Structure and permeability of Low-Methoxyl Pectin (LMP)-Sodium alginates (NaAlg) films

Abstract

Alginate and pectin form synergistic mixed gels that lead to a microstructure totally different from that of pure biopolymers. Synergistic mixed gels are of particular interest as makers of films with new improved characteristics. The objectives of the present work were, (a) to develop composite biodegradable films based on sodium alginate (NaAlg) and low-methoxyl pectin (LMP), (b) to evaluate the water vapor (WVP) and oxygen permeability, and (c) to characterize film microstructure by electron microscopy. A film-forming solution was prepared by adding 2% w/w of NaAlg, LMP or their mixtures, in a constantly stirred solution of NaCl (0,1M), during 4 h and then poured into Petri glass plates and allowed to dry at ambient, resulting in about 40 jam films. WVTR was determined gravimetrically using a modified ASTM Method E 96-95, and oxygen permeability of the prepared films was measured according to ASTM D3985. Films structure was characterized using both transmission (TEM) and scanning (SEM) electron microscopy. Results indicated that water vapor permeation follows Henry's law after two weeks of assay. Water permeability reduced in mixed biopolymer films, indicating NaAlg/LMP synergism. Pure pectin films resulted a better water vapor barrier, indicating the molecular weight has not effect on permeability. On the other hand, the reduction in permeability by alginate adding was assumed to be a result of the filling of pectin net. Permeability to oxygen was irrelevant. Confirming permeability assays, SEM micrographs of 1:1 alginate/pectin films show surface with lower porosity than pure (pectin or alginate) films. Moreover, TEM studies at the higher magnification (140,000X) also show a closer and denser nature in the case of mixed networks.