4.5.1. Viscosity and Texture
Texture is one of the most important parameters in fermented dairy products including yogurt. The methods of analyzing rheological properties of yogurt vary depending on the type of yogurt; stirred, drinking or set-yogurt and the investigated information. Many studies have been done to better understand the rheology of yogurt using the rheometer, texture analyzer, or viscometer (Bourne, 2002). Viscosity is the common term used to define “flowability” of a product. Viscosity or so called „absolute viscosity‟ is the tendency of a product to resist flow and is defined by equation (1) (Bourne, 2002);
η = viscosity (milliPascal second (mPa.s) or centipoises (cP)) σ = shear stress (Pascal or Newton meter-2
) γ = shear rate (s-1
)
Absolute viscosity is used for a Newtonian fluid at which the flow of fluid is directly proportional to the applied stress and that viscosity is constant at changing shear rates. Water is the best example of Newtonian fluid and its viscosity at 20°C is 1 mPa.s (Bourne, 2002). Since yogurt is rather a complex system consisting of lactose, proteins, fats and other suspended matters, its behaviour is categorised as non-Newtonian fluid and has apparent viscosity (Bourne, 2002). The best method of measuring viscosity is by point measurements, at which viscosity is measured at various rates of shear stress and shear rate (N. Shah, personal communication with Brookfield™, August 26, 2010). This allows the investigation of the flow and the overall rheology of the product, particularly if the product of interest follows pseudoplastic behaviour (N. Shah, personal communication with Brookfield™, August 26, 2010).
As food contains various chemical components, the behaviour (flow) of non-Newtonian fluid is categorised into different classes according to its shear stress-strain relationship. Unlike the Newtonian fluid which flows freely, certain amount of pressure is required to force non- Newtonian fluid such as tomato sauces and condiments to flow. Once the fluids start to flow, the shear stress is proportional to shear rate. The minimum force required to begin the flow is called yield stress and the flow behaviour of such product follows a plastic or Bingham model (Figure 8). A slightly different model of Bingham flow is pseudoplastic fluid. The Bingham model of pseudoplastic fluid shows Newtonian behaviour at low shear rate but an increase of shear force causes an incline in shear rate, resulting in changes of viscosity (Figure 8). Typically, salad dressings fall into this category. The last flow behaviour that has been successfully quantified is dilatants or frequently called „shear thickening‟ at which equal increments of shear stress gives less increments in shear rate (Figure 8). The application of this behaviour, however, is rarely applied in the food industry (Bourne, 2002). Yogurt, either in stirred or set-form, follows non- Newtonian behaviour that shows yield stress with „shear thinning‟ and time dependency behaviour (Ozer & Kirmaci, 2010). Shear thinning is described as non-Newtonian behaviour where apparent viscosity decreases with time of shearing and the change is irreversible; that is, it
stays in the thinner state even if shear stress is removed. In contrast, fluid that reverts to its original state after shear stress removal is called thixotropic. The plot of apparent viscosity with time to describe these flows is shown in Figure 9.
Figure 8. Different fluid flows at various shear stress and shear rate (Bourne, 2002)
Figure 9. Apparent viscosity over time of various flow (a); shear rate versus time of various flow (b) (Bourne, 2002).
Many factors can contribute to the viscosity of a product which include temperature, solute concentration, molecular weight of the solution, and suspended matter of solution (e.g. fruit pulp,
fibre, etc) (Bourne, 2002). Temperature, however, plays the most important role in viscosity measurements. Regardless of the type of behaviour of the fluid (Newtonian or non-Newtonian), measurements of viscosity are significantly affected by an increase in temperature, where the relationship between viscosity and temperature is (typically) inversely proportional (Bourne, 2002).
Texture is an important factor in set-yogurt as it describes the strength of gel network. The strength of gel in set-yogurt contributes to its firmness, which is defined as the force attained at a given deformity of a product, hence attributed to the degree of syneresis. With many scientific studies have been done to improve textural properties of yogurt, texture is a single crucial factor in manufacturing yogurt. Such studies include improving yogurt quality by reducing oxygen level (Horiuchi et al., 2009), incorporating lentil flour (Zare et al., 2011), skim milk/starch (Tamime et al., 1996), and whey protein (Augustin et al., 2003; Guzmán-González et al., 1999).
4.5.2. Sensory Evaluation and Consumer Acceptance
The objective of this study was to develop a stable probiotic rich dehydrated yogurt base which can be used to produce acceptable liquid yogurts by potential consumers at home, restaurants, etc. Although many important factors can be measured by instruments, this is still insufficient in characterising the liquid yogurt products. Consumer acceptance tests, which require 50-100 panellists, give food manufacturers important information about consumer likeliness towards the product based on the product‟s sensory properties. Information can be obtained for single products without comparing to other products. A nine-point-hedonic scale is the commonly used rating method for sensory evaluation as the scales can easily be interpreted using statistical approaches of analysis of variance (ANOVA), regression and correlation analysis (Resurreccion, 1998). With many yogurt products available in the market nowadays, consumer acceptance tests provide an indication of product acceptance without the effect of other factors, such as packaging, price, and health claims, which may enhance its acceptance among target consumers thus minimizing product failure once it is on the market (Resurreccion, 1998). According to the objective of the analysis, it is not uncommon that panellists are screened and selected based on their organoleptic abilities. Panellists are then asked to generate important attributes that best
describe the products, called descriptive analysis. With so many varieties of yogurt products available in the market, descriptive attributes may differ between products. For example, descriptive attributes of peach-flavoured yogurt drink were overall acceptability, colour, flavour, and mouthfeel (Gonzalez et al., 2011). Meanwhile, for stirred yogurt, smoothness, graininess, flavour, overall acceptance, and colour (Zare et al., 2011) or taste, characteristic yogurt flavour, colour, and consistency may be considered as important attributes (Obi et al., 2010). For set- yogurt, texture and mouthfeel (Tamime et al., 2006) can be added to the appearance, colour, smoothness, sweetness, sourness, and overall acceptance (Hashim et al., 2009).