Effect of pretreatment on the film forming properties of potato and barley starch dispersions

The effects of preheating temperature, glycerol concentration and acid modification on the film-forming properties of potato and barley starches were studied in extension of the starch films and by light microscopy. In addition, the effects of heating pretreatment and acid modification were studied by size exclusion chromatography (SEC). With both starches, the starch:glycerol ratio ranged 100:5 to 100:67 of 3 wt % starch dispersions. Increasing the glycerol content increased the elongation and decreased the tensile strength of potato and barley starch films prepared after preheating at 121 degrees C. In the case of potato starch at 100:33 starch:glycerol ratio, raising the preheating temperature from 100 to 180 degrees C caused the tensile strength to increase from 2.4 to 5.0 MPa and the elongation to decrease from 104 to 556. The mechanical properties of potato starch films underwent a notable change when the preheating temperature was 100 or 121 degrees C. The molecular mass of amylose decreased from 1100,000 to 670,000. A major change in the molecular mass of potato amylopectin could be detected only after heat treatment at 180 degrees C. At 121 degrees C and at 180 degrees C, starch dispersions exhibited a clear separation into amylose- and amylopectin-rich phases. No continuous films were obtained from acid-modified potato starches with molecular mass of amylose 20,000. With barley starch, as the preheating temperature increased from 100 to 180 degrees C the tensile strength increased from 2.6 to 4.9 MPa and the elongation decreased from 52 to 5%. At the same time the molecular mass of amylose decreased from 300,000 to 80,000. At 121 degrees C the granular structure remained, but at 180 degrees C no granules were left and instead the film consisted of amylopectin droplets and small amylose-rich domains. The strongest films (9.4 MPa), with 7% elongation, were obtained with acid-modified barley starch in which the molecular mass of amylose was 45,000. The tensile strength of both potato and barley starch films increased during storage reaching 9 MPa after 15 days. Elongation of both films decreased markedly during storage.