JP2950647B2 - Heat treatment method for solid packaged food - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of heat-treating a solid packaged food, and more particularly to a method of heat-treating a food at a desired desired temperature.

[0002]

2. Description of the Related Art Heat treatment of foods is performed for various purposes such as heat sterilization, protein denaturation, separation of active ingredients, and cooling and freezing for suppressing various reactions. At this time, depending on the temperature and time, unwelcome changes in the food may occur at the same time as the heat treatment. Therefore, the temperature history must be controlled so that the desired temperature and the processing time required to achieve the respective targets can be achieved without any excess or shortage. However, when solid heat-dissipating food that must maintain a lump of geometric shape, such as a retort food, cannot be transformed into a shape convenient for processing, and heat capacity is distributed, When the heat treatment temperature reaches the target value during the heating process, the process shifts to the cooling process. In the initial stage, the temperature at the center of the food, where heat conduction is most delayed, is still rising due to inertia. For this reason, the final required processing temperature and time significantly exceed the heat treatment, resulting in excessive heat treatment, and conversely, if cooling is started early in anticipation of inertia, the temperature and time at the center of the food will not reach the target values, Temperature control is extremely difficult due to insufficient heat treatment.

[0003] Therefore, for example, when heat-sterilizing retort food, the reference temperature is set to 121.1 ° C by an output of a measuring instrument called a “F value (sterilization time at reference temperature) computer” (a sensor inserted in the center of the food). Although the F value is calculated at a thermal lethal curve gradient of 10 ° C.), the F value at the time of measurement is displayed, but the state in advance cannot be predicted. Further, since the F value changes exponentially with temperature, the heating process proceeds and the center temperature rises, and
As the value approaches the target value, a large change in the F value is observed in a short time. Therefore, it is very difficult to monitor the F value and operate the apparatus. Further, if the process shifts to the cooling step when the F value reaches the target value, the center temperature still continues to rise at the beginning of the cooling step, so that the final F
The value is in a state of over-sterilization far exceeding the target value. Therefore, the result of the heat sterilization work normally performed according to the time schedule determined by trial and error can only confirm how much the result was converted into the F value.

In view of the above, conventionally, the temperature profile has been theoretically calculated from the physical properties (density, specific heat, thermal conductivity) of the above-mentioned food and the value of the heat transfer coefficient derived from the shape and dimensions of the apparatus and the food.
The cooling start time for the F value at the center of the food with the slowest heat conduction to reach the target value at the end of the entire heating and cooling process is predicted before the end of the heating process, and the operating valve of the heating medium is controlled at that time. Was. ("News from All Water Processing Federation", March 1989, No. 102, Issued by the Japan Federation of Fishery Processing Cooperatives)

[0005]

The problem to be solved is that it is considerably difficult to know in advance the physical properties such as the density, specific heat, and thermal conductivity of a solid packaged food with the precision required for practical use. Is that

[0006]

According to the present invention, when heat-treating a solid packaged food, one or more parameters which characterize the heat transfer characteristic inherent to the food by observing the aspect of temperature change in the early stage of the heating step. , And solve the heat conduction equation including the above parameters to predict the temporal temperature change after the noted position inside the food, and set the target heat treatment temperature and time of the food at the end of all the heating and cooling processes to the food. It is characterized in that heat treatment is performed by predicting a cooling start time for realizing an adequate amount even before the completion of heating.

[0007]

According to the above technical means, one or a plurality of parameters characterizing the inherent heat transfer characteristic of the solid packaged food are specified, and a heat conduction equation including the parameters is solved to determine a target position in the food. Predict the subsequent temporal temperature change, and predict the cooling start time before the end of heating to achieve the desired heat treatment temperature and time of the food at the end of the entire heating and cooling process even within the food. I do.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The heat treatment method for solid individual packaged food according to the present invention will be described below by taking a heat sterilization step of a retort food as an example. First, for simplicity, the food is a square plate, and
Thickness (D) is sufficiently smaller than width (W) (D << W)
And In recent years, foods in such a form have been increasing due to distribution needs and rationalization of heat treatment. In this case, the heat flow is generated perpendicularly to the flat plate along the thickness (D) from the surface of the square flat plate, and the following one-dimensional heat conduction equation holds.

[0009]

(Equation 1)

Where T is temperature, X is position, t is time,
ρ is density, Cp is specific heat, and λ is thermal conductivity.

The boundary condition in the heating step can be expressed by the following equation, where h is the surface heat transfer coefficient, Ti is the initial temperature, Th is the heating medium temperature, and 2D is the thickness of the food.

[0012]

(Equation 2)

Therefore, the basic concept of the method of the present invention will be described by considering the heat transfer characteristics of the solid plate-shaped individually packaged food based on the above basic formula. First, there is an internal time constant τ as a guide to know the time delay of the heat conduction phenomenon inside one food. The internal time constant τ is given by the following equation, where Ga is the internal heat conductance indicating the amount of heat transfer per unit temperature difference, C is the heat capacity characterizing the food, and S (= W2) the plate area.

[0014]

(Equation 3)

Similarly, there is an external time constant τs as a measure for knowing the time delay of the heat conduction phenomenon on the outer surface of the food. The external time constant τs is given by the surface thermal conductance Gs.
It is given by the following equation.

[0016]

(Equation 4)

Therefore, the independent and dependent variables of the heat conduction equation are given by the following equations to make the dimensionless.

[0018]

(Equation 5)

Then, the following general equations are obtained for the heat conduction equation and the boundary conditions, respectively.

[0020]

(Equation 6)

The solution is shown by the following equation.

[0022]

(Equation 7)

Here, Pn is an n-th root satisfying the following equation. (Depending on boundary condition (7a))

[0024]

(Equation 8)

An is given by the following equation. (Depending on boundary condition (7b))

[0026]

(Equation 9)

The solution (8) shown above is formally as follows.

Η = η (θ, ξ, τ, Bi) (11) That is, according to the above equation (11), regarding the temperature η at the position 内部 inside one solid flat plate-shaped packaged food, When the two parameters τ and Bi are specified, it indicates that the relationship with θ, that is, the temporal change is uniquely determined. Since the above parameters τ and Bi are characteristic values characterizing the heat transfer characteristic inherent to the food and not the physical properties of the food material, it is not necessary to know the details of the physical properties of the food material. Therefore, when heat-treating the solid plate-shaped individually packaged food, if the characteristic value τ, Bi unique to the food is specified by observing the aspect of the temperature change in the early stage of the heating process, the inside of the food in the heat sterilization process, particularly the most heat conduction, The temperature history of the delayed central part is known, and its highly accurate temporal control becomes possible. Therefore, when the food is heated with a heat medium (usually hot water) at the temperature Th, a temporal change in temperature inside the food is predicted in view of a delay in heat conduction, and the F value is determined at the end of the entire heating / cooling process. The cooling start time to reach the target value is predicted before the end of heating. Then, when the heating is completed at that time and the process proceeds to the cooling step, it becomes possible to realize the F value without excess and deficiency even in the inside of the food, especially in the center, and to perform the heat treatment.

Next, the error of the temperature sensor is examined, and it is shown that the true value can be estimated even if the observed value contains an error, thereby facilitating the practical use of the present invention.

First, the temperature at an arbitrary position X in the food is measured at least three times, ie, k times, every few seconds from the start of the heating step. .., Tk, and the observed values of the temperatures corresponding to the observation times are represented by T1, T2,.
Dimensionless as 1, T2,..., Tj,.
It becomes like the following formula.

Θj = tj / τ, Yj = (Tj−Th) / (Ti−Th), ξ = X / D (12)

Also, η (θj, ξ, τ, Bi) described by the true parameters (unknown) and the observed Yj
Is defined as Ej (Ej is an observation error and is 0 if there is no error in the observation), and Ej is given by the following equation.

Ej = η (θj, ξ, τ, Bi) −Yj (13) Therefore, the known approximate values of τ, Bi, に 関 す る with respect to the target food (however, twice or half the true value) May be wrong.) As τa, Bia, ξa, and around it,

Τ = τa + δτ, Bi = Bia + δBi,
ξ = {a + δ} (14) Then, equation (1)
When 3) is tailored to take the first term, Ej is formally given by the following equation.

[0035]

(Equation 10) Here, Rj in equation (15) is a residual regarding the approximate function,
It can be calculated from the observed value, and δη is the total derivative of the solution (8) of the basic equation. When the sum of squares of Ej is minimized, the following equation is established.

[0036]

[Equation 11] Further rearranging is as follows.

[0037]

(Equation 12) In the above equations (17a), (17b) and (17c), Rj can be calculated from the measured value, and the other partial differential equations are as follows.

[0038]

(Equation 13) Thus, a value can be obtained by numerically differentiating or analytically differentiating minute dτ, dBi, dξ. Accordingly, since δτ, δBi, δ 決定 are determined by solving the above matrix, the true values τ, Bi,
ξ is found.

Τ = τa + δτ, Bi = Bia + δBi, ξ = {a + δ} (19) That is, in the above operation, the sum of squares of the error Ej between the true value and the observed value is minimized. Even if it is included, the true value can be estimated. In the above operation, since only the first term when the Taylor expansion is performed around the known parameter is used, a higher-order error remains in the parameter obtained as a true value in the above equation. In this case, the same operation may be repeated several times using the true value obtained by the above equation as a new known parameter.

Next, based on the heat transfer characteristics specified as described above, a cooling start time tq at which the heating is terminated in the heating sterilization step and the process shifts to the cooling step is determined. In this case, the time t
Assuming that the heating is performed up to q, and thereafter, the process shifts to the cooling process using the cold water at the cooling water temperature Tc, the basic formula in the cooling process corresponding to the formula (1) is as follows.

[0041]

[Equation 14] The boundary condition in the cooling step is as follows. However, Tq
(X) is a temperature distribution obtained by substituting the time tq at the end of the heating step into the equation (8).

[0042]

(Equation 15) Using the above equations (20) to (22), the temperature at the center of the food in the cooling step is as follows.

[0043]

(Equation 16) Here, regarding the heat resistance of bacteria, for simplicity, Big was used.
Apply the log law of elow and assume the most typical case where the heat lethal curve is straight on common semilogarithmic graph paper. In this case, lethal time at reference temperature Tr be Fa
(F value), at an arbitrary temperature T other than the reference temperature, the temperature drops by Z degrees (gradient of the thermal lethal curve) and the required sterilization time F
(T) is multiplied by 10, and the following equation is satisfied.

[0044]

[Equation 17] Further, a small sterilization achievement ratio df that progresses to a small dt time at an arbitrary temperature T is a ratio of the small operation time dt to F (T), and the following expression is established.

[0045]

(Equation 18) The above equation (26) is integrated throughout the heating and cooling steps as shown in the following equation,

[Equation 19] The cooling start time tq is determined so that

FIGS. 1 to 3 show graphs in which the estimated value and the actually measured value according to the present invention are obtained and drawn for the model food. The model food used in the experiment was an acrylic sample in FIGS. 1 and 2, and as a konjac material in FIG. 3, the model food was stored in a heating drum filled with boiling water, heated, and stored in a cooling drum filled with running water. And cool. Further, a thermocouple is used as a temperature sensor for measuring the hot water temperature Th and the cooling water temperature Tc, respectively. Further, a temperature sensor Ts for inputting an experimental system for specifying a heat transfer characteristic value, and a verification for the predicted center temperature. The temperature sensor Tm is embedded in the food. The insertion position ξ = 0.5 in FIG. 1, ξ = 0.16 in FIG. 2, and ξ in FIG.
= 1.0 (outer surface).

In the figure, the horizontal axis represents the elapsed time from the start of heating, and the vertical axis represents the temperature and the Fp value (F value). In the figure, the crosses indicate input data (in the figure, data) collected by the experimental system to specify the heat transfer characteristics. 21 at an interval of 10 seconds, which is determined by the temperature sensor Ts at an unknown insertion position. Further, all the curves drawn by dotted lines in the figure represent the predicted values, and are three types of the predicted temperature of the sensor insertion position and the center temperature, and the predicted Fp value obtained from the predicted center temperature. In addition, all the curves drawn by solid lines in the figure represent actual measured values, and are two types of central temperatures Tm and actual measured Fp values obtained from Tm. Since the measured value and the estimated value are in good agreement, there are some parts where the solid line and the dotted line cannot be distinguished.

As shown, the error of Fp is small in each case, and the initial purpose is achieved.

[0049]

According to the present invention, when heat-treating a solid individual packaged food, the internal time constant and the external time constant that characterize the heat transfer characteristic of the food are specified, the heat conduction equation is solved, and the internal Precisely predicts the temperature change over time, and predicts the cooling start time before the end of heating in order to achieve the target heat treatment temperature and time for the food at the end of the entire heating and cooling process, even within the food. Since the heat treatment is performed at that temperature and time, the operation result can be accurately predicted during the operation, and the optimization control of the heat treatment of the solid packaged food can be easily and accurately performed. Further, since the time constant can be specified even when the insertion position of the temperature sensor is unknown, the workability of mounting the sensor is improved. Alternatively, the time constant can be specified by the temperature change of the food surface without inserting the sensor inside the food, so that the nondestructive inspection of all the products can be performed.

[Brief description of the drawings]

FIG. 1 is a graph showing an estimated value and an actually measured value according to the present invention using an acrylic sample.

FIG. 2 is a graph showing an estimated value and an actually measured value according to the present invention using an acrylic sample.

FIG. 3 is a graph showing estimated values and measured values according to the present invention using konjac materials.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-281042 (JP, A) JP-A-3-216990 (JP, A) JP-A-4-273916 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) G05D 23/00-23/32

Claims (1)

(57) [Claims] 1. When heat-treating a solid individual packaged food,
Observe the aspect of the temperature change in the early stage of the heating process, identify one or more parameters that characterize the heat transfer characteristic of the food, solve the heat conduction equation including the above parameters, Temporal temperature change is predicted, and at the end of the entire heating / cooling process, the target heat treatment temperature and time of the food are predicted even before and after the cooling start time for realizing the target heat treatment temperature and time within the food without any shortage. A method for heat-treating solid, individually packaged foods.