Scorched particles are generally accepted to be a measure for any deposits
in the drying chamber having been exposed to high temperatures thus getting
scorched, discoloured and at the same time insoluble.
However, it is not only the dryer that contributes to the scorched
particles, as even the raw milk may contain some dirt or sediment, and if not
clarified in a separator these will be found in the powder.
Also from the evaporator brown, insoluble, jelly lumps may contribute to the
scorched particles, if deposits have been formed in the tubes due to
insufficient coverage of the tubes, (remedies for this have been discussed
earlier, page 37) or insufficient cleaning.
If it has been concluded that the scorched particles originate from the
dryer, the reason is very often deposits in the wheel or around the nozzles or
in the air disperser. How to solve the problem may differ from case to case,
but adjustment of the air disperser will usually help in most cases.
The test for determining scorched particles is simple and rapid:
25 g skim milk powder, 32.5 g whole milk powder or 15 g whey powder
(or equivalent amount of concentrate depending on total solids), is mixed with
250 ml of water of 18-28ºC in 60 sec. in the same kind of mixer as used for
insolubility index. The milk solution is filtered and the filter pad is
compared with a standard for classification. The scorched particles are
expressed as A, B, C, or D depending on the intensity and colour of the
particles left on the filter. See Fig. 108.
Fig. 108 Determination of
scorched particles
If scorched particles cannot be traced to the evaporator, see page 189 or
the spray dryer, they may originate from milk powder used in "compounded"
products like baby food.