The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.

Abstract

This review is intended to give insight into the factors which influence the flavour volatiles of cocoa beans. Cocoa bean flavour is one of the most important quality attributes of cocoa bean and cocoa bean products such as chocolate. The composition of the cocoa bean depends on bean genotype, postharvest treatments such as pulp pre-conditioning, fermentation, drying, industrial processes such as roasting, as well as type of soil and age of cocoa tree. With increasing demand for sustainable production of high quality cocoa beans, better understanding of factors which contribute to the variation in flavour character would have significant commercial implications. This paper was received in 2015, and published in 2016.

Introduction

Cocoa (Theobroma cacao L.) is a crop of huge economic significance in the world, and the key ingredient for chocolate manufacturing. It forms the major agricultural export commodity for several producing countries in West and Central Africa including Cote d’Ivoire, Ghana, Nigeria and Cameroon (Afoakwa et al., 2011a).

Cocoa belongs in the family Sterculiaceae, which includes about 70 Genera including the genus Theobroma, which is where the species cacao is found. The seeds of this plant are what are referred to as cocoa beans, and are found in the fruit-filled pod. The pods are oval in shape, and can measure from 12-30 cm long, and contain around 30 to 40 seeds. The seeds are surrounded by a mucilaginous pulp, which comprises about 40% of the bean’s fresh weight (Schwan and Wheals, 2004; Lima et al., 2011). The pulp is rich in fermentable sugars of roughly 9-13% w/w (weight for weight) such as glucose, fructose, and sucrose (Lefeber et al., 2010), high acidity (pH 3.0-3.5) due to diverse organic acids but mainly citric acid (Guehi et al., 2010), and a protein content in the range of 0.4% to 0.6% w/w (Lima et al., 2011).

Cocoa is cultivated around the world over an area of roughly 70,000 km² (Kim et al., 2011) between 20° north and south of the equator in areas with an environment cacao can grow (Fowler, 1999). Roughly 70% of the world’s cacao production occurs in equatorial region of West Africa, with the rest of the production in equatorial regions of Central and South America, West Indies, and tropical regions in Asia (Dillinger et al., 2000).

The cocoa tree is a perennial, and can grow 8 to 15 meters high (Fowler, 1999). The growing environment should be hot and moist. Cacao cannot withstand long drought conditions without greatly impacting it’s growth and production levels. The fruit (the pods) vary in size, shape, external colour, and appearance.

There are about 5-6 million farmers in countries across Africa, Asia, and Latin America who produce about 90% of the world’s cocoa. The number of people who depend on cocoa for their livelihoods worldwide is roughly 40-50 million (World Cocoa Foundation, 2010). In West and Central Africa, cocoa exports generate over $8 billion for the region’s national economies (IFDC, 2014) and supports around 2 million smallholder farm households. In Ghana, the industry employs about 70% of the national agricultural labour force (COCOBOD, 2013). For these farmers, cocoa contributes about 70-100% of their annual household incomes (Anang et al., 2013; Nunoo et al., 2014).

Consumption of chocolate and other cocoa products is reported to contribute health. Polyphenols in cocoa beans have been reported to exhibit anticarcinogenic (Rodriquez-Ramiro et al., 2011; Oleaga et al., 2012), anti-atherogenic (Wollgast and Anklam, 2000), and vasodilatory (Gomez-Juaristi et al., 2011; Khawaja et al., 2011) effects mainly as antioxidants (Schinella et al., 2010; Martorell et al., 2011).

ICCO (International Cocoa Organization) reported 4.4 million tons of cacao production in the 2013/2014 crop season (ICCO, 2015a). The global demand for sustainable cocoa is growing by 2-3% annually. West Africa is confronted with a 2% annual decline in production due to poor farm management practices, planting low-yield varieties, pests and diseases, aging cocoa trees, and loss of soil fertility due to inadequate or no use of fertilizers (Abekoe et al., 2002; Baah and Anchirinah, 2011; Akrofi et al., 2015). Sustaining production of high quality cocoa beans in West Africa is crucial for the millions of family farmers that depend on it for their livelihoods. The cocoa industry wants to see improvements and growth in the quantity and quality of cocoa beans produced and the standard of living enjoyed by the growers.

Sustainable production of cocoa also involves the quality of the cocoa beans themselves, and there many components which add to the cocoa bean quality such as flavour volatiles, nutritional composition, polyphenolic content and fermentative quality. The flavour volatiles are impacted by many factors, which will be discussed here based on the research available when this was published.

Factors affecting flavour quality of the cocoa beans

There are several indicators used to measure cocoa bean quality including bean size, bean count (in the pod), bean colour, and acidity of the beans. The most important indicator is the amount and type of volatile flavour compounds. The characteristic flavours of cocoa beans are due to a very rich volatile fraction composed of a mixture of hundreds of compounds (Magi et al., 2012). There are over 600 flavour compounds identified from cocoa beans and cocoa products (Crafack et al., 2014). These compounds are comprised of nitrogen and oxygen heterocyclic compounds, aldehydes and ketones, esters, alcohols, hydrocarbons, nitriles and sulphides, pyrazines, ethers, furans, thiazoles, pyrones, acids, phenols, imines, amines, oxazoles, and pyrroles.

Some of these compounds have a particular flavour/aroma characteristic. For instance, most esters confer a fruity/flowery attribute while pyrazines carry a more earthy/roasted flavour (Owusu, 2010). Many of these flavour compounds are formed during roasting from flavour precursors that were created during the fermentation and drying process. These flavour compounds are also influenced by factors such as cocoa genotype, bean composition, soil type, age of cocoa tree, postharvest treatments such as pulp pre-conditioning, fermentation, drying, industrial processes such as roasting, as well as storage and transportation (Afoakwa et al., 2008, Afoakwa, 2010; Owusu et al., 2011; Crafack et al., 2014).

Effect of genotype and origin of cocoa tree on cocoa bean flavour quality

The flavour quality of the cocoa bean depends on the genotype of the tree which produced it. Cocoa beans of different genotypes and origin have their own distinct flavour characteristics (Table 1). At the time of this publication, namely three broad cultivars of cacao were recognized: Forastero, Criollo, and Trinitario. A fourth variety grown in Ecuador called Nacional is also another recognized variety. (Side note from Geoseph: Today there are additional and different terms associated with the recognized cultivars of cacao not mentioned in this paper). These differences are due to inherent genetic composition of the beans, but also botanical origin, location of growth, climate, amount and time of sunshine and rainfall, soil conditions, ripening, time of harvesting, and time between harvesting and fermentation all contribute to variations in the flavour of the bean.

Three broad cultivars of cacao

Forastero comprises 95% of the world’s cocoa production (Saltini et al., 2013) and is commonly referred to ask “bulk cacao” in the chocolate trade. It is cultivated predominately in Cote d’Ivoire, Ghana, Nigeria, and Cameroon. The seeds are flat, astringent, and purple in colour (more rarely ivory or pale) due to the anthocyanins contained within the bean. Forastero trees are highly productive and moderately resistant to pests and diseases (Ferrao, 2002; Barley, 2005; Lima et al., 2011). Forastero have a higher pH after fermentation when compared with Criollo beans (Ortiz de Bertorelli et al., 2009).

Criollo is the original cultivated cacao indigenous to Northern, South, and Central America. The colour of the beans is white to ivory or a very pale purple colour due to an anthocyanin inhibitor gene (Fowler, 199; Ferrao, 2002). They are susceptible to many diseases, produce a low yield, and therefore are less commonly cultivated. Today, cultivation is found in Central America and some regions in Asia (Fowler, 1999; Ferrao, 2002; Thompson et al., 2007). Criollo is reported to contain high levels of pyrazine type flavour compounds and a low pH.

The Trinitario type originated in Trinidad and is the product of natural hybridization and recombination of the Criollo and Forastero cacao (Fowler, 1999; Ferrao, 2002). The beans vary in colour, but are rarely white. The trees can be susceptible to pests and diseases at a level between Forastero and Criollo populations (Fowler, 1999; Ferrao, 2002; Bartley, 2005). It has strong chocolate characteristics not found in other varieties such as winery type aromas (Afoakwa et al., 2008).

Trinitario and Criollo varieties can produce what is termed “fine” cocoa, and share a total production of below 5% (ICCO, 2015b). These, along with Nacional, are used to make high quality dark chocolate.

Clapperton et al. (1994) observed consistent differences in cocoa flavour, intensity, acidity, sourness, bitterness, and astringency among West Africa Amelonado variety (AML), four Upper Amazon clones [Iquitos Mixed Calabacillo 67 (IMC67), Nanay 33 (NA33), Parinari 7 (PA7), Scavina 12 (SCA120)], and unidentified Trinitario (UIT1) grown in Sabah, Malaysia. Ziegleder (1990) suggested monoterpens (a group of volatile flavour compounds) such as linalool are part of the components responsible for fine flavour in cocoa, and concluded that fine flavour cocoas contain higher amounts of linalool than bulk cocoa.

The genotype dictates the type and quantity of stored proteins, carbohydrates, and polyphenols (Afoakwa et al., 2008), which in turn determine the precursor compounds formed during fermentation and post-fermented drying and thereby impacting the flavour and intensity of the cocoa bean (Taylor, 2002; Luna et al., 2002; Counet et al., 2004; Taylor and Roberts, 2004).

Cocoa bean anatomy and constituents

The cocoa bean is made up of two parts, the testa (shell or “husk” or seed coat), and the embryo (see Fig. 1). Attached to the outside of the testa is the sugary, white mucilaginous pulp that encompasses the seed. The embryo (the main part of the seed we consume) is made up of two parts: the two folded cotyledons and the embryonic axis which connects them.

Figure 1. At the very top is a whole cocoa bean that has been fermented, dried, and roasted. The bottom left is the seed with the testa removed, revealing the cotyledon. The bottom right image has to cotyledon broken up (AKA nibs) with the embryonic axis (AKA the germ) circled in green. Image by Bean To Bar World.

The dry weight of a cocoa bean is on average 1-1.2 g. The fresh cocoa bean is about 32-39% water, 30-32% fat, 10-15% proteins, 5-6% polyphenols, 4-6% starch, 4-6% pentosans, 2-3% cellulose, 2-3% sucrose, 1-2% theobromine, 1% acids and 1% caffeine (Lopez and Dimick, 1995; Bertazzo et al., 2011). The main sugars in cocoa beans are sucrose (90%), and fructose and glucose (6%) (Biehl and Ziegleder, 2003).

Cocoa bean fat is made up of 95% triacylglycerols, 2% diacylglycerols, <1% monoacylglycerols, 1% polar lipids, and 1% free fatty acids (Biehl and Ziegleder, 2003). The main fatty acids in cocoa butter are stearic (35%), palmitic (25%), and oleic (35%), with the rest of the fat being mostly polyunsaturated linoleic (3%) (Bracco, 1994).

The cotyledon is made up of two types of parenchyma storage cells: polyphenolic cells (14-20% dry bean weight - a single large vacuole filled with polyphenols and alkaloids such as caffeine, theobromine, and theophylline), and lipid-protein cells which have cytoplasms tightly packed with many small protein and lipid vacuoles as well as starch granules - all which play a role in cocoa bean flavour and aroma (Kim and Keeney, 1984; Nazaruddin et al., 2001).

The polyphenols are stored in the pigment cells (AKA polyphenol-storage cells) of the cotyledons, and depending on the amount of anthocyanins are white to purple. There are three groups polyphenols in cocoa beans:

catechins (flavan-3-ols) 37%
- The main catechin being (-)-epicatechin (35%)
- Other catechins in small amounts include (+)-catechin, (+)-gallocatechin, and (-)-epigallocatechin
anthocyanins 4%
- Mainly cyanidin-3-α-L-arabinoside and cyanidin-3-β-D-galactoside
and proanthocyanidins 58%
- mainly flavan-3,4-diols

Other polyphenols identified in cocoa beans are glycosides: quercetin-3-O-α-D-arabinoside and quercetin-3-O-β-D-glucopuranoside. About 17 phenolic acids and esters have been reported, and 7 of them comprise no more than 23 ppm of the seed dry weight (phloroglucinol, protocatechuic acid, vanillic acid, o-hydroxyphenylacetic acid, p-coumaric acid, caffeic acid, ferulic acid). Epicatechin and smaller procyanidins are soluble and therefore cause the astringent taste sensation of cocoa, but molecules longer than 3 subunits are insoluble and cause no astringency (Ziegleder, 2009).

In regards to proteins, there are four main kinds found in cocoa beans that make up 95% (w/w) of the total seed proteins. These include: albumins (52%, water-soluble), globulins (43%, salt-soluble), prolamins (alcohol-soluble) and glutelins (soluble in dilute acids and alkali). Albumin is not degraded during fermentation, and Voigt and Biehl (1995) demonstrated that incubating cocoa albumin with cocoa protein enzymes did not produce any cocoa-related aromas after roasting. Some globulins (vicilin-class) are degraded during fermentation and into peptides and amino acids (flavour precursors), which will take part of the Maillard reactions during drying and roasting to develop aroma compounds (Voigt et al., 1993; Amin et al., 1998; Hue et al., 2016).

Effect of postharvest treatment of cocoa on bean flavour quality

Postharvest treatment is anything that is done to the cocoa beans before the beans are ready to be shipped to a chocolate manufacturer. These are carried out in the country of origin where the cacao was growing, and are critical to the flavour profile of the beans. Although genotype dictates to a high degree the flavour of the cocoa bean, it’s easy to develop poor bean flavour from poor postharvest treatment practices.

Pulp pre-conditioning

This involves altering the properties of the fruit pulp before microorganisms develop. During fermentation, the pulp itself is metabolized by a sequence of microorganisms. Since the properties of the pulp determine the microbial development and metabolism, changes to this pulp substrate may impact the production of:

alcohols by yeasts
acids by lactic acid bacteria
acids by acetic acid bacteria

The changes can include altering the moisture content of the pulp, altering the sugar content, and altering the volume of the pulp per seed, and altering the pH and acidity of the pulp. For instance, removing some of the pulp or reducing the fermentable sugar content can lead to less acid production and less acidic beans (Afoakwa et al., 2012). Other studies have also shown that pre-conditioning have significant effects on bean acidity and polyphenol content, which results an altering the flavour (Meyer et al., 1989, Nazaruddin et al., 2006; Afoakwa et al., 2012).

The methods for pulp pre-conditioning include:

pod storage
depulping
bean spreading

The pulp can be pre-conditioned inside the pod as with pod storage before being brought to the fermentation facility, r outside the pod as with depulping and bean spreading.

Depulping of cocoa beans

Excessive pulp on the bean may lead to excessive sour beans (Afoakwa and Patterson, 2010). Removing some of the pulp reduces the amount of fermentable sugars, which leads to less acid production (Duncan et al., 1989; Sanagi et al., 1997; Afoakwa et al., 2012). Schwan and Wheals (2004) reported that removing up to 20% of the pulp from fresh Brazilian cocoa beans significantly improved the flavour. Depulping can be done mechanically or enzymatically.

Mechanical methods include a press (to press the juice out of the beans), centrifuges, or spreading them on a flat surface for several hours (which allows for more “sweating” during the first 24 hours of fermentation). However, the processes causes bruising of the beans and leads to the activation of enzymes which might influence various biochemical processes during fermentation. The amount of pulp removed can be from 10-30%, ideally 20-25% by weight based on original total combined weight of beans and pulp.

Enzymatic depulping involves adding pectin degrading enzymes to the cocoa bean-pulp mass before fermentation to breakdown the pectin in the pulp. This reduces the volume of the pulp and increases the aeration of the fermentation mass.

Benefits of de-pulping, in addition to reducing acidity, include shorter fermentations, increased efficiency, and the opportunity to use the excess pulp in manufacturing jams, marmalades, pulp juices, wines, or cocoa soft drinks (Schwan and Wheals 2004; Dias et al., 2007; Afoakwa 2010).

Pod storage

Pod storage is as simple as it sounds: storing the harvested cocoa pods for a specific duration before opening them and fermenting the seeds. Afoakwa et al. (2011b) suggest that this can have a beneficial effect to the chemical composition of the beans and resulting flavour. Meyer et al. (1989) reported that pod storage of Malaysian cocoa pods led to reduced nib acidification and a reduction in the acidity notes and increase in cocoa flavour in the beans. Afoakwa et al. (1989) also noted that increasing pod-storage consistently reduced the non-volatile acidity and increased the pH during fermentation of cocoa beans in Ghana. Nazaruddin et al. (2006) found that that pulp pre-conditioning significantly changed bean acidity as well as a significant reduction in polyphenol compounds, thereby reducing astringency and bitterness in the cocoa and cocoa products.

Fermentation

Fermentation of the cocoa beans is crucial to flavour development, as it promotes biochemical changes and flavour precursors. Raw unfermented cocoa beans have a very astringent and unfavorable flavour, so they are both fermented with the fruit pulp and then dried afterwards to reduce these unfavorable characteristics. However, there are correct methods of fermenting and drying that are required.

During fermentation, microbes populate the mass as the microenvironment changes (temperature, pH, oxygen availability). Fermentation allows the generation of free amino acids and peptides (flavour precursor compounds) which occur from the enzymatic degradation of cocoa proteins, as well as the generation of reducing sugars from enzymatic degradation of sucrose. The typical cocoa aroma is generated by the combining of these amino acids, peptides, and reducing sugars during roasting (Frauendorfer and Schieberle, 2008). In addition to the precursors formed during fermentation, there is a significant increase in volatile compounds (alcohols, organic acids, esters, and aldehydes) after fermentation (Frauendorfer and Schieberle, 2008; Magi et al., 2012). Phenolic compounds are oxidized and polymerized to insoluble higher molecular-weight compounds (tannins) which reduces the concentration of Phenolic compounds and thereby reducing the bitterness and astringency to acceptable levels (Misnawi, 2008).

Cotyledon protein degradation is central to flavour formation. Proteolysis (breaking down proteins) in the cocoa bean takes place within 24 hours after the destruction of the cells and acidification by acetic acid (Ziegleder, 2009). Carboxypeptidase (a result of proteolysis) plays an important role in converting hydrophobic oligopeptides into cocoa specific aroma precursors, namely leucine, valine, alanine, isoleucine, and phenylalanine, which are required for cocoa aroma when combined with reducing sugars during roasting (Voigt et al., 1994).

The duration and method of fermentation is crucial as well. Aculey et al. (2010) noted an increased level of organic acids (propanoic acid, 2-methylpropanoic acid, 3-methylbutanoic acid and acetic acid) after 72 hours of cocoa fermentation, all 4 of which are important odour-active compounds in cocoa (Bonvehi, 2005; Frauendorfer and Schierberle, 2006). Unfermented cocoa beans do not develop a good cocoa flavour even after roasting, and over-fermented cocoa beans produce hammy and putrid off-flavours (Afoakwa et al., 2008; Afoakwa, 2015). Other important flavour components produced during fermentation include ethyl-2-methylbutanoate, tetramethylpyrazine, and certain other pyrazines. Bitter notes come from theobromine and caffeine, together with diketopiperazines formed from roasting through thermal decompositions of proteins. Other flavour precursor compounds which come about from amino acids being released during fermentations include 3-methylbutanol, phenylactaldehyde, 2-methyl-3-(methyldithio)furan, 2-ethyl-3,5-dimethyl, and 2,3-diethyl-5-methylpyrazine (Taylor, 2002; Afoakwa, 2015).

Drying

The cocoa beans are dried after fermentation is complete. They are dried to reduce their moisture content from 60% to about 6-8% which will reduce the risk of mould growth during storage. Drying also continues on some of the chemical changes that were occurring during fermentation, which in the end will improve flavour development (Kyi et al., 2005). It initiates major polyphenol oxidizing reactions which are catalyzed by polyphenol oxidase, and allows for the formation of new flavour components. It also causes a loss of membrane integrity, which induces the brown colour post drying. This reduces bitterness and astringency and forms the “chocolate brown” colour of well fermented beans. Oxidation of acetic acid continues during drying, and reducing sugars participate in the thermal treatment of non enzymatic browning reactions (Maillard reactions) to form volatile fractions of pyrazines (Hashim and Chaveron, 1994; Cros and Jeanjean, 1995). Oberparleiter and Ziegleder confirm these findings by identifying Amadori compounds, which are the first intermediates of the Maillard reaction in dried, unroasted cocoa beans. These Amadori compounds are the first intermediates of the reaction between the free amino acids (from degradation of proteins) and glucose (from degradation of sugars).

Both the rate of drying and the method chosen are crucial to the final quality of the cocoa beans. If drying occurs too quickly, the beans will retain too much acetic acid and become too acidic. This is due to the fact that the rapid drying hardens the testa which prevents acetic acid from migrating out from the cocoa bean. If the drying rate is too slow, the beans will become low in acidity, have poor colour, and have a high presence of moulds (Hashim et al., 1999; Hii et al., 2006; Bharath and Bowen-O’Connor, 2008; Zahouli et al., 2010).

Effect of industrial processing of cocoa on bean flavour quality

roasting

During roasting, volatile acids evaporate from the cocoa beans, and results in a reduction in acidity, and therefore reduces the sourness and bitterness of the post-roasted cocoa beans (Afoakwa et al., 2008). High roasting temperature reduces acidity, more specifically the volatile acids with low boiling points such as acetic acid. The less volatile acids (do not vaporize at room temperature) such as oxalic, citric, tartaric, succinic, and lactic acid remain more or less unchanged by roasting.

During roasting, the free amino acids, short-chain peptides, and reducing sugars produced during fermentation and drying produce desirable flavour compounds via the Maillard reaction and Strecker degradation reaction to produce pyrazines, alcohols, esters, aldehydes, ketones, furans, thiazoles, pyrones, acids, imines, amines, oxazoles, pyrroles, and ethers. For the Maillard reaction to take place, it requires a high temperature and low moisture environment, which is why it occurs during roasting (Fennema, 1996). The Maillard reaction produces carbonyls which react with free amino acids during the Strecker degradation reaction. This causes a degradation of the amino acids to aldehydes, and these aldehydes contribute to aroma. Strecker degradation of each unique amino acid produces a unique aldehyde with a unique aroma (Fennema, 1996).

Roasting parameters determines the chemical and physical processes that occur inside the bean, and dictates the final quality and flavour of the bean. Both temperature and duration of roasting significantly impact the chemical and physical changes which occur. Cocoa beans are roasted from 15-45 minutes with temperatures that range from 130°C to 150°C or 265°C to 300°F (Krysiak and Motyl-Patelska, 2006; Krysiak, 2006; Krysiak et al., 2013). The time and temperature depends on what is being roasted (whole beans, nibs, or cocoa liquor), the final product (dark or mil chocolate) and the type of cocoa (Criollo, Forastero, etc.) (Kothe et al., 2013). Ramli et al. (2006) reported that fine cocoa varieties require lower roasting temperatures versus bulk cacao.

Soil chemical composition and cocoa bean flavour quality

Cocoa trees are very particular about where they will grow and thrive, including the type of soil they prefer. Soil for cocoa trees needs to contain coarse particles with a reasonable quantity of nutrients to a depth of about 1.5m to allow for a good root system to develop (ICCO, 2015c). Cocoa trees are very sensitive to moisture stress and lack of water, so the soil must contain both nutrients and good water retention properties as well as good drainage. Cocoa trees can grow in soils with a pH of 5.0-7.5, but pH lower or higher should be avoided (ICCO, 2015c). The soil pH impacts the solubility of minerals and nutrients taken up by the tree, and is so pH is a useful indicator of other soil parameters (Ololade et al., 2010).

Soil pH provides information about availabilities of exchangeable cations (e.g. Ca2+, Mg2+, K+, etc.) in soil (Ololade et al., 2010). The soil should contain at least 3.5% organic matter in the top 15 cm of the soil. The organic matter includes remains of plants, animals, and microorganisms in all stages of decomposition. The optimum nitrogen/phosphorus ratio should be around 1.5 (ICCO, 2015c).

There are many studies done on soil type, soil chemical composition, and nutrient requirements for cocoa production (Amusan et al., 2005; Ololade et al., 2010; Baah et al., 2011; Adewole et al., 2011), however all the research conducted on soil effects are focused on yield with little to no work on the effect of soil on the flavour quality of cocoa beans. Therefore, there is a big gap in terms of research required to connect the effect of soil chemical composition on the flavour quality of cocoa beans.

Age of cocoa tree and flavour quality

Cocoa trees go through different stages in their life cycle. Production life cycle of cocoa trees occurs in four stages according to Mahrizal et al. (2013):

No yield in the first few years, normally 3-4 years
Increased yield at an increasing rate
Increased yield at a decreasing rate
Decreasing yield (trees past their prime)

Binam et al. (2008) reported trees to be productive from about year 4 to year 18, after which yields begin to decline due to exhaustion of soil nutrients, erosion, and increasing occurrence of pests and plant diseases (Vekua, 2013). Again, the studies that have reported on the impact of aging trees focused in on yield (Binam et al., 2008; Hainmueller et al., 2011; Vekua, 2013), not flavour quality. Therefore, there is a big research gap to look at the influence of the cocoa tree age on the formation of flavour precursors during cocoa bean fermentation as well as the formation of flavour volatiles.

Conclusion

Sustainable production of cocoa is important to both small holder farmers and their families that depend on cocoa for income, and also the millions of people who enjoy or work with chocolate and other cocoa products. However, sustainable production depends on the quantity of cocoa beans produced and their quality which includes cocoa bean flavour. More work needs to be done on the impact of tree age and soil composition on the development of flavour precursors and overall flavour volatile compound development in cocoa beans and products derived from them.

Core findings 

Cocoa bean flavour is an important quality attribute which determines acceptability of cocoa beans and cocoa products such as chocolate. 
The complex composition of cocoa bean flavour depends on the bean genotype specifically on contents of bean storage proteins, polysaccharides and polyphenols. 
Works need to be done on the impact of age of cocoa tree and soil chemical compositions on the development of flavour precursors during fermentation and drying and their subsequent formation of flavour volatile compounds during cocoa bean roasting

Factors influencing quality variation in cocoa (Theobroma cacao) bean flavour profile - a review