Evaluation of Moisturizing Effect of Metasilicate Extracted from Rice Hull

In Japan, where many resources are imported, recycling wastes is one of the most concerning issues. This is consistent with the Sustainable Development Goals (SDGs), which call for the secondary use of agricultural by-products that are regularly generated. Rice hull, particularly, generated during rice production are one of the agricultural by-products. In this study, the utilization of metasilicate which is produced during the extraction of silicic acid present in rice hulls was focused. Metasilicate is mainly a free component of hot springs and is reported to have a moisturizing effect. Thus, the metasilicate extracted from rice hulls would expected also have a similar effect. To examine the use of rice hulls as a moisturizing component, the elution characteristics of metasilicate present in rice hulls in tap water were first evaluated. The concentration of the dissolved metasilicate was determined colorimetrically using the molybdenum yellow method according to the guidelines of the mineral spring analysis method. As a result, 546 mg/kg of metasilicate was eluted from the rice hull in tap water at a water temperature of 40 °C. This value was much higher than 50 mg/kg of metasilicate, which is defined as a mineral spring. Next, the moisturizing effect of metasilicate extracted from rice hulls was evaluated by taking a warm bath in the water in which the hulls were immersed. As a result, the ratio of change of the moisture content of the skin remained higher than the value before the experiment even 30 minutes after the warm bath. In the corresponding t-test at a significance level of 5%, there was no significant difference in the rate of change of the moisture content of the skin 5 and 30 minutes after the warm bath, suggesting that the rice hull may have a moisturizing effect.


Introduction
Yamagata Prefecture, which is the only place in Japan where there are hot springs in every city, town and village. Zao Onsen and Ginzan Onsen are famous throughout Japan, and many tourists visit them every year. In addition to these, there are many other hot springs throughout the prefecture that are said to have health benefits, however these local resources are not fully utilized. Aside from this, the organic utilization of waste through recycling is an important issue for the future in Japan, which has to rely on imports for many resources. This has become a mainstream issue in the world, as Target 12.2 of the Sustainable Development Goals (SDGs) adopted at the United Nations Summit (2015) sets out the goal of "achieving sustainable management and efficient use of natural resources by 2030 (1) ." Upon analyzing domestic situations against social background, the inedible agricultural parts proceed from the cultivation of rice production, which is a staple food in Japan, is the first thing that comes to mind. In particular, approximately 12% of the 1.7 million tons of rice hulls generated every year is incinerated (2) , and it is necessary to establish the methods for secondary utilization of rice hulls. Previous studies have focused on silicic acid, which accounts for about 19 to 22 wt.% of the components of rice hulls, and performed its selective extraction. The extraction methods are classified into calcination method via the incineration of rice hulls and wet method using raw rice hulls as raw materials. For example, the extraction of silicic acid using the calcination method has been considered the mainstream method because of the simplicity of the extraction process (3,4) . However, the metasilicate extracted, using high-temperature calcination, converts into a carcinogen, which may affect health. In contrast, the wet process, which uses raw rice hulls as raw materials, can reduce environmental burden because the hulls are not calcined, and the silicic acid extracted is non-toxic and highly convenient. This makes it possible to use silicic acid in a variety of applications that were previously limited. One example is the use of metasilicate, which is a hydrate of silicic acid; metasilicate is known as a free constituent of hot springs and has moisturizing effects, in addition to its applications as an adhesive and metal surface preservative (5,6) . Therefore, metasilicate extracted from rice hulls were expected also have a moisturization effect. If the use it can contribute to cosmetic effects, it is considered to lead to proposals for new regional revitalization, such as linking waste to secondary income for agricultural workers.
Thus, to examine the use of rice hulls as a moisturizing agent, the elution characteristics of metasilicate contained in rice hulls in tap water was evaluated. Subsequently, the effect of metasilicate extracted from rice hulls on moisture retention was evaluated.

Sample
Rice hulls (variety: Tsuya-hime) harvested in Tsuruoka City, Yamagata Prefecture, in FY 2015 were used as samples. The rice hulls were dried in a constant temperature dryer (FS-43, ADVANTEC) at 75 ℃ for 24 h, and then ground for 30 s in a grinding machine (HS-23, Labonect Co., Ltd.). Figure 1 shows the procedure for elution and quantification of metasilicate. The test powder (0.5 g) was added to tap water (10.0 ml) and stirred in a thermostatic shaking tank (T-N22S, THOMAS) at 40 °C for 24 h. Subsequently, the solution was filtered through five types of C filter paper. Eluted metasilicate was determined according to the Guideline for Mineral Spring Analysis Method (Revised in 2014) 7-31 Determination of metasilicate (2) Colorimetric method using the molybdenum yellow method. Take 1.0 ml of the filtrate, add 9.0 ml of deionized water, 0.2 ml of hydrochloric acid (5+4) and 0.4 ml of 10 % ammonium molybdate solution, and stir. After 10 min, the absorbance of the solution was measured using a spectrophotometer (ASV11D, AZ-1) at a wavelength of 430 nm. The silicon dioxide content in the solution was determined by this method, and the metasilicate (H2SiO3) content in the solution was calculated using the following equation,

Elution characteristics of metasilicate into tap water
where W is the silicon dioxide (SiO2) content in the solution [mg/kg], and V is the volume of sample collected [ml]. The number of samples was set to five.    Table 1 lists the experimental conditions for moisturizing effect. First, the subjects were acclimatized to the experimental environment by maintaining a chair-rest state for 15 min. Then, the sample (50.0 g) was immersed in plain water (1.0L) at 40 °C for 24 h and bathed for 15 min in a solvent in which metasilicate was eluted. The area to be bathed was limited to the wrist. The amount of water in the skin was measured before and every 5 min after the warm bath and the changes were observed. Baths were also conducted in plain water at 40 °C and in water in which a chemical sodium silicate solution (No. 3, Kishida Chemical Co., Ltd.) was dissolved (metasilicate concentration: 400 mg/kg). A moisture meter (TR-3, Sato Shoji) was used to measure the skin moisture content. The principle of moisture content measurement is shown in Figure 2. The dielectric constant of water was remarkably higher than that of other substances, and the corresponding moisture content of the skin was indirectly calculated by measuring the capacitance of the stratum corneum. These values are quantified as relative values from 0 to 99 based on the trend of 200 people by age, which is the data acquired by the moisture meter. Skin moisture content was measured by the same person for all subjects in order to standardize the contact pressure of the electrodes to the skin. The skin moisture content was obtained five times per measurement, and the average value was used as the measurement data. Significant differences in skin moisture content between 5 and 30 min after warm bathing in each solution were evaluated by the corresponding t-test using SPSS (SPSS version 21; SPSS, Inc.) at a significance level of 5%.

Evaluation of moisturizing effect of rice hull
The purpose of this research and method of the experiment were explained to the subjects beforehand, and sufficient understanding was obtained. Figure 3 shows the elution amount of metasilicate in each solvent. The amount of metasilicate eluted from rice hulls was approximately 546 mg/kg in tap water at a water temperature of 40 °C, and the elution rate was approximately in the previous experiment, the amount of metasilicate eluted from rice hulls in ion-exchange water (pH = 7.6) was approximately 100 mg/kg at 25 °C and 400 mg/kg at 90 °C. These results confirmed that metasilicate was more easily eluted from tap water than from deionized water, even at relatively low water temperatures. It is said that the amount of metasilicate defined as a hot spring is 50 mg/kg, and that the amount expected to have high moisturizing effect is 100 mg/kg. Therefore, the use of tap water as a solvent is expected to lead to a simple extraction of metasilicate for use as a moisturizing component. Figure 4 shows the temporal changes in the rate of change in skin moisture content due to warm bathing. This represents the ratio of the change in the skin water content after and before the warm bath. The mean values of the moisture content of the skin of all the subjects were +3.29%, +0.188%, and +6.98%, respectively, when the solution was plain water, water in which a reagent was dissolved (hereinafter referred to as "chemical"), and water in which rice hulls were immersed (hereinafter referred to as "rice hulls"), and water in which rice hulls were immersed (hereinafter referred to as "rice hulls"),and remained higher than the values before the experiment until 10 min after the warm bath. However, when the solution was plain water or chemical, the water content of the skin was lower than the value before the experiment after 15 min and 20 min, respectively, and decreased to -2.62% and -3.19% after 30 min. On the other hand, the moisture content of the skin of the rice hull remained higher than that before the experiment (+1.67%) for 30 min after the warm bath. Figure 5 shows the Percent change in skin moisture content 5 and 30 minutes after warm bath. As in Figure 4, this represents the ratio of the change in skin moisture content after the warm bath to that before the experiment. There was a significant difference (p < 0.05) between the rate of change in the skin water content 5 and 30 min after the warm bath when the solution was plain water and chemical. On the other hand, when the solution was rice hull, there was no significant difference in the rate of change in water content between 5 and 30 min after the warm bath.

Moisturizing Effect of Metasilicate
There was no significant difference in the rate of change in the amount of moisture in the skin between 5 and 30 min after the warm baths in the water in which rice hulls were immersed, suggesting that rice hulls may have a moisturizing effect. However, the moisturizing effect could not be obtained in plain water in which the chemical sodium silicate solution (which was originally expected to have a moisturizing effect) was dissolved. There are two reasons for this. The first is the pH of the solution. When the pH of each solution was measured, the pH of the plane water and the water in which rice hulls were immersed was near neutral (pH = 6-8), while the bath water in which the chemicals were dissolved was slightly alkaline (pH = 9.5). Alkaline solutions dissolve proteins when they come into contact with the skin and eventually cause intracellular dehydration (8) . This effect is thought to cause the skin to dry more easily than in other solutions. Second, the experimental conditions were different from those used in the previous study. In the previous study, metasilicate and calcium ions were identified as the components of hot spring water that significantly promoted the formation of keratinocyte membranes among hot spring water from all over Japan (6) . In comparison with the experimental conditions of the moisturizing effect in this study, the hot spring water used in the experiment focused on the effect of metasilicate alone and therefore contained no components other than the chemical sodium silicate solution and tap water. On the other hand, hot spring water contains not only metasilicate but also many other substances such as bicarbonate ions and chloride ions. Therefore, it is thought that the components eluted from the rice hull affected the skin moisture content through interaction with metasilicate.
In the present experiment, the rate of change in the water content of the skin varied significantly. In particular, in the solution in which the chemical and rice hull were immersed, the water content increased in the water in which the chemical was dissolved but decreased in the water in which the rice hull was immersed and vice versa in some subjects. In this experiment, only ten subjects were tested; however, it is essential to conduct more experiments with more subjects to control the variation of data and to clarify the effect of rice hulls. Table 2 lists the components of the solution in which rice hulls were immersed, and the components contained in the sample with tap water as the solvent in 2.1 were determined using an ICP optical emission spectrometer (SPS3500 DD, HITACHI). The results confirmed that calcium, potassium, magnesium, and sodium were eluted from the solution in which rice hulls were immersed in addition to silicon. These components are contained in soil and irrigation water, and it can be said that they were absorbed by the rice plants during the growth process and leached into the tap water. Since these components are also contained in hot springs, the interaction with metasilicate eluted from the rice hull seemingly affected the water content of skin. The components eluted from the rice hulls are said to be affected by the environmental conditions of rice production, such as the silicon concentration in the soil and irrigation water, although the composition of the hulls differs little among rice varieties (9) . Therefore, changes in the composition of rice hulls due to the cultivation environment may affect the effects of rice hulls.

Conclusions
In this study, the elution characteristics of metasilicate contained in rice hulls into tap water were evaluated in order to examine the use of rice hulls as a moisturizing component. In addition, the moisturizing effect of metasilicate extracted from rice hulls was evaluated. The summary of the obtained result is given was shown as follow.
1) When tap water at 40 °C was used as a solvent, 546 mg/kg of metasilicate was eluted, which was much higher than the definition of mineral spring. 2) In the case of a warm bath in water in which rice hulls were immersed, the moisture content of the skin 30 minutes after the warm bath remained higher on average than before the experiment, suggesting that rice hulls may have a moisturizing effect on the skin. 3) The moisturizing effect of metasilicate may be obtained by interaction with other substances. 4) The composition of rice hull depending on the cultivation environment of rice may affect the effect of rice hulls.