Dairy Chemistry And Biochemistry

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 389

Milk sample

Figure 10.5 Apparatus for visual determination of the rennet coagulation time of milk.

of apparatus have been developed. The most popular of these, although with
limited use, is the Formograph (Foss Electric, Denmark), a diagram of
which is shown in Figure 10.6a. Samples of milk to be analysed are placed
in small beakers which are placed in cavities in an electrically heated metal
block. Rennet is added and the loop-shaped pendulum of the instrument
placed in the milk. The metal block is moved back and forth, creating a
‘drag’ on the pendulum in the milk. The arm to which the pendulum is
attached contains a mirror from which a flashing light is reflected on to
photosensitive paper, creating a mark. While the milk is fluid, the viscosity
is low and the drag on the pendulum is slight and it scarcely moves from
its normal position; hence a single straight line appears on the paper. As the
milk coagulates, the viscosity increases and the pendulum is dragged out of
position, resulting in bifurcation of the trace. The rate and extent to which
the arms of the trace move apart is an indicator of the strength (firmness)
of the gel. A typical trace is shown in Figure 10.6b. A low value of r indicates
a short rennet coagulation time while high values of a3, and k,, indicate a
milk with good gel-forming properties.
A recently developed, and apparently industrially useful, apparatus is the
hot wire sensor. A diagram of the original assay cell is shown in Figure
10.7a. A sample of milk is placed in a cylindrical vessel containing a wire of
uniform dimensions. A current is passed through the wire, generating heat
which is dissipated readily while the milk is liquid. As the milk coagulates,
generated heat is no longer readily dissipated and the temperature of the