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.