Resumen:
There is little information on the antioxidant composition of green or roasted coffee. This work evaluated the antioxidant capacity (AC) and the content of total phenolic compounds (TPC) in the ethanolic extracts of deep-frozen green coffee (DFGC), green coffee without deep-freezing (GCWDF), and roasted coffee (RC). The AC was evaluated by the ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) and
DPPH (2,2-diphenyl-1-picrylhydrazyl) methods, while the content of phenolic compounds was determined
by the Folin-Ciocalteau method. The average values of the AC (mmol Trolox/g coffee) were 1.35 for DFGC
at a temperature interval between -40ºC and -10 ºC, 0.22 for GCWDF, and 0.15 for RC between100 ºC and 250 ºC by the ABTS method. These values correspond to a % inhibition of this radical of 90.5, 81.5, and 62.3, respectively. By the DPPH method, the AC (mg caffeic acid/g coffee) was 21.31 for DFGC, 20.91 for
GCWDF, and 7.72 for RC. These values correspond to a % inhibition of this radical of 94.4, 94.4, and 87.6, respectively. The TPC content (mg gallic acid/g coffee) was, on average, 158.4 for DFGC, 132.6 for GCWDF, and 99.3 for RC. To know the content of antioxidant compounds considering the rate and speed at which they can be extracted, the effective diffusion coefficient (Deff) in m2/s, and the first-order kinetic constant (k) in s-1 of the TPC extraction process were also determined. The results of Deff were 6.8 10-11, 4.7 10-11, and 5.83 10-11, and those of k were 2.15 10-3, 1.57 10-3, and 1.58 10-3 on average for DFGC, GCWDF and RC, respectively.
Descripción:
The ethanolic extract obtained from DFGC has a higher AC than that obtained from GCWDF and RC. Expressed in mmol Trolox per gram of coffee, the AC of DFGC is six and nine times higher than that of GCWDF and RC, respectively. This gives the DFGC extract a percentage of inhibition of the ABTS radical of 90.5 %, which is 11 and 45 % higher than that obtained from GCWDF and RC extracts. In grams of caffeic acid per gram of coffee, the AC of green coffee is barely 2 % greater than that of GCWDF, but it is almost three times greater than that of RC. In this case, DFGC shows a percentage inhibition of the DPPH radical of 94.4 %, which is practically equal to that of GCWDF and almost 8 % higher than that of RC.
Similarly, the content of TPC in DFGC is, on average, 19 and 59 % higher than that of GCWDF and RC. The polyphenolic compound extraction process satisfactorily fits the first-order kinetic model, and both the kinetic coefficient and the effective diffusion coefficient are related to the moisture content of the coffee sample. The diffusion and extraction speed of the compounds that give the AC to coffee, whether frozen or roasted, is related to the ethanol-water ratio of the
solvent. Because of the effect of the polarity of the solvent, the extraction of phenolic compounds in frozen coffee is favored in coffee samples with higher moisture content. Therefore, the deep-freezing process is highly recommended to preserve the AC of green coffee. Moreover, there remains the challenge of designing new products from deep-frozen green coffee
for the technology of coffee drinks.