Sunflower hulls are the by-product of the dehulling of sunflower seeds before they are used for oil extraction or as bakery ingredients (AAFC, 2007). Sunflower seeds contain about 20-30% hulls that are often removed before oil extraction due to their deleterious effects on oil presses and because they reduce the quality of both oil and meal (Kartika, 2005). Decreasing the hull content by 1% improves pressing capacity by 2.5%. A well-managed dehulling process yields seeds with 8-12% hulls remaining on the kernels (Campbell, 1983). 100 kg of seeds having 25% hulls yielded 16.5 kg of hulls in an experiment where 66% of the hulls were removed (Carré, 2009).
Dehulling is done after cleaning the seeds and drying them down to 5% moisture, which facilitates kernel-hull separation (Kartika, 2005). The usual process consists in cracking the seeds by the mechanical action of centrifugal or pneumatic shellers. The resulting blend is winnowed to separate the hulls from the kernels. In new sunflower varieties, breeders have enhanced oil content at the expense of hulls, resulting in seeds with thinner hulls that are difficult to remove: these varieties remain undecorticated and do not yield sunflower hulls (Carré, 2009; Grompone, 2005; Campbell, 1983).
Sunflower hulls are light in weight and bulky, and are, therefore, costly and impractical to transport. They are burned as fuel to power oil mills but only half of the hulls available can be used on-site for energy production. The remaining half has to be transported off-site, to provide energy or for other purposes, such as composting, bedding material, or as a low-quality roughage for livestock (Dorrell et al., 1997; Carré, 2009).
Sunflower screenings are a mixture of variable amounts of residues including hulls, groats, lightweight or broken seeds, heads, sclerotia bodies, weed seeds, chaff, joints, straw, elevator dust and floor sweepings (Lardy et al., 2009).