Eur Food Res Technol (2006) 223: 91–95 DOI 10.1007/s00217-005-0140-9 ORIGINAL PAPER S. Guckian · C. Dwyer · M. O’Sullivan · E. D. O’Riordan · F. J. Monahan Properties of and mechanisms of protein interactions in films formed from different proportions of heated and unheated whey protein solutions Received: 31 May 2005 / Revised: 21 July 2005 / Accepted: 14 August 2005 / Published online: 20 December 2005 C  Springer-Verlag 2005 Abstract A range of films were formed using different proportions of heated (80 ◦ C for 30 min) and unheated whey protein isolate (WPI) solutions. The films, with lower proportions of heated WPI solution had lower percentage elongation, tensile strength and Young’s modulus, higher solubility and, in general, similar water vapour permeabil- ity, compared with a film formed from a heated WPI solu- tion. Hydrophobic interactions and hydrogen bonding dom- inated in the formation of the films with lower proportions of heated WPI solution, whereas disulphide bonding played a more important role in the formation of films with higher proportions of heated WPI solution. Keywords Edible films . Whey protein isolate . Water vapour permeability . Tensile properties . Disulphide bonding Introduction Edible films and coatings can prevent quality changes in foods by acting as barriers to unwanted migration of mois- ture, lipids and gaseous components (O 2 , CO 2 , volatile flavours) and they may also protect foods from mechani- cal damage and breakage [1, 2]. Whey proteins have been intensively investigated in the past decade as a material for use in the manufacture of edible films and coatings. Whey protein edible films are transparent, bland and flex- ible and possess excellent oxygen, aroma and lipid barrier properties at low relative humidity [2–4]. It has been suggested that to produce intact films, heat- ing of the whey proteins is necessary [5]. Heating of whey proteins is believed to induce molecular unfolding of the globular whey proteins, which leads to exposure of their S. Guckian · C. Dwyer · M. O’Sullivan () · E. D. O’Riordan · F. J. Monahan Department of Food Science, University College Dublin, Belfield, Dublin, 4 Ireland e-mail: michael.osullivan@ucd.ie Tel.: +353-1-7167158 Fax: +353-1-7161147 internal sulphydryl and hydrophobic groups. The unfolded proteins may then undergo intermolecular protein–protein interactions (entropic forces, dipolar and electrostatic inter- actions), which together with sulphydryl/disulphide inter- change reactions, are involved in the formation of whey pro- tein film matrices [6–9]. Films formed from heated whey protein are insoluble in water [2, 4], which can be a valuable attribute when the preservation of both film and food in- tegrity is a requirement. However, the insolubility in water of heated whey protein films is not always a desirable qual- ity and, for example, the films may be perceptible to the con- sumer during consumption of foods that contain such films. In order for edible films to fulfil functions such as forming a barrier between adjacent food layers, the film must be easily applied to the food product and remain on the product during storage, use and consumption. However, it should also disintegrate or dissolve to some degree during the cooking or mastication process, in order to achieve acceptable levels of sensory perception of the film within the food product. In the case of protein films, the disintegration process will most likely depend on intrinsic properties of the protein used for casting the film, for example solubility in water [10]. For such applications, the edible films used need a certain degree of solubility, which cannot be attained using films formed from heated whey protein isolate (WPI) solutions [2, 4]. Previously, films formed from unheated WPI solutions, as well as from heated WPI solutions were produced by others [4]. It was found that films formed from unheated WPI so- lutions had lower tensile properties and were water-soluble, in comparison with the water-insoluble films formed from heated WPI solutions. It was postulated that films formed from unheated WPI solutions had a less cohesive structure than films formed from heated WPI solutions, in which cohesion relies principally on covalent interactions, such as disulphide bonding. However, no experimental work has been reported to confirm this. It is possible that films manufactured from increasing ratios of heated to unheated WPI protein may have incrementally different mechanical, solubility and barrier properties, which may be useful for producing films for particular applications.