Removal of seed coat mucilage
Camelina seeds are oblong in shape and comparatively small in size (Figure 1A) with average dimensions of 2.2 mm in length and 1.2 mm in width. Specialized cells in the seed coat of crucifers, such asArabidopsis thaliana, Capsella bursa-pastoris andLepidium flavum (Soukoulis et al., 2018), produce extracellular, pectinaceous mucilage that absorbs water and expands and forms a gel around the seed coat. Mucilage aids in fruit and seed dispersal, defends against pathogens, enhances water uptake during germination and acts as a barrier to water and oxygen diffusion into inner tissues to prevent seed germination during undesirable environment conditions (Soukoulis et al., 2018). Interference of seed coat mucilage with protein extraction and recovery has been reported for flax (Wanasundara and Shahidi, 1997) and yellow mustard (Wanasundara et al., 2012), and the same was expected for camelina as the seed contains about 6.7% mucilage by weight.
Li et al. (2016) showed that camelina mucilage is composed of galactose (58.1%), glucose (25%), rhamnose (11.6%) and xylose (5.2%), whereas arabinose, glucose and xylose are more abundant in flax, yellow mustard and chia mucilage, respectively (Soukoulis, et al., 2018). Viscozyme®, an enzyme complex derived from Aspergillus spp. containing arabinase, cellulase, β-glucanase, hemicellulase and xylanase, was more effective in breaking down camelina seed coat mucilage than other carbohydrases (pectinases and cellulases) or mild alkaline (NaHCO3) solutions (data not shown). The optimum conditions for Viscozyme® pre-treatment for camelina seeds were 3 h of soaking with 0.1 mL enzyme/g seed with vigorous stirring to remove/reduce seed coat mucilage. Microscopic examination of soaked, untreated seed showed water-absorbed mucilage as a halo around the seed (Figure 1B) compared to dry, non-hydrated seed (Figure 1A) or hydrated seed after enzyme treatment (Figure 1C). Although non-enzymatic approaches such as heating of hydrated camelina seed (as slurries) to temperatures up to 85°C to reduce slurry viscosity have been used, the possibility of partial denaturation of proteins at ~50°C during hot-pressing of camelina seed, as has been reported by Boyle et al. (2018), indicates the importance of low temperatures (< 50°C) during seed pre-treatments to minimize heat-induced alterations in proteins.