Unraveling phytochemistry and biofunctionality of black garlic: a systematic review

Abstract

Black garlic is a processed product prepared by ageing raw garlic bulbs under high–temperature and high-humidity conditions for several days or weeks. Black garlic has a better sensory appeal, with a sweet–sour taste, a characteristic sulfur-sweet-roasted aroma, and a soft, elastic, jelly like texture. In the last decade, black garlic has acquired a reputation in the functional food market due to its antioxidative, anticancer, anti-inflammatory, cardioprotective, hepatoprotective, and anti-diabetic activities. This review provides an insight into processing techniques, including pretreatments for the production of black garlic, phytochemical changes during the ageing, and their effect on the quality parameters of black garlic, Maillard reaction products, changes in the aroma profile of black garlic, and physiological role of constituents of black garlic in various diseases. This review will be helpful in expanding the industrial applications of black garlic as a functional food.

Introduction

Garlic (Allium sativum L.) is used as a condiment/seasoning, particularly in traditional Indian, Chinese, and Oriental cuisine. However, beyond its role as a spice/condiment, garlic has tremendous therapeutic potential as a functional food. In many traditional medicine systems, including Chinese, Egyptian, Greek, Roman, and Ayurveda of India, garlic has been employed for the treatment of a variety of diseases, such as respiratory ailments, skin diseases, intestinal disorders, antiseptics for wounds, and the prevention of fungal and bacterial infections (Savairam et al., 2023). In many recent studies, the pharmacological values of raw garlic, including its role in managing blood pressure, lowering cholesterol, and its antimicrobial, anticarcinogen, and cardiovascular protective effects, have been extensively investigated (Okaro et al., 2023). Despite various health benefits of raw garlic, its consumption as intact garlic clove is limited owing to its very harsh, pungent odor. Raw garlic also causes bad breath and body odor after digestion and may sometimes cause gastrointestinal health issues, such as flatulence, stomach upset, and diarrhea (Tahir et al. 2021). Although garlic after processing (garlic paste/powder/oil) is used as a food ingredient, processing operations alter many of the health benefits of raw garlic and negatively influence the efficacy and bioavailability of its bioactive components, primarily organosulfur compounds. Therefore, alternative safe and healthy garlic products should be explored.

Black garlic is traditionally consumed in Southeast Asian countries such as Korea, Japan, and Thailand, and has recently been found to be an appealing functional food globally (Kimura et al., 2017). Black garlic is prepared by ageing raw garlic bulbs under high temperature and high humidity conditions for several days or weeks. The processing conditions commonly employed during black garlic production include temperatures in the range of 40–90°C and relative humidity of 60–90% (Angeles et al., 2016; Bae et al., 2014; Sun and Wang, 2018). Unlike raw garlic, black garlic is non-spicy/pungent. The characteristic pungent aroma of raw garlic is mellowed during the ageing by heat inactivation of the allinase enzyme in intact garlic cloves, which further restricts the production of pungent flavor-producing volatile sulfur compounds (Bae et al., 2014; Yang et al., 2019; Liang et al., 2015). Black garlic also has a sweeter and light sour taste, soft and rubbery texture, is non-allergic, and does not cause any gastrointestinal problems after consumption. Black garlic cloves can either be eaten directly after peeling the outer coat of the clove or its extract can be incorporated into other processed foods, such as beverages, meat products, and bakery products (Aoudeh et al., 2023; Chang et al., 2023; Kim et al., 2008, ; Kim and Rho, 2011; Wang et al., 2013). In addition, owing to its high antioxidant capacity, black garlic has been used in cosmetic products such as shampoos, facial creams, and soaps.

Numerous scientific studies have demonstrated diverse biological actions of black garlic including anticancer, anti-inflammatory, antimicrobial, antioxidant, hepatoprotective, antidiabetic, cardioprotective, and immunomodulatory activities (Al-Shehri, 2021; Chen et al., 2018; Ho et al., 2016; Kim et al. 2017a, 2017b; Valls et al., 2022). S-allyl-1-L-cysteine (SAC), a water-soluble organosulfur compound, is a major bioactive component of black garlic (Rais et al., 2023). In addition, the total polyphenol, and flavonoid content and antioxidant potential of black garlic are greater than those of raw garlic (Kodera et al. 2020; Kim et al. 2013). Thus, with enhanced biofunctionality and better sensory appeal, black garlic becomes a promising functional food ingredient. However, wider consumer acceptance of this novel functional ingredient is limited due to the absence of a standard quality index and variability in the product quality of black garlic commercially available in the market. There is a lack of scientific knowledge regarding the manufacturing process of black garlic. In this review, different quality aspects of black garlic, including phytochemical changes, organoleptic properties, aroma profiles, and biofunctionality are discussed. Furthermore, various health supplements of garlic were investigated to identify quality marker sulfur components responsible for its health benefits. In addition, various thermal and non-thermal pretreatments of garlic were analyzed for its effect on ageing period of black garlic and influence on final quality of the product. Thus, the study may help to improve the scientific knowledge and prospects of black garlic as an industrial product.

Bioactive components of garlic based health supplements

A variety of garlic-based health supplements and processed products, such as garlic powder, garlic oil, garlic oil macerate, garlic paste, black garlic, and aged garlic extract, are commercially available. However, the processing conditions have major implications on the stability, efficacy, safety, and sensory appeal of these garlic-based health supplements. During the manufacture of many garlic-based health supplements, the bioactive potential of fresh garlic may be altered, and products with different bioactive components are developed. Sulfur-containing compounds are considered to be the major unstable bioactive constituents of garlic preparations. The non-sulfur components of garlic, such as fructans, inulin, saponins, flavonoids, organoselenium compounds, and phenolic compounds, act synergistically with sulfur compounds to enhance the bioactive potential of garlic and garlic products (Diretto et al., 2017; Jiang et al., 2021; Shang et al., 2019). However, different garlic-based functional foods/health supplements exhibit varying levels of bioactivity and bioavailability of their sulfur components. Therefore, it is important to identify biomarker sulfur compounds/bioactive components in garlic-based health supplements/functional foods.

The composition of bioactive sulfur compounds differs considerably in garlic-based health supplements, depending on the processing conditions employed. The primary sulfur-containing compounds in raw garlic are γ-glutamyl-S-allyl-L-cysteine and oxidized forms of cysteine sulfoxides, such as alliin (S-allyl-L-cysteine sulfoxide), methiin (S-methyl-L-cysteine sulfoxide), and propiin (S-propyl-L-cysteine sulfoxide) (Rais et al., 2023). Allicin (diallyl thiosulfinate) is not originally present in intact garlic, but is generated during processing operations such as crushing/cutting of garlic cloves due to the action of the vaculor enzyme alliinase on cytosolic alliin. Allicin is highly unstable and readily transforms into volatile organosulfur components such as diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), allyl methyl sulfide, allyl methyl disulfide, allyl mercaptan, dimethyl trisulfide, methyl allyl thiosulfinate, 2-vinyl-4-H-1,3-dithiin, 3-vinyl-4-H-1,2-dithiin and ajoene (Bastaki et al., 2021). Some studies have reported that allicin and its derivatives, such as diallyl sulfide (DAS) and diallyl disulfide (DADS), are physiologically active components of garlic with antibacterial, antifungal, and antiviral activities (Ozma et al., 2023). However, owing to their instability in garlic-based health supplements, the health claims of allicin and its derivatives require further investigation. Most garlic-based commercial health supplements currently available in the market, such as garlic oil, garlic oil macerates, and garlic powder, do not contain allicin (Santhosha et al., 2013). Furthermore, a few studies on the bioavailability of garlic constituents have also reported that blood, urine, and liver do not contain allicin and its derivatives, even when a significant amount of raw garlic is consumed (Lawson and Wang, 2005). It is assumed that allicin is trapped before its absorption by binding to proteins and fatty acids in the plasma membrane, and is thus not present in the blood (Amagase, 2006). Another sulfur component of garlic is alliin, a precursor of allicin. After the direct consumption of garlic cloves, alliin is not converted into allicin during the digestion process, which may be due to the inhibition of allinase activity under acidic conditions in the stomach (Amagase, 2006; Santhosha et al., 2013). It is thus derived that the health benefits of garlic-based health supplements may be associated with compounds other than allicin.

During clinical studies of garlic and garlic products, water-soluble sulfur compounds such as S-allyl-L-cysteine (SAC), S-methyl-cysteine (SMC), and S-allyl-mercaptocysteine (SAMC) have been established as important bioactive components because of their stability, safety, and bioavailability (Kimura et al., 2017; Manoonphol et al., 2023; Moreno-Ortega et al., 2023). SAC has been reported to exhibit strong antioxidant, anti-cancer, cardioprotective, anti-inflammatory, hepatoprotective, and neurotrophic activities (Anandasadagopan et al., 2017; Asdaq, 2015; Chatterjee et al., 2019; Imai et al., 2016; Kim et al., 2017b). After oral consumption, SAC was detected in the liver, kidney, and plasma. SAC, a water-soluble cysteine-containing compound, is readily absorbed into the gastrointestinal tract through amino acid transporters in the intestinal lumen (Rose et al., 2005). SAC has bioavailability of up to 98 percent and is physiologically active (Rais et al., 2023). Higher concentrations of SAC, such as aged garlic extract and black garlic, have been found in novel functional foods derived from garlic. Because of its stability and health benefits, SAC can be used as a quality marker for black garlic. Black garlic also contains other water-soluble bioactive sulfur compounds, such as S-methyl-L-cysteine (SMC), S-allylmercapto-L-cysteine (SAMC), and small amounts of oil-soluble sulfur compounds (diallyl disulfide, diallyl trisulfide, diallyl sulfide, and allyl methyl disulfide). Therefore, to further establish the biofunctionality of garlic-based functional foods, the phytochemistry and processing of black garlic requires a detailed investigation.

Production of black garlic

Black garlic is a processed product of garlic, manufactured at controlled temperature and humidity conditions for a period of several days or weeks. The process is often referred to as ageing or fermentation; therefore, the term ‘aged garlic’ or ‘aged black garlic’ is also used interchangeably to refer to black garlic. In a few studies, the ageing process for black garlic is alternatively referred to as fermentation; however, the role of microorganisms in catabolic changes during the ageing process has not been established. Thus, the ageing of garlic to produce black garlic is thought to be an entirely thermally dependent event with prominent roles in various enzymatic and non-enzymatic reactions, such as depolymerization of fructans, Maillard reaction, caramelization, and oxidation of phenols, which results in favorable organoleptic and functional properties of black garlic (Abe et al., 2020). As reported in the literature, the temperature suitable for ageing black garlic ranges from 40 to 90°C, while a relative humidity (RH) of 60 to 90% needs to be maintained during ageing (Ai and Huong, 2018; Bae et al., 2014; Chang et al., 2020; Kang, 2016). The ageing period of black garlic varies from 15 to 70 days, depending on the temperature of ageing and pretreatments (Angeles et al., 2016; Hue et al., 2022; Zhang et al., 2016).

The ageing temperature is the most crucial factor affecting the final quality of black garlic. Enzymatic activity, water content in the final product, and most of the ageing-related phytochemical changes in garlic are affected by ageing temperature. The temperature required during the ageing of black garlic depends on the garlic type and the desired quality of the end product. In a few studies, temperatures as low as 40°C have been reported for ageing, while to accelerate ageing-related changes, high temperature (up to 90°C) can be achieved (Zhang et al., 2016). A temperature above 60°C results in the inactivation of the allinase enzyme; thus, in many studies, higher temperatures are preferred for black garlic production. According to the published results of black garlic processing, temperatures in the range of 70–75°C are considered to be the most favorable temperature for the desired quality attributes of the final product, such as uniform black color, sweet–sour taste, and chewy texture (Chua et al., 2022; Hue et al., 2022). Higher temperatures result in curtailing the ageing period of black garlic, but it may also cause a hard texture and bitter taste in the final product due to higher moisture removal and a higher rate of Maillard browning. Low-temperature ageing is beneficial for enhancing the formation of bioactive sulfur compounds such as SAC. The higher amount of SAC in black garlic aged at low temperatures is due to the more favorable conditions for γ-GTP enzyme activity at lower temperatures (Bae et al., 2014).

The relative humidity of the surroundings was maintained during the ageing process of black garlic to control the moisture content of the final product and the rate of many biochemical reactions during ageing. The most commonly employed relative humidity conditions for black garlic production range from 70 to 90% (Angeles et al., 2016; Chang et al., 2020; Sun and Wang, 2018). Higher humidity during ageing slows down the rate of browning reaction, and thus moist products with sweet tastes are developed after ageing garlic (Sun and Wang, 2018; Gonzalez-Ramirez et al., 2022). Black garlic aged under high-humidity conditions has a better physical appearance, texture, and browning. In any case, low-humidity conditions are avoided to prevent excess drying and shriveling.

Pretreatments for production of black garlic

The conventional thermal treatment for converting raw whole garlic bulbs into black garlic is very time-consuming and involves higher energy costs. Therefore, various pretreatments, such as freezing, ohmic heating, high-pressure processing, pulsed electric fields, ultrasonication, and microwave, have been explored to reduce the ageing time and energy cost and improve the quality of black garlic (Li et al., 2015; Rios-Rios et al., 2021). Many of the pretreatments are responsible for disrupting the cell structures of raw garlic, causing a higher rate of interaction among the chemical constituents in the damaged cell structures, which accelerates the rate of physicochemical reactions and shortens the ageing time. Freezing is the most explored pretreatment for the accelerated ageing of black garlic (Li et al., 2015; Kandemirli et al., 2020; Chan et al., 2022). The freezing injury to the plant cells results in damage to the cell membranes and oozing of intracellular substances into the surrounding environment, thus increasing the rate of Maillard browning and shortening the ageing period (Li et al., 2015).

Non-thermal pre-treatments, such as high-hydrostatic pressure (HHP) and ultrasonication, also positively influence the processing of black garlic by regulating enzyme activity and Maillard reactions. Application of high pressure during HHP pretreatment results in the destruction of cellular structure and thus promotes enzymatic reactions in damaged cells during the initial stage of ageing (Li et al., 2020). At lower pressures (100–300 MPa), enzyme activity increases, while at higher pressures (above 400 MPa), the enzymes become inactivated. Thus, HPP at pressures below 400 MPa is more effective for accelerating the ageing of black garlic (Zhao et al., 2021). Also, a greater amount of reducing sugars is generated during the initial stage of ageing in HHP-treated garlic, which accelerates the Maillard browning rate (Li et al., 2020). Novel thermal processing pre-treatments, such as ohmic heating (OH), have also been explored for the accelerated ageing of black garlic. It is postulated that the reduction in ageing time after ohmic heating may be due to electroporation of the cells. Electroporation causes more soluble sugars to be released from cells, thereby enhancing the rate of the Maillard reaction. Furthermore, tissue compression in garlic and solute diffusion through the membrane are enhanced with the application of OH pretreatment, and thus fructans can readily participate in the Maillard reaction with the formation of reducing sugars (Rios-Rios et al., 2021). Many of these pretreatments not only help to curtail the manufacturing cost of black garlic with reduced energy requirements but may also positively influence the functionality of aged garlic products. For example, ultrasonication pre-treatment before ageing may result in a greater amount of SAC, a bioactive functional component, in the final aged product (Chan et al., 2022). Pretreatments such as freezing and HHP also generate higher amounts of polyphenols and flavonoids in aged black garlic. The sensory attributes of black garlic are also positively influenced by pretreatment before the ageing. However, there are limited studies on the effects of different pretreatments on the quality characteristics of black garlic. Therefore, in order to establish better control over the quality of the final product, more comprehensive studies are required to understand the mechanism of compositional changes in black garlic with the application of different pretreatments.

Quality indicators of black garlic monitored during the ageing process:

Black garlic has a better sensory appeal than its native counterpart, as aroma, taste, and texture are improved after the ageing process. Sensory quality attributes are thus important quality indicators of black garlic, and their monitoring during the ageing process helps to develop a suitable quality index for black garlic. The phytochemical changes that contribute to the sensory quality of black garlic are shown in Fig. 1.

Fig. 1.

Biochemical changes in black garlic responsible for its sensory attributes and bioactive potential

Fructans are the major carbohydrates in raw garlic and account for 85% of the total carbohydrate content (Yuan et al., 2016). Ageing of black garlic results in the degradation of fructans to other oligosaccharides and monosaccharides, mainly glucose and fructose. Enzymatic degradation catalyzed by fructan exohydrolase plays a key role in the depolymerization of fructan, followed by a thermal effect on depolymerization (Ding et al., 2021; Lu et al., 2018). The total sugar content in black garlic increased up to 80% of dry matter, with fructose as the major sugar, followed by sucrose, glucose, arabinose, and maltose (Li et al., 2015; Zhang et al., 2015). With fructose as a major reducing sugar (1.7 times that of sucrose), black garlic is one of the sweetest functional food ingredients with a better sensory appeal. Owing to its effect on sensory attributes, the reducing sugar content is an important quality indicator during black garlic production. Therefore, controlled production of reducing sugars is required during the ageing. High-temperature ageing resulted in a sudden increase in reducing sugar content, followed by a decline in sugar content in the later stage of ageing. This is because a higher amount of sugar is consumed during Maillard browning at elevated temperatures during ageing. Thus, black garlic aged at high temperatures may have a burnt/caramelized flavor. However, low-temperature ageing provides a sweeter and more aromatic taste due to a gradual increase in reducing sugar content throughout the ageing period (Zhang et al., 2016).

Black garlic has a relatively longer shelf life and better sensory appeal than its native counterpart due to a shift in the pH of raw garlic from 6.0–6.40 to 3.0–4.0 after the ageing (Bae et al., 2014; Chang et al., 2020; Najman et al., 2020). The rate of change of pH during the ageing of black garlic is supposed to occur in two stages, with an initial sharp decline followed by a gradual change in pH during the later stage of ageing (Hue et al., 2022). In addition, ageing temperature has a major influence on the pH of black garlic, as lower pH values are recorded for black garlic aged at high temperatures. The change in pH during black garlic processing is closely related to the Maillard reaction products formed during the ageing period. The ageing process of garlic may cause oxidation of the aldehyde group in aldohexose, resulting in the formation of carboxylic acids and the low pH of black garlic (Bae et al., 2014). Acetic and propanoic acids are generated during the ageing of black garlic by the α-dicarbonyl and β-dicarbonyl cleavage of hexose or pentose (Zhang et al., 2015). Acetic acid, 3-methylbutanoic acid, and propanoic acid are primarily responsible for the flavor of black garlic (Yang et al., 2019). In fact, a few organic acids in black garlic, such as acetic acid, formic acid, and succinic acid, are not present in raw garlic. Thus, the generation of organic acids during agieng followed by low pH helps to improve the shelf stability of black garlic by restricting microbial growth during storage, and it also improves the acceptability of black garlic with its characteristic sweet–sour taste.

Moisture content has a significant impact on the textural properties of black garlic and thus becomes an important quality parameter (Botas et al., 2019). Because ageing of black garlic is performed under high temperature and humidity conditions for a longer duration, moisture migration during ageing is an important event to be controlled. Raw garlic contains approximately 60–65% water. As ageing progresses, moisture declines continuously, with 21% to 37% loss of water during ageing (Bedrnicek et al., 2021; Najman et al., 2021). Moisture content in the range of 40–50% is ideal for black garlic with better sensory appeal, as it results in a soft, elastic, and chewy texture of the product (Najman et al., 2021; Zhang et al., 2016). A moisture content between 35 and 40% results in a slightly drier product with poor elasticity (Jing, 2020). Moisture content below 35% may cause very dry, brittle, and hard-textured black garlic (Najman et al., 2020).

Maillard reaction products in black garlic

Maillard reactions play a key role in deriving black garlic with a uniform black color. Phytochemical changes associated with the Maillard reaction also enhance the aroma and taste of black garlic and may alter the nutritive value and bioactive potential of the final product. During the initial stage of the Maillard reaction, colorless compounds with sugar-amine condensation are generated and continue until the Amadori products (i.e., with glucose) or Heyn’s products (i.e., from fructose) are formed (Ahmed and Wang, 2021). The intermediate stage produces colorless or yellow intermediates, most notably dicarbonyl compounds, through several reactions such as sugar dehydration, sugar fragmentation, and amino acid degradation (Strecker degradation). The final stage of the Maillard reaction involves aldol condensation, aldehyde-amine condensation, and the formation of heterocyclic nitro compounds. High-molecular-weight polymeric compounds, such as melanoidins, develop at the end of the reaction (Chang et al., 2020).

Amadori and Heyns compounds act as precursors for the aroma and color components of black garlic (Yuan et al., 2016). Amadori compounds are produced by a glucose-amino acid reaction, whereas Heyns compounds are derived from fructose-amino acid reactions during the ageing of black garlic. Since aldose (glucose) reacts faster with amino acids than ketose sugars (fructose), Amadori compounds are generated at a faster rate than Heyns compounds during the Maillard reaction (Yuan et al., 2016). However, Heyns compounds are equally important in black garlic because fructose is quantitatively greater than glucose in aged black garlic. In addition, ageing of black garlic spans a longer timeline, and thus the rate of production of Amadori and Heyns compounds varies during ageing. During the initial stage of ageing, the rate of formation of Amadori and Heyns compounds is greater than the rate of degradation because of the higher amounts of reducing sugars and amino acids. Heyns compounds, such as N-fructosylglutamyl-phenylalanine and N-fructosylisoleucine, also increased during the initial stage of ageing (Chang et al., 2020). However, at a later stage of ageing, Amadori and Heyns compounds decline rapidly (Yuan et al., 2016).

Fru-Arg is a major Amadori compound, followed by Fru-Ala, Fru-Leu, Fru-Val, Fru-Phe, Fru-Met, Fru-His, and Fru-Glu in aged black garlic (Yu et al., 2020). In addition, Amadori rearrangement products with carboline structures, such as 5-carboline (γ-carboline), norharman (β-carbolin), harmane (1-methyl-β-carbolin), harmol (7- hydroxyharman), ethyl-β-carboline, and L-oxonoreleagnine (1,2,3,4- tetrahydronorharman-1-one), are generated during ageing via the reactions of tryptophan with reducing sugars or its degradation products (Chang et al., 2020). Tryptophan is an important amino acid found in raw garlic. During garlic ageing, tryptophan reacts with aldehyde or pyruvic acid to generate neo-formed alkaloids, tetrahydro-β-carboline. Norharman and harmane are oxidation products of tetrahydro-β-carboline. Other tetrahydro-β-carboline derivatives, such as harmol, ethyl-β-carboline, and L-oxonoreleagnine, are generated by the reaction of tryptophan with reducing sugars or its degradation products (Chang et al., 2020).

Other crucial intermediate products of the Maillard reaction are 5-hydroxymethyl furfural (5-HMF) and other furfural derivatives such as 5-(hydroxymethyl)−2-furoic acid (5-HMFA). The 5-HMF content of black garlic is comparable to that of other products, such as instant coffee. The 5-HMF formation during garlic ageing is linked to the intensity of the black color of the product. In a few studies, HMF formation during black garlic production has been used as a quality indicator for the completion of the ageing process (Liang et al., 2015). High concentrations of HMF, generated at high temperatures, of ageing result in a bitter taste in the final product. Furthermore, HMF formation is influenced by the type of sugar, pH, and water activity of garlic during ageing. Fructose is a more reactive sugar than glucose and sucrose during enolization reaction for the formation of HMF (Anese and Suman, 2013). The fructose content in garlic increases during ageing, and acidity decreases simultaneously, both of which are favorable for the formation of HMF. However, the high water activity of garlic limits the rate of HMF formation during ageing (Zhang et al., 2015). Melanoidins, dark brown polymeric compounds with high molecular weights (HMW), are the end products of the Maillard reaction. In general, polymerization of low-molecular-weight (LMW) Maillard reaction products or their crosslinking with proteins leads to the formation of melanoidin in black garlic (Zhao et al., 2021). Melanoidins impart a bitter taste to black garlic and therefore negatively influence its sensory quality.

Aroma components of black garlic

Aroma is an important quality indicator of black garlic. The aroma profile of black garlic changes profoundly as the undesirable pungent aroma of raw garlic is mellowed, and a sweet, sour-roasted flavor develops during the ageing process. One of the major reasons for the loss of the pungent odor in black garlic is the thermal inactivation of the alliinase enzyme during the ageing process. In addition, thermal treatment lowers the concentration of sulfur volatiles in black garlic, whereas sweet and roasted volatiles increase with the Maillard reaction. Roasted and sweet aroma active substances are concentrated in black garlic, whereas raw garlic exhibits a greater concentration of greenish/floral and sulfurous aroma active substances. These aroma-active substances can be broadly categorized as alcohols, aldehydes, ketones, heterocyclic compounds, organic acids, and sulfurous compounds. The following section discusses sulfurous and non-sulfurous aroma-active substances.

Sulfurous aroma active substance in black garlic

The characteristic pungent flavor of raw garlic is associated with its sulfur-containing compounds. The ageing of black garlic affects unstable sulfur compounds, and thus, the flavor profile of black garlic evolves with compositional changes in sulfur compounds. The garlic-like or sulfur-like aroma of black garlic is due to aroma-active substances, such as allyl methyl trisulfide, 2-vinyl-4H- 1,3-dithiine, diallyl disulfide, diallyl trisulfide, 3-vinyl-1,2-dithiacyclohex-4-ene, diallyl sulfide, 3H-1,2-dithiole, and dimethyl sulfoxide (Yang et al., 2019). The volatile sulfur compounds identified in raw garlic, such as allyl mercaptan, and 1,3-dithiane were not detected in black garlic, while other sulfur compounds, such as dimethyl disulfide and allyl methyl disulfide, were reported to be greatly reduced in black garlic (Martinez-Casas et al., 2017). Diallyl disulfide and diallyl trisulfides are the major sulfur volatiles in raw garlic, whereas allyl methyl sulfide is a major contributor to the sulfur aroma in black garlic because of its higher odor activity value (Molina-Calle et al., 2017; Yang et al., 2019; Abe et al., 2020). Another flavor component produced during the thermal processing of garlic is allyl alcohol (2-propen-1-ol), which is responsible for the sulfurous odor (Martinez-Casas et al., 2017). Certain aromatic substances in raw garlic, such as 1-pentanehiol, 3-mercapto-3-methylbutyl formate, allyl 1-propenyl sulfide, methyl 1-propenyl disulfide, dimethyl sulfide, and 1-decanethiol sulfide, do not appear in black garlic after ageing (Molina-Calle et al., 2017; Najman et al., 2020).

Non-sulfurous aroma active substance in black garlic

Many non-sulfurous aroma active substances, particularly products of the Maillard reaction, are active participants in the characteristic sweet–sour-roasted aroma of black garlic. Furfural (roasted flavor), a Maillard reaction product, is the second most concentrated aroma active substance in black garlic after allyl methyl sulfide (Molina-Calle et al., 2017). Other Maillard reaction products in black garlic, such as 3-(methylthio)-propioaldehyde, impart cooked potato flavor to black garlic (Yang et al., 2019). 2-methylothio-ethanol, 2-methyl-2-propanethiol, 2-octanone, 2-furanmethanol, α-pinene, 2,3,-dimethyl-pyrazine, 5-methylfurfural and 4-hydroxy-5-methyl-3-furanone contributed to the roasted, caramelized, burnt and fruity aroma of black garlic (Najman et al., 2020). The furfural derivatives such as 5-methylfurfural and 2-acetylfuran are also associated with the characteristic sweet aroma of black garlic (Kilic-Buyukkurt et al. 2023). Other heterocyclic compounds, such as furaneol, 1-(2-furanyl)-ethanone, 2-acetyl-1-pyrroline, 5-heptyldihydro-2(3H)-furanone, and butyrolactone, contribute to the roasted and sweet flavor of black garlic (Yang et al., 2019). 2-methylpropanol is another major aroma-active substance responsible for the spicy flavor of black garlic. 5-HMF has a slightly bitter taste compared with black garlic (Jing, 2020; Yang et al., 2019). Most of the aroma active substances responsible for the sweet and roasted aroma of black garlic appear in the later stage of ageing, while acetone, 2-propane-1-ol, and 2-methylene-pentalanal had the maximum concentration during the intermediate stage of ageing (Molina-Calle et al., 2017).

Bioactive components of black garlic

The ageing of garlic to produce black garlic has a significant impact on its bioactive components, particularly organosulfur compounds, polyphenols, and flavonoids; thus, the biofunctionality of this novel functional food ingredient is improved (Table 1). The major bioactive components of black garlic are discussed below.

Table 1.

Bioactive potential of Black garlic

Bioactive potential	Bioactive component	Mechanism of action	References
Anticancer	SAC, SAMC	Induce apoptosis Inhibiting cancer cell proliferation Target oxidative stress	Jikihara et al. (2015) Park et al. (2014) Vijayan et al. (2022)
Anti-inflammatory	Polyphenols, 5-HMF, 2-linoleoyglycerol, Pyruvate	Suppression of TNF-α formation Suppression of cell adhesion molecules activated by cytokines Inhibition of reactive nitrogen species	Kim et al. (2017a) Kim et al. (2011) You et al. (2019)
Anti-oxidative	SAC, Polyphenols, carboline alkaloids 5-HMF	Inhibit free radical formation Increase SOD activity Suppress superoxide production	Lu et al. (2014) Chang et al. (2020)
Cardioprotective	SAC, Polyphenols	Inhibit platelet aggregation Lowers cholesterol, triglycerides Inhibit lipid peroxidation Improve vasodilation	Villalon et al. (2016) Liu et al. (2018)
Hepatoprotective	SAC, SAMC	Reduced oxidative stress Increased activity of SOD and CAT Inhibit lipid peroxidation	Ahmad and Wang (2021) Tsai et al. (2019)
Anti-diabetic	SAC	Elevated serum insulin Enhanced lipid metabolism Decreased TBAR, SOD, CAT	Thomson et al., (2015) Lee et al. (2009)

Organosulfur compounds

During the ageing of black garlic, the thermal stability of the organosulfur compounds is an important factor. The content of alliin, a precursor of allicin, decreased during thermal treatment, probably due to the conversion of alliin to other stable organosulfur compounds. The allicin content also decreases during ageing (Liang et al., 2015). In most studies on black garlic, allicin is not detected in the final product. However, some authors have reported trace amounts of allicin at lower temperatures of ageing (50°C and 60°C), whereas at higher temperatures of ageing (70°C and 80°C), allicin is completely lost in aged black garlic (Chang et al., 2020). The content of water-soluble organosulfur compounds such as S-allyl cysteine (SAC) increases during ageing of black garlic. In fact, a five to sevenfold increase in SAC content has been reported for garlic aged at different temperatures (raw garlic: 20–30 µg/g SAC; Black garlic: 80-150 µg/g SAC) (Ai & Huong, 2018; Sasmaz et al., 2023). γ-glutamyl-S-allyl cysteine (GSAC), a precursor of SAC, is enzymatically hydrolyzed by γ-glutamyl transpeptidase (γ-GTP) to produce SAC during ageing. The temperature employed during ageing affects the activity of γ-GTP and thus controls the production of SAC. Garlic samples aged at low temperatures have higher amounts of SAC than those aged at higher temperatures (Bae et al., 2014). It has also been proposed that diallyl disulfide combines with glutathione to generate S-allyl-glutathione, which is one of the building blocks of SAC (Chang et al., 2020). Meanwhile, S-(2-carboxyl)-glutathione can be produced by glutathione and methacrylic acid, and this compound can subsequently be transformed into GSAC by removing the glycine structure. The higher amount of GSAC generated during ageing, along with its enzymatic hydrolysis by γ-glutamyl transpeptidase, is responsible for the increased level of SAC in black garlic. SAC formation of SAC is also affected by the water-facilitated reaction between GSAC and γ-GTP. Water in garlic facilitates the enzymatic activity of γ-GTP, and higher amounts of water support the transformation of GSAC into SAC (Bae et al., 2014). Garlic samples heated at higher temperatures and low humidity lose more water, which reduces the activity of the γ-GTP enzyme compared to samples heated at lower temperatures and high humidity. Few studies have reported that SAC formation in black garlic is an entirely thermally dependent event with no role in enzymatic hydrolysis by γ-glutamyl transpeptidase (Zhang et al., 2019). It is also postulated that the production of SAC during ageing is closely associated with the thermal degradation of alliin, since the chemical groups in alliin provide the basis for the synthesis of SAC and SAMC (Liu et al., 2022). Pretreatments also influence SAC content during ageing (Table 2). The enzymatic reaction of γ-GTP and GSAC is promoted during the pretreatment period due to cell disruption; thus, a lower quantity of SAC is generated during the subsequent ageing process. However, SAC content after ultrasonication is higher than that of other pretreatments, such as freezing and High Hydrostatic Pressure, due to a less pronounced effect on cell structure (Chan et al., 2022).

Table 2.

Changes in nutritional and functional components of black garlic during ageing process

Processing conditions	Parameters							References
	Moisture (%)/aW	pH/Acidity	Reducing sugars	Total phenols (GAE)	Total flavonoids	Antioxidant capacity	SAC
Temp: 60–70°CRH: 80–90% Duration: 42 days	34.74		42.1 g/100 g	1.82%		3262 µg/ml	159.81 mg/kg	Ai and Huong (2018)
Temp: 72 °C/75 °C/78 °C RH: 90% Duration: 14/21/33 days	aW 72 °C:0.91 75 °C:0.94 78 °C:0.93	72 °C:3.49 75 °C:3.53 78 °C:3.80		72 °C:14.85 g/kg 75 °C:16.32 g/kg 78 °C:14.78 g/kg		DPPH (Trolox mmol/kg) 72 °C:109.65 75 °C:97.74 78 °C:100.47		Angeles et al. (2016)
Temp: 60–85 °C RH: 70% Duration: 45 days							97.74–114.27 µg/g	Bae et al. (2012)
Temp: 40 °C/55 °C/70 °C/85 °C RH: 70% Duration:45 days	45.3–53.4	4.09–5.94				DPPH (%) 40 °C:22.50 55 °C:30.34 70 °C:33.93 85 °C:44.77	40 °C:124.67 55 °C:113.25 70 °C:113.25 85 °C:85.46 µg/g	Bae et al. (2014)
Step1:82 °C/3 days Step2:30 °C/1 day Step3:72 °C/11 days	38.26	4.39		14.03 g/kg		DPPH:12.93 gTE/kg FRAP:12.893 gTE/kg		Bedrnicek et al. (2021)
Temp:50 °C/60 °C/70 °C/80 °C RH: 85% Duration: 35 days	50 °C:45.53 60 °C:37.73 70 °C:30.77 80 °C:29.90	50 °C:4.19 60 °C:3.53 70 °C:3.52 80 °C:3.52		50 °C:11.80 g/kg 60 °C:13.20 g/kg 70 °C:16.97 g/kg 80 °C:15.90 g/kg		DPPH (%) 50 °C:38.46 60 °C:61.55 70 °C:70.95 80 °C:69.87		Chang et al. (2020)
Temp: 70 °C RH: 90% Duration: 35 days	29.88	3.74	16.07 g/kg	48.35 mg/g	15.7mgRE/g			Choi et al. (2014)
Temp: 70–75 °C RH: 80–90% Duration: 8–12 days	56.1	3.8				DPPH:3.91 µg/ml FRAP: 26.54 µg/ml		Chua et al. (2014)
Temp: 60 °C/65 °C/70 °C/75 °C RH: 70% Duration:42 days	60 °C:50.86 65 °C:48.06 70 °C:44.08 75 °C:40.55	60 °C:4.74 65 °C:4.50 70 °C:3.57 75 °C:3.29	60 °C:10.51 65 °C:12.71 70 °C:16.79	60 °C: 14.39 mg/g 65 °C:21.05 mg/g 70 °C:44.27 mg/g 75 °C: 38.24 mg/g	10.64mgQE/g	DPPH 60 °C:114.81 65 °C:73.74 70 °C: 69.93 75 °C:75.69 µg/ml		Hue et al. (2022)
Step1:90 °C/100% RH/34 h;Step2: 60 °C/60% RH/6 h; Step3:75 °C/70% RH/48 h;Step4: 70 °C/60% RH/60 h; Step5:65 °C/50% RH/192 h	43.66	4.22	4010.59 mg/100 g					Kang (2016)
Temp: 70 °C RH: 80% Duration: 45 days	45.54–47.59	4.01–4.15		12.50–15.10 mg/g	52.53–63.32 mg/100 g	546.26–654.93 µmol/g		Najman et al. (2020)
Temp: 70 °C RH: 80% Duration: 45 days		3.43–3.95	39-45 g/100 g	13.64–17.24 mg/g	57.08–68.08 mg/100 g	707.28–828.42 µmol/g		Najman et al. (2021)
Temp: 65–85 °C RH: 70–85% Duration: 24–50 days	52.93			1152.46 mg/100 g		DPPH:2855.73 µmolTrolox/100 g ABTS: 4374.60 µmolTrolox/100 g	80.21 mg/100 g	Sasmaz et al. (2023)
Temp: 65–85 °C RH: 70–80% Duration: 20–30 days	29.5–31.35	4.09–4.18		2529.82–2886.73 mg/g	121-191mgRE/100 g	DPPH: 47–54% FRAP: 56.3–67.8%		Tahir et al. (2022)
Temp: 60 °C/70 °C/80 °C/90 °C RH: 80% Duration:15–69 days	60 °C:49.02 70 °C:46.06 80 °C:47.11 90 °C:47.08	Total Acid (g/kg) 60 °C:33.61 70 °C:37.50 80 °C:30.96 90 °C:36.37	60 °C:335.21 70 °C:357.96 80 °C:348.93 90 °C:154.91 g/kg	60 °C:13 g/kg 70 °C:12.35 g/kg 80 °C:15.12 g/kg 90 °C:13.27 g/kg				Zhang et al. (2016)
Temp: 65 °C/75 °C/85 °C RH:70%/75%/80%/85% Duration:16 days	31.28	Acidity4.030%	357.946 mg/g	4.499 mg/g		46.708%		Sun and Wang (2018)
Temp: 70 °C RH: 90% Duration: 30 days Ohmic Heating Pretreatment (OH)			TT: 360.3 mg/g dm OH: 253.4 mg/g dm	TT: 5.1 mg/g OH: 5.3 mg/g		DPPH (%) TT: 52.5 OH: 48.9		Rios-Rios et al. (2021)
Step1: 45 °C/90%RH/3 days Step2: 70 °C/90%RH/27 days Pretreatments: HHP, HPCD	Control: 59.05 HHP: 54.87 HPCD: 60.45	Control: 3.78 HHP: 4.06 HPCD: 4.05	Control: 465.79 mg/g HHP: 408.38 mg/g HPCD: 424.22 mg/g	Control: 1183.1 mg/g HHP: 943.56 mg/g HPCD: 1205.54 mg/g			Control: 1264.74 µg/g HHP: 1033.92 µg/g HPCD: 1115.42 µg/g	Li et al. (2020)

Other water-soluble organosulfur compounds, such as S-allyl-mercapto-L-cysteine (SAMC), S-methyl cysteiene (SMC), and cycloalliin, are also generated during the ageing of black garlic (Zhang et al., 2012). SAMC are generated by the transformation of allyl sulfide or by the reaction of allicin and cysteine during ageing (Molski, 2024; Utama et al., 2024). SAMC exhibits potent antiradical, hepatoprotective, and anticancer activities (Rios-Rios et al., 2021). SMC, an amino acid derivative of black garlic, has also been reported to have anti-carcinogenic, anti-diabetic, and cardiovascular effects in experimental animals (Edmands et al., 2013). Cycloalliin, a derivative of isoalliin, was also found in higher concentrations in black garlic than in raw garlic. Cycloalliin enhances the bioactive potential of black garlic through its anticancer and hypocholesterolemic activities (Liang et al., 2015). Other organosulfur compounds, such as methionine sulfoxide, S-propyl-1-propanesulfinothioate, diphenyl disulfide, and thiacremonone, are present at significantly higher levels in black garlic than in raw garlic (Molina-Calle et al., 2017). Thiacremonone has also exhibited different bioactive properties, such as anti-obesity, anticancer, and anti-inflammatory effects (Molina-Calle et al., 2017). Thermal treatment also results in the production of trace amounts of diallyl sulfides (DAS) (diallyl monosulfide, diallyl disulfide, diallyl trisulfide) in black garlic (Vinayagam et al., 2023). DAS possesses anticancer, hypoglycemic, antibacterial, anti-atherosclerotic, and hypolipidemic properties (Okoro et al., 2023). The metabolic pathway of organosulfur compounds during the black garlic production is shown in Fig. 2.

Fig. 2.

Metabolic pathway of organosulfur compounds in black garlic

Polyphenols

Garlic is one of the richest sources of phenolic compounds among spice/vegetable crops (3.4–10.8 mg GAE/g) (Afzaal et al., 2021). Hydroxycinnamic, β-resorcylic, p-hydroxybenzoic, gallic, caffeic, vanillic, chlorogenic, coumaric, and ferulic acids are the major phenolic compounds in garlic (Rais et al., 2023). Ageing results in an approximately four-to ten-fold increase in the total polyphenol content in black garlic (Choi et al., 2014). The increase in polyphenol content is also associated with the enhanced antioxidant capacity of black garlic and ultimately defines its enhanced biofunctionality. Thermal treatment causes polyphenols to be released more easily from their bound forms (esterified and glycosylated); thus, the increase in polyphenol content was recorded for black garlic after ageing with an increase in the free fraction of phenolic acids (Ai and Huong, 2018). It has also been proposed that ageing results in the heat inactivation of enzymes involved in the oxidation of phenolic substances, resulting in a higher phenol content in black garlic (Kim et al., 2013). The temperature of ageing influences the phenolic content in black garlic, with higher phenolic content at higher temperatures of ageing (Zhang et al., 2015; Hue et al., 2022). However, during the later stage of ageing, the total polyphenol content decreases owing to a higher rate of loss of phenolic compounds than its rate of accumulation (Ai and Huong, 2018; Zhang et al., 2015). In contrast, few studies have reported the continuous formation of complex phenolic compounds even during the later stage of ageing with the progress of the Maillard reaction, which may ultimately result in increased polyphenol content (Kang, 2016).

Maillard reaction products

Although the generation of Maillard reaction products may lead to the loss of nutritional value of the processed products, the bioactive potential of these neo-formed bioactive compounds has also been reported in many recent studies. The antioxidative properties of Amadori and Heyns compounds and their role in enhancing the biofunctionality of black garlic have been well established in the case of black garlic (Yu et al., 2020). N-(1-deoxy-D-fructose-1-yl)-L-arginine (Fru-Arg) has been identified as a major antioxidant in aged garlic extracts. In addition, Amadori rearrangement compounds, such as arginyl-fructose and arginyl-fructose-glucose, have shown antihyperglycemic effects (Ha et al., 2015). In addition, few studies have reported the protective effects of Fru-Arg in aged black garlic extract on neuroinflammation and neurodegradation (Qu et al., 2016). HMF is another important product of the Maillard reaction. HMF is considered a harmful substance owing to its mutagenic effect, although no conclusive research on humans regarding the carcinogenic effects of HMF is available. In fact, the anticancer effect of HMF has been reported in a few studies. HMF also exhibits anti-allergic, anti-inflammatory, anti-diabetic, and anti-oxidative properties (Choudhary et al., 2020; Cao et al., 2013). Melanoidins are end products of the Maillard reaction, which not only influences the sensory quality of the final product but also enhances the biological activity of black garlic. In thermally processed foods such as coffee, melanoidins have proven to be a potential functional ingredient with antioxidative, anticancer, antidiabetic, and anti-inflammatory properties (Mesias and Delgado-Andrade, 2017). Melanoidins have shown high antioxidant ability in black garlic (Kang, 2016; Zhao et al., 2021). The tetrahydro-β-carboline derivatives generated during the ageing of black garlic have shown important biological functions in the prevention of neurodegenerative diseases (Parkinson’s and Alzheimer’s disease) (Chang et al., 2020). Tetrahydro-β-carboline derivatives also have preventive effects on thrombosis owing to their ability to inhibit platelet aggregation (Sato et al., 2006). The structural configurations of the major bioactive components of black garlic are shown in Fig. 3.

Fig. 3.

Structural configurations of major bioactive components in Black Garlic

Biofunctionality of black garlic

Antioxidant activity

The majority of the health benefits of black garlic are attributed to its higher antioxidant potential. The antioxidant properties of black garlic are attributed to the increased levels of polyphenols, flavonoids, and sulfur compounds, such as SAC. In addition, carboline alkaloids, such as 1-methyl-1,2,3,4-tetrahydro-β-carboline derivatives and 5-HMF in black garlic, have shown strong antioxidant potential (Lu et al., 2018). The antioxidant activity of black garlic is influenced by the temperature used during ageing. Bae et al. (2014) reported that the DPPH radical scavenging activity of black garlic increased with increasing ageing temperatures from 40 to 85°C. In this study, the free radical scavenging activity of raw garlic was reported to be 6.21%, while for black garlic aged at 40°C and 85°C, the DPPH activity was 22.50% and 44.77%, respectively (Bae et al., 2014). The Maillard reactions and their products may have a greater influence on the antioxidant properties of black garlic, as the higher antioxidant activity at higher temperatures of ageing is also correlated with a higher rate of browning. In a study by Chang et al. (2020), the antioxidant potential of black garlic samples aged at 70°C (70.95%) was greater than that of samples aged at 50°C (38.46%), 60°C (61.55%), and 80°C (69.87%). Choi et al. (2014) reported that the antioxidant activity increased during the initial period of ageing (4.65% for raw garlic to 37.32% on the 7th day to 74.48% on the 21 st day of ageing), followed by a slight decrease until the final stage (63.09%). It was considered that the unstable organosulfur compounds present in raw garlic were transformed into stable, water-soluble substances, such as SAC, during the ageing period, resulting in very high antioxidant activity during the initial stage of thermal treatment. However, the temperatures that resulted in the highest SAC content and antioxidant activity did not match in the study by Bae et al. (2014). The authors reported that SAC content increased to a much greater extent in garlic samples heated at low temperatures, whereas antioxidant activity increased in garlic samples heated at high temperatures. Therefore, other active compounds, such as polyphenols, flavonoids, and products of Maillard reactions, might be responsible for the higher antioxidant potential of black garlic (Fig. 4).

Fig. 4.

Evolution from white to black garlic during ageing process

Anticancer activity

Consuming black garlic has been associated with a lower risk of developing cancer, as it exhibits excellent chemopreventive effects. In vitro studies have shown that black garlic and its extract can suppress malignant cell growth in lung, liver, uterus, gastric, breast, and colon cancers (Dong et al., 2014; Kim et al., 2018; Jikihara et al., 2015). Several studies have clarified the mechanisms underlying the anticancer activity of black garlic and the chemopreventive role of different bioactive components. High levels of oxidation, which cause stress and ROS formation, are necessary for signal transduction in malignant cells to develop tumors. One novel therapeutic approach for cancer treatment is targeting oxidative stress (Wang et al., 2015). A significant portion of the anticancer properties of black garlic is ascribed to water-soluble organosulfur compounds such as SAC because of its antioxidant properties and ability to act as ROS scavengers (Rais et al., 2023). SAC has been shown to be effective during both the early stages and the later stages of carcinogenic processes (Vijayan et al., 2022). According to Penislusshiyan et al. (2020), SAC improves the activity of antioxidant enzymes, such as GST, which leads to better carcinogen detoxification and inhibition of carcinogen formation. SAC decreases the sensitivity of stomach malignancies to lipid peroxidation by concurrently boosting the antioxidant state and inhibiting cell proliferation (Castrillon et al., 2019; Rais et al., 2023). SAMC, another bioactive component of black garlic, has been found to inhibit cancer cell growth and promote apoptosis in several cancer cell lines (Xiao et al., 2013; Yudhistira et al., 2022). SAMC has recently been shown to have anticancer properties against bladder, ovarian, liver, colon, and prostate cancers (Okoro et al., 2023). Both SAC and SAMC were more abundant in black garlic than in raw garlic. Therefore, the antitumor potential of black garlic is considered higher than that of raw garlic.

Anti-inflammatory activity

Many bioactive components from plant/herbal sources have been used as anti-inflammatory agents. The anti-inflammatory properties of black garlic are further linked to its preventive effects against cardiovascular diseases and cancers. The anti-inflammatory efficacy of black garlic is attributed to polyphenols, alkaloids, melanoidins, polysaccharides, and organosulfur compounds (Qiu et al., 2020; Tak et al., 2014). Other bioactive components such as 5-hydroxymethylfurfural, 2-linoleoylglycerol, and pyruvate also exhibit anti-inflammatory properties in black garlic (Kim et al., 2011; Afzaal et al., 2021). Very few studies have been carried out to identify the molecular mechanism underlying the anti-inflammatory properties of black garlic. The anti-inflammatory properties of black garlic are due to the presence of 5-HMF, which suppresses cell adhesion molecules that are activated by cytokines such as TNF-α (Kim and Rho, 2011). In another study, Kim et al. (2017a) reported that aged black garlic extract inhibited the production of MAPKs and extracellular-signal regulated kinase (ERK), resulting in its anti-inflammatory effect. The 2-linoleoylglycerol in black garlic reduces the levels of inflammatory mediators, such as nitrous oxide (NO), prostaglandin, IL-6, IL-1β, and tumor necrosis factor (TNF-α), as well as the expression of pro-inflammatory enzymes, such as nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) (Kim et al. 2017a).

Cardioprotective activity

Numerous studies have demonstrated the efficacy of black garlic in the prevention and treatment of various cardiovascular diseases. In a study by Morihara et al. (2016), aged garlic extract suppressed the development of atherosclerosis in mice, with lower concentrations of total cholesterol and triglycerides (TGs) and reduced relative abundance of CD11b + cells (Kang and Shin, 2012). SAC and polyphenols in black garlic may exert cardioprotective effects against atherosclerosis by inhibiting lipid peroxidation and preventing oxidative changes in LDL. Recently, hydrogen sulfide (H₂S) was shown to be a novel gasomediator in the cardiovascular system, where it acts as a vasodilator and cardiovascular protector (Gong et al., 2018). The cardioprotective effect of SAC may also be attributed to its ability to increase H₂S production. H₂S is produced by the enzymatic action of cystathionine lyase and cystathionine synthetase on the cysteine component of SAC. In another mechanism for the cardioprotective effect of SAC, it has been demonstrated that SAC blocks NF-κB activation, thus restricting a variety of signaling pathways leading to atherosclerosis (Anandasadagopan et al., 2017). According to García-Villalón et al. (2016), black garlic extract has a relaxing effect on coronary arteries and improves the antioxidant function of the heart. Jung et al. (2011) reported that aged black garlic improved the blood lipid profile (triglycerides, LDL, and HDL) in patients with hypercholesterolemia. Ha et al. (2015) demonstrated the mechanism of action of black garlic on lipid metabolism. The authors reported that black garlic supplementation improves the blood lipid profile by decreasing the expression of SREBP-1c and its related enzymes and increasing the expression of CPT-1. It has also been reported that black garlic consumption is associated with increased fecal excretion of triglycerides and cholesterol, and is thus attributed to the hypocholesterolomic effect of black garlic. In another study by Valls et al. (2022), treatment for six weeks with aged black garlic tablets (250 mg/tablet) to the hypercholesterolemic patients showed a reduction in high blood pressure.

Hepatoprotective activity

Liver fibrosis, non-alcoholic fatty liver disease, liver cirrhosis, and alcohol-induced liver diseases are the major causes of morbidity. Liver enzymes, such as alanine transaminase (ALT), aspartate transaminase (AST), and alkaline phosphatase (ALP), usually increase during hepatotoxicity; therefore, these enzymes are markers of liver function. The development of liver disorders is aided by oxidative stress. Black garlic prevents oxidative stress and other liver disorders by decreasing the levels of ALT, ALP, and AST and increasing SOD and CAT (Ahmed and Wang, 2021). Water-soluble organosulfur compounds, such as SAC and SAMC, actively contribute to the hepatoprotective role of black garlic (Xiao et al. 2013). The therapeutic effect of SAC is due to its ability to improve antioxidant enzyme activities, such as SOD and CAT, and decrease the end products of oxidative stress, such as malondialdehyde or 4-hydroxynonenal (Colin-Gonzalez et al., 2012). Tsai et al. (2019) showed that administration of SAC and polysaccharide-enriched black garlic extract to ICR mice prevented CCl₄-induced liver damage by inhibiting lipid peroxidation and inflammation. In another study by Xiao et al. (2013), SAMC was shown to inhibit lipogenesis markers and restore lipolysis markers in nonalcoholic fatty liver disease. Thus, black garlic consumption may help slow the progression of liver diseases.

Anti-diabetic activity

Diabetes mellitus (DM) is a metabolic disorder that occurs as a result of metabolic disturbances in carbohydrates, lipids, and proteins. Metabolic malfunction in patients with diabetes is due to defects in insulin secretion, insulin action, or both. Black garlic may exert antidiabetic effects by increasing pancreatic insulin secretion and enhancing insulin sensitivity. In streptozotocin-induced diabetic mice, black garlic extracts considerably reduced blood glucose levels and markedly elevated serum insulin (Thomson et al., 2015). In another study, black garlic powder was used to treat diabetic male Wistar rats. Black garlic powder has been shown to reduce blood sugar, prevent glycogen buildup in the liver, and enhance lipid metabolism by boosting the activity of glutamic pyruvic transaminase, glutamic oxaloacetic transaminase, and GTP (Kang et al., 2013). Srinivasan (2005) ascribed the hypoglycemic effect of garlic to bioactive components, such as SAC, which stimulate insulin production or interfere with glucose absorption. SAC interacts with endogenous thiol-containing molecules, such as cysteine or glutathione, to protect insulin from sulfhydryl (-SH) inactivation (Zhai et al., 2018). The ability of black garlic extract to reduce oxidative stress is also linked to the prevention of the rapid progression of diabetic conditions.

Anti-obesity activity

Obesity is a major health concern not only in developed countries but also in many developing countries. Obesity is also associated with the development of other health risks, such as diabetes, cardiovascular diseases, hepatic disorders, hyperlipidemia, and arthritis. Although different therapeutic approaches have been investigated for the control of obesity and weight management, adverse side effects of clinical treatments for obesity control are a matter of concern. Plant-based health ingredients, such as black garlic, have been extensively investigated for the treatment of obesity and other health issues. The positive effect of black garlic on obesity control is due to its ability to regulate adipogenesis, lipolysis, lipid peroxidation, and the accumulation of body fat. High levels of phenolic compounds and flavonoids in black garlic are potential bioactive constituents that prevent obesity by regulating lipid metabolism (Chen et al., 2014). It is further stated that the high-fat diet induced obesity in rats can be controlled by downregulating CD36 transport protein activity, as the protein is responsible for the accumulation of lipids in adipocytes (Chen et al., 2014).The mechanism for obesity control with supplementation of black garlic in the diet may also involve its ability to upregulate adiponectin, peroxisome proliferator-activated receptor, and downregulation of SREBP-1C, acetyl-CoA-carboxylase, hepatic hydroxy-3-methylglutaryl coenzyme A reductase, and acyl-CoA cholesterol acyltransferase mRNA as well as an increase in the expression of carnitine palmitoyltransferase-1 mRNA (Seo et al., 2009; Qiu et al., 2020; Ha et al., 2015). In another study by Chang et al. (2017), supplementation of black garlic in high-fat diet (HFD)-fed rats was found to be beneficial for the control of obesity along with suppression of liver, peritoneal, and epididymal fat weights, serum lipid triglyceride levels, and hepatic lipid profiles induced by an HFD.

Conclusion

Black garlic is one of the promising ingredients for the functional food market owing to its better nutritional and sensory attributes as well as higher bioactive potential than raw garlic. Anticancer, anti-inflammatory, anti-allergic, anti-obesity, hepatoprotective, and cardioprotective functions were well established, and the role of different bioactive constituents in improving biofunctionality was also elaborated in this study. The ageing of garlic results in improved organoleptic properties of black garlic through rate determination of Maillard browning and degradation of pungent aroma generating sulfur compounds. However, metabolic changes in key organosulfur compounds during ageing and the physiological role of bioactive organosulfur components require more detailed investigation to establish the bioactive potential of black garlic. The Maillard reaction is the most crucial aspect during ageing of black garlic, and the number of Maillard reaction products determines the organoleptic properties and antioxidative potential of black garlic. Future studies addressing role of Maillard reaction products in bioactive potential need to be done. The high energy costs incurred during the long duration of ageing and selection of a suitable quality index are major challenges affecting the commercial viability of black garlic. In the future, ageing of garlic could be assisted with suitable pretreatments to reduce the ageing period, curtail the cost of production, and improve the techno-functional properties of black garlic. The study also help to identify major quality attributes of black garlic and guides evolutionary changes in the quality attributes during ageing process. Thus, industrial application of black garlic as novel functional food becomes more promising with control of quality and functionality of the product during the ageing process.

Author contribution

Harshad Mandge: Conceptualization, writing original draft, Validation, Investigation. Jagbir Rehal: Conceptualization, Review & editing of the manuscript. Deepika Goswami: Writing—review & editing, Supervision. Aditi Choudhary: Conceptualization, Review & editing of the manuscript. Preetiender Kaur: Review & editing of the manuscript, validation, methodology.

Funding

This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

Declarations

Conflict of interest

The authors declare that they have no conflicts of interest.