Submitted to: Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/29/2014
Publication Date: 1/6/2015
Publication URL: http://handle.nal.usda.gov/10113/60321
Citation: Bonnaillie, L., Tomasula, P.M. 2015. Application of humidity-controlled dynamic mechanical analysis (DMA-RH) to moisture-sensitive edible casein films for use in food packaging. Polymers. 7(1):91-114. DOI: 10.3390/polym7010091.
Interpretive Summary: Edible packaging films made from milk proteins are a 'green' potential solution to replace some petroleum-based food-packaging films and improve the quality and preservation of food, while at the same time reduce waste, since the films biodegrade and can be eaten with the food they are protecting. Thin films manufactured from casein proteins are strong, transparent, tasteless, and an excellent barrier against oxidation, but are also extremely sensitive to ambient humidity, as well as temperature, during storage and utilization in the food-packaging plant or at home. In this work, we explored a new technology that combines the precise control of environmental conditions (humidity, RH, and temperature, T) to the high-tech measurement of dynamic mechanical data (DMA) of polymeric films, to simulate various environmental conditions that may be applied to casein films during storage and utilization – from 'normal' ambient conditions at 20 deg C and 50% RH, to refrigeration, or extreme 'abuse' conditions – and quantify the different effects of moisture and heat on the strength, flexibility and shelf-life of the films. This versatile new technology will help determine the range of conditions in which casein films and other moisture-sensitive, edible films can be stored and used; which types of food packaging they are appropriate for; and how they may be modified to improve their strength, elasticity, resistance to environmental conditions and shelf-life, to promote their use in the food industry.
Technical Abstract: Protein-based and other hydrophilic thin films are promising materials for the manufacture of edible food packaging and other food and non-food applications. Calcium caseinate (CaCas) films are highly hygroscopic and physical characterization under broad environmental conditions is critical to application development and film optimization. A new technology, humidity-controlled dynamic mechanical analysis (DMA-RH) was explored to characterize CaCas/glycerol films (3:1 ratio) during isohume temperature (T) ramps and steps, and isothermal RH ramps and steps, to determine their mechanical and moisture-sorption properties during extensive T and RH variations. When RH and/or T increased, CaCas/Gly films became strongly plasticized and underwent several primary and secondary humidity-dependent transition temperatures (or transition humidities), when the CaCas/Gly network hypothetically rearranged itself to adapt to the increased water-content and heat-induced molecular mobility. Between 5-40 deg C and 20-61% RH, moisture-sorption was rapid and proportional to humidity between transition points and accelerated greatly during transitions. CaCas/Gly films seemed unsuitable for storage or utilization in warm/humid conditions as they lost their mechanical integrity around Tm~40 deg C at 50% RH and Tm decreased greatly with increased RH. However, below Tm, both moisture- and heat-induced structural changes in the films were fully reversible and casein films may withstand a variety of moderate abuse conditions.