Submitted to: Journal of Applied Polymer Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 15, 2007
Publication Date: N/A
Interpretive Summary: Casein is one of the major milk proteins. It has high nutritional value and the ability to form cohesive films and coatings. Casein films are edible and they have properties similar to traditional packaging materials such as polyethylene. There is growing interest in the use of edible films for biodegradable packaging material. Edible films are an environmentally-friendly alternative to petroleum-based packaging. By hindering oxygen transfer into and out of the product, preventing moisture loss, adding some extra physical strength or imparting antimicrobial activity, the casein films and coatings can lengthen the shelf-life of food products and non-food products. However to develop the edible film technology, lengthy experimentation is required to test the properties of the many combinations of films that can be made. To help advance the technology and find commercial applications of casein films, models that predict their functional properties based on formulation would be beneficial. In this study, three different types of casein that differ by structure and solubility in water were examined to investigated how they each interact with glycerol - a common plasticizer added to improve the flexibility of films. A connection between the microstructure of an edible film and the measured physical properties of casein films was identified. We also found that there are empirical equations that can model how the casein films change with increasing plasticizer content. These models may be useful for predicting casein film function and designing applications.
Technical Abstract: Casein films have the potential to be used to improve the quality and safety of food or to be used as biodegradable packaging. Plasticizers are incorporated into protein films to lower the glass transition temperature (Tg) and to improve tensile properties relative to pure films. The extent that plasticizers improve the flexibility of brittle films is dependent on plasticizer content. Predictive models that link film property with composition will aid in the development of applications for protein films. The objective of this study was to examine the tensile properties and glass transition temperature of casein films as affected by glycerol content and to develop empirical models that describe the composition-function relationship in the films. Films made from casein precipitated using high-pressure CO2 (CO2CAS), calcium caseinate (CaCAS) or sodium caseinate (NaCAS) were plasticized with glycerol (Gly) at three levels - 10%, 20% and 30%. Tensile strength, elastic modulus and glass transition temperature of the films were measured, and the microstructure of the films was also investigated with TEM imaging. A mathematical relationship based on the Gordon-Taylor equation was used to determine plasticizer efficiency ratings, k, of glycerol in the protein films. TEM showed differences in microstructure among the films. NaCAS film was homogenous while the CaCAS and CO2CAS films had similarly aggregated structures. It is concluded that k values for tensile and Tg properties are characteristic for a given protein and reflect the differences in microstructure. This correlation between definitive imaging results and empirical plasticizer efficiency ratings shows validity of the approach to model protein film function. The model may eliminate the need for lengthy experimentation and help design casein films with tailored functional properties.