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GEA Niro Method No. A 6 a |
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Revised: September 2005 |
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1.
Definition
The dispersibility of a powder in
water is its ability to break down into particles passing through a 150 µm
sieve.
2.
Scope
This method is used for any kind
of dried dairy product, but is especially applicable to skim milk and whole
milk.
3.
Principle
A powder sample of known water
content is evenly spread on the surface of 25°C water. The mixture is stirred
manually for a short time and part of the mixture is filtered through a sieve.
The total solids content of the collected liquid is determined. Dispersibility
is calculated from the mass of the test portion and the values for water
content and total solids.
4.
Apparatus
4.1
Balance, sensitivity - 0.1 g.
4.2
Analytical balance - sensitivity 0.1 mg.
4.3
Drying oven without forced air circulation, with a thermostatic control
capable of maintaining the temperature at 102°C ± 2°
C.
4.3
Desiccator.
4.4
Beaker - 600 ml, inside diameter 88 mm, height 123 mm, with graduation
mark at 150 ml.
4.5 Glass
plate - 120 x 120 mm, thickness 2.5 mm.
4.6 Metal
tubing - inside diameter 73 mm, height 50 mm.
4.7 Stand
and clamp for holding the glass tubing.
4.8
Spatula - stainless steel, thickness 1 mm, overall length 250 mm, length
of blade 135 mm, width of blade 25 mm (see Fig. 1).
4.9 Stop
watch.
4.10 Test sieve -
diameter 200 mm, 150m woven metal wire cloth complying
with ISO 565.
4.11 Erlenmeyer flask
- 250 ml with stopper.
4.12 Glass funnel
(see Fig. 1).
4.13
Pipette.
4.14 Glass weighing
dish.
4.15
Pumice.
4.16
Brush.
5
Reagents
N/A
6.
Procedure
6.1
Determine the moisture content of the powder as described in method No.
A 1 b.
6.2 Weigh
out 26 ± 0.1 g of skim milk or 34
± 0.1 g of whole milk. Transfer the powder to the glass
plate within the metal tubing and spread evenly with the spatula.
6.3 Weigh
out 250 ± 0.1 g of deionised water at 25°C ± 0.2°C in a
dry glass beaker, taking care that the inside of the beaker above the final
water level remains dry.
6.4 Place
the beaker on the stand, place the glass plate on top of the beaker and mount
the metal tubing over the glass plate, clamping it so that it is centrally
located above the beaker and leaving the glass plate free enough to be
withdrawn.
6.5 Start
the stop watch.
6.6 At
the same time, start the withdrawal of the glass plate. This should be
performed with a gentle and continuous movement and should be accomplished in
2.5 sec.
6.7
Immediately remove the beaker from the stand and insert the spatula down
the side of the beaker until it touch the bottom, this should take exactly 5
sec. During the next 5 sec. stir the content with the spatula making one
complete stirring movement per sec., i.e. a smooth and continuous movement of
the spatula back and forth across the diameter of the beaker and occupying 1
sec., with the end of the spatula in continuous contact with the bottom of the
beaker and slightly tilted away from the side of the beaker at the end of each
half stirring movement. This will minimize the accumulation of unwetted dried
milk on the side of the beaker. Without interruption, continue stirring in the
same manner for 15 sec., but hold the spatula in a vertical position
throughout. While making the 20 complete stirring movements in 20 sec.,
continuously rotate the beaker on its base so that approx. one complete turn
(360°) is achieved during the stirring.
6.8 Allow
the beaker to stand for 30 sec. (i.e. until the stop watch shows 55 sec.) and
then, without disturbing any sediment, quickly pour some of the liquid (down to
the 150 ml graduation mark) into the test sieve. Do not tilt or move the sieve
during filtration. Use a wet sieve which has been wiped with a towel to remove
excess water. During filtration, the sieve should be placed on a dry and clean
receiver.
6.9 30
sec. after start of the filtration (i.e. at 1 min. 25 sec. on the stop watch),
transfer the contents of the receiver to the conical flask and insert a
stopper.
6.10 Mix the content
thoroughly. Determine the dry matter in duplicate, as described in step
11-13.
6.11 Pipette approx.
8-12 g of the milk into a dry, glass weighing dish containing dried pumice or
sea sand.
6.12 Dry to constant
weight at 102°C in an oven. Cool to room temperature in a desiccator and weigh.
The difference in weight indicates amount of milk dry matter in the
reconstituted milk.
6.13 Measurements are
carried out in duplicate.
7.
Result
D = Dispersibility in
%.
T = Total solids in % of
the liquid (4.10).
W = Moisture in %
(4.1).
The mean value of two tests should
be calculated to the nearest 1%.
If the difference between two
tests exceeds 4%, the determination is not valid and another set of duplicate
values should be obtained as described in 4.
The formulas are derived as
follows:
If p grams (dry matter and water)
of the test portion (P grams) are dispersed in the 250 g of water,
then
This can be simplified as
follows:
With instant dried milk where P is
26,
8.
Reproducibility
N/A
9.
Remarks
N/A
10.
Reference
• GEA Niro Research Laboratory
• IDF Standard
87:1979.
