Chemistry Reference
In-Depth Information
I.7.
Effect of Crystal Size on the Viscosity of Sweetened
Condensed Milk
Properties of lactose related to its solubility also affect the density, viscosity,
water activity, freezing and melting points and the susceptibility of the food
product to browning. Apart from the concern about sandiness, there are
other aspects related to the crystal size that need further consideration, i.e.,
sedimentability of the crystals and the effect of the number of crystals on the
viscosity of the finished product. Generally, sedimentation of lactose crystals
in SCM occurs within 1 month when the viscosity of product is less than
5000 mPa s (5000 cps) and crystal size is greater than 40 mm.
According to Stokes' law, the rate of sedimentation of lactose (v) is affected
more significantly by its crystal size than by the viscosity of the medium (
). v is
proportional to r
2
,wherer is the radius of the crystal. For instance, the rate of
sedimentation is increased four times when crystal size is doubled, but only
twofold when viscosity is halved. The two combined (doubling crystal size and
reducing viscosity by 50%) increases the rate of sedimentation by eightfold.
Stokes' law
¼
2r
2
g
ð
p
f
Þ
9
where
is the rate of sedimentation, r the radius of the particle, g the gravita-
tional acceleration,
p
-
f
the difference in density between particle
and medium and
the viscosity of the medium.
Stokes' equation indicates that the sedimentation of lactose reduces the
viscosity of the system, and the reduction in the viscosity increases the rate of
sedimentation; both are affected by crystal size.
The effect of crystal size on viscosity can be observed during the crystal-
lization of lactose in SCM or milk concentrates. Viscosity increases to a
maximum during the first half of the crystallization process because lactose
crystals are still very small (large specific surface) and the mother liquor still
has a relatively high solids content, which means that the friction between the
crystals and the mother liquor is high. As lactose crystals grow and the solids
content of the mother liquor decreases, viscosity decreases.
I.7.1.
Heat Treatment Before Evaporation
Even though the size and number of lactose crystals affect the viscosity
of SCM, heating during processing has a greater effect. Factors that affect the
viscosity of SCM during processing include preheating of milk before
