In the mixing-technology processing of pet food, both a continuous mixer and a batch mixer can achieve high mixing quality. The differences lie less in a fundamentally better or worse mixing quality, and more in how the homogeneity is generated, controlled, documented, and safeguarded during product changes. Recipe diversity, batch size, the proportion and criticality of micro-components, production campaigns, changeover frequency, and the requirements for traceability and carry-over protection are especially decisive for the choice.

In feed manufacturing, mixing quality is usually assessed with the coefficient of variation, also called CoV or CV. It describes the relative scatter of the concentration of an examined component across several representative individual samples. The CoV is calculated from the standard deviation and the mean:

CV=sxˉ×100%

The lower the CV, the more even the distribution of the examined component. Which target values are required depends on the type of feed and on the criticality of the component. For complete feeds, values below 10 percent are often used as a guideline. Stricter requirements can apply to mineral, vitamin or medicated premixes, and to particularly low-dose active substances. For diluted medicated premixes, the Canadian Food Inspection Agency, for example, names a maximum of 5 percent CV, while 10 percent is named for micro- and macro-premixes and complementary feeds, and 15 percent for complete feeds or total mixed rations. What is always decisive is that the marker used reflects the real mixing risk, and that the sampling is representative across the entire discharge run.

In batch mixing, a fully defined recipe is weighed in, dosed, mixed and then discharged as a demarcated batch. The entire batch remains in the same mixing chamber for a fixed mixing time. This allows fill level, order of addition, mixing time, rotational speed and, where applicable, liquid additions to be stored exactly as a batch recipe. Homogeneity can be checked after every batch or as part of a defined validation plan. This is particularly advantageous when pet food comprises many recipe variants, or when vitamins, minerals, amino acids, enzymes, medicated additives, flavourings or other low-dose components need to be mixed in evenly.

A batch mixer can generate very high and well reproducible homogeneity when dosing, fill level and mixing parameters suit the recipe. Short-term fluctuations in the input of individual components can be partly offset within a batch, because the entire product volume is repeatedly circulated during the mixing time. This does not mean, however, that dosing errors remain without consequence: if a component is weighed in incorrectly overall, the recipe for the entire batch remains faulty. The real strength of the batch system is therefore the clearly demarcated, documentable and verifiable batch. In feed production, several individual samples are often drawn from the discharge to assess mixing uniformity, for example ten samples taken in succession over time. The CV determined from these makes it visible whether the mixture is even across the entire batch.

A continuous mixer, by contrast, works with a continuous product stream. All components are continuously dosed, fed into the mixer, and discharged again as a continuously mixed stream. Under steady-state conditions, a continuous mixer can likewise achieve very good homogeneity. The prerequisite is that all dosing streams operate stably and that the mass flows of the individual components continuously match the target recipe. Precise gravimetric feeders, uniform feeding, stable bulk densities, controlled flow properties of the raw materials and a matched residence time in the mixer are particularly important.

The continuous process, however, reacts immediately to fluctuations. Bridging in the storage vessel, pulsating screw feeders, differing bulk densities, refilling operations, fluctuating moisture or changing particle sizes can alter the instantaneous dosing performance. This deviation does not become visible only after a batch is complete; it can occur directly in the product stream. The quality of continuous systems must therefore preferably be monitored with reference to time and mass flow. Sampling takes place at defined time intervals, as close as possible to the mixer discharge. Only in this way can it be assessed whether the process runs at steady state after start-up, and whether the mixing quality remains stable across the entire production campaign. The Canadian Food Inspection Agency likewise recommends, for continuous mixing plants, taking samples during ongoing operation at even time intervals directly at the mixer discharge.

For complex pet food recipes with many components dosed in small quantities, a batch mixer is often more robust, because a defined total quantity is processed over a complete mixing cycle. For large quantities of a few standard recipes, on the other hand, a continuous mixer can offer economic advantages. It works without batch-related interruptions and can enable a very high, constant throughput. Its quality, however, depends more heavily on the dosing, the raw-material supply and the process parameters remaining permanently stable. The statement that a batch mixer is fundamentally more homogeneous would therefore be too sweeping. A well-designed continuous mixer can achieve comparable homogeneity with stable dosing. The batch mixer, however, is often simpler to validate and easier to safeguard in day-to-day operation with recipes involving many variants.

The second major difference concerns product changeover times and carry-over. With a batch mixer, the recipe boundary is unambiguous. After a batch has been completely discharged, the mixer can be cleaned, rinsed, or charged directly with the next batch. The product changeover time depends above all on residual discharge, cleaning requirements, any rinse material needed, and the downstream conveying sections. For production operations with many varieties, small to medium lot sizes and frequent changes, this is a considerable advantage. The batch is clearly assigned, the raw materials and process parameters used can be documented, and release can take place on a batch-specific basis.

Carry-over can also occur with batch mixing. Residues in the mixer, at the discharge, in screw conveyors, elevators, filters, conveying lines, buffer silos and the filling plant are particularly critical. Fine, adhesive, fatty or electrostatically charged components can remain on surfaces after discharge. In pet food, changes between recipes containing medicated additives, coccidiostats, allergens, different animal species, different protein sources or strongly differing nutrient profiles are particularly sensitive. A visibly empty mixer is therefore not sufficient as proof of a safe product changeover. What matters is the actual residual quantity across the entire plant and the analytical demonstration that critical components do not pass into the following product at an impermissible concentration. Fine particles increase the risk that residues remain in the equipment and affect subsequent mixtures.

With continuous mixing, a product changeover is usually more complex, because not only the mixer itself but the entire process line must be included in the changeover. Pre-vessels, feeders, conveying units, the mixer, downstream conveying sections, buffers, screens, separators and filling technology all contain material with a certain residence time. When switching from recipe A to recipe B, a transition zone is therefore created in which both recipes overlap. This transition material must be identified, collected separately, returned, repurposed or, where necessary, discarded. Only once the entire plant is completely filled with the new recipe and the process parameters are again running at steady state can the product be released as a regular batch.

Continuous plants are therefore particularly economical when long campaigns with few recipe changes are run. The changeover losses, the time required to run down and restart, and the quantity of transition product are then spread over a large production quantity. With many short runs and frequent SKU changes, this advantage can be lost. Batch mixers are usually more flexible in such situations, because they offer clear batch boundaries and the cleaning and rinsing steps can be better integrated into a batch-related sequence.

Regardless of the mixing principle, the plant design for product changeovers and hygiene is at least as important as the mixer type. Low-dead-space, readily accessible and, wherever possible, fully dischargeable mixing chambers, discharges, conveying sections and dosing units reduce residual quantities and simplify cleaning. Production sequencing is likewise an important tool. Uncritical recipes can be scheduled ahead of sensitive recipes. Products with similar raw materials or compatible ingredients can be grouped together as far as possible. For medicated additives, a production sequence from a higher to a lower inclusion rate of the same active substance can be sensible, provided this is permissible from a regulatory and quality-assurance perspective. In addition, a validated rinse material can be run through the mixer and downstream process sections. Its effectiveness must be analytically demonstrated for the respective critical component. Production sequencing and rinsing procedures are regarded in feed guidelines as central measures for limiting carry-over of active substances.