Chemistry Reference
In-Depth Information
(Mimouni et al., 2005) or by centrifugation and measuring the concentration
of lactose in the supernatant (Zhizhin et al., 1971).
Since SCM contains
10.8 g lactose per 100 g, and 8.37 g are expected
to crystallize, a maximum of 77.5% of the lactose is expected to crystallize.
Since the microbial stability of SCM depends solely on its water activ-
ity, the knowledge that final water activity of SCM is reached only upon
complete crystallization of lactose becomes very important.
I.8.2.
Cooling After Complete Crystallization of Lactose
The viscosity of SCM increases rapidly during the first 4-12 h after
seeding, followed by a less rapid increase during the next 24 h, and a slower
increase during subsequent storage. In the absence of seed, there is also an
increase in viscosity during the first few hours after manufacture, but the
magnitude is much smaller. Hence, in addition to the increase caused by
lactose crystallization, there are other changes that contribute to such
increases in its viscosity. In a study on concentrated milk, where the effect
of lactose crystallization was not considered, Bienvenue et al. (2003) showed
that the viscosity increased steeply after 4 h of storage and was largely
reversible under high shear, but it became progressively less reversible upon
longer storage. The appearance of yield stress suggests the presence of rever-
sible flocculation arising from weak attraction between casein micelles, but as
concentrated milk ages, there is a progressive transition from reversible to
irreversible aggregation; particle size analysis confirmed irreversible aggrega-
tion and fusion of casein micelles during storage of concentrated milk (see
Bienvenue et al., 2003). Hence, as SCM ages, it loses its ability to recover from
shear thinning.
Since gel network formation and its transition to irreversible aggrega-
tion increases with temperature, the ability of SCM to recover from shear
thinning is reduced by storage at higher temperatures. Hunziker (1946)
reported that storage at or below 158C retarded age thickening, and on
storage at or below 78C, it retained its initial viscosity permanently.
Since pumping of product from the tank to the canning line causes
significant shear thinning, it is prudent to chill the product soon after the
crystallization of lactose has reached completion. In addition, chilling the
product may be desirable because it increases its viscosity, thereby reducing
its tendency to foam; but the increase in viscosity must be such that it does not
become too viscous to pump.
The longer the SCM remains in the tank, or the later the product is packed
into cans, the greater will be the shear thinning (irreversible recovery of gel
structure). The viscosity of SCM is always 20-50% lower throughout storage
when it was packed 48 h after manufacture than when packed after 12-24 h.
