The test results obtained in this study are reported in three main parts. The first part discusses the optimum parameters of the HANDY. The second part describes the results of the experiments which are designed to compare the effectiveness and applicability of BI and BS. BI and BS are utilized in evaluating the mechanical crushing resistance of kernels. The third part analyzes the applicability of the HANDY to evaluate the threshing quality of maize kernels.

## Working condition of HANDY

During the testing, a slight irregular vibration of the frame is observed. The impact part can generate a small noise though it is fixed to the ground by the rubber casters. The operation of the HANDY is not as laborious as traditional testers for measuring maize crushing resistance. Great flexibility in adjusting operation parameters is achieved with the HANDY. However, the device needs to improve its intelligence in picking un-sieve broken kernels.

## Test for establishing optimum operating parameters of HANDY

## Peripheral speed of the centrifuge disc

In order to ensure the device to have a stable performance and repeatable results, the tests using the straight type centrifuge disc are conducted at three peripheral speeds (23.6 m/s and 28.3 m/s and 33 m/s correspondings to 1500 r/min, 1800 r/min, and 2100 r/min rotating speed). The speed range has been determined by the baseline test result. The BI at three rotating speeds is depicted in Fig.4a. The BI measured with three rotating speeds (1500 r/min, 1800 r/min and 2100 r/min) are 0 -13.7%, 1%-34.7%, 7.6%-51.7%, respectively. The average BI is 2.8%, 8.5% and 22.7%, respectively. Statistical analysis of the BI shows a remarkable consistency in the results when the speed is different: the three curves of BI present a similar trend. Specifically, the BI decreased with the moisture content increased under the overall, which is almost in long-tailed distribution. In order to obtain the BI with smaller variability, both the variation of coefficient and distribution dispersion under different rotating speeds are discussed.

Fig.4b shows the coefficient of variation of the BI at different rotating speeds. The average coefficients of variation of the results are 56.6, 16.3, 19.1 when rotating speeds are 1500 and 1800 and 2100 r/min, respectively. Note that the BI is close to 0 when the rotating speed is 1500 r/min and the kernel moisture is more than 21%. In this case, small numerical changes of the BI can also have a significant effect on the coefficient of variation, resulting in low repeatability. This is the reason why the coefficient of variation of BI is larger when the rotating speed is 1500 r/min than others. As a result, the speed at 1500 r/min is too low for the HANDY operation. The coefficient of variation of the BI obtained at the speed of 2100 r/min is worse than those at the speed of 1800 r/min. This also shows that when the HANDY works at the rotating speed of 1800 r/min exhibited better repeatability and higher precision in discovering the kernels with different level of mechanical crushing resistance.

Fig.4c shows the distribution of the BI at different rotating speeds. When the rotating speed is 1500 r/min, 50% of the BI is less than 1.5%, which indicates that the BI has significant uneven distribution. This further shows that the HANDY operated at this speed causes less measurable damage to kernels. When the rotating speed is 1800 r/min, 57.1% of the BI is within 3%-7%, 50% of which are within 2%-3%. The BI varied among different-moisture intervals, which ensure the continuity of BI. The HANDY works at the rotating of 2100 r/min can produce a substantial amount of damage to kernels which have various crushing resistance.

In addition to the above discussions, the time to pick broken kernels also needs to be considered. When the rotating speed is lower than 1500 r/min, the impact energy cannot be transmitted from the centrifugal disc to the kernels sufficiently. As a result, few kernels are broken completely and it is hard to pick them out. On the contrary, when the rotating speed is higher than 2100 r/min, it will take about additional 2 minutes to pick the broken kernels out. It even produces massive maize flour or juice. However, when the rotating speed is about 1800 r/min, a number of the kernels are broken with obviously broken characteristics and then easy to be picked. Moreover, the time to pick broken kernels is acceptable. Thus, when the speed is around 1800 r/min, the test results are conducive to evaluate the crushing resistance of kernels.

## Type of the centrifuge disc

From a machine design perspective, the design objective of the centrifugal disc should make all the kernels be subjected to identical impact, and produce little random splatter of kernels. From the angle of kinematics, the type of discs can affect the magnitude and direction when kernels departing from the discs [26]. Thus, the objective of the analysis is to find an optimal centrifuge disc type. Three types of centrifuge disc (straight, curved and oblique) are selected and designed in this study (Fig.5). Tests are conducted at rotating speeds of 1800 r/min by the HANDY.

Fig.6a shows the value of BI when using different types of centrifugal discs. For all the tested maize varieties, the BI for different types of centrifuge discs (straight, curved and oblique) are within 1%-34.7%, 1.2%-36.6%, 1.1%-34.2%, respectively. The BI decreases with the increase in moisture content. Note that, when the moisture content increases to about 23%, the BI decreases to a minimum. When the moisture content continues to increase, the BI changes within a small range.

The coefficient of variation and the distribution dispersion of the BI measured with different centrifuge disc types are discussed. As Fig.6b shows, the curved disc produces the BI with a lower coefficient of variation compared to the straight and oblique disc. The average coefficient of variation for the straight and oblique disc is 1 and 1.7 times greater than that shown by the curved disc, respectively.

Fig.6c shows the distribution of the BI for a different type of centrifugal discs. The SPSS 23.0 is used, the influence of centrifugal disc type on BI is analyzed through two methods: descriptive statistics analysis and difference testing. There is no significant difference in data distribution among the three centrifugal discs. This means that for the kernels with the same crushing resistance, all centrifugal discs can cause a considerable amount of damage to them. However, the results of the different tests show that more sensibility for the same test kernel samples by using the curved centrifugal disc (Fig.7). This further shows that in comparison with the straight and oblique centrifugal discs, the curved type has superior sensitivity and is suitable for assessing to mechanical crushing resistance of maize kernel. It is more effective to distinguish the maize varieties with a small difference in crushing resistance. Therefore, it is appropriate to choose the curve type as the optimum centrifugal disc.

## Repeatability of results

The plots of BI moisture content for the maize kernel (Fig.8) show that the graphs for the five replicate tests are similar. The maximum difference of BI within any set is 1.2% which shows that the HANDY can produce repeatable results (speed: 1800 r/min, disc type: curved type).

## Breakage Susceptibility test results

Fig.9a shows the results of the Breakage Susceptibility tests. As expected, the results follow the normal breakage behavior of kernels. That is the BS of kernels decreases as the moisture content increases. When the BS is at maximum value, the maize kernels show higher mechanical crushing resistance. The change rules of BS of maize kernels obtained by HANDY are similar to those obtained by using the Stein Crush resistance tester, Wisconsin Tester, and Centrifugal Corn Crush resistance tester in previous studies [27-30].

## HANDY test results

The rotating speed of HANDY is 1800 r/min with the curved centrifugal disc. The BI obtained for the same maize kernels, As shown in Fig.9b, the change rules of BI obtained by HANDY tests and BS obtained by the Breakage Susceptibility tests are similar. For both Breakage Susceptibility tests and HANDY tests, the moisture content of the kernel is used as a variable. With the increasing of moisture content, the overall changing rules of BI and BS can be divided into three stages: stage I, stage II and stage III. The moisture content ranges of the three stages are 14%-18%, 18%-25%, and 25%-31%.

For stage I (14%-18%), stage II (18%-25%), and stage III, both BS and BI drop sharply, drop slowly, and keep stable with the increase of moisture content, respectively. For maize kernel materials, the lower the moisture content, the higher the hardness and brittleness [14]. Therefore, the mechanical characteristics of kernels are brittle and hard in stage I, which makes it more likely to break into small pieces under the impact forces. In stage III, the kernels show plasticity, high elasticity and flexibility. In stage II, the mechanical characteristics of kernels are between that in stage I and stage III.

An obvious difference between BS and BI in stage III is observed. Specifically, the results of BI increases to the maximum value and then decreases with the further increases in moisture content [31-32]. In contrast, the BS in stage III is reduced close to zero. Consider the energy absorption capability, the wet kernels are higher than dry ones, the kernels achieve greater flexibility at high moisture, thus making the kernels absorb more deformation energy before crack [33-34]. It cannot be neglected that numerous kernels are split into parts but still connected by a seed coat. As a result, those broken kernels connected by seed coat cannot pass the circular sieve (12/64 inch in diameter), resulting in the BS reduced close to zero in stage III (as shown in Fig.9a). However, in the HANDY tests, BI shows a bell-shaped curve in stage III (as shown in Fig.9b). As a result, compared with the BS, the BI can effectively reflect and evaluate the mechanical crushing resistance of the kernels at this stage.

## Results of mechanical threshing tests

Generally, mechanical damage is induced by impact during harvesting which can debase the quality and shortens the storage period of maize kernels [35-36]. In order to prove that HANDY can be used to predict the impact damage severity of maize during harvesting, the mechanical threshing test of maize ear is carried out. The HANDY test, meanwhile, is conducted at the optimum parameters.

The results of BR and BI are shown in Fig.10a. For all the tested hybrids, the ranges of the BR and BI are 1.3%-13.5%, 1.1%-34.2%, respectively. Fig.10a indicates that the relationship between the BR and the BI is diverse in three moisture content ranges. When the moisture content is less than 18%, both BR and BI decreased with the increase of moisture content, but the decreasing rate of BI is higher than BR. When the moisture content is 18%-25%, the BI continuously decreasing but the BR is increasing. When the moisture content is more than 25%, the change regularity of BR and BI is similar and increasing overall. In aggregate, with the increase of kernel moisture content, the broken rate first decreased and then increased, which is close to the result obtained before [37].

Fig.10b shows the relationship between the BR and the BI which eliminates the information of kernel moisture compared with Fig.10a. The plot procedure is as follows: First, select a single moisture content of kernel on both curves of BI and BS (denoted by the straight line in Fig.10a). The intersection of this straight line with the BI curve becomes the x-coordinate and the intersection of the straight line with the BS curve becomes the y-coordinate of the Fig.10b. Plot this point in a separate graph (point X in Fig.10b), which represents the relationship between the BR and the BI and no longer possesses a kernel moisture element. Repeat this process for each kernel moisture to construct the remainder of Fig.10b. As a result, the BI (independent variable) is used to generate a subsection linear regression functions that could be used to predict BR (dependent variable) achieved using the HANDY. The equation is:

The R2 of the regression model at three moisture content is 0.86, 0.87 and 0.87, respectively, which further illustrates the BI prove capable of explaining on average about 86.7% of the BR of maize kernel in threshing. These values indicate that the subsection linear regression model may be considered satisfactory, however, it is necessary to check the linear regression model in Eq. (4) to evaluate whether it can provide an acceptable approximation or not. The approximation precision level of the linear regression model is evaluated through the calculation of relative errors between the results obtained from the HANDY and threshing tests. The evaluation results are given in Fig.11. As expected, the results show that the BR predicted values and the measured ones are in good conformity at three kernel moisture ranges (Fig.11a). As shown in Fig.11b, it’s seen that the range of relative error is calculated between 2% and 32.8%. The average relative error for three moisture range is 11.1%, 23.7% and 17.86%, respectively. A relative error of less than 20% is observed for 15 out of 24 experiments (i.e. for 62.5% of experiments). As a result, considering the variability of physiological characters in maize kernel the linear regression model the coefficient of determinations of these regression equations are satisfactory.

## Evaluation of maize kernel crushing resistance

In this study, BI is found to have remarkable correlativity with crushing resistance and be treated as an index to represent crushing resistance for maize kernel. It is not negligible that different cultivars expressed different sensitivity to impact force, this is related to their different capability of crushing resistance. Thus, the BI and BR of maize kernel under different cultivars and test parameters are discussed in this section. 21 test varieties are classed into three groups by different moisture contents. The BI is tested by HANDY at the decided working parameters (curved, 1800 r/min). The BR of the test verities is estimated using Eq.4. The breakage data are shown in Fig.12.

Five maize cultivars are identified with higher damage resistance among 21 tested candidate varieties. Cultivars 6 (DH618) at group one, 7 (XD20) and 8 (XY335) at group two,16 (ZD909) and 21 (DH605) at group three have the lowest BR of 0.9%, 1.7%, 1.9%, 3.0% and 3.2%, respectively. Those cultivars have better damage resistance than the others in the same group when suffering the same load. Breakage data of selected cultivars under different testing conditions are shown in Fig.13.

As shown in Fig.13, when using the curved type centrifugal disc, with the increase of rotation speed, the BI increased obviously. It can also be found that the increments of BI of varieties 6 (DH618), 7 (XD20), and 8 (XY335) are similar under different speeds. For cultivars 16 (ZD909) and 21 (DH605), the BI of the oblique centrifugal disk is greater than that of the curved type.