Oral processing of semi-solid and soft-solid foods

Scientists define solid food and liquid food as two extremes of one continuum, with semi-solid food (semi-liquid food) in the middle (Bourne, 2003). As yet, there is no generally agreed definition of semi-solid foods, as it is not always possible to determine whether a material is behaving as a liquid or a solid. Semi-solid food is related to the rate of deformation when stress is applied. Therefore, many semi-solid foods display viscous and/or elastic properties (McKenna, 2003). From previous studies, it is well known that semi-solid foods include some emulsions, and food emulsions demonstrate a great range of rheological characteristics. Thus, the oral manipulation and swallowing of semi-solid food is quite different from solid food (Hiiemae & Palmer, 2003). For semi-solid food generally only the tongue is used to compress food on the hard palate and spread it. The mashed food is propelled or pushed into the pharynx by the tongue in the stage II transport cycles. A bolus accumulates in the oropharynx during multiple transport cycles (oropharyngeal aggregation time, which may last up to about 10 or 12 seconds in healthy individuals). Subsequently, the swallowing process is more like a liquid: the tongue surface sweeps the remaining food from the oral cavity into the pharynx (squeeze-back), and the pharyngeal surface of the tongue pushes backward to propel food through the pharynx (tongue base retraction) (Hiiemae & Palmer, 1999, 2003).

Chen and associates conducted a series of experiments to study oral processing in relation to the food properties of a full range of liquid, semi-solid and solid foods. They tested food bolus properties during oral processing, focusing on the swallowing phase. Regarding solid foods, Chen, Karlsson, and Povey (2005) marked the acoustic ranking

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of six different biscuits, and their results from instrumental assessment were consistent with results from sensory panel tests. Regarding liquid and semi-solid foods, such as lab-constituted liquid or gel and commercial foods, Chen and Moschakis (2006) discovered the heat-set whey protein gel had a smoother surface with salt addition than without salt addition. They further concluded that the pressure drop and cavitation of a suddenly stretched fluid could be critical in influencing perception of food stickiness in another study (Chen, Feng, Gonzalez, & Pugnaloni, 2008). The viscoelasticity of biopolymer fluids composed of casein and waxy maize starch was a factor influencing the stretching of biopolymer fluids, which was quantified by stretch-ability (Chan et al., 2009). Chen and Lolivret (2011) used eighteen commercial fluid foods and ten lab- constituted foods (liquid and semi-solid foods) to investigate the importance of food bolus rheological properties on swallowing. They found that the bolus rheology, especially its extensional stretch-ability, had the critical influence on the ease of swallowing. Chen and Stokes (2012) found that tribology was another key aspect to understand food oral processing, texture and mouthfeel, because it involved fluids’ rheological properties as well as the surface properties of interacting substrates in relative motion. In addition, Alsanei and Chen (2014) found a positive correlation between the maximum tongue pressure and the maximum consistency of bolus that the subject swallowed for those who had lower tongue pressure generation capacity (< 40 kPa).

Chen (2007) also reviewed the surface texture as an important sensory perception factor for consumers. After that, Chen and Lolivret (2011) explained the relationship between sensory perception and food physical properties. Recently, Chen and Eaton (2012) confirmed that creaminess was not a key sensory property but an integrated sensory experience.

In summary, Chen’s studies demonstrated that the oral processing of semi-solid foods is impacted by food properties, especially food rheological properties and tribology in the oral cavity. He also related food properties to human sensory perception during oral processing of semi-solid foods, which provides important information for food development.

Besides food properties, oral sensitivity, tongue movements, temperature, saliva composition and oral physiology are also important to the perception of semi-solid food

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(Engelen & Van Der Bilt, 2008). Furthermore, all these factors impact the oral processing of semisolid food.

Shear in oral cavity

Shear is the key factor in food rheology. It is considered to be the main force of oral processing of semi-solid foods. Shear is a force that one plane exerts on a neighbouring plane per unit area of contact, and which causes a deformation in a direction related to the direction of the applied force (International Food Information, 2009). Shear forces are applied during food processing, both in vitro and in human oral cavity, such as mixing, extrusion or pressing, and mastication. Shear forces affect the texture of final product or ready-to-swallow bolus.

Shear stress is the stress component applied tangentially to the plane on which the force acts. It is expressed in units of force per unit area. Shear stress is a force vector that possesses both magnitude and direction (Bourne, 1982a). Shear related parameters are used to predict or describe the texture property of semi-solid foods (Terpstra et al., 2005; Terpstra et al., 2009). Terpstra et al. (2005, 2009) investigated the relationship between orally perceived thickness and calculated shear stress on the tongue for two types of viscous semi-solid food: mayonnaise and custard. They found a linear relationship between calculated shear stress and thickness within a limited range of shear stresses (mayonnaise < 150Pa; custard < 30Pa). This result is similar to the work of Kokini (Elejalde & Kokini, 1992; Kokini, Kadane, & Cussler, 1977). Outside these limited ranges, the linear relationship broke down and the perceived thickness levelled off with shear stress for both mayonnaise and custard (Terpstra, et al., 2005).

Shear rate is an important parameter to assess food rheological properties in the mouth. It is the velocity gradient established in a fluid as a result of an applied shear stress (Bourne, 1982b). In food processing, the effective shear rate is representative of oral deformation (Cutler, Morris, & Taylor, 1983; Houska et al., 1998; Shama & Sherman, 1973; Terpstra, et al., 2005). Various researchers have tried to determine it and their studies can be classified into two main types: large-deformation (steady-shear) measurements which were used prior to 1982; and small deformation (dynamic) viscosity measurements (Stanley & Taylor, 1993). During oral processing, a force is applied by the tongue and teeth, causing shear stress and food break-up. The shear rate

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operating in the mouth during eating is not constant (Shama & Sherman, 1973) and varies over several orders of magnitude depending on the food. The shear rates in the

mouth for various foods range from 10 to 500 s-l (Elejalde & Kokini, 1992); for milk,

the shear rate was 416 s-l. Shearing results in little difference in liquid model systems.

The shearing effect is especially high for a solid food that has to be broken up for the release of flavour compounds (Roberts & Acree, 1995).

Shear strength is another term to describe the oral shearing in oral processing. It is the maximum shear stress which a material can withstand (Kramer & Szczesniak, 1973). However, shear rate and shear stress are more commonly used terms.