Effect of some active compounds on microstructure and thermal properties of starch-chitosan blend films

MARÍA JULIETA BOF; DELIA ELISA LOCASO; MARÍA ALEJANDRA GARCÍA; ALBERTO JIMÉNEZ MARCO; AMPARO CHIRALT

Donostia - San Sebastián

Conferencia; 5th International Conference on Biobased and Biodegradable Polymers; 2015

Traditionally films and coatings derived from petroleum have been widely used in the pharmaceutical and food industry. Nowadays, there is an increasing trend and an emerging technology to replace them by biodegradable materials that could act as carriers of active compounds such as antimicrobials and/or antioxidants. In this sense, some natural and synthetic polymers have been studied for this purpose. Among natural polymers available to produce films, starch is one of the most commonly used materials due to its low cost, renewability and good film forming capacity. However, corn starch films are a poor water vapour barrier and exhibit poor mechanical properties. In order to improve these characteristics, a blend with chitosan is promising. Chitosan is a natural polymer obtained as by-product from the fishing industry. This polymer is extracted from crab shells and it is obtained by deacetilation of chitin. It has good film forming capacity and its particular interest is due to its antimicrobial activity attributed to its -NH groups presents in the molecular structure. Some natural compounds such as grapefruit seed extract (GSE) and lemon essential oil (LEO) can be added to polymer?s matrix to improve its antimicrobial/antioxidant activity. However, additives incorporation may affect physical properties of the materials, which are closely related with their microstructure. Thus, the aim of this work was to study the effect of LEO and GSE addition on the microstructure and thermal properties of starch-chitosan blend films.Film forming dispersions (FFD) were obtained by mixing a 4% w/w solution of gelatinized corn starch (CS) (Glutal, Buenos Aires, Argentine) and a 2.5% w/w suspension of chitosan (CH) (Parafarm, Buenos Aires, Argentine) with glycerol (Panreac, Barcelona, Spain) at 25% w/w of solids as plasticizer. The chitosan?s suspension was prepared by dissolving the powder in 1.25% acetic acid, maintaining under stirring and filtering in a mesh to remove impurities. The proportion of corn starch solution and chitosan suspension was 75:25. Then, 1 and 3% w/w of GSE (Euma, Buenos Aires, Argentine) and LEO (Litoral Citrus, Buenos Aires, Argentine) were added to FFD. The formulations were homogenized in a rotor-stator (Ultraturrax T25, Janke and Kunkel, Germany) connected to a venturi vacuum pump, spread onto teflon plates and left until total drying. Films were conditioned for one week before tests at 25°C and 53% RH. The microstructural analysis of films was carried out by SEM using a scanning electron microscope (JEOL JSM-5410, Japan). Films were cryofractured by using liquid N2 to observe their cross-section, then were fixed on copper tubs, gold coated and observed, using an accelerating voltage of 10 kV. Thermal analysis was performed, in duplicate, in a thermogravimetric analyzer (TGA 1 Star® System Analyzer, Mettler-Toledo, Inc., Switzerland) from 25°C to 600°C at 10°C/min, using a nitrogen flow (250 mL/min). The thermal degradation temperature (Tpeak) was obtained at the minimum of the DTG curves. Differential scanning calorimetry analysis was performed using a DSC 1 Stare System (Mettler-Toledo, Inc., Switzerland). The samples (in duplicate) were put into aluminium pans and heated from 0° to 200°C at 10°C/min rate. Then, they were cooled to 0°C and finally heated from 0° to 200°C at the same heating rate. Glass transition temperature was obtained as the midpoint of the glass transition.Microstructural analysis reveals how components are arranged in the dry films. SEM images of control, GSE1% and GSE3% films show a homogeneous phase which can be attributed to the compatibility and miscibility of both polymers. Same results were reported for other authors in wheat starch-chitosan blend films. However, in LEO1% and LEO3% samples, the presence of oil droplets embedded in the polymers? matrix can be observed. The images also show the differences in films? thickness due to the greater total solid content in the film forming dispersions with LEO. The high retention of the essential oil in the films can be appreciated from both, SEM images and TGA curves, where a first weight loss step (T<250ºC) could be observed in films containing these component. From DSC analysis, the only phase transition observed was glass transition of the starch phase in the temperature range previously observed by other authors. Tg values were not significantly affected by the incorporation of active compounds. Likewise, no notable effect of these compounds were observed for the peak degradation temperature (Tpeak) of the polymer blend, although when essential oil was present at the highest ratio, the TGA curve revealed a marked weight loss (about 50%) at temperature before 260, associates to the volatilization of its components from the polymer matrix. This weight loss partially overlaps with the start of degradation of the polymer blend and affected the peak temperature.