According to our previous research, the three generally-used species in
Panax genus could be distinguished by the presence of some characteristic ginsenosides such as the notoginsenoside R
1, which is a characteristic marker for
P. notoginseng, while the ginsenoside Rf is a marker for
P. ginseng. Furthermore, the ocotillol-type triterpene 24(R)-pseudo-ginsenoside F
11 presents in high amounts in
P. quinquefolius and in very minute amounts in
P. ginseng, and hence a high ginsenoside Rf/24(R)-pseudo-ginsenoside F
11 ratio (>700) clearly differentiates
P. ginseng and
P. quinquefolius [11]. Likewise, the ratio between Rb
1 and Rg
1 is a very clear marker as the high Rb
1/Rg
1 ratio (around 10 or greater) indicates
P. quinquefolius, while low (1–3) indicates
P. ginseng [12]. Additionally, the ginseng plant contains some other important secondary metabolites such as ginseng oils, phytosterol, carbohydrates, amino acids, peptides, vitamins, minerals, certain enzymes and phenolic compounds (caffeic acid, syringic acid, p-coumaric acid, ferulic acid and cinnamic acid)
[13].
Due to black ginseng having originated and being distributed in Asia, the term black ginseng in this article refers to black P. ginsing, unless otherwise stated.
The preparation of balck ginseng depends on the steaming and drying for several times (usually 9). The steaming process casues the transformation of several secondary metabolites in to other forms through different chemical reactions. The polar ginsenosides transform into specific less polar ginsenosides by hydrolysis, dehydration, decarboxylation and isomerization reactions. The notable structural changes are the hydrolysis of sugar moieties at C-3, C-6 or C-20 and subsequent dehydration at C-20. These reactions took place to the major primary ginsenosides (Rb
1, Rb
2, Rc, Rd, Re and Rg
1) as well as to other minor ginsenosides
[14]. As shown in Figure 2, the ginsenosides Ra
1, Ra
2, Ra
3, Rb
1, Rb
2, Rb
3, Rc and Rd are converted to the ginsenosides Rg
3, F
2, compound K and Rh
2 in black ginseng through hydrolysis reactions for sugar moieties at C-3 and C-20. On the other side, the ginsenosides Rk
1 and Rg
5 are produced as a result of the dehydration reactions of Rg
3 [15] . The R-epimers of Rg
2, Rg
3 and Rh
1 were produced through isomerization of the corresponding ginsenosides and through addition reactions (selective attack of the hydroxyl group) for the dehydrated corresponding ginsenosides (Rk
1 and Rg
5 in case of 20(R)-Rg
3 )
[16].
Rh
1 and Rh
4 were deduced to be generated from the ginsenoside Rg
1 through hydrolysis and dehydration reactions, respectively. Interestingly, four ginsenoside (Rg
6, F
4, Rk
3, and Rh
4) were produced from the ginsenoside Rg
2 through hydrolysis and dehydration reactions
[17]. The acetylated ginsenosides (20(S)-Rs
3 and 20(R)-Rs
3) were produced from the malonyl derivatives of Rb
1, Rb
2, Rc and Rd the by hydrolysis of glycosyl moiety at C-20 and decarboxylation of malonyl moiety attached to glycosyl linkage at C-3, 20(S)-Rs
3 further underwent dehydration to generate Rs
4 and Rs
5 . Generally, The steaming process causes an increase in the protopanaxadiol group to protopanaxatriol group ratio (PD/PT) from 1.9 to 8.4 in white and black ginseng, respectively
[15] in addition, it also led to the production of some specific ginsenosides such as (20(S)-, 20(R)-Rg
3, Rk
3, Rh
4, Rk
1, Rg
5, …etc.) which are absent from white ginseng . As black ginseng was subjected to much more steaming, the concentration of these ginsenosides is much higher than red ginseng. Similarly, the steaming and drying processes led to some chemical changes in the other secondary metabolites. Although steaming causes a decrease in the contents of polysaccharides from 29.1% in fresh ginseng to be only 11.1% in black ginseng
[18], it causes an increase in reducing sugars and acidic polysaccharides contents. The increase of reducing sugars and acidic polysaccharides contents in black ginseng with percentages of 128 % and 187.5 %, respectively, comparing the white ginseng was reported
[16]. Similarly, the phenolic compounds content was increased more than three folds by steaming from 3.1 mg/g in white ginseng to be 10.6 mg/g in black ginseng
[16]. In another comparative study, the phenolic contents of white ginseng and black ginseng roots of P
anax ginseng, P. notoginseng and
P. quinquefolium were evaluated to be 20.4±0.90 mg/g, 17.12±0.56 mg/g, 14.45±0.13 mg/g in white ginseng roots and to 34.3±0.18, 44.15±1.45, and 34.05±2.03 mg/g in the black ginseng roots, respectively. The identified phenolics included; ferulic, gentisic, cinnamic, syringic, and p-hydroxybenzoic acids combined with arginine and maltose due to Maillard reaction
[19]. Another study reported the increase of salicylic acid, vanillic acid and p-coumaric acid contents from 0.121, 0.404 and 0.522 mg/100g in white ginseng to 0.394, 0.628 and 0.737 mg/100g in black ginseng, respectively, because of the steaming process
[20]. In fact, the black color of black ginseng is a result of a chemical reaction named Maillard reaction, which is achemical reaction between reducing sugars and amino acids resulting in glycosylamines and or ketosamines
[21]. The reported Maillard reaction products in ginseng due to steaming included: Argin-maltose
[20] Arg-fru-glc, Arg-fru, maltol-3-O-β -D-glucoside in addition to maltol which increased from 2.598 mg/100g in white ginseng to 94.007 mg/100g by steaming
[19]. The content of maltol was reported to be much higher in black ginseng than red and white ginseng
[22]. The significant decrease of free amino acids from 17.9 mg/g in white ginseng to 2.79 mg/g after steaming has been reported
[23]. Arginine’s content (the most predominant amino acid in ginseng) was reduced from 10.4 to 1.38 mg/g and b-N-oxalyl-L-a,b-diaminopropionic acid (b-ODAP), a famous neurotoxin, was decreased also by 92.9%. The decrease of amino acid content in black ginseng is believed to be a result of Maillard reaction due to the detection of increased levels of Maillard reaction products
[22]. These results have been authenticated by a recent study used multiple ultra-performance liquid chromatography with mass spectrometry (UPLC-MS) assay methods and proved the decrease of amino acids contents during the steaming process. The concentrations of 29 amino acids were less in red ginseng than white ginseng and were the least in black ginseng
[24]. On the other hand, benzo(a)pyrene hydrocarbon was detected only in the black ginseng with a content of 0.17 μg/kg
[16]. Benzo(a)pyrene is a polycyclic aromatic hydrocarbon produced as a byproduct of the incomplete burning of organic materials. It is a potent carcinogenic and listed as Group 1 carcinogens by the International Agency for Research on Cancer
[24]. According to the Korea Food and Drug Administration (KFDA), the maximum acceptable level for benzo(a)pyrene on any food products is 5.0 μg/kg
[16]. Accordingly, although reporting the presence of benzo(a)pyrene hydrocarbon in black ginseng, it’s still in very minute amounts and much lower than the accepted limit.