Moisture
All milk powder has to meet a
requirement for residual moisture. For skim milk it is usually 4% and for whole
milk usually 2.5%. There may naturally be deviations from country to country.
The moisture content will have an influence on the keeping quality of
the powder. High moisture content (high water activity Aw) will thus decrease
the keeping quality, as the proteins will denaturate and the lactose, which is
found in an amorphous stage, will crystallize causing the free fat to increase
in whole milk powders, and oxidation of the fat will be the result. The
Maillard reaction, which is a reaction between the NH2 group in the aminoacid
lysine, and lactose, becomes more pronounced, and the powder may even become
brown and lumpy. The Maillard reaction is directly proportional to the storage
time, temperature and residual moisture content. The moisture can be controlled
by the outlet temperature of the dryer or by applying more heat to the
Vibro-Fluidizer. Moisture absorption should be avoided, and dehumidification of
the cooling air is recommended in humid areas.
The packing material
should be of such a quality that very little vapour will penetrate the bag or
container. As there will always be some vapour diffusion (and the diffusion
direction is determined by the water vapour pressure) it is recommended to
store the powder in a dry, cool place, where the water vapour pressure will be
low.
Residual moisture is determined by a simple drying oven method.
The powder is dried at 102-105șC for three hours. The difference in weight
(i.e. weight loss) is determined and the moisture calculated in per cent of the
powder weight.
Various quick methods for determination of moisture have also
been developed. They usually work with a powerful heating lamp, the voltage of
which can be adjusted. This type of equipment will never be so accurate as the
drying oven method, but is a great help during operation of a plant, as the
operator can have quick response from the laboratory enabling him to find
suitable drying parameters.
Automatic control of the moisture content
is measured with infrared light. The reflection from the sample is direct
proportional to the moisture content, and the output is used to control the
outlet temperature by regulation of either the feed pump or the heat applied to
the heating section of the Vibro-Fluidizer.
Bulk
Density The bulk density is an economically, commercially,
and functionally important property. When shipping powders over long distances
the producers are of course inter-ested in a high bulk density in order to
reduce the shipping volume. A high bulk density also saves packing material and
storage capacity.
For some powders the aim is a low bulk density,
obtained by agglomeration, for optical reasons, or because of requirements to
instant powder production.
Bulk density is defined as the weight of a
given volume of powder and is expressed in g/ml, g/100 ml, or g/l. The
reciprocal value is the bulk volume which is expressed in ml/100 g or ml/g. The
bulk volume is usually used when a graduated cylinder glass is used for the
determination. The volume of 100 g of powder is then measured in the cylinder.
As to the other method giving the bulk density, the weight of the powder in a
100 ml cylinder is measured. Both results can naturally be converted to the
other expression. The value may either be expressed as tapped 0 times (loose),
tapped 10 times (poured), 100 times, or 1250 times. Various types of equipment
can be used for the tapping. Also manual tapping is used. The intensity of the
tapping is naturally influencing the value.
The bulk density of milk
powders is a very complex property, as it is a result of several other
properties. However, the primary factors determining the bulk density are:
- Particle density, given by:
- the solids density, a
function of product composition
- the content of
occluded air in the particles
- Amount of interstitial air, i.e. air between particles (agglomeration)
- Flowability
These properties are discussed in the following.
Particle Density/Occluded Air
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 den-sity 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 20șC |
| 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.
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.
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.
Interstitial Air
This is a very complex property, too. The less
interstitial air (the air that occupies the space between
particles/agglomerates) the higher bulk density.
The amount of
interstitial air is determined by the particle size distribution and the degree
of agglomeration. The content of interstitial air can be calculated as follows:
Via = 100 / Dpowder - 100 / Dparticle (21)
where:
Via =
Volume of interstitial air in ccm/100 g powder
D powder = Powder bulk
density (tapped 100x) in g/ccm, page 193
D particle = Particle density in
g/ccm, see formula (19)
A powder with particles of the same diameter
would be ideal from a drying point of view, but undesirable from a bulk density
point of view, as the air space (the interstitial air) between the particles
will be very large thus resulting in low bulk density. The ideal is a wide
particle size distribution with enough small particles to fill out the space
between the medium and large particles thus resulting in a powder with high
bulk density. There is, however, a limit as to how small particles are wanted
from a recovery point of view, plus the fact that a powder with many small
particles will be dusty. Furthermore, they will affect the flowability
negatively.
A wider particle size distribution, but in the bigger
particle size spectrum is therefore wanted. This can be obtained by using high
solids content and/or viscosity, reducing the velocity of the wheel or pressure
of the pressure nozzles, or using bigger nozzle size. The result will however
be very dubious in a single-stage dryer where the bigger particles call for
higher outlet temperature thus increasing the occluded air content due to
reasons already discussed (case hardening). Powders with extremely high bulk
density can therefore only be achieved in two-stage dryers.
The powder
leaving the chamber will, as earlier discussed, be slightly agglomerated due to
the primary agglomeration. In a one-stage dryer equipped with pneumatic
conveying system, the problem does not occur due to the mechanical treatment it
is exposed to. But in two-stage dryers the primary agglomeration is
significant. The agglomeration is developed due to the powder being more
thermoplastic. As the mechanical treatment in the Vibro-Fluidizer is very
gentle, the agglomerates are not broken up. A pressure conveying system is
therefore recommended, if a powder with very high density is wanted. It should
however be pointed out that the primary agglomeration has a positive influence
on the flowability of the powder.
It has been observed that freshly
made powder often exhibits low bulk density which increases several days after
the production. This is caused by the electrostatic charge of the powder making
the particles stick together, forming "agglomerates". As the time passes the
powder will lose the charge and behave normally. An effective earth connection
of all parts of the drying equipment can to some extent solve this problem.
Flowability
The flowability of
a powder is not fully understood. Two free-flowing powders mixed together will
not necessarily be free-flowing. A good flowability is obtained from large
particles or agglomerates without small particles - this will, however, tend to
decrease the bulk density. Also the particle surface plays an important role
and especially the content of free fat. Nozzles are generally believed to
produce particles with better flow-properties than the wheel, especially in
whole milk powder. A powder with a good flowability will increase especially
the poured and loose bulk density.
Many attempts have been made to
develop a suitable method for measuring the flow-ability - some methods are
measuring the angle of repose for a given amount of powder and some methods the
time it takes the powder to pass through a hole in a funnel with a given
diameter. Common for these methods are, however, that they are suitable for
powders with a good flowability, whereas they cannot be used if the powder is
not free-flowing. Further, the result is influenced by the ambient conditions
especially the humidity of the air.
A method developed by NIRO is
however suitable for any kind of powder. In this method the time is measured by
a given volume of powder to flow through well de-fined slits in a drum rotating
with a given revolution/min.
Solubility
That milk powder has to be soluble in water is obvious. However, not
all of the components in the powders are soluble when reconstituted in water.
In powders produced in modern dryers, this amount is very small and approaching
100% solubility. Nevertheless, powders with a bad solubility are still produced
and any dryer can in fact be maloperated resulting in a powder with bad
solubility.
The method for measuring the solubility is very simple,
well defined, and easy to perform:
10 g of skim milk powder, 13 g of
whole milk powder or 6 g of whey powder (or equivalent amount of concentrate
depending on solids content) is mixed with 100 ml of water at approx. 24șC in a
mixer at high speed for 90 sec. The milk is then left for 15 min. after which
it is stirred with a spatula. 50 ml is filled into a graduated 50 ml centrifuge
glass with conically graduated bottom. The glass is spun in a centrifuge for 5
min., the sediment-free liquid is sucked off, the glass is filled up again with
water (to make the reading easier), and the content is stirred up. Then the
glass is put into the centrifuge and spun for 5 min. after which the sediment
is read.
The sediment is expressed in ml and is termed Insolubility
Index. It is usually below 0.2 ml in powder from good quality milk dried in
modern well-designed evaporators and dryers.
The reasons for high
Insolubility Index (i.e. bad solubility) in a powder may be many. It is usually
denatured caseins or very complex combinations of casein-whey protein and
lactose, the chemistry of which is not fully understood. The main contributing
factors are:
- Bad quality milk with a high development of lactic acid, i.e. bacterial
activity will result in high Insolubility Index, as any extensive
heat-treatment will cause an irreversible protein denaturation, especially of
the caseins.
- High temperatures of the concentrate during the evaporation will cause a
pronounced age-thickening resulting in viscosity increase and bad atomization,
i.e. high temperatures during the drying.
- Generally it may be said that the higher temperatures and viscosities
during the processing, the higher Insolubility Index may be expected. Powders
with a high lactose content such as baby food will practically never get a high
Insolubility Index, as lactose protects the proteins from denaturation.
- Powders dried according to the one-stage drying principle will more easily
get a high Insolubility Index than from the two-stage drying principle.
It is not only the dryer, which is to blame for high Insolubility Index.
Also the evaporator may harm the concentrate. It is, however, measured very
rarely. But if a factory has untraceable problems, it is recommended to
investigate the concentrate. This is done by using the same method as described
above, but with an amount of concentrate depending on the solids content and
corresponding to the specified amount of powder. See page 189. If milk powders
with high Insolubility Index are used in "compounded" products like baby food,
a correspondingly higher Insolubility Index should be expected.
Scorched Particles
Scorched
particles are generally accepted to be a measure for any deposits in the
dry-ing 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
18-28șC 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.
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.
Total Fat
The total fat in the whole milk powder is a question of
standardizing the raw milk prior to the processing and has got nothing to do
with the drying process.
The standardizing is carried out either by adding
skim milk or cream to the milk, or removing cream from the milk, depending on
the content of fat in the raw milk and the fat content aimed at in the final
powder. Standardizing tanks equipped with agitators are in most cases used, but
other methods are also recommendable.
As the fat content in the raw
milk in practically all cases is too high when producing whole milk powder,
skim milk powder is sometimes used for standardizing. The equipment needed is
an in-line powder/liquid blender known from recombining plants. As the solids
content will increase by adding skim milk powder, the evaporator should be
designed accordingly.
For an accurate determination of the fat in whole
milk powder the Rose-Gottlieb method is used and for a quick determination the
Gerber method is used.
Surface Free Fat
In whole milk powder the fat is present as fine globules covered
with a membrane substance and distributed evenly in the particles. However, not
all the fat is protected by a membrane, especially on the surface of the
particle, but it is also found inside the particles. This type of fat is termed
Free Fat, and it will have a direct influence on the shelf-life of the powder
and is directly responsible for the non-wettable surface when the powder is
mixed with cold water.
Free fat in the whole milk powder cannot be
avoided but reduced consider
ably. This is done by:
Avoiding excessive pumping and agitation of the
raw uncondensed milk. Recirculation in the evaporator should be avoided by all
means.
The pasteurization of the milk prior to the evaporator plays a role.
Direct pas-teurization, especially at low temperature, results in low viscosity
of the concen-trate and a fine atomization with a big surface to mass ratio
leading to increased free fat content.
The free fat is most efficiently
reduced by homogenization of the concentrate, preferably in a two-stage
homogenizer. In the first stage a pressure drop of 70-100 kg/cm2 is applied.
The fat globules will disintegrate into small globules which might - due to
static electricity - agglomerate again, i.e. they will consist of many small
fat globules. In the second stage a pressure drop of 25-50 kg/cm2 is applied
breaking up above mentioned agglomerates.
It is a general rule that nozzles
produce a powder with a lower free fat content than with the wheel, mainly due
to the homogenization effect of the nozzle.
Any strong mechanical handling
of the powder should be avoided, and then it is not astonishing that the
two-stage drying gives a powder with a lower free fat content than the
one-stage drying.
In plants with integrated fluid beds the free fat will
increase, if the bed temperature is too low signifying too high moisture
content in the powder which results in lactose crystallization. See however
page 311, where production of whole milk powder with high free fat used in the
chocolate industry is discussed.
To determine the free fat in the
powder 50 ml of petroleumether and 10 g of powder are mixed slowly for exactly
15 min. The mixture is filtrated and 25 ml of the filtrate is evaporated, the
residue weighed and the free fat percentage is calculated either based on total
fat or more commonly based on the powder.
In another method for determination of the free fat toluene is used, and the
extraction time is sometimes as long as 24 hours. The result will naturally be
different from that obtained by the above method.
It is generally
accepted that the first method gives results representing the surface free fat,
whereas the other methods represent the total free fat, i.e. also what is
inside the pores and capillary network.
Wettability
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 particle
s 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 coex istence 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
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
20șC ± 2șC. 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 25șC.
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.
Dispersibility
Another
important property of instant powders is the ability to disperse in water by
gentle stirring. This means that the powder should disintegrate into
agglomerates which again should disintegrate into the single primary particles.
To obtain a good dispersibility of a powder it is necessary that the
powder is wettable and that the agglomeration is optimal, i.e. no fine
particles should be present.
The analytical method is very difficult to
define and perform and the reproducibility is very poor. There are numerous
methods, and the results cannot be compared.
Being aware of that, IDF has
developed a new dispersibility test. This test is based on determining the
capability of a powder (25 g of skim or 34 g of whole milk powder) being poured
on a surface of water (250 g, 25șC) to disintegrate into particles capable of
passing through a 150-micron sieve when applying the prescribed manual stirring
for 20 sec. The amount of powder passing the sieve and being dissolved or
dispersed is found by the determination of total solids of the filtrate and
expressed in percentage as dispersibility.
The powder is considered
instant by IDF, if the dispersibility is at least 85% (whole milk) or 90% (skim
milk). However, plants with new drying technology easily produce powders with a
dispersibility of 95%.
There is no doubt that this test presents a more
reliable basis for assessment of instant milk powders than the wettability
test. On the other hand, it is a test requiring relatively high expenditure of
work, so it can hardly be used as a routine test. Besides, even when using
skilled workers the reproducibility is rather poor.
A more simple
method is to pour 10 g of skim milk powder or 13 g of whole milk powder
i nto 100 ml of water at room temperature and then manually stir with a
teaspoon until the powder is dispersed leaving no lumps on the bottom of the
glass. The time used is measured by means of a stop watch.
After some
training the reproducibility is fairly good, and the method is quick.
Furthermore, it has the supreme advantage that it is just what the housewife
does when she wants to prepare a glass of milk.
Sludge
Similar to the IDF
dispersibility (used only for instant cold-water soluble whole milk), but only
12.5 g powder in 100 ml water at 25șC and 85șC is used. The sieve used is
600-micron. The residue on the sieve after filtration is weighed and recorded.
Slowly Dispersible Particles (SDP)
The same procedure as for Sludge is used. After filtration through
the 600-micron sieve the milk is poured into a test tube which is emptied again
immediately. The remaining film with undissolved particles/agglomerates is
compared with a photograph 5-grade scale. The SDP is determined in both 25șC
and 85șC warm water. The remedy to improve the SDP value is agglomeration.