Key words:
soy protein, oxidized starch, edible films, mechanical properties, water vapour permeability

Summary:                                                                                                                                                                                 
The influence of oxidized starch on the physicochemical properties of cast soy protein isolate films is determined in this study. Films were cast from heated (70 °C for 20 min) alkaline (pH=10) aqueous solutions of 7 % soy protein isolate containing 50 % (by mass) glycerol as a plasticizer and different levels of added oxidized starch (0, 5, 10, 15, and 20 %, by mass). For all types of films, opacity, contact angle, tensile strength, elongation at break, water vapour permeability, measured at 25 °C for four relative humidity differentials (30–53, 30–75, 30–84 and 30–100 %), differential scanning calorimetry and microstructure were determined after conditioning film specimens at 25 °C and 30 % relative humidity for 48 h. Oxidized starch content significantly affected (p<0.05) tensile strength, from 0.95 to 1.51 MPa, temperature at maximum degradation rate from 64.8 to 74.2 °C and water vapour permeability from 1.08·10^–10 to 3.89·10^–10 g/(m·Pa·s) at relative humidity differentials of 30–50 and 30–100 %, respectively. Various internal arrangements were observed as a function of film composition (percentage of oxidized starch). 

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