To achieve an RSD below 5 per cent with short mixing times in baking mixes containing baking extracts, lecithin, fat and several functional powder additives, liquid addition, distribution of the powder additives, and mixing kinematics must be designed as a single, integrated process. The critical point is not the pre-mixing of trace elements, but the even, lump-free incorporation of several liquid components and functional powders, without producing local over-wetting, fat lumps or later segregation.

Assessing the starting situation correctly

Baking mixes can contain numerous functional powder components, such as flour fractions, starch, sugar, leavening agents, acidulants, emulsifiers, milk or egg powder, cocoa, flavourings, salt, enzymes, fibre, hydrocolloids, colourants and baking extracts. In addition, lecithin, liquid or molten fats, oils, flavourings, extracts or other liquid additives can be introduced. These components often differ considerably in particle size, bulk density, wettability, fat content, cohesiveness and moisture sensitivity.

A very high mixing quality therefore means more than an even distribution of a single marker. As far as possible, the liquids must be present as a thin, even distribution on the moving powder bed. At the same time, functional powder additives must be distributed without agglomerate formation and without local over-concentrations. Even small amounts of a poorly distributed liquid can create sticky nests to which powder adheres. Such agglomerates impair the visual and functional properties of the finished baked product. Liquid additions can cause agglomerates in baking mixes; an even liquid distribution in the powder is therefore particularly important.

Dry pre-mixing first

Before liquid addition, the dry main formulation should first be homogenised. This applies in particular to the functional powder additives that differ significantly in particle size, bulk density or cohesiveness from the main matrix. A short, defined dry-mixing phase produces an even starting distribution and prevents dry islands or locally over-concentrated functional additives from remaining once wetting begins.

For particularly cohesive, fatty or fine powder components, a targeted pre-mix with a suitable proportion of the main matrix can be worthwhile. This is not about classical trace-element dilution, but about breaking up agglomerates, improving dosability, and evenly pre-distributing functional constituents. The pre-mix is then introduced into the moving main batch. An appropriate order of powder addition reduces the risk of fine components adhering to walls, seals or poorly circulated areas.

Introducing liquids finely

Lecithin, fat, oil, baking extracts and other liquid additives should not be added as a jet, a surge, or individual large drops onto a stationary powder bed. What matters is a finely atomised, precisely dosed addition distributed over an area into an active mixing zone. Two-fluid nozzles or suitable spray nozzles produce a controlled droplet spectrum and can distribute more viscous liquids better on the product, provided liquid temperature, viscosity, pressure and nozzle geometry are set appropriately. For baking mixes, the use of two-fluid nozzles is recommended, because the liquid must be distributed as evenly as possible onto the particles.

Dosing preferably takes place during stable, three-dimensional product circulation. Each liquid phase should be introduced into a zone through which fresh powder continuously passes. This reduces local over-wetting and prevents fat or lecithin from adhering to walls, tools or lumps that have already formed. The spray pattern must not hit the vessel wall or stationary product areas. The spraying time should be long enough to distribute the liquid quantity over the entire product volume; an addition that is too fast increases the risk of lumping. After addition, a defined post-mixing phase is required so the liquid can continue to distribute. Technical guidance on liquid addition likewise recommends a preceding dry mix, a sufficiently long spray phase into moving product, and a subsequent wet-mixing time.

With several liquids, everything does not necessarily have to be added at the same time. A staged dosing sequence is often advisable. For example, part of the fat or oil can first be used to bind certain dust fractions or to adjust the wettability of the main matrix. Depending on its form and the formulation, lecithin can be dissolved or diluted in a suitable carrier fat or oil and then finely sprayed. In dry food mixes, lecithin is used both as a powder and diluted in oil, to coat particles with a thin phospholipid layer. The permissible sequence, however, must be established through formulation trials, because it affects flowability, instantisation, agglomeration, water uptake and subsequent baking properties.

Suitable mixing technology

Short mixing times with several liquid and powder additives require a mixer that continuously moves the entire batch volume into an active mixing zone. A convective mixer with intensive three-dimensional product circulation is fundamentally suitable for this. Supplementary cutting rotors, choppers or high-shear tools can be used to specifically break up agglomerates formed by liquid addition. This intensive stage, however, should only be operated for a limited time and only as intensively as required. Otherwise, unwanted fines formation, heating, or a change in bulk material properties can occur.

The optimal mixing technology depends on the ratio between powder and liquid quantity, the viscosity of the additives, the melting temperature of the fat, the required coating, the sensitivity of functional constituents, and the desired particle structure. At low liquid quantities, even wetting is the priority. At higher fat or binder fractions, a transition to agglomerating or pasty product states can occur. In that case, the mixing tool, torque reserve, temperature control, nozzle concept and, where applicable, a separate de-agglomeration stage must be designed for this state.

Temperature and process control

Temperature is often a decisive lever with lecithin and fat. It affects viscosity, pumpability, atomisation, droplet size and the distributability of the liquid. If the fat is too cold, it can be viscous, partially crystalline, or difficult to atomise. If it is too warm, it can penetrate the powder too quickly, alter the desired particle surface, or cause quality losses with temperature-sensitive constituents. The temperature of the mixer, liquid, lines and nozzles should therefore be defined and monitored.

A temperature-controlled mixer jacket or temperature-controlled liquid lines can help keep the viscosity of the liquid phase stable. The process temperature, however, should only be set as high as necessary for a stable spraying process. With fats that crystallise again after addition, the cooling curve can also affect the subsequent flow condition and storage stability of the baking mix.

The mixing time should be determined through a mixing kinetics study. First, the dry-mixing time is examined, then the optimal spraying duration for each liquid, and finally the required wet-mixing time after liquid addition has ended. The goal is the shortest robust process sequence with which the targeted RSD, the desired fat and lecithin distribution, and the required free flowability are reliably achieved. An unnecessarily long wet-mixing time can lead to further agglomeration, fat smearing, wall build-up, or a change in particle structure.

Correctly demonstrating homogeneity

An RSD below 5 per cent must be demonstrated for the formulation characteristics that are actually critical. A single salt or dye marker can assess the general dry mix, but does not automatically prove the even distribution of lecithin, fat, baking extract or functional powders. Several analyses should therefore be combined. Suitable methods include, for example, fat determination, moisture determination, specific analysis of individual functional additives, extract analytics, image analysis of visible agglomerates, sieve analysis to check for lumps, and sensory and baking-technology tests.

Sampling must cover the entire batch and the discharge. Samples from different zones of the mixing chamber, as well as time-staggered discharge samples from the start, middle and end of emptying, are advisable. The mean, standard deviation and RSD can be calculated from at least ten individual samples:

RSD=sxˉ×100%

In addition to RSD and the mean, maximum, minimum, range, fat or lecithin distribution, lump content, free flowability and stability after storage should be considered. A mixture can be homogeneous immediately after mixing and still change due to fat crystallisation, vibration, conveying routes or prolonged storage.

Practical process sequence

A robust process sequence begins with controlled dosing and brief homogenisation of the dry main components and functional powder additives. The liquid phases – baking extracts, lecithin, fat, oil or flavourings – are then introduced into the actively moving product in a defined sequence, temperature-controlled and finely atomised. Each addition is followed by a sufficiently long but not excessively long post-mixing phase. If lumps form or cohesive additives need to be broken up, a local intensive mixing stage is switched on only for the time required. The product is then discharged and filled with as little waiting time, drop height or strong vibration as possible.