(178f) Steam Stripping of Aroma from Coffee: Multi Scale Modelling of Extraction Behaviour

Aroma is a major factor in consumer satisfaction in drinking coffee, and more than 1000 compounds have been identified n coffee spanning various chemical groups. During instant coffee manufacture, aroma is extracted from fresh roast and ground coffee, to be added back before the final drying stage, preventing evaporative and thermal losses.

One means of aroma extraction can be steam stripping of a coffee bed, the moisture content of which may range from dry to fully saturated. The product aroma profile is largely determined by this process, therefore, there is a need to understand how key aroma compounds extract upon contact with water and steam at different conditions.

A theoretical model of steam stripping a coffee bed has been constructed, consisting of four stages of mass transfer, at the scale both of the individual particle and the whole bed; oil-to-water aroma release; diffusion through the porous coffee grains; partition into the gas phase; and advection up the packed column. Simulations showed three rate-limiting extraction mechanisms related to oil-water partitioning, diffusivity and water-air partitioning.

Experimental data has provided evidence for these three extraction behaviours but there are compounds limited by other factors not anticipated by the model. These are believed to arise from interactions with the soluble coffee components and the solid coffee matrix, a hypothesis supported by further experiments.

Process variables including the grind (particle) size, moisture level and distribution in a coffee bed have been tested and reveal a range of responses depending on the chemistry of the aroma. Moisture may help mobilize the soluble residues on the surface of the coffee grain which may contain aroma and thus assist its extraction, or it may enable the aforementioned interactions to take place, thus limiting volatization of aroma.

Experimental and modelling results have been combined to reveal a wide range of extraction kinetics of volatile aromas in three phase steam stripping of roast and ground coffee. This knowledge reveals how manipulation of the process conditions - time, moisture, grind size, geometry - can influence select chemical groups, enabling the optimization of the process in tailoring an instant coffee aroma profile through process engineering.