Mechanical and permeability properties of sodium caseinate and chitosan films

M. PEREDA; M. I. ARANGUREN; N. MARCOVICH

Viña del Mar, Chile

Simposio; ARCHIPOL ' 07 IV Simposio Argentino- Chileno de Polímeros; 2007

ARCHIPOL

MECHANICAL AND PERMEABILITY PROPERTIES OF SODIUM CASEINATE AND CHITOSAN FILMS Introduction Edible films from renewable natural products can extend the food shelf- life, thus improving the quality of food [1,2]. Caseinate, obtained by acid precipitation of casein, easily forms aqueous solutions due to their random- coil nature and ability to form extensive intermolecular hydrogen bonds [3]. Caseinate derived edible film wraps may be able to partially replace some conventional synthetic packaging materials used to preserve and protect foods due to their renewable and biodegradable nature [1,2]. On the other hand, chitosan is available from waste products of the shellfish industry [4]. Its importance resides in its antimicrobial properties in conjunction with its cationicity and its film-forming properties [5]. Chitosan has been reported to be biodegradable, non- toxic, biocompatible and bioadhesive [6]. The objective of this work is to manufacture and characterize edible films made from sodium caseinate and chitosan. In order to use the films as food packaging, it is necessary to have an understanding of the permeability characteristics, as well as their mechanical properties. The effect of different glycerol concentration on WVP measurements and mechanical properties was studied. MATERIALS AND Methods Reagents: Sodium caseinate (90 g protein/100 g NaCas) was obtained from Lactoprot Deutschland GMBH, Germany. Chitosan (deacetylation degree 90%) was supplied by ACOFAR, Mar del Plata, Argentina. Glycerol (gly) was obtained from DEM Chemicals, Mar del Plata, Argentina. Film Formation: NaCas solutions with protein (pro) concentrations of 2.5 % (w/v) were prepared in distilled water. Chitosan (Chit) solutions (2 %, w/v) were prepared by dispersion in acetic acid solution (1 %, v/v). Different glycerol weigth ratios were added to both solutions. Chitosan/NaCas composite films were prepared by mixing chitosan solution with sodium caseinate solution. Glycerol was added to achieve gly/(pro+ Chit) weigth ratio of 0.28. The films were obtained by casting of the solutions. The obtained films were peeled off from the plates after the excess water was evaporated and kept in a closed reservoir at 64.5 % RH and 23 ± 2ºC for three days. Mechanical tests: Tensile tests were performed at room temperature (23 ± 2ºC) according to ASTM D1708-93, using an Instron Universal Testing Machine model 8501. Water vapor permeability (WVP): The water vapor transfer rate [g/(s.m2)] through films was determined gravimetrically using the ASTM Method E96-95. Film specimens were mounted on acrylic cups containing Cl2Ca (0% RH). The cups were placed in a controlled chamber at 25ºC and 64.5 % RH for at least two days before testing. Six specimens were tested for each film type. Permeability values are reported as water permeability coefficient in [g /(m s Pa)]. Results and discussion The addition of glycerol reduces modulus and maximum stress in tensile tests, while it largely improves the ultimate deformation of the films. The samples are less rigid and much more flexible as the glycerol concentration increases (Figure 1). The effect of the plasticizer addition on the tensile behavior of Chit films is similar to that observed for NaCas films (not shown). Figure 1.- Stress- strain curves of NaCas films prepared with different glycerol contents and conditioned at 21 °C and 50 % RH. Figure 2 shows that water vapor permeability values have an exponential functionality with glycerol content, in both films. Addition of glycerol to the polymer network facilitates the diffusion of water molecules through the film. At the same glycerol content (28 % w/w), the WVP of the hybrid film Chitosan/NaCas takes an intermediate value between those of the Chit and NaCas pure films, while it exhibits a higher tensile strength than those of both pure materials. Figure 2.- WVP as a function of glycerol content. for a) chitosan, b) sodium caseinate films (<, experimental values; solid line, model). Acknowledgements CONICET (Fellowship awarded to M. Pereda and Grant PIP 6250/05) and the Science and Technology National Promotion Agency – National University of Mar del Plata (ANPCyT-UNMdP, grant PICTO No. 14-381). References 1. Audic J.; Chaufer B., Eur. Pol. J., 2005, 41,1934. 2. Schou M.; Longares A.; Montesinos-Herrero C. et al., Food Sc. and Tech., 2005, 38, 605. 3. Mc Hugh T.H,; Krotcha J.M, Food Technology, 1994, 97. 4. Wong D.W.S.; Gastineau F.A.; Gregorski K.S. et al., J. Agric. Food Chem., 1992, 40, 540. 5. Arvanitoyannis S.; Nakayama A.; Aiba S., Carb. Pol., 1998, 37, 371. 6. Remuñán-López C., Bodmeier R., J. contr. rel., 1997, 44, 215.7.