Microwave vacuum drying (MVD) was investigated for apple slices enriched with quercetin derivatives
by vacuum impregnation (VI). Additional freeze drying (FD) and air drying (AD) were conducted.
Compared to native apples, the impregnated tissue resulted in higher moisture content, elevation of
weight and significant browning, due to the incorporated VI solution. The total quercetin content and
quercetin glycoside composition were not affected by MVD and FD. The vacuum conditions protect the
polyphenols from oxygen dependent degradation and browning reactions. AD resulted in an average
quercetin glycoside loss of 44% and undesirable changes, particularly discoloration. The degradation is
caused by both non-enzymatic and enzymatic reactions. The pulsed microwave energy intake improved
the drying result in structure and led to a faster drying process of 130 min. The bulk density of MVD
apple chips (0.69 g/ml) ranged between 0.33 g/ml for FD and 0.75 g/ml for AD. The final moisture content
was the lowest after FD (6.8 g/100 g), followed by 9.0 g/100 g after MVD and 12.7 g/100 g after AD. The
shelf life was significantly influenced by storage temperature and time. After 12 month at 20 C, the total
quercetin content decreased by 21%.