The change in milk viscosity upon addition of rennet was used to study the
effect of various milk treatments on cenneUng reaction. The change in milk viscosity
after the addition of rennet when recorded versus time gave a curve that passed
through 4 turning points (T, to T .). An induction period with no change in viscosity (T,)
ended with a rise in the viscosity to a first maximum (T2) then viscosity decreased to a
minimum with milk still in the liquid form (TJ) followed by a rapid increase to reach a
final maximum forming a plateau (T 4) when coagulation occurred. The effect of milk cooling, freezing. heat treatments, calcium addition. rennet
concentration, addition of NeCI and homogenization on vlscosity-tlme curve was
studied. Milk cold storage. freezing. heat treatments and the addition of NaCI elongated
the periods of 4 phases periods than of (aw mil~ periods. The delaying effect in
descending order was the addition of NaCI > freezing :> cooling > heat treatments.
Mosllhe delay was in the tag phase (T,). Heal treatments delay of T, was proportional
to the temperature. There was a good correlation with r = 0.98 (p < O.01) between T\
and the temperature. Increasing the concentration of rennel was more effective in
correcting the effect of heating than the addition of calcium. Heat treatments caused a
large increase in viscosity of the first maximum than the raw milk value.
Homogenization was very effective in correcting the effect of milk heat treatments and
shortened TI than the value of raw milk. The end of the lag phase (Tr) leading to the
first rise in viscosity occurred at a certain amount of casein hydrolysis depending on to
milk treatment.