Interaction of high pressure-stressed soy protein isolate and 5-methyltetrahydrofolate and its impact on the stability of 5-methyltetrahydrofolate

Abstract

5-Methyltetrahydrofolate (5-MTHF) is considered as a safer fortifier than folic acid (FA), but still unstable and less bioavailable. To investigated the soy protein isolate (SPI) or high hydrostatic pressure (HHP) treated SPI on the thermal stability of 5-MTHF and interaction of protein with 5-MTHF, the thermal stability of 5-MTHF in SPI-5MTHF complexes and HHP treated SPI-5MTHF complex was firstly investigated, and then revealed the effect of HHP on the structure and micromorphology of SPI by circular dichroism spectroscopy, free sulfhydryl content and scanning electron microscopy. Finally, the effect of HHP on the ability of the SPI to bind to 5-MTHF was also investigated by fluorescence spectrometry and isothermal titration. The results showed that the SPI-5MTHF complex was more thermally stable than free 5-MTHF in the thermal environment, and the retention of 5-MTHF was increased by about 306-fold. HHP also improved 5-MTHF stability in the HPP-SPI-5MTHF complex, with the highest retention of 5-MTHF of 104.77 ± 0.22 % at 300MPa. HHP increased apparent binding constants (4.83-26.78-fold) and binding sites (9.22 %-15.60 %) of SPI with 5-MTHF at pressures of 100-300 MPa. The interaction between SPI and 5-MTHF is a spontaneous exothermic reaction driven by enthalpy change. 5-MTHF is mainly bound to SPI by van der Waals forces and hydrogen bonding to form a complex. This study lays the foundation for further application of SPI or HHP-SPI as an encapsulation material to improve the stability and bioaccessibility of 5-MTHF.

Keywords: 5-Methyltetrahydrofolate; High hydrostatic pressure; Interaction; Soy protein isolate; Stability.

Graphical abstract

Fig. 1

TGA curves of SPI under different HHP treatments from 0 to 500 MPa.

Fig. 2

DSC curves of SPI under different HHP treatments from 0 to 500 MPa.

Fig. 3

CD curves of SPI under different HHP treatments from 0 to 500 MPa.

Fig. 4

Sulfhydryl group concentration of SPI under different HHP treatments Data were expressed as means ± SD (n = 3). Values with different letters indicated significant differences (p < 0.05).

Fig. 5

Micro-morphology of SPI treated with different HHP. Note: (A)→(F) indicates control, 100, 200, 300, 400, and 500 MPa/10 min, respectively.

Fig. 6

Fluorescence spectra of HHP-SPI.

Fig. 7

Fluorescence quenching curves of HHP-SPI. Note: (A)→(F) is 100, 200, 300, 400, 500 MPa/10 min and control. The curves are a→f from top to bottom, representing 5-MTHF concentrations of 0, 2.26 × 10⁻⁸, 4.53 × 10⁻⁸, 6.79 × 10⁻⁸, 9.06 × 10⁻⁸, and 11.3 × 10⁻⁸ mol/L.

Fig. 8

Double logarithm plot of fluorescence quenching of native and SPI in the presence of 5-MTHF. Note: The Q values were 2.26 × 10⁻⁸, 4.53 × 10⁻⁸, 6.79 × 10⁻⁸, 9.06 × 10⁻⁸, and 11.3 × 10⁻⁸ mol/L.

Fig. 9

ITC spectrum of HHP-SPI titrated with 5-MTHF. Note: (A)→(F) indicates control; 100 MPa/10 min; 200 MPa/10 min; 300 MPa/10 min; 400 MPa/10 min; 500 MPa/10 min, respectively.