Seed microstructure and meal chemical composition
TEM microscopy of camelina seed cotyledon cells showed distinct separation of OBs from protein-rich areas or protein storage vacuoles (PSVs) that were, more or less, spherical in shape and evenly distributed within the cytoplasm (Figures 1D and E). The discrete dark areas inside PSVs may be globoids containing phytic acid crystals as reported for other seeds (Weber and Neumann, 1980). Camelina OBs had an average diameter of 0.68 μm with a packing density of ~3 OBs/μm2. The OBs of Brassica seeds, including rapeseed, have diameters between ~0.2 and 3.0 μm (Katavic et al., 2006). The comparatively small size of camelina OBs and the high packing density may facilitate larger oil storage capacity within the cotyledon cells. The distinct membrane of camelina OBs (Figure 1E) may prevent coalescence and provide steric hindrance or electrostatic repulsion that may stabilize the OBs similar to other oil-bearing seeds (Tzen, 2012). Mechanical pressing during oil extraction may rupture these membranes and allow oil leakage. Subsequent extraction with non-polar solvents, such as hexane, removes any oil that remains trapped within the cellular matrix. Cellular components remaining after oil extraction, including organelle membranes, cell wall membranes and proteins, collectively make up the spent meal.
The mucilage-reduced, oil-free meal contained 51.3% protein and 6.8% ash (mineral) (Table 1); these levels were higher than those reported for camelina pressed cake meal [~40-45% protein (Boyle et al. 2018; Li et al., 2014) and 5% ash (Zubr, 1997)]. The levels of phytate and phenolic compounds, which are considered anti-nutrients in many seed -derived meals (Wanasundara, 2011), were at 6.1% and 1.0% (as sinapic acid equivalents), respectively (Table 1).