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Application of Nondestructive Testing Technology in Determination of Fruit Quality
The main evaluation indicators of fruit quality generally include various physical and chemical indicators such as sugar content, acidity, maturity, freshness, and hardness. Traditional methods for assessing the quality of fruits are mainly chemical analysis methods, which require considerable chemical reagents. The testing and analysis process is cumbersome and slow. Usually, a small amount of samples are used to replace the quality of batches of fruit, and the fruits must be destroyed during the test. The practical significance of quality grading is not significant. In recent years, the non-destructive testing method, based on physical, mechanical, optical, electrical, and biological characteristics of fruits, has determined the physical and chemical indicators of fruit without destroying the fruit itself, Deng Ganran (Institute of Agricultural Machinery, Chinese Academy of Tropical Agricultural Sciences), and does not require chemical reagents. The cumbersome analysis process has a simple and fast method, high accuracy of test data, and is conducive to achieving on-line hierarchical control of internal quality of fruits. Therefore, it has a wide range of application prospects. Non-destructive testing methods for fruit quality have been studied at home and abroad, and some have been applied commercially. This article will introduce the testing principle of several kinds of fruit non-destructive testing methods, such as electrical characteristic test method, dynamic characteristic test method, near-infrared test method, acoustic characteristics analysis method and so on. It is recommended to use the instrument: a soil tester.
1 Electrical characteristics test method
There are a large number of bioelectric fields formed by charged particles inside the biological tissues of fruits. The biochemical reactions of fruits in the mature, damaged, and spoilage processes will be accompanied by the conversion of substances and energy, resulting in the amount of charge of various chemical substances in biological tissues. And changes in the spatial distribution of the charge. The distribution and intensity of the bioelectric field affect the electrical properties of the fruit macroscopically. Thompson et al. studied the variation of the electrical characteristics of apples in the frequency range of 300-900 MHz and found that the electrical properties of apples are closely related to their maturity. Mclendon et al. found that the dielectric constant and loss tangent of pears in the frequency range of 0.5 to 5 kHz showed a decreasing trend with increasing frequency. However, as the pears mature, both gradually increase. Nelson et al. studied a variety of fruits in the frequency range of 0.2 to 20 GHz and found that the dielectric constant of the tested fruits decreased steadily with frequency. In the frequency range of 10 Hz to 13 MHz, Kato Haruo performed a comparative analysis of the electrical characteristics of damaged fruits and normal fruits. The results showed that the string, parallel equivalent resistance and impedance of damaged fruits were lower than normal fruits in the frequency band, and the series connection etc. The effective capacitance and loss factor values ​​are larger than normal fruits, and the series equivalent resistance is related to the freshness or maturity of the fruit.
The relationship between the internal quality of fruits and the macroscopic electrical properties is as follows: The electrical properties of fruits are tested by methods such as slicing, spurs, contact and non-contact. The first two are lossy tests, and the latter two are directly placed between the flat electrodes. The electrical property parameters are determined and are non-destructive testing.
Non-destructive testing of electrical parameters of fruit electrical circuit shown in Figure 1. At a given measurement frequency ω, the fruit under test is placed between the parallel electrodes as the internal medium of the capacitor, and the drive current I output from the sine wave generator flows through the capacitor with the calibration resistor R0 and the tested fruit as the medium. The series circuit, through the output voltage E1 and E2 of two differential amplifiers with the same gain K, through a simple algebraic operation, the complex impedance of the fruit is
Where: ω is the angular frequency, ω = 2 π f ; f is the frequency of the electric field; R s , C s is the serial equivalent resistance and capacitance of the fruit. As long as the real part and the imaginary part of the ratio of E1 and E2 are calculated, the series equivalent resistance and the series equivalent capacitance can be obtained, and then the parallel equivalent resistance, the parallel equivalent capacitance and the complex permittivity can be obtained. This measurement method has an important relationship with the outside temperature and the shape of the fruit. The electrical characteristics of fruit at low frequencies are more affected by temperature, and as the temperature rises, the impedance will gradually decrease. At frequencies above a certain value, the effect of temperature will become very small; Generally speaking, the size of the fruit The smaller the impedance is, the larger the impedance is below 15 kHz. Because the electrode spacing is constant, the size of the fruit directly affects the amount of media filling between the electrodes.
The relationship between the internal quality of fruit and the electrical parameters of fruit: When the fruit is rot or damaged, the relative permittivity will increase significantly. The reason is that after the fruit rots, the cell membrane is often damaged, the ion permeability increases, and the equivalent impedance decreases. Small, at the same time, the colloidal bound water becomes free water, and the permittivity of free water is far greater than that of bound water, resulting in an increase in capacitance and relative permittivity; as the freshness of fruit decreases, its free water is consumed with respiration. As the content of organic acids and protopectin gradually decreases, the relative permittivity decreases, and the equivalent impedance tends to increase.
2 dynamic characteristics test
The elastic modulus and phase angle of the dynamic test were measured under different conditions of preload, excitation power, maturity, and the like. The test principle is that when sinusoidal alternating stress-strain dynamic test is used for axial loading, the sinusoidal force is generated by a sine wave generator through an amplifier and is generated by an electric oscillator. A piezoelectric acceleration sensor is fixed on the oscillator shaft and the sample is clamped. Between two discs, one disc is connected to the vibrator, and the other is connected to the rigid support by a dynamic force sensor to determine the static preload. Acceleration and force sensor signals are amplified and input through horizontal and vertical channels Storage oscilloscope, by adjusting the power amplifier to change the excitation power (determine the input exciter power to obtain excitation power), see Figure 2.
During the experiment, the symmetric sides of the fruit were applied with a certain preload/preload up-and-down fluctuation of +0.25 N. Then the signals such as force, acceleration and phase angle between force and acceleration displayed on the oscilloscope were quickly read and stored, and were stored at the same frequency at a certain frequency. After the end of the experiment, the average and the standard deviation were read on the oscilloscope.
3 Near-infrared test method
Near-infrared wavelengths range from 800 to 2500 nm, and near-infrared radiation is applied to the fruits. The functional groups (-OH, -CH2, -NH) that make up sugars and acids absorb specific light that corresponds to the natural vibrations of the corresponding molecules. Near infrared The spectroscopic method is a technique that utilizes the above characteristics to detect components such as sugar, acid, moisture, and chlorophyll from the amount of light absorbed. This method only destroys the fruit to determine its chemical composition when the calibration line is established. After the calibration line is made, only the near-infrared spectroscopy spectrum of the sample can be measured, the predicted value of the component can be obtained, and multiple components can be measured at the same time in an instant.
The in-line near-infrared internal quality inspection technology is classified into a reflected light type and a transmitted light type, and a light-transmitting type is classified into incomplete light shielding type and completely light-shielded type. Reflected light measurement is a method in which near-infrared rays are irradiated near the equator of the fruit and the light reflected from the surface of the fruit after diffusion is measured. A detection principle is shown in FIG. 3 . The conveying device conveys the fruit to the sugar content measuring device, and the near-infrared light irradiating on the fruit is reflected by the fruit and then collected by the light collector, and then passes through a beam splitter (filter) to take out only a real wavelength of near-infrared light. The filtered light is converted into an electric signal by the photoelectric element, so that the reflection intensity at different wavelengths is obtained, and the sugar content is calculated from the reflection intensity based on a multivariate regression sugar meter calculation model (calibration line) established in advance.
The principle of incomplete shading transmission light measurement test is shown in Figure 4. The near infrared ray A is irradiated on the surface of the fruit, and the diffused light B passing through the inside of the fruit is detected by the sensitization sensor provided on the other side. This method is a reflection-type improvement, since the irradiation light and the light sensor are arranged on both sides of the fruit, reducing the influence of the surface reflection light on the light sensor. During operation, the illumination light is continuously illuminated, and the light sensor located on the optical axis is controlled by an electronic shutter, which is detected only when the fruit is in place. The front part is provided with a fruit diameter detection sensor. When the fruit passes through, the light source is blocked, the fruit diameter is obtained based on the time difference between the beginning of light blocking and the end of light blocking at the back, and the center of the fruit is determined. The computer takes the center position as a reference to determine the start. , End the measurement time, perform spectrum measurements several times within this time interval, and then use the average value for internal quality analysis.
The principle of complete shading transmission light measurement test is shown in Fig. 5. Two sets of horizontally arc-shaped light sources are located above the conveying device and symmetrically arranged on both sides of the conveying device. Fruits are manually placed on the conveying device. Fruit bucket. There is a vertical through hole in the middle of the bucket, and the inside of the surface in contact with the fruit is a sponge material, which can make the fruit surface closely fit with the fruit bucket; the transmitted light measurement part is located below the conveying device. When the fruit moves to the measuring device along with the conveying device, the light arranged in an arc around the fruit is uniformly irradiated on the surface of the fruit, and the light passing through the inside of the fruit is detected by the photosensitive sensor through the through hole in the middle of the fruit bucket.
4 Acoustic characteristics analysis
The acoustic characteristics of fruit refer to the reflection characteristics, scattering characteristics, transmission characteristics, absorption characteristics, attenuation coefficients and propagation speeds of fruits under acoustic waves, and their own acoustic impedances and natural frequencies. These reflect the basic interaction between sound waves and fruits. Regularity, so the fruit can be judged according to the acoustic characteristics of the fruit. Stone ML uses sound pulse impedance technology to perform non-destructive measurement of watermelon ripeness in the field, and compares the frequency response parameters of the pulse response with the sugar, pulp color, hardness, and quality of a single watermelon. Duprat F et al. used acoustic pulse response technology to detect the change in hardness of growing pears before and after harvest, and established a first-order model of pear hardness transformation. Mizrath et al. applied ultrasound technology to the non-destructive quality testing of avocados and established a relationship between ultrasound parameters and the maturity of avocados. Ultrasonically applied non-destructive tests on mangoes with ultrasonic probes and peels. Contact, measurement of attenuation of ultrasonic waves in mango pulp, establishment of equations between ultrasonic attenuation and hardness values ​​and physiological indicators.
5 Application Prospects of Fruit NDT
China is a big country for fruit production and consumption, but the per capita share is still at a relatively low level. With the continuous improvement of the people’s living standards, the demand for fruits is increasing, and higher requirements are placed on the quality of fruits. People’s choice of fruits has gradually shifted from the traditional vegetable markets and street hawkers to large supermarkets. Fruits that are well packaged and grading have been difficult to reach the high-quality market of supermarkets to obtain higher prices. Therefore, the classification of fruits is an inevitable requirement for market development. However, in addition to the low ergonomics, the traditional artificial grading is also affected by the human factors of the operator. The grading is difficult to be standardized, and the grading basis can only be the appearance of the fruit. The internal damage and internal quality indicators of the fruit cannot be identified. The basic meaning of the classification is not reached - the classification should first be considered from the internal quality of fruit. The deficiencies of chemical analysis are also very obvious. In addition to its low efficiency and complicated process, it is costly. Therefore, it can only be used in a more laboratory sense rather than a commercial application. Since these two methods are not suitable, and the fruit must be graded, we can expect that in the near future, the non-destructive testing technology of fruits will certainly be widely used.