Casein Films: The Effects of Formulation, Environmental Conditions and the Addition of Citric Pectin on the Structure and Mechanical Properties

Abstract

Thin casein films for food packaging applications reportedly possess good strength and low oxygen permeability, but low elasticity and high sensitivity to moisture. Modifying the films to target specific behaviors depending on environmental conditions can enable a variety of commercial applications for casein-based films. The mechanical properties of solvent-cast (15% solids) calcium-caseinate/glycerol films (CaCas:Gly ratio of 3:1) were characterized as a function of processing and environmental conditions, including film thickness, solution formulation and ambient humidity (from 22% to 70% relative humidity (RH) at ~20 °C). At constant RH, the elongation at break (EAB) had a strong positive dependence on the film thickness. When RH increased, the tensile strength (TS) and modulus (E) decreased approximately linearly, while EAB increased. From 0.05% to 1% (w/w) of citric pectin (CP) was then incorporated into CaCas/Gly films following seven different formulations (mixing sequences), to alter the protein network and to evaluate the effects of CP on the tensile properties of CaCas/Gly/CP films. At constant film thickness and ~60% RH, the addition of 0.1% or 1.0% CP to the films considerably increased or decreased EAB, TS and E in different directions and to different extents, depending on the formulation, while optical micrographs also showed vastly differing OPEN ACCESSPolymers 2014, 6 2019 network configurations, suggesting complex formulation-and stoichiometry-dependent casein-pectin interactions within the dried films. Depending on the desired film properties and utilization conditions, pectin may be a useful addition to casein film formulations for food packaging applications.

Application of Humidity-Controlled Dynamic Mechanical Analysis (DMA-RH) to Moisture-Sensitive Edible Casein Films for Use in Food Packaging

“…Films were cast on silicone baking mats (Weston Products LLC, Strongsville, OH, USA) [3]. In between film-casting, the mats were cleaned with diluted Contrex detergent, wiped dry, then a 70/30 percent ethanol/DI water solution was applied to remove any detergent residue.…”

“…After spreading the solution, silicone mats were transferred to level tables at ambient temperature and relative humidity (T and RH) for an hour to set. Since variations in ambient RH during drying and storage reportedly have drastic effects on the properties of casein films [3], the films were then placed to dry in an environmental chamber at 50% RH and 20 °C (Model 6020-1, Caron, Marietta, OH, USA). After 1 day, the films were peeled from the mats, then placed between sheets of paper and stored in the chamber for a minimum of 4 days before testing.…”

“…The proline residues are also hydrophobic and their distribution pattern along the proteins contributes to the caseinates' amphiphilic character that is exploited in the film-making process. Films manufactured from CaCas or NaCas blended with glycerol (Gly) are strong, transparent, tasteless and excellent oxygen barriers [2], but their mechanical properties vary strongly with the ambient humidity present during their manufacture, storage and utilization [3]. To date, edible films intended for food packaging applications have been prepared from a variety of proteins, carbohydrates, and protein/carbohydrate combinations, and are generally highly hydrophilic, soluble in water and extremely sensitive to moisture during storage and utilization; however, the tensile properties of hydrophilic edible films are typically measured at normal temperature (T) and relative humidity (RH) conditions (23 ± 2 °C and 50% ± 5% RH) only.…”

“…For example, Cas/Gly film strips are stored in normal conditions for at least 40 h according to the standard ASTM method for testing tensile properties of thin films [4], then mounted on an Instron-type texture analyzer or an opened dynamic mechanical analyzer (DMA), in a room or chamber also controlled at ~50% ± 5% RH and 23 ± 2 °C, to measure the stress-strain curves of Cas/Gly films in normal conditions [5][6][7]. In order to measure the stress-strain curves under different RH conditions and obtain the Young's modulus, Y, tensile strength, TS, and elongation at break, D (%), of casein films as a function of RH, films have been left to equilibrate in the open in the laboratory and tested at different times of the year, with RH varying from as low as ~10% to as high as ~70% RH [3,8]. At constant film thickness, the elongation at break of CaCas/Gly films increased considerably with RH, while the tensile strength and elastic modulus decreased somewhat linearly with RH and could in theory be extrapolated to ~0 MPa at ~80% RH [3].…”

“…In order to measure the stress-strain curves under different RH conditions and obtain the Young's modulus, Y, tensile strength, TS, and elongation at break, D (%), of casein films as a function of RH, films have been left to equilibrate in the open in the laboratory and tested at different times of the year, with RH varying from as low as ~10% to as high as ~70% RH [3,8]. At constant film thickness, the elongation at break of CaCas/Gly films increased considerably with RH, while the tensile strength and elastic modulus decreased somewhat linearly with RH and could in theory be extrapolated to ~0 MPa at ~80% RH [3]. Using casein films grafted with acrylonitrile, Somanathan et al found that the transient tensile properties (i.e., stress-strain curves) of casein-based films were strongly affected by the amount of moisture absorbed by the films [9].…”