Effects of vacuum impregnation and drying methods on physiccochemical, antioxidation and sensory properties of fig Ficus carica L.

Abstract

This research aimed to study the rapid osmotic dehydration (OD) and drying process which is time-saving and effective in maintaining the quality attributes especially antioxidant and sensory properties of the prepared figs. Firstly, effects of vacuum impregnation (VI) (6.8 or 13.5 kPa) for 10 min, followed by atmospheric restoration until 4 h, in comparison with conventional OD for 4 h were studied. All the prepared samples had no significantly different (p > 0.05) color values and physical texture values and it was evident that the vacuum impregnated samples had the reduced values of total phenolics (TP), total monomeric anthocyanins (TMA) and ferric reducing ability of plasma (FRAP) by 10-20, 30-40 and 8-45%, respectively, when compared with those values of fresh figs. However, the OD helped to decrease water activity (aw) and moisture content of the samples. The vacuum pressure (VP) of 13.5 kPa resulted in the significantly (p ≤ 0.05) highest water loss (WL), weight reduction (WR), TP content, TMA content and FRAP value, thus, the sample pretreated at this VP was used further for a study of influences of drying temperature (55, 60, 70 and 90°C) and fruit shape (whole, pressed and half). It was found that an increasing temperature yielded higher drying rate and the half fruits had higher drying rate in comparison with those values of the pressed and the whole fruits, respectively. In addition, the half fruit also had the highest TP and FRAP values. Thus, it was selected to further investigate for the effects of microwave-vacuum drying (MWVD) at 960 W, VP of 8 kPa for 20 min, in compassion with multistage drying at 90°C for 90 min, followed by 70 °C for 150 min and 60°C and the hot-air drying at 60°C until the sample having moisture content less than 30% (w.b.). It was evident that either the MWVD or the multistage drying helped reduce the drying time. The resultant VI and dehydrated samples had not significantly different values of L* and hue angle. The multistage-dried samples had the highest a*, b* and chroma values, whereas the hot-air dried samples had the lowest values. MWVD gave the samples with the highest hardness, cutting work and adhesiveness, whereas hot-air drying gave the lowest hardness value. TP and TMA contents and FRAP value were reduced by drying when compared with fresh or VI samples. However, MWVD was effective enough for maintaining the highest antioxidant activity and yielding the sample with the highest sensory scores including texture and overall preference.