Abstract
Gingerols and shogaols are compounds found in ginger (Zingiber officinale Roscoe); shogaols are found in lower concentration than gingerols but exhibit higher biological activities. This work studied the effects of different drying methods including open sun drying (OSD) solar tunnel drying (STD) and hot air drying (HAD) with various temperature on the formation of six main active compounds in ginger rhizomes, namely 6-, 8-, and 10-gingerols and 6-, 8-, and 10-shogaols, as well as essential oil content. Antioxidant and antimicrobial activity of dried ginger was also evaluated. High performance liquid chromatography (HPLC) analysis showed that after HAD with variable temperature (120, 150 and 180 °C), contents of 6-, 8-, and 10-gingerols decreased, while contents of 6-, 8-, and 10-shogaol increased. High formation of 6-, 8-, and 10-shogaol contents were observed in HAD (at 150 °C for 6 h) followed by STD and OSD, respectively. OSD exhibited high content of essential oil followed by STD and HAD method. Ginger-treated with HAD exhibited the highest DPPH (IC50 of 57.8 mg/g DW) and FRAP (493.8 µM of Fe(II)/g DM) activity, compared to STD and OSD method. HAD ginger exhibited potent antimicrobial activity with lower minimum inhibition concentration (MIC) value against bacteria strains followed by STD and OSD, respectively. Ginger extracts showed more potent antimicrobial activity against Gram positive bacteria than Gram negative bacteria strains. Result of this study confirmed that conversion of gingerols to shogaols was significantly affected by different drying temperature and time. HAD at 150 °C for 6 h, provides a method for enhancing shogaols content in ginger rhizomes with improving antioxidant and antimicrobial activities.
Highlights
Ginger (Zingiber officinale Roscoe) is a well-known spice belonging to the family Zingiberaceae, and is cultivated across the world but especially in Asian countries [1]
The contents of gingerols and shogaols in different drying method and temperature are shown in method with increasing of temperature rate from 120 ◦ C to 150 ◦ C, concentration of 6- and 8-gingerols increased significantly, but interestingly, 10-gingerol, which was identified at 120 ◦ C, was not detected in 150 ◦ C and 180 ◦ C
Resulted in decreasing of all gingerols and shogaols content. These results confirm the impact and advantage of temperature in the conversion of gingerols to shogaols, which is consistent with the results of Huang et al [17] and Cheng et al [18]
Summary
Ginger (Zingiber officinale Roscoe) is a well-known spice belonging to the family Zingiberaceae, and is cultivated across the world but especially in Asian countries [1]. Several phytochemicals are found in ginger, including 4-, 6-, 8-, 10-, and 12-gingerols and 6-, 8-, and 10-shogaols, flavonoids, and phenolics [2]. Previous studies have revealed that shogaols are the dehydration products of gingerols and exhibit higher biological activities including anticancer and antioxidant activities [3,4]. Gingerols are the principal constituents of fresh ginger roots, whereas shogaols are found in low amounts in dried and thermally treated roots [5,6]. Previous study by current authors on other ginger variety 6- and 8-shogaol content and antioxidant activity compared to shade (average temperature of 15 ◦ C). Freeze (−60 ◦ C) drying method [7]. Different methods and techniques such as microwave drying, Molecules 2018, 23, 1646; doi:10.3390/molecules23071646 www.mdpi.com/journal/molecules
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