Formation of semi-solid lipid phases by aggregation of protein microspheres in water-in-oil emulsions Shahid Iqbal a , Gulzar Hameed a , Musa Kaleem Baloch a , David Julian McClements b, ⁎ a Department of Chemistry, Gomal University, Dera Ismail Khan, Pakistan b Biopolymers and Colloids Research Laboratory, Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA abstract article info Article history: Received 20 February 2012 Accepted 11 April 2012 Keywords: Emulsions Microspheres Proteins Reduced fat Reduced trans‐fat Gelation WPI Rheology Delivery systems Controlled aggregation of protein microspheres in water-in-oil (W/O) emulsions was used to form semi-solid lipid materials. The aqueous phase consisted of 10 wt% whey protein isolate (WPI) in buffer solution (pH 7.0, 100 mM NaCl). The oil phase consisted of a lipophilic nonionic surfactant (8 wt % polyglycerol poly- ricinoleate, PGPR) dispersed in a liquid oil (soybean oil). Lipid phases containing protein microspheres were formed by homogenization of the oil and aqueous phases to form a W/O emulsion followed by heating (90 °C for 30 min) to promote gelation of the WPI in the aqueous phase. Temperature-scanning dynamic shear measurements showed that the W/O emulsions underwent an irreversible liquid-to-solid transition when heated above the thermal denaturation temperature of WPI, which was attributed to protein gelation and microsphere aggregation. Optical microscopy indicated that a three-dimensional network of aggregated protein microspheres was formed at high aqueous phase contents (> 30 wt %). Shear rheology measurements (shear stress versus shear rate) indicated that these structured emulsions were non-ideal plastic-like materials. The apparent shear viscosity increased with thermal treatment, increasing aqueous phase content, and decreas- ing shear rate. The structured W/O emulsions developed in this study may be useful materials for the develop- ment of foods with highly viscous or gel-like lipid phases, but low saturated or trans-fat contents. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction A number of chronic diseases have been associated with over- consumption of total fat, saturated fats, and trans‐fatty acids, such as obesity, coronary heart disease, diabetes and hypertension (Abete, Goyenechea, Zulet, & Martinez, 2011; Cotton, Burley, Weststrate, & Blundell, 2007; Dhaka, Gulia, Ahlawat, & Khatkar, 2011). There has therefore been considerable emphasis on the development of lipid- based food products with reduced levels of saturated and trans-fats (Nehir El & Simsek, 2012; Wassell, Bonwick, Smith, Almiron-Roig, & Young, 2010). One of the major challenges in developing these products is that saturated and trans-fats play an important role in determining the desirable textural and sensory properties of many food products (Bayarri, Taylor, & Joanne, 2006; McClements & Demetriades, 1998). Saturated and trans-fats have relatively high melting points, which mean they are fully or partly crystalline at ambient temperatures (Acevedo, Peyronel, & Marangoni, 2011; Narine & Marangoni, 1999; Wassell et al., 2010). Under appropriate conditions, these high melting fats form three-dimensional networks of aggregated fat crystals in lipid- based products (such as butter, margarine, and shortenings), leading to characteristic solid-like properties, such as plasticity and spreadability (Narine & Marangoni, 1999; Rogers, 2009; Smith, Bhaggan, Talbot, & van Malssen, 2011). When high melting fats are removed from the lipid phases of fatty foods it is often difficult to obtain the desired rheo- logical characteristics. There is considerable interest in developing new strategies to create lipid phases with solid-like characteristics without using satu- rated or trans‐fats (Nehir El & Simsek, 2012; Wassell et al., 2010). A number of structural design approaches have been developed to achieve this goal, including the utilization of alternative sources of high melting lipids that form crystal networks (such as phytosterols, phytostanols, and waxes), the formation of gel-like association col- loids within the oil phase using surface-active lipids (such as mono- glycerides, phospholipids, or surfactants), and the utilization of multiple emulsions (Hughes, Marangoni, Wright, Rogers, & Rush, 2009; Lupi, Gabriele, de Cindio, Sanchez, & Gallegos, 2011; Rogers, 2009; Rogers, Wright, & Marangoni, 2009; Surh, Vladisavljevic, Mun, & McClements, 2007; Wassell et al., 2010). The current study examined the possibility of creating solid-like lipid phases by inducing aggregation of protein microspheres within water- in-oil (W/O) emulsions. The protein microspheres were produced by thermal denaturation of globular proteins (whey protein isolate) dis- persed within the aqueous phase of W/O emulsions. Thermal denatur- ation of whey proteins under appropriate pH and ionic strength conditions promotes protein aggregation, which leads to gelation at pro- tein concentrations high enough to form a three-dimensional network Food Research International 48 (2012) 544–550 ⁎ Corresponding author. Tel.: + 1 413 545 1019. E-mail address: mcclements@foodsci.umass.edu (D.J. McClements). 0963-9969/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2012.04.020 Contents lists available at SciVerse ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres