Eectofsoyaoilandglycerolonphysicalpropertiesof compositeWPI®lms N.B.Shaw,F.J.Monahan * ,E.D.O'Riordan,M.O'Sullivan Department of Food Science, University College Dublin, Agriculture Building, Bel®eld, Dublin 4, Ireland Received1June2000;accepted29March2001 Abstract The water vapour permeability WVP), mechanical strength, equilibrium moisture content MC), glass transition tempera- ture and opacity of whey protein isolate ®lms plasticised with dierent levels of glycerol Gly) glycerol:protein Gly:Pr) 0.5, 0.6 and 0.7) and containing dierent levels of added soya oil oil:protein Oil:Pr) ratio 0.0, 0.2, 0.3 and 0.4) were measured. Increasing oil concentration led to increases in percent elongation EL), glass transition T g temperature, and de- creases in MC, tensile strength TS), and elastic modulus EM), but did not aect the WVP of ®lms. Increasing levels of Gly led to increases in percent EL, MC and decreases in TS, EM, T g and ®lm opacity. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: CompositeWPI®lm;Soyaoil;Watervapourpermeability;Mechanicalproperties;Glasstransitiontemperature 1. Introduction The formation of ¯exible ®lms from hydrocolloids requires two essential ingredients: a ®lm forming polymeric material to provide structural support, and aplasticiser,suchasalowmolecularweightpolyol,to reduce ®lm brittleness McHugh & Krochta, 1994a). In ®lms manufactured from whey protein isolate WPI), increasing levels of glycerol Gly) and sorbitol have been reported to increase ®lm extensibility and reduce ®lm strength McHugh, Aujard, & Krochta, 1994; McHugh & Krochta, 1994b). However, high water vapour transfer is commonly a feature of ®lms containing high levels of these plasticisers McHugh et al., 1994). In contrast, waxes and fatty acids were found to be eective barriers to water vapour transfer Greener & Fennema, 1989a,b; Kester & Fennema, 1989a,b,c) but lacked sucient strength to form self- supporting ®lms. It has been proposed that composite ®lms, which contain both hydrocolloid and lipid components, should possess good water barrier prop- erties with structural strength Greener & Fennema, 1994). Previousstudiesofcomposite®lmshavefocussedon incorporating fatty acids into hydrocolloid ®lms Kamper & Fennema, 1984a,b; Greener & Fennema, 1989a,b, 1992; Kester & Fennema, 1989a; Hagenmaier & Shaw, 1990; Martin-Polo, Mauguin, & Voilley, 1992a,b; Fennema, Donhowe, & Kester, 1994; McHugh & Krochta, 1994c; Park, Testin, Park, Verg- ano, & Weller, 1994) and have shown that the lipid component eectively reduced ®lm water vapour per- meability WVP). Shellhammer and Krochta 1997) found that water vapour barrier properties and degree of viscoelasticity of the lipid component aected the barrier and mechanical properties of composite WPI ®lms incorporating a variety of waxes and an anhy- drous milkfat fraction. Information regarding the physical properties of WPI composite ®lms containing naturallyoccurringfoodoilssuchassoyaoilislimited. Incorporation of these low melting point lipids into composite ®lms may provide an alternative means of plasticising ®lms whilst limiting water vapour transfer. Such ®lms may also be more organoleptically accept- able than those containing higher melting point lipids, and lend themselves to a wider range of food appli- cations. The objective of this study was to investigate the in¯uence of varying proportions of Gly and soya oil on the physical properties of ®lms formed from WPI. JournalofFoodEngineering512002)299±304 www.elsevier.com/locate/jfoodeng * Correspondingauthor.Tel.:+353-1-7067090;fax:+353-1-7067090. E-mail address: frank.monahan@ucd.ieF.J.Monahan). 0260-8774/02/$-seefrontmatter Ó 2002ElsevierScienceLtd.Allrightsreserved. PII:S0260-877401)00071-1