Background
Silflower (Silphium integrifolium) is a perennial oilseed crop, native to the central United States. It is under growing interest of domestication owing to its drought tolerance and competitiveness with annual weeds. Silflower seeds are rich in oil (15.2%) and protein (32.2%), suggesting the feasibility of producing silflower protein isolate (SiPI) following oil extraction. However, the abundant phenolics in silflower seeds gives them dark color and astringent tastes, both of which limit the practical use of SiPI in food systems.

Methods
Two strategies were developed and optimized to produce dephenolized SiPI. The first method (M1) encompassed extracting defatted silflower meal (DSM) at pH 6.0 with 2 M NaCl and treating the suspension with XAD-16 nonionic resin. The second approach (M2) involved dephenolizing DSM with 70% ethanol, extracting the residue at pH 9.0, and precipitating the proteins with 70% ethanol. A conventional method of extracting at pH 9.0 followed by isoelectric precipitation was employed as the control. Products were analyzed for protein yield, purity, total phenolic content (TPC by Folin-Ciocalteu method), color (L*a*b* values), subunit composition via SDS-PAGE, and thermal properties by differential scanning calorimetry (DSC).

Results
DSM exhibited high TPC of 542 mg GAE/100 g. The control method led to the highest yield (25%) but imparted SiPI a dark color (L* = 33.1) and a TPC level of 333 mg GAE/100 g. Method M1 lowered the yield to 18%, but it achieved nearly complete removal of phenolics (TPC = 0.06 mg GAE/100 g) and concomitantly increased the L* value to 80.3. Method M2 resulted in higher TPC (2.70 mg GAE/100 g) and lower L* value (72.2) than M1. However, this method gave a better yield of 23% and produced a crude extract that recovered 81.1% of phenolics from DSM. All SiPI’s had protein content of >90%. SDS-PAGE revealed that SiPI consisted mainly of a protein resembling helianthinin in sunflower seeds, and an albumin fraction with lower abundance. DSC measurements indicated peak thermal denaturation of SiPI at 93.6 °C.

Significance
This study demonstrates the feasibility of producing protein isolates from silflower, a novel specialty crop with attractive growth traits. It addresses a key hurdle to their application in food industry, namely, the high phenolic content. Moreover, it provides an integrated approach to recovering both the health-benefiting phenolics and nutritious proteins from silflower seeds besides oil.

James kenar

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