Rheological Behaviors of Edible Casein-Based Packaging Films Under Extreme Environmental Conditions, Using Humidity-Controlled Dynamic Mechanical Analysis
Thin casein films for food packaging applications possess good strength and low oxygen permeability but low water-resistance and elasticity. Customizing the mechanical properties of the films to target specific behaviors depending on temperature and humidity changes would enable a variety of commercial applications for casein-based films. The mechanical properties of edible films are vitally important to determine possible uses, such as replacement for plastic films. Dynamic mechanical analyses under controlled humidity (DMA-RH) can supply useful information about the mechanical properties and network-structure of hydrophilic protein films, including the storage modulus (E'), loss modulus (E''), deformation (swelling and shrinkage), damping behavior (tan-δ), as well as various transition temperatures and humidities. The dynamic mechanical properties of solvent-cast (15% solids) calcium-caseinate/glycerol films (CaCas:Gly ratio 3:1) were fully characterized on a broad range of temperature (T=5-90°C) and humidity (RH=0-80%) using DMA-RH technology to study behaviors under normal or extreme environmental conditions. Citric pectin (CP, 0.05 to 1 wt.%) was then incorporated into CaCas films as a crosslinker using 3 different formulations (A, F, G) to examine CP effects on the properties and macrostructure of CaCas/Gly/CP films, and improve the stability of CaCas film under high humidity or temperature. The mechanical properties of casein films were extremely sensitive to formulation, CP content, T and RH, and DMA-RH technology proved a precise and effective tool to characterize composition/properties trends and point out various transition temperatures during T-ramps at 50% RH, and 'transition humidities' during RH-ramps at 20°C. Transition-T and RH were considered to correspond to different critical points at which sufficient plasticization with water and/or heat triggered various rearrangements of the CP/protein/water network, until ultimate film-failure at the 'melting point'. DMA-RH enabled to precisely characterize shifts in E’, E’’ and tan-δ, as well as shifts in the various transition-T and transition-RH caused by different formulations or increased CP contents. Generally, the addition of CP improved the environmental stability of CaCas films: after addition of 1% CP, the melting-T increased from ~40°C to ~60°C, and the 'melting-RH' from ~58% to ~67%, depending on formulation. F films showed the most drastic improvement with increased CP content, while G films appeared stronger and generally more T- and RH-stable at all CP contents. Improving the strength and environmental stability of casein-based films will broaden their possible range of application, such as use in edible food packaging.
Keywords: Calcium caseinate, thin films, DMA