# Flavour in black garlic: a comprehensive review

Thomas A Vilgis

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Thomas A Vilgis. Flavour in black garlic: a comprehensive review. International Journal of Molecular and Physical Gastronomy, 2025, 10.17180/ijmpg-2024-art07. . hal-05147620

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# Flavour in black garlic:
a comprehensive review

Thomas A. Vilgis

Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz, Germany

*Correspondence: vilgis@mpip-mainz.mpg.de

## Abstract

Black garlic has grown in popularity over the last few years and is now widely used, particularly in fine dining restaurants, but it is also available in shops. After several days to weeks of treatment of the entire bulbs at temperatures between 60 °C and 70 °C, its flavour bears little resemblance to the fresh clove. It has an non-crunchy, soft consistency, a slight acidity and a caramel-like sweetness. It can be used in a variety of applications, including desserts and even chocolate. The purpose of this review is a deeper understanding of the fermentation processes. It is argued that this knowledge provides a basis for systematic temperature controlled process during preparation, which improves the flavour of black garlic better.

## Keywords

black garlic, flavour, heat treatment, enzymatic reactions, glycation

## 1. Introduction

Black garlic has an distinct flavour and has little in common with fresh, white garlic (Allium sativum

L.). For example, black garlic shows a caramel odour with braised beef-saucy flavour, and has a spicy, caramel-like, occasionally slightly burnt, slightly mushroom-like odor and even a touch of dried fruit and nuts (Kilic-Buyukkurt et al., 2023). Although the sulphur notes are reminiscent of its origins, it is by no means pungent or even lachrymatory (Hu et al., 2024). The irritating, cold trigeminal irritation of white garlic has disappeared, as has the prolonged "garlic breathe". Instead, the flavour of black garlic dominates, with a savoury flavour and a taste combination of sweet, sour and umami.

On the other hand, Mediterranean, Indian and Asian cuisine would be inconceivable without fresh, white garlic. Whether raw, e.g. in aioli, roasted or cooked, e.g. in stews, curries or wok recipes, garlic contributes significantly to the final flavour of many traditional dishes. Fresh garlic contains about 63 % water, 28 % carbohydrates (main component storage carbohydrates are fructans), 2.3 % organosulphur compounds, 2 % proteins (including the enzyme alliinase), 1.2 % free amino acids, and 1.5 % lipids (Santhosha et al., 2013). Due to sulphur compounds, fresh garlic is characterised by strong trigeminal
International Journal of Molecular and Physical Gastronomy

Review

<figure><img src="image_2.png" /><figcaption>Figure 1. Schematic representation of the chemical processes involved in the formation of the odourant compound allicin (6) from alliin (1). An important by-product is pyruvate (4), which is responsible for a large part of the initial (fermentation) processes in black garlic. The structure of alliinase is shown at bottom left (from Databank Swiss-Model).</figcaption></figure>

stimuli associated with orthonasal and lacrimatory irritation, and a painful cold sensation on the tongue, similar to onions (Bautista *et al.*, 2005; MacPheroson *et al.*, 2005). Fresh garlic is dominated by the intense, pungent, sulphur-like odour of the Allioideae subfamily, which many people dislike. It persists for a long time after consumption. The degradation products of the sulphur compounds can be detected in the breath for hours after consumption and are even excreted through the skin (Taucher *et al.*, 1996).

Most of these irritations are caused by sulphur compounds that are produced when the plant cells are damaged during cutting and oral processing. Garlic contains both alliin and the alliin-cleaving enzyme alliinase (also known as alliin lyase), but not in the same cells or cell spaces. Alliin, which is polar and therefore water soluble (Figure 1), is stored in the cytoplasm of intact plant cells. The enzyme alliinase is located in the vacuoles.

The biological function of alliin in garlic and onion plants is a defensive mechanism: when garlic cells are destroyed by herbivores, alliin and alliinase interact, which results in chemical reactions sketched in Figure 1. Water-soluble alliin (1) is converted by alliinase to allylsulfenic

acid (2) and dehydroalanine (3). Dehydroalanine is unstable and decomposes to ammonia $NH_3$ and pyruvic acid. In the next step, allylsulfenic acid (4) reacts in an aqueous medium to form the volatile molecule allicin (5), which is responsible for the typical garlic odour. The degradation of dehydroalinin (3) is important in the context of fermentation, since the anion of pyruvic acid is pyruvate.

When exposed to heat, such as during steaming, cooking or frying, most pungent trigeminal sensations disappear. The strong trigeminal cold pain stimuli are also lost. The alliinases are inactivated when exposed to heat, which prevents the process shown in Figure 1. The strong trigeminal stimuli and the typical garlic odour are also significantly reduced if garlic is first blanched and then pickled. The blanching temperature plays a decisive role. Blanching at 60 °C up to 30 min has only a minor effect, whereas the pungent sensations decrease significantly with increasing blanching temperature (Rejano *et al.*, 2004). This is due to the increasing inactivation of the allinase enzyme with increasing blanching temperature (and blanching time), as shown in Figure 2. The maximum activity is between 30 °C and 40 °C. In
International Journal of Molecular and Physical Gastronomy

Review

this range, garlic can be fermented by the
classical lactic acid process. Above 80 °C, the
enzyme is irreversibly inactive and no significant
concentrations of pungent odours are formed. The
odour is then largely determined by the odorants
that can form under the particular thermal
conditions (Bi *et al.*, 2023).

The transition range between 50 °C and 70 °C is therefore of interest for black garlic processing, as the activity of alliinase is still present in this temperature interval, while the higher thermal energy allows the flavour development of black garlic. The strong changes occur as a result of several weeks of ageing, maturing, or accelerated curing at higher temperatures between 60 °C and 70 °C.

## 2. Phenomenology of black garlic

The production of black garlic involves the ageing (curing) of entire bulbs at temperatures between 60 °C and 90 °C, under conditions of sufficient high humidity (between 70 % and 90 %). The changes that occur during the heat treatment of garlic are visible to the naked eye at a macroscopic level, as shown in Figure 3. In addition, the garlic cloves have shrunk considerably, indicating a corresponding loss of water. Even at this stage, the cloves are no longer crunchy but rather leathery (Choi et al., 2014). As the process continues, the loss of water decreases and the individual cloves have a rubbery, almost elastic texture. In the early stages, only a small proportion of the free water

evaporates; the remaining water is required for
the enzymatic conversion of alliin to allicin, as
shown in Figure 1. If the moisture content, and
thus the water activity $a_w$ becomes too low,
enzymatic reactions are no longer possible,
because most of the remaining water is largely
bound (Troller and Christian, 2012). However,
the colour changes continue, indicating
significant changes in taste and odour. A
glycation reaction is possible even at
temperatures around 70 °C if enzymatically
reactive precursors with sufficient water activity
are formed, as shown in more detail in section 3.
These are likely to react over time to form
corresponding brown and odourant substances
even at comparatively low temperatures. Choi *et
al.* (2024) have measured the colour intensity
accordingly.

<figure><img src="image_3.png" /><figcaption>Figure 3. The activity of the enzyme alliinase shows a maximum between 30 and 40 °C (Wang et al., 2011).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2. Change of colour of garlic during an experiment conducted over 35 days at a temperature of 70 °C under 90 % humidity (Choi et al., 2014).</figcaption></figure>

<figure><img src="image_5.png" /></figure>
<table><caption>Table 1. Water and acid content, pH and reducing sugar concentration during garlic ripening at 70 °C (data from Choi et al., 2014). Values indicated by different letters in the same row are significantly different by Duncan's multiple range test (p < 0.05).</caption><thead><tr><th rowspan="2">Component</th><th colspan="6">Time (d)</th></tr><tr><th>0</th><th>7</th><th>14</th><th>21</th><th>28</th><th>35</th></tr></thead><tbody><tr><td>Water content (%)</td><td>64.21 ±<br>1.48 a</td><td>32.72 ±<br>0.97 b</td><td>31.77 ±<br>2.60 b</td><td>31.12 ±<br>0.17 b,c</td><td>29.55 ±<br>0.39 c</td><td>29.88 ±<br>0.49 c</td></tr><tr><td>Acidity (mg/kg)</td><td>0.40 ± 0.01<br>e</td><td>1.30 ±<br>0.01 d,e</td><td>1.50 ±<br>0.02 c,d</td><td>1.70 ±<br>0.03 c</td><td>2.30 ±<br>0.06 b</td><td>2.60 ±<br>0.03 a</td></tr><tr><td>pH value</td><td>6.33 ± 0.07<br>e</td><td>5.49 ±<br>0.09 d,e</td><td>4.41 ±<br>0.17 c,d</td><td>4.22 ±<br>0.08</td><td>4.07 ±<br>0.02 b</td><td>3.74 ±<br>0.062 a</td></tr><tr><td>Reducing sugars<br>(g/kg)</td><td>1.52 ± 0.01<br>d</td><td>2.73 ±<br>0.32 c</td><td>12.42 ±<br>0.85 b</td><td>15.96 ±<br>0.29 a</td><td>15.98 ±<br>0.23 a</td><td>16.07 ±<br>0.38 a</td></tr></tbody></table>

The data from Choi *et al.* (2014) shown in Table 1 support these subjective perceptions. The water content decreases the most in the first 7 days; the pH value drops from values just above 6 to values below 4. The clearest differences can be seen in the content in reducing sugars, which increase by a factor of 10 during temperature induced curing. The reducing sugars are essentially mono-, di- and oligosaccharides, which contribute to the perception of sweetness, albeit with varying degrees of sweetness. Black garlic therefore dominates in the basic taste qualities of sweet and sour (Choi *et al.*, 2014).

The flavour of garlic changes significantly during the thermal process. Recently, the taste of black garlic has also been investigated using electronic tongues, in particular the sour, bitter and umami

tastes (Najman *et al.*, 2022). The results of a flavour comparison between fresh garlic (0 days) and thermally processed garlic (in this case 45 days at 70 °C and 80 % humidity) are shown in Figure 4. Electronic tongues work with well defined sensors, but the results are not always directly transferable to sensory experiments with (trained) panels (Najman *et al.*, 2022). For example, salty compounds are detected in fresh garlic, which have less to do with the salty taste and more to do with free cations dissolved in the cell plasma, such as sodium and calcium (Najman *et al.*, 2022). Nevertheless, the differences are visible and correspond to the impressions given by the analytical data in Table 1. The changes can be clearly assigned to sour, bitter, sweet and umami flavours (Najman *et al.*,

The data from Choi *et al.* (2014) shown in Table 1 support these subjective perceptions. The water content decreases the most in the first 7 days; the pH value drops from values just above 6 to values below 4. The clearest differences can be seen in the content in reducing sugars, which increase by a factor of 10 during temperature induced curing. The reducing sugars are essentially mono-, di- and oligosaccharides, which contribute to the perception of sweetness, albeit with varying degrees of sweetness. Black garlic therefore dominates in the basic taste qualities of sweet and sour (Choi *et al.*, 2014).

The flavour of garlic changes significantly during the thermal process. Recently, the taste of black garlic has also been investigated using electronic tongues, in particular the sour, bitter and umami
2022). The sour taste correlates with the changes in pH and acid concentration in Table 1, as does the increase in sweetness due to the increasing concentration of low molecular weight sugars, while the increase in umami flavour corresponds to protein hydrolysis. The pronounced bitter taste of fresh garlic is mainly due to the glycosinolates (Jones et al., 2004) and other sulphur compounds (Kubec et al., 2018), which react and degrade during garlic processing. The increase in acidity is reminiscent of lactic acid fermentation inside the gloves during the very first stage of processing, as indicated by the decrease in pH according to Table 1. These mainly phenomenological observations suggested a taste spider diagram to illustrate the pronounced differences between fresh and black garlic as shown in Figure 4.

# 3 Molecular aspects of flavour changes

## 3.1 Sugars and sweetness

Fresh garlic contains free, low molecular weight sugars with varying numbers of carbon atoms, such as arabinose, galactose, glucose, fructose, sucrose and maltose. There is also a high proportion of fructans, which is the main storage carbohydrate (Baumgartner et al., 2000) in garlic. Fructans consist of fructose residues, usually with a sucrose unit at what would otherwise be the reducing endpoint (Figure 5).

The repeating number *n* in Figure 5 defines the chain length (degree of polymerisation) and the molecular weight. It can take values between 10 and 200; in garlic, chain lengths around *n* = 9 dominate (Baumgartner et al., 2000). This means that both short, oligomeric structures and longer, polymerised chains can be present in fresh garlic. These polymerised fructan chains are too large to be recognised by sweet taste receptors and do not trigger a taste stimulus. However, if the fructan chains are cleaved during the thermal process due to prolonged exposure to temperature and sufficient humidity (Lu et al., 2018), more and more sugars are released, and the sweet taste increases. Chua et al. (2022) performed a

detailed analysis on mono- and disaccharides (Table 2). This study is limited to 12 days of garlic ripening at 75 °C and 85 % humidity, as the sugar content changes significantly only during the first 14 days (Table 2).

During the first 8 days, there is a sharp increase in fructose. After that, however, the fructose content drops sharply. Glucose, on the other hand, only increases after 10 days, then decreases slightly and remains at a high level of just under 20 %. The decrease in fructose content indicates participation in the glycation and amino-carbonyl reactions. It is also interesting to note that sucrose, which is present in practically all fructans, largely disappears. The explanation for this is in line with everyday experience: even if the sucrose is cleaved from the end group fructan, it is still split into glucose and fructose because the pH drops at the same time during the process. The high humidity prevents the water content from dropping too low, so in the acidic environment the sucrose forms inverted sugar, a mixture of glucose and fructose (Thavarajah and Low, 2006).

<figure><img src="image_6.png" /><figcaption>Figure 4. Schematic phenomenological taste to visualize the differences between fresh garlic (light grey) and thermally processed garlic (45 d / 70 °C, dark grey) shows significant differences (based on the descriptions by Jones et al., 2004; Kubec et al., 2018).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5. Two typical examples of (a) inulins and (b) garlic fructans are shown. The fructose-based repeating unit (monomer) is shown in green. The reducing end contains sucrose, at the other end closes with fructose (alpha-D-glucopyranosyl-[beta-D-fructofuranosyl](n-1)-D-fructofuranoside). Garlic contains other similarly structured polysaccharides that are not shown here (Jiang et al., 2022).</figcaption></figure>

<table><caption>Table 2. The development of glucose, fructose and sucrose (% per weight) during the first 12 days of temperature-induced garlic ripening (from Chua et al., 2022).</caption><thead><tr><th></th><th colspan="4">Time (d)</th></tr></thead><tbody><tr><th>Sugar (%)</th><td>0</td><td>8</td><td>10</td><td>12</td></tr><tr><th>Glucose</th><td>&lt; 0.1</td><td>1.5</td><td>23.3</td><td>19.8</td></tr><tr><th>Fructose</th><td>&lt; 0.1</td><td>20</td><td>&lt; 0.1</td><td>&lt; 0.1</td></tr><tr><th>Sucrose</th><td>&lt; 0.1</td><td>0.3</td><td>&lt; 0.1</td><td>&lt; 0.1</td></tr></tbody></table>

<figure><img src="image_8.png" /></figure>
Review

<table><caption>Table 3. The development of free amino acids over time during the thermal process (rounded values only, from Choi et al. 2014) recorded not all 20 amino acids. Values followed by different letters in the same row are significantly different by Duncan's multiple range test (p < 0.05); -: non detected.</caption><thead><tr><th rowspan="2">Amino acid (mg/100g)</th><th colspan="6">Time (d)</th></tr><tr><th>0</th><th>7</th><th>14</th><th>21</th><th>28</th><th>35</th></tr></thead><tbody><tr><td>Leucine</td><td>58.62 ± 3.37 b</td><td>73.44 ± 1.79 a</td><td>62.81 ± 1.51 b</td><td>62.12 ± 5.64 b</td><td>61.23 ± 4.32 b</td><td>59.19 ± 3.94 b</td></tr><tr><td>Isoleucine</td><td>50.04 ± 13.47 b</td><td>89.25 ± 18.93 a</td><td>86.45 ± 8.23 a</td><td>83.79 ± 2.37 a</td><td>79.44 ± 1.14 a</td><td>71.07 ± 2.25 a</td></tr><tr><td>Valine</td><td>47.74 ± 0.19 a</td><td>47.68 ± 0.64 a</td><td>36.71 ± 4.64 b</td><td>35.23 ± 5.61 b</td><td>34.73 ± 8.61 b</td><td>33.91 ± 6.61 b</td></tr><tr><td>Methionine</td><td>31.56 ± 1.40 b</td><td>82.51 ± 4.70 a</td><td>80.73 ± 6.37 a</td><td>78.11 ± 2.33 a</td><td>73.59 ± 9.71 a</td><td>71.11 ± 3.55 a</td></tr><tr><td>Cysteine</td><td>81.06 ± 0.95 a</td><td>69.43 ± 0.94 b</td><td>49.20 ± 5.42 c</td><td>46.90 ± 8.98 c</td><td>43.44 ± 3.38 c</td><td>42.14 ± 7.18 c</td></tr><tr><td>Phenylalanine</td><td>55.64 ± 0.74 c</td><td>82.38 ± 8.35 b</td><td>70.20 ± 4.41 b</td><td>135.16 ± 7.10 a</td><td>136.25 ± 12.76 a</td><td>143.07 ± 6.32 a</td></tr><tr><td>Tyrosine</td><td>449.95 ± 6.29 a</td><td>109.13 ± 26.09 b</td><td>102.33 ± 0.38 b</td><td>82.28 ± 7.41 b</td><td>78.35 ± 4.34 b</td><td>77.31 ± 7.54 b</td></tr><tr><td>Aspartic acid</td><td>90.12 ± 2.55 b</td><td>117.50 ± 9.07 a</td><td>64.53 ± 5.84 c</td><td>62.43 ± 4.34 c</td><td>61.65 ± 9.12 c</td><td>60.19 ± 8.16 c</td></tr><tr><td>Glutamic acid</td><td>286.60 ± 8.09 a</td><td>128.87 ± 9.09 b</td><td>112.81 ± 3.02 b</td><td>108.11 ± 9.12 b</td><td>101.88 ± 7.71 b</td><td>100.11 ± 6.09 b</td></tr><tr><td>Arginine</td><td>409.05 ± 3.00 a</td><td>340.20 ± 75.68 b</td><td>119.30 ± 0.52 d</td><td>208.71 ± 51.36 c</td><td>71.92 ± 0.65 d,e</td><td>40.34 ± 1.10 e</td></tr><tr><td>Lysine</td><td>61.68 ± 5.29 a</td><td>57.96 ± 4.93 a</td><td>47.39 ± 2.24 b</td><td>42.50 ± 2.27 b</td><td>41.53 ± 3.67 b</td><td>40.50 ± 9.22 b</td></tr><tr><td>Histidine</td><td>89.12 ± 1.88 b</td><td>191.69 ± 18.89 a</td><td>58.76 ± 13.02 c</td><td>58.76 ± 13.02 c</td><td>57.62 ± 5.51 c</td><td>57.89 ± 8.34 c</td></tr><tr><td>Threonine</td><td>81.25 ± 13.59 a</td><td>46.30 ± 0.87 b</td><td>53.53 ± 8.36 b</td><td>57.54 ± 2.22 b</td><td>58.36 ± 11.17 b</td><td>59.36 ± 5.22 b</td></tr><tr><td>Serine</td><td>38.53 ± 0.82 a</td><td>25.72 ± 0.87 b</td><td>25.84 ± 0.33 b</td><td>24.78 ± 1.12 b</td><td>24.78 ± 1.12 b</td><td>23.71 ± 5.41 b</td></tr><tr><td>Glycine</td><td>21.50 ± 2.51 b</td><td>37.41 ± 8.54 a</td><td>9.60 ± 1.78 c</td><td>8.63 ± 1.18 c</td><td>-</td><td>-</td></tr><tr><td>Alanine</td><td>89.72 ± 11.44 b</td><td>239.13 ± 17.84 a</td><td>67.01 ± 2.39 c</td><td>83.59 ± 5.71 b</td><td>47.38 ± 1.74 d</td><td>32.74 ± 5.37 d</td></tr></tbody></table>

<caption>Table 3. The development of free amino acids over time during the thermal process (rounded values only, from Choi et al. 2014) recorded not all 20 amino acids. Values followed by different letters in the same row are significantly different by Duncan's multiple range test (p < 0.05); -: non detected.</caption>
Review

A detailed analysis of free amino acids provides clues to the umami flavour of black garlic. A detailed analysis of the amino acid spectrum is necessary because the main trigger for the umami flavour is free glutamic acid, which is present to a greater or lesser extent in most proteins. Choi et al. (2014) found considerable differences in the temporal changes of free amino acids, which are summarised in Table 3. For reasons of readability, only rounded values without statistical and systematic deviations are given here, which can be viewed in the original publication if interested. The table, which at first glance appears confusing, nevertheless provides some useful information on flavour formation.

For example, it is noticeable that the concentrations of some free amino acids such as leucine, isoleucine, aspartic acid, histidine and alanine initially increase over time, but then decrease again. Proteins are partially broken down in the first few days, and amino acids are released in the process.

The decrease in these amino acids over time indicates reactions to aminocarbony products, which leads the odour of black garlic. Other amino acids, such as methionine, increase and remain at a high level. This is surprising at first glance, as this amino acid is known to react to form the odourant methional (Methven et al., 2007) also known as 3-(methylsulfanyl)propanal (sulphurous, cooked vegetables, cooked eggs). The water required for this reaction is lacking for periods of more than 7 days, and the temperatures are too low for strong methional formation.

Glutamic acid, which triggers umami taste, also decreases significantly over time, which would contradict the increase in umami taste shown in Figure 4. On the other hand, it is possible that glutamic acid (1) is converted into pyroglutamic acid (2), also known as 5-oxoproline, under certain conditions, as outlined in Figure 6.

Pyroglutamic acid contributes strongly to the umami taste. For instance, pyroglutamic acid has been found as a taste component in long cooked meat broths (Schieberle et al., 1998), which is attributed to the umami taste (Zhang et al., 2017). It is also found in long fermented products such

as fish sauces (Park et al., 2002). In particular, the presence of heat-resistant (thermophilic) lactobacilli seems to favour the formation of pyroglutamic acid. This has been studied in cheese ripening and provoked with appropriate microorganisms (Mucchetti et al., 2002). It was also confirmed for the classic lactofermentation of kimchi (Eom et al., 2023): when thermophilic lactobacilli are inoculated, more polyglutamic acid is produced. It is also known for its salty, umami and sour taste (Gazme et al., 2019). High concentrations of polyglutamic acid can also be clearly detected in black garlic (Liang et al., 2015).

The molecular mechanisms of salty and umami tastes through the activation of individual taste receptors has also been confirmed (Frerot and Chen, 2013; Yao and Udenigwe, 2018). These examples already show the importance of pyroglutamic acid for thermal and fermentative processes, especially when they are prolonged.

Thus, as Table 3 suggests, the strong degradation of glutamic acid is not a mandatory criterion for the final umami flavour of black garlic. Although glutamic acid, the umami tastant, is usually formed by proteolysis during the fermentation process, it can also be formed enzymatically (by glutaminase activities) from glutamine by certain lacatobacilli (Zhao et al., 2016).

Almost all fermentation processes produce the umami aromatic pyroglutamic acid as well as certain tripeptides that may contain pyroglutamic acid, proline and another hydrophobic amino acid. Interestingly, even threshold concentrations, i.e. individual components of various pyroglutamyl peptides and precursors such as

<figure><img src="image_9.png" /><figcaption>Figure 6. The conversion of glutamic acid (1) to pyroglutamic acid (5-oxoproline) (2).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 7. Amadori and Heyns rearrangements between a fructose and any amino group -NH₂, which can be part of a protein or peptide, described by the residue R (after Ríos-Ríos et al., 2019).</figcaption></figure>

Amadori products, enhance the umami taste of soy sauce (Kaneko et al., 2011).

### 3.3. Amadori-Heyns rearrangement and glycation

Black garlic is produced neither by fermentation nor by a pure non-enzymatic browning reaction, but by a mixed form during the different stages of thermal treatment. At the relatively high temperatures between 60 °C and 90 °C, allinases are partially inactivated, as shown in Figure 2. On the other hand, the water content decreases and the cell walls become unstable (Wei and Lintilhac, 2003). Protein-cleaving enzymes, proteases, stored there are released and cleave the proteins into peptides of different lengths at a very early stage, as the simultaneously high humidity prevents the water activity from decreasing. Over time, the proteases are also inactivated, while the

fructan are thermally cleaved. The free fructose that is formed (Baumgartner et al., 2000) can react with proteins and peptides to form Amadori and Heyns products, which play a central role in flavour formation both during heating (frying) and

<figure><img src="image_11.png" /><figcaption>Figure 8. Time course of the formation of Amadori and Heyn products at 55 °C and 80 % humidity (Yuan et al., 2018).</figcaption></figure>
around 3. The acidic flavour is based on acetic acid, succinic acid, formic acid, fumaric acid and its isomer maleic acid in different concentration ratios (Chua *et al.*, 2022).

It should also be remembered that pyroglutamic acid, as mentioned above, stimulates both umami and acid receptors and contributes to the sour taste.

<figure><img src="image_12.png" /><figcaption>Figure 10. The structure of γ-glutamyl cysteine. The hydrogen sulphide group of cysteine is highlighted in yellow and the carboxylic acid group COOH of glutamic acid is highlighted/framed in red.</figcaption></figure>

### 3.5 Mouth fullness (kokumi)

Mouth fullness (kokumi) is associated with a strong development of the umami taste, e.g. during fermentation (soya and fish sauces), ripening, maturing, curing (cheese, ham), or prolonged cooking (Tolstede *et al.*, 2009; Kuroda *et al.*, 2013; Araya-Morice *et al.*, 2021; Heres *et al.*, 2023; Li *et al.*, 2023).

This sensation is caused by γ-glutamyl peptides, the best known of which is glutathione, GSH, γ-L-glutamylcysteinylglycine, a tripeptide consisting of the three amino acids glutamic acid, cysteine and glycine, which is found in most cells (Li *et al.*, 2022; Dutta *et al.*, 2022; Forde and Stieger, 2024). However, the kokumi sensation itself is not a taste perception, because mouth fullness is not perceived via the known taste receptors, but via special calcium channels, the so-called calcium-sensitive receptors (Maruyama *et al.*, 2021).

Liu *et al.* (2022) showed that a precursor of glutathione, namely, γ-glutamyl cysteine (Figure 10), is mainly responsible for the kokumi sensation in black garlic.

However, first, the two amino acids must be linked and second the proteinogenic α-bond enzymatically transformed into an α-bond, by the two enzymes γ-glutamyl cysteine synthetase and γ-glutamyl transpeptidase. Therefore, kokumi intensity in black garlic depends strongly on the rate of these enzymatic processes during the process temperature and time. The maturation of garlic at high temperatures will therefore contain less kokumi than at low temperatures, such as the 50 °C just mentioned, at which the enzymes are still active for some time. Lui *et al.* (2022) have determined the temperature-time-dependent activity of the relevant enzymes and showed that at 50 °C their activity drops within the first 8 to 10 hours, where enzymes γ-glutamyl cysteine synthetase, showed the largest resistance with highest activity within 12 to 14 hours. Moreover, it seems that the activity of the enzymes is reduced by half between 5 and 6 h. After 18 h, no significant activity remains for all relevant enzymes. At higher temperatures, the decrease in activity is significantly faster. However, the systematically higher activity of γ-glutamyl cysteine synthetase compared to the other enzymes is striking. This underlines the formation of significant concentrations of the kokumi-stimulating γ-glutamylpeptides, provided the process temperature is not much higher than 50 °C. As an important result, this means that the processes that are relevant to kokumi take place at lower temperatures and in the early stages of garlic processing.

### 3.6. Formation of odorants

A striking feature of the odorant spectrum of black garlic is a deep, caramel-like flavour, which in most cases can be attributed to furans as reaction products from fructose (Leng *et al.*, 2020). For example, 5-hydroxymethylfurfural (1), often abbreviated as 5-HMF, whose odour attributes are caramel-like, buttery, musty and reminiscent of hay and tobacco in the mouth. In
Review

<figure><img src="image_13.png" /><figcaption>Figure 11. Most important part of the aroma spectrum of black garlic (Vierich and Vilgis, 2021). The numbers under the structures of the most odour-active odorants correspond to the numbering in the text.</figcaption></figure>

addition to 5-HMF, furaneol (2), 2(5H)-furanone (3) and 5-heptyldihydro-2(3H)-furanone (4) stand out among the caramel flavours with high odour activity. They add odours ranging from dried fruit, caramelised fruit to burnt odours, such as those that occur when roasting onions and indicate the origin of fructose, too. Roasty odours are also formed, which are caused by 3-vinyl-1,2-dithiacyclohex-4-ene (5) and 2-vinyl-4H-1,3-dithiin (6) and 3H-1,2-dithiol (7), i.e. heterocyclic sulphur compounds that are reminiscent of sauces, roasted meat and rotten eggs. Similarly, 2-acetyl-1-pyrroline (8) adds the odour of fried rice and popcorn, and γ-butyrolactone (9) contributes a creamy impression of caramelised, burned milk.

The sulphurous character of black garlic is enhanced by linear sulphurous odorants such as diallyl trisulphide (10), allyl ditrisulphide (11) and dimethyl sulphoxide (12), with odour attributes

such as cooked garlic, boiled potatoes and boiled eggs.

Sulphurous odorant compounds are of course also characteristic of black garlic, albeit much less pungent. These compounds are also widespread in allium vegetables such as garlic, onions, leeks, etc.

Alliin and its derivatives (isoalliin, methiin and propiin) are the most important precursors of these sulphur-containing compounds (Vierich and Vilgis, 2021). Alliin and its derivatives are cleaved by alliinase to form allicin, which in turn is a precursor of allicin. Alliin and its derivatives are cleaved by alliinase to allicin, which is rapidly converted to all types of sulfur compounds, including diallyl disulfide, diallyl trisulfide and diallyl sulfide. These compounds are responsible for the strong, often painfully cold and tear-irritating odour of fresh garlic. Compared to fresh garlic, this strong odour, accompanied by

<figure><img src="image_14.png" /></figure>
trigeminal stimulation, is much less pronounced in
black garlic due to the much lower concentrations
of diallyl disulfide and diallyl trisulfide.

Yang et al. (2019) found a high odour activity of diallyl disulfid in black garlic. The equally high odour activity of allylmethyltrisulfide also contributes to the "fresh", sulphurous aroma spectrum of black garlic. Fresh garlic contains an average of 13 % proteins and 75 % carbohydrates (mostly fructans). These two components form browning substances during the glycation reaction at a later stage, the garlic discolours and forms typical glycation reaction products during ripening (Najman et al., 2022; Kilic-Buyukkurt et al., 2023). Thus, heterocyclic compounds such as thiophenes, alkylpyrazines, furanones and furans are identified in black garlic, which are formed by the glycation during ripening in the later stages. Furaneol and other furans are derived from sugars via Amadori intermediates (Schwab, 2013). In fresh garlic, as mentioned above, fructan has a high proportion as a storage carbohydrate, but free, low-molecular sugars with different numbers of carbon atoms, such as arabinose, galactose, glucose, fructose, sucrose and maltose. From these, caramelisation reactions take place, which lead to various furans and furaneol.

The amino-carbonyl reaction can be followed
directly using examples: 2-acetylfuran, which
smells of roasted nuts, is formed by cyclisation of
1,4-dideoxyosone through dehydration from
glucose and glycine. The odour-intensive
compound 2(5H)-furanone is also formed by
glycation. 3-(Methylthio)-propioaldehyde, which
smells like boiled potatoes, is also a typical
product, as it is formed from the amino acid
phenylalanine by Strecker degradation (Gijs et al.,
2000) and also plays an important role in malting
and during the brewing process in beer, for
example.

The organic acids responsible for the sour taste
are formed via both glycation reactions and lipid
oxidation (Martínez-Casas et al., 2017). Acetic
acid and propionic acid are formed from hexoses
or pentoses during thermal processing by further
cleavage (Yang et al., 2019). In addition, lipid

oxidation in combination with the glycation and
amino-carbonyl reactions contributes to the
overall flavour of black garlic by providing
intermediates from different reactions. The
structure of the compound 5-
heptyldihydro-2(3H)-furanone is reminiscent of a
lactone (see (9) for the γ-butyrolactone formed
here), and is therefore a odorant compound
derived from a saturated fatty acid. The typical
cucumber odor (E,Z)-2,6-nonadien-1-ol, on the
other hand, is oxidised directly from unsaturated
C18 fatty acids.

For a better understanding, it is important to
know the origin of the odourants. The most
odour-active compounds are summarized in
Figure 11. They have been included in the
flavour scheme developed by the author to
provide a better overview. Obviously, fresh garlic
occupies the same aroma groups, but with a
significantly different weighting. This shows that
no new odorant types, such as esters, terpenes,
or aromatics are formed during garlic maturation
at temperature, with the exception of the process
induced odorants due to caramelisation and the
glycation.

Most of the compounds formed by the glycation
reaction contribute to the distinct roasted flavour
of black garlic. Further improvement of the
flavour could therefore be achieved by
enhancing the glycation, e.g. through a selective
temperature profile during ripening. This is
discussed further below.

3.7. Influence of temperature, humidity, and
time

Temperature, humidity and process time are the
main factors that determine the sensory
properties of the black garlic. This can already
be observed in the formation of the caramel
odour 5-HMF. Leng et al. (2020) presented
experiments on this, which are summarised in
Figure 13. Relatively short process times of up to
15 days were defined, but samples were taken
every other day to determine the 5-HMF
concentration. Maturation was analysed at

<figure><img src="image_15.png" /></figure>
Review

<figure><img src="image_16.png" /><figcaption>Figure 12. The residual moisture (a) and the total concentration of free amino acids (b) as a function of the process time (from Zhang et al., 2016).</figcaption></figure>

process temperatures of 50 °C to 90 °C, and the humidity was varied from 30 % to 70 %. Leng et al. (2020) found that at high process temperatures, the formation of the concentration of 5-HMF increases sharply between two and four days and also reaches its highest values. At low temperatures, the concentration remains lower and the increase is less. High humidity also favours rapid formation of the odorant compounds. However, it is noticeable that the concentration of 5-HMF decreases again at high temperatures and higher humidity, as the compound can continue to react.

Zhang et al. (2016) have performed a deeper and more thorough investigation, but limited themselves to the process temperature, albeit with process times of up to 72 days. Unfortunately, the air humidity was not controlled during these experiments, as the experiments were carried out in a drying oven and no further details of the prevailing humidity inside were reported. Nevertheless, especially as they confirm the results already mentioned. However, their results can used to optimise black garlic processing for better culinary solutions, which can be seen during the following discussion.

Figure 12 shows the residual moisture in the garlic and the number of free amino acids at the

respective process time. At high temperatures of 90 °C or 80 °C, the residual moisture content drops by a factor of almost 2 in the first 12 or 21 days respectively. The garlic cloves quickly acquire a leathery to tough consistency.

At the same time, the concentration of free amino acids decreases rapidly during this time. They react relatively quickly to form aromatic substances with the risk of an unbalanced flavour. At 70 °C, both the residual moisture and the free amino acids concentrations decrease to a lesser extent. Even after 30 days, the residual moisture is still high enough to guarantee a pleasantly pasty, still not too leathery texture. At a process temperature of 60 °C, there is still enough residual moisture after 72 days to ensure a pleasant texture. The amino acid concentration also remains constant between 15 and 72 days. The low temperature reduces the glycation, resulting in less roasted and bitter products, while the remaining amino acids support the flavour with their balanced sweet, sour and umami taste.

The progression of the colour change during browning over time, objectively measured by colorimetry (Anzalone et al., 2013), and the associated concentration of the caramel ingredient and indicator 5-hydroxymethylfurfural
International Journal of Molecular and Physical Gastronomy

Review

<figure><img src="image_17.png" /><figcaption>Figure 14. A possible temperature (red curve) and humidity profile (blue curve) for black garlic in five zones: I: activation of enzymes, II: promotion of enzymatic processes, III: glycation zone, promotion of non-enzymatic browning reaction, IV: cooling phase with stopping of reactions, V: drying phase for safe storage and longer shelf life.</figcaption></figure>

black garlic can be controlled much more precisely than if temperature and humidity are kept constant for weeks.

At low temperatures between 50 °C and 60 °C, allinases and other enzymes are responsible for breaking down the pungent and bitter flavour components. These must first be activated, which happens at temperatures around 50 °C. However, this requires more time and it is necessary to keep the garlic at this temperature for a certain amount of time to allow sufficient time for these processes to take place. Heating the garlic too quickly would quickly inactivate the enzymes. At the same time, the humidity must be kept high during this phase, as drying out reduces the water activity of the garlic and restricts the enzyme activity.

Therefore, two temperature levels are required in the first phase, which lasts around 5 to 7 days: around 50 °C for activation and just under 60 °C for the enzymatic processes, as well as high

humidity. At temperatures around 60 °C and higher humidity, however, the fructans also begin to decompose.

The number of reducing sugars increases, which is intrinsic to the rapid formation of Amadori and Heyns products as precursors for flavour and aroma formation. Higher temperatures are favourable for the glycation reactions. The subsequent glycation zone can therefore be extended at 80 °C over a longer period of time. During this time, the caramel flavours and glycation products are formed. Even browning (blackening) is also complete. These chemical reactions also require a high level of humidity, which can be up to 95 %. The flavour yield is then also possible with a shorter total ripening time despite relatively low temperatures for good flavour formation. Excessively long process times are counterproductive anyway, as indicated by the experiments of Zhang et al. (2016).
Figure 14 summarises these conclusive considerations. However, the times and temperatures shown in this figure do not necessarily have to be adhered to. This is merely an illustrative example of possible flavour optimisations. The drying phase V could be omitted when the black garlic does not to be stored for a long time. The advantage of such a staggered process control was already shown in the experiments of Liu et al. (2022). The concentration of the kokumi relevant peptides increased by employing lower initial activation temperatures and enzymatic processes for the enzymes during phases I and II.

As suggested from the results summarized in Figures 12 and 13, the variation of temperatures may have strong impact of the variation and composition of aroma compounds. Lower temperatures at the glycation phase III may pronounce more caramel type aroma compounds, higher temperatures and times could strengthen the savoury flavours. These experiments could be conducted by varying the humidity also, since quite some influence on the aroma formation is expected.

Further analytic research certainly helps to understand the processes in more details. Moreover, the processes described here are universal and can be applied straight forward to other bulbous plants from the onion family (Alliaceae), as has been shown with onions recently (Moreno-Ortega et al., 2020).

**Conflict of Interest**

The author declares no conflicts.

**Funding**

This review work is not part of any funding.

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**Edited by:**

Anonymous

**Reviewers:**

1. Anonymous

2. Christophe Lavelle, Muséum national d'histoire naturelle, Paris (France)

**Received:**

28 August 2024

**Accepted :**

14 April 2025

**Published:**

8 June 2024.

<figure><img src="image_18.png" /></figure>
International Journal of Molecular and Physical Gastronomy

Review

**Section:**

This article was published in the “Reviews” section of the *International Journal of Molecular and Physical Gastronomy*.

**Citation:**

Vilgis TA. 2024. Flavour in black garlic: a comprehensive review, *International Journal of Molecular and Physical Gastronomy*, 11(1), 7, 1-21. doi: 10.17180/ijmpg-2024-art07.