Physical properties
Silflower seeds are flat, mostly ovate or cordate in shape, with wings on the periphery (Fig. 1a). The seed size varied considerably. The average seed length was 15.55 mm and ranged from 11.54 to 20.75 mm (Table 1). The average seed width was 9.42 mm and varied from 4.61 to 11.76 mm. The wing accounted for about 55% of the seed’s length and width. The seed averaged 1.34 mm in thickness and ranged from 0.92 to 1.63 mm. The hull covering the kernel is 0.16 mm thick or 12% of the seed’s thickness. Given the wide variation in seed size, seed cleaning and grading can be performed by using a combination of screen sizes and configurations. Cleaning and grading remove inferior seeds and other dockage that will reduce the quality of the final products.
Seed weight is an indicator of seed quality. This is determined from the weight of 100 or 1,000 seeds depending on the size of the seeds. Major crops like corn, soybean, wheat, and others have established standard test weights (bulk density) specified in pounds/bushel (USDA, Agricultural Marketing Service). For silflower, the seed weights and densities may be utilized as indicators of the progress of the crop development. A thousand silflower seeds weighed 23.84 g (Table 1). This is within the range of 100 seed weight of 2.2 to 7.9 g, but well below the 4.4 g and 5.2 g average obtained from two sunflower germplasm studies (Mogali and Virupakshappa, 1994; Sudrik et al., 2014). Seed bulk density varied from 145.27 g/L (loose) to 189.58 g/L (tapped). Tapping reduced the volume by 23.3%. This considerable shrinkage in volume is important in sizing bin capacity for storage or transport. The true density of the seeds, however, is 7.2 and 9.5 times higher than the tapped and loose bulk densities, respectively. With low bulk density and high true density values, the porosity of the bulk seeds ranged from 86.2 to 89.4%. The presence of seed wings is largely responsible for the significant interparticle void volume. From previous work on the likewise perennial milkweed seed, which have the same shape as silflower seeds, dewinging reduced the seed volume by 46% and increased the bulk density by 63% (Evangelista, 2007). Porosities of flat seeds with no wings have been reported: pumpkin - 65.73%, luffa - 58.23%, and watermelon - 39.14 to 51.68% (Joshni et al., 1993; Aliyu et al., 2017; Araki et al., 2018).
Manual dehulling of the seed also removed the seed coat (Fig. 1b). Like the seeds, the teardrop-shaped kernel also varied widely in size. The kernel length ranged from 5.44 to 8.49 mm, width from 2.81 to 5.63 mm, and thickness from 0.72 to 1.28 mm (Table 1). The kernel accounted for 56.14% of the seed weight. The true density of the kernel (1,198.8 g/L) is lower than that of the hull (1,581.5 g/L). This observation is also true for pumpkin seed kernel (Joshi et al., 1993), and watermelon seed kernel (Teotia and Ramakrisna, 1989). However, because of its fibrous nature, loose silflower hulls are light and can be compressed to reduce their volume.