by Fig. 2, a container 4 was filled with 1% aqueous solution of gelatin and placed in a thermostat so as to maintain said solution at a temperature of approximately 50°C. There were uprightly provided in said aqueous solution a heat generating element 1a and a thermoresistor 1b having a construction identical to that of said heat generating element 1a and both of them were electrically connected by lead wires 9 to a constant DC (direct electric current) source 6, a digital volt meter 7 and a controller 8. Electric resistance values of 1a and 1b, respectively, were continuously measured utilizing the four-terminal method while said aqueous solution 5 was being cooled and thereby the average temperature ϑ w of heat generating element as well as the fluid temperature ϑ ∞ were continuously measured. Specifically, the heat generating element 1a was supplied with constant DC current ( 0.4 A in this experiment ) causing a self-heating thereof and simultaneously the average temperature ϑ w of heat generating element was derived from the resistance value Rw calculated on the basis of the voltage value Vw and the current value iw. At the same time, the thermoresistor 1b was supplied with feeble constant DC current ( 1mA in this experiment ) and the temperature of the thermoresistor 1b was derived from the resistance value calculated on the basis of the voltage value and the current value thereof as the temperature ϑ ∞ of the aqueous solution 5 surrounding said thermoresistor 1b. By the using these values of ϑ w and ϑ ∞ , the surface temperature ϑ s of heat generating element was calculated according to the equation 3. The average temperature ϑ f of the stagnant fluid layer around the heat generating element 1a was calculated utilizing the definition equation 7. The experiment as has been mentioned above was conducted while the sample solution was being cooled at a rate of 24°C/hr and it was found that, as shown by Fig. 3, the temperature difference ϑ s - ϑ ∞ abruptly changes at a point substantially corresponding to the stagnant fluid layer average temperature ϑ f of 12.4°C. Based on this result, a gel-point temperature of 12.4°C was obtained for the case in which said 1% aqueous solution of gelatin ( Sigma Chemical Co., U.S.A. No. G-2500 ) was treated at the cooling rate of 24°C/hr. EXAMPLE 2 Said gel-point temperature was measured at various cooling rates. As seen from Fig. 4, the lower the cooling rate is, the higher the gel-point temperature is. A reference gel-point temperature of approximately 27°C at the cooling rates zero was derived from extrapolation values based on a regression curve of gel-point temperature at the cooling rates of 24°C/hr, 11.2°C/hr and 0.5°C/hr.

Claims: A method for measuring a gel-point temperature comprising steps of stationarily providing in a quantity of fluid to be measured a heat generating element (1a) and a fluid temperature measuring element (1b); measuring a temperature Θ ∞ of said fluid; measuring an average temperature Θ w of said heat generating element; characterised by steps of determining a surface temperature Θ s of said heat generating element from said average temperature Θ w , specific constant values k₁ and k₂ of the heat generating element and said fluid temperature Θ ∞ ; determining an average temperature Θ f of an imaginary layer of stagnant fluid around the heat generating element from the diameter d of said heat generating element, the thermal conductivity λ of said fluid, said fluid temperature Θ ∞ , the surface temperature Θ s of said heat generating element and the heat transfer coefficient α at the surface of said heat generating elment; calculating temperature differences Θ s - Θ ∞ with a series of temperatures of the fluid to be measured; and detecting a significant change in said temperature difference Θ s - Θ ∞ so that the fluid temperature Θ ∞ or the average temperature Θ f of said stagnant fluid layer at the moment of such detection may be obtained as a gel-point temperature. A method according to claim 1, wherein a quantity of fluid around a fixed heat generating element is non-gelled fluid which is flowing or standing still and the measurement is performed during gelation of this fluid. A method according to claim 1, wherein a quantity of fluid around a fixed heat generating element is in a gelled state which is flowing or standing still and the measurement is performed during melting of this fluid. A method according to one of the preceding claims, wherein two or more cooling or heating rates are used to determine temperatures Θ ∞ and Θ s causing gelation or melting of the fluid and using an extrapolation value of gel-point temperature at the cooling or heating rate of zero for determining the specific gel-point temperature of this fluid.

Inventors: HORI TOMOSHIGE, ITOH KENSUKE, HORI, TOMOSHIGE, ITOH, KENSUKE
Applicants: SNOW BRAND MILK PROD CO LTD, SNOW BRAND MILK PRODUCTS CO., LTD.
Application year: 1989
Main classification: G01N25
IPC codes: G01N33, G01N11, G01N25