**2. Chemical composition of garlic**

Garlic is considered one of the 20 most important vegetables with various uses around the world, either as a raw vegetable for culinary purposes or as part of traditional and modern medicine [14].

Its health-promoting properties are attributed to its chemical composition. The chemical composition of garlic is significantly influenced by the variety, growing conditions, and growing practices. In particular, the fertilization regime and soil properties can have a significant impact on quality properties, such as mineral composition, dry matter, protein content, and total soluble solids content [15]. Due to the fact that garlic is commonly consumed cooked and as a dried spice, several authors have also investigated and found that heating can affect the chemical composition of garlic, including its antioxidant properties. The health benefits of garlic depend on the content of biologically active compounds that differ between varieties and geographical areas [16].

Fresh garlic contains approximately 63% water, 28% carbohydrates (fructans), 2.3% organosulfur compounds, 2% protein (allinase), 1.2% free amino acids (arginine), and 1.5% fiber [1, 17]. Other authors [18] reported approximately 65% water, 28% carbohydrates, 2.3% organosulfur compounds, 2% protein, 1.2% free amino acids, and 1.5% fiber in garlic.

Garlic contains 17 amino acids, which include lysine, histidine, arginine, threonine, aspartic acid, glutamine, proline, serine, glycine, alanine, cysteine, valine, methionine, isoleucine, leucine, tryptophan, and phenylalanine. A very effective component of garlic is polysaccharides, which are becoming more and more attractive due to their various pharmacological effects. They exhibit various biological activities, including antioxidant and hepatoprotective effects. Garlic polysaccharides consist of monosaccharides, such as fructose, glucose, and sucrose [19]. These polysaccharides have a high molecular weight, form one of the active components of garlic, and are characterized by low toxicity. In garlic, they represent approximately 26−30% of the fresh weight [20].

Study [21] reported that the main carbohydrate in kitchen garlic is sucrose, which ranged from 1.99 to 3.29 g.100 g-1 fresh mass and represented up to 97% of the total carbohydrate content in garlic. Fructose was detected in relatively low amounts, and glucose was not recorded in the monitored garlic varieties. Dried garlic heads contain mainly water (62−68%) and carbohydrates (26−30%), while proteins are detected in a relatively smaller amount (1.5−2.1%). The energy content is approximately 140 kcal.100 g-1 of fresh mass.

Garlic contains a high content of vitamins, especially vitamin C and group B vitamins. Vitamin C has beneficial effects on the immune system. Ensuring sufficient intake of vitamin C through the diet is essential for the body's resistance to infections [22].

Garlic contains relatively large amounts of vital minerals and trace elements, such as calcium, phosphorus, potassium, sodium, magnesium, aluminum, iron, copper, manganese, chromium, molybdenum, selenium, germanium, and iodine. The most represented minerals in garlic are potassium, phosphorus, and calcium [23]. Other authors [24–26] state the rich content of sodium, magnesium, potassium as well as vitamin C and group B in garlic. Potassium is one of the main blood minerals called

electrolyte, it is important for cellular function; a high concentration of potassium helps to prevent hypertension [27].

The health effects of garlic depend on the level of some mineral elements, such as Se, K, Zn, and Ca [28]. Lee et al. [29] reported that potassium, phosphorus, magnesium, and calcium are the main constituents of garlic. Sulfur, selenium, and germanium are also found in kitchen garlic [30].

Turan et al. [31] reported that consumption of 100 g of garlic represents 3−5% of the recommended daily intake of Zn and Mn for adults. Mineral elements prevent various diseases, such as osteoporosis, cardiovascular disease, and aging.

The vitamins and minerals found in kitchen garlic are essential for proper functioning and health, so they must be obtained through an adequate balanced diet. They are considered micronutrients and act mainly as cofactors [32]. Garlic is an important source of nicotinic acid, thiamin, and riboflavin. It also contains citral, linalool, geraniol, α-phellandrene, valeraldehyde, and propionaldehyde [33]. A wide range of other organic compounds in garlic, such as steroid glycosides, essential oils, lectins, prostaglandins, adenosine, glycolipids, and phospholipids are reported [34].

Garlic is a source of other nonvolatile phytonutrients with significant medicinal and therapeutic properties, of which special emphasis is placed on flavonoids, saponins and sapogenins, phenolic compounds, nitrogen oxides and amides, and proteins [35].

#### **2.1 Sulfur compounds in garlic**

The specific smell and taste of garlic caused by sulfur-containing phytochemicals [36, 37]. The most characteristic volatile and odorous organosulfur compounds of garlic are released after disruption of the cell membrane. Garlic contains bioactive compounds, such as organic sulfur compounds, phenols, and saponins, which exhibit various biological activities, namely antioxidant, immunomodulatory, antiinflammatory, anticancer, hepatoprotective, cardiovascular protective, antidiabetic, antiobesity, renal, neuroprotective, antibacterial, and antifungal activities [38–40]. The synergistic interactions between the individual components present contribute to these remarkable health effects of garlic [41].

Garlic contains at least 33 sulfur compounds. Organosulfur compounds are effective scavengers of free radicals. A unique group of plants of the genus Allium are alk(en)yl-cysteine sulfoxides, which are responsible for their typical smell and taste. These sulfoxides include S-methyl-L-cysteine sulfoxide (methine), S-allyl-L-cysteine sulfoxide (alliine), S-propyl-L-cysteine sulfoxide (propiine), S-ethyl-L-cysteine sulfoxide (ethiine), and S-butyl-L-cysteine sulfoxide (butyin) [42, 43]. Untreated fresh garlic also contains high amounts of glutamylcysteine. These compounds can be hydrolyzed and oxidized to form alliin. In fresh garlic, alliin is present in the amount of 10 mg/g, while in dried garlic its level is 30 mg/g [44].

After processing, such as cutting, crushing, chewing, or dehydration, allinase rapidly cleaves cytotoxic cysteine sulfoxides (alliin) to form cytotoxic and aromatic alkylalkane thiosulfinates, such as allicin, which is the main biologically active component of garlic extract. It represents about 70% of all thiosulfinates formed in crushed garlic [45, 46]. According to Ref. [47] uncrushed garlic contains approximately 16 organosulfur compounds, while crushed garlic contains 23 organosulfur compounds. Allicin contributes to the characteristic flavor and aroma of garlic [16]. Allicin is a thio-ester of sulfenic acid and its beneficial effect on health is attributed to its antioxidant activity as well as its interaction with thiol-containing proteins (**Figure 1**) [48].

*Garlic (*Allium sativum *L.): Characterization of Bioactive Compounds and Related Health… DOI: http://dx.doi.org/10.5772/intechopen.108844*

**Figure 1.**

*Enzymatic reaction of sulfur-substituted cysteine sulfoxides. Source: [48, 49].*

Garlic also contains S-propylcysteine-sulfoxide (PCSO) and S-methylcysteinesulfoxide (MCSO). Other sulfur compounds found in garlic include diallyl sulfide, diallyl disulfide, diallyl trisulfide, S-allyl cysteine allyl mercaptan, allyl methyl disulfide, allyl methyl trisulfide and Ref. [6].

These active compounds are mainly responsible for protecting tissue from damage and various disorders [1]. Some phenolic compounds and thiosulfinates are related to the defense function of garlic, and these compounds serve as nonenzymatic antioxidants and antifungal metabolites [50]. These organosulfur compounds are very sensitive and chemically unstable; they are prone to degradation and easily undergo oxidation when exposed to light, oxygen, and high temperatures [40]. The different content of sulfur compounds in garlic varieties is determined genetically [51].

#### **2.2 Polyphenol content and antioxidant activity of garlic**

Polyphenolic compounds are a group of biologically active compounds in plant foods. Polyphenolic compounds have been identified as nutrients, plant secondary metabolites, phytonutrients, antioxidants, dietary bioactive substances, and protective factors [52].

Secondary metabolites, such as phenolics, including flavonoids and anthocyanins, have been shown to be significantly involved in plant responses to various abiotic factors, including temperature [53]. In addition to temperature, the influence of light is also considered to be one of the main influences that affect the biosynthesis of phenols in plants during growth and development. The consumption of polyphenolic compounds may play an important role in health prevention through the regulation


#### **Table 1.**

*Content of phenolic acids in garlic [55, 56].*

of metabolism, weight, chronic diseases, and cell proliferation. Kitchen garlic (*Allium sativum* L.) is one of the important vegetables, which is characterized by a significantly high content of polyphenolic compounds that has a positive effect on the human body. The largest share of polyphenolic compounds in garlic is represented by phenolic acids and flavonoids. Garlic is ranked second in total polyphenol content out of 23 commonly consumed vegetables [54]. Phenolic acids are among the most important antioxidants that protect the human organism against the negative effects of free radicals (**Table 1**) [55, 56].

Among the phenolic acids in garlic, caffeic acid, o-, m-, p-coumaric acid and chlorogenic acid, were identified. Of the derivatives of hydroxybenzoic acids, mainly gallic acid, protocatechuic acid, β-resorcylic acid, vanillic acid, and syringic acid. Of the other phenolic substances, mainly pyrogallol, rutin, hesperidin, resveratrol, and naringenin were identified in garlic [34]. Garlic flavonoids, such as nobiletin, tangeretin, and rutin, also significantly contribute to the pharmacological activity of garlic (**Table 2**) [57].

Beato et al. [54] mentions the presence of caffeic acid, ferulic acid, vanillic acid, p-hydroxybenzoic acid, and p-coumaric acid in garlic. Colorful varieties of garlic are rich in anthocyanins. The antioxidant properties of garlic are widely proven. Some polyphenols and sulfur-containing compounds are antioxidants and participate in protecting cells from oxidative damage. Garlic oil has been found to contain more than 30 organic sulfur-containing compounds, including diallyl trisulfide, diallyl disulfide, and diallyl sulfide with excellent antioxidant properties [58, 59]. Anwar et al. [60] attributes the antioxidant activity of garlic to organosulfur compounds that increase glutathione levels and glutathione-S-transferase activity. Garlic oil contains compounds based on sulfhydryl groups (–SH) that directly trap ROS and modulate the redox state of cells.


#### **Table 2.** *Content of flavonoids in garlic [56, 57].*

*Garlic (*Allium sativum *L.): Characterization of Bioactive Compounds and Related Health… DOI: http://dx.doi.org/10.5772/intechopen.108844*

Bozin et al. [61] describe a strong relationship between the content of polyphenolic compounds and the value of antioxidant activity in cooking garlic. The collective of authors [62] analyzed the correlation between the content of total polyphenols and antioxidant activity in 43 varieties of garlic and showed significant positive correlations. Consumption of garlic has antioxidant and antigenotoxic properties, which demonstrates a chemopreventive role against the harmful effects of hydrogen peroxide [63].

## **3. Health benefits of garlic**

The phytochemicals found in garlic exhibit several biological effects, play a key role in maintaining health and have the potential to reduce the risk of civilization diseases. The pharmacological effects of garlic on human health are in a certain correlation with the content of bioactive compounds present in garlic. Garlic preparations, such as oils, powders, and pills, are already widely used in several cardiovascular diseases, as they effectively reduce blood lipid profiles and blood pressure [64, 65]. Antibacterial properties were first attributed to garlic by L. Pasteur. Hippocrates already observed that garlic inhibited cancer cell lines and also prevented intestinal diseases [66].

Garlic extracts are characterized by antioxidant, antidiabetic, antimicrobial, antiasthmatic, and neuroprotective effects [36, 67]. Shang et al. [68] describes the anticancer, hepatoprotective, antidiabetic, anti-obesity effects of garlic. It is known that secondary metabolites as well as organic sulfur compounds, mainly allicin and thiosulfinates, are responsible for most of the biological activities of garlic, such as antibiotic, antiviral, and antifungal properties [69, 70].

Consumption of garlic helps in the metabolism of fats, reduces the level of cholesterol and triglycerides in the blood. *In vitro* and animal studies have shown that garlic can suppress many types of cancer, such as blood, bladder, stomach, breast, oral cavity, lung cancer, colorectal cancer, skin cancer, and uterine cancer [71–73]. Epidemiological studies have shown that garlic has a role in the prevention of cardiovascular diseases due to the presence of these bioactive components [74]. Garlic consumption may protect the brain from loss of intellectual capacity and may reduce the risk of Alzheimer's disease [75].

These properties of garlic are primarily due to the beneficial content of vitamins, phenolic compounds, anthocyanins, flavonoids, and tannins in cooking garlic. Garlic has been shown to have potential anticancer properties, which is due to the content of diverse sulfur-containing compounds that have an inhibitory effect on the carcinogenic process [76]. Choi and Park [77] reported that garlic oil has an anticancer role by inducing apoptosis and inhibiting differentiation and tumor angiogenesis.

Garlic polysaccharides are characterized by antiviral as well as anti-inflammatory effects [78]. Allylsulfides have shown a hepatoprotective effect and improve liver damage. Research has confirmed that diallyl disulfide increases the activity of antioxidant enzymes and reduces lipid peroxidation [79]. Allicin has antimicrobial, anti-inflammatory, antithrombotic, and anticancer properties [46].

Diallyl sulfide suppresses the enzymatic activities of cytochrome P450-2E1, reducing the formation of reactive oxygen and nitrogen species [80]. Studies indicate that diallyl trisulfide (DATS) is responsible for the anticancer effect in garlic consumers, especially organic sulfur compounds, such as peptides of glutamylcysteine, alliin, and their degradation products, which arise when cell membranes are broken, can interact with cytochromes and P-glycoprotein, and can effectively reduce cholesterol levels [81].

Garlic extracts are also effective against *Helicobacter pylori* [82]. Consumption of garlic has been reported to have some beneficial effects in preventing changes in the lipid profile induced by the heavy metals, nickel, and chromium [83].

Ajoene present in garlic has many health benefits. It works as an antioxidant. Ajoene also has antithrombotic properties that help prevent blood clots, which reduces the risk of heart disease and stroke in humans. Ajoene exhibits potential virucidal properties against a range of viruses, including vesicular stomatitis, human rhinovirus, and herpes. Ajoene has a wide spectrum of antimicrobial (antibacterial and antifungal) properties. Ajoene reduces the tumor size of basal carcinoma cells by inducing apoptosis, it is effective in inhibiting the growth of tumor cells [84]. Regular consumption of garlic may have a chemo-preventive role against various human cancers including colon, liver, breast, stomach, and pancreatic cancer. Garlic polyphenols lower blood glucose levels in a variety of ways, such as preventing β-cell damage, reducing insulin resistance, increasing insulin sensitivity and secretion, and suppressing the activity of glucosidase enzymes. Bioactive substances found in garlic suppress lipid peroxidation, nitric oxide synthetase activity, alleviate cardiovascular complications, allergic reactions, and delay aging [66]. Garlic oil inhibits proliferation and induces apoptosis in a pancreatic cancer cell line [85].
