Sodium dodecyl sulfate treatment improves properties of cast films from soy protein isolate

The manufacture of edible/biodegradable films or coatings can potentially add value to soy protein. This study was conducted to determine the effect of sodium dodecyl sulfate (SDS) on selected physical properties of glycerin-plasticized soy protein isolate (SPI) films. Films were cast from heated (70 °C for 20 min), alkaline (pH 10) aqueous solutions of SPI (5 g/100 ml water), glycerin (50% w/w of SPI), and SDS (0, 5, 10, 20, 30, or 40% w/w of SPI). Tensile strength (TS), elongation at break (E), moisture content (MC), total soluble matter (TSM), water vapor permeability (WVP), and color values (L, a, and b) were determined after conditioning film specimens at 25 °C and 50% relative humidity (RH) for 2 days. SDS reduced (P<0.05) film TS by as much as 43% for films with 40% SDS (6.2 vs. 10.9 MPa for control SPI films). In contrast, film E increased (P<0.05) notably with addition of SDS even at 5%. Films with SDS had smaller (P<0.05) MC and larger (P<0.05) TSM values than control SPI films. Films containing 10% or more SDS had lower WVP values than control SPI films by as much as 50%. Increased yellowness, evidenced by greater (P<0.05) +b color values, was noted for films with high amounts (20, 30, or 40%) of SDS. Changes in tensile, solubility, and water vapor barrier properties of SPI films due to the addition of SDS were largely attributed to disruption of hydrophobic associations among neighboring protein molecules as the non-polar portions of the SDS molecules attached onto hydrophobic amino acid residues within the film structure. It was demonstrated that adding anionic surfactant SDS to film-forming solutions prior to casting could greatly modify the properties of SPI films. In particular, SDS improved the water vapor barrier ability and the extendibility of SPI films, both desirable attributes when assessing the potential of such films for packaging applications.