Study on the relationship between tea concentration and Tyndall effect

: The quality of tea is relative with its concentration. In this paper, we present a method to identify the concentration of tea based on Tyndall effect. The tea is illuminated by 3-watt LED light sources with the main wavelength in ultraviolet and visible bands. The intensities of transmitted light are detected using the distributed photometer and the images of light distribution are captured by CCD camera at vertical direction to observe the Tyndall effect. The intensities and images for different light sources are compared. The experimental results show that there exist Tyndall phenomenon in tea. In addition, the Tyndall effect becomes weaker as the tea concentration increases when the tea is illuminated by the LED light sources with the main wavelength in the ultraviolet band. The method presented in this paper should be helpful for rapid identification of the tea quality


Introduction
After thousands of years of development, tea has become one of the three major non-alcoholic beverages and tea is widely loved by people all over the world because of its excellent flavor and health benefits. The flavor and health care effect of tea is the result of the joint action of taste and aroma. Its components mainly include amino acids, sugars, fats, vitamins, chlorophyll, carotene, tea pigments, tea polyphenols, tea polysaccharides, caffeine, etc. These taste and aroma substance determines the quality of tea [1,2]. At present, the research on the quality of the tea leaves is mainly divided into two types. The first is to study the tea leaves directly, and the second is to study the tea. Less research has been done on tea leaves. Bacterial species [3,4], pest and disease disasters [5] are detected to study the tea leaves . More research has been done on tea soup as follows. The function of genetic traits [6] is studied by the use of molecular marker technology. and the structure of active ingredients [7]is determined by the use of liquid chromatography. Aroma substances [8] is analyzed by the use of mass spectrometry, and raw material varieties is identified by the use of near-infrared [9]. .In addition, absorption spectrum of tea is analyzed by using ultraviolet spectrophotometer [10]. To sum up, the current research on both tea leaves and tea requires professional equipment, and the operation is difficult. Therefore, there is an urgent need for a way to quickly identify tea. It was found that tea contains a variety of compounds, and the particle size of these compounds is generally in the range of colloids.
When light passes through these colloidal particles, the Tyndall effect occurred. The concentration of tea is closely related to the content of ingredients in tea. Therefore, the relationship between the tea concentration and the Tyndall effect is explored in the paper for the first time.
In this article, in order to explore the relationship between tea and the Tyndall effect, two types of samples and light sources provided by SMC (Shenzhen) Co., Ltd. have been prepared for testing. In the section 1, the propagation mechanism of light in tea are introduced. In the section 2, the material preparation and experimental procedure of this study are introduced. In the section 3, the experimental data is analyzed and discussed.

Principle
When light encounters an object, Reflection, Scattering, Absorption and Transmission will occur. Reflection of light occur when light changes the direction of propagation at the interface between two substances and returns to the original substance. Scattering of light refers to the effect that part of the light travels away from the original direction when light passes through an inhomogeneous medium [11]. Transmission of light is that incident light is refracted through an object. The absorption of light refers to the effect that atoms absorb the energy of photons and transition from a low-energy state to a high-energy state when exposed to light. In the solution, the particle diameter of the dispersoid is less than 1 nm. When the visible light enters the solution, the solution has a very weak scattering effect on the light, which mainly produces the transmission of light. While in the turbid liquid, the particle diameter of the dispersoid is generally larger than the wavelength of visible light. Therefore, when visible light enters the turbid liquid, the phenomenon of reflection mainly occurs. In colloids, the diameter of colloidal particles is less than the wavelength of visible light. When visible light enters the colloid, light scattering will occur to produce the Tyndall effect [12]. The Tyndall effect is formed by the scattering of visible light (wavelength 400 ~ 700 nm) [13]. In the process of light propagation, reflection or scattering of light will occur when light irradiates the particle. If the particle is many times larger than the wavelength of the incident light, light reflection occurs. If the particle is smaller than the wavelength of the incident light, light scattering occurs. The diameter of colloidal particles is between 1nm and 100nm. It is smaller than the wavelength of visible light (400nm ~ 700nm), it will produce obvious scattering effect when visible light passes through the colloid. For a true solution, although the molecules or ions are smaller, the intensity of scattered light decreases significantly with the decrease in the volume of the scattering particles. Thus there is a very weak scattering effect when the true solution is illuminated by light. In addition, the intensity of scattered light also increases with increasing particle concentration in most cases. It is worth noting that the frequency of scattered light does not change in the Tyndall effect. And the Tyndall effect in the pure water and colloidal solution is shown in Figure 1. Luminous intensity is a physical quantity used to express the intensity of luminescence within a unit solid angle and a given direction of a light source. In this experiment, the intensity of light reflection in visible band is very small and can be ignored. Therefore, the intensity of light scattering in visible band is indirectly defined by testing the Luminous intensity of transmitted light, which is the intensity of Tyndall effect. The distributed photometer was used to test the Luminous intensity. The specific test principle is shown in Figure 2.

Material preparation
The light sources used in this experiment are all 3535 vertical structure light sources. The power of these light sources is 3w and the divergence angle of these light sources is controlled within 120°. The specific wavelengths are shown in Table 1, the picture of 3535 vertical structure light source is shown as Figure 3. In this experiment, the tea water ratio of both two teas is 1g tea: 100ml water. Both teas were soaked in water at 80°C and placed at room temperature for 4 to 6 hours to fully dissolve the soluble molecules in the tea leaves, and then the tea leaves were taken out to obtain a filtrate. The filtered pictures of Xinyang Maojian Tea and Raw Pu'er Tea are shown in Figure 4.

Methods
1、 Use the 12 groups of light sources in Table 1

Results and discussions
Under the darkroom environment, the light sources in Table 1 are used to irradiate the tea in Xinyang Maojian Tea and Raw Pu'er Tea, and their Tyndall effect presented are shown in Figure 5 and Figure 6. According to Figure  5 and Figure 6, it is found that the Tyndall effect is existed in Xinyang Maojian Tea and Raw Pu'er Tea between 345nm to 660nm. Xinyang Maojian Tea and Raw Pu'er Tea have very obvious Tyndall effect at 345nm, 365nm, 395nm which are ultraviolet bands. At the same time, Xinyang Maojian Tea also has obvious Tyndall effect at 405nm, 470nm and 560nm, while Raw Pu'er Tea is at 470nm, 530nm and 560nm.  According to the definition of Tyndall effect, there should be no Tyndall effect in the ultraviolet band. However, it can be clearly found that not only the Tyndall effect exists in the ultraviolet band, but also the intensity of the Tyndall effect is very obvious through the naked eye observation. There are two reasons for this problem. One is that the ultraviolet light sources will be added with light in other wavelength bands during production to ensure the safety of the use of ultraviolet light sources. The second is that due to the emission of the tea in the ultraviolet band, the photons of the higher band excited show the Tyndall effect. Therefore, for 345nm, 365nm, and 395nm, the test of the light source spectrum and the test of the emission spectrum are carried out in order to distinguish between the scattering of visible light and the emission of tea. This part will be discussed after the analysis of the Tyndall effect caused by visible light. The absorption of Xinyang Maojian Tea and Raw Pu'er Tea was tested, and the results are shown in Figure 7. As shown in the figure, the absorbance of Xinyang Maojian Tea and Raw Pu'er Tea is basically the same between 400nm and 700nm, the variances of Xinyang Maojian Tea and Raw Pu'er Tea are 0.004 and 0.01, respectively. Therefore, the absorbance in this between 400nm and 700nm can be ruled out as an average error when testing Luminous intensity. Since there is no reflection in tea and tea is only excited in the ultraviolet band, after excluding the average error of absorption, the result of the Luminous intensity test represents the intensity of the Tyndall effect to a certain extent. The results of the Luminous intensity test in visible light using the LED 626 are shown in Figure 8. Figure 8 shows the difference between the Luminous intensity passing through the water and the Luminous intensity passing through the teas. As shown in Figure 8, the Tyndall effect exhibited by 560nm light source in the Xinyang Maojian Tea is the strongest, followed by 505nm, and then 530nm, which is basically consistent with the effect observed by the naked eye. At the same time, the best Tyndall effect exhibited by Raw Pu'er Tea is at 505nm, followed by 530nm and 560nm. The bands with the strongest Tyndall effect values of different teas have a slight deviation, but they are generally in the green light band. For the ultraviolet band, spectral tests and emission tests are added. The spectral test is from 420nm to 700nm, and the band in this range is tested to determine the visible light band mixed in the ultraviolet light source. The emission test tests the emission spectrum of Xinyang Maojian Tea and Raw Pu'er Tea at 345nm, 365nm, and 395nm respectively, to determine the emission band produced by ultraviolet to tea. The result of the spectrum is shown in Figure 9, the color coordinates and color temperature are shown in Table 2 and the result of emission spectrum is shown in Figure 10.  As shown in Figure 9, it can be observed that there are visible light bands in 345nm, 365nm, and 395nm. According to the spectrum and color coordinates, the visible light band mixed with 345nm is biased towards green-white light, 365nm is biased towards blue-white light, and 395nm is biased towards violet light. In addition to the scattering in the visible light band, the Tyndall effect produced by the ultraviolet band also has an emission effect. The stray light at 345nm is mainly green, so it can be clearly observed in Figure 5 and Figure 6 that the color of the Tyndall effect is mainly green, and according to the emission spectrum of Figure 10, the emission peak at 345nm is at 450nm. Therefore, the Tyndall effect observed in Figure 5 and Figure  Raw Pu'er Tea 525nm in Raw Pu'er Tea and at 460nm in Xinyang Maojian Tea. Therefore, the color of Tyndall effect at 395nm in Xinyang Maojian Tea is white with a litter blue while the color of Tyndall effect in Raw Pu'er Tea is green and purple mixed in white. So, for the Tyndall effect in the ultraviolet band, it should be a combination of the scattering of the stray light in the visible band and the emission effect of ultraviolet band to tea. In addition, it can be observed from Figure 5 and Figure 6 that the Tyndall effect of a single band of visible light on tea is obviously not as strong as that of the ultraviolet band. There are two reasons for this. The first is that there is no obvious contrast between the background color generated by the visible light band and the color presented by the Tyndall effect, so the Tyndall effect is not obvious to the naked eye. Secondly, the wavelength of the light frequency in the visible light band is too narrow and the energy is low, compared with the emission and stray light in the ultraviolet band, its color rendering is slightly inferior. Based on the above two reasons, the Tyndall effect produced by the ultraviolet band in tea is the most obvious by naked eye.
Since the Tyndall effect is a physical phenomenon, the Tyndall effect generated in the ultraviolet band should be observed in the daylight environment. The Tyndall effect in daylight environment in ultraviolet band is shown in Figure 11. In Figure 11. As shown in the figure, the Tyndall effect of 395nm is the most obvious to the naked eye. Therefore, 395nm light has the strongest correlation with substances in tea. In order to test the influence of tea concentration on the intensity of Tyndall effect, the 395nm light source with the best Tyndall effect was used to test the Luminous intensity of Xinyang Maojian Tea with different concentrations. The concentration of Xinyang Maojian Tea is controlled by changing the ratio between Xinyang Maojian and water. The specific ratio is shown in Table 3. In a daylight environment, 395nm light source was used to irradiate Xinyang Maojian Tea with different ratios of tea to water, and the Tyndall effect was observed with the naked eye, as shown in Figure 12. Use the same method as above to test the Luminous intensity of Xinyang Maojian Tea with different concentrations. This time, the test Luminous intensity only discusses the relationship between concentration and Luminous intensity, so the scattering and emission effects in tea will affect the results, which is different from the visible light band. The test results are shown in Figure 12.  As shown in Figure 13. It can be seen that the higher the concentration of tea, the smaller the Luminous intensity transmitted. This result proves that as the concentration increases, the transmitted light intensity gradually becomes weaker. The weaker the transmitted light, the stronger the Tyndall effect to a certain extent. This shows that there is a correlation between the Tyndall effect and tea concentration. The more flavor substances in the tea, the greater the concentration of the tea. And the flavor substances in the tea are the main factors that lead to the taste of tea. Therefore, the Tyndall effect can also be used to identify the taste and flavor of tea. In addition, the Tyndall effect light path of different concentrations of tea is analyzed by using grayscale with MATLAB. One of the biggest benefits of using grayscale for Tyndall effect light path is that it can eliminate chromatic aberrations, exposure and some other inevitable matters caused by hardware during shooting. As shown in Figure 14, the light path of Tyndall effect after grayscale processing is very obvious, and the interference factors such as chromatic aberration, exposure are removed. As shown in Figure 14 and Figure 15, it can be clearly observed that the higher the concentration of tea, the weaker the scattered light (transmitted light) in the horizontal direction of the light source. The gray values of the red areas in Figure 15 are 107.3467 (0.5) and 88.3922 (1.5), respectively. This conclusion also confirms that the higher the concentration of tea, the weaker the transmitted light. Figure 15. The gray area corresponding to the gray value.

Conclusion
In conclusion, an optical method based on the Tyndall effect is proposed to explore the relationship between tea concentration and scattered light. By testing the visibleultraviolet spectrum and light intensity data, the conclusion was obtained that the Tyndall effect in the ultraviolet band is easier to observe than the visible band by using LED light sources with power 3w, because the ultraviolet LED has the scattering of visible light and the excitation of ultraviolet light. In addition, according to the results of image processing, the higher the tea concentration, the weaker the Tyndall effect. The changes in the concentration in tea leaves are mainly due to changes in the content of flavor substances and some compounds that determine the flavor of tea leaves. Therefore, the Tyndall effect can also be used to indirectly explore scattered light and tea flavor. This paper proposes a new research idea for using optical methods to study tea. In the future, the relationship between light and matter in tea will be studied.