CONICET | Buscador de Institutos y Recursos Humanos

The traditional sorghum-milling involves a decortication step, which removes the bran mainly. After that, the endosperm is reduced by hammer-milling. Recently, roller-mills plus a vibrating-screen sieving devices are used, and a pre-conditioning is helpful to improve the extraction. Therefore, sorghum grain hardness is one of the most important parameter to assess the dry-milling quality. The mechanical-properties of biological-materials obtained by nano-mechanical tests are very useful to understand how nanoscale structures and the resulting set of these structures derive in the macroscopic features of the systems. The objective of this study was to evaluate the changes of mechanical-properties of sorghum grains at different pre-moistening conditions by nano-indentation AFM test. The surface of the whole sorghum grains and their cross-section areas at two different moisture conditions (10% and 15%) were studied. High and low tannin and white sorghum cultivars were tested. The cutting-seeds were cut transversely and polished in a microtome. The Bio-AFM (Bioscope-Catalyst, Bruker, USA) and a DNISP diamond Bruker cantilever were used in QNM mode. The deflection-cantilever was 100 nm (load: 21.6 μN). In all cases 3 seeds were indented and 150 indentations in each seed were performed. Force-distance curves were collected. Young´s-modulus (E) was estimated with the software Nanoscope Analysis v.1.40 using Hertz?s model. The E values range of the whole seeds pericarp pre-moistened at 10 and 15% was 905-1522 MPa and 1134-1608 MPa, respectively. The E range of the pericarp and endosperm of the cutting-seeds at 10% was 1628-2630 MPa and 1278-1432 MPa, respectively; and at 15% was 1476-1747 MPa and 1471-1684 MPa, respectively. Regarding to the whole seeds evaluations, the sorghum-cultivars did not present significant differences in the pericarp resistance at the same moisture content. Also, the change in the moisture content of the seeds did not modify the pericarp stiffness significantly in any sorghum-cultivars. In relation to the cutting-seeds analysis, the pericarp showed higher E than endosperm at both moisture conditions, thus, the pericarp had a harder structure than the endosperm. The sorghum-cultivars did not show significant differences in the pericarp and endosperm stiffness at the same moisture content. However, the white sorghum cultivar showed that high water proportions resulted in stiffer endosperm. The modification in the pre-moistening conditions affected the nano-mechanical properties of low tannin sorghum pericarp, a reduction in the E value was observed. In general, comparing the pericarp E values between whole seeds and cutting-seeds, it was observed that the pericarp stiffness of the whole seeds were lower than the cutting-seeds pericarp. The results obtained suggested that high moisture produced stiffer endosperms in white sorghum cultivar, however, no significant changes were found in the pericarp in the moisture range tested. This method thus appears an interesting alternative to characterize any type of grains.