Research And Application On Preparation Technology And Edible Film’s Forming Property Of Purple Sweet Potato Starches Modified By Dry Heating Method

Purple sweet potato is a new type of sweet potato with abundant nutrition, good color and tasty flavor. The main component of purple sweet potato is starch with high amylose content, which means purple sweet potato starch has a good film forming property. In this paper purple sweet potato starch, mixed with xanthan gum, was taken as the raw material to prepare modified purple sweet potato starch using dry heating method. The preparation technology and characteristics of modified purple sweet potato starch were studied. The film-forming process was optimized based on the modified purple sweet potato starch. Then the antimicrobial experiment of the modified purple sweet potato starch was carried out and this edible coating film was applied to the storage of fresh-cut durian fruit. The results provided a theoretical guidance for the application and utilization of modified purple sweet potato starch and its edible film in industrial production, and expanded the application of purple sweet potato starch edible film in fresh-cut fruit storage. The main contents are as follows:1. The effects of xanthan gum content, mixture pH, drying time and drying temperature on the characteristics of modified purple sweet potato starch were studied. The preparation conditions of purple sweet potato starches modified by dry-heating were optimized with the orthogonal experiment. The results showed that the optimal processing parameters were 1.0% of xanthan gum, mixture pH at 6,3 hours of drying time and drying temperature at 130℃. Under the optimal condition, the tensile strength, elongation at break and water vapor permeability of modified purple sweet potato starch film were tested as 2.58 MPa,43.46% and 1.96×10-12 g·cm/(cm2·s·Pa) respectively. Compared to the unmodified purple sweet potato starch film (with 1.77MPa of tensile strength,36.49% of elongation at break and 3.53 X 10"12 g-cm/ (cm2·s·Pa) of water vapor permeability), the properties of modified purple sweet potato starch film were greatly improved.2. The granule structure of the purple sweet potato starch and the physical-chemical properties of the starch paste were determined after modified with xanthan gum and dry heating method. The effect of dry heating treatment on the characteristics of the modified purple sweet potato starch was studied. The results showed that the modified starch granules were slightly expanded as well as adhered to each other, and the sizes of starch granules were not uniformly distributed. The modified starch had a lower onset gelatinization temperature, higher peak viscosity and final viscosity, and higher thermal stability. The modified starch paste presented significantly higher freeze-thaw stability, lower solubility and expansion degree, and decreased transparency. The rheological curve of the modified starch paste was shown as a counter clockwise loop, and the yield stress was significantly increased. The formed gel showed solid features with an increasing of storage modulus G’, and its resilience of shear structure was improved. The X-ray diffraction demonstrated that the modified starch, no matter before or after the dry-heating treatment, belonged to the ’C’ type, but the crystallinity of the starch granules increased after a dry heat treatment with xanthan gum.3. The effects of the adding amounts of modified purple sweet potato starch, glycerol and sodium alginate on the characteristics of modified purple sweet potato starch film were studied, and the parameters of the film forming process were optimized using response surface methodology. The optimal parameters of the film forming process were determined with 3.99% of modified purple sweet potato starch, 1.51% of glycerol and 0.60% of sodium alginate.4. The effects of the antibacterial agents concentrations and pH on the antimicrobial characteristics of dry-heating modified purple sweet potato starch edible films were studied, and the best antimicrobial condition was determined. Then the edible modified starch coating was applied to the fresh-cut durian fruit preservation. The results showed that the edible modified starch coating could inhibit the growth of Bacillus subtilis, Staphylococcus aureus, Escherichia coli and Salmonella with the addition of 0.025% of Nisin and 0.06% of Natamytin at pH 7. The edible coating with antimicrobial agents could effectively decrease the water loss, keep the nutrients and vivid color, delay the softening, and inhibit the microorganisms’ growth during the process of storage. Therefore a good flavor was maintained and the shelf life of fresh-cut durian fruit was extended.