Effects of High-Intensity Ultrasound Pretreatment on Structure, Properties, and Enzymolysis of Walnut Protein Isolate.

Fei Zhao,Xiaosong Zhai,Xuemei Liu,Jingxiang Cui,Guoting Liang,Haizhou Dong,Meng Lian,Wentao Wang

doi:10.3390/molecules27010208

Abstract

The purpose of this paper was to investigate the effect of high-intensity ultrasonication (HIU) pretreatment before enzymolysis on structural conformations of walnut protein isolate (WPI) and antioxidant activity of its hydrolysates. Aqueous WPI suspensions were subjected to ultrasonic processing at different power levels (600–2000 W) and times (5–30 min), and then changes in the particle size, zeta (ζ) potential, and structure of WPI were investigated, and antioxidant activity of its hydrolysates was determined. The particle size of the particles of aqueous WPI suspensions was decreased after ultrasound, indicating that sonication destroyed protein aggregates. The ζ-potential values of a protein solution significantly changed after sonication, demonstrating that the original dense structure of the protein was destroyed. Fourier transform infrared spectroscopy indicated a change in the secondary structure of WPI after sonication, with a decrease in β-turn and an increase in α-helix, β-sheet, and random coil content. Two absorption peaks of WPI were generated, and the fluorescence emission intensity of the proteins decreased after ultrasonic treatment, indicating that the changes in protein tertiary structure occurred. Moreover, the degree of hydrolysis and the antioxidant activity of the WPI hydrolysates increased after sonication. These results suggest that HIU pretreatment is a potential tool for improving the functional properties of walnut proteins.

Highlights

Walnuts (Juglans regia L.) are increasingly consumed for their nutritional attributes and health profile [1,2]
The data indicated that over-processing of the walnut protein isolate (WPI) dispersions could lead to an increase in the particle size distribution of walnut protein dispersions
The results showed that protein peptides with high antioxidant activity were obtained by low-power ultrasonic treatment, and WPI peptides with higher antioxidant activity depended on suitable ultrasonic treatment conditions

Summary

Introduction

Walnuts (Juglans regia L.) are increasingly consumed for their nutritional attributes and health profile [1,2]. Walnuts have high economic value due to the high level of lipids (67% on a dry basis) [4]. Lipids are rich in essential polyunsaturated fatty acids ( linoleic acid), which have nutritional advantages such as anti-oxidizing properties and the ability to lower blood cholesterol [5]. The increasing market demand for walnut lipids has led to a large amount of a by-product: nutritional walnut proteins, which are used as animal feed or discarded [7]. It is necessary to increase the economic value of defatted walnut proteins, as the development of the walnut industry is discouraged by the underutilization of the by-product [5]. The main group of proteins (about 70%) found in walnuts are glutelins, whose poor aqueous solubility limits their functional features as water-based food products

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Conclusion