Storage-induced caking of cocoa powder

- Storage-induced caking of cocoa powders occurred only if fat is present at particle surface and is in molten state.
- No influence of powder water activity on caking was observed.
- Caking mechanism implies the formation of liquid fat bridges between particles, which solidify upon cooling.

Cocoa powders are highly subjected to caking, a phenomenon of solid particles agglomeration that impairs powder functionalities such as rehydration and flowing properties. This study aimed at identifying the main caking mechanisms occurring during storage of cocoa powders, according to their fat content and water activity, as well as storage temperature. The formation of caked powder was monitored at macroscopic scale by sieve analysis, showing that caking was significant only for fatty powders at 40 °C, in agreement with a caking mechanism controlled by fat melting. The similarity of results obtained at 0.2 and 0.7 water activity indicated that the humidity caking mechanism was not significant for these powders. Observations performed by TEM evidenced the formation of fat bridges, confirming the occurrence of the fat melting mechanism. Then, solvent extraction techniques designed to quantify fat fractions permitted to highlight that storage caused the conversion of encapsulated fat into free fat. At temperatures sufficient to melt cocoa fat (40 °C), this newly formed free fat migrates toward cocoa particle surface, enhancing its fat coverage, hence making cocoa particle sticky and prone to cake. Finally, XPS analysis of the extreme surface of cocoa powders confirmed that two conditions should be met to trigger significant cocoa powder caking: high fat content and elevated storage temperature.