Effect of vacuum freeze-drying on the antioxidant properties of eggplants (Solanum melongena L.)

Abstract

The Vacuum Freeze Drying (VFD) process is a low temperature drying technique that can be used for food preservation. The aim of this work is to evaluate how VFD operating conditions affect the amount of some nutritional compounds of eggplants (Solanum melongena L.). The product samples were freeze-dried under different pressure and temperature conditions and had their ascorbic acid (AA) concentration, total polyphenol content (TPC) and antioxidant capacity (AC) measured before and after processing. Temperature was found to have a significative effect on primary drying times: considering results obtained at 40 Pa, the duration moved from 8.6 ± 2.5 h when the process was carried out at 0 °C, to 20.9 ± 7.8 h at −30 °C. Chamber pressure had a less significant effect, being drying time, at 0 °C, ranging from 6.2 ± 2.3 h, for the test at 20 Pa, to 8.6 ± 2.5 h, for test done at 40 Pa. Very low temperatures resulted in higher AC retentions, being the AC loss 68.6 ± 1.5% at 0 °C and 49.9 ± 3.2% at −30 °C (at 10 Pa), while having a negative impact on the AA content, being the AA loss 12.2 ± 1.7% at 0 °C and 37.9 ± 3.4% at −30 °C (at 10 Pa). Higher pressures were favorable for TPC retention in the dried product: being the TPC loss 47.7 ± 5.5% at 10 Pa and 32.5 ± 8.5% at 40 Pa (at 0 °C). A similar trend can be observed for AC, where the AC loss was 68.8 ± 1.5% at 10 Pa and 35.7 ± 12.1% at 40 Pa (at 0 °C). Freezing stage had a non-significant effect on the targeted compounds.

Keywords:

• Vacuum freeze-drying
• eggplant
• ascorbic acid
• polyphenol
• antioxidant compounds

Acknowledgments

Valuable suggestions of Dr. Chiara Mollea (Politecnico di Torino) for the spectrophotometric analysis are gratefully acknowledged.