Mechanical Properties of Grains Sorghum Subjected to Compression at Different Moisture Contents

This study aimed to evaluate the influence of drying at different temperatures on the mechanical properties of grains of grain sorghum subjected to compression at the natural rest position. Grains dried at temperatures of 60, 80 and 100 °C with different moisture contents (0.515; 0.408; 0.315; 0.234; 0.162 and 0.099 (d.b.)) were subjected to uniaxial compression between two parallel plates, applied at their natural rest position, at a rate of 0.001 m s. The force required to rupture in grains of grain sorghum increased as their moisture contents decreased, with values of 47.17 to 78.44 N, 61.81 to 69.66 N and 52.07 to 70.89 N for the temperatures of 60, 80 and 100 °C, respectively. The compression force required to deform grain sorghum decreased with the increment in moisture content, and the proportional deformation modulus increases with moisture content reduction. Within the studied range of moisture content, the values were 87 × 10 to 354.99 × 10 Pa, 132.63 × 10 to 465.98 × 10 Pa and 80.18 × 10 to 429.85 × 10 Pa for the temperatures of 60, 80 and 100 °C, respectively.


Introduction
Along the production process, grains are subjected to strong impacts.Steps like harvest, handling, transport and storage can cause damages such as: cracks, fissures and breaks, resulting in a series of problems in the subsequent steps (Amaral, Dobis, & Carvalho, 2018).Damaged grains cause greater deterioration during storage because they facilitate the action of secondary insects and microorganisms.In addition, dry matter loss is intensified and, consequently, there is loss of nutritional quality, viability and storage potential of the product.
Compression test is a simple experiment where a convex body is compressed between two parallel plates, which is carried out to obtain force-deformation data.This test is performed to obtain parameters which predict the response of the material when subjected to a certain load (Goneli, 2008).Knowing the resistance of grains is indispensable to reduce possible damages and maintain product quality until the processing.Thus, studying mechanical properties becomes necessary to know the behavior and resistance of each product under certain condition.
Several factors can influence the mechanical properties of grains, including drying temperature, moisture content, rigidity and region of the grain where the force is applied (Mohsenin, 1986).Fernandes, Corrêa, Diniz, Leite, and Vasconsellos (2014) observed that the compression force required to deform wheat grain decreases with the increment in moisture content and varied between 139.8 and 21.4 N. Thus, grains with higher moisture contents offer lower resistance to compression.Ribeiro, Corrêa, Furtado, Goneli, and Resende (2007), evaluating the mechanical properties of soybean grains at different positions, observed that the compression force required to deform the grain by 2 mm decreased with increasing moisture content.For the position P 1 (natural rest position), the mean force varied from 302.72 to 72.5 N; for the position P 2 (on the hilum), from 127.64 to 41.19 N; and for the position P 3 (vertical position), there was variation from 110.86 to 21.72 N, as a function of different moisture contents.
Thus, this study aimed to assess the influence of drying at different temperatures on the mechanical properties of grains of grain sorghum subjected to compression at the natural rest position.jas.ccsenet.

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Results
Radiuses of curvature (R, r) are obtained based on physical dimensions.Thus, grain size has great influence on these estimates.Such variation observed in the data of radiuses at different moisture contents may have been caused by heterogeneity in the shape and size of sorghum grains.Oliveira et al. (2017), evaluating the mechanical properties of baru fruits, and Fernandes et al. (2014), working with wheat grains, obtained similar results.
Table 3 presents the values of total deformation (D), elasticity modulus (E), maximum compression force (F), tangent and secant at the different moisture contents and drying temperatures evaluated.It can be noted that, at all drying temperatures, the values of the respective coefficients analyzed tended to increase with moisture content reduction.In addition, the lowest values in general were found at temperature of 60 °C, indicating that high drying temperatures may cause alterations in the characteristics of hardness and elasticity of the grains.
At temperatures of 80 and 100 °C, the values of maximum secant showed the same trend as those of maximum tangent, but with lower magnitude.According to Goneli (2008), the maximum secant, besides indicating the maximum value of the force-deformation curve where the tangent passes through the origin, also indicates the point at which the curve begins to drop, which represents the moment in which the behavior of the product changes from elastic to plastic.For Henry, Su and Zhang (2000), the maximum value of the secant represents the ability of the material to resist to deformation before the maximum elastic value is found.
For all drying temperatures evaluated, there was an increase in rupture force with moisture content reduction, as presented in Figure 4.These values ranged from 78.45 to 47.18 N for temperature of 60 °C, 66.66 to 61.82 N for 80 °C and 70.90 to 52.08 N for 100 °C.
Resende, Corrêa, Ribeiro, and Neto Figueiredo (2013), evaluating rough and dehulled rice grains, observed similar behavior, where the reduction in moisture content from 0.30 to 0.12 (d.b.) resulted in increment in rupture force from 48 to 79.5 N for rough rice and 37.2 to 70.6 N for dehulled rice grains.Such variation in the force required to cause rupture in the physical structure of the grains is directly associated with the moisture content of the biological material, as well as with its physical resistance.

Table 1 .
Values of the physical dimensions of sorghum grains, length (L), width (W) and thickness (T) at temperatures of 60, 80 and 100 °C Table2shows the mean values of radius of curvature of the grains of grain sorghum for different drying temperatures and moisture contents used in the determination of the proportional deformation modulus.Radius values oscillated among the different moisture contents, not showing a clear behavior for the different drying temperatures.

Table 2 .
Mean values of radiuses of curvature of the grains of grain sorghum (m) for each moisture content(%  d.b.)