H. Luyten

Fracture and yielding of gels.

Publisher Summary This chapter focuses on the fracture and yielding behavior of food gels. The fracture and yielding behavior of food gels is an important mark of quality, affecting aspects, such as (i) eating quality; (ii) usage properties, for example, ease of cutting and spreading; (iii) handling properties during storage and/or further processing in connection with, for example, shape retention and pumping characteristics. These aspects pertain to the fracture and yielding behavior of gels under widely varying conditions and time-scales. During eating, for example, a gel is quickly deformed until it fractures; the time-scale of this deformation is less than a second. On the other hand, the time-scale of shape retention during storage may be of the order of days or (much) longer. Fracture and yielding are properties that are concerned with large deformations. However, most of the fundamental rheological studies have been done at small deformations. Often, there is no direct relation between them. General characteristics of a macroscopic fracture are: (1) simultaneous breaking of bonds between the structural elements forming the network (for example, atoms, molecules, crystals) in a certain macroscopic plane; (2) breakdown of the structure of the material over length-scales clearly larger than the structural elements, resulting in the formation of cracks; and (3) ultimately, the falling apart of the material into pieces. The first characteristic also applies to viscous flow. Yielding does not include the third characteristic of macroscopic fracture; it results in a material that flows.

Yielding behaviour of mixtures of xanthan and enzyme-modified galactomannans.

Abstract The rheologicai behaviour as well as the stabilization against sedimentation of particles was studied for aqueous mixtures of xanthan with enzyme modified galactomannans. Both properties depended strongly on the galactomannan used. The shear modulus increased while the strain at yielding and the sedimentation velocity decreased with decreasing galactose content of the galactomannan. Results are compared with those for mixtures of xanthan with locust bean gum.

Small and large deformation behaviour of mixtures of xanthan and enzyme modified galactomannans

Small and large deformation properties of aqueous mixtures of xanthan with enzyme modified galactomannans at low ionic strength are discussed in terms of the theory of rubber elasticity and the structure of the galactomannans. The linear deformation region of the gels is small indicating that the deformation behaviour is dominated primarily by the rigid xanthan molecule. Galactomannans with a lower galactose content have a smaller region of linear behaviour.

Sedimentation of a single particle in systems of xanthan with locust bean gum in relation to the rheological behaviour

Abstract The sedimentation behaviour of single inert particles in aqueous mixtures of xanthan with locust bean gum is semi-quantitatively compared to the rheological behaviour of these materials as determined with various rheometers. It is concluded that for these mixtures the existence of a yield stress combined with a further slow breakdown of the xanthan locust bean gum network may explain the relatively slow sedimentation observed.

Sedimentation in Aqueous Xanthan + Galactomannan Mixtures

Publisher Summary The sedimentation of suspended particles in a liquid dispersion is regulated by the rheological properties of the dispersion at low stresses and low shear rates. This chapter discusses sedimentation in aqueous xanthan + galactomannan mixtures. It presents a comparison of some experimental results for the viscosity of xanthan + galactomannan mixtures as determined by sedimentation and with a rheometer. For that comparison, sedimentation experiments were performed with glass beads of 0.2-5 mm diameter in a glass tube of 18 mm diameter. Sedimentation viscosities were calculated with the Stokes equation corrected for the wall effect and for non-laminar flow. Apparatus viscosity was measured with a Deer Rheometer PDR 81 equipped with concentric cylinders. The result of the comparison is that the apparent viscosity of glycerol + water mixtures, as well as of dispersions of xanthan in 0.1 wt% aqueous NaCl, as determined by sedimentation or in the rheometer, were, at comparable shear stresses, equal within the experimental accuracy. However, for xanthan + galactomannan mixtures, the apparent sedimentation viscosities were considerably higher than those measured in the rheometer at the same stress.

Chapter 34:Mechanisms Determining Crispness and Its Retention in Foods with a Dry Crust

Various popular food products consist of a crispy crust and a non-crispy moist inner part. Well-known examples are many bread types and deep-fried battered snacks like fried fish. For the first type the crust has originally the same chemical composition as the rest of the food product, while for the...

Individual Fracture Events in Cellular Foods

Crispy foods like biscuits and breads crusts are cellular solids with relatively large pores of a wide size distribution. Important for the crispy character of such products are both the fracture behaviour and the acoustic emission (Luyten et al. [1]). With aging under deteriorating conditions, these products become less crispy. This is often ascribed to an increase in water activity of the crusts resulting in a change in the mobility of the different molecules, and thus a change in the solid material properties. However, it is difficult to relate directly the change in molecular properties to changes in crispy behaviour. One of the reasons for this is the product morphology, dry crispy foods are irregular built cellular solids. An example is given in Fig. 1.