3.1. Physicochemical Properties
Color is an important physicochemical property that affects consumers’ perception of food. The pure white/light yellow color of FRG is changed to dark brown and eventually black during its aging process (
Figure 2). Contents of moisture, protein, lipid, carbohydrate, and ash in garlics are determined according to the Association of Official Analytical Chemists (AOAC) method. There were commonalities in analytical methods among studies, and the methods used in each study are illustrated in
Table 1,
Table 2 and
Table 3, except studies not mentioning analytical methods. Data found within studies are statistically significant (
Table 1,
Table 2 and
Table 3). The concentrations of total and reducing sugars were detected by colorimetric method using phenol-sulfuric acid (PSA) and 3,5-dinitrosalicylic acid (DNS), respectively. ABG shows low moisture content and pH and high browning intensity compared to FRG. In addition, the contents of protein, lipid, carbohydrate, ash, total sugar, and reducing sugar are high in ABG (
Table 1). The total and reducing sugar contents from both FRG and ABG show a significant difference throughout the literature. These differences could result from using different cultivars of garlic. Overall, the total and reducing sugar contents are higher in ABG than those in FRG.
Table 2 shows the changes in phytochemical components in ABG compared to FRG. Allicin and SAC contents were determined by high performance liquid chromatography (HPLC). Flavonoid, pyruvate, thiosulfate, and total phenol contents were determined by colorimetric methods. ABG is abundant in flavonoids, pyruvate, and phenols, but less abundant in allicin compared to FRG. The marked variation in concentrations of flavonoids and total phenols throughout the literature results from different bases used for calculation, such as quercetin, rutin, caffeic acid, garlic acid, and tannic acid. Changes in thiosulfate contents in ABG are controversial among researchers. The amount of SAC increased during ABG process is four- to eight-fold higher than that in FRG [
14,
15,
34]. The amount of SAC is affected by the aging period rather than by temperature. The SAC contents in garlic aged at 40 °C and 85 °C for 24 h are 4.31 ± 0.01 and 2.88 ± 0.16 mg/100 g dry weight, respectively. However, the contents markedly increase to 12.47 ± 0.16 and 8.55 ± 0.08 mg/100 g dry weight in garlic aged at 40 °C and 85 °C, respectively for 45 days. Interestingly, the contents of SAC highly increase over time, but the contents decrease as temperature increases [
34]. There are also variations in the detection levels of the SAC contents in ABG based on each method [
15] (see
Table 2). Generally, HPLC represents HPLC-ultraviolet detection (UVD). Some studies used HPLC-fluorescence detection (FLD) method. In a comparative study of the different analytical methods for analysis of SAC in garlic [
15], HPLC-FLD and HPLC-UVD showed 2.2 and 2.3 mg/100 g SAC content in FRG, respectively. However, the SAC contents greatly vary in ABG depending on the method. The contents of SAC in ABG detected by HPLC-FLD and HPLC-UVD methods are 9.8 ± 0.2 mg/100 g and 11.4 ± 0.9 mg/100 g, respectively. Despite variation in the data due to analytical methods, SAC content in ABG is still higher than that in FRG. In addition, DPPH radical scavenging ability is increased by a high temperature and a long aging time [
34]. Choi et al. [
37] report that antioxidant contents in ABG, such as total polyphenol and total flavonoid, and antioxidant activities are high in garlic aged for 21 days compared to 35 days.
The concentrations of free sugars and minerals in garlic are shown in
Table 3. The concentrations of free sugars and minerals increase in ABG. The concentrations were measured by HPLC with appropriate pretreatment. The high concentration of free sugars, in particular fructose, is closely related to the sweetness of ABG [
31]. The rate of change in fructose concentration is very high compared to other free sugars, such as arabinose, galactose, glucose, sucrose, and maltose. ABG shows a trend of increase in concentration of amino acids, such as aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, methionine, isoleucine, leucine, tyrosine, and phenylalanine [
14,
25,
31]. Free amino acids are detected in ABG, but their concentrations have not yet been compared between ABG and FRG.
3.2. Antioxidant Activity, Compounds, and Therapeutic Effects
Antioxidant activity is the strongest property of ABG. The antioxidant and anti-inflammatory activities are generally determined by colorimetric methods with different substrates. Ferrous (Fe
2+)-chelating ability, ferricyanide reducing power, free radical and nitrite scavenging activity, and superoxide dismutase (SOD) activity of ABG were analyzed and compared to those of FRG [
9,
11,
36,
39,
40,
41]. Comparative studies on biological activities between FRG and ABG show that ABG has strong DPPH, ABTS, hydroxy radical scavenging activities, reducing power, and SOD activity compared to FRG [
9,
14,
16,
36,
38,
39]. However, nitrite radical scavenging activity of FRG and ABG is controversial between two published studies [
16,
38]. One of two controversial studies used water extract of garlic for comparison of nitrite radical scavenging activity of FRG and ABG [
38], and the other study used ethanol extract [
16]. Water extract of ABG shows high nitrite radical scavenging activity compared to FRG, but ethanol extract of ABG shows the opposite. The ethanol extract of garlic increases nitrite radical scavenging activity in a dose-dependent manner. However, the scavenging activities vary according to concentration. The nitrite radical scavenging activity is detected low at concentrations below 10 mg/mL of ABG compared to FRG, but high at concentrations above 20 mg/mL. ABG shows low Fe
2+-chelating activity compared to FRG (see
Table 4). The number of studies regarding nitrite radical scavenging and Fe
2+-chelating activities is small. The antioxidant activity of ABG is comparable to
N-acetyl-
l-cysteine (NAC), a representative ROS scavenger. These antioxidant activities result from various phytochemicals in ABG, giving several therapeutic advantages.
Representative antioxidant components in ABG are total phenols and flavonoids [
31,
36,
41]. OSCs, such as SAC, SAMC, DAS, DADS, and DATS, are also important antioxidants in AGE [
22,
32,
42], but there is no report about their presence and concentration in ABG, an aged garlic. Additional studies are needed to identify these components in ABG. ABG contains many kinds of antioxidants, such as tetrahydro-β-carboline derivatives [
43,
44],
N-fructosyl glutamate,
N-fructosyl arginine [
45], allixin, selenium [
5], and
N-alpha-(1-deoxy-
d-fructos-1-yl)-
l-arginine (Fru-Arg) [
46]. Pyruvate is the main antioxidant molecule [
47,
48], which is abundant in ABG [
9,
31,
36]. The concentration of pyruvate in ABG is higher than that in FRG (see
Table 2). Pyruvate reduces H
2O
2-induced ROS levels in RAW264.7 cells [
9].
ABG inhibits H
2O
2-induced ROS generation in RAW264.7 cells [
9,
41] and
tert-butyl hydroperoxide-induced lipid peroxidation in isolated rat hepatocytes [
49]. ABG also shows antioxidant activity in animal models. Ultraviolet B-induced oxidative damage in mice skin is reduced by ABG treatment [
50]. ABG reduces chronic alcohol-induced oxidative liver damage [
51] and hangover symptoms [
52] in rats. Lipid peroxidation is downregulated by ABG in high-fat-diet (HFD) rats [
17,
53,
54] and restraint stressed rats [
55].
3.3. Anti-Inflammatory Activity, Compounds and Therapeutic Effects
Compared to studies regarding antioxidant activity of ABG, a relatively small number of studies have considered the anti-inflammatory activity of ABG. Some compounds showing anti-inflammatory effects in ABG are identified as pyruvate, 2-linoleoylglycerol, and 5-hydroxymethylfurfural [
9,
18,
19]. Pyruvate has anti-inflammatory activity as well as antioxidant activity [
47,
48]. Pyruvate reduces lipopolysaccharide (LPS)-induced nitric oxide (NO), and prostaglandin E
2 (PGE
2) is released [
9]. The 2-linoleoylglycerol isolated from ABG suppresses the levels of NO, PGE
2, and pro-inflammatory cytokines via inhibition of mitogen activated protein kinases signaling pathways in LPS-induced RAW264.7 cells [
19]. The 5-hydroxymethylfurfural reduces TNF-α-induced monocytic cell adhesion to human umbilical vein endothelial cells (HUVECs) through suppression of vascular cell adhesion molecule-1 (VCAM-1) expression, ROS generation, and nuclear factor kappa B (NF-κB) activation [
18].
ABG decreases the production of NO and pro-inflammatory cytokines in LPS-induced RAW264.7 cells and in septicemic mice [
56], TNF-α-induced NF-κB activation in HUVECs [
12], and phorbol 12-myristate-13-acetate-induced production in COX-2 and PGE
2 through inactivation of NF-κB [
57]. Hexane extract of ABG reduces cell proliferation and expression of ICAM-1 and VCAM-1 in TNF-α-activated human endometrial stromal cells [
20].
Lactobacillus rhamnosus fermented ABG inhibits generation of NO, PGE
2, COX-2, and pro-inflammatory cytokines through upregulation of heme oxygenase-1 in RAW264.7 cells [
58]. ABG exerts an anti-inflammatory effect by inhibiting the COX-2 and 5-lipooxygenase activities, pro-inflammatory cytokines, and leukotrienes in LPS-induced RAW264.7 cells [
9]. The anti-inflammatory effect of ABG is lower than that of FRG (see
Table 4). Generally, anti-inflammatory activity is directly proportional to antioxidant activity, but in ABG it is not [
9].
3.4. Other Biological Effects
In addition to the above-mentioned antioxidant and anti-inflammatory effects on cells and animals, ABG has anti-cancer, anti-diabetic and obesity, anti-allergic, hepatoprotective, cardioprotective, neuroprotective, and anti-thrombotic effects. The therapeutic effects of ABG have been demonstrated in many types of cells and animal models.
ABG shows dose-dependent chemopreventive effect in several cancers in vitro and in vivo. ABG inhibits cell proliferation and induces apoptosis in SGC-7901 human gastric cancer cells. In Kunming mice inoculated with murine fore-gastric carcinoma cell lines, ABG inhibits growth of inoculated tumors [
59]. ABG reduces cell motility, invasiveness, and activities of matrix metalloproteinase-2 (MMP-2) and MMP-9 in AGS human gastric cancer cells [
60]. The hexane extract of ABG induces caspase-dependent apoptosis through both intrinsic and extrinsic pathways in U937 leukemic cells [
61]. ABG inhibits HT29 colon cancer cell growth via the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway [
62]. The 70% and 90% ethanol extracts of ABG have cytotoxicity in several human cancer cell lines: AGS, A549 lung cancer, HepG2 liver cancer, and MCF-7 breast cancer cells [
63].
ABG also shows protective effects on diabetes and obese animals. Administration of yeast (
Saccharomyces cerevisiae) fermented ABG attenuated HFD-increased body fat and plasma lipids in diabetic obese mice [
64]. ABG reduces insulin resistance and serum total cholesterol and triglyceride levels, and increases the high density lipoprotein (HDL) cholesterol levels in db/db mice through antioxidant activity [
21]. The SAC enriched black garlic juice gives anti-diabetic effects in streptozotocin-induced insulin deficient mice [
65]. ABG reduces thiobarbituric acid reactive substances (TBARS) in serum, liver, and kidneys through increasing activities of antioxidant enzyme in streptozotocin-induced diabetic rats [
66]. ABG supplementation for 12 weeks reduces blood lipid parameters in patients with mild hypercholesterolemia [
67]. In vitro, ABG inhibits adipocyte differentiation and adipogenesis by suppressing the pro-adipogenic transcription factors in 3T3-L1 preadipocytes [
68].
ABG has anti-allergic activities inhibiting β-hexosaminidase release in RBL-2H3 rat basophilic leukemia cells [
39]. It is also shown to inhibit immunoglobulin E mediated allergic response in RBL-2H3 cells as well as in vivo passive cutaneous anaphylaxis [
69].
ABG has protective effects from liver, heart, and brain damage. ABG shows hepatoprotective effect on carbon tetrachloride- or
d-galactosamine-induced liver damage and HFD-induced hepatic steatosis and subsequent liver injury in rat [
26]. ABG extract enriched with SAC and polyphenols (ABG10+) exerts cardioprotective effects. In rat hearts, ABG10+ induces a relaxing effect on coronary arteries before and after ischemia reperfusion (IR) and prevents the IR-induced decrease in myocardial contractility [
70]. ABG ethanol extract increases spatial memory and the number of Purkinje cells in rats treated with monosodium glutamate [
71,
72], indicating that ABG might have a neuroprotective effect. ABG shows anti-thrombotic effects on thrombin-induced platelet aggregation in rat [
73] and human [
38]. ABG downregulates heat shock proteins 70 and COX-2 expression levels [
74], and reduces TBARS in aerobic exercise rats [
74,
75]. These various therapeutic effects result from antioxidant and/or anti-inflammatory activities of ABG as shown in aerobic exercise rats. Some hepatoprotective and anti-obesity effects are described in
Section 3.2 as anti-oxidative activity.