Lubricating properties of human whole saliva as affected by b-lactoglobulin B. Vardhanabhuti a, * , P.W. Cox b , I.T. Norton b , E.A. Foegeding c a Food Science Program, University of Missouri, Columbia, MO 65211-5160, USA b Department of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK c Department of Food, Bioprocessing and Nutrition Sciences, North Carolina State University, Raleigh, NC 27695, USA article info Article history: Received 21 December 2009 Accepted 20 February 2011 Keywords: Astringency Saliva Lubrication Whey proteins Tribology abstract The effect of b-lactoglobulin (b-LG) at pH 3.5 and 7.0 on lubricating property of saliva as related to astringency perception was investigated using tribology. Saliva was adsorbed onto surfaces of a rotating poly dimethylsiloxane (PDMS) ball and disc to form a film under conditions that mimic the rubbing contacts in the oral cavity (Bongaerts, Rossetti, & Stokes, 2007) and the lubricity of saliva films upon exposure to astringent compounds was measured. While addition of non-astringent b-LG at pH 7.0 slowly increased friction of saliva film between tribopair surfaces, b-LG at pH 3.5 rapidly increased the friction coefficients of saliva, similar to other astringent compounds (epigallocatechin gallate and alum). This supports the hypothesis that astringency of b-LG arises from the loss of lubrication of saliva which is in agreement with the well-accepted astringency model of polyphenols. Increasing b-LG concentration at pH 3.5 (0.5e10% w/w) caused a rapid increase in friction coefficient; however, at the highest protein concentration, the friction coefficient, although higher than observed for water, was below the values observed for the lower protein concentrations. This suggests that static tribology testing is different from the dynamic in-mouth system such that a simple relationship between friction and sensory astringency cannot be found for all conditions. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Astringency is defined as the complex sensation due to shrinking, drawing or puckering of the epithelium as a result of exposure to substances such as alums or tannins (ASTM, 2004). Though some studies have suggested astringency to be mediated through taste-like receptors (Kawamura, Funakoshi, Kasahara, & Yamamoto, 1969; Schiffman, Suggs, Sostman, & Simon, 1992), it has been more widely accepted that astringency is a tactile perception (Bate-Smith, 1973; Breslin, Gilmore, Beauchamp, & Green, 1993; Lyman & Green, 1990; Smith, June, & Noble, 1996). Astringency of polyphenols, the most widely studied astringent compounds, has been proposed to arise from complexation with proline-rich salivary proteins, leading to the loss of saliva lubrica- tion (Baxter, Lilley, Haslam, & Williamson, 1997; Charlton et al., 2002; Kallithraka, Bakker, & Clifford, 1998; Kallithraka, Bakker, & Clifford, 2001). In a comprehensive review of astringency mecha- nisms and perception, Bajec and Pickering (2008) demonstrated that astringency is a complex, multifaceted sensation and that the investigation of its mechanisms is complicated by a number of variables. They concluded that distinct astringent compounds may utilize different pathways in eliciting astringency and that both tactile and chemical stimulation may contribute to the sensation. Tribology has been used to study the role of surface properties on adhesiveness and lubricity in engineering applications such as coating, bearing, and engine oil design (Erdemir, 2005; Holmberg, Matthews, & Ronkainen, 1998), development of artificial joints (Unsworth, 1995), and cosmetic industry (Dowson, 1997). Recently, food researchers have used tribology in order to understand how foods and beverages are perceived in the oral cavity, the lubricating properties of saliva, as well as how foods influence oral lubrication (Dresselhuis, de Hoog, Cohen Stuart, & van Aken, 2008; Giasson, Israelachvili, & Yoshizawa, 1997; de Hoog, Prinz, Huntjens, Dresselhuis, & van Aken, 2006; Luengo, Tsuchiya, Heuberger, & Israelachili, 1997; Malone, Appelqvist, & Norton, 2003; Rossetti, Bongaerts, Wantling, Stokes, & Williamson, 2009; Stokes et al., 2008; de Vicente, Stokes, & Spikes, 2006). While the mechanism for astringency is often ascribed to delubrication, there is generally no direct evidence of changes in lubrication properties. The loss of lubrication is a logical deduction since one major function of saliva is lubrication (Mandel, 1987) and, if astringency is a tactile sensa- tion, then it should manifest itself by some physical means. Using a tribometer, frictional properties can be determined by measuring the traction force between two surfaces in rubbing motion. * Corresponding author. Tel.: þ1 573 882 1374; fax: þ1 573 884 7964. E-mail address: Vardhanabhutib@missouri.edu (B. Vardhanabhuti). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.02.021 Food Hydrocolloids 25 (2011) 1499e1506