Tomato is one of the most widespread horticultural species in the world. Used in a wide and diverse range of forms, from being suitable for consumption fresh to use as a manufactured derivative, e.g. sauce, peeled, juices, ketchup, etc., it is hard to imagine tomato-free cuisine. With many national traditions and dishes based on this culinary vegetable, it is said to be one of the symbols of Mediterranean cuisine.
This book looks at the many changes that are taking place in the tomato market and industry; tomato producers are combining tomato origin, tradition, territory, quality, service and supply chain to adapt to the needs of the new consumers. It deals with the topics that are pertinent to the current industry: rheology and mechanical properties; origin determination; innovation and new product development; market research; sensory and consumer preference; quality control and new methods; volatile compounds and aroma; non-conventional processing technologies; functional and healthy compounds; waste and by-product valorization; and sustainability and traditional products.
Providing a comprehensive overview of the actual tomato industry; how it ensures product authenticity; new product development, particularly focused on consumer demands; the presence of bio-active substances able to prevent chronic diseases (carotenoids, phenolic and flavonoids); and how to convert industrial waste into added value by-products; this book will appeal to professionals and food product developers.
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Rheological Properties of Tomato Products
MIRIAM T. K. KUBO, MELIZA L. ROJAS, ALBERTO C. MIANO AND PEDRO E. D. AUGUSTO
Rheology is the science that studies the flow and deformations of materials when they are subjected to mechanical forces. Rheological study of food is necessary for the determination of engineering parameters, design of manufacturing machineries and unit operations, quality control and product development, definition of packaging and storage strategies, and more. Knowledge of rheological behavior is required all the way from manufacturing to product consumption. Consistency and mouthfeel are particularly valuable attributes in tomato products, highlighting the importance of rheology in sensory quality and consumer acceptance.
Fluid foods exhibit a wide variety of rheological behavior, ranging from Newtonian to time-dependent and viscoelastic. Since most tomato products are dispersions composed of suspended particles (pulp) and aqueous medium (serum), the content and characteristics of both phases play an important role in the complex rheology of tomato products. By causing structural and physicochemical changes in pulp and serum, food processing and its conditions consequently affect the rheological properties of the product as well.
In this chapter the principles of rheology, including the description of fundamental concepts and classification of fluids, are introduced. In addition, steady-state shear, time-dependent, and viscoelastic properties are described. Each is discussed separately, showing examples of tomato products and presenting the equations and their respective parameters usually employed to model the rheological behavior. Further, the influence of product composition, processing conditions, and operations on these properties is also discussed.
1.2 Fundaments of Rheology
Rheology studies the deformation and flow of materials subjected to mechanical forces. Depending on the material characteristics and the mechanical events, different rheological properties are obtained. The determination of rheological properties of food is important because they are useful for studying food quality and for designing equipment and food processing. In addition, the rheological parameters are crucial for calculating unit operations that involve phenomena not only of momentum transfer, but also of heat and mass transfer.
The rheological properties are determined by studying the deformation of the material during the application of a stress ([sIGMA]), or vice versa. The stress consists of applying a force (F) in a determined superficial area (A). Many types of stresses can be applied, depending on the food characteristics and processing, thus leading to different analyses.
For instance, in solid materials (Figure 1.1), normal stress (uniaxial compression or extension, where the applied force is perpendicular to the cross-section) or shear stress (where the applied force is parallel to the cross-section) can be applied. In fact, pure solid materials have elastic behavior, which means that when a stress applied to the solid is released, the solid recovers its shape from any deformation. However, "solid" foods do not behave either as a pure solid or as a pure fluid: they have an intermediate behavior known as viscoelastic behavior.
The rheological properties of fluids are commonly evaluated by applying shear stress. Figure 1.2 illustrates an ideal experiment where a fluid sample is in contact with two parallel slabs of known area A separated by a distance dy. While one slab is fixed, the other one moves at constant velocity v due to an applied force F. The fluid layer close to the upper slab will move with the same velocity, while the fluid close to the lower slab will remain at rest. Therefore, when the steady-state condition is obtained, the fluid will move following a velocity profile in the x direction. In fact, the shear stress can be calculated by eqn (1.1), resulting in a fluid velocity gradient called shear rate ([??}, eqn (1.2)).
σ = F/A (1.1)
[??] = dv/dy (1.2)
The relation between the shear stress and the shear rate gives the information necessary to recognize the type of fluid and its behavior during processing. This is detailed in the following section.
1.2.1 Fluid Flow
Fluid materials can be classified rheologically according to their flow behavior (Figure 1.3). First, a perfect fluid is one whose shear rate is linearly proportional to the shear stress and whose constant of proportionality is called viscosity (η), which represents the resistance of the fluid to flow. These fluids follow Newton's law (eqn (1.3)), so they are called Newtonian fluids. Example of Newtonian fluids are air, water, dilute solutions, oil, milk, clarified juices, and the juice serum.
σ = η · [??] (1.3)
However, most liquid foods do not follow Newton's law, due to structural changes during flow. They are known as non-Newtonian fluids, but depending on the fluid, they have different behaviors (Figure 1.3). These fluids can also be classified as time-independent or time-dependent non-Newtonian fluids. Time-dependent means that the fluid structure changes as the flow time increases. For instance, some particles of the fluid may aggregate to form bigger particles or may be destroyed to form smaller particles during flow, changing the rheological behavior as the fluid flows.
Time-independent non-Newtonian fluids can usually present four different behaviors: dilatant, pseudoplastic, Bingham, and Herschel–Bulkley (Figure 1.4). In most of these fluids, the "viscosity" is not a constant property in relation to the shear rate. This property is therefore known as apparent viscosity, as its value is a function of the shear rate.
Dilatant fluids, or shear-thickening fluids, are characterized by an increase in apparent viscosity as the shear rate increases. Examples are concentrated suspensions of starch in water, crystalized honey, and suspensions of sand in water. This behavior is due to the collision of the suspended particles when the fluid is sheared, increasing the resistance to flow (i.e., causing an increase of the apparent viscosity).
In contrast, pseudoplastic fluids, or shear-thinning fluids, present an opposite behavior: as the shear rate is increased, the apparent viscosity is reduced. This behavior is caused by the alignment of the suspended particles due to the flow when they are subjected to shear. For instance, when these fluids are at rest, they seem to be very consistent; however, their consistency is reduced when the container they are in is shaken. Familiar examples of such fluids are fruit purées, mayonnaise, mustard, and ketchup.
These two first fluids (dilatant and pseudoplastic) can be mathematically described by eqn (1.4), known as the Ostwald–de Waele model or power law model. In this equation k represents the consistency coefficient and n represents the flow behavior index. For Newtonian fluids n = 1, for dilatant fluids n > 1, and for pseudoplastic fluids n< 1.
σ = k · [??]n (1.4)
There are other types of fluids that need a minimum shear stress to start to flow. This minimum shear stress is known as yield stress (σ0). The presence of yield stress is characteristic of multiphase materials such as fruit pulps and juices, which are formed by particles in suspension (cells, cellular wall, fibers) in an aqueous solution of sugars, proteins, soluble polysaccharides, and minerals (the serum phase). After the application of a shear stress higher than the yield stress, the fluid flows. When flowing, some fluids (including some juices) behave similarly to Newtonian fluids; these are known as Bingham plastic fluids. On the other hand, if after the application of the yield stress the fluid behaves similarly to a pseudoplastic fluid, the fluid is called a Herschel–Bulkley fluid. The most common mathematical model used to describe fluids with a yield stress is the Herschel–Bulkley model (eqn (1.5)). Note that this model can be used as a general model for describing all the previously described fluid behaviors:
σ = σ0 + k · [??]n (1.5)
The characterization of time-dependent non-Newtonian fluids is important for understanding possible changes during processing. There are two common fluid behaviors: fluids whose consistency (apparent viscosity) is reduced over the shearing time, known as thixotropic fluids, and others whose consistency is increased over the shearing time, known as rheopectic fluids. Rheopectic behavior is characterized by the reorganization of the fluid structure during flow and it is unusual to find in food products. Thixotropic behavior, on the other hand, is characterized by the rupture and disaggregation of suspended particles and molecules in the food. Therefore, the stress is reduced as the flow time passes. This behavior is very common in foods such as fruit derivatives, tomato products being a typical example.
1.2.2 Viscoelastic Properties
Most food does not behave either as an ideal fluid (with pure viscous behavior, described by Newton's law) or as an ideal solid (with pure elastic behavior, described by Hooke's law). Food products have an behavior intermediate between these two ideals, thus being classified as viscoelastic products. The viscoelastic properties of a food are important in studying the product stability, the properties of which can be correlated with the structure to explain the product changes during processing. Assessing the viscoelastic properties of food can be carried out by methods such as the dynamic oscillatory procedure or creep and recovery procedure.
The dynamic oscillatory procedure consists of applying a sinusoidal shear stress with a determined amplitude within the linear behavior (<5%). Three parameters are involved during the procedure, where one of them is kept constant, another is varied, and the third is measured: shear stress (σ), strain (γ), and oscillatory frequency (ω). In most cases, an oscillatory movement is applied to the product, and the strain is measured as response. Depending on the phase difference (δ) between the input and output (Figure 1.5), the viscoelastic properties are determined. When there is no phase difference between the strain and the stress sinusoids, it means that the product behaves as a pure elastic solid; when the phase difference is 90°, it means that the product behaves as a pure viscous fluid. If the phase difference is between 0° and 90°, this means that the product has viscoelastic behavior.
Consequently, viscoelastic products can be described by eqn (1.6). This equation introduces the parameters G' and G". The parameter G' is known as the storage modulus and describes the elastic behavior of the product (eqn (1.7)). G" is the loss modulus and describes the viscous behavior of the product (eqn (1.8)).
[MATHEMATICAL EXPRESSION OMITTED] (1.6)
G' = (σamplitude/γamplitude) · cos δ (1.7)
G" = (σamplitude/γamplitude) · cos δ (1.8)
In addition, other viscoelastic parameters can be obtained from this procedure: the complex modulus (G*, (eqn (1.9)) and the complex viscosity (η*, eqn (1.10)), which represent the overall resistant of the product to flow:
G* = [square root of ((G')2 + (G")2)] (1.9)
η* = G*/ω (1.10)
Another frequently used procedure to determine the viscoelastic properties of food is the creep-compliance procedure. This consists of applying an instantaneous stress (σ) to the product, which is then kept constant for a period of time when the strain change (γ) is measured. Then, the stress is released, and the recovery behavior of the product is observed. When compliance (J, the inverse of the modulus of elasticity) against time is plotted (eqn (1.11)), a creep-recovery profile similar to Figure 1.6A is obtained. This method combines fundamental mechanical models (Figure 1.6B) to describe the viscoelastic properties: Hooke's elasticity model (represented by a spring), Newton's viscosity model (represented by a dashpot), and their combinations.
Some of the most commonly used models to describe viscoelastic behavior are the Maxwell model, which combines a spring and a dashpot placed in series; the Kelvin–Voigt model, which combines a spring and a dashpot placed in parallel; and the Burger model, which combines a Maxwell body and a Kelvin–Voigt body in series. All these models make it possible to isolate the elastic and viscous contribution of a viscoelastic product, being a very interesting approach for studying these products. The Burger model is the most complete and best describes the viscoelastic complexity of foods (eqn (1.12)). Its parameters are useful to isolate viscous and elastic behavior: G0 and η0 refer to the instantaneous elastic modulus and viscosity component associated with the Maxwell spring, G1 refers to the retarded elastic modulus associated with the Kelvin–Voigt body, and η1 is the viscosity component associated with the retarded elasticity of the Kelvin–Voigt body.
J(t) = γ(t)/σapplied (1.11)
[MATHEMATICAL EXPRESSION OMITTED] (1.12)
For more detailed information about rheology concepts and analysis, the reader may consult the following references: Ahmed et al., Augusto and Vitali, Ibarz and Barbosa-Canovas, Rao, Rao and Steffe, Singh and Heldman, and Steffe.
1.3 Steady-state Shear Behavior
A steady flow curve (shear stress as a function of shear rate) is a valuable way to characterize the rheological behavior of fluids and this information is very useful in various industrial applications. The steady-state shear properties are related to the product flow behavior. From an engineering standpoint, these properties are important for the design of machinery such as fillers, pumps, and impellers and the design of several unit operations, including fluid moving, mixing, and heat transfer processes.
As already discussed, from flow curves and use of rheological models, the behavior of fluids can be classified as Newtonian or non-Newtonian. The latter can be divided into four categories: pseudoplastic (shear-thinning), dilatant (shear-thickening), Bingham, and Herschel–Bulkley. Different flow models have been employed to describe properties under steady-shear over wide ranges of shear rates. For tomato products, the power law model (eqn (1.4)) and, when yield stress is considered, the Herschel–Bulkley model (eqn (1.5)) have been extensively used. In addition, other models, such as Casson, Carreau, and Falguera–Ibarz, have also been employed. Table 1.1 presents the equations of these models and some examples of tomato-based products in which the successful use of these models has been reported.(Continues…)
Excerpted from "Tomato Chemistry, Industrial Processing and Product Development"
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