RHEOLOGY of SOLID FOODS M. Anandha Ra Foods May Be
Total Page：16
File Type：pdf, Size：1020Kb
1 RHEOLOGY OF SOLID FOODS M. Anandha Ra Foods may be classified in terms of their rheological properties and sensory attributes as liquids, semi-solids, soft solids and hard-solids (Van Vliet et al., 2009; Foegeding et al., 2011). It may be noted that while there is no universal definition for absolute transitions from one state to another, based on chewing criteria, it can generally be considered that: (1) fluid foods flow and do not require any chewing during oral processing (e.g., milk); (2) semisolid foods are processed in the mouth by squeezing the tongue and palate (e.g., pudding); (3) soft-solid foods require chewing but do not have “crispy” attributes (e.g., cheese); and (4) hard-solid foods are crispy and associated with an acoustic emission (e.g., crackers). We have learned earlier that with many liquid foods, we work with their viscosity or apparent viscosity because of their shear-dependent nature. Foods that are semisolids, soft solids, and hard solids are viscoelastic materials, with varying levels of elastic (solid-like) behavior being predominant. One definition of a solid is that it is a state of matter characterized by particles arranged such that their shape and volume are relatively stable. Further, the constituents of a solid tend to be packed together much closer than the particles in a gas or liquid. In contrast, a liquid is one of the states of matter. The particles in a liquid are free to flow and while a liquid has a definite volume, it does not have a definite shape (http://chemistry.about.com/od/chemistryglossary/a/liquiddef.htm). In many studies on solid foods, the results are expressed in force and deformation units, e.g., slope of force-deformation curve (often, erroneously referred to as Young’s modulus), fracture force, etc. As noted by various authors (Baltsavias et al., 1999; Dobraszczyk et al., 1987), such an approach does not allow comparison of results because the force-deformation relationship is strongly affected by specimen dimensions. Fundamental properties, such as Young’s modulus, derived from stress vs. strain data, are independent of specimen size. Fundamental mechanical parameters of solids include fracture stress and strain, yield stress, elastic modulus, Poisson’s ratio, coefficient of friction and fracture toughness. Lillford (2001) reviewed the fracture properties of solid foods with particular emphasis on the role of microstructure. It was pointed out that the microscopic structure, and particularly 2 the examination of fracture surfaces, give significant insight into the origin of the materials’ properties. He noted that, “the microscopic structure, and particularly the examination of fracture surfaces, give significant insight into the origin of the materials’ properties.” In addition, Lillford (2001) pointed out several interesting relationships, outlined next, between mechanical properties and structural characteristics of solid foods. Bourne (2002) described fracture as occurring under Types 1, 2 and 3. Type 1 is described as a simple fracture, where the imposed stress has exceeded the strength of the material and the body has separated into 2 or more pieces or occasionally the fracture may be partial and the body may not separate into pieces. Type 2 is described as brittle fracture, where there is little or no deformation before fracture and the original un-deformed body may result in many pieces. Type 3 is described as ductile fracture where there is substantial plastic deformation and low energy absorption prior to fracture. In a pioneering study, Griffith (1921) showed that the specific fracture energy (fracture toughness), �!, the elastic modulus, E, the fracture stress, �!, and the critical crack length, l, are related by: � = !!!! (9.1) ! !" �! can be measured directly from the experiment, and an estimate of the modulus, E, can be made, but l and �! are unknown, (the work of fracture can also be estimated from the area under the force-displacement curve, and is referred to as "fracture toughness"). Fracture toughness is a property of a material that describes the ability of the material containing a crack to resist fracture. The fracture toughness, to be discussed later, is determined from the stress intensity factor. Ashby (1983) has presented relations for the moduli and fracture stresses in air filled foams, obtaining: ! ! !! = �! (9.2) !! !! Where, E = foam modulus, �! = cell wall modulus, �! = density of cell wall material, �! = bulk density of the foam, and C1 = a constant. 3 The crushing stress of a brittle foam, �!", and the bending fracture stress of cell wall, �!, are related according to: ! !!" !! ! = �! (9.3) !! !! Where �! is a constant. For the elastic buckling stress, �!", the relationship is: ! !!" !! = �! (9.4) !! !! Where �! is a constant. In agreement with the above relationship, the modulus of a cake, E, was related to the bulk density, �!: ! � = �!�! (9.5) The constant, �!, varies with water activity. At a given water activity, assuming that the modulus of the cell wall modulus, �!, and the density of the cell wall material, �!, do not change significantly over a range of bulk densities: !!!! �! = ! (9.6) !! The above equation predicts that the modulus and fracture stress decrease as air content is increased, in agreement with the softer texture of aerated bread and cake, loss of brittleness (jaggedness in stress-strain curves) with increased humidity (Lillford, 2001). 4 SOME RHEOLOGICAL TESTS ON SOLID FOODSÍ The rheological tests on solid foods can be classified based on the type of strain used, such as: (1) extension/compression, (2) flexure, (3) torsion, and (4) small-amplitude oscillatory (dynamic). Commercial instruments capable of measuring force and deformation simultaneously can be used for conducting tests of types 1 and 2. For conducting a torsion strain test, a sample must be shaped like a capstan. Commercially available dynamic mechanical analyzers can BE used for conducting dynamic rheological tests. Fracture and crack propagation are important in quality assessment of many foods, such as fruits and vegetables, biscuits, and cheeses; the fracture mechanics approach using concepts of energy is valid after the onset of failure (Lillford, 2001; Luyten et al., 1992). Systematic studies have been conducted to understand the relationship between composition and/or microstructure on fracture and crack propagation, as well as on the mechanical properties of a number of foods. Vincent et al. (1991) noted that a crack can be propagated by torsion (giving out-of plane tearing), by tension (giving crack-opening) or shear (giving edge-sliding), as well as the wedge fracture test. The choice of a fracture technique depends on material limitations and other constraints, such as specimen size, shape and homogeneity of the sample and its preparation, as well as additional preparation of the sample to fit in the chosen experimental set up (Foegeding et al., 2011). Studies conducted on specific foods should be consulted in order to examine the unique details of the experimental techniques. Poisson's Ratio Poisson's Ratio, ν, is a basic rheological property of ideal elastic solids and viscoelastic solids. It is defined as the ratio of lateral strain to axial strain in a specimen subjected to axial deformation. For very small deformation of an incompressible homogeneous material (rubbery), i.e., a material that does not change its volume when subjected to stress or strain, the Poisson’s ratio tends to 0.5. For compressible materials that show a certain reduction of volume under stress or strain are characterized by 0 < � < 0.5; specifically, for porous materials � tends to zero. It is the constant that relates modulus of rigidity to Young's modulus in the equation: � = 2� � + 1 5 where, E is the Young's modulus; G, is the shear modulus (modulus of rigidity); and �, Poisson's ratio. The formula is valid only within the elastic limit of a material. For incompressible materials, E=3G. Rohm et al. (1997) used a video-based method to monitor lateral specimen expansion during compression of selected solid foods continuously and to establish a procedure for the calculation of average compressional stress, based on actual values of a specimen’s cross- section. The test specimens were compressed to failure at constant crosshead speed of 10 mm/min between parallel stainless steel plates; the plate-food interfaces were generously lubricated with a low-viscosity paraffin oil. The apparent Poisson’s ratio �!, which refers to the ratio of lateral expansion to uniaxial compressional strain, was calculated at specific values of deformation according to: �!=�� �!/�! �� ℎ! ℎ! ] (9.7) Where the subscripts o and t refer to the initial and actual values. Values of the Poisson’s ratio for various foods are shown in Table 9.1 (Rohm et al., 1997). 6 Table 9.1. Values of the apparent Poisson ratio, �!, of selected food materials at different values of deformation (Rohm et al., 1997). ____________________________________________________________________________ Sample Apparent Poisson ratios at different values of axial Hencky strain, ϵH: ϵH~0.05 ϵH~0.12 ϵH~0.30 ϵH~0.65 ϵH~0.90 ____________________________________________________________________________ Apples Red Delicious 0.21 0.25 f Jonagold 0.17 0.22 f Bread Rye bread A 0.28 0.22 0.21 0.19 0.20 Rye bread B 0.30 0.23 0.19 0.19 0.21 White bread C 0.17 0.14 0.11 0.07 0.07 Butter Sample A 0.42 0.44 0.43 f Sample B 0.44 0.45 0.45 f Potatoes Raw 0.38 0.43 0.46 f Steamed 0.40 0.42 f f, beyond specimen fracture 7 EXTENSIONAL STRAIN STUDIES ON FILMS AND SKINS The rheological properties of a number of solid, thin, ductile, foods, such as edible films and fruit skins can be determined in extension. First, we consider tests in which a sample is tested as-is, as opposed to tests in which a notch is made.