Abstract :
[en] The present study investigated effects of high hydrostatic pressure (HHP) combined with four main factors on structure and gelation properties of sweet potato protein (SPP), including a modulation of pH conditions, addition of salt ions, enzyme microbial transglutaminase and sulfur-containing amino acids respectively.
First of all, effects of HHP (250, 400 and 550 MPa) combined with different pH (3.0, 6.0 and 9.0) on chemical forces, structure and gelation properties of SPP were investigated. Surface hydrophobicity (Ho) and zeta potential of SPP significantly increased from 250 to 550 MPa at all three pH conditions (p < 0.05). The total amount of sulfhydryl (-SH-) groups in SPP decreased after HHP treatment at pH 9.0, whereas the amount of free -SH- increased. High molecular mass aggregates (>180 kDa) were observed in SPP after HHP treatment at pH 6.0 and 9.0 by SDS-PAGE. Storage modulus (G′) of SPP were significantly increased after HHP treatment. Due to the covalent cross-linking of SPP by the formation of disulfide bonds, textural properties and water-holding capacity of gels made from SPP after 250 and 400 MPa treatment at pH 9.0 were significantly improved, and the gels showed a compact and uniform gel network with the contribution of immobilized water fractions.
Next, effects of 400 MPa treatment combined with monovalent (NaCl) or divalent (CaCl2 and MgCl2) salts in different concentrations on structure and gelation properties of SPP were evaluated. Surface hydrophobicity and zeta potential of SPP significantly decreased as salts concentration increased. The total amount of sulfhydryl (-SH-) groups in SPP decreased with addition of NaCl, CaCl2 and MgCl2 under HHP, whereas the disulfide bonds increased from 0.59 to 0.80 and 0.74 μmol/g with addition of 0.1 and 0.2 mol/L of CaCl2 combined with HHP, respectively. The α-helical content of SPP was increased by all three salts, but decreased as salts combined with HHP, while β-sheet content was increased at high ionic strength with HHP, especially in the presence of CaCl2 and MgCl2, being increased from 30.40 to 33.45 and 33.55 %, respectively. Storage modulus (G′) of SPP was enhanced by salts and HHP, even though it decreased at high ionic strength. Textural properties and water holding capacity of SPP gels were improved with salt ions by HHP, being attributed to more bound water.
After that, effects of HHP (250, 400 and 550 MPa) combined with microbial transglutaminase (MTGase) on structure and gelation properties of SPP were investigated at pH 7.0. MTGase induced the formation of high molecular mass aggregates of SPP by the inter-molecular cross-linking. The average particle size of SPP was increased by HHP at 250-550 MPa, being increased from 249.5 nm (0.1 MPa) to 270.2 nm (550 MPa). While it was decreased by the subsequent MTGase catalysis and showed the lowest value of 200.3 nm at 550 MPa, indicating the formation of intra-molecular linkages. The amount of sulfhydryl groups and thermal denaturation temperature were increased by HHP and MTGase, which showed the highest value of 3.5 μmol/g protein and 80.4 °C in MTGase catalyzed SPP with 400 MPa pretreatment. Textural properties of the gels made from HHP treated SPP or those with further MTGase catalysis were both enhanced due to the covalent linkage of disulfide bonds (-S-S-) and ε-(γ-glutaminyl) lysine isopeptide bonds. In addition, low-field NMR results showed an increase in corresponding area fractions of A2b and A21, being attributed to more bound or immobilized water trapped in SPP gel matrix.
Furthermore, effects of 400 MPa treatment combined with sulfur-containing amino acids (L-cysteine or L-cystine) in different concentrations on structure and gelation properties of SPP were studied. Additives altered the denaturation temperature and reduced the denaturation enthalpy of SPP. Higher α-helical contents were observed in untreated or HHP treated SPP with L-cysteine or L-cystine, while β-sheet and random coil structure unit were decreased. FTIR spectra showed a weak absorbance in HHP treated SPP with L-cysteine and L-cystine. Storage modulus (G′) of untreated or HHP treated SPP was enhanced by L-cysteine and L-cystine. Textural properties of SPP gels were improved by the addition of sulfur-containing amino acids under HHP corresponding to the disulfide linkages, especially for L-cysteine. Low-field NMR indicated that water bound more closely to SPP molecules in the presence of L-cysteine and L-cystine, and more immobilized water fraction was trapped in SPP gel matrix, suggesting an improvement in water holding capacity.
