Rheology and microstructure of bimodal particulate dispersions: Model for foods containing fat droplets and starch granules Cheryl Chung a , Brian Degner b , David Julian McClements a, ⁎ a Department of Food Science, University of Massachusetts, Amherst, MA, 01003, United States b ConAgra Foods, Six ConAgra Drive, Omaha, NE 68102, United States abstract article info Article history: Received 29 March 2012 Accepted 11 June 2012 Keywords: Bimodal suspensions Mathematical modeling Viscosity Yield stress Emulsion Fat Starch Sauces The influence of fat content and emulsifier type on the rheology, optical properties, stability, and microstruc- ture of bimodal particle suspensions containing small fat droplets and large starch particles was studied. Fat droplets were coated by either small molecule surfactants (non-ionic/anionic mixture) or globular proteins (whey protein). Mixed systems containing oil droplets (0 to 30 wt.%) and modified starch (5 wt.%) were heat- ed (90 °C for 5 min) to promote starch gelatinization. The flow behavior, yield stress, and apparent viscosity of these systems were determined from shear stress versus shear rate measurements. For both emulsifiers, the yield stress of the mixed systems increased with increasing fat content, indicating that a close-packed system was formed. The apparent viscosity (at 20 s -1 ) increased with increasing fat content for protein-coated fat droplets as expected from theory, but it increased and then decreased for surfactant-coated lipid droplets. The unexpected behavior of the surfactant-based mixed systems was attributed to droplet coalescence and phase separation, as shown by microscopy and visual observation. Overall, these results have important impli- cations for designing high quality food products (such as sauces, dressings, and desserts) with reduced calories. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recent increases in the number of individuals in developed and de- veloping countries that are either overweight or obese has prompted governments and industry to identify effective strategies to tackle this problem (Brambila-Macias et al., 2011). The government has developed and promoted dietary and lifestyle guidelines to encourage consumers to eat fewer calories, consume more nutrient rich foods, and increase physical activity (Brawer, Brisbon, & Plumb, 2009). The food industry has responded by developing a variety of foods that pro- vide consumers with healthier options in their diets, such as reduced calorie foods, reduced fat foods, reduced carbohydrate foods, and foods designed to promote satiety and satiation (Nehir El & Simsek, 2012). The rational design of these foods relies on a fundamental understanding of the role of food composition and structure on their physicochemical, sensory, and nutritional properties. Many commonly consumed high calorie foods are complex multi- phasic materials containing mixtures of different kinds of particles. For example, many sauces, desserts, and dressings consist of a mixture of starch granules and fat droplets (de Wijk & Prinz, 2007; McClements, 2005; Sikora, Badrie, Deisingh, & Kowalski, 2008). The starch granules are usually used as a thickening agent due to their ability to swell upon heating and form a highly viscous or gel-like material (Tattiyakul, Liao, & Rao, 2009). The fat droplets provide a variety of desirable physical and sensory attributes to these foods: they lead to a creamy appearance due to their ability to scatter light (McClements, 2002b); they increase the viscosity due to their ability to alter the flow profile (Bayarri, Taylor, & Joanne, 2006); they provide a desirable mouthfeel due to their interac- tions with the tongue and palette (Bellamy, Godinot, Mischler, Martin, & Hartmann, 2009; Dresselhuis, Stuart, van Aken, Schipper, & de Hoog, 2008); they enhance the flavor profile due to their ability to solubilize non-polar flavor molecules (Bellamy et al., 2009; Dresselhuis et al., 2008). Both fat droplets and starch granules are a major source of calories in the human diet, and their overconsumption has been implicated in chronic diseases such as obesity, diabetes, and heart disease (Swinburn, Caterson, Seidell, & James, 2004). The design of reduced calorie versions of traditional foods requires that some or all of the fat and/or starch is re- moved while maintaining their desirable physicochemical and sensory attributes. In addition, the approaches used must be commercially viable and economically feasible. The development of reduced calorie versions of these foods therefore depends on improved knowledge of the role that fat droplets and starch granules play in determining their overall properties. The purpose of this study was to improve our understanding of the influence of fat droplets and starch granules on the appearance, rheology, and stability of model sauces, desserts, and dressings. To a first approximation these semi-solid food products can be considered to be bimodal particulate suspensions that contain a mixture of large Food Research International 48 (2012) 641–649 ⁎ 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.2012.06.011 Contents lists available at SciVerse ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres