JP3328501B2 - Gas sensor for food quality detection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting the quality of food, which is used effectively for quality control and production control of processed foods such as soups in the field of food industry, and more particularly, to a flavor fraction of food. The present invention relates to a gas sensor exhibiting excellent response characteristics to dimethyl disulfide, which is a component found in US Pat.

[0002]

2. Description of the Related Art Soups that are processed foods include aliphatic hydrocarbons, aromatic hydrocarbons, aldehydes, ketones, furans, esters, sulfur compounds, nitrogen compounds, phenols,
Contains various taste components and odor components such as oils and fats. For example, in consommé soup, important odor components are esters contained in meat, ketones generated when vegetables are boiled, aldehydes generated when meat is boiled, and pyrazines, which are fragrant components of meat. It contains a sulfur compound that is a refreshing scent when vegetables are boiled. Each of these groups includes representative components such as ethyl acetate, acetone, caproaldehyde, 2-methylpyrazine, and dimethyl disulfide, and these components are mixed to form a unique smell of consomme soup.

[0003] In the production of soups, its taste,
It is important to accurately evaluate the aroma and control each process based on the results. Conventionally, quality control for taste and smell in the manufacturing process of processed foods such as soups has centered on sensory tests relying on human sensation, along with evaluation methods such as measuring sugar content and transparency. Is the current situation.

However, in recent years, strict requirements have been made on the quality of processed foods, and it has become difficult to perform sufficient quality control with conventional evaluation methods. Especially for sensory tests of taste and smell, it is necessary to train specialized panels or evaluate them by multiple persons if objective evaluation is to be performed, and it is difficult to immediately respond in the manufacturing process. It is the current situation that professionals rely on intuition for many years. In addition, in the sensory test, it is difficult to make an accurate determination of taste and smell depending on individual differences and physical condition.

On the other hand, it is conceivable to control the quality of food by analyzing typical components of taste and smell with instruments. Taste analysis means include high-performance liquid chromatographs and umami component sensors such as enzyme sensors and microbial sensors, and odor analysis means include gas chromatographs. However, high-performance liquid chromatographs and gas chromatographs are unsuitable for on-site management because the equipment is complicated, large and expensive, and the operation is complicated and time is required for measurement. Further, in the above-mentioned sensor for taste component, there are problems in life and sensitivity, and sufficient results have not been obtained.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended for detecting the quality of processed foods, such as soups, in a simple, quick and inexpensive manner. It is an object to provide a gas sensor.

[0007]

Means for Solving the Problems The present inventors have studied the odor components contained in soups, and found that these odor components have a great effect on the sensory evaluation of the soups, and therefore the odor contained in the soups. We focused on the fact that if we developed a gas sensor that responds to components with high sensitivity, we could easily and accurately control the quality of soups. Then, a semiconductor gas sensor material exhibiting excellent response characteristics to dimethyl disulfide, which is a component found in the flavor fraction of soups, was searched.

In this case, a sensor for hydrogen sulfide and a sensor for mercaptan have been proposed as sensors for a sulfur-based gas using a metal oxide as an element material. As the sensor element material, tin oxide for hydrogen sulfide is used. It has been proposed to use indium oxide for mercaptans (T. Maekawa et al, Chemistry Letters, pp. 575-578.199).
1, Maekawa et al., Report of Graduate School of Science and Engineering, Kyushu University, No. 15
Volume, No. 3, 273-279, December 1993, T. Maekawa et al, Jour
nal of Material Chemistry, 1994, 4 (8), 1259-1262).
On the other hand, as a sensor responding to dimethyl disulfide, one of the aroma components of food, a sensor element material made of an iron oxide-based material to which praseodymium is added has been proposed, but this sensor has a sensitivity and selectivity. (Abu et al., Surface Chemistry, Vol. 16, No. 8, 474-479, 1995). Therefore,
The present inventors have developed a sensor for dimethyl disulfide using indium oxide responsive to organic sulfur compounds based on these findings.

As a result, the present inventors have found that when indium oxide [In ₂ O ₃ ] in which lithium sulfate [Li ₂ SO ₄ ] or sodium sulfate [Na ₂ SO ₄ ] is dispersed is used as a sensor element material, The present inventors have found that a semiconductor gas sensor that exhibits excellent response characteristics to dimethyl sulfide [(CH ₃ S) ₂ ] and is useful as a sensor for evaluating the quality of soups and the like can be obtained, and has led to the present invention. .

Therefore, the present invention provides a gas sensor for detecting the quality of food, characterized in that it has responsiveness to dimethyl disulfide, using indium oxide obtained by dispersing lithium sulfate or sodium sulfate as a sensor element material.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the dispersion amount of lithium sulfate or sodium sulfate relative to indium oxide [(lithium sulfate or sodium sulfate) / {indium oxide + (lithium sulfate or sodium sulfate)] is:
It is preferably 0.1 to 10% by weight, particularly preferably 1 to 5% by weight. By setting the dispersion amount of lithium sulfate or sodium sulfate to 0.1 to 10% by weight, a sensor having extremely good response characteristics to dimethyl disulfide can be obtained. In this case, lithium sulfate or sodium sulfate can be dispersed in indium oxide by a known method such as an impregnation method, a colloid adsorption method, and a coprecipitation method. The structure of the sensor of the present invention is not limited, and may be the same as that of a normal semiconductor gas sensor. Note that lithium sulfate and sodium sulfate can be used in combination. In this case, the total amount of lithium sulfate and sodium sulfate added to indium oxide may be within the above range.

[0012]

Next, the present invention will be specifically described by way of examples.
Fabricate various semiconductor gas sensors as described below,
The characteristics of these sensors for odor components in soups were investigated.

(1) Preparation of Sensor Element A powder sample (sensor element material) was prepared by using In ₂ O ₃ as a base material and adding 12 kinds of sulfates to each. A sensor element was manufactured using the obtained powder sample.

Preparation of Powder Sample A powder sample was prepared by adding a sulfate to In ₂ O ₃ powder (base material). In ₂ O ₃ of the base material is InCl ₃
Was hydrolyzed with a 28% NH ₃ aqueous solution, and the obtained precipitate was filtered, washed with water and dried, calcined in air at 850 ° C. for 5 hours, and further pulverized with a ball mill. Sulfate was added by the impregnation method. First, the sulfate was mixed with the base material at a predetermined ratio. Further, distilled water was added thereto, and the water was evaporated with stirring until the sample became a paste.
Dry for 2 hours. Then, pulverize in air for 7
It was baked at 00 ° C. for 5 hours. The sulfates added in this manner include Li ₂ SO ₄ , Na ₂ SO ₄ , K ₂ SO ₄ , MgS
O ₄ , CaSO ₄ , SrSO ₄ , BaSO ₄ , Al ₂ (S
O ₄ ) ₃ , CdSO ₄ , ZnSO ₄ , Ag ₂ SO ₄ , Ce (S
O ₄ ) ₂ . The amount of sulfate added to the base material [sulfate / (base material + sulfate)] was 5% by weight for all powder samples.

Fabrication of Sensor Element As shown in FIG. 1, an alumina insulating tube (with an inner diameter of 0.4 mm,
Two Pt wires (0.3 m
mφ) is applied at a distance of 3.0 mm, a sensor material (powder sample) kneaded with water and made into a paste is applied, dried at room temperature, and baked at 700 ° C. for 4 hours in air to obtain a sensor element. Was prepared.

(2) Measurement of sensor characteristics Measurement method a. Apparatus The response of the sensor element was measured by a method (current detection method) for obtaining the output voltage across the reference resistance of the circuit. In this case, the sensor element was incorporated in the measurement circuit shown in FIG.
The sensor element was connected to the element holder by spot welding, and the element holder was attached to the reaction tube. FIG. 3 shows a reaction tube to which the element holder is attached.

B. Procedure Air was passed through the reaction tube equipped with the element holder at a flow rate of 200 cm ³ / min. In this case, before proceeding to the measurement temperature, 6
A pretreatment with dry air at 00 ° C. for 30 minutes was performed. Thereafter, the sample gas is cooled to a measurement temperature in a humid air atmosphere, a steady-state value (R _a ) of the element resistance in the humid air is obtained, and a test gas into which a target gas component is introduced is circulated in a carrier gas (humid air). The steady-state value (R _g ) of the element resistance was determined. Thereafter, the gas was switched to wet air, and after the element resistance was restored to a steady value to some extent, the temperature was changed to 600 ° C. × 3 in dry air.
After performing the process for 0 minutes, the process proceeded to the next measurement. In this case, the carrier gas for introducing the target component was humid air.If the measurement is performed using a gas generated from a food sample such as an actual soup as a test gas, it is considered that the influence of water vapor is inevitable. Because.

C. Analysis of gas sensitivity An example of a response curve of a sensor obtained at the time of measurement is shown in FIG. When the output voltages in the air and the test gas in this figure are obtained, the respective element resistances R can be obtained using the following two equations. R _a = r (E / V _a -1) R _g = r (E / V _g -1) where r is a reference resistance, E is an applied voltage, V is an output voltage,
The subscripts a and g indicate in air and gas, respectively. further,
The gas sensitivity S is represented by the ratio of the element resistance in the test gas to the element resistance in air. S = R _a / R _g

Measurement of sensor characteristics for target gas In ₂ O ₃ alone device and the added In ₂ O ₃ measured temperature 200 ° C. The sensitivity to dimethyl disulphide of elements sulfate (5 wt%), 300 ℃, were measured at 400 ° C..
The gas phase concentration of dimethyl disulfide was 20 ppm. FIG. 5 shows the results. FIG. 5 shows that the sensitivity to dimethyl disulfide in humid air was Li ₂ S in a temperature range around 300 ° C.
O ₄ -In ₂ O ₃ is best. It was found that Na ₂ SO ₄ -In ₂ O ₃ best in a temperature range of about 400 ° C..

B. In ₂ O ₃ single element, Li ₂ SO ₄ -In
_The dependency of the sensitivity of the ₂ O ₃ element and the Na ₂ SO ₄ —In ₂ O ₃ element on the dimethyl disulfide concentration was examined. The measurement temperature was 200 ° C. for the In ₂ O ₃ single element, and Li ₂ SO ₄ -In ₂
The O ₃ element is 300 ° C., the Na ₂ SO ₄ —In ₂ O ₃ element is 40 ° C.
0 ° C. FIG. 6 shows the results. Good linearity was obtained between the logarithmic value of the dimethyl disulfide concentration and the logarithmic value of the dimethyl disulfide sensitivity, indicating that the Li ₂ SO ₄ —In ₂ O ₃ element and the N ₂
a ₂ SO ₄ -In ₂ O ₃ elements showed good concentration dependence, and it was found that the sensitivity is higher than the In ₂ O ₃ alone device. Note that the sensitivity in FIG. 6 is a value that gives the dimethyl disulfide concentration by multiplying by a specific value.

C. Li ₂ SO ₄ —In ₂ O ₃ element and Na ₂
Dimethyl disulfide (DMDS) of SO ₄ —In ₂ O ₃ element,
The selectivity for methylpyrazine (MP), acetone (Ac), capronaldehyde (CA), ethanol (EtOH), and ethyl acetate (EA) was examined. The sensor temperature is 300 ° C or 400 ° C and the gas phase concentration of the target component is 20
ppm. The results measured at 300 ° C. are shown in FIGS.
FIG. 8 shows the result of measurement at 00 ° C. 7 and 8, the Li ₂ SO ₄ —In ₂ O ₃ element and the Na ₂ SO ₄ —In ₂
The O ₃ element responds with high sensitivity to dimethyl disulfide, but is much less sensitive to other odor components, especially when the Na ₂ SO ₄ —In ₂ O ₃ element is used at 400 ° C. Was found to have very good selectivity.

D. The response speed and reproducibility of the Na ₂ SO ₄ —In ₂ O ₃ element to dimethyl disulfide were examined. The sensor temperature is 400 ℃, the gas phase concentration of dimethyl disulfide is 20pp
m, and the number of measurements was 5. As a result, the 90% response time was 77 seconds, the reproducibility was 4.78% CV, and the Na ₂ SO ₄
-In ₂ O ₃ elements, it was observed response speed and reproducibility in dimethyl disulfide measurement is good. An example of the response curve at this time is shown in FIG. 9, and it is recognized that both the response characteristics and the recovery characteristics are good.

[Experimental Example] The concentration of dimethyl disulfide in the same sample (soup) was measured using the gas sensor of the present invention and a gas chromatograph. As a sensor element of the gas sensor of the present invention, a Na ₂ SO ₄ —In ₂ O ₃ element (Na ₂ S
The amount of O ₄ added was 5% by weight), and the sensor temperature was 400 ° C.

The test gas was prepared by introducing a target gas component into a carrier gas (wet air) using the sample introduction device shown in FIG. The sample introduction device of FIG. 10 includes a sealable sample injection cell having a sample liquid injection port, a filter paper disposed in the sample liquid injection cell, and impregnated with the sample liquid (soup) injected from the sample liquid injection port. In the sample injection cell, the sample liquid impregnated in the filter paper is vaporized, and the vaporized sample is mixed with a carrier gas and introduced into a reaction tube. By using this apparatus, the influence of the odor component in the measurement environment can be cut and a clean carrier gas can be obtained, so that the target component, dimethyl disulfide derived from soup, can be accurately measured.

FIG. 11 shows the correlation between the measured values. as a result,
The value measured by the gas sensor of the present invention using the Na ₂ SO ₄ —In ₂ O ₃ element showed a good correlation with the value measured by gas chromatography. In addition, Li ₂ SO ₄ −
The same measurement was performed using an In ₂ O ₃ element (Li ₂ SO ₄ added amount 5% by weight) and a sensor temperature of 300 ° C.
Again, the measured values showed a good correlation with the values measured by gas chromatography. Therefore, this experiment confirmed the reliability of the gas sensor of the present invention.

[0026]

As described above, the gas sensor for detecting food quality of the present invention has excellent response characteristics to dimethyl disulfide as an odor component contained in foods, and particularly, quality control and production of soups such as consommé soup. It can be used effectively for management.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view showing an example of a sensor element.

FIG. 2 is a circuit diagram showing a circuit for measuring sensor element characteristics.

FIG. 3 is a schematic view showing a reaction tube used for measuring sensor element characteristics.

FIG. 4 is a graph showing an example of a response curve of a sensor element.

FIG. 5: In ₂ O ₃ element alone and In ₂ added with sulfate
4 is a graph showing the sensitivity of an O ₃ element to dimethyl disulfide.

FIG. 6 shows the relationship between the logarithmic value of the dimethyl disulfide concentration and the logarithmic value of the dimethyl disulfide sensitivity in the In ₂ O ₃ element, the Li ₂ SO ₄ —In ₂ O ₃ element, and the Na ₂ SO ₄ —In ₂ O ₃ element. It is a graph which shows a relationship.

FIG. 7 shows a Li ₂ SO ₄ —In ₂ O ₃ element and Na ₂ SO ₄ —I
4 is a graph showing the sensitivity of an n ₂ O ₃ element to various gases at 300 ° C.

FIG. 8: Li ₂ SO ₄ —In ₂ O ₃ element and Na ₂ SO ₄ —I
4 is a graph showing the sensitivity of an n ₂ O ₃ element to various gases at 400 ° C.

FIG. 9 is a graph showing an example of a response curve of a Na ₂ SO ₄ —In ₂ O ₃ element.

FIG. 10 is a schematic view showing an example of a device for introducing a test gas into a reaction tube provided with a sensor element.

FIG. 11 is a graph showing the correlation between the dimethyl disulfide concentration measured by the gas sensor of the present invention and the dimethyl disulfide concentration measured by gas chromatography.

Continued on the front page (72) Inventor Norio Miura 301-17-301, Hirao, Chuo-ku, Fukuoka City, Fukuoka Prefecture (72) Inventor Naomi Funasaki 4- 13-14 Kichijoji Kitamachi, Musashino City, Tokyo Electrochemical Instruments Inc. (72) Inventor Taiichi Asano 4- 13-14 Kichijoji Kitamachi, Musashino City, Tokyo Electrochemical Instruments Co., Ltd. (72) Inventor Kenji Hayashi 3-11-1 Natsumidai, Funabashi City, Chiba Prefecture Nichire I Funabashi Heim 308 (72) Inventor Hiroaki Kozuka 16-16-29, Yachiyo-Taipei, Yachiyo-shi, Chiba (56) References JP-A-9-26406 (JP, A) JP-A-8-15200 (JP, A) JP-A-8- 15201 (JP, A) JP-A-6-34592 (JP, A) JP-A-3-223660 (JP, A) JP-A 1-2232247 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) G01N 27/12 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A gas sensor for detecting the quality of food, characterized in that it has responsiveness to dimethyl disulfide and uses indium oxide, in which lithium sulfate or sodium sulfate is dispersed, as a sensor element material.