Current Opinion in Colloid & Interface Science 18 (2013) 349–359 Contents lists available at SciVerse ScienceDirect Current Opinion in Colloid & Interface Science journal homepage: www.elsevier.com/locate/cocis Oral processing, texture and mouthfeel: From rheology to tribology and beyond☆ Jason R. Stokes a,b,⁎, Michael W. Boehm a, Stefan K. Baier c a School of Chemical Engineering, The University of Queensland, Brisbane 4072, Queensland, Australia b ARC Centre of Excellence of Plant Cell Walls, The University of Queensland, Brisbane 4072, Queensland, Australia c PepsiCo Long Term Research, 3 Skyline Dr., Hawthorne, NY 10532, USA article info abstract Article history: Texture and mouthfeel arising from the consumption of food and beverages are critical to consumer choice and Received 1 March 2013 acceptability. While the food structure design rules for many existing products have been well established, al- Received in revised form 18 April 2013 though not necessarily understood, the current drive to produce healthy consumer acceptable food and beverages Accepted 18 April 2013 is pushing products into a formulation space whereby these design rules no longer apply. Both subtle and large Available online 30 April 2013 scale alterations to formulations can result in significant changes in texture and mouthfeel, even when measurable texture-related quantities such as rheology are the same. However, we are only able to predict sensations at the Keywords: Oral processing initial stages of consumption from knowledge of material properties of intact food. Texture Research is now on going to develop strategies to capture the dynamic aspects of oral processing, including: from Mouthfeel a sensory perspective, the recent development of Temporal Dominance Sensation; from a material science per- Tribology spective, development of new in vitro techniques in thin film rheology and tribology as well as consideration of Lubrication the multifaceted effect of saliva. While in vivo, ex vivo, imitative and empirical approaches to studying oral pro- Rheology cessing are very insightful, they either do not lend themselves to routine use or are too complex to be able to as- fi Thin lm certain the mechanism for an observed behaviour or correlation with sensory. For these reasons, we consider that Gap dependent fundamental in vitro techniques are vital for rational design of food, provided they are designed appropriately to Saliva Fat capture the important physics taking place during oral processing. We map the oral breakdown trajectory Oil through 6 stages and suggest a dynamic multi-scale approach to capture underlying physics. The ultimate goal Emulsions is to use fundamental insights and techniques to design new food and beverages that are healthy yet acceptable Hydrocolloid to consumers. Polysaccharide © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. Microstructure Health Obesity 1. Introduction a good enough understanding exists to re-design different types of foods to have largely the same rheological features [1,2]. Therefore, it is possible Both natural and processed foods contain hierarchical structures and to design food rationally to meet rheological criteria and for meeting spe- multiples phase, ranging in length scale from the nanoscopic to the mac- cific nutritional requirements; for instance, the role of hydrocolloids in roscopic. These structures are present to provide certain functionality nutrition and digestion is covered elsewhere in this issue [3]. However, such as nutritional value and texture control or to aid processing and foods created in this rational way still fail to meet consumer expecta- shelf-stability. Rheology is used as an essential design tool in engineering tions: consumers are let down by the overall sensory experience, which food as it is important to processing, shelf stability and sensory percep- is strongly influenced by the food and beverages' organoleptic properties. tion, including texture and mouthfeel, and it can probe the overall struc- We address here consideration of in vitro strategies that provide insights ture as well as the interplay between individual colloidal components. into oral processing and, when coupled to in vivo studies, will better There is extensive knowledge on the complex relationships between rhe- enable rational design of foods and beverages. ology and the dominant underlying structure of foods and beverages, and Texture and mouthfeel play pivotal roles in product acceptability, and except for the point at which food enters the mouth (e.g., first bite of ☆ This is an open-access article distributed under the terms of the Creative Commons solids, initial thickness of liquids), we cannot currently predict these per- Attribution-NonCommercial-No Derivative Works License, which permits non-commercial cepts using fundamental rheological properties of the food and beverage •• use, distribution, and reproduction in any medium, provided the original author and source [4 ] or through measurements derived from imitative or empirical tech- are credited. niques such as “texture profile analysis” (TPA) using a texture analyser ⁎ Corresponding author at: School of Chemical Engineering, The University of • [5,6 ]. Consequently, replicating foods with healthier formulations has Queensland, Brisbane 4072, Queensland, Australia. Tel.: +61 7 336 54361; fax: +61 7 fi 336 54199. proven dif cult, and important questions arise as to what role ingredients E-mail address: [email protected] (J.R. Stokes). like fat play that makes it so desired in food and renders the texture of 1359-0294/$ – see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cocis.2013.04.010 350 J.R. Stokes et al. / Current Opinion in Colloid & Interface Science 18 (2013) 349–359 products more acceptable [7]. Similarly in beverages, when basic tastants food, such as astringency and mouthcoating; after-feel is a term common- such as bulk sweeteners (e.g., high fructose corn syrup) are substituted ly used to describe these mechanical sensations that are also inherently with alternative sweeteners (e.g., aspartame), even if apparent sweetness part of texture perception. Hence, texture is not just about rheology, but level is the same, the perceived mouthfeel is substantially altered [8,9]. texture also encompasses tactile mechano-sensations associated with New insights are needed into the physical drivers for texture and mouth- the contact between the food, food residue and human oral surfaces [17]. feel if food manufacturers are going to be in a position to rationally design These accepted definitions of texture highlight its truly multi- food with an enhanced nutritional profile that is also acceptable to dimensional nature and emphasise that it is not a simple food property consumers. that can be measured instrumentally [17].Regardless,aconsiderable Food Oral Processing involves comminuting solid food to small parti- amount of effort has been expended to do just this using imitative me- cle sizes, mixing with saliva, and forming a bolus that is then swallowed chanical tests and rheology, as covered in the next section. However, and transferred to the stomach [4••,10];thefirst book on the subject has these endeavours rarely consider contributions to texture from structural recently been published [11••]. Regardless of the initial state of food, it and surface properties, and they are also unable to consider cross-modal undergoes a conversion to a form that is rheologically suitable for influences from the different senses. Tactile mechano-sensation plays a swallowing in a highly sophisticated dynamic process [12,13••].The major role in the perception of texture and mouthfeel, yet this is unlikely organoleptic properties of food, including texture perception, should de- to be captured through rheology measurements; of closer relevance is pend on the constantly changing status of the food during oral process- measurement techniques in tribology, which considers the forces associ- ing [14] as well as the changing status of the salivary film coating oral ated with interacting surfaces in relative motion (covered later in this surfaces and saliva itself [11••,15•,16•]. Utilising knowledge of oral pro- review). In addition, what is often not considered or quantified in both cessing in the rational design of foods is challenging and relevant in sensory and mechanical measurements is that food undergoes a major vitro measurement techniques are needed that provide mechanistic in- transformation upon entering the mouth, so exactly what structural, me- sights into texture/mouthfeel and can be used in food structure design, chanical and surface properties of the food and the food bolus are relevant but these also require validation using in vivo studies and sensory to the perception at any particular time point is open for debate. science. Fig. 1 depicts the transition in film thickness of fluid-like foods or This review will consider oral processing and texture/mouthfeel beverages between oral surface as they are consumed, indicating it with particular emphasis on developing in vitro strategies to capture goes from a rheology-dominant deformation process to one where the dynamic nature of oral processing and the changing status of food tribology (surface properties) dominates. during consumption, as well as the underlying physics/mechanics Sensory profiling traditionally involves a descriptive approach and taking place during this process. quantification of intensity after eating