increase in temperature was observed in the grape by-product (initial
temperature = 0 C). Moreover, no mass loss in of plant material
was observed. This phase ended with the appearance of the
first drop of water outside the microwave cavity. During the next
phase (phase B), the water inside the plant material was heated
up, then diffused out of plant matrix and moved downward under
the influence of earth gravity. At this point, the temperature
reached a plateau at 100 C and remained constant until the complete
extraction of unbound water. A sharp decline in mass of plant
material was observed during this phase. At last, when there was
only tightly bound water left, the temperature started to further
increase, which led to the burning phase (phase C) and no further
mass loss was observed.
3.3. Identification and quantification of MHGE by HPLC
HPLC analyses allowed the identification of seven different
anthocyanins at 530 nm (Table 2). Whatever the microwave power
used, the most abundant anthocyanin in the grape by-product extract
was malvidin-3-O-glucoside in agreement with the literature
(Abert-Vian et al., 2006). The other anthocyanins identified were
peonidin-3-O-glucoside, petunidin-3-O-glucoside, delphinidin-3-
O-glucoside and malvidin-3-O-(6-O-p-coumaryl) glucoside. Cyanidin-
3-O-glucoside and malvidin-3-O-(6-O-acetyl) glucoside were
also detected in traces in MHGE. The total anthocyanin content
estimated by HPLC was consistent with the value determined by
UV–visible spectroscopy (Luque-Rodríguez et al., 2007). In addition
to anthocyanins, gallic acid, caffeic acid and catechin were the
main phenolic compounds identified and quantified in MHGE at
280 nm (see Table 2).