4. Conclusions
The SFE of green tea leaves was studied at 343K and 30 MPa,
using pure CO2 and different green cosolvents for food processing.
In comparison with ethyl acetate and ethanol cosolvents,
ethyl lactate resulted in superior capacity for caffeine extraction.
The highest yields of caffeine were obtained with ethyl
lactate both in SFE-static mode (the cosolvent soaking the
vegetal material before CO2 pumping) and SFE-dynamic mode
(the mixture of CO2 + cosolvent was continuously pumped into
the extraction cell). The general trend of cosolvent effect was
ethyl lactate > ethanol > ethyl acetate, which corresponds with the
behaviour observed in the pressurized liquid extraction of caffeine
from green coffee beans [32].
The analysis of the overall extraction curves obtained in the
SFE-static approach indicate that extraction velocity in the early
extraction stages is around 7 times higher using CO2 + EL than with
pure supercritical CO2. Consequently, the convective mass transfer
coefficients resulted for the model of Sovová, indicate the highest
value for the CO2 + EL supercritical solvent (kYA = 6.5 × 10−2 s−1)
around 2.6 times higher than for supercritical CO2.
Thus, ethyl lactate is a suitable green alternative to be employed
as cosolventin SFE to remove caffeine from natural matrices, reducing
the extraction time and/or the amount of CO2 employed.