The particle density is given by the density of the powder solids and the
occluded air in the particles. The powder solids density expresses the density
of solids without any air and is given by the composition of the powder. When
the composition and the density of the single components are known the density
of the solids (D solids) can be calculated using the following formula:
D solids = 
(18)
where %A, %B, %C are equivalent to the composition and DA,
DB, and DC the corre-sponding solids density. %W is the
percentage of moisture. The solids densities of various typical components in
milk powders are as follows:
| Solids, air and moisture free: |
Density, g/ml at 20ºC |
| Milk fat |
0.94 |
| Non-fat milk solids |
1.52 |
| Calcium caseinate phosphate complex |
1.39 |
| Amorphous lactose |
1.52 |
| Beta-lactose |
1.59 |
| Alpha-lactose monohydrate |
1.545 |
Powder solids density cannot be changed without changing the composition and
is thus for a given product constant.
The particle density may be measured in an air pycnometer. However, as this
equipment is not available in all laboratories, the petroleumether method will
be discussed. A given amount of powder is mixed with a given volume of
petroleumether in a graduated measuring cylinder:
particle = W / V1- V2 (19)
where:
D particle: particle density in g/ccm
W
particle: weight of powder in g
V1
particle: volume of powder + petroleumether in ml
V2
particle: volume of petroleumether in ml
The occluded air content is calculated as follows:
Voa = 100 / D particle - 100 / D solids (20)
where:
Voa = Volume of occluded air in ccm/100
g powder
D particle = Particle density, see formula (19)
D
solids = Density of solids, see formula (18)
The particle density for the reciprocal value of the occluded air content is
influenced by many factors previously discussed. They are summarized here:
- Pasteurization temperature of the milk prior to evaporation
- Amount of air in the concentrate
- Foaming ability of the concentrate
- Type of wheel used or size of nozzle
- Solids content in the concentrate
- Drying conditions (one-stage or two-stage)
Pasteurization temperature of the milk prior to
evaporation
The pasteurization temperature of the milk prior
to the evaporation changes the denaturation degree of the whey proteins and
thereby their physical stage and behaviour during drying. High pasteurization
temperature results in many denatured whey proteins being very compact and
different from undenatured whey proteins which are spongelike. Undenatured whey
proteins have a higher "water binding power". A bigger t or driving force is
therefore necessary to evaporate the last moisture with case-hardening as a
result. A high degree of denaturation will therefore give low occluded air
content (high particle and bulk density) and vice-versa. See Fig. 105.
Amount of air in the concentrate
The amount of
air in the feed naturally gives a high content of occluded air, especially if
the surrounding air temperature during the critical stage of the drying is high
causing case-hardening. See page 125.
Foaming ability of the concentrate
The foaming
ability of the feed is determining how much of the air whipped into the
concentrate will remain there and in the created droplets. See page 125 and
page 189.
Type of wheel used or size of nozzle
Besides
the foaming ability of the concentrate, the type of wheel and nozzle is
decisive as to the amount of air that will be whipped into the concentrate.
Solids content in the concentrate
Feed
concentration plays an important role and high concentration gives less
occluded air content.
Drying conditions (one-stage or two-stage)
The
drying conditions and temperature of the particle during the drying are one of
the main factors. Gentle drying, i.e. low surrounding temperatures as in
two-stage drying results in low occluded air.