Ultrasound-assisted extraction ameliorates the physicochemical properties of defatted mulberry seed protein to promote lipid production in Schizochytrium sp. SR21

Abstract

To improve protein extraction rate and the utilization of mulberry seed protein (MSP) for biorefinery purposes, ultrasound-assisted extraction was used to treating defatted mulberry seed protein (DMSP) for enhancing lipid production in Schizochytrium sp. SR21. Under ultrasound irradiation with solid-liquid ratio of 1:40, temperature of 60 °C, time of 9 min, and output power of 600 W, the extraction rate of DMSP reached 77.05%, which was significantly improved than the blank control (18.70%) and microwave treatment (22.50%). Moreover, the solubility and emulsibility of DMSP treated by ultrasound were significantly improved; the assay results measured by near-infrared Raman Spectroscopy showed that ultrasound-assisted treatment could reduce the α-helix in the protein (reduced by 9.75%); correspondingly, structures of β-sheet and β-turn increased 1.33% and 3.01%; it indicated that ultrasound treatment changed the spatial arrangement of protein main chain atoms. The antioxidant activity test and the amino acid composition of defatted mulberry seed protein hydrolysate (DMSPH) showed that ultrasound-assisted treatment enhanced the antioxidant activity of the protein hydrolysate and there was no significant difference in amino acid composition between the DMSP and yeast extract. The DMSPH was used as nitrogen source to cultivate Schizochytrium sp. SR21 which showed that DMSPH promoted lipid production and the DHA content was 35.55%, which was better than the results of cultured with traditional nitrogen source; it indicates that DMSP can be used as nitrogen source in culture of microorganisms and further explanation of ultrasound-assisted treatment of modified MSP as an alternative nitrogen source achieved the goal of biorefinery.