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Summary Biodegradable food packaging promises a more sustainable future. Poly(lactic acid) (PLA) is a promising alternative for petroleum-derived polymers. However, PLA films suffer from poor barrier properties compared with petroleum-derived ones. To address this issue, we designed bilayer and sandwich-architectured trilayer films based on PLA and gliadin. We reported firstly the direct formation of PLA–gliadin bilayer/trilayer films without surface modification on PLA film. The films were compact and uniform, and double/triple layers were combined firmly, preventing delamination. This strategy enhanced mechanical resistance, ductility and moisture barrier of gliadin films and concomitantly enhanced the oxygen barrier for PLA films. Thymol loadings endowed bilayer/trilayer films with antimicrobial activity and antioxidant activity. The antimicrobial activity of the films depended on film types, and gliadin layer presented larger inhibition zone than PLA layer, hinting that the films possessed directional releasing role. This study opens a promising route to fabricate bilayer/trilayer architecture helping to create synergism of the biopolymers.

Summary The aim of this investigation was to explore the pH-induced conformational changes of phaseolin using differential scanning calorimetry (DSC), fluorescence and circular dichroism (CD) spectra. The surface charges at the varying pH and ionic strength were also determined to probe the possible relationship between the surface charges of phaseolin and its conformation. The ζ-potential of phaseolin decreased from 27.9 mV at pH 3.0 to –44 mV at pH 9.4, and the isoelectric point (iep) occurred at pH 4.21, which were comparable with the theoretical estimations derived from amino acid composition and pK values. The iep of phaseolin was independent of ionic strength, while the magnitude of ζ-potential decreased as the ionic strength increased. The near-UV CD, DSC and fluorescence spectra analyses indicated highly acidic and alkaline pH conditions induced the loss of tertiary conformation of phaseolin. The far-UV CD spectra analysis confirmed pH-shift led to the decreases in ordered secondary structure compositions. The present results suggested the loss of secondary and/or tertiary conformation of phaseolin as a result of the varying pattern in electrostatic interactions as pH shifted from 7.0 to acidic or alkalic pH.

Summary The effect of pHs and heating on the protein–polysaccharide complexation between the 0.5 wt% soy globulin (7S or 11S) and 0.1 wt% chitosan was studied. Electrophoretic and light scattering techniques were used to examine the electrical charge and aggregation of the individual biopolymers and complexes. At pH 3.0–6.5, 7S (or 11S) globulin in the presence of chitosan had significantly higher ζ-potentials and lower particles size than 7S (or 11S) globulin alone did (e.g. 600–6000 nm at pH 5.5), indicating the formation of complexes. After heating 7S (or 11S)–chitosan mixtures had higher positive value of ζ-potential. 7S (or 11S)–chitosan mixtures exhibited a significant increase in positive value of ζ-potential and stability after heating at lower pH values (pH 3.3 instead of pH 4.5). Compared with other mixtures, at pH 2.5–6.0, the most remarkable decrease in aggregation was obtained for 11S–chitosan mixtures after heating at pH 3.3.

A simple two step precipitation method was used to investigate the effect of MgCl 2 instead of CaCl 2 on fractionation of soybean glycinin and β-conglycinin. Compositional and physicochemical properties of the resulting protein fractions were characterised. The optimised procedure, in terms of protein yield, purity, phytate content and physicochemical properties, was obtained when the addition of 5 mM MgCl 2 was used. After application of 5 mM MgCl 2 , the phytate content of the glycinin-rich and β-conglycinin-rich fractions was about 0.4% and 1.3%, respectively, but the addition of 5 mM CaCl 2 increased the phytate content of the glycinin-rich fraction to 1.25% and decreased that of β-conglycinin-rich fraction to 0.67%. Low phytate protein product was suitable for use in infant formula and acidic food. The solubility of the glycinin-rich fractions with MgCl 2 was significantly higher than that with CaCl 2 at pH < 4.5. Application of MgCl 2 improved thermal stability of the β-conglycinin-rich fraction.

The effects of succinylation and acetylation on some functional, structural properties and in vitro trypsin digestibility of hemp protein isolate (HPI) were investigated. The extent of acylation gradually increased from 0 to 60―70%, with the anhydride-to-protein ratio increasing from 0 to 1.0 g g ―1 . Size exclusion chromatography showed that succinylation led to formation of more soluble protein aggregate than acetylation, especially at anhydride levels higher than 0.1 g g ―1 . Succinylation led to gradual increase in protein solubility (PS) from 30 to 85―90%, while in the acetylation case, the PS was improved only at low anhydride levels, increasing from 30 to about 50% with anhydride-to-protein ratio increasing from 0 to 0.2 g g ―1 . At neutral pH, the emulsifying activity indexes (EAI) of HPI was 22.1 m 2 g ―1 , and the EAI linearly and significantly increased with the extent of acylation. The EAIs of succinylated and acetylated HPI (1.0 g g ―1 ) were 119.0 and 54.4 m 2 g ―1 , respectively. Differential scanning calorimetry (DSC) and intrinsic fluorescence spectrum analyses indicated gradual structural unfolding of proteins, or exposure of hydrophobic clusters to the solvent, especially at higher anhydride levels. Additionally, the in vitro trypsin digestibility was significantly improved by the succinylation. The results indicated that the chemical acylation treatment (especially succinylation) could be applied to modify some selected functional properties of hemp proteins, especially PS and emulsifying ability.