Lesson 36. TECHNOLOGY OF DRY MILKS
Module 13. Technology of dried milks
TECHNOLOGY OF DRY MILKS
Milk is unique in its content of valuable nutrients. Both the chemical composition and the physical properties of the milk powder play an important role in its use with quick and complete reconstitution adding convenience to the products. Every day, millions of liters of milk are processed into dry products. By far the largest part ends up as ordinary skim milk and whole milk powders. These products are market commodities with little requirement for product functionality. Consumer demand for specific product properties, particularly in the food industry, has resulted in the development of many different dry dairy products, ranging from instant whole milk powder to speciality food ingredients. The dairy industry invests heavily in the development and production of such products.
During the last few decades there has been a growing market for powders which are instantly soluble in cold water. Ordinary non-agglomerated powders tend to lump when mixed with water, and if strong mechanical stirring is not applied, it may result in an inhomogeneous mixture which is not attractive to the consumer. The quality requirements to instant milk powders are constantly getting stricter as an increasing number of properties must be optimized simultaneously and controlled within still narrower limits in order to obtain a high quality product with the highest degree of uniformity, fulfilling the requirements set by costumers, organizations and industrial and/or legislative standards.
The historic development started with the pioneering research of Mr. David D. Peebles in the beginning of the fifties, and instantized non-fat dry milk was marketed from 1954. Soon it replaced the regular spray dried products on the retail market. Lecithinizing during the drying in the fluid bed is not always applied; it is meant to obtain instant properties.
The principle of instantization is the agglomeration of individual spherical milk particles into clusters and the conversion of lactose from the glass into a microcrystalline form which makes the powder more wettable and less hygroscopic. In the instantization process, the surface of milk powder particles are humidified, or the particles are only partially dried during manufacturing, so that the surface is tacky and partial crystallization of lactose leading to formation of microcrystals occurs before the particles are redried. This produces a clustering of the particles in loose spongy aggregates of low density (known as agglomerates / conglomerates / granulates ) which flow freely and disperse readily in cold water, as water penetrates the spongy structure of these aggregates and allow them to sink and disperse.
The principal purpose of agglomeration, also called instantizing, is to improve the rate and completeness of the reconstitutability of dry milk products. Small single particles dissolve instantly in water. Powder consisting of small particles is, however, difficult to disperse. Big particles are easy to disperse in water, but dissolve only partially.
Agglomeration is a result of wet and/or semi-dry particle collision with size >125 µm. Control is achieved by returning dry fine powder to the wet spray during different stages of spray drying. Mastering the agglomeration techniques is the art of modern spray drying. Plant operation and economics are other important parameters.
In addition to nonfat dry milk, the instantizing process has been applied to dry whole milk and other milk fat containing dry dairy products with only limited improvement in reconstitutability resulting. The milk fat adversely affects wettability because of its hydrophobic property.
36.4 Major Systems
The process may be carried out using either of these two methods:
- Spray Drying Agglomeration
- Rewet Agglomeration
36.4.1 Spray drying aggomeration
In Forced Secondary agglomeration the small dry particles are introduced into the dryer near the atomizing device, where they will meet and collide with atomized wet droplets thus forming agglomerates consisting of many particles stuck together having a size of 100-500 µ, depending on the parameters selected.
Due to the special air flow pattern in a Multi Stage Dryer, a considerable spontaneous, secondary agglomeration takes place. For production of high quality instant whole or skim milk powder this spontaneous agglomeration suffices, and the fines are just returned to the integrated fluid bed, from where they will get airborne again and reach the atomizing zone again. However, the agglomeration may be further enhanced by forced, primary agglomeration (collision of sprays overlapping each other from different nozzles in a multi-nozzle atomization unit) and/or by returning the fines to the atomization zone (forced, secondary agglomeration). Further flexibility can be gained by designing the atomization unit in a way that allows the distance between the single nozzles or between the nozzles and the fines return tube to be altered. Depending on the atomization device the fines return is designed in different ways as shown in Figures 36.5 to 36.7.
The fines return is an integral part of the nozzle unit with the fines duct in the centre surrounded by nozzles at the periphery. The fines are introduced tangentially into the fines distribution duct or through a center pipe. The nozzles can be welded to the nozzle rod at a certain angle, so that by turning the nozzle rod around its axis the collision point can be altered.
It is the process of separating the part of fines which is entrained in the main drying air leaving the drying chamber. The efficiency of separation is determined by the air flow pattern and air velocities in the drying chamber.
It is defined as the partial break down of agglomerates in fluid beds or powder conveying systems resulting in creation of either fines, smaller agglomerates (abrasion) or of a number of smaller sized particles (fragmentation). Factors affecting the extent of attrition are the jet velocity, determined by the pressure difference across the perforated plate, the fluidization velocity and the actual design of the perforated plate.
It is defined as the separation of fines in fluid beds. The efficiency of classification is mainly determined by the fluidization air velocity and also the fluid bed design.
After the final drying, the powder enters the cooling section where the powder is cooled by means of air at ambient temperature followed by cooled, dehumidified air. The powder is finally passed over a sifter where any oversize particles are removed. It is also possible to install a sifter with two nets thus removing any remaining particles/agglomerates of small diameter. Together with the fines, this fraction may be returned to the atomizing device thus producing a powder with a well defined agglomerate size distribution.
The agglomeration is improved by:
¨ High solids content in the concentrate
¨ Bigger quantity of fines returned to the atomizing device
¨ Fines introduction closer to the atomizing device
¨ Shorter distance from nozzle to fluidized layer in a static fluid bed
¨ Higher moisture content from the primary drying stage
¨ Bigger primary particles
¨ Lower pasteurization temperature of the milk prior to the evaporation
When leaving the sifter, the powder should not be exposed to strong mechanical conveying, neither by means of air nor by fast moving mechanical screws. However, today's lenient vacuum-low speed air systems are used without too much damage to the agglomerates. The best thing, however, is to install the plant so high that filling into bags or bins is possible by gravity.
36.4.2 Rewet agglomeration
Since many powders may become instant by the mere agglomeration, many processes have been developed during the past years to agglomerate ordinary powders consisting of single spray particles usually produced in plants with pneumatic conveying system.
The major systems of agglomeration are the Peebles (
Each varies in equipment and operation details. The general features in common are:
- Wetting of surface of the particles with steam, atomized water, or a mixture of both.
- Agglomeration, whereby the particles collide due to the turbulence and adhere to each other forming clusters.
- Redrying with hot air.
- Cooling and sizing to eliminate the very large agglomerates and the very small particles.
Wetting of the surface of the particles is done with humid air, steam, atomized water either pure or containing milk solids, sugar or other soluble components. The water may further contain additives such as vitamins (water soluble), minerals, colour and surface-active agents. The atomization of the moistening agent can be carried out by means of nozzles or a rotary atomizer. If a rotary atomizer with two feed pipes is used, it is possible to use a combination of steam and water or use two moistening agents, which cannot be mixed for various reasons. If the product is insoluble, an adhesive can be dissolved in the moistening agent. When doing so products otherwise impossible to agglomerate can be agglomerated with a good result.
Agglomeration, whereby the moist sticky particles collide due to the turbulence and adhere to each other forming agglomerates, is essential for the rewet process. As powders with different compositions do not behave in the same way during the rewetting and agglomeration process, different equipment is needed to obtain an optimal agglomeration. In principle there are two ways of performing the agglomeration:
¨ Droplet agglomeration
¨ Surface agglomeration
In the droplet agglomeration process, the powder particles are wetted with droplets of liquid atomized by means of a nozzle or a rotary atomizer while suspended in air. The powder may either be introduced around the rotary atomizer or the nozzle by means of gravity or pressure air conveying, or from below by means of pressure conveying.
The droplet agglomeration process is especially used for powders containing fat such as whole milk powder and powders with a high content of sugar such as cocoa-milk-sugar mixtures. If cold-water instant whole milk powder is produced, a lecithin dosing equipment is installed between two Vibro-Fluidizers. However, the final product quality will never be as good as that produced on a drying plant equipped with fines return, fluid beds and lecithin dosing equipment.
In the surface agglomeration, either steam or warm moist air with a high relative humidity is used as the moistening agent. The surface of the individual dry particles is wetted due to condensation of the water vapour on the colder particles, whereby the stickiness required for the agglomeration is created. The subsequent agglomeration will take place, if the particles are exposed to sufficient mechanical impact. The warm humid air is usually made by spraying steam into warm air at a given temperature to obtain a relative humidity of 100%.
Table 36.3 Specifications of basis powder for agglomeration
The surface agglomeration can also be used for whole milk powder, but the agglomerates get too compact for obtaining a powder with good rehydrating properties.
As the basis powder used for agglomeration is remoistened to obtain the desired surface characteristic for an optimal stickiness, this additional moisture has to be evaporated again in order to reach the specified moisture content. The agglomerates may break down again, if they are exposed to extensive mechanical handling, such as in a pneumatic conveying system. It is therefore necessary to perform the drying in a Vibro-Fluidizer.
Like the redrying, the cooling is performed best in a Vibro-Fluidizer.