Lesson 40. PHYSICAL PROPERTIES OF DRIED MILKS-I
PHYSICAL PROPERTIES OF DRIED MILKS-I
Good-quality dried milk should flow readily, be free from lumps or caking and be uniformly white or light cream in colour. Related Documents: Several properties of powdered products affect the quality and the suitability of the powder in specific applications. The physical properties of the powder may confer different properties upon powders of identical chemical composition.
A milk powder particle generally consists of a continuous mass of amorphous lactose and other low-molar-mass components in which fat globules, casein micelles, and serum protein molecules are embedded. The lactose generally is in a glassy state, the time available for its crystallization being too short. If, however, precrystallization has taken place, large lactose crystals may be present. When precrystallized whey powder is examined microscopically, most of the particles look more like lactose crystals of the tomahawk shape to which other material adheres. If lactose has been allowed to crystallize afterward due to water absorption, its crystals are generally small (about 1 µ m).
40.2 Roller-Dried Milk Powder
It consists of solid, irregular masses containing little enclosed air. The fat globule structure is broken by contact of the milk with the very hot rollers with the pressure of the scraper knives, so that free fat becomes scattered throughout the powder - some of it on the surface and is readily extracted by solvents. Roller-dried milk looks completely different from spray powder in the microscope. It consists of fair-sized flakes. Due to the intense heat treatment during the drying it has a brownish color, a strong cooked flavor, and the availability of lysine has been considerably reduced, by 20 to 50%.
40.3 Freeze-Dried Milk
It consists of coarse, irregularly shaped, and very voluminous powder particles, which dissolve readily and completely. However, the fat globules show considerable coalescence, unless intense homogenization has been applied. In most cases, damage due to heat treatment is minimal.
40.4 Spray Dried Milk Powder
40.4.1 Particles shape
Spray dried milk are found to be round with different surface structures, as revealed by electron microscopy. Some particle may be smooth, but most of them are severely wrinkled with deep surface folds and having "apple-like structure" caused by an implosion during the last stage of drying process or during the cooling of particles. The deep surface folds are formed due to the presence of casein in the spray dried material.
The air may be present as a single cell or as a number of small bubbles, and usually occupies 15% to 25% of the volume of the particle. In addition, there are always some solid particles, the proportion varying considerably (10 to 70% of the total) between different atomizers. The air cell volume may influence the keeping quality of the powder.
The body of the particle of whole milk powder is usually porous , whereas in case of skim milk powder the body in most particles is compact. Particles prepared from unhomogenized concentrated whole milk are more porous than particles made from homogenized concen trated milk. Similarly, small particles are more porous than large particles. High porosity is usually associated with the occurrence of cracks and capillaries in the particles.
Structure of agglomerated skim milk powder ranges from smooth comprising of fused primary globular particles with very small rosettes like lactose crystals on the surface to having rough surfaces covered with relatively large lactose crystals. Interior of these particles is found to be hollow and the crust is compact. The surface of the instant milk particles is very fragile and needs careful handling to avoid shattering and dust formation.
40.4.2 Particle size distribution
Particle size is of importance for the reconstitution of powder, its flowability and appearance. Particle size distributions of milk powder are usually between 20 and 60 µ m, and the distribution is relatively wide. The particles in agglomerated powder are much larger, up to 1 mm, and are irregular in shape. Such a powder usually contains very few separate particles smaller than 10 µ m. Within one sample of nonagglomerated powder the larger-sized particles have on average a higher vacuole volume, partly because a drying droplet shrinks more strongly if it contains no vacuoles. The particle size distribution of a milk powder depends on a number of factors in the production process. These are:
(a) Speed of rotation or pressure applied
(b) Feed rate of the concentrate
(c) Velocity of concentrate through the orifice (in case of nozzle type atomizer)
(d) Feed concentration and its viscosity.
(e) Temperature difference between the drying droplet and the hot air in the drier
40.4.3 Effect of atomizers
With a given liquid, pressure-jet atomizers tend to produce the lowest air-cell volume and the highest proportion of solid particles. On the other hand, air cell volume is also closely related to the concentration of the liquid being sprayed, and thus pre-concentration of the milk to a high solids content give less trapped air in the powder. In general, highly concentrated milks are sprayed more readily with centrifugal atomizers, and this factor tends to be the important one. After drying, further air diffuses into the particle for about 24 h owing to cooling and contraction.
40.4.4 Status of lactose
Lactose is the major constituent of the particle itself and comprises about 38 % of full cream powder and 50% of separated milk powder. It forms an amorphous glass-like envelop entrained in the particle and is also in the continuous phase within it. The outer envelope is only and slightly permeable and retains enclosed air and gases and seals in the fat which is only partially extractable by solvents.
The proportion of free fat is very variable and it is stated to range from 3% to 10% in spray dried milk as compared with 43% to 75% in freeze dried milk and about 90 % in roller-dried milk.
Lactose is, however very hygroscopic and readily absorbs moisture from air (over 50% RH) or from within the powder if its moisture content exceeds 5%. The amorphous lactose then crystallizes as ά- lactose mono-hydrate and forms a crystal lattice which renders the particle permeable. The fat is than freed, and at critical moisture content between 8.6 to 9.2% is almost complete extractable by solvents. The powder itself also gradually cakes into hard lumps. In spray powder the fat globule structure is largely retained although pressure-jet atomizers may produce some degree of homogenization to globule of smaller size.
Milk powder properties are divided into two main groups:
1. General properties: Includes properties mentioned under International Standards ensuing the required bacteriological quality, freedom from defect, and composition.
2. Properties specifically related to milk powders: Includes such properties as are directly influenced by the special technology and processes applied in the milk drying industry. These properties can be further divided into-
a. Physical properties which define the structure of the powdered product.
b. Functional properties which define the consumer's requirements that ensure that the product is suitable for a given purpose.
c. Product faults which indicate some und esi rable though often unavoid able deterioration caused by processing.
The distinction between th ese groups is often not sharp. For example, high free fat is a defect of instant whole milk powder for household use, but is an important functional property of whole milk powder for the chocolate industry. Bulk density which is listed among the physical properties is also an important functional property.
In order to achieve the desired characteristics and functional properties, it is essential to know the influence of the operating parameters on the individual properties. The properties of the powder are affected by the milk quality, the design of the evaporator and dryer, and by the process conditions. Some properties are governed by conditions outside the evaporator and dryer. This is the case when whey is dried, as the desired non-hygroscopic nature of the powder is chiefly governed by crystallization of the lactose between evaporator and dryer. Heat classification of the powder is determined outside the evaporator and dryer, as it is a f unction of the heat treatment prior to evaporation.
- The moisture content of a milk powder is defined as that part of the water contained by the solid which is in a form capable of taking part in deterioration of the powder. Thus water which is bound in the lactose crystal is not normally considered to be part of the moisture content of milk powder.
- Residual moisture is one of the most important properties of milk powder both from a quality and an economic point of view. The quality specifi¬cations lay down the maximum permissible moisture to achieve the desired shelf life. The economic aspect demands that maximum moisture content be approached as closely as possible, while at the same time ensuring that no portion of the product will exceed this moisture level.
- High moisture content has a major influence upon the development of most storage defects except fat oxidation. Normal roller-dried milk contains 1.5 to 2.5% and spray-dried milk 2.0 to 3.5% moisture. In the manufacture of powder of minimum moisture content, the nature of the packing is extremely important owing to the hygroscopic nature of dried milk solids. There appears to be critical level around 5.0% moisture, and every effort should be made to maintain a level not exceeding 4.0%.
- 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 denature 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 amino acid 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.
- In two-stage drying, it is also important to control the intermediate moisture, i.e. the moisture of the powder at exit from the drying chamber, because it influences many other properties including solubility index, particle density, bulk density, agglomeration, etc.
- It is also important to check occasionally how moisture content changes through the fluid bed system, to ensure that extensive overdrying followed by re-humidification to the specified final moisture content in the cooling section is not taking place.
- In addition to the grading standards, numerous customer specificat¬ions call for even lower moisture contents than one indicated above.
- Because there is some gain in moisture content during pneumatic conveying and blending and to a lesser extent during storage, it is normal to produce powder from the drier at a lower moisture content than that called for by the specification so that the final powder remains within specifi¬cation.
- 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 it is recommended to store the powder in a dry, cool place, where the water vapour pressure will be low.
The moisture content of a powder is estimated by
(a) Oven drying at 102°C ± 2°C to constant weight.
(b) Toluene distillation method
(c) Karl Fischer titration method.
Because of the importance of powder moisture content, many efforts have been made to control this parameter automatically. Automatic moisture control based on infrared absorption is in operation on many plants, sometimes achieving standard deviation less than 0.1%. Even better results have been achieved when combining the infrared control with feed-forward loops in which the set point of outlet air temperature is adjusted according to variations of the inlet pa rameters.
For process control, a modified quick oven-drying method and laboratory infrared apparatus are used.
(d) The outlet temperature is usually used as the parameter by which the final moisture of the product is controlled, thus compensating for the unavoidable (and often unknown) variations of other factors.
- The changes of the outlet temperature on a given dryer for a given product can be expressed by following linear equation, which is valid within the range of normal running conditions.
- It is observed that each 1 % rise in powder moisture makes it possible to decrease the outlet temperature by 5°C or to increase the inlet temperature by 50°C at a constant outlet temperature, while at the same time, keeping the particle temperature almost unchanged.
40.5 Bulk Density
The density of a powder may be defined in various ways. The density of the particle material, i.e., excluding the vacuoles, is called the true density . The weight of powder which can be packed into a given volume - known as the bulk, apparent, or packing density can vary considerably.
- Bulk density is important economically, commercially and functionally because it affects the size of containers, storage space and transport space.
- When transporting powders over long distances, the producers are interested in high bulk density to reduce the volume. Also, high bulk density saves in packaging material. In some instances producers may be interested in low bulk density to supply optically larger amounts of powder on the retail market than that of their competitors. Low bulk density, as influenced by agglomeration, is also an important part of powder properties, influencing the instant characteristics.
- The bulk density of milk powders is a very complex property being the result of several other properties, and being influenced by a number of factors as shown in Fig. 40.3 below. The primary factors determining the bulk density are
(a) The particle density, given by
a. Product mass density
b. The content of occluded air inside the particles.
(b) The content of interstitial air, i.e. the air between the particles, given by
a. Particle size distribution
(c) 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. The value may either be expressed as tapped 0 times (loose), tapped 10 times (poured), 100 times (Tapped), or 1250 times (Tapped-to-extreme).
Spray Dried Powder
Powder particles are dispersed in air and spray particles also contain air with in them. The density of air free milk solids is 1.32 g/ml. for whole milk and 1.46 g/ml for separated milk, but the bulk density of normal spray powder may very between 0.5 and 0.8 g/ml.
The bulk density of spray powder depends
(1) Partly upon particle size because small spheres pack more closely together than large ones, although a mixture of particles can also give a heavy powder owing to packing of small particles between large ones. In this respect jet atomizers, particularly pneumatic types, have the advantage, whereas centrifugal atomizers tend to produce large particles and lighter powders.
(2) On the other hand, bulk density is also influenced by the air content of the particles and consequently there is a direct relationship between bulk density and the degree of pre-concentration which is advisable when a heavy powder is required. This factor is probably the most important one and favours the centrifugal atomizer.
(3) It is also possible to increase bulk density by de-aeration of the liquid before drying.
The bulk density of roller powder depends upon
(1) The fineness of grinding and is therefore variable. In general, it is lighter than spray powder and varies between 0.3 and 0.5 g/ml.
(2) Although roller powder contains little entrapped air, there are considerable voids between the irregular particles, which pack less closely than regular spheres.
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 discussed as follows: Related Documents:
40.6 Particle Density/ Occluded Air
- The particle density depends on many factors. Composition of the solids plays an important part, first of all because it defines the product mass density (for instance the density of whole milk solids is less than that of non-fat milk solids). High protein content tends to reduce particle density as it increases the tendency of the feed to foam. This foaming can be suppressed somewhat by high heat pre-treatment (denaturing the whey proteins) and also by high concentration combined with heating the feed.
- To achieve high particle density, it is important to avoid any treat¬ment which may incorporate air into the feed, such as excessive agitation, etc. Rotary atomizers tend to incorporate air into the droplets, and pressure nozzles produce much higher particle density than rotary wheels. However, special vane-shaped rotary wheels, sealed disc atomizer, steam-flushing of the air space in a disc atomizer are now available with less tendency to entr¬ain air in product droplets.
- The presence of air in the atomized droplets causes occluded air in dried particles. Depending on drying conditions, or to be precise on the particle temperature during the drying process, those air bubbles initially pres¬ent may expand and further reduce the particle density.
- The droplet formed on atomization of a concentrate with high total solids contains more solids and hence less water than droplets of the same size produced from a concentrate with lower solids . The void size and amount of vapour available to expand is therefore less in droplets from high solids concentrate .
- A high inlet air temperature causes rapid formation of a solid surface/crust on the particle, thus increasing the void size and the quantity of vapour, and tends to cause ballooning of the particle. The effect of high inlet air temperature can be counteracted by decreasing the outlet air temperature and by the use of secondary drying. Therefore, if all other conditions are the same, the two stage drying process provides higher particle density than single-stage drying. The resultant increase in bulk density is due to the more rapid cooling of the inlet air thus delaying the formation of a solid surface on the particles.
- The influence of inlet air temperature, outlet air temperature, atomizer speed on bulk density of milk powder is depicted in the Tables 40.1, 40.2 and 40.3 respectively.
Table 40.2 Influence of the temperature of the drying air on whole milk powder
Table 40.3 Influence of the number of revolutions of the atomizer on whole milk
Table 40.4 Effect of different processing condition on the density of whole milk powders
- The ways in which particles pack together also influence the bulk density. It depends on the particle size range. The wider the range of sizes, the more likely it is that small particles will pack in the voids left between large particles and the higher will be the bulk density. As smaller particles contain proportionately less occluded air than large particles, the removal of fine material, for instance in the manufacture of instant whole milk powder, affects the bulk density both by decreasing the average particle density and by decreasing the number of small particles available to fill the voids between the larger particles. If a powder is subjected to severe mechanical action, there may be a breakdown of individual particles, forming more fine material and decreasing the number of internal voids, thus increasing the bulk density.
- The poro¬ sity is defined as the percentage, by volume, of the powder mass occupied by air surrounding the individual particles. Table 40. 5 shows the effect of different processing conditions on the porosity of whole milk powder.
- 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. The solids densities of various typical components in milk powders are shown in Table 40.6 as follows
- Powder solids density cannot be changed without changing the composition and is thus for a given product constant.
The particle density for the reciprocal value of the occluded air content is influenced by many factors. They are summarized here:
1. 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 is sponge like. A high degree of denaturation will give low occluded air content (high particle and bulk density) and vice-versa.
2. 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.
3. 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.
4. 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.
5. Solids content in the concentrate: Feed concentration plays an important role and high concentration gives less occluded air content.
6. 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.
40.7 Interstitial Air
This is a very complex property, too. The less interstitial air the higher the bulk density.
- The amount of interstitial air is determined by the particle size distribution and the degree of agglomeration.
- 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 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. Powders with extremely high bulk density can be achieved in two-stage dryers.