Key Aromatic Volatile Compounds from Roasted Cocoa Beans, Cocoa Liquor, and Chocolate

: The characteristic aromas at each stage of chocolate processing change in quantity and quality depending on the cocoa variety, the chemical composition of the beans, the specific protein storage content, and the polysaccharides and polyphenols determining the type and quantity of the precursors formed during the fermentation and drying process, leading to the formation of specific chocolate aromas in the subsequent roasting and conching processes. Bean aroma is frequently pro-filed, identified, and semiquantified by headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPMEGC-MS) and by gas chromatography olfactometry (GC-O). In general, the flavors generated in chocolate processing include fruity, floral, chocolate, woody, caramel, earthy, and undesirable notes. Each processing stage contributes to or depletes the aroma compounds that may be desirable or undesirable, as discussed in this report.


Introduction
The volatile aromatic components of cocoa beans vary depending on whether the beans are dry, roasted, or processed in cocoa liquor or chocolate.Different concentrations of the most relevant volatile components in a particular variety (criollo, forastero, and trinitario) rely on geographical origin, the chemical composition of the beans, the postharvest techniques used, such as fermentation and drying, and industrial processes like roasting and conching [1].Regarding the varieties and genotypes, criollo, trinitario, and national cocoa have higher fine aroma concentrations as fruit (fresh and ripe), floral, herbaceous, wood, nuts, and caramel notes [2][3][4][5]; forastero cocoa on the other hand (considered bulk grade), possess higher concentrations of the predominant aromas of malt, honey, roasted, caramel, cocoa, and chocolate [6], as well as low acidic and alcoholic notes [7].Finally, the high yielding CCN51 cocoa genotype (Colección Castro Naranjal) grown in Ecuador (classified as bulk grade), has volatiles that exude a sweet and fruity or fresh character to the beans [5].
The unique aromatic profiles and diversity of aromas are linked to the number of components exhibited in the beans; the greater the number of components, the more complex the overall aroma of a specific cocoa sample.For instance, the aromatic profiles of criollo and trinitario cocoa from Mexico comprise 46 and 47 of the most relevant volatile components, respectively [8].Likewise, 69 of the most important volatile components represent Ecuador's national cocoa [5], with 67 to 89% being aromatically desirable components, and the remaining fraction (11 to 33%) exudes unpleasant aromas that may originate during fermentation and drying [9][10][11][12].The main cause of unpleasant aromas comes from excessive fermentation that originates compounds that impart smoky and ham aromas, which have significantly high concentrations that affect the final quality of the beans.The suggested limits for some unpalatable volatile components in fermented beans are 2 µg/kg for 3-ethylphenol (smoked) and 3-propylphenol (smoked, phenolic), 12.20 µg/kg for 4-methylphenol (fecal), 3-methylphenol (smoked) and 4-ethylphenol (smoked), and 70 µg/kg for 2-methoxyphenol [13].If the drying process is performed slowly, there is a loss of volatile acids and water [14].This is because the porosity of the husk enclosing the cotyledons increases [15], facilitating volatilization and implying that the dried beans are less acidic and have a higher pH, providing their aromatic potential [14].As the moisture level decreases, the level of cocoa butter accumulates.Cocoa butter is the most abundant component in dried cocoa beans [16], accounting for half of the components.This butter fraction may include fatty acids (myristic, palmitic, palmitoleic, stearic, oleic, linoleic, and arachidic) and triacylglycerols (composed of about 42% of 1-palmitate-2-oleate-3-stearate triacylglycerol, 24% of 1,3-diestearate-2-oleate triacylglycerol, and 22% of 1,3dipalmitate-2-oleate triacylglycerol), as the major components [17,18] have a serious impact on the perception of the chocolate flavor and aroma.The release of chocolate aromas during tasting is linked to lipophilic volatile compounds, mainly alkylpyrazines, which are considered to be more complex and more substituted with the alkyl group, therefore having higher lipophilicity [19].The higher degree of saturation and hydrophobicity of cocoa butter may be associated with decreased pyrazine release [17].
Most aromatic volatile compounds are released during the roasting process.The number of volatile compounds varies with roasting intensity; therefore, a greater amount is formed when roasting at higher temperatures [20].Several studies detail the diversity of these compounds within the temperature ranges of 95 to 160 °C [6,9,21,22].The most volatile compounds found in roasted beans correspond to esters, followed by acids, alcohols, aldehydes, and ketones, which are characteristic of a given cocoa variety, and depending on the roasting time and temperature, other compounds, such as pyrroles, pyrazines, and Strecker aldehydes, are reduced or generated [6,9,21,22].Higher roasting temperatures may generate unpleasant compounds with burnt or smoky notes and a dark brown color in processed chocolate.In the next process in which the roasted beans are ground, the cocoa liquor has particles measuring between 22-26.5 µm [9,23], driving the extra release of aroma compounds.Similarly, during conching, the removal of the remaining moisture and volatile unpleasant acetic acid compounds occurs, acquiring acceptable levels [24].Prolonged conching (6-10 h at 80 °C) results in the loss of both undesirable volatile flavorings and other flavorings that provide desirable characteristic aromas like sweet, fruity, floral, and chocolate [25].It has also been reported that the aromatic volatile compound profiles of dark chocolates are strongly affected by the brand-related formulation and processing conditions.In some cases, differences within the same brand have even been shown [26].
Chocolate production generally involves a combination of fermentation, drying, roasting, and conching processes [25]; thus, it is crucial to investigate how these processes impact the levels and types of compounds that determine the chocolate aroma and its origin.Therefore, this review provides information on more relevant volatile organic compounds arising from postharvest and bean manufacturing methods to their implication in the final quality of the chocolate.In addition, the active odor components in dried beans, roasted beans, liquor, and chocolate are identified and traced with emphasis on the most desirable compounds.

Extraction and Quantification of Volatile Cocoa Compounds
Several studies have been published on the aromatic components contained in cocoa beans, especially the forastero, national, criollo, and trinitario varieties.Forastero cocoa represents about 85% of world production [27].It is characterized by producing seeds that, after being fermented, generate volatile floral and sweet aromas [28,29].The national cocoa grown in Ecuador classified as "fine and aroma grade", produces seeds with fruity, green, and woody volatile aromas.This variety represents 63% of world production [28,30].On the other hand, Criollo cocoa represents only 5% [27] and its fermented beans are associated with floral, fruity, and woody, volatile aromas [28].Finally, trinitario cocoa is a criollo hybrid variety and has strong basic chocolate characteristics together with fruity and floral aromas [28].The characterization and comparison of the seeds in the different varieties are complex since the list of representative genotypes in all cocoa-producing countries is extensive [29,[31][32][33], making the standardization of the preharvest, postharvest, and cocoa manufacturing processes derivatives since each variety has a unique flavor and aroma potential [34].
Cocoa aromas and flavors are detailed through several volatile aroma profiles and samples, such as fermented and roasted dry beans, cocoa liquor, and chocolate.Although there are several methods of characterization, the two most common are detailed.On the one hand, the analysis of volatile compounds is carried out by headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPMEGC-MS) [4,11,35,36].
The use of divinylbenzene carboxene polydimethylsiloxane (DVB/CAR/PDMS) fiber has been reported to define the organic volatiles in cocoa [8,12,21,22,28,[37][38][39].Optimal fiber coating is related to the number of peaks in the chromatogram and generated intensity.When using CAR-PDMS, up to 100 peaks have been detected more intensely, while in PDMS-DVB, the number of peaks is less than 75 [40], implying that a higher number of volatiles are extracted from a specific cocoa matrix.
In contrast, olfactometry, which collects data that are obtained from gas chromatography olfactometry (GC-O), helps to calculate to what extent a certain compound influences the overall aroma depending on its dilution factor (DF), [6,10,[41][42][43]; it detects odors and describes them by means of an "aromagram"-a representation of the dilution factor logarithm versus retention time [10,42,43].In order to illustrate this, [6] reported the dilution factors of roasted and unroasted forastero cocoa beans.In Table 1, 2-and 3-methylbutanoic acid (FD 8192; sweaty), acetic acid (FD 2048; acidic), and 3-hydroxy-4,5-dimethyl-2 (5H)-furanone (FD 1024; spiced) were detected at the highest dilutions, suggesting that these compounds are among those contributing to the aroma of unroasted cocoa beans.Both techniques focus on cocoa matrix aroma profile characterization and the evaluation of the most relevant compounds.Volatile profiles can be estimated by the odor activity value (OAV) of a compound present in a specific cocoa matrix.The OAV is defined as the ratio between the compound concentration and the odor threshold value (OTV), for which the contribution to the overall aroma of the cocoa matrix is to be evaluated.The OTV found in the literature determined in a fatty medium (sunflower oil) is a reference for estimating the OTV in cocoa beans, for which the fat content is between 53.3 to 55 % in dry fermented beans.If the concentration of a volatile compound is higher than its respective odor threshold and its OAV > 1, this compound contributes to the overall aroma and flavor of the cocoa matrix.[9,12,[44][45][46][9], determined that the concentration of 3methylbutanal (provides malty, cocoa and chocolate notes) in cocoa liquor was 721.44 ng/g, much higher than its OUV: 5.4-80 ng/g.This compound clearly contributes to the overall aroma of cocoa liquor, with an OAV from 9.02 to 133.60.On the other hand, [12] reported OAV = 0.55 for linalool (floral notes) in the chocolate processed from cocoa liquor of the national variety; its concentration was 20.2 ng/g and its OTV = 37 ng/g, meaning this compound does not contribute to the final chocolate aroma.The volatile profiles of other matrixes studied are detailed in Tables 2-6.

Flavor Precursors in Cocoa Beans
Aromas and flavors are generated during fermentation.This is a postharvest process carried out by various micro-organisms (yeasts, lactic acid bacteria, and acetic acid bacteria) generating flavor and aroma precursors, like the reducing sugars (glucose and fructose) formed by the action of cotyledon invertase and free amino acids release by carboxypeptidase (optimum pH 5.6) and aspartic protease (optimum pH 3.5), for which, later, during the drying and roasting of the beans, is combined in Maillard reactions to produce a high potential of aromas of different chemical classes [5,[47][48][49][50][51][52].According to the cut test method, the cotyledons of well-fermented and dry beans from a set of 100 have at least 60% brown beans and a smaller percentage of slaty and defective beans [49,50].For example, [21] reported that the percentage of forastero brown beans ranged from 75.33 to 84%.Further, the fermentation index (FI) estimates the fermentation quality and provides a fermented bean final state analytical evaluation.According to [53], well-fermented beans have a (FI) ≥ 1. [21] reported a FI from 1.04 to 1.22 in preconditioned and fermented beans.Likewise, [54] reported values from 0.37 to 1.05 in beans fermented from 0 to 6 days, respectively.The FI correlates with the amount of reducing sugars, free amino acids, pH, and cocoa bean cotyledon color [55].Therefore, the evaluation of precursors formed during fermentation is crucial to determine the quality of cocoa since well-fermented beans have a higher amount of precursors [56].
Regarding sugar reduction, a 78% and 93.5% increase at the end of fermentation was reported for trinitario and forastero cocoa beans, respectively [5,21].Similarly, [16] reported a 60% to 70% increase for a forastero cocoa hybrid in Ghana.The difference is due to the postharvest process in which the cob is stored in order to reduce the moisture and the amount of pulp adhering to the beans; this method reduces the concentration of fermentable sugars and, therefore, the acidity at the end of fermentation.Additionally, the increment in reducing sugars is related to the method and fermentation duration, where the greatest amount of fructose is produced with respect to glucose [5,21,47].A fructoseto-glucose ratio of 2:1 is expected when using the heap and tray fermentation method, 4:1 when the pods have been preconditioned and fermented in heap [5,57], and 4:1 when the pods have been preconditioned and fermented in heap [21].This ratio gives an indication of fermentation quality in terms of the reducing sugars in cocoa varieties [58].
Likewise, the proteolysis of cocoa globulin (similar to vicilin) (storage protein of 566 amino acids) leads to the breakdown and abundance of free amino acids from 300 to 800 di-and tripeptide units, highlighting acidic, hydrophobic, basic, etc. [59,60].[57] reported that hydrophobic amino acids comprised 68-73% of the total amount of amino acids, with leucine, phenylalanine, and alanine being the most abundant in dried forastero cocoa beans.Similarly, [56] indicated that leucine, phenylalanine, valine, alanine, and isoleucine were the predominant hydrophobic amino acids in dried criollo cocoa beans, comprising 56 to 70% of the total amino acids, while [61] quantified 50% of leucine and phenylalanine in fermented and dried forastero cocoa beans.When roasting the beans, this hydrophobic amino acid fraction partially descends through Maillard reactions, increasing the pyrazine levels [52].In beans, these hydrophobic amino acid fractions partially decrease via Maillard reactions, increasing the pyrazine levels [62].Therefore, the level of pyrazines can be used to evaluate the efficiency of roasting during this stage because of its pronounced influence on the cocoa's final aroma [56].
The abundance of peptides depends on geographical origin.[63], and [64], reported that high levels of oligopeptides and amino acids are found in cocoa bean samples from Central America, Caribbean Islands, Mexico, Santo Domingo, and Peru, together with samples from Papua New Guinea and the criollo variety from Mexico; in contrast, the samples from Flores, Sulawesi, Malaysia, and Ivory Coast showed low peptide concentrations and free amino acids.The difference in peptide concentration in relation to the origin of the cocoa is due to the nitrogen fertilization of the plants and mainly the pH found in the soil [65].Furthermore, the season in which the pods are harvested particularly affects the pH and nitrogen concentration of the beans.[66] reported that the pods harvested in summer had beans with a lower pH than those harvested in winter; likewise, the beans harvested in the dry season had a higher concentration of nitrogen than the ripe beans during the rainy season.

Volatile Compounds Generated from Precursors
Table 2 indicates the number of compounds in the different chemical groups and cocoa matrixes.It is noted that the availability of the compounds increases as the beans are turned into chocolate.Overall, acids, alcohol, aldehydes, aldehydes, ketones, and esters are present throughout the processing chain.The compounds found represent sour, vinegar, rancid, sweaty, fruity, floral, soapy, creamy, green leafy, herbal, and spicy notes.Obviously, unfermented beans do not have pyrazines, furans, furanones, pyrans, pyrones, or pyrroles as they are more common in well-fermented roasted beans, except for the lower concentrations of pyrazines that are found in dry beans, a metabolic product of B. subtilis or Bacillus megaterium, which are present at the end of cocoa fermentation [5,38,67].These compounds have cocoa, chocolate, walnut, popcorn, coconut, and candies notes, although they also have fruity notes like furans (furaneol) [45].
Dried beans have organic acids that volatilize during roasting [39].It has been reported that 2-and 3-methylbutanoic acid and acetic acid have been found to be most relevant in dried forastero cocoa beans.During roasting, the acid level decreases significantly depending on the roasting temperature.Thus, at 135 °C and 160 °C, a significant increase in acids is observed, while at lower temperatures, the increase is negligible.The 2-and 3methylbutanoic acids persist at the end of roasting at a temperature of 95 °C and their concentration reaches 17300 ng/g ( [6].This concentration exceeds the odor threshold of 203 and 11 ng/g, respectively, indicating that these acids greatly affect the final chocolate aroma.Likewise, [44] reported that only acetic acid and isovaleric acid are of an active odor in bitter chocolate samples, especially from beans roasted at 100 °C to 140 °C and in concentration rises from 171.26 to 381.4 ng/g.Figure 1 illustrates the compound percentage generated from foreign cocoa beans from Africa, America, and Southeast Asia when roasted at 140 °C for 30 min.There is a rise in aldehydes, phenols, pyrazines, furans, pyrans, and pyrroles at the end of roasting.A reduction in undesirable compounds, like acids and alcohols, is clearly evident.However, not all acids change at the end of the process.Cocoa liquor is the product obtained from fermented, dried, roasted, and ground cocoa beans.The concentration of total aromatic volatiles in cocoa liquor is higher than in chocolates.[12]reported that cocoa liquor had four to seven times more volatiles than the processed national and forastero variety chocolates than cocoa liquor, respectively.These included acids, alcohols, aldehydes, esters, ketones, pyrazines, lactones, and terpenes.Similarly, [9] highlighted acids, alcohols, esters, terpenes and terpenoids, aldehydes, ketones, pyrazines, furans, furanones, pyrans, pyrones, and pyrroles in the cocoa liquor of the forastero variety.Conversely, in the criollo cocoa variety, [68] reported that the main components were alcohol, esters, aldehydes, ketones, hydrocarbons, nitrogen and oxygen heterocycles, nitriles, and sulfides.Many of the compounds are similar in each variety, and what differentiates them is the contribution of each of them to the overall aroma of a particular variety.Hence the need to know the aromatic profiles of the varieties. Finally, processed chocolates have a wide variety of compounds.Table 2 shows that 85 compounds present in chocolate are subdivided into several chemical groups, including acids, alcohols, aldehydes, ketones, esters, and pyrazines as the major compounds.Pyrazines (16 compounds) are most abundant in chocolates and impart their malty, roasted cocoa, nutty, almond, and hazelnut notes.In contrast, Table 2

Fermented and Dried Beans
Table 3 indicates the contribution of the individual compounds to the overall aroma of the beans; their OAV > 1. 20 and 36% out of the compounds impart undesirable aromas.Acetic acid, 2-methylbutanoic acid, 3-methylbutanoic acid, and 2-methylpropanoic acid were common in both studies.They differ by approximately 55 times for acetic acid and 48 times for 3-methylbutanoic acid due to the fermentation method used (wood box vs. pile fermentation), days fermented (5 d vs. 3 d with prefermentation), and the origin of the beans (Costa Rica vs. Ecuador).These compounds are present in the over-fermented beans from leucine metabolism; such acids are undesirable in any cocoa matrix, imparting vinegar, pungent, sweaty, and rancid flavors [5,42]; it was reported in separate and complementary studies that box fermentation offers less aeration compared to the heap method.It was expected that the acetic acid concentration in the heap method would be higher, as aeration drives the emergence of acetic acid bacteria; however, the box method offers a higher concentration.The prefermentation of the beans helped to reduce their moisture and sugar content, thus decreasing the acid content at the end of fermentation.Prefermentation was absent in the box fermentation study.2-methoxyphenol and 3ethylphenol are also compounds that impart an unpleasant smoky flavor and odor and have been reported as a crucial marker of this aroma.In this study, these compounds are very significant, presenting concentrations of 221,00 and 7,66 µg/kg and an OAV of 122,8 and 3,5, respectively.[13] proposed the maximum tolerable concentration based on the rounded threshold value of 2 µg/kg for 3-ethylphenol.Sometimes, the aroma imparted by 2-methoxyphenol goes unnoticed as it is masked by the more odorous compounds in cocoa.[13] reported a concentration of 70 µg/kg for 2-methoxyphenol, and the reason for this depends on compound concentration and the authentic cocoa odors contributing to the pleasant aroma of cocoa masking the foul odor.
Table 3 shows the difference between the two studies on fruit aromas.The highest amount of these aromas is present in the study of [5], who used beans of the Ecuadorian trinitario variety in their study.In contrast, [42] proposed a single relevant fruit aroma, as is the case for ethyl phenylacetate.In both studies, the same variety (trinitario) was analyzed; however, there were large differences in the amount of its volatile compounds.The difference may be due to geographical origin, postharvest techniques, the chemical composition of the beans, and the method used for their quantification [22].Fruity volatile compounds, such as isoamyl acetate, 2-nonanone, 2-heptanol, 2-heptanone, 2-pentyl acetate, and ethyl, in descending order of their OAV, contribute to the overall aroma of the dried beans.These fruity aroma compounds are most relevant in the trinitario cocoa variety from Ecuador, known for aromatic cocoa.The esters synthesized during fermentation are dependent on environmental precursors, as well as on aeration and the presence of alcohol, such as ethanol.For example, the production of isoamyl acetate by yeasts (Pichia fermentants) uses isoamyl alcohol as a precursor [69].According to Table 2, this compound is also odor active (OAV = 4), and its concentration exceeds its OTV; therefore, it remains in the aromatic profile and serves as a precursor for other compounds.2-pentyl acetate (OAV = 5.47) and ethyl acetate (OAV = 1) can act as the precursors of secondary alcohols, such as 2-pentanol and 2-heptanol [70].The latter contributes its relevant citrus aroma (OAV = 11.08) to Ecuadorian trinitario cocoa beans.
Floral aromas in both studies include, in descending order, linalool, 2-phenylethanol, 2-phenylacetaldehyde, 2-phenylethyl acetate, acetophenone, and 2-methyl-3-buten-2-ol.[71] proposed linalool as a grade indicator (fine or basic grade) in some varieties.A linalool/benzaldehyde ratio of greater than 0.3 indicates fine-grade cocoa.Our review showed linalool and benzaldehyde OAVs of 17.38 and 35.15, respectively, with both compounds aromatically active at a 0.3 ratio.[20] reported a 0.56-to-0.89linalool/benzaldehyde ratio in roasted Criollo cocoa beans.As linalool is a biosynthesis product, its creation depends on plant varieties, growing, and fermentation conditions.During the roasting process, the linalool content decreases slightly due to volatility, but the relative difference between basic and fine-grade cocoa remains [71].Schluter et al. (2020) [42] reported a high OAV with a prevalence of chocolate and malt flavors, likely related to the high concentrations of isoleucine, leucine, and phenylalanine during fermentation.They attributed such aromas to Strecker aldehydes, such as 2and 3-methylbutanal (chocolate, malt flavor) and phenylacetaldehyde (honey flavor), respectively.On the contrary, 2-phenylethyl acetate, trimethylpyrazine, ethyl acetate, acetophenone, 2-methyl-3-buten-2-ol, 2-octanol, tetramethylpyrazine, ethanol, and 2-methyl-1-propanol show a low OAV, which, individually, would not contribute to their respective aromas, but, as a whole, would present a range of odors from wine to floral and honey.

Roasted Beans
Roasting the beans boosts the intensity of some of the volatile compounds present in dried beans, generating the appearance of new compounds.[6] reported 31 compounds in forastero cocoa beans roasted at 95 °C, 71 compounds in beans (of the same variety) roasted at 140 °C, while [9] reported 78 compounds in forastero cocoa beans roasted at 160 °C.They found 34 compounds that had increased in their concentration during roasting, nine compounds that had increased in concentration up to 140 °C and had then decreased or were undetectable; A total of 26 new compounds were added, and the concentration of nine compounds decreased significantly.After roasting, the distribution of the different classes of compounds changed, increasing in pyrazines (22.79%) and aldehydes (15.62%) and forming new compounds from Maillard reactions, such as pyrroles, furans, and pyrans [22].Some pyrrole derivatives are formed at moderate roasting temperatures and relatively high humidity [40].These compounds come from sugar precursor degradation

OAV
in cocoa and decrease during roasting.[73] reported that these volatiles are a useful indicator for which their level can be used to monitor the early stages of roasting.
The presence of undesirable compounds, Figure 3, in both studies represents 42% and 27% of the total aroma profile, indicating the presence of acidic, musty/earthy, burnt, and smoky flavors [74].Its high intensity and overall contribution are reflected in the OAVs ranging from 1.77 for 2-methoxyphenol to 4920 for acetic acid.The concentration of acetic acid is more than half in the unroasted beans when roasted at 130 °C for 30 mins.Roasting at higher temperatures or for a longer period does not significantly affect the acetic acid concentration.On the contrary, the concentration is higher when roasted at 160 °C for 30 min [75].Other compounds, such as sulfur derivatives, are also present in cocoa beans roasted at temperatures >160 °C.These compounds generate unpleasant smoky, onion, cabbage, and gasoline notes that persist in the final chocolates, albeit in lower concentrations [39,44].A percentage greater than 50% of the compounds in Table 4 indicates a contribution of desirable aromas, such as fruity, floral, cocoa, and chocolate, including the Strecker aldehydes (2-and 3-methylbutanal, phenylacetaldehyde) present in dry beans in smaller quantities.3-methylbutanal increases its contribution during roasting, especially at 95 and 110 °C, while at 125 °C, there is a return to the initial values.This is due to the prevalence of the volatilization phenomenon compared to that of generation [76].Furthermore, the concentration of benzaldehyde increases during fermentation [5]and during roasting, making it susceptible to losses at temperatures above 125 °C [76].Throughout the processing of the beans, the concentration of this compound decreases.In dry beans, the concentration is 1.08 µg/g, and its OAV is 18.08, whereas, in roasted beans, it drops to 1.01 µg/g and presents OAV = 16.90 [9,21].Other compounds that turn up after roasting are 4-hydroxy-2,5-dimethyl-3 (2H)furanone, 2-acetyl-1-pyrroline, 2,3-dimethyl-5-ethylpyrazine, trimethylpyrazine, and 2,3dimethylpyrazine. 2-acetyl-1-pyrroline, which is typically associated with roasting (Parker, 2015), exudes a popcorn and cracker-like aroma, having a low threshold in oil 0,1 ng/g and a concentration above OTV = 39 ng/g.Therefore, the contribution to the overall aroma of the roasted beans is significant (Table 3).Likewise, the pyrazines generated during roasting at temperatures above 100 °C exude notes of cocoa, baked potato, and chocolate.[45] reported that simple unsubstituted or monosubstituted pyrazines have a roasted, biscuit aroma and relatively high aroma thresholds, but as the substitution increases, the odor threshold decreases.It is evident from Table 3 that some of the pyrazines have low OTVs: 2,3-diethyl-5-methylpyrazine and 2-ethyl-3,5-dimethylpyrazines, and high OTVs: 2,3,5-trimethylpyrazine and 2,3-dimethyl-5-ethylpyrazine.These compounds exude baked potato, chocolate, earthy, popcorn, and cocoa aromas, respectively.However, their OTVs are inversely related to their OAVs.There are a few fruity and floral aromas, as shown in Table 3, including linalool, 2-phenylethanol, and 2-phenylethyl acetate, in descending order of their aromatic contribution.This deficiency in fruity aromas is common in the forastero variety [28].Finally, the 59 volatile compounds present in the roasted beans include alkyl pyrazines and aldehydes from reactions between oligopeptides derived from vicilin class globulin (7S) and reducing sugars of the cocoa beans [36].The author reported eight unidentified volatile compounds likely to expel specific cocoa notes.
Smoky, acidic, hammy, or musty aromas are the common compounds in cocoa liquors, and are present throughout the postharvest process [79].From 8 to 24% of the total active odor aromas (OAV > 1) correspond to undesirable compounds in cocoa liquors and dried beans, such as acetic acid, 2,3-pentanedione, 2-and 3-methylbutanoic acid, and 2methoxyphenol.In addition, 2,3-butanedione, dimethyl di-and trisulfide, methylpropanoic acid, and butanoic acid are in roasted beans and persist in cocoa liquor.During chocolate processing with these liquors, the OAV partially decreases or is undetected.For instance, in acetic acid and 2-nonanone, the OAV decreases by 5 to 14 times, and the concentration of 2,3-butanedione is undetected, so their OAV cannot be calculated [12]

Chocolate
The flavor of chocolate depends on the way the series of processes described above are carried out.Conching is the last of these processes, whereby the manufacturer can obtain the flavor and aroma required for a particular product.However, this process cannot correct previous mistakes, like an unpleasant smoky or moldy flavor due to poor drying, nor can it turn an inferior cocoa taste into a perfect one [80].Specifically, the function of conching is to evaporate volatile acids, achieve adequate viscosity, remove excess moisture, and develop a desirable color [81,82], as well as remove off-flavors and aromas while retaining desirable ones [80].Chocolate producers often use different conching temperatures and times depending on cocoa bean varieties and the origin of the chocolate products with the desired aromatic properties [83].In particular, levels of the most important odors decrease significantly by rising conching duration (from 6 to 10 h at 80 °C).Prolonged times reduce most pyrazines (including 2,5-dimethylpyrazine, 2-ethyl-5methylpyrazine, and 2,3,5-trimethylpyrazine) and the levels of alcohol, acid, aldehydes, and small esters [25].Similarly, [11] reported that prolonged conching reduces volatile acid concentration, alcohol, 3-methylbutanal, benzaldehyde, and several lesser volatile pyrazines, like trimethylpyrazine, tetramethylpyrazine, and acetylpyrrole.They also noted that this treatment increases the furfural content and does not affect the isobutanal, 2-methylbutanal, and phenylacetaldehyde levels because of the additional reaction compensation to form Strecker aldehydes during conching.Other components with significant contributions to chocolate (Figure 5) and that are highly odorous based on their low odor thresholds are 2-methylbutanal (2.2 ng/g), 3,5-diethyl-2-methylpyraniza, furaneol (27 ng/g), 2,3-diethyl-5-methylpyrazine (7.2 ng/g), ethyl 2-and 3-methylbutanoate (0.37 and 0.98 ng/g), 2-methylpropanal (3.4 ng/g), 3-isobutyl-methoxypyrazine (0.04 ng/g), 3isopropyl-2-methoxypyrazine (0.01 ng/g), and linalool (37 ng/g) [10][11][12]44,84].
The greater the availability of the compounds, the more complex the chocolate aroma is due to a wide volatile matrix.The studies in this paper show that acetic acid is the most abundant compound, and the rest of the compounds represent a mixture from different families (acids, alcohols, pyrans, aldehydes, esters, furans, and pyrazines).Table 6 shows that the chocolates made with 40 and 90% cocoa have an almost complete distribution of aromas ranging from fruity to undesirable in a range of 13 to 25%.Many of the compounds that remain in chocolate have an increased concentration along the processing chain or remained unchanged, as has been reported; the influence of the manufacturing process is greater than that of the difference in the cocoa production area, providing a diversity of aroma profiles [86].Particularly, criollo cocoa roasting increases acetophenone, tetramethylpyrazine, 2,3,5-trimethylpyrazine, and 2,5-dimethylpyrazine concentration, whereas 2-heptanol, phenylethyl alcohol, 2,3-butanedione, 2-phenylphenylpyrazine, 2,3-butanedione, 2,3-butanedione and 2,3-butanedione, 2,3-butanedione, 2-phenyl-2-butenal, 5-methyl-2-phenyl-2-hexanal, ethyl octanoate, ethyl phenylacetate, ethyl decanoate, and trans-linalool oxide remain stable during roasting [2].In addition, some of the samples' volatile compounds are affected by each brand formulation, masking or enhancing a specific volatile.[26] considered three of the predominant masses of chocolates, namely, mass 33, 43, and 61, which were identified as methanol, a fragment of diverse origin, and acetic acid, respectively, and because of their high concentration, he suggested these three masses can have a huge impact on the fingerprint analysis that differentiates them by regions and brands.

Conclusions
Throughout the cocoa processing chain, aromatic precursor compounds give rise to characteristic aromas in each cocoa matrix.The way aroma compounds contribute to a specific matrix is estimated by the compound odor activity value.Initially, the dry fermented beans had acidic notes for which the OAV was high.The diversity of acids depends on the fermentation method used and also on bean preconditioning to reduce the fermentable sugar and, therefore, the final acidity of the bean, whereas fruity and floral aromas were characteristic of dry fermented beans.However, their concentration and abundance of compounds depends on the variety used.Compounds that exude chocolate aromas are scarce in dry fermented beans as they arise from Maillard reactions during roasting.The roasting parameters, like temperature, roasting time, and the roasting method, influence the appearance of new compounds and the preservation of those already found in the dry beans.Temperatures above 160 °C for a period of 35 min favor the appearance of pyrazines but reduce the compound concentration responsible for fruity and floral aromas, such as esters and ketones.In cocoa liquor, on average, 61.84% of the compounds represent fruity and floral aromas.The abundance of compounds in chocolate is directly related to the conching process, whereby the remaining fraction of moisture and undesirable aroma is eliminated, and the desirable aromas are concentrated.

Figure 1 .
Figure 1.Relative percentage of volatile compounds before and after roasting.Source: taken and adapted from[22].

Table 1 .
Dilution factors of active odor compounds from roasted and unroasted forastero cocoa beans.

Table 2 .
Number of common volatile compounds in the cocoa bean processing chain.

Table 3 .
Odor activity values of key aroma compounds in fermented dried beans.

Table 4 .
Odor activity values of key aroma compounds in roasted beans.
. The OAV same occurs with butyric acid and 3-methylbutyric acid, which are only detected in cocoa liquors and disappear in processed chocolate.It is worth mentioning that although acids present a lower number of compounds in cocoa liquors, they have the highest VAOs, affecting the liquor's pH.Cocoa liquor with a low pH (4.75 to 5.19) is more likely to have off-flavors.

Table 5 .
Odor activity values of key aroma compounds in cocoa liquor.

Table 6 .
Odor activity values of key aroma compounds in dark chocolate.Made with Ecuadorian national cocoa liquor.** Made from African cocoa liquor. *