centrifugal acceleration and duration, and tube dimensions) be to make the test
meaningful? (b) It is generally observed that the test underestimates the times needed
for creaming during storage. In some cases the discrepancy is very large. What can be
the possible causes and how can it be established which of these actually holds? (c) In
one of the emulsions, with quite a small volume fraction of oil, the creaming during
storage is much faster than predicted. What could be the cause and how could it be
established?
Answers
(a) According to Stokes, the sedimentation velocity is proportional to the
acceleration, i.e., toRo^2 versusg. However, Eq. (13.24) shows that the creaming
rate will be proportional to acceleration overH, the creaming distance. The latter
will generally be larger in the vessels in which the emulsions are stored (HV) than in
the centrifuge tubes (HT). Moreover, the times involved in the gravity creaming and
the test should be such thattT?Ro^2 =HT¼tV?g=HV. In this way the proportion
creamed in the test and in the vessel will be equal, provided that Eq. (13.24) holds.
FIGURE13.13 Flow curves (shear stress versus shear rate) of soya milk. Curves 1
and 2 are for milk made from dehulled beans (about 6%dry matter), 3 and 4 from
whole beans (about 7%dry matter). Soaking was overnight at room temperature
(curves 1 and 3); or 4 hours at 60 8 C (2 and 4). (From results by A. O. Oguntunde et
al. Proc. 7thWorld Congr. Food Sci. Technol. Singapore, 1987, p. 307.)