Encyclopedia
Please note this is a comparison between Version 1 by Jacek Tabarkiewicz and Version 2 by Rita Xu.
Black garlic (BG) is a fermented form of garlic (
Allium sativum L
.), produced at precisely defined temperatures, humidities, and time periods. Although garlic has been used for thousands of years, black garlic is a relatively new discovery. There are many bioactive compounds in black garlic that give it medicinal properties, including anti-inflammatory and anti-cancer properties.
- black garlic
- anti-inflammatory
- anti-cancer
1. Introduction
Garlic (Allium sativum L.) is a shallow-rooted vegetable plant belonging to the Alliaceae family [1]. Native to western Asia and the Mediterranean coast, this plant is widely distributed around the world [2]. Allium sativum includes two subspecies: A. sativum variety sativum (softneck garlic) and A. sativum variety ophioscorodon (hardneck garlic) [1]. There are differences between both subspecies in terms of structure. The head of hard garlic has a hard neck and six to eleven cloves surrounding a woody stem, while soft garlic has no flower top, contains up to twenty-four cloves, and has a stem that is soft and central [1]. Garlic can grow in temperate and warm climate zones and is perennial [1][3][1,3].
Garlic has been used in traditional medicine around the world since ancient times due to its valuable health-promoting properties [4]. Scientific research results indicate a number of health-promoting properties: hepatoprotective, nephroprotective, immunomodulatory, anti-allergic, antioxidant and anti-cancer properties (Figure 1) [5][6][7][8][9][5,6,7,8,9]. Its unfavorable taste and smell have recently significantly reduced consumption all over the world, with the exception of China and India [10]. Consuming raw garlic in quantities to achieve enormous health benefits for the patient is difficult due to its pungent taste and odor [11].
Figure 1. Selected properties of black garlic.
Garlic consumption is accompanied by an unpleasant odor from the mouth and body. There is evidence that garlic consumed in large doses has toxic effects [8]. Additionally, a significant number of consumers have stopped consuming garlic due to gastrointestinal discomfort, including damage to the stomach and intestinal walls [10][11][10,11]. When large doses of garlic are used, negative side effects are observed: anemia, calcium deficiency and contact allergies [8].
In recent years, there has been an increase in interest in the health-promoting properties of black garlic (BG) as a rich source of several bioactive compounds, mainly those with antioxidant properties [4][12][4,12]. It is obtained from raw garlic subjected to aging processes [13][14][13,14]. Black garlic is produced by fermentation and ripening under strictly defined conditions of a temperature of 60–90 °C and humidity of 60–90% for 10 to 80 days [14]. As it ages under precisely defined processing parameters, e.g., temperature or humidity, the color of garlic changes from white to dark brown/black [15]. The color change is the result of enzymatic browning in the Millard reaction—condensation between the reducing carbonyl group of sugar and the amino group [10][15][16][17][10,15,16,17]. In Figure 2, rwesearchers can visualize the difference between fresh garlic and aged garlic. During the fermentation process, white garlic loses its sharp taste due to the alliin content in favor of a sweet or sweet–sour taste and becomes odorless and has a consistency ranging from rubbery and stringy to gelatinous [4][14][18][4,14,18]. Moreover, new S-allicin compounds resulting from the transformation of allicin have very strong antioxidant properties as a result of the Maillard reaction between reducing sugars and allicin [19]. During garlic aging, the chemical oxidation of phenols and thermal degradation of organic sulfur compounds also occur [4][20][4,20].
Figure 2. Fresh garlic and black garlic heads and cloves comparison: (a) fresh garlic (Allium sativum) head; (b) black garlic (Allium sativum) head; (c) fresh garlic head and gloves; (d) black garlic head and cloves; (e,f) a side-by-side comparison of a clove of fresh garlic (left) and black garlic (right) (photograph by Julia Trojniak).
2. Active Compounds of Black Garlic
The aging process of black garlic causes a change in physicochemical properties associated with an increase in the content of antioxidant compounds [21]. Compared to fresh garlic, black garlic has a significantly different chemical composition. Black garlic is characterized by a high concentration of many antioxidant compounds, including the following: phenols, flavonoids, pyruvate, S-Allyl-Cysteine (SAC), S-allyl-Mercapto-Cysteine (SAMC), and 5-hydroxymethylfurfural (5-HMF). It also contained allicin-derived organosulfur compounds (OSC): diallyl sulfides (DAS), diallyl disulfides (DADS), diallyl trisulfides (DATS), and diallyl tetrasulfide [10][22][10,22]—as shown in Figure 3a–h. Scientific research results have shown that the content of phenols differs significantly between black and fresh garlic [23]. An increase in temperature and a decrease in humidity during aging significantly increases the level of polyphenols, thiosulfonates, and allicin [24].
Figure 3. Chemical structures of several of the most important compounds found in black garlic (BG): (a) S-Allyl-Cysteine (SAC); (b) S-Allyl-Mercapto-Cysteine (SAMC); (c) 5-Hydroxymethylfurfural (5-HMF); (d) pyruvate/pyruvic acid; (e) dialyl sulfides (DAS); (f) dialyl disulfides (DADS); (g) dialyl trisulfides (DATS); (h) dialyl tetrasulfide.
3. Anti-Inflammatory Properties of Black Garlic
It has been shown that black garlic significantly reduces blood sugar levels, lipid peroxidation, and antioxidant defenses by activating downstream nuclear factor erythroid 2-related factor 2 (Nrf2) and Nrf2 targets such as quinone-oxidoreductase-1 (NQO1), heme oxygenase-1 (HO-1), and glutathione S-transferase alpha 2 (GSTA2) [25][49].
There are several compounds in black garlic that could have anti-inflammatory effects, including pyruvate, S-Allyl-Cysteine (SAC), 2-linoleoylglycerol, and 5-hydroxymethylfurfural (5-HMF) [10][26][27][10,48,50].
It was shown that 5-hydroxymethylfurfural suppressed cell adhesion by human umbilical vein endothelial cells (HUVEC) by inhibiting the expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule (ICAM-1), ROS generation, and nuclear factor kappa B activation [28][51]. Kong et al. investigated 5-Hydroxymethylfurfural’s (5-HMF) effects on LPS-stimulated RAW 264.7 macrophage inflammatory responses. It was found that 5-HMF suppressed the phosphorylation of proteins connected to MAPK, NF-κB, and Akt/mTOR signaling pathways. The inhibition of these pathways was mediated through inhibition of pro-inflammatory mediators (NO, PGE2, TNF-α, IL-6 and IL-1β) and reactive oxygen species (ROS) [29][52].
Using HaCaT keratinocytes as a model system, it was found that S-Allyl-Cysteine (SAC) from black garlic (BG) induces an anti-inflammatory response by inhibiting the production of pro-inflammatory cytokines TNF-α and IL-1β. In addition, S-Allyl-Cysteine significantly inhibited TNF-α-induced activation of P38 and JNK MAP kinases and NF-κB [30][53].
A mouse model of contact dermatitis showed that black garlic decreased the activation of macrophages, as well as the release of inflammatory mediators like nitric oxide II (NO), tumor necrosis factor (TNF-α), and interleukin 6 (IL-6) [31][54]. A reduction in inflammatory mediators was achieved by inhibiting iNOS, COX-2, and NF-κB. Additionally, the fraction of black garlic extract (BG10) showed a stronger anti-inflammatory effect against 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced contact dermatitis in RAW264.7 cells compared to crude black garlic extract (ABG) [31][54]. In a study involving RAW 264.7 macrophages that had been stimulated with LPS, Kim et al. investigated the anti-inflammatory properties of BG. In aged black garlic, one compound (AGE-1) inhibited the production of pro-inflammatory mediators (NO, PGE2, IL-1β, IL-6 and TNF-α). In contrast, the second compound (AGE-2) did not show such an effect [26][48].
The anti-inflammatory and hepatoprotective effects of black garlic extracts were demonstrated in a mouse model of acute hepatitis by reducing the levels of alanine aminotransferase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP), and maldialdehyde (MDA). Additionally, black garlic extracts improved the activity of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and glutathione reductase (GSH-Rd), while reducing tumor necrosis factor alpha (TNF-α) and interleukin-1 (IL-1β) levels in mouse liver, indicating a significant anti-inflammatory effect [32][55]. Increasing activity of SOD in hepatocytes during a toxic liver injury stay, in opposition to the in vitro results described above, can suggest that the influence of BG extracts on SOD activity could be dependent on the model reswearchers checked SOD activity in, that is, whether it was a simple chemical reaction or in complex model of whole organs or organisms.
The anti-inflammatory properties of black garlic were also demonstrated in a study by Lee et al. using mice with colistin-induced nephritis. An aged black garlic extract inhibited the expression of the TGF-β1 protein, which induced the NF-κB signaling pathway. CD68+ cells (“mouse macrophages”), infiltrated in kidneys, were also reduced, as were levels of IL-1β and TNF-α [33][56].
Using RT-PCR analysis, Recinella et al. examined the effect of aged black garlic extract on pro-inflammatory and pro-oxidant mediators (COX-2, TNF-α, IL-6, NF-kB) and iNOS mRNA levels on isolated LPS-stimulated heart samples. In this study, ABGE inhibited all the inflammatory and prooxidative mediators mentioned, which that suggests black garlic has anti-inflammatory properties [23].
Moreover, black garlic combined with vitamins D, C, and B12 had greater protective effects by inhibiting more inflammatory and oxidative pathways related to stress in LPS-exposed mice’s hearts than each vitamin alone [23].
The chloroform extract of aged black garlic (CEABG) inhibited TNF-α-stimulated VCAM-1 expression by decreasing reactive oxygen species (ROS) production and inhibiting the activation of the redox-sensitive transcription factor NF-κB in human umbilical vein endothelial cells (HUVEC) [34][57].
4. Anti-Cancer Properties of Black Garlic (BG)
Throughout the world, cancer is one of the leading causes of death. Despite scientific progress, cancer therapy remains a challenge for many types of cancer. Oncological therapies often cause unfavorable side effects for patients, leading to reductions in their quality of life. Often, it also affects the deterioration of health.
In recent years, scientists have been focusing on researching the role of phytochemicals, mainly antioxidants, present in plants to determine new and effective methods for supporting anticancer therapy, especially in terms of limiting side effects. Intensive research is being conducted to find new substances with anti-cancer properties and therapeutic potential.
Carcinogenesis is influenced by internal and environmental factors. The consequences of the action of free radicals, e.g., reactive oxygen species generated in the human body, are one of the main internal etiological factors of cancer [35][36]. Oxidative stress is frequently associated with chronic inflammation, which can be followed by neighboring cell mutation and increased proliferation, often creating an environment that is conducive to the development of cancer. The antioxidative and anti-inflammatory properties of black garlic described above are also indirect anti-cancer mechanisms. In the text below, rwesearchers focused on the direct anti-cancer features of aged garlic.
It has been shown that mature black garlic extract inhibits the proliferation, migration, invasion, and metastasis of ER+ breast cancer cells in the MCF-7 and MDA-MB-361 cell lines [16]. Moreover, it stimulates apoptosis in ER+ breast cancer cells via the inhibition of the expression of anti-apoptotic proteins MCL-1 and BCL-2, while stimulating the expression of pro-apoptotic proteins BIM and BAK [16]. The reduction in MCL-1 expression was mediated by JNK activation caused by an increase in the amount of reactive oxygen species in cancer cells [16].
It has been shown that hexane extract from ripening black garlic induces apoptosis of the human leukemic cells (U937). The process of caspase-dependent apoptosis was initiated by both intrinsic and extrinsic pathways [36][58].
The use of ABGE in the treatment of colon cancer also has potential therapeutic value. In the study by Dong et al., it was observed that ABGE inhibited proliferation and stimulated the apoptosis of HT29 colon cancer cells. The possible mechanism of the anticancer effect is the modulation of the PI3K/Akt signaling pathway, increasing the expression of PTEN and reducing the expression of Akt and p-Akt [37][59].
The effects of mature black garlic on 1,2-dimethylhydrazine (DMH)-induced colon cancer models in rats and the anti-proliferative mechanisms of action were determined. Its inhibitory effect on the proliferative activity in cancerous lesions was observed without affecting the normal colonic mucosa. AGE (aged garlic extract) gradually inhibited the progression of DLD-1 by delaying cell proliferation by reducing the expression level of cyclin B1 and cdk1, which in turn was caused by the weakening of NF-κB activity [38][60].
The anticancer effects of aqueous extract of aged garlic on diethylnitrosamine (DEN)-induced liver cancer in rats were observed. The administration of this extract for 7 weeks resulted in a reduction in liver mass, with a significant reduction in the levels of alanine aminotransferase (ALT), aminotransferase (AST), and total bilirubin (TBIL), and an increase in antioxidant activity (TEAC test). The authors emphasize its extraordinary hepatoprotective and antioxidant effects in rats with DEN-induced liver cancer [39][31].
Next to S-Allyl-Cysteine (SAC), S-Allyl-Mercapto-Cysteine (SAMC) is another compound in black garlic with health-promoting properties. According to Zhang et al.’s study, SAMC induced apoptosis in human colon cancer cell line SW620 in vitro, which may explain garlic’s antiproliferative properties. Also, these results suggested that S-Allyl-Mercapto-Cysteine induces apoptosis through the JNK and p38 pathways, which activate Bax and p53 [40][61].
Hep-G2 cells, prostate cancer (PC-3), MCF-7 breast cancer, and mouse macrophage line (TIB-71) were inhibited by 80%–90% after 72 h by inhibiting cell proliferation and the cell cycle and causing apoptosis [41][42][35,62]. Black garlic showed dose-dependent cytotoxic effects on HL-60 leukemia cells. Unlike fresh garlic, black garlic did not induce pro-apoptotic internucleosomal DNA fragmentation; therefore, cytotoxic activity was induced in a pro-apoptotic manner only in white garlic [43][63]. It is worth noting that black garlic was not found to be genotoxic when tested on fruit flies (Drosophila melanogaster) and is even antigenotoxic [43][63].
Both in vitro and in vivo, black garlic extract also demonstrated anticancer and immunomodulatory properties against SGC-7901 human gastric cancer cells as well as in the mouse model. In a dose-dependent manner, ABGE significantly increased SOD and GSH-Px activity compared to the negative control. By inducing apoptosis in vitro, ABGE inhibited the growth of cancer cells [44][64].
On the other hand, no anti-proliferative and pro-apoptotic effects were demonstrated in the MCF-10A breast cancer cell line [16]. In a study on three lung cancer cell lines (H1975, H520, A549), water and alcohol extracts also showed negligible anti-proliferative properties [45][46]. A study by Kim et al. found that black garlic extract had no cytotoxic effects when applied to RAW264.7 and RBL-2H3 cells [8].
Angiogenesis is an important process in tumor development and the inhibition of new blood vessel formation could be effective in cancer treatment. Arianingrum R et al. performed in vivo experiments using the Chorio-Allantoic Membrane Assay (CAM) for the evaluation of blood vessel formation as well as in silico study via docking analysis of BG compounds on the VEGF receptor [46][65]. The authors showed that ethanol extract, ethyl acetate, and n-hexane fractions of BG inhibit angiogenesis, with n-hexane fraction having the highest efficacy. The molecular docking assay indicated some possible inhibitory interaction between the black garlic bioactive compounds and VEGFR. On the other hand, the induction of angiogenesis was described in vivo in a zebrafish model or male Wistar rat model and in vitro with the use of HUVEC cells [47][48][66,67].
