Beyond surface selection: The impact of different methodologies on tribological measurements Helen S. Joyner (Melito) a,⇑ , Chris W. Pernell b,1 , Christopher R. Daubert b,2 a School of Food Science, University of Idaho, 606 Rayburn St., MS 2312, Moscow, ID 83843, United States b Department of Food, Bioprocessing, and Nutrition Sciences, North Carolina State University, Campus Box 7624, Raleigh, NC 27695, United States article info Article history: Received 10 January 2014 Received in revised form 24 February 2014 Accepted 2 March 2014 Available online 12 March 2014 Keywords: Soft tribology Measurement Friction coefficient abstract Emerging links between food friction and sensory texture have prompted an increase in food tribology studies. Soft tribological surfaces (e.g. elastomers) are used in these studies to mimic conditions in the oral cavity. However, measurement protocols and tribological surfaces vary among food tribological stud- ies. Because the majority of these studies use empirical measurements, it is difficult to quantitatively compare their results. Although the effects of surface hydrophobicity, roughness, and modulus on friction coefficient measurements have been examined, there has been little study of variability due to differ- ences in measurement parameters. Therefore, this study examined the effect of different measurement parameters on friction coefficient magnitude and variation. Tribological data were affected by multiple measurement parameters. A measurement protocol that yielded repeatable measurements with low var- iation was proposed based on these results. It is suggested that a standard protocol for food tribological measurements be adopted to enable proper data comparison among studies. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Tribology has been used for decades by the chemical and material engineering industries to determine frictional properties of various substances, such as lubricants and rubber (Dresselhuis et al., 2007). More recently, tribology has been used in food research to study the frictional properties of oil-in-water emulsions (Bellamy et al., 2009; Chojnicka et al., 2008; de Hoog et al., 2006; Dresselhuis et al., 2007; Dresselhuis et al., 2008a,b), mayonnaise (de Wijk and Prinz, 2005; Giasson et al., 1997; Terpstra et al., 2009), chocolate (Carvalho-da-Silva et al., 2013; Luengo et al., 1997), and various dairy products (Chojnicka-Paszun et al., 2012; de Wijk and Prinz, 2005; de Wijk et al., 2006; Meyer et al., 2011). There has also been an effort to relate food frictional properties to sensory behavior (Bellamy et al., 2009; Chojnicka-Paszun et al., 2012; de Wijk and Prinz, 2005; de Wijk et al., 2006; Dresselhuis et al., 2008a; Giasson et al., 1997). It has been suggested that tribo- logical study may provide information on food sensory behavior that cannot be determined by standard rheometry (Chen and Stokes, 2012; Chojnicka-Paszun et al., 2012; Dresselhuis et al., 2008a; Giasson et al., 1997; Luengo et al., 1997; Malone et al., 2003; Terpstra et al., 2009). Current methodology for measuring tribological properties of foods has been adapted from traditional methodology in an effort to better mimic the movement of food in the oral cavity. A variety of tribometers, including Mini-Traction Machines (Chojnicka et al., 2008; Chojnicka-Paszun et al., 2012; de Vicente et al., 2006a,b; Garrec and Norton, 2012; Myant et al., 2010a), rheometers equipped with tribological cells (Carvalho-da-Silva et al., 2013; Goh et al., 2010; Krzeminski et al., 2012), atomic force devices (Harvey et al., 2012; Luengo et al., 1997), and custom-made labo- ratory apparatus (de Hoog et al., 2006; de Wijk and Prinz, 2005; de Wijk et al., 2006; Dresselhuis et al., 2007; Giasson et al., 1997; Prinz et al., 2007), have been used to determine the tribological properties of foods and other materials. While traditional tribolog- ical methodology is performed using steel-on-steel or other hard surfaces (Dresselhuis et al., 2007), tribological measurements of foods are generally performed on softer surfaces, such as silicone gels (Bellamy et al., 2009; de Vicente et al., 2006a,b; Dresselhuis et al., 2007; Garrec and Norton, 2012; Myant et al., 2010a,b), rub- ber (Chojnicka-Paszun et al., 2012; de Wijk et al., 2006; Giasson et al., 1997; Krzeminski et al., 2012), or cadaver tissue (de Hoog et al., 2006; Dresselhuis et al., 2007, 2008a,b; Prinz et al., 2007). These softer surfaces are more similar to oral surfaces and are used to more accurately mimic conditions in the oral cavity (Chen and Stokes, 2012; de Hoog et al., 2006; Garrec and Norton, 2012). http://dx.doi.org/10.1016/j.jfoodeng.2014.03.003 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 208 885 9683. E-mail addresses: hjoyner@uidaho.edu (H.S. Joyner (Melito)), cwpernel@ncsu. edu (C.W. Pernell), cdaubert@ncsu.edu (C.R. Daubert). 1 Tel.: +1 919 513 7674. 2 Tel.: +1 919 515 2951. Journal of Food Engineering 134 (2014) 45–58 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng