FOOD HYDROCOLLOIDS Food Hydrocolloids 22 (2008) 485–491 Segregative interactions between gelatin and polymerised whey protein Sinead M. Fitzsimons, Daniel M. Mulvihill, Edwin R. Morris à Department of Food and Nutritional Sciences, University College Cork, Cork, Ireland Received 12 September 2006; accepted 16 January 2007 Abstract Polymerisation of whey proteins increases their potential use as food ingredients. In this investigation, mixtures of 6 wt% whey protein isolate (WPI) with 1–4 wt% type B (alkaline extract) gelatin, before and after polymerisation of WPI (heating to 85 1C and quenching to 5 1C), were characterised by measurements of solution viscosity at 45 1C (above the gelation temperature of gelatin) and by changes in optical rotation and turbidity (measured as optical density at 600 nm) over the temperature range 45–5 1C. Viscosities for mixtures of gelatin with un-polymerised WPI were only slightly higher than for gelatin alone, but after polymerisation they rose steeply above those of the individual components as the gelatin concentration was increased, giving values of 1.84 mPa s for 6 wt% polymerised WPI, 4.07 mPa s for 4 wt% gelatin, and 36.5 mPa s for the mixture of these. The large enhancements in viscosity are attributed to self- association of polymerised WPI in response to segregative interactions with gelatin. On cooling to 5 1C to gel the gelatin component, the unheated mixtures containing un-polymerised WPI showed only slight increases in turbidity, similar to those observed for WPI alone (polymerised or un-polymerised). Cooled mixtures of gelatin and polymerised WPI, however, gave very large increases in turbidity, which were fully reversible on re-heating to 45 1C. Optical rotation measurements showed no indication of direct association between gelatin and polymerised WPI at any temperature in the range studied (45–5 1C). It is suggested that association of gelatin molecules in the early stages of gelation triggers phase separation, giving a continuous matrix of gelatin gel with the polymerised whey protein dispersed through it in small droplets (which increase turbidity by scattering light). r 2007 Elsevier Ltd. All rights reserved. Keywords: Biopolymer mixtures; Thermodynamic incompatibility; Segregation; Whey protein polymers; Gelatin 1. Introduction Whey proteins, once considered the waste product of cheese manufacture, have been the subject of increasing interest (Mulvihill & Ennis, 2003) due to their high nutritional value. Because of their low molecular weight and compact, approximately spherical, globular structure they generate much lower solution viscosities than ex- panded polysaccharide coils, and have not traditionally been used as thickening agents. However, the viscosity of solutions containing whey proteins can be increased by heating under controlled conditions to form soluble whey protein polymers of high molecular weight. These have a much larger effective hydrodynamic volume than native globular proteins, giving rise to the increase in viscosity (Vardhanabhuti & Foegeding, 1999). Vardhanabhuti, Foegeding, McGuffey, Daubert, and Swaisgood (2001) defined ‘‘polymerised whey proteins’’ or ‘‘whey protein polymers’’ as soluble aggregates of whey protein that are formed on heating at temperatures and protein concentrations that would normally give a gel, but at low salt concentrations which prevent formation of a continuous network. Other terms that have been used to describe polymerised whey proteins are ‘‘pre-heated whey protein’’ (Barbut & Foegeding, 1993), ‘‘process whey protein’’ (Kinekawa, Fuyuki, & Kitabatake, 1998), and ‘‘heat denatured whey protein’’ (Bryant & McClements, 2000a). Whey protein polymers are formed by heating solutions to between 70 and 90 1C for 5–60 min at low salt concentrations and pH values well above the isoelectric point (usually at pH 7.0) to avoid immediate aggregation of the protein. The maximum salt concentrations normally used at pH 7.0 are 50 mM NaCl or 10 mM CaCl 2 (Bryant & McClements, 1998). Vardhanabhuti and Foegeding (1999) ARTICLE IN PRESS www.elsevier.com/locate/foodhyd 0268-005X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2007.01.005 à Corresponding author. Tel.: +353 21 4903625; fax: +353 21 4270001. E-mail address: ed.morris@ucc.ie (E.R. Morris).