Solubility Differences of Major Storage Proteins of Brassicaceae Oilseeds

Seeds of six commercially produced Brassica juncea, Brassica napus and Sinapis alba varieties representing high-glucosinolate condiment-type and low-glucosinolate canola-type were studied for solubility characteristics of the predominant seed storage proteins (SSPs). The non-protein nitrogen components such as glucosinolates, nucleic acids, betaine, choline and sinapine contributed 3.1–5.2% and 7.9–10.8% for the total N content of low- and high-glucosinolate meals, respectively. The cruciferin and napin which are the predominant SSPs of crucifers were purified from these seeds and used to confirm soluble protein types under the conditions provided. The napins were soluble between pH 2 and 4 but not the cruciferins. Strong alkaline pH brought both cruciferin and napin into solution. In general, the SSP solubility was increased due to the presence of NaCl or CaCl₂ salts in the medium. The effect of CaCl₂ on solubility was more positive than NaCl for all the seed types except S. alba at neutral and alkaline pH. Presence of salts indeed reduced solubility of S. alba SSPs at alkaline pH. The medium pH and salt ions and their ionic strength can be manipulated to achieve selective solubility of napin and cruciferin of Brassicaceae seed meals.