Phase transitions in starch based films containing fatty acids. Effect on water sorption and mechanical behaviour

The water sorption capacity (WSC) of starch films containing, or not, glycerol (1:0.25 starch:glycerol ratio) and fatty (palmitic, stearic and oleic) acids (1:0.15 starch:fatty acid ratio) was analysed at 10, 25 and 40 °C in the entire range of aw. Starch re-crystallization during the equilibration time was also analysed by X-ray diffraction. Likewise, the glass transition and lipid melting properties as a function of the film water activity were determined. Mechanical behaviour was also analysed at 25 °C for film samples equilibrated at 0, 53, 68 and 75% RH. Crystallization was promoted at low temperature and high relative humidity which modifies the expected tendencies in WSC as a function of the temperature. In relationship with this, V-type structures were formed during time, mainly in fatty acids containing films. Glass transition of the starch matrix at low moisture contents was affected by the presence of saturated fatty acids. Microstructural observations seem to corroborate the complex formation between these and the polymer chains. The mechanical behaviour of the films, as a function of the moisture content, is coherent with the results of the phase transition analysis and microstructural observations. The plasticization effects and the structural discontinuities in the polymer matrices determine the value of the elastic modulus and the behaviour at break.