Pigging of production systems minimises product losses and reduces the cleaning effort. Usually, cleaning pigs made of plastic are used, but these have specific demands concerning system design and carry the risk of product contamination with foreign materials. The Chair of Food Engineering at Technische Universität Dresden and the Fraunhofer Institute for Process Engineering and Packaging IVV are currently developing alternative approaches for cleaning processes in the chocolate industry.

The cleaning of food processing equipment is a huge cost factor, as the strict hygiene requirements are associated with a high expenditure of time and energy. In the chocolate industry, cleaning pipes with water is nearly impossible as chocolate is not water-soluble and tends to form clumps that may clog the pipes. In practice, the successor product is therefore frequently used as a purging material. This displaces the predecessor product, but generates a large quantity of mixed phase. Alternatively, the pigging of pipes enables effective cleaning and a reliable separation of the predecessor and successor products.

However, cleaning with solid pigs is limited by the design of the pipework, such as the presence of bends, changes in cross-section and fittings. High investment costs in piggable pipes and the constant risk of foreign material contamination due to abrasion or defects of the cleaning pig limit the use of plastic materials. Ice pigging was therefore developed for the water-based CIP cleaning sector, in which an ice-water mixture is used as a flexible, contour-adaptive pigging suspension. Compared to conventional rinsing this requires less purging material, and a significantly higher cleaning effect is achieved. This process is already used for the cleaning of drinking water pipes, but has also been tested for foodstuffs such as ketchup. But, as it is not possible to prevent the ice from melting during the process, the length of the pipes to be cleaned is limited.

This is where the IGF project 01IF22267N (FlexPig) of the Fraunhofer Institute for Process Engineering and Packaging IVV and the Chair of Food Technology at TU Dresden comes in: Using the chocolate industry as an example, research is focussing on dense sugar-fat suspensions as pigs, which are suitable for cleaning a complex pipework carrying fat-based foods. The pig suspension can be considered as a cost-effective replacement for the mixed phase of the predecessor and successor products and is discharged at the end of the pipework. As it should only consist of the product's own components, slight mixing with the pigging suspension does neither jeopardise food safety nor affect labelling issues of the successor product.

Knowledge of the rheological properties of the pig suspensions is essential for optimising the cleaning behaviour. Highly concentrated suspensions exhibit shear-thickening or shear-blocking behaviour, known as shear jamming. This behaviour is known, for example, from water with a high proportion of starch grains, and also forms the basis for cleaning using ice pigging. Shear jamming is the basic reason for plug flow, meaning that the suspension moves through the pipe like a solid body and pushes forward the predecessor product. However, the tendency to shear thickening behaviour depends not only on the volume fraction of the solid phase, but also on the particle shape and size as well as on interfacial effects between the solids and the continuous phase of the suspension. Apart from the composition of the pigging suspension itself, the cleaning effect and the cohesion of the pigging suspensions is also affected by the properties of the predecessor and successor product.

The Chair of Food Technology at TU Dresden is responsible for carrying out rheological investigations to determine the influence of the particle size distribution of the dispersed sugar particles and the influence of surface-active substances on the flow properties of highly concentrated sugar-fat suspensions. This knowledge makes it possible to specifically determine viscosity, yield stress, viscoelastic properties and shear jamming behaviour of the pig suspensions.

At the Fraunhofer Institute for Process Engineering and Packaging IVV, cleaning tests are carried out on pilot plant scale with pig suspensions of different composition. Furthermore, various methods for applying and detecting the pig suspensions are investigated to ensure they are properly introduced into the pipeline, the cleaning success is characterised, and the pig suspensions are fully removed from the pipelines afterwards.

The knowledge concerning rheological properties of highly concentrated suspensions and their influencing factors is combined with the findings on the cleaning efficiency of selected pig compositions. This enables to specifically target pig properties and thus to individually adapting its rheological properties to the predecessor and successor product. In the future, the findings will also be transferred to pig suspensions composed of other fats as a continuous phase or other solid phases. This will allow the technology to be transferred to other product categories, e.g. in the delicatessen or cosmetics sector, where the pigs are also based on hydrophilic, product-specific raw materials.