Question
In Emmentaler (‘‘Swiss’’) cheese, gas formation causes the formation of holes in the
cheese mass. The gas is CO 2 , produced from the lactate in the cheese by propionic
acid bacteria. If al goes well, spherical openings with a smooth surface (‘‘eyes’’) are
formed, diameter about 2 cm; the time over which the fastest growth of the holes
occurs is about 10 days. Sometimes, slits rather than eyes are formed; the slits or
cracks are shaped like a flat disk, with a rough surface and a sharp edge. Two
experiments are done with respect to hole formation. (1) Two cheeses are made from
the same milk in the same manner, but one has a higher concentration (number per
unit volume) of the bacteria mentioned. For the lower concentration, eyes are
formed, for the higher one slits. (2) Two cheeses are made, of which one has a shorter
consistency than the other. The short one gives slits, the ‘‘long’’ one eyes. Explain in
a qualitative sense the differences in (1) and in (2). (3) Why does a slit not grow
throughout the whole cheese? See also Question 2 in Section 5.1.3.
Answer
(1) Gas is formed and will diffuse to any small hole present in the cheese. This means
that gas pressure in the hole may become larger than atmospheric, by an amountss.
If nowss>sy, the hole will expand. Sincesyis quite small in cheese, this will readily
occur; the cheese mass around the hole will flow, and an eye is formed. However, if
the gas production is very fast (more bacteria), the stress in the hole will become
higher andss>sfrmay be reached; consequently, fracture occurs, leading to a slit.
This is also likely because a higher gas production generally implies a faster growth
rate. The elongational strain rate around the hole is given byC¼dlnR=dt. For the
situation given, this meansC& 10 ^6 s^1 , and for faster growth it may perhaps be by
10 times higher. A glance at Figure 17.8b shows that a higher strain rate may cause a
much smaller fracture strain and hence—for the same material—a smaller fracture
stress. (2) Now the gas production rate will be the same, and thessvalue in the holes
will differ little between the cheeses. As illustrated in Figure 17.8, however, a short
cheese will have a lowersfrvalue than a long one. (3) Ifss>sfr, a roughly circular
fracture plane is formed and the hole becomes larger, since the two faces can bulge
farther. This implies thatsssharply decreases, and fracture will stop; in other words,
insufficient energy is available for slit growth. Naturally, the CO 2 pressure can build
up again, and the process may be repeated a few times. After a while, the lactate will
be fully converted and no further CO 2 will be formed, generally before the slits can
grow quite large.
17.1.4 Texture Perception
The texture of a food, i.e., its consistency and physical inhomogeneity as
perceived by the consumer, often is an important quality mark. When