Microbial transglutaminase induced modification of wheat gliadin based nanoparticles and its impact on their air-water interfacial properties

Abstract

There is a growing interest in plant protein based nanoparticles (NPs) such as wheat gliadin NPs (GNPs) for stabilizing foams in food systems. To increase the range of their applications, it is necessary to understand the role of different gliadin types and to tailor their behavior at air-water (A-W) interfaces. The latter is here done by modifying GNPs with microbial transglutaminase (TG). The most pronounced solubility loss of ω-gliadins (by 36.1%) after TG treatment at pH 4.5, measured by reversed phase high performance liquid chromatography (RP-HPLC), indicated that this protein type was more susceptible to TG action than α-gliadins and γ-gliadins, the solubility losses of which were 11.2 and 11.4% respectively. The lack of intramolecular disulfide bonds in ω-gliadins but not in the other gliadin types likely resulted in a greater accessibility of the lysine residues and thus more pronounced susceptibility of ω-gliadins to the enzyme. Interestingly, TG treatment of GNPs notably improved their foaming properties at pH 4.5 (and thus below the gliadin pI). The high surface dilatational modulus value of TG treated GNPs, when compared to control GNPs (53.9 and 32.7 mN/m, respectively), indicated formation of a strong viscoelastic A-W interfacial protein film, presumably due to the presence of TG catalyzed intermolecular isopeptide bonds. Fractionation of foams stabilized by (un-)treated GNPs and subsequent RP-HPLC analysis of the foam and liquid fractions showed that γ-gliadins were more important for A-W interface stability than ω-gliadins even if the latter were most affected by the TG treatment. Elimination of positively charged (at pH 4.5) lysine residues and conformational changes (both as a result of TG treatment) may have altered the ability of ω-gliadins to adsorb at the interface and to interact with other proteins thereat. Overall, TG treatment of GNPs resulted in formation of very strong protein films at the A-W interface stabilized by both covalent bonds and non-covalent interactions which in turn led to highly stable foams.