Lesson 41. PHYSICAL PROPERTIES OF DRIED MILKS-II
PHYSICAL PROPERTIES OF DRIED MILKS-II
Dry milks exhibit so many other physical properties also. They are discussed here.
41.2 Lump Formation
Spray powder may contain small hard lumps when some of the milk droplets are only partially dried under following different conditions:
(1) If the milk feed to the atomizer is increased whilst the volume and temperature of the hot air supply remains unchanged.
(2) When the larger particles remain moist and develop lumps at points where they are distributed in the powder mass.
(3) Similar effect may result if the speed of a centrifugal atomizer is reduced (e.g. by a slipping belt drive) as a reduced rate of rotation increases particle size
(4) A falling air temperature may also cause incomplete drying of the large particles. Changes in feed rate are usually indicated by a fluctuating outlet air temperature.
Control instruments should be placed conveniently to permit continuous observation (and when possible recording) of atomizer speed, air flow rate, inlet temperature, and outlet temperature, whilst the milk feed should be calibrated to permit very small known alterations of setting to be made.
The flowability of a powder refers to the ease with which the powder particles move with respect to one another. This is an important but complex property. The main factors controlling flowability are:
- 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.
- A powder with a good flowability will increase especially the poured and loose bulk density.
- Milk fat is an important factor in cohesion. Increasing fat content up to 20% increases the resistance to flow. It is suggested that once a fat content of 20% is reached, the amount of surface fat on the particles is sufficiently high to cause maximum resistance to flow and therefore further increase in fat content apparently does not cause increase in cohesion and thus decrease in flowability.
- Pressure nozzle powders are superior in this respect to rotary ato¬mizer powders.
- Two - Stage drying gives better results than Single-Stage drying.
- Particle size distribution - cohesion or resistance to flow, of spray dried milk powder increases with decreasing particle size.
- Other factors improving flowability are
(b) Low percentage of f ines
(c) Addition of f ree - flowing agent (e.g. SiO2, Na-Al-silicate, or Ca3(PO4)2 ) in milk powder for coffee machines.
(d) Addition of components by dry mixing (e g. sugar, whey powder)
(e) Low relative humidity of atmospheric air
41.4 Reconstitution / Instant Properties
The phenomena involved in reconstitution process and corresponding powder-properties are shown below in Table 41.2.
Splitting the reconstitution process into the above steps is helpful for understanding the process, for instance to be able to find the reasons for lack of instant property. On the other hand there is no sharp borderline between these individual reconstitution steps, i.e. they do not p roceed successively but simultaneously, at least two at a time and the individual properties influence each other. Therefore, it is difficult to determine the individual properties independently of the others.
- The wettability is a measure for the ability of a powder to be wetted with water at a given temperature. This is only used when producing instant powders. 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.
- The fine powder produced by the spray process is difficult to wet and. tend to produce lumps which are slow to disperse in water. It appears that very small particles under ~ 50µ swells within initial contact with water and blocks the interstices and hinders the access of more water. A 1arge particles size is preferable, and a figure of 100-150µ is generally considered to be an ideal size.
- 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 is not only possible but usually unavoidable.
- 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 < 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.
- 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 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.
- The reconstitution of a mass of powder is more complicated. 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 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.
- 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.
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.
- 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%.
To obtain 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.
Milk powder has to be soluble in water. 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.
The solubility (more strictly, reconstitutabi1ity) of dried milk is a very important commercial property. Loss of solubility is due to denaturation of protein by heat treatment during the manufacturing process.
- The severe heat treatment used in atmospheric roller- drying reduces the solubility of the powder to 80 to 85% by damage to the fat globule. Structure also releases some drops of oil, and the reconstituted milk has a less attractive appearance. Some makers employ preliminary homogenization of the milk to secure improvement.
- Spray dried milk (once it has been wetted) should be highly soluble up to 98 to 99% and the fat globules should be largely undamaged. The solubility of the powder is determined by the heat treatments used at all stages of the manufacturing process.
- The use of temperatures above 74°C for pre- heating the milk reduces the solubility of the powder as the temperature rises. At 88°C as used for "high-heat" powder, the reduction is definite but of little real importance to most purposes.
- Usually the evaporation process is of less significance, but some reduction of solubility may result if a high-temperature effect is used or the milk is over concentrated.
- Powder solubility is also affected by the temperature of the hot air in the drying chamber and the time of exposure of the powder to it. The maximum desirable air inlet temperature varies with the degree of air turbulence and the speed of removal of the powder. It should not exceed 170°C. Rapid removal of the powder is essential, and recovery in cyclones is preferable to bag collectors.
- The final solubility tends to be the cumulative result of relatively small effects at each succeeding stage of the whole process. Protein insolubility due to heating of the liquid milk probably differs from that induced by exposure of the powder to hot air. The former is an irreversible change, while the latter is at least partly reversible if the powder is reconstituted in water at 50°C instead of 20°C.
- A spray powder of poor solubility tends to form three layers when reconstituted. The top layer consists, mainly of fat globules which have carried some insoluble protein to the surface. A sediment at the base consists of settled insoluble protein which has also entangled some fat globules, whilst the middle layer of soluble protein also permeates both top and bottom layers.
- During storage, loss of solubility may occur, depending mainly upon the moisture content of the powder and the storage temperature. Large-scale reconstitution of spray dried milk for consumption as fluid milk or for manufacturing purposes requires considerable care and even dispersal of the milk solids. The complete absorption of water by the powder takes some time; hence it is desirable for the reconstituted milk to stand undisturbed for 3 to 4 hrs after preparation. If the product is to be consumed as fluid milk, preliminary cold storage for 24 hrs is needed fully to regain the flavour and characteristics of fluid milk. In addition, homogenization is often employed.
- 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:
2. 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.
3. Generally it may be said that the higher the 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.
4. Powders dried according to the one-stage drying principle will more easily get a high Insolubility Index than from the two-stage drying principle.
41.5 Agglomerate Structure and Powder Properties
Depending on the design and adjustment of the fines return system - particularly the location of the introduction of the fines in relation to the atomization device - different agglomerate structures result ( ), which influences certain powder properties, such as bulk density, mechanical stability, dispersibility and slowly dispersible particles as shown in Fig. 41.4.
¨ If the fines are introduced close to the atomizing devise, the moisture content of the primary spray particles is high and thereby their plasticity and stickiness, and the fines particles may penetrate primary particles or be completely covered by concentrate. Such agglomerates have been termed 'Onion'-structured (Fig. 41.5). 'Onion'-structured agglomerates are characterized by a high mechanical stability and a high bulk density, but they will often appear as slowly dispersible particles after reconstitution.
¨ With progressively looser agglomerate structures, the bulk density and mechanical stability decrease gradually, and the overall instant properties improve. However, if a 'Loose Grape'-structure is eventually obtained, the mechanical stability may be so low that the powder becomes very susceptible to attrition resulting in deteriorated instant properties.
41.6 Scorched Particles
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.
- 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, or insufficient cleaning.
- If 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 which involves taking of 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) and mixing 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 then 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 as shown in Fig. 41.7 below
41.7 Total Fat
- Most fat globules are less than 2 µ m, but a small proportion of the fat (e.g., 2% of it) is to be found as a thin layer on parts of the surface of the powder particles. The vacuole volume of most powders varies from 50 to 400 ml . kg − 1 .
- 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.
- 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. As the solids content will increase by adding skim milk powder, the evaporator should be designed accordingly.
41.8 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 is the portion of the fat content of a milk powder which can be extracted by organic solvents (e.g. carbon tetrachloride).
- Free fat does not exist in globules protected by the amorphous lactose, but occurs as "pools" on the external and internal surfaces (craters) of powder particles.
- The free fat content is expressed as a percentage either of the total fat content or of the total mass of powder. A free fat content of 10% is excessive if it refers to a powder basis but may be acceptable if it refers to a total fat basis.
- Free fat content of milk powders affect both reconstitution and powder stability.
Free fat in the whole milk powder cannot be avoided but reduced considerably by
(a) Direct pasteurization, especially at low temperature, results in low viscosity of the concentrate and a fine atomization with a big surface to mass ratio leading to increased free fat content.
(b) The f ormation of free fat is reduced by a high content of amorphous lactose, and is increased by high protein content.
(c) Low melting fats tend to produce higher levels of free fat.
(d) As total fat content increases above ~ 28%, t he free fat rises progressively
(e) The free fat is most efficiently reduced by homogenization of the concentrate, preferably in a two-stage homogenizer.
(f) Avoiding excessive pumping and agitation of the raw uncondensed milk. Recirculation in the evaporator should be avoided by all means.
(g) Gentle drying conditions: Effects of drying variable such as inlet air temperature, outlet air temperature, atomizer speed etc. are shown in Tables 39.1, 39.2 and 39.3 respectively. 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.
( h) 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.
(i) Gentle powder treatment:
a. Any strong mechanical handling of the powder should be avoided.
b. The two-stage drying gives a powder with a lower free fat content than the one-stage drying.
c. Avoiding pneumatic transport
d. Low pressure drop in cyclones
e. Effective cooling of powder by fluid bed.
(j) The powder moisture should not be too low.
(k) Avoid lactose crystallization
(l) Addit ion of emulsifier
41.9 Whey Powder
When producing whey powder, especially the non-caking type, the following properties, and some of those mentioned earlier for Milk Powder, should be considered:
41.9.1 Total moisture and free moisture
During the manufacture of non-caking whey powder, the lactose is crystallized during which process the lactose picks up one molecule of water.
41.9.2 Hygroscopicity and caking properties
The hygroscopicity, i.e. the ability to absorb moisture, of the whey powders is determined by the degree of crystallization of the lactose. But also the salts and even the proteins absorb water, however, only in a limited amount compared with the lactose.