Which mix treatments are suitable for muesli and cereals to prevent segregation in the packaging?
With muesli and cereals, segregation in the packaging is most effectively avoided through a combination of gentle base mixing, short and controlled product paths, and dosing of the components close to the packaging. It is particularly robust to bring critical components – such as nuts, dried fruit, chocolate pieces or fine spice and dust fractions – together as late as possible and close to the multihead weigher or the packaging machine. This leaves only a short time and distance between mixing and the bag in which the components can separate again.
Why segregation occurs
Muesli and cereal mixtures are particularly prone to segregation because their components often have markedly different particle sizes, shapes, bulk densities and surfaces. Fine flakes, sugar, powder or fragments can trickle down through the gaps between larger constituents. This mechanism is known as percolation. Round or compact components can roll further down a heap than flat flakes or irregularly shaped dried fruit. Free fall also produces different trajectories; vibration during conveying and transport can likewise re-sort the constituents. Differences in size, shape and density are key drivers of such segregation effects.
Good mixing quality at the end of the mixing process alone is therefore not sufficient. The mixture can segregate again during emptying, in intermediate vessels, at transfer points, on vibratory conveyors, in pneumatic conveying lines, along drop paths, or even during transport and storage in the sealed bag. The entire process chain through to the finished container must therefore be regarded as part of the mixing task.
Suitable treatments of the mix
A first effective measure is harmonising the particle structure. Very fine, freely mobile fractions promote percolation in coarse mixtures. Where the formulation and product profile allow, fine fractions can be reduced, agglomerated upstream, or bound to coarser carrier particles. A narrower particle size distribution can also lower the tendency to segregate. This must not, however, come at the expense of mouthfeel, instant behaviour, sensory quality or labelling. Generally reducing the size of all constituents is usually not a solution for muesli, because the desired chunkiness and visual product quality would be lost.
In suitable formulations, a light coating step can stabilise the mixture. Small quantities of oil, sugar solution, syrup, lecithin or another binder that is legally and sensorially suitable for food can bind fine particles to flakes, nuts, extrudates or other coarser carriers. This reduces the number of freely mobile fine particles and can lower the risk of percolation. Binders, however, change texture, crispness, water activity, shelf life, flow behaviour and, where applicable, the allergen or labelling situation. They should therefore only be used where the formulation target allows it. Deliberately increasing cohesion through liquids or binders is cited as one way to reduce the tendency of bulk materials to segregate.
Another suitable treatment is a defined premix of critical minor components. Fine spices, vitamins, minerals, sugar, salt, powdered flavours or dyes can first be homogenised with a partial quantity of a suitable carrier component. This premix is then introduced into the main mixer. This prevents small quantities of a fine constituent from remaining locally concentrated or moving through the product as an independent fines fraction. With very sensitive dried fruit, chocolate pieces or large nuts, by contrast, it can be sensible to add these only late and with particularly low mixing intensity.
Process control through to the bag
For basic homogenisation, gently operated batch mixers, for example drum or other low-shear mixers, are suitable where the components flow well and are largely free of agglomerates. With more complex mixtures involving many components or larger differences in density, shape and particle size, a slow-running convective mixer with controlled three-dimensional circulation is often more robust. Rotational speed, mixing time, fill level, addition sequence and discharge should be set so that the required homogeneity is achieved without causing breakage, fines formation or over-mixing.
After mixing, free drop heights should be consistently minimised. Cascades, chutes, spouts, telescopic tubes or controlled transfer points reduce the differing trajectories of light, flat, large and compact components. The conveying equipment should generate as little vibration as possible. Large drop distances, fast conveyor belts, strong shaking motion and long pneumatic transport paths are particularly critical for muesli mixtures, because they can promote percolation, rolling segregation and breakage. Reducing conveying speed and drop height is an established measure for limiting segregation.
Buffer vessels, silos and hoppers should be designed for mass flow. With mass flow, the material moves toward the discharge across the entire vessel cross-section. This reduces zones of long residence time, funnel or channel flow, and the selective discharge of certain components. Important parameters are sufficient wall inclination, suitable wall materials, a sufficiently large outlet, a flow-favourable geometry and a matched discharge element. Mass-flow vessels can substantially reduce segregation-related problems.
With continuous production, a continuous mixer with gravimetrically dosed individual components can be useful, provided the mixture is fed directly to packaging without large intermediate buffers. This reduces residence time and the number of transfer operations. For many premium muesli and cereal products, however, separate dosing close to the packaging is the safest method. The main components are here assembled portion-wise via a multihead weigher or a comparable combination weighing system and only brought together immediately before the bag. Individual components can be weighed separately and combined in a targeted way. Such solutions are used in the cereal industry to dose different sub-components in a controlled manner and bring them together close to the packaging.
Quality control
Effectiveness should not be assessed from a single sample taken from the mixer alone. Time-staggered examination of the discharge, as well as samples from bags taken at the start, middle and end of a packaging run, are advisable. Depending on the formulation, the proportions of critical components can be counted, weighed, or determined via image analysis and marker analytics. Suitable metrics are the coefficient of variation of individual ingredients, the range between minimum and maximum, and a visual assessment of layering, breakage proportion and fines enrichment.
KoneSlid® for stable muesli mixtures
Muesli and cereal mixtures often consist of components with strongly differing particle sizes, shapes and bulk densities. Flakes, extruded cereals, nuts, dried fruit, sugar, fine spices and dust therefore tend to segregate again after the actual mixing – during discharge, along drop paths, in buffers, or on the way to the packaging machine. The amixon® KoneSlid® mixer KS combines gentle, actively generated product circulation with short mixing and discharge times. Special wetting devices can additionally be used to bind fine dust specifically to larger carrier particles and reduce the formulation's tendency to segregate.
The KS is designed for sensitive bulk materials with differing structures, for example instant beverage powders, dried milk derivatives, instant soups, muesli, tea, spray-dried products, fluid-bed granulated products, belt-dried products or components of frozen foods. The central mixing tool produces three-dimensional forced restratification. The product is conveyed upward near the wall, flows downward under gravity at the centre, and is there guided back toward the outer mixing zone by a conical displacer body. This continuously captures and spatially distributes the entire batch volume.
After around four tool revolutions, the entire volume can be fully restratified once. Depending on the formulation, fill level, particle structure and desired homogeneity, the technically ideal random mixture can be achieved after approximately 20 to 40 revolutions. The mixing time required for this and the actual mixing quality must be verified with the original product. The controlled, comparatively low-speed product movement limits the mechanical energy input and can help preserve the structure of sensitive flakes, agglomerates, coatings and coarse constituents.
A key component in preventing segregation is the targeted wetting of fine constituents. amixon® muesli and cereal mixers can be equipped with special wetting devices. These dose small quantities of a suitable liquid binder – for example oil, sugar solution, syrup, lecithin or another formulation-compliant liquid – finely and as evenly as possible into the moving mixture. Fine dust, sugar fractions, spice powder or small fragments can thereby adhere to coarser carrier particles such as flakes, extrudates, nuts or dried fruit. The number of freely mobile fine particles is reduced, which can in particular lower the percolation of fines through the gaps between large components.
The wetting must be precisely matched to the product and the formulation. Too little liquid can fail to achieve the desired binding effect; too much liquid can cause local clumping, change crispness and flow behaviour, raise water activity or impair shelf life. Besides the quantity and composition of the binder, droplet size, spray pattern, dosing position, addition timing, product temperature and mixing duration after addition are decisive. The mixer ensures that the binder is distributed as evenly as possible without excessively stressing sensitive components. Binding fine fractions with suitable binders is a recognised approach for reducing the tendency of granular mixtures to segregate.
Unlike free-fall and tumble mixers such as double-cone or V-mixers, the KS generates an active, defined product movement. Free-fall systems work through repeated lifting and falling back of the bulk material. They can be suitable for free-flowing components with similar particle characteristics, but for complex mixtures they often require longer mixing times and offer only limited scope for the targeted incorporation of small liquid quantities. With strong differences in particle size, shape or bulk density, segregation can also increase during mixing or especially during emptying. The KoneSlid®, by contrast, enables controlled convective circulation and targeted liquid incorporation.
Discharge is also important for the stability of the mixture. On the KS, the central closure system can lower to open a large discharge cross-section. For sufficiently free-flowing muesli mixtures, the batch can be emptied within a few seconds. The short discharge process can reduce the time in which fine particles trickle downward, coarse constituents roll, or components sort themselves on a heap. A residual discharge of up to 99.98 per cent or better, however, is always related to the specific product, the fill level and the apparatus design. For strongly cohesive, moist, sticky or very unevenly structured mixtures, the residual quantity and emptying time can differ. Even the claim of a fully segregation-free discharge must be demonstrated for each formulation through time-staggered discharge samples.
After mixing, the process paths through to the bag remain decisive. Drop heights, vibratory conveyors, heaps in buffers, long transport distances and repeated transfer can segregate a formulation again despite optimal mixing. The outlet, transfer points, conveying equipment and packaging machine should therefore be designed with minimal drop heights, the shortest possible paths and mass-flow-oriented material transport. For formulations with particularly diverse components, separate dosing of the constituents close to packaging via a multihead weigher can be sensible. This shortens the path between combination and the bag and limits the opportunity for re-segregation.
The KS can be built project-specifically for hygienically and safety-critically demanding food applications. This includes a mixing chamber designed for ATEX Zone 20, as well as options for vacuum or inerting. A design to EHEDG, FDA or 3-A requirements is possible depending on the agreed construction. Surfaces welded free of crevices and ground smooth, a mixing tool supported only at the top, large CleverCut® inspection doors with OmgaSeal® seals, low-dead-space discharge elements and integrated washing lances improve accessibility, cleaning and the prerequisites for fast product changes. Dry and wet cleaning can be designed to be validatable depending on spices, allergens, sugar or fat content.
Before the investment, mixing and packaging trials with the original product can be carried out in the amixon® pilot plant. More than 30 test units in various sizes are available at the Paderborn headquarters; pilot plants in Japan, India, Thailand, China, South Korea and the USA are additionally available. Mixing quality, breakage rate, fines content, wetting, binding of fine fractions, coating distribution, discharge behaviour, segregation stability, cleanability and reproducibility are examined. Particle size analyses before and after the trial, as well as time-staggered samples from the discharge and the packaging, provide the basis for designing the mixer, wetting system, discharge and packaging line to suit.