The cleanability of mixers for sticky powders and fat-containing recipes, for example cocoa powder, chocolate powder, spice blends or fat-coated powders, must already be taken into account during plant planning. Good cleanability does not result from a single measure but from the interplay of hygienic design, suitable discharge, a matched cleaning procedure and a traceable check of cleaning performance. Hygienic design requires that product-contact and cleaning-relevant surfaces can be effectively cleaned, disinfected where necessary, and fully drained.

Sticky powders can deposit on the vessel wall, mixing tools, shafts, seals, flaps and discharge elements. With fat- or oil-containing recipes, coherent deposits often form that bind dust and become further consolidated through temperature, pressure or long standstill times. Particularly critical are transitions, weld seams, tool connections, shaft passages, seal seats, outlet areas and inaccessible points behind internals. Such deposits increase the risk of cross-contamination, allergen carry-over, microbiological problems after wet cleaning and inconsistent product quality.

A readily cleanable mixing plant has smooth, corrosion-resistant and readily accessible product-contact surfaces. Weld seams must be flush and finished. Overlaps, open gaps, sharp internal corners, long branch lines, non-drainable recesses and unnecessary threads in the product space are to be avoided. Inspection openings must be large enough to allow visual inspection of critical areas and, where necessary, manual re-cleaning. Sufficiently complete product discharge reduces the amount of material remaining before cleaning even begins.

Self-draining is particularly important with wet cleaning. After cleaning, no residual pools may remain in vessels, pipework, nozzles, seal spaces or discharge elements. Hygienic-design guidelines therefore require a suitable slope, fully drainable internals and the avoidance of horizontal surfaces on which cleaning or product liquid could stand. This requirement relates not only to product-contact interior surfaces but also to surfaces outside the product space from which liquid could reach critical areas.

Shaft passages and seals are among the particularly critical design areas. The seal design must suit the product, pressure, temperature, cleaning method and the required level of hygiene. Options include, for example, gas- or liquid-flushed sealing systems, product-side barrier seals or seal spaces that are accessible for cleaning. What matters is not that a seal is generally described as "free of dead space", but that it is reachable, drainable and controllable during operation for the intended cleaning. Bearings and drives must be reliably separated from the product space.

With sticky recipes, process-side measures can significantly ease later cleaning. Scrapers or wall-close mixing tools can limit build-up during mixing and discharging. Small, product-appropriately designed gaps between tool and vessel wall can promote self-cleaning during operation; gaps that are too tight, however, can also cause pressing-in and adhesion with fat-containing powders. Tool geometry, rotational speed, temperature and recipe must therefore be assessed together. Long standing times before cleaning should be avoided where possible, since fats can crystallise, oxidise or form hard deposits together with powder particles.

With water-sensitive products or products that must be processed dry, it should first be checked whether dry cleaning is sufficient. This includes controlled vacuuming, brushing at accessible points, where applicable approved compressed-air methods, and manual removal of product residues. Compressed air must not be used uncontrolled, however, because it can raise dust, carry it into seal or machine areas, and increase the risk of a dust explosion or cross-contamination. Dry cleaning is frequently preferred for dry bulk materials; where wet cleaning is necessary, the plant, the surroundings and the subsequent drying process must be designed for it.

With fatty, oily or strongly adhering deposits, wet cleaning is often required. This can include pre-rinsing, alkaline cleaning with suitable surfactants or cleaning boosters, a defined temperature, mechanical cleaning action and a final rinse. Whether rotating spray heads, static spray balls or targeted jet nozzles are suitable depends on vessel geometry, internals, product adhesion and the required impact energy. A spray ball does not automatically guarantee the cleaning of every surface; areas behind mixing tools, seals and discharge elements in particular must be considered separately. After wet cleaning, complete draining and, where required, validated drying are necessary.

The cleaning strategy should also take account of the sequence of batches produced. A sequence from light to dark products, from neutral to strongly flavoured recipes, or from allergen-low to allergen-containing products is often sensible. The specific sequence, however, must be assessed on the basis of risk; with critical allergens, strongly colouring ingredients, animal-derived constituents or microbiologically sensitive recipes, a complete intermediate clean is required. A production-side batch sequence does not replace validated cleaning.

Cleaning performance must be checked against predefined acceptance criteria. Visual inspection is indispensable but is not always sufficient for critical product changes. Depending on product and risk, swab samples at difficult-to-reach points, rinse samples, TOC analysis, product-specific residue analysis, allergen analysis and microbiological testing may be required. A riboflavin test can be useful for qualifying wetting during wet cleaning, but only shows the liquid coverage achieved and is not direct proof of the removal of a specific cocoa deposit or fat film. The FDA expects written cleaning validation procedures with defined acceptance criteria, sampling plans and sufficiently sensitive analytical methods.