Abstract:
A heat-generating element and a fluid temperature measuring element are immersed in a molten or gelled fluid sample to measure continuously both temperature Σw of the said heat-generating element and fluid temperature Θ∞ during the cooling or heating of the fluid sample at one or several constant cooling or heating rates. An abrupt change in Θw-Θ∞ value is detected to obtain the practical gel-point temperature of a characteristic gel-point temperature, which is defined as the extrapolated gel-point temperature at a cooling or heating rate of zero.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for measuring gel-point temperatures of gelled and/or molten fluids such as foodstuffs and high molecular compounds. 
     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. 
     The gel-point temperature measurement of prior art includes, 
     (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 and a temperature at which the iron ball thus begins to sink into the mass of gel is measured as a gel-point temperature; and 
     (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 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 while 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 a common weak point such that it is impossible for these methods to measure gel-point temperatures on a variety of substances. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a method by which a gel-point temperature of substantially any substance is measured with high accuracy and in an objective manner. 
     Accordingly, the present invention resides in a method for measuring a gel-point temperature comprising steps of stationarily providing a heat generating element adapted a generate heat and at the same time to measure a temperature of itself and a fluid temperature measuring element within a quantity of fluid to be measured; continuously measuring factors such as an average temperature θw of the heat generating element, a surface temperature θs of said heat generating element, a temperature difference between the average temperatures θw of said heat generating element and a temperature θ.sub.∞ of the fluid, a temperature difference between the surface temperature θs of said heat generating element and the temperature θ.sub.∞ of the fluid, and a heat transfer coefficient α at the surface of said heat generating element while a temperature of the fluid to be measured is increased or decreased; and detecting an abrupt change in an average temperature θ f  of an imaginary layer of stagnant fluid formed around said heat generating element, said average temperature θ f  being calculated from the measurements set forth above, and thereby determining a gel-point temperature. 
     Such method of this invention provides affects as follow: 
     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 measurement is possible selectively at various temperature changing rates and it is possible to calculate a gel-point temperature at the temperature changing rate of 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 the fluid objectively by the apparatus itself and, therefore, with high accuracy; 
     D. Both the off-line measurement by fluid sampling and the on-line measurement during the manufacturing process are possible; 
     E. The method according to this invention is free from structural destruction of gel as has been inevitably caused by the mechanical measuring method of prior art and, therefore, applicable to a fluid for which the gel-point temperature measurement has been difficult due to such structural destruction; 
     F. Any size of the heat generating element is selective, so that even a small quantity of sample can be subjected to the measurement; and 
     G. The measurement is possible even under special conditions such as 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 θs of said heat generating element stationarily provided in 1% aqueous solution of gelatin which is being cooled and the fluid temperature θ.sub.∞ 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 is stationarily provided in a mass of molten or gelled fluid a heat generating element 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 θw of heat generating element, said surface temperature θs of heat generating element, the temperature difference between said average temperature θw and said fluid temperature θ.sub.∞, the temperature difference between said surface temperature θs and said fluid temperature θ.sub.∞, 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 a 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 θw 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 the following equation: ##EQU1## 
     
         and 
    
     
         Rw=Vw/iw                                                   (2) 
    
     where R0 through R2 represent temperature coefficients of electric resistance. 
     The fluid temperature θ.sub.∞ 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 a 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 θw of heat generating element can be used to express the surface temperature θs of heat generating element by the following equation: 
     
         θ.sub.s =θ.sub.∞ +k.sub.1 (θ.sub.w -θ.sub.∞)k.sub.2                              (3) 
    
     where k 1  and k 2  represent constants specific value to the heat generating element. 
     Accordingly, the surface temperature θs of heat generating element can be calculated from the average temperature θw of the heat generating element and the fluid temperature θ.sub.∞. 
     Thus, it is possible to determine or to calculate the average temperature θw of the heat generating element, the surface temperature θs of the heat generating element, the temperature difference between said average temperature θw of the heat generating element and said fluid temperature θ.sub.∞ and the temperature difference between said surface temperature θs of the heat generating element and said fluid temperature θ.sub.∞. 
     The heat transfer coefficient α is given by a following equation: 
     
         α=Q/S(θs-θ.sub.∞)                  (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).sup.2                                             (5) 
    
     The average temperature θw of the heat generating element, the surface temperature θs of the heat generating element, the temperature difference between said average temperature θw of the heat generating element and the fluid temperature θ.sub.∞, the temperature difference between said surface temperature θs of the heat generating element and said fluid temperature θ.sub.∞, 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 the 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.sub.∞ 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.sub.∞ 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 ##EQU2## and θf is given as an integrated average temperature of said temperature distribution by ##EQU3## 
     The relationship in the equation 7 between d f  and δ are respectively expressed by following equations: 
     
         df=d+2δ                                              (8) ##EQU4## where δ represents a thickness of said stagnant fluid layer. 
    
     Thus the average temperature θ f  of the stagnant fluid layer is claculated from said diameter d of the heat generating element, the thermal conductivity λ of the stagnant fluid, the fluid temperature θ.sub.∞, the surface temperature θs of the heat generating element and the heat transfer coefficient α. For an aqueous gel, λ  is practically constant and can be assumed as 0.6 w/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 θs of the heat generating element and the fluid temperature θ.sub.∞ 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 5 cm 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 2 mm and a length of 10 cm 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 thermostatically controlled environment so as to maintain said solution at a temperature of approximately 50° C. There was 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 resistors contained within 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 θw of heat generating element as well as the fluid temperature θ.sub.∞ were continuously measured. 
     Specifically, the heat generating element 1a was supplied with constant DC (0.4 A in this experiment) causing a self-heating thereof and simultaneously the average temperature θw of the heat generating element was derived from the resistance value Rw calculated on the basis of the voltage Vw value and the current value iw. At the same time, the thermoresistor 1b was supplied with feeble constant DC (1 mA 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.sub.∞ of the aqueous solution 5 surrounding said thermoresistor 1b. By the using these values of θw and θ.sub.∞, the surface temperature θs 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 θw-θ.sub.∞ 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 θ 9P  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 θ 9P  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. 
     While the invention has been particularly shown and described with reference to preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention.