. Characterisation of the feed solution
The viscosity of SMC from 12.5 to 40.0% TS, with and without
rennet-treatment, is shown at a shear rate of 100 s1 (Fig. 2). For
12.5e35.0% TS, only a small difference was detected between the
chymosin-treated sample and the untreated sample because of the
reduced total volume of the para-casein micelles. It can be
concluded that no measurable aggregation of para-casein micelles
occurred for the enzymatically treated samples over this range of
TS. Above 35% TS, the chymosin-treated sample showed a steep
increase in viscosity. This can be explained by a higher probability
of contact between the hydrolysed casein micelles at such high TS
levels, which induced the aggregation of casein micelles. This aggregation
can be induced by pumping the SMC to the spray nozzle,
which makes the process at TS contents above 35% difficult to
control.
Apart from viscosity, the gelation temperature of the SMC for
different TS contents was determined before and after enzymatic
treatment. The intersection points for G0 and G00 for increasing
temperatures are plotted as a function of the TS content (Fig. 3). The
temperature at the gelation point of the concentrates decreased
with increasing TS content due to the higher protein content. For
subsequent spray-drying experiments, the gelation temperatures
are important as gelation should take place after atomisation and
not during pumping to the spray nozzle.
These results (Fig. 3), together with the results on viscosity
limitations (Fig. 2), presume that 35.0% is the highest TS content for
a practicable spray-drying encapsulation process. At this TS content
a shear-induced gelation, brought about by pumping, can be avoided,
and a temperature-induced gelation only occurs at a temperature
higher than 30 C. Consequently, at room temperature, no
premature aggregation prior to drying is expected. This is a prerequisite
to spray-drying the sample using a nozzle.