Properties of flour films as affected by the flour's source and particle size

Wheat (W) and rye flour (R, R1 and R2) films were developed. The rye flours used differed in their particle size. R1 and R2 resulted from jet milling of the R flour at two different feed rates. The mean particle size of W, R, R1 and R2 flours was 173.87, 100.91, 54.59 and 35.66 μm, respectively. Glycerol was used as plasticizer at concentrations of 0.5, 0.7 and 1 g/g of flour. Films made from mixtures of flours (50:50 w/w) were also studied in the presence of 0.7 g glycerol/g of flour. Colour, moisture content, solubility, thickness, viscosity, microstructure and mechanical properties were assessed. In most cases the films were homogeneous. Film moisture content, thickness, opacity and mechanical properties were correlated to glycerol concentration. Particle size was found important for the mechanical properties. Wheat films were lighter ([L*] ≈ 49) and less opaque (135). For both single and composite films, moisture content varied from 24 to 35%, solubility from 43.5-58%, opacity from 135 to 371, thickness from 0.66-0.72 mm and mechanical strength from 0.68-1.7 MPa. All film-forming solutions exhibited shear thinning behaviour. Wheat flour dominated most of the studied properties in composite films. Rye films were more opaque, with relatively high solubility but greater mechanical strength.