Abstract
A rapeseed has been the second largest oil crop in the world, especially in Canada and China. The main by-products after oil extraction, rapeseed meals have been normally used for animal feed or agricultural fertilizer. However, a low economic benefit has resulted from rapeseed meals with plenty of proteins. Among them, the protein from the rapeseed cold-pressed cake can be used to develop high value-added products, due to the light color and low degree of denaturation. The rapeseed protein also showed better nutrition for the body health of humans, due to the nutritionally balanced amino acid composition and high S-containing amino acid. Conventionally, the alkali-solution and acid-isolation extraction are widely used for protein extraction, where a strong alkali or acid can be selected to adjust the pH, but the water quality can deteriorate. Fortunately, the alkaline electrolyzed water (a by-product of acid electrolyzed water) has been served as an excellent solvent, because of the high reducing power, small molecule cluster structure, low surface tension, and strong permeability. Moreover, it is easy to gradually return to the normal water after use. In this present study, an electrolyzed water extraction of protein was optimized for the cold-pressed cake of double-low rapeseed in a highly efficient and environmentally friendly way. A one-factor Box-Behnken test was also designed to measure the extraction rate, structural and functional properties of rapeseed protein, further to compare with the conventional alkali dissolution and acid precipitation. The results showed that there was the most influential of pH values on the extraction rate, followed by the extraction temperature, solid-to-liquid ratio, and extraction time. An optimal combination of parameters was also achieved, where the extraction temperature was 45 ℃, the pH value of alkaline electrolyzed water was 11.5, the solid-to-liquid ratio was 1:10 (g/mL), the extraction time was 60min, and the extraction rate of rapeseed protein was 59.34%. An excellent extraction performance of alkaline electrolyzed water was obtained with the increasing extraction rate from 50.72% to 59.34% (P<0.05), the improved solubility (from 30.62% to 38.40%, P<0.05), and the high absolute value of zeta potential (from 10.17 mV to 11.90 mV, P<0.05), compared with the alkali-dissolved acid precipitation (using the ultrapure water extracted rapeseed protein, URP). Likewise, the numberr of sulfhydryl groups and free sulfhydryl groups increased from 20.43 to 22.64 μmol/g (P<0.05), and 15.00 to 16.88 μmol/g, (P<0.05), respectively, compared with the URP. The Fourier transform infrared spectrum showed that the random coil and β-turn decreased significantly, whereas, the β-sheet increased, compared with the URP. The surface tension at the air-water interface was reduced, because the surface hydrophobicity increased from 40.70 to 50.92 μg (P<0.05), where the particle size was reduced, but the fluorescence intensity increased significantly. Furthermore, the emulsifying activity, foaming capacity, and fat absorption activity increased from 32.92 m2/g to 35.13 m2/g (P<0.05), 47.17% to 52.92% (P<0.05), and 1.86 to 2.41 g/g (P<0.05), respectively. Therefore, the extraction of rapeseed protein from the cold-pressed cake with the alkaline electrolyzed water can be widely expected to improve the extraction rate, further reducing the usage of strong alkali reagent. This finding can also provide a new strategy to extract the rapeseed protein for less impact on the structure and the better functioning of rapeseed protein.
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