In the case of whole milk powder it is required that the water is >40ºC, as whole milk powder particles are always covered by a thin layer of fat making the powder repellent in cold water. However, there has in the last decades been an ever increasing demand for cold water instant whole milk powder, so besides the agglomeration it is necessary to apply a surface active agent.
For this purpose lecithin (originating from soy beans) dissolved in pure butter-oil, in order to make a liquid, may be used. Lecithin is superior as to the functional performance, i.e. achieving of instant properties. The butter-oil is chosen also in order to use a natural milk component, as using a vegetable fat, even it is done in many cases, could be considered a falsification.
The amount of lecithin and of the total free fat (i.e. original free fat + added butter-oil + lecithin) in the final powder may vary from 0.1-0.3% and 1-2%, respectively. However, variations within these limits result in rather big differences as to the desired properties.
One must be aware that a high amount of total free fat together with high amount of lecithin improves the wettability, but on the other hand it is affecting the flowability and may seriously affect the dispersibility. At lecithin levels >0.5% it is possible to detect the characteristic soy flavour. The structure of the powder and the degree of agglomeration are of importance too, as poorly agglomerated powders require higher amount of wetting agent than well agglomerated products.
Fig. 117 Process diagram. Instant whole milk powder
The processing diagram for the production of instant whole milk powder is shown in Fig. 117. It can be seen that the whole process has been split into two subsequent process lines, i.e. the manufacture of a basic powder in a plant as shown in Fig. 84, followed by the lecithination and packing line, Fig. 118.
Fig. 118 Two-stage lecithination plant
The splitting of the process line into two is the most advantageous way of operation, as it will be explained below.
In the two-stage process the basic powder is collected for intermediate storage. It is important to prevent any damage to the powder by mechanical treatment. The intermediate storage is therefore preferably accomplished in tote-bins or similar containers of 1-2 m3.
The basic powder is then transferred from the totebins into the supply silo and is metered into the first Vibro-Fluidizer by means of a dosing screw. The powder is heated and at the same time any fines are blown off.
The lecithin dosing equipment consists of two vessels, dosing pump, powder trap with two-fluid nozzle, and control panel. The first vessel serves for the preparation of wetting agent, and the second one as supply vessel, from where the wetting agent is metered to the two-fluid nozzle for spraying on to the powder. See Fig. 119.
Fig. 119 Lecithin dosing equipment
The flows, temperatures and pressures of the wetting agent and of the atomizing air are recorded. Interlocking in the control panel ensures that the flow of powder will stop automatically, if for some reason no lecithin is applied. Consequently no powder will leave the plant without a proper lecithin coating. The second Vibro-Fluidizer, also supplied with warm air, ensures a gentle but proper mixing of the powder to obtain a uniform distribution of the lecithin mix over the particle surface.
The powder leaving the lecithination unit is packed into retail packages. The filling machine is placed preferably directly below the lecithination unit with a hopper for short intermediate storage to avoid any unnecessary transport.
As shown on the process diagram on Fig. 117 (the dotted line) the powder production and lecithination can be made in one continuous process. In this case the powder trap with lecithin nozzle is placed between the integrated fluid bed and the Vibro-Fluidizer, Fig. 120. On this flow sheet reception, pretreatment, standardization and evaporation equipment are shown. The product quality can be compared with the one achieved by the split process operation described above.
Fig. 120 Complete plant for production of instant whole milk powder (MSD)
However, there are many reasons for preferring the split process operation, as retail packing of milk powder is never a fully continuous operation, since there is always a natural break between the powder production and the packing. During this break the powder must be stored in bulk for one to several days, preferably in tote-bins to avoid damage.
For quality reasons it is better to store unlecithinated powder. The lecithination process therefore fits best as a part of the packing line forming one continuous operation.
The intermediate storage of the powder after production makes it possible to analyze the product to classify it and to calculate the composition and quantity of wetting agent in order to achieve the desired properties.
A further advantage of the split process operation is that fines created during storage and transport can be blown off in the first Vibro-Fluidizer of the lecithination unit.
The quality of this rejected fines portion corresponds to ordinary (non-agglomerated) whole milk powder, and can be sold as such. The amount is usually less than 5%.
Today, however, most instant whole milk is produced in MSD plants equipped with a lecithin dosing equipment, placed between the integrated fluid bed and the Vibro-Fluidizer, in one processing step, see Fig. 119. The final powder is conveyed to silos by lenient low speed vacuum conveying systems - being very gentle to the agglomerated product - before packing either in retail packs or 25 kg bagging lines. The conveying lines may be equipped with pre-gassing by N2/CO2 for prolonging the shelf life.