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 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.
Highlights
Interest has grown for edible packaging films as a replacement for synthetic plastic films driven by a societal desire for environmentally-friendly, biodegradable and sustainable food packaging
At pH 7, the film formulation protocol strongly affected the macro-structure and tensile properties of the films, even at very low pectin contents, a phenomenon that can be exploited to tailor some of the properties of casein films in various ways to broaden their range of potential commercial applications, such as edible wrappers or soluble pouches for single-serve foods, packaging films for cheese or oxygen-sensitive foods, coatings for nuts and cereals or multi-layered packaging films
Numerous Control A and F films of different thicknesses (d = 0.01–0.18 mm) were repeatedly prepared throughout the year, and multiple samples were tested on many different days to take advantage of the entire range of relative humidity (RH) available throughout the year, in order to quantify both the effects of RH and d on the mechanical properties of calcium-caseinate/glycerol films (CaCas)/Gly films and to characterize these films on the full range of environmental RH and thickness conditions that might be encountered during the commercial development and end-use of CaCas/Gly food-packaging films
Summary
Interest has grown for edible packaging films as a replacement for synthetic plastic films driven by a societal desire for environmentally-friendly, biodegradable and sustainable food packaging. Tomasula et al developed a method to produce more hydrophobic casein through high pressure processing with CO2, producing casein films with considerably lower solubility [14,16,19,20] Hydrocolloids, such as gelatin, were used to enhance the mechanical strength, as well as the water vapor barrier properties. Chemical methods, such as alkali-treatment or the addition of various cross-linkers, were reported to improve the elongation and strength and decrease the solubility of casein films [22] Additives, such as stearic acid or various enzymes, were helpful to improve the water-resistance and mechanical properties of casein films, while DL-glyceraldehyde and glutaraldehyde enhanced the microstructure of glycerol plasticized casein films [13,25,26]. At pH 7, the film formulation protocol strongly affected the macro-structure and tensile properties of the films, even at very low pectin contents, a phenomenon that can be exploited to tailor some of the properties of casein films in various ways to broaden their range of potential commercial applications, such as edible wrappers or soluble pouches for single-serve foods, packaging films for cheese or oxygen-sensitive foods, coatings for nuts and cereals or multi-layered packaging films
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