matrix. Furthermore, this is in accordance with the fact that a very
short constant extractionrateperiod was observedinthe extraction
curves (tCER values were 0.96, 0.16 and 0.30 min, respectively, for
EA, EL and ETOH) and thus, mass transport within the solid phase
dominated extraction rate almost from the beginning of the process.
In this respect, the solubility value (Y*) calculated as the slope
of a theoretical linear behavior up to 14 min of extraction should
be considered as an estimated value.
The quality ofthe model regression can be observed in Fig. 1a–d,
in which the falling extraction rate period (FER) is depicted with
dashed lines. A comparison ofthe OECs corresponding to the different
supercriticalfluids shows thatif no cosolvents are employed the
extraction entered the FER period at 54.5 min. Using the cosolvents,
the caffeine extraction rate is considerably higher at the beginning
of the extraction and thus, the FER period started at 23.6 min for
CO2 + EAC, 11.3 min for CO2 + ETOH and 7.5 min for CO2 + EL.
The solid phase mass transfer coefficients (kXA) resulted rather
similar in the fitting of all OECs (see Table 4). In general, kXA values
were two orders of magnitude lower than kYA values, indicating a
strong limitation of caffeine mass transfer in the solid phase. This
limitation become evidentfrom the beginning ofthe extraction and
thus, it is possible that the saturation of the supercritical phase was
not attained. Furthermore, kXA values resulted reasonably similar
for all cosolvents used, representing similar mass transfer in the
solid phase. Regarding fluid phase mass transfer coefficient (kYA),
values were 1.8 and 2.6 times higher for, respectively, CO2 + ETOH
and CO2 + EL, with respect to pure CO2 or CO2 + EA.