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Partic. vol. 56 pp. 75-83 (June 2021)
doi: 10.1016/j.partic.2020.10.012

Measurement and quantification of caking in excipients and food products with emphasis on the non-homogeneous interaction with ambient moisture

Katrina Brockbank*, Brian Armstrong, Jamie Clayton

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    • Moisture absorption may result in non-homogenous caking. • Homogeneous and non-homogeneous crusting can be assessed using powder rheology. • Powder cakes can strengthen even after being removed from high humidity conditions. • More accurate measures of different types of caking are necessary.


Whilst caking occurs via several different mechanisms, absorption and migration of moisture is frequently the most dominant mechanism within the food and pharmaceutical industry. Fully understanding the propensity to cake is important for minimising down-stream process issues, however most characterisation techniques assume that moisture induced caking occurs homogenously through the sample resulting in a uniformly caked powder bed. In this study, the effect of moisture induced caking on powder flowability was investigated using powder rheology. Several materials, including skimmed milk powder (SMP) and sulphated methyl ester (SME) were stored for several days under controlled humidity conditions. The flow energies, a measure of the resistance to flow, were measured at 24 h intervals using an FT4 Powder Rheometer. As the energy is measured as a function of the bed height, variations in the powder bed are also captured. The results demonstrated that caking does not always occur uniformly, instead a caked region (or crust) forms at the air-powder interface and then progresses through the powder bed. Furthermore, the strength of this caked region was shown to increase over several days before stabilising.

Graphical abstract


Powder; Caking; Crust formation; Powder rheometry; Relative humidity