Cheese. Several fully ripened cheese varieties can be considered
closely packed systems. When freshly made, the structure resembles that of a
paracasein micelle gel as discussed in Section 17.2.3, although with quite a
high particle volume fraction; moreover, the ‘‘gel’’ contains unbound filler
particles, i.e., the milk fat globules. In a few days, the micelles disappear,
and now a more or less continuous mass of much smaller proteinaceous
particles, size of the order of 10 nm, can be observed. Presumably, these
particles attract each other at least somewhat, since the cheese mass can be
considerably deformed without yielding or fracturing: see Figure 17.8,
cheese 2. Upon further aging, considerable proteolysis occurs, resulting in a
mass of peptides and even smaller molecules. The system now may be
considered as a paste, albeit with very small building blocks (and, of course,
filler particles). If the moisture content of the cheese is relatively high, a
viscous liquid results, as can be observed in several well-ripened soft cheeses
with a surface flora. For a lower moisture content (semihard and hard
cheeses), the consistency becomes more like that of a brittle solid, as
illustrated in Figure 17.8 for cheese 1.
17.5 CELLULAR SYSTEMS
Several foods have cellular structures: nearly all fruits and vegetables, bread
and cake, products made by high temperature extrusion, some types of
candy bars, etc. A cellular system can be defined as a collection of closely
fitting cells. The cells are enveloped by a soft-solid matrix and are closely
packed with gas or liquid. Generally,the matrix provides the stiffness, this in
contrast to the closely packed systems of the previous section, where the
solid character is primarily due to the particles.
The cells of the structure can be either closed or open. The former type
is like a foam, in that the matrix can geometrically be compared with the
continuous phase of the foam, which consists of thin lamellae and Plateau
borders. Here we have thicker lamellae orwalls, andbeams(struts, ribs)
where two lamellae meet; if the cells are filled with gas we can call the system
asolid foam. Open cells occur when the lamellae contain holes; now we
speak of asponge. Some types of sponge structures merely consist of beams.
Mostplant tissues(see, e.g., Figure 9.4) consist of closed cells that are mainly
filled with an aqueous liquid.
The volume fraction of cellsjis defined as one minus the volume
fraction of the matrix material. Generally,jvaries between 0.5 and 0.85. In
most cases, the cells are not very anisometric and not very polydisperse. This
makes it easier to develop theory for the rheological properties.