Incorporating flavours, prebiotics, vitamins and trace elements into milk powder requires mixing technology that is both gentle on the product and reproducible. Especially with agglomerated or instantised milk powders, flowability, wettability, reconstitution behaviour and particle structure must be preserved. At the same time, sensitive ingredients must be protected against heat, moisture, oxygen and mechanical stress.

What matters is not the mixer type alone, but the interplay of mixing principle, fill level, rotational speed, mixing time, energy input, product feed and dosing quantities. The ideal plant achieves reliable circulation of the entire powder without causing unnecessary abrasion or a relevant temperature rise through high shear forces. Blanket limit values for mixing time, temperature rise or energy input are of little validity; they must be determined on the basis of the specific milk powder quality, the recipe, the batch size and the desired end product.

Flavours can be present dry, encapsulated, oil-bound or liquid. Liquid flavours are usually introduced directly into the main mixer via a finely dosing spray lance or nozzle. The nozzle should be positioned in an intensively moving product zone so that the liquid distributes quickly across many particles. Excessively large droplets, too high a local dosing rate or an unsuitable nozzle position can cause caking and unwanted agglomerates. With hygroscopic milk powder, the introduction of water or other unwanted liquid fractions must be particularly limited.

Prebiotics such as inulin, fructo-oligosaccharides or galacto-oligosaccharides are generally used in substantially larger quantities than vitamins. Given sufficient similarity of particle size and bulk density, they can therefore usually be dosed directly into the main batch. A premix is not fundamentally required. However, it can be sensible if the addition quantity is small, the prebiotic tends to generate dust, there are large differences in bulk density or particle size, or a particularly high dosing accuracy is required.

The situation is different for vitamins and trace elements. These are often dosed in very small quantities and can segregate given an unfavourable particle-size or density distribution. For this reason they are usually prepared as standardised premixes with a suitable carrier substance or with a partial quantity of the milk powder. The premix is then introduced into the main batch. Stepwise dilution of the vitamin mix with milk powder before mixing into the total quantity is a proven approach for improving homogeneity.

Several mixing systems come into consideration for the main mixture. Vertical twin-shaft mixers combine efficient, three-dimensional product movement with comparatively gentle treatment of sensitive powders. They are used worldwide for demanding mixing tasks with milk powder, nutrient components, flavours and other functional ingredients. Because of the high circulation performance, the entire mixed product can be reliably captured. With a suitable design, small quantities, dry substances and liquids introduced via nozzles can be distributed evenly without unnecessarily impairing the sensitive agglomerate structure. Whether a premix is required for a particular ingredient depends above all on its dosing quantity and on differences in particle size, bulk density and flow behaviour.

Twin-shaft paddle mixers enable rapid mixing. They are suitable for larger batches, the distribution of directly dosed prebiotics and the controlled addition of liquid flavours. Their rotational speed, tool geometry and fill level must be designed so that sensitive agglomerates are not excessively stressed.

Horizontal ploughshare mixers offer intensive circulation and can distribute small quantities as well as liquids well. They are particularly suitable where preserving the particle structure is less important. However, ploughshare mixers are not automatically low in shear. At high rotational speed or when size-reduction tools are used, the energy input can rise considerably. Their suitability for instantised milk powders must therefore be checked in terms of abrasion, temperature rise and reconstitution behaviour.

Vertical cone-screw mixers work at low rotational speeds and are particularly gentle on sensitive particles. They are well suited to dry milk powder mixtures, encapsulated flavours and recipes with sensitive ingredients. Their comparatively long mixing times can be a disadvantage with oxidation- or moisture-sensitive components. With low-dosed vitamins and trace elements, a premix in this mixer type is usually particularly important.

Container and free-fall mixers cause only low mechanical stress and are therefore suitable for sensitive dry mixtures. Their suitability is particularly good where the components have similar particle properties and no liquids need to be added. With very small dosing quantities or marked differences in density and particle size, however, longer mixing times or premixes may be necessary.

For the selection of the mixing system, hygienic properties are decisive alongside mixing quality. Milk powder is dusty and moisture-sensitive; product deposits and cleaning residues can impair the quality of subsequent batches. Product-contact surfaces should therefore be readily accessible, smooth and as free as possible of areas prone to deposits. In dry powder plants, dry cleaning is frequently used to avoid the introduction of moisture. Where wet cleaning is required, the plant, and in particular areas that are difficult to access, must be dried reliably. EHEDG describes specific hygienic-design principles for this purpose for plants processing dry bulk materials.

For light- or oxygen-sensitive vitamins, in particular vitamin A and β-carotene, an enclosed, low-oxygen process can be sensible. Shorter open product paths, suitable packaging and, where applicable, nitrogen inerting can help to limit oxidative losses. However, the protective effect must be validated for the recipe, storage time and packaging.