The wettability is a measure for the ability of a powder to be wetted with
water at a given temperature. This analytical method is only used when
producing instant powders. It is obvious that the wettability depends on
the surfaces of the agglomerates or single particles - are they water repellent
or will they absorb water too quickly thus forming a film through which the
water cannot penetrate.
Generally speaking, wetting is a process in which the gaseous phase at
the surface of the solid phase is replaced by a liquid phase, all three
phases coexisting for some time, so that a certain amount of intermixtures and
solutions (mainly of the solid and the liquid phase) is not only possible but
usually unavoidable.
Besides this, milk powder must be considered as a composite surface with the
separately enclosed surfaces connected by more or less stable 'bridges' to form
a complex capillary network. For simplification, let us first discuss the
mechanism of wetting a single surface.
The factor deciding if there will be any wetting at all is the
interfacial tension between the particle surface and the water. Skim milk
powder particles will usually be wetted easily (provided there is less than
0.03% fat on the surface), as the powder material is mainly lactose being in an
amorphous phase and protein, both absorbing water readily. However, whole milk
powder particles are always covered by a layer of fat, making them water
repellent. The amount of this surface free fat varies between 0.5 and 3% of the
powder.
This water repellence of the particles caused by their fat coating may be
overcome, and an interfacial tension facilitating the wetting may be achieved
by adding a surface active agent to the surface free fat. It has been known for
years that phospholipids such as lecithin are well suited for this purpose.
Lecithin has the advantage of being a natural product and even a natural
component of milk, and being both lipophilic and hydrophilic it is able to
absorb water. See page 249.
When the particles have been wetted, the individual components of the milk
powder start dissolving and dispersing, thus forming a concentrated solution of
milk around the particles. At the same time the particles start sinking to the
bottom, but it should be mentioned that, in order to make the particles sink,
the density of the particles has to be greater than that of the water.
The density of a particle depends on its composition and amount of occluded
air. During the first stages of reconstitution the density of the particles
decreases, mainly because the lactose and the minerals, which are the heaviest
milk components, start dissolving faster than the other components. At the same
time, the density of the solution being formed is increased because of the
dissolving lactose, so that the difference between the densities of the
particles and of the surrounding liquid is reduced. The particle density may
even become the same or lower than that of the liquid, so that, after the
initial sinking, the particles start to rise again. Thus to prevent this, the
particle density should be high, i.e. the content of occluded air should be
low. See page 211.
The reconstitution of a mass of powder is more complicated. As already
mentioned, powder is a composite surface with a greatly ramified system of
capillaries of various dimensions and a complicated geometrical pattern thus
having different capillary attraction effects.
Under these conditions there will be wetting not only on the surface of the
water, but also of particles lying above the surface, as the water is drawn
toward them by capillary attraction. This replacement of interstitial air by
water through capillary penetration is very often incomplete, as the amount of
penetrating water is insufficient, thus leaving air bubbles between the wetted
particles. In this way we have all three phases going on simultaneously,
resulting in the coexistence of their products of varying concentrations. This
coexistence is very dangerous, because after a short time the space between the
particles will be filled with milk of different, including high,
concentrations. This results in a sticky jelly with islands of unwetted powder
and residual air. Furthermore, lumps, that are wet and swollen outside and dry
inside, are created. As these are impervious to water, their complete
reconstitution is extremely difficult even with strong agitation.
To obtain a fully reconstituted milk in a reasonably short time and with
minimum effort, capillary penetration of water into the powder must therefore
be avoided. The capillary effect depends on the structure of the powder, i.e.
the size of the agglomerates, the size and the amount of non-agglomerated
particles, the amount of interstitial air and the specific surface area of the
powder. Penetration of water into the powder is easily avoided/delayed - to
allow dispersion before dissolution - when the powder consists of large
agglomerates.
The agglomeration is discussed on page 153.
The analytical method is simple and easy to perform:
10 g skim milk powder or 13 g whole milk is poured into 100 ml water at a
given temperature, usually 20ºC ± 2ºC. The time required for all the powder to
be wetted is measured by means of a stop watch. IDF prescribes the use of 10 g
skim milk or whole milk powder in 250 ml water at a temperature of 25ºC. See
Fig. 110.
Fig. 110 Determination of
wettability
Skim milk powder should be wetted within 15 sec. to be termed
instant. For whole milk powder there is no requirement, but many producers of
instant whole milk powder manufacture the powder to the same standard as valid
for the skim milk powder. However, for the subsequent dispersing process,
especially for whole milk powder, it is advantageous that the wettability is
about 30-60 sec., as it eases the subsequent dispersion of the powder into the
water.