High moisture extrusion of pea protein: Effect of l-cysteine on product properties and the process forming a fibrous structure

In this study, the pea protein isolates were mixed with l-cysteine and then extruded under the temperature of 169 °C and a moisture content of 54% conditions. The effect of l-cysteine (0, 0.03, 0.06, 0.09, 0.12, 0.15% (w/w)) addition on the functional and structural properties of texturized pea protein was studied. The results showed that water holding capacity descended but oil holding capacity and nitrogen solubility index increased since the hydrophobic group of pea protein was exposed during the high-moisture extrusion. l-cysteine significantly affected the textural properties and fibrous degree of texturized pea protein by changing the cross-linking effect on the disulfide bonds between pea protein molecules. However, l-cysteine would have a negative effect on the fibrous structure when the addition amount exceeded 0.09%. The unstable α-helical structure in the protein secondary structure was transformed into a relatively stable β-sheet by fourier transform infrared spectroscopy. The content of sulfhydryl group decreased due to oxidation reaction during extrusion, which was conducive to the formation of disulfide bond, and the protein rearranged to form a more robust new chemical cross-linking in the die. The extruded pea protein had a dense structure, and the lamellar fiber orientation was obvious according to the observation of scanning electron microscope. It was expected to provide a reference for the development and utilization of pea protein, broaden the research of pea protein high-moisture extrusion technology, and improve the quality of extruded pea protein.