Physical and mechanical properties of lotus kernel for plumule removal of fresh lotus seeds

This study aims to reveal the effect of the soaking time of the lotus kernel on the quality of the plumule removal in the fresh lotus seeds. The physical and mechanical properties of the lotus kernel were investigated under different soaking times. The "Taikong 36" variety lotus seed was chosen as the research object. Firstly, a series of experiments were carried out to establish the relationship between soaking time and moisture content of the lotus kernel. The moisture contents of the lotus kernel at soaking times of 0-180 min were obtained as 60.75%, 62.06%, 62.54%, 62.85%, 63.08%, 63.23%, and 63.45%, respectively. Secondly, the geometric parameters of the lotus kernel and lotus plumules under different soaking times were measured using an vernier caliper. It was found that the maximum short diameter and length of the lotus kernel, and the length and the maximum width of the pointed fissure at the kernel tip increased with an increasing soaking time, as did the length, root diameter, maximum thickness, and maximum width of the lotus plumule. Thirdly, the hardness of the lotus kernel under different soaking times was measured using a Universal TA texture analyzer. The results showed that their hardness gradually decreased, as the soaking time of the lotus kernel increased. Once the soaking time increased from 0 to 180 min, the hardness of the lotus kernel decreased from 8.76 to 7.83 N. Mechanical properties tests of lotus kernel under different soaking times (including elastic modulus test, compressive strength test, and the force measurement for removing lotus plumule) were conducted using the UTS-4.5KN/T162 universal testing system, in order to obtain the stress-strain curve and load-displacement curve of the lotus kernel material. The relationship between physical and mechanical performance indicators of lotus kernels and their soaking time was established using regression analysis. It was found that the elastic modulus, compressive strength of the lotus kernel, and the force of removing lotus plumules decreased with the increase of soaking time. Once the soaking time increased from 0 to 180 min, the longitudinal compressive modulus of lotus kernel decreased from 5.44 to 5.01 MPa, while the compressive strength decreased from 5.15 to 4.75 MPa, the lateral compressive modulus of lotus kernel decreased from 4.48 to 3.74 MPa, and the compressive strength decreased from 4.07 to 3.54 MPa, the force of removing the lotus plumules using a solid punch needle decreased from 18.26 to 16.16 N, and the force of removing the lotus plumules using a hollow punch needle decreased from 14.54 to 12.31 N. Finally, the air extrusion plumule-removing machine of fresh lotus seed was developed to conduct the plumule-removing experiment of lotus kernel with different soaking times. It was found that the soaking time of the lotus kernel had a direct impact on the plumule complete removal rate, kernel breakage rate, and plumule intactness rate. When the soaking time for the lotus kernel was relatively short, there was also the smaller length and width of the fissure at the tip of the lotus kernel, indicating that the lotus plumules could not squeeze out of the compressed air. The lotus kernel presented the higher hardness, elasticity modulus, compressive strength, and the force of removing the plumule, which can easily cause the breakage of the lotus kernel and plumule. Therefore, the plumule complete removal rate and the plumule intactness rate of the lotus kernel were lower, resulting in a higher lotus kernel breakage rate. The length and width of the fissure at the tip of the lotus kernel also increased, as the soaking time increased. The hardness of the lotus kernel, elastic modulus, compressive strength, and the force of removing the plumule decrease with the increase in soaking time, making it easier for the lotus plumule and lotus kernel to completely separate. Furthermore, the lotus kernel breakage rate remained relatively low with the increase in the plumule complete removal rate and plumule intactness rate. If the soaking time was too long, there was an increase in the moisture content, the breakage rate, as well as the length and width of the fissure at the tip of the lotus kernel, as the soaking time increased. Although compressed air can easily extrude the lotus plumule for the higher plumule complete removal rate, the hardness, elastic modulus, compressive strength, and force of removing the lotus plumule continued to decrease, with the increase in soaking time. The breakage of the lotus kernel and plumule was easily caused by the cracking at the interface between the lotus kernel and embryo, thus increasing the lotus kernel breakage rate with the low intactness rate of the lotus plumule. The optimal removing plumule of lotus kernel can be achieved when the soaking time was 120 mins (the plumule complete removal rate was 94.67%, the plumule intactness rate was 84.00%, and the lotus kernel breakage rate was 3.33%). The finding can provide the basic data support to improve the lotus kernel quality in the efficient and low-damage removal processing of plumule in fresh lotus seeds.