Plasticizer eect on mechanical properties of b-lactoglobulin ®lms Rungsinee Sothornvit a , John M. Krochta a,b, * a Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA b Department of Food Science and Technology, University of California, Davis, CA 95616, USA Received 1 March 2000; received in revised form 21 November 2000; accepted 29 November 2000 Abstract The mechanical properties elastic modulus, EM; tensile strength, TS and % elongation, %E) of b-lactoglobulin b-Lg) ®lms plasticized with dierent plasticizers were determined. Six plasticizer types were studied over a range of concentrations. Propylene- glycol-plasticized b-Lg ®lms were the most brittle, with mechanical properties independent of plasticizer content. Films with other plasticizers studied glycerol, Gly; sorbitol, Sor; polyethylene glycol, PEG 200 and PEG 400 and sucrose, Suc) exhibited negative exponential dependence on plasticizer concentration for EM and TS, while they exhibited linear dependence on plasticizer con- centration for %E. The EM and TS data for each plasticizer were ®tted with an exponential model, while %E data were ®tted with a linear model to quantify the plasticizer eect. The EM 0 , TS 0 and %E 0 of b-Lg ®lms without plasticizer determined from the ®tted equations were 1500, 37.28 MPa and 0, respectively. The k EM ; k TS and k E values determined from the ®tted EM, TS and %E data, respectively, re¯ect the eciency of plasticizers. The k EM ; k TS and k E values indicate that plasticizer eciency generally decreased in the order Gly, PEG 200, PEG 400, Sor and Suc, on the bases of mole plasticizer-oxygen-atom/mole b-Lg and mass plasticizer/mass b-Lg. These results re¯ect the eect of plasticizer composition, size and shape. The k EM ; k TS and k E value order was reversed when the basis was changed to mole plasticizer/mole b-Lg. The latter results clearly re¯ect the eect of plasticizer number of O atoms. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: b-lactoglobulin; Edible ®lms; Plasticizer; Mechanical properties 1. Introduction Edible ®lms and coatings are of interest since they have potential to improve food shelf life and quality and protect food from deterioration by microorganisms and physical damage. However, lack of edible ®lm and coating property data has limited their use in food ap- plications. In particular, basic research is still necessary to increase understanding of ®lm composition±struc- ture±function relations and thus enable food applica- tions. Whey protein isolate WPI) has been found capable of forming transparent ®lms and coatings that provide excellent oxygen, aroma and oil barrier properties at low relative humidity RH) McHugh & Krochta, 1994a; Mate & Krochta, 1996; Miller & Krochta, 1997; DeMulder-Johnston, 1999). Whey protein consists of b- lactoglobulin b-Lg), a-lactalbumin a-La), bovine se- rum albumin BSA) and some immunoglobulins. b-Lg is the major component 50±60% of whey protein) Dy- bing & Smith, 1991). The properties of ®lms made from b-Lg and WPI were compared by Mate and Krochta 1996). They found no signi®cant dierences in perme- ability of water vapor and oxygen between b-Lg and WPI ®lms. To simplify interpretation of results, b-Lg was selected to study eect of plasticizer on ®lm me- chanical properties in our study. b-Lg and WPI are generally heated to denature pro- tein and expose the internal sulfhydryl groups to allow formation of intermolecular disul®de bonds which aect the ®lm structure McHugh, Aujard, & Krochta, 1994b; Mate & Krochta, 1996). The combination of resulting intermolecular disul®de bonds and intermolecular in- teractions between protein chains based on hydrogen bonding, hydrophobic interactions and electrostatic forces produces brittle ®lms. Thus, plasticizers must be added at a certain amount to reduce protein chain-to- chain interaction and increase ®lm ¯exibility. Plasticizers are small molecular weight M W ) compounds Banker, 1966). Hydrophilic plasticizers, which can interfere with protein chain-to-chain hydrogen bonding, e.g., glycerol Gly), polyethylene glycol PEG), sorbitol Sor) and Journal of Food Engineering 50 2001) 149±155 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +1-530-752-2164; fax: +1-530-752- 4759. E-mail address: jmkrochta@ucdavis.edu J.M. Krochta). 0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4  0 0 ) 0 0 2 3 7 - 5