(745f) Using Powder Rheometry to Identify and Quantify Different Humidity Induced Caking Behaviours

“Caking” is the phenomenon where materials gain strength during storage due to changes in particle-particle interactions. Whilst caking occurs via several different mechanisms, for example, mechanical, thermal, environmental and/or chemical, the absorption and migration of moisture is frequently the most dominant mechanism. Fully understanding the propensity to cake is important for minimising downstream process issues, such as blockages, which may have a detrimental impact on the productivity and final product quantity.

It is often assumed that caking occurs uniformly throughout a powder bed, however recent advances in powder rheometry techniques have demonstrated that powders can exhibit a wide range of caking behaviour, including non-homogenous caking (crusting) and even a “hybrid” response dependent on storage conditions. Furthermore, as powder rheometry can quantify the resistance to flow throughout the powder bed, progression of the cake/crust and its resultant strength can be assessed using this method.

To demonstrate the ability of powder rheometry to capture the impact of different caking behaviours, several materials including skimmed milk powder (SMP), sulphated methyl ester (SME) and three food flavours were stored for several days under controlled humidity conditions. SMP exhibited non-homogeneous behaviour with a clearly defined caked region which progressed through the sample over several days. In contrast SME exhibited the aforementioned hybrid behaviour, with the bulk of caking localised, but further caking occurring below the defined crust. In both cases, the strength of the crust was shown to increases over time and demonstrating that caking may require several days to fully develop. Analysis of the food flavours indicated that some materials can also switch between the observed caking regimes, with one material initially exhibiting hybrid behaviour before transitioning to a more homogeneous response.

Caking behaviour may be further complicated by variations in the environmental conditions. To further understand this, samples were stored at 75%RH for 3 days and followed by 3 days at ambient (33%RH). As expected, the return to ambient conditions decreased the degree of caking for some samples. However, for SMP, drying the sample had limited impact on the crusted region but more interestingly, caking continued to occur in the previously uncaked region below the crust. More notably one of the food flavourings presented a significant increase in the degree of caking on return to ambient conditions, demonstrating that for some materials, drying can actually exacerbate caking behaviour.

Overall, the results demonstrated the humidity induced caking is complex, manifesting in a diverse range of different behaviours. In order to fully understand the caking behaviour of a given material, it is therefore necessary to use a technique capable of capturing variations in the flowability throughout the powder bed. Though the use of powder rheometry it is not just possible to identify the caking regime but also quantify the impact, allowing the progression of the cake over an extended period be determined.