The results for global extraction yield (Yg), total phenolic compounds (TPC), and antioxidant activity (AA) are shown in Table 1.
Different letters in the columns represent significant differences in Tukey test with a significance level of 95%. Lowercase letters consider all the treatments and uppercase compare samples extracted by the conventional method (Sox).
3.3.2 Total phenolic content (TPC)
Extracts of fresh ginger showed similar TPC between themselves and OD samples at 60 oC (Table 1). Comparatively, dried ginger extracts were richer in TPC but had the lowest Yg, indicating a higher extraction selectivity. VMD resulted in slightly higher TPC concentrations between the drying methods coupled with conventional extraction (Sox) but without significant difference compared to OD samples. Opposite, Ghafoor et al. [28] and An et al. [13] observed reductions in TPC of microwave-dried ginger of 20.3 and 13.2%, respectively, compared to OD at 60 oC. These results could be due to the vacuum absence and a consequent increase in the temperature above 120 oC. Also, inadequate sample homogenization could have generated hotspots that favored the degradation of phenolics.
UAE at 20 or 80 oC promoted different TPC concentrations between themselves for fixed drying methods (F80, F120, or Mic). There is a trend that UAE at 20 oC (Ult20) promotes an increase of TPC concentration, which was more pronounced at 80 oC oven drying (F80Ult20), the opposite behavior of Yg. That supports the selectivity of Ult20. VMD associated with UAE at 20 oC (MicUlt20) produced the extract with the highest TPC, significantly higher than the treatments F60Sox, F80Sox, and F80Ult80. After water removal and cellular disruption by MAE drying, UAE at 20 oC extracted phenolics, avoiding unwanted molecules such as starch and fibers and their degradation products. Furthermore, ultrasonic cavitation destroys cellular structures of ginger releasing phenolic compounds. Also, the lower temperature of this treatment avoids the thermal degradation of phenolic compounds [29].
3.3.4 Components profile
The compositions of the ginger extracts are shown in Table S3 (Online resource 2). In fresh ginger (FreSox), 22 compounds were identified. The major constituents were the sesquiterpenes, followed by alkenes, phenolic compounds, a monoterpene, an aldehyde, and a fatty acid. In FreMic, 17 compounds were identified, and the contents of major sesquiterpenes (α-Zingiberene, α-Farnesene, β-Sesquiphellandrene, and α-Curcumene) were not significantly different from FreSox. Higher differences in the total alkenes (-135.1%) and phenolics (+ 78.3%) were observed. Phenolics in FreMic were not significantly different from FreSox. FreMic’s temperature (150 oC) is enough to produce 6-shogaol from 6-gingerol, but the production was not significant, probably due to the short exposure time. Jacotet-Navarro et al. [19] verified increases of 5.20 and 450% in the concentration of 6-gingerol and 6-shogaol, respectively, by microwave treatment of ginger press cake, but no statistical analysis was carried out.
OD at any temperature did not affect the main volatile compounds compared to the fresh ginger (FreSox). F60Sox, F80Sox, and F120Sox showed 14, 16, and 18 sesquiterpenes, respectively, and only 13 compounds were identified in the fresh sample (FreSox). The content of sesquiterpenes in F60Sox (77.1%) was higher than FreSox (71.2%). The monoterpenes eucalyptol, α-terpineol, (R)-(+)-β-citronellol, and β-citral were identified in OD samples. Those were absent in fresh-extracted samples. The water removal during drying may favor the liberation of low-polarity compounds to extraction, such as mono- and sesquiterpenes. Besides, higher drying temperatures may convert sesquiterpenes to monoterpenes [13]. However, while the monoterpenes in dried samples could have been produced from sesquiterpenes, there was no significant proportional reduction in sesquiterpenes.
MicSox had a lower content of α-zingiberene and β-sesquiphellandrene (lower at 26.72 and 26.15%, respectively) than F60Sox. That may be caused by thermal degradation by the microwaves. Besides, as VMD samples were submitted to low pressure, these could have a higher loss of sesquiterpenes by volatilization compared to less volatile components, such as phenolics. This hypothesis is supported by the sum of phenolic compounds identified in MicSox (18.75%), which was higher in 35.3, 2.6, and 24.0% than the samples F60Sox, F80Sox, and F120Sox, respectively. Simultaneously, the sum of identified sesquiterpenes of MicSox (60.80%) was lower by 20.7, 13.2, and 14.1% than in the same respective treatments. MicUlt20 and MicUlt80 had contents of α-curcumene lower by 43.21 and 43.79%, respectively than F60Sox. That indicates that the combined effect of microwaves and ultrasound can reduce the content of this compound due to volatilization and degradation. Losses caused by vacuum drying were reported by Osae et al. [30], but the association of this technique to MD reduced the processing time and consequently lowered the losses by volatilization to acceptable levels.
The highest contents of monoterpenes were shown by F80Ult20 and MicUlt20. That suggests that OD at 80 oC or VMD reduced monoterpenes loss, and their association with UAE at 20 oC promoted a higher extraction due to ultrasonic cavitation while avoiding thermal degradation and volatilization at the extraction stage. OD at 120 oC associated with hot extraction (F120Sox, F120Ult80) led to a lower content of monoterpenes, possibly caused by thermal degradation and volatilization during drying.
The contents of phenolics in dried ginger compounds comprised 13.86–22.35% of the extracts. The molecules were: 6-shogaol, 6-paradol, zingerone, and its stereoisomer butan-2-one, 4-(3-hydroxy-2-methoxyphenyl)-. These gingerol-derived molecules are all bioactive compounds with several benefits for human health and food preservation and safety [4, 8]. The highest content of 6-gingerol was detected in FreMic, while in FreSox it was about half that value. The high temperature (150 oC) and pressure (11.8 bar) reached by FreMic may have destroyed the cellular components to which 6-gingerol is attached, enabling its extraction. And, temperatures higher than 80 oC can trigger a higher conversion of 6-gingerol to 6-shogaol. OD at 60 oC possibly did not damage cell structures enough to enable a higher liberation of this compound from cell structures for extraction. Those results are consistent with the contents of 6-shogaol observed. The highest contents of 6-shogaol were shown by F120Ult80 (3.95%), MicSox (3.49%), F120Ult20 (2.72%), F80Ult80 (2.52%), F120Sox (2.01%), and MicUlt20 (1.77%). So, hot drying was crucial to increase shogaol production. That was expected, since the formation of 6-shogaol is triggered from 80 oC, and those processes subjected the samples to high temperatures during both drying and extraction. However, despite shogaol being more antioxidant than shogaol [4, 9], the mildest conditions of Ult20 extractions were adequate to avoid thermal and volatilization losses of other antioxidant compounds such as sesquiterpenes, monoterpenes, and phenolics, making MicUlt20 the most antioxidant extract.
Minor compounds found in the ginger extracts were aldehydes, ketones, alkenes, and one pyrone. The aldehydes and ketones are degradation products of phenolic compounds produced by higher processing temperatures [31]. 4H-pyran-4-one,2,3-dihydro-3,5-dihydroxy-6-methyl- is a bioactive pyrone decurrent from Maillard’s reaction of hexoses [32]. This compound was detected at F120Sox, F120Ult80, and MicSox. The higher temperatures of those processes induced Maillard’s reaction, generating this compound.