Themodulusof the particles. This is clearly shown by the difference
between curves (b) and (c), where the particles have about the same
anisometry but differ in agar concentration and hence in modulus.
The systems discussed here also have a yield stress; unfortunately,
results on large deformation rheology of the systems seem to be lacking.
There are alsoliquid systemsof a structure similar to ‘‘fluid gels.’’ A
case in point is stirred yogurt. The original set yogurt is effectively an acid
casein particle gel. By controlled stirring, a thick viscoelastic liquid is
obtained, i.e., without a significant yield stress. The rheological properties
are for the most part due to the presence of network fragments that are
highly anisometric and where protrusions on these fragments may hook into
each other: see Figure 17.26d.
Pastes. Fluid gels may to some extent be suitable models for
various foods that can be described as pastes, i.e., systems that have a yield
stress but are readily deformable and that contain finely dispersed material.
Anyway, the qualitative relations listed above for fluid gels also apply to
most pastes. We will briefly mention some of the numerous foods in this
category.
Quarkand comparable fresh cheese varieties. This product is similar to
the stirred yoghurt just mentioned, but at a lower water content. This gives
it a significant yield stress.
Liver paˆte ́and comparable meat products. It consists of small tissue
fragments of various kinds, some separate cells, fat particles, etc.
Presumably, interaction forces between the particles, e.g., due to the
presence of some gelatin, contribute to the firmness.
Fruit pure ́es, such as apple sauce and tomato pure ́e. These products
consist for the most part of tissue fragments and whole cells in an aqueous
liquid. Most vegetable tissues are pretty stiff, because the cell walls are stiff
and because of cell turgor. However, the material is heated to high
temperatures and this process leads to loss of turgor and cell wall rigidity;
see Section 17.5, ‘‘Plant Tissues.’’ Especially in tomato products, the cells
become highly deformable, which is the reason that tomato pure ́e is made of
concentrated ‘‘juice,’’ to realize a high packing density. Apple sauce tends to
be firmer, especially if not cooked too intensively.
Peanut butteris a concentrated suspension of particles obtained by
grinding roasted peanuts in the oil that leaks out of the fragments upon
grinding. The fragments are relatively large, say a millimeter, anisometric,
and rigid, which gives the product some firmness despite the relatively high
fraction of free oil. To enhance firmness, a little highly saturated fat is
sometimes added, to form a fat crystal network in the oil.