Effects of the geometry of reinforcement on physical properties of sodium caseinate/TiO2 nanocomposite films for applications in food packaging.

Plastic materials for food packaging are being replaced by biodegradable films based on biopolymers due to the adverse effects they have had on animal life and the environment. In this study, nanocomposite films containing 2.5 wt% sodium caseinate and 2 wt% glycerol were reinforced with 0.1 or 0.2 wt% nano TiO2 prepared in two forms: spheres (P25) and tubes. The effects of nanoreinforcement geometry on mechanical, tensile, barrier, thermogravimetric, and optical properties, and distribution of nanoparticles were described. The interactions among film components were analyzed by Fourier transform infrared spectroscopy (FTIR). Addition of nanotubes significantly increased E' (341 wt%) and E" (395 wt%) moduli, the Young modulus E (660 wt%), the residual mass at 500°C (38 wt%), and color change (6.78) compared to control film. The compositional mapping studies showed that P25 nanoparticles were homogeneously distributed between the surfaces of the film while nanotubes were found on the bottom surface. The changes in position of the FTIR spectra signals as compared to pure protein signals indicated strong matrix/reinforcement interactions. In addition, the changes in intensity in 1100, 1033, and 1638 cm-1 FTIR signals suggested formation of a protein/Tween 20 ester. The geometry of reinforcement was highly relevant regarding physical properties, showing nanotubes as being very successful for enhancing tensile properties.

Nanoreinforcement as a strategy to improve physical properties of biodegradable composite films based on biopolymers.

Food packaging is evolving from inert plastic to renewable biopolymer film that acts as barrier against gases, light radiation, and microorganisms, reducing food waste without environmental damage. Distinct starting systems were selected to prepare films: single polymer matrix, blend of polymers, cross-linked polymers, and emulsion-based matrix. The blend of polymers was one of the best approaches to improve mechanical and barrier properties of films, especially when one of the polymers was pectin, gelatin or xanthan gum. These polymers can form a gel and increase the viscosity of the starting systems leading to a more elastic matrix. Although some of these films showed potential to replace plastic materials, their physical properties were poor compared to plastics. Thus, several strategies were used to strengthen matrix building block connections or interactions between nanoreiforcement and matrix compounds with the aim of improving physical properties. Among metal oxides, TiO2, ZnO, CaO, and MgO were the most studied, alone or in combinations with other reinforcements. Natural fillers, like chitosan and cellulose nanofibers were also added to improve the biopolymer's performance. Several of these systems successfully extended the shelf life of food systems by retarding spoilage, showing great potential to improve food quality and reduce waste. However, most of the studies were carried out on a laboratory scale and it would be necessary to explore the feasibility of producing these films on an industrial scale.

Structural characterization of nanoemulsions stabilized with sodium caseinate and of the hydrogels prepared from them by acid-induced gelation.

Hydrogels obtained by acidification with glucono-δ-lactone (GDL), starting from nanoemulsions formulated with different concentrations of sodium caseinate (1-4 wt%) or 4 wt% sodium caseinate and sucrose (2-8 wt%), were prepared with the aim of quantifying structural parameters of both, initial nanoemulsions and hydrogels after 2.5 h of GDL addition, using the Guinier-Porod (GP) or the generalized GP models. Gelation process was followed by performing in situ temperature-controlled X-ray small angle scattering experiments (SAXS) using synchrotron radiation. In nanoemulsions, the calculated radius of gyration for oil nanodroplets (Rg oil ) decreased with increasing protein concentration and for the 4 wt% protein nanoemulsion, with increasing sucrose content. Calculated values of Rg oil were validated correlating them with experimental Z-average values as measured by dynamic light scattering (DLS). For hydrogels, radii of gyration for the sphere equivalent to the hydrogel scattering object (R gsph ) were close to 3 nm while correlation distances among building blocks (R g2 ) were dependent on formulation. They increased with increasing contents of sodium caseinate and sucrose. R g2 parameter linearly correlated with hydrogel strength (G' ∞ ): a more connected nanostructure led to a stronger hydrogel.

Relationship between nano/micro structure and physical properties of TiO2-sodium caseinate composite films.

Films obtained by casting, starting from conventional emulsions (CE), nanoemulsions (NE) or their gels, which led to different structures, with the aim of explore the relationship between structure and physical properties, were prepared. Sodium caseinate was used as the matrix, glycerol as plasticizer, glucono-delta-lactone as acidulant to form the gels, and TiO2 nanoparticles as reinforcement to improve physical behavior. Structural characterization was performed by SAXS and WAXS (Small and Wide Angle X-ray Scattering, respectively), combined with confocal and scanning electron microscopy. The results demonstrate that the incorporation of the lipid phase does not notably modify the mechanical properties of the films compared to solution films. Films from NE were more stable against oil release than those from CE. Incorporation of TiO2 improved mechanical properties as measured by dynamical mechanical analysis (DMA) and uniaxial tensile tests. TiO2 macroscopic spatial distribution homogeneity and the nanostructure character of NE films were confirmed by mapping the q-dependent scattering intensity in scanning SAXS experiments. SAXS microscopies indicated a higher intrinsic homogeneity of NE films compared to CE films, independently of the TiO2 load. NE-films containing structures with smaller and more homogeneously distributed building blocks showed greater potential for food applications than the films prepared from sodium caseinate solutions, which are the best known films.

New insights about flocculation process in sodium caseinate-stabilized emulsions.

Flocculation process was studied in emulsions formulated with 10wt.% sunflower oil, 2, 5 or 7.5wt.% NaCas, and with or without addition of sucrose (0, 5, 10, 15, 20 or 30wt.%). Two different processing conditions were used to prepare emulsions: ultraturrax homogenization or further homogenization by ultrasound. Emulsions with droplets with diameters above (coarse) or below (fine) 1µm were obtained. Emulsions were analyzed for droplet size distribution by static light scattering (SLS), stability by Turbiscan, and structure by confocal laser scanning microscopy (CLSM) and small angle X-ray scattering (SAXS). SAXS data were fitted by a theoretical model that considered a system composed of poly dispersed spheres with repulsive interaction and presence of aggregates. Flocculation behavior was caused by the self-assembly properties of NaCas, but the process was more closely related to interfacial protein content than micelles concentration in the aqueous phase. The results indicated that casein aggregation was strongly affected by disaccharide addition, hydrophobic interaction of the emulsion droplets, and interactions among interfacial protein molecules. The structural changes detected in the protein micelles in different environments allowed understanding the macroscopic physical behavior observed in concentrated NaCas emulsions.