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Chocolate and Cocoa, Flavor and Quality

  1. Emmanuel Ohene Afoakwa

Published Online: 13 JAN 2012

DOI: 10.1002/0471238961.chocafoa.a01

Kirk-Othmer Encyclopedia of Chemical Technology

Kirk-Othmer Encyclopedia of Chemical Technology

How to Cite

Afoakwa, E. O. 2012. Chocolate and Cocoa, Flavor and Quality. Kirk-Othmer Encyclopedia of Chemical Technology. 1–19.

Author Information

  1. University of Ghana

Publication History

  1. Published Online: 13 JAN 2012

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Abstract

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

Cocoa is the fundamental ingredient in chocolate manufacture and is derived from the fruit of the cocoa tree. Variations in cocoa bean genotype exhibit distinct differences in flavor characteristics during subsequent processing of the beans into cocoa liquor. These chocolate characteristics not only originate in flavor precursors present in cocoa beans, but also they are generated during post-harvest treatments and transformed into desirable odor notes in the manufacturing processes. Complex biochemical modifications of bean constituents are further altered by thermal reactions in roasting and conching and in alkalization. However, the extent to which the inherent bean constituents from the cocoa genotype, environmental factors, post-harvest treatment, and processing technologies influence chocolate flavor formation and relationships with final chocolate flavor quality requires more in-depth understanding. With increasing specialty niche products in chocolate confectionery, greater understanding of factors contributing to variations in cocoa genotypes and their relationships with final chocolate flavor character would have significant commercial implications.

1 Introduction

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

The cocoa tree (Theobroma cacao L. of the Sterculiaceae family) is generally known to have originated from Central and South America, with the most important area being the foot of the Andes in the upper reaches of the Amazon River. Cocoa is one of the most important agricultural export commodities in the world and forms the backbone of the economies of some countries in West Africa, Southeast Asia, and South America. Cocoa beans are the fermented and dried seeds of Theobroma cacao, derived from the fruit/pod of the tropical cocoa tree, and they form the fundamental ingredient in chocolate manufacture.

Currently, four broad cultivars are commonly recognized: Criollo, forming approximately 5% of the world production; and the more common Forastero, with smaller, flatter, and purple beans; Nacional, also known as “fine” flavor cocoa and grown mainly in Ecuador; and Trinitario, a more disease-resistant hybrid of Criollo and Forastero. The Criollo and Nacional are known as fine flavor cocoas and produced mainly in the West Indies and the Ecuadorian region, whereas the Forastero, known as “basic” or “bulk” cocoa accounts for 90% of the world cocoa market and are produced mainly in West Africa (1). The cultivars exhibit differences in the appearance of pods, yields of beans, and in resistance to pests and diseases (2). World annual cocoa bean production is approximately 3.5 million metric tons, and the major producers are the Ivory Coast, Ghana, Indonesia, Brazil, Nigeria, Cameroon, Malaysia, and Ecuador. There are also a number of smaller producers, particularly of “fine” cocoa, which forms less than 5% of world trade (3).

Chocolate has a distinctive flavor character, with specific notes related to bean genotype, growing conditions, and processing factors (4). Fermentation is a key processing stage that causes the death of the bean and facilitates removal of the pulp and subsequent drying. During this stage, there is initiation of flavor precursor formation and color development, as well as a significant reduction in bitterness. During the thermal reactions of roasting, important modifications occur, including Maillard reactions with contributions from reducing sugars and amino acids, each showing variations. Conching is also important for flavor development and final texture in chocolate, effecting elimination of volatile acids, removal of moisture, viscosity modifications, and color changes resulting from emulsification and tannin oxidation (5, 6). Chocolate consumption has possible health benefits with specific claims recently identified and studied (7, 8). Cocoa beans and derived products are rich in antioxidants, including catechins, epicatechins, and procyanidins, and these polyphenols are similar to those found in wine, vegetables, and tea (9-11). These contribute as precursors to flavor formation in cocoa and chocolate (12-14).

The chemistry of cocoa beans in fermentations is still under study (14-16), as are contributions from roasting and alkalization (17-19) and conching (6, 20, 21). The key flavor compounds in chocolate have been identified (3, 13). However, the biochemical and chemical processes leading to chocolate flavor formation and development, and their relationships to the final character and perceptions of quality, are not fully understood. This article discusses the variations in cocoa genotypes, their biochemical characters and post-harvest treatment (fermentation and drying), and how they influence the formation and development of chocolate flavors during subsequent industrial manufacture.

2 Cocoa Cultivation and Practices

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

2.1 Cultivation of Cocoa

Cocoa cultivation requires an appropriate climate that is mostly found within the area bounded by the tropics of Cancer and Capricorn. The majority of the world's cocoa is grown as small or large plantations within 10° North and South of the equator, and it is best suited for sea level up to a maximum of approximately 1000 m, although most of the world's cocoa grows at an altitude of less than 300 m. Cultivation requires temperatures generally within 18–32°C (65–90°F) and rainfall well distributed across the year, with a range between 1000 and 4000 mm (40 and 160 in) per year, but preferably it is between 1500 and 2500 mm (60 and 100 in).

During cultivation, cocoa prefers high humidity, typically ranging between 70% and 80% during the day and 90% and 100% at night. Cocoa trees are usually planted to achieve a final density of 600–1200 trees/hectare (1500–3000 trees/acre) and intercropped with food crops. As a result of the fragility of the cocoa trees during the early stages of growth, they are mostly protected from strong winds using food crops; for instance, plantain trees are used as a wind shield on plantations in Ghana. The trees grow well on most soil but prefer well-aerated soils with good drainage and a pH of neutral to slightly acidic (5–7.5); pests and diseases also should be carefully controlled (22). Cocoa trees used to grow to a height of ∼10 m tall at maturity, preferably under the shades of other trees. However, modern breeding methods have led to the development of trees to a standard of ∼3 m tall to allow for easy harvesting.

2.2 Flowering and Pod Development

The emergence of the bud through the bark of the tree marks the beginning of the cocoa bean development. This takes approximately 30 days from its histological beginnings to its culmination on the bark surface, and within hours of its emergence, the bud matures, the sepals split, and the flower continues to mature during the first night after the budding. On the next morning after budding, the flower is fully opened and the anthers release their pollens. If not pollinated and fertilized on this day by insects, the flowers continue to abscission on the next day. It is interesting to note that a single healthy cocoa tree produces approximately 20,000 to 100,000 flowers yearly, but only 1% to 5% of these get pollinated and develop into pods.

Once successfully pollinated and fertilized, the various stages of embryo and ovule growth continue, the pods reaching a maximum size after approximately 75 days after pollination. The pods then mature for another 65 days, making a total of approximately 140 days after pollination. The fruit are then allowed to ripen for ∼10 days and the pods harvested. The matured cocoa fruit measure between 100 and 350 mm (4 and 14 in) long and have a wet weight of ∼200 g to ∼1 kg (23). A key determinant of properly ripened cocoa fruit is the external appearance. There are considerable variations in the shape, color, and surface texture on the pods depending on genotype. The ripening is visible as changes in the colors of the external pods walls occur, and the nature of color changes is dictated by the genotype of cocoa involved. However, cocoa fruit ripening is generally thought to be from green or purple to varied shades of red, orange, or yellow depending on genotype. The composition of the internal content, comprising the bean and the pulp, will be extensively discussed in the next section, with emphasis on the bean composition and its influence on chocolate flavor precursor formation and development.

2.3 Harvesting and Pod Opening

Harvesting of cocoa fruits involves the removal of pods from the trees and the extraction of the beans and pulp from the interior of the pod. While the ripening process occurs in a 7–10-day period, the pods can safely be left on the trees for up to 2 weeks before harvesting. Thus, a 3-week window exists during which the cocoa may be considered fit to harvest. There are two concerns that dictate how quickly the harvest is completed—the potential for pod diseases and the possibility of bean germination in the pod, if delayed for too long.

During harvesting, a knife or cutlass is normally used to remove the pod from the tree, but a special long-handled tool is available for removing pods that are higher up the tree. After removing the pods from the trees, they may be gathered into heaps and opened immediately or allowed to sit for a few days before opening, which is a technique known as pod storage that has been reported to have significant beneficial effects on the flavor quality of the bean during subsequent fermentation and processing. Much of this depends on the geographic and historical practices encountered in the various growing regions. The actual splitting of the pods is done by a variety of means depending on location, including the use of cutlasses or machetes, or cracking with a wooden billet or club. The practice of cutting with cutlass or machetes requires considerable skill as the beans can easily be damaged during the process and during subsequent penetration by mold and stored product pests, rendering them as defects. Figures 1 and 2 show cocoa pods with their constituent cross-sectional and longitudinal bean arrangements.

original image

Figure 1. Matured ripe cocoa pods.

original image

Figure 2. (a) Vertical sections of unripe and ripe cocoa pods showing arrangements and color of fruits and seeds. (b) Opened unripe cocoa pod showing longitudinal arrangement of fruits.

There are 30–40 beans or seeds inside the pod attached to a central placenta. The beans are oval in shape and enveloped in a sweet, white mucilaginous pulp. After breaking the pod, the beans are then separated by hand and the placenta is removed. A seed coat or testa separates the seed cotyledons from the pulp. Beans taken directly from the pod to controlled drying conditions develop virtually no chocolate flavor after processing, and fresh beans are free from compounds necessary for the development of chocolate flavor. The process of fermentation is therefore necessary for the formation of constituents or flavor precursors that undergo further development during the roasting process. Thus, the mature in-pod cocoa bean is made up of three components—pulp, testa, and cotyledons.

3 Chemical and Biochemical Composition of Cocoa Beans

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

The shell (testa) represents 10–14% dry weight of the cocoa bean, while the kernel or cotyledon is made up of most of the remaining 86–90% (Table 1). The cotyledon confers characteristic flavors and aromas of chocolate (24) and is composed of two types of parenchyma storage cells. Polyphenolic cells (14–20% dry bean weight) contain a single large vacuole filled with polyphenols and alkaloids, including caffeine, theobromine, and theophylline (24). The pigmented polyphenols, when undisturbed, confer a deep purple color to fresh Forastero cotyledons. Lipid-protein cells, on the other hand, have cytoplasms tightly packed with multiple small protein and lipid vacuoles and other components such as starch granules—all of which play roles in defining cocoa flavor and aroma characteristics (9, 25).

Table 1. Bean Composition of Unfermented West African (Forastero) Cocoaa
Constituents Dried beans (%) Fat-free materials (%)
  1. a

    From Refs. 3 and 6.

cotyledons 89.60 
shell 9.63 
germ 0.77 
fat 53.05 
water 3.65 
ash (total) 2.63 6.07
nitrogen    

total nitrogen

 2.28 5.27

protein nitrogen

 1.50 3.46

theobromine

 1.71 3.95

caffeine

 0.085 0.196
carbohydrates    

glucose

 0.30 0.69

sucrose

 1.58 3.86

starch

 6.10 14.09

pectins

 2.25 5.20

fiber

 2.09 4.83

pentosans

 1.27 2.93

mucilage and gums

 0.38 0.88
polyphenols 7.54 17.43
acids    

acetic (free)

 0.014 0.032

oxalic

 0.29 0.67

Reineccius and co-workers (26) reported that fresh unfermented cocoa beans contained 15.8-mg/g sucrose and trace amounts of fructose, sorbose, mannitol, and inositol. Berbert (27) suggested a sucrose content at 24.8-mg/g unfermented beans formed approximately 90% of total sugars (27.1 mg/g). The reducing sugars, fructose, and glucose form approximately 6% (0.9 and 0.7 mg/g, respectively) and others (including mannitol and inositol) at <0.50 mg/g. Differences have been attributed to method and time of harvesting, type, and origin of cocoa beans (22). Tissue components remain compartmentalized, separating flavor constituents that may interact with cell membrane and wall breakdown during the subsequent fermentation.

3.1 Polyphenols and Chocolate Flavor Quality

Cocoa is rich in polyphenols, specifically catechins (flavan-3-ols) and procyanidins, stored in cotyledon pigment cells and cocoa leaves (24). Depending on anthocyanin content, pigmentation in polyphenol-storage cells ranges from white to deep purple. Polyphenol and alkaloids, ca 14–20% bean weight, are central to the bean flavor character (9). Three groups of polyphenols can be differentiated: catechins or flavan-3-ols (ca 37%), anthocyanins (ca 4%), and proanthocyanidins (ca 58%). The primary catechin is (−)-epicatechin, up to 35% of total polyphenols and from 34.65 to 43.27 mg/g of defatted freshly harvested Criollo and Forastero beans (9). Less abundant is (+)-catechin with only traces of (+)-gallocatechin and (−)-epigallocatechin. Nazaruddin and co-workers (25) reported total polyphenols ranged from 45 to 52 mg/g in cocoa liquor, 34 to 60 in beans, and 20 to 62 in powder: (−)-epicatechin contents were 2.53, 4.61, and 3.81 mg/g, respectively.

The anthocyanin fraction is dominated by cyanidin-3-α-L-arabinoside and cyanidin-3-β-D-galactoside. Procyanidins are mostly flavan-3,4-diols that are 4 to 8 or 4 to 6 bound to form dimers, trimers, or oligomers with epicatechin as a main extension sub-unit (24). Fat-soluble polyphenols in dried fat-free fresh Forastero cocoa from 15% to 20%, which falls to approximately 5% after fermentation. Contents of 10% or greater are considered a sign of poor fermentation. Higher concentrations of polyphenols lead to very astringent tasting chocolate. Criollo cocoa beans have approximately two thirds of this content of polyphenols, and anthocyanins have not been found (28). Polyphenol reactions with sugar and amino acids contribute flavor and color to cocoa beans and alkaloids to the bitterness (9). During fermentation, protein breakdown occurs partly by hydrolysis to peptides and amino acids and partly by conversion to insoluble forms by the actions of polyphenols. Polyphenol oxidase promotes oxidative browning to give the characteristic chocolate brown color of well-fermented Forastero beans.

3.2 Effects of Proteins and Sugars on Flavor Precursor Formation

Cocoa cotyledons contain as storage proteins single albumin and globulin species. The globulin, with two polypeptides of 47 and 31 kDa (29, 30), is degraded in fermentation, whereas the albumin (21 kDa) is not. Cocoa-specific aroma precursors can be generated in vitro from globulin in partially purified bean fractions by aspartic endoprotease and carboxypeptidase activities (31). Cotyledon protein degradation into peptides and free amino acids appears central to flavor formation. The consensus is that the combined action of two proteases, namely aspartic endopeptidase and serine carboxy-(exo)peptidase, on vicilin (7S)-class globulin (VCG) storage polypeptide yield cocoa-specific precursors. The aspartic endopeptidase (EC 3.4.23) hydrolyses peptide bonds in VCG at hydrophobic amino acid residues and forming hydrophobic oligopeptides—substrates for the serine exopeptidase (EC 3.4.16.1) that removes carboxyl terminal hydrophobic amino acid residues (32, 33).

Kirchhoff and co-workers (34) observed a correlation between free amino acids accumulation and generation of specific aroma precursors, with pH-dependent proteolytic processes. Activities in both key enzymes are pH-dependent, near to pH 3.8—optimum for aspartic endopeptidase—more hydrophobic oligopeptides and less free amino acids are produced. Whereas close to 5.8—the optimum for serine exopeptidase—there are increases in hydrophilic oligopeptides and hydrophobic amino acids. Related storage proteins or alternative peptidases both failed to produce appropriate flavor precursors. With a rapid fall to low pH (<4.5), reduction in flavor precursors is observed and slow diffusion of organic acids through cotyledons, timing of initial entry, duration of period of optimum pH, and final pH are crucial for final flavor (33). Thus, bean composition interacts with fermentation in formation of cocoa flavor quality. Analysis of VCG proteins and proteolytic degradation products in five popular genotypes (Forastero, Criollo, Trinitario, SCA 12, and UIT1) concluded that the character of chocolate may vary, but all genotypes had the potential for abundant aroma content in raw cocoa (35).

Electrophoretic (SDS-PAGE) analyses showed polypeptide species at 47, 31, and approximately 14.5 kDa all derived from post-translational modification of a vicilin (7S) storage protein precursor observed in vivo as a 139-kDa trimer (32). Polypeptide and cDNA sequence data showed considerable homology to other 7S class storage proteins and, specifically, to α-globulin in cotton seeds (36). Specific cocoa aroma was obtained in vitro when this vicilin globulin was successively degraded by an aspartic endoprotease and a carboxypeptidase and products were roasted in the presence of reducing sugars (31). Acidification during fermentation is critical for final cocoa quality because the different pH optima of endoprotease and carboxypeptidase activities determine efficiency and products of proteolysis. The outcome produces mixtures of hydrophobic and hydrophilic peptides, the latter more important for formation of typical aroma notes. In summary it can be concluded that proteolysis of globulin is central to cocoa flavor formation.

Low-molecular-weight protein breakdown products and reducing sugars all contribute to Maillard reactions that produce cocoa flavor in roasting (7). Peptides and hydrophobic free amino acids, specifically leucine, alanine, phenylalanine, and tyrosine released during fermentation by aspartic proteinase and carboxypeptidase activities (30-32), contribute to flavor (37) by reacting with fructose and glucose (38).

4 Effect of Genotype on Cocoa Bean Flavors

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

Genotype influences both flavor quality and intensity in chocolate (13, 39), likely determining the quantities of precursors and the activity of enzymes, and thus contributions to flavor formation. Reineccius (6) concluded that varietal differences were primarily a result of quantitative (as opposed to qualitative) differences in flavor precursor and polyphenol contents. The contents of sugars and the enzymic breakdown of polysaccharides form an important source of precursors. However, post-harvest processes (fermentation and drying) and roasting have a strong influence on final flavors (4, 40). Four primary cocoa types: forastero (bulk grade), criollo (fine grade), and hybrid, trinitario (fine grade) show wide variations in final flavor (1, 22). Nacional cacao is viewed as a third fine variety: producing the well-known Arriba beans with distinctive floral and spicy flavor notes (1). These differences in flavor can be ascribed to bean composition variation from a botanical origin, the location of growth, and the farming conditions. Bulk varieties dominate blends, while fine grades, used in lesser quantities, are selected to make specific contributions to overall flavor profile.

Each bean variety has a unique potential flavor characteristic. But growing conditions such as climate, amount, and time of sunshine and rainfall, soil conditions, ripening, time of harvesting, and time between harvesting and bean fermentation all contribute to variations in final flavor formation. Table 2 summarizes how differences in genetic origin, cocoa variety, and duration of fermentation influence the flavor profile but how different conditions may lead to significant differences in flavor from a single cocoa variety. A good example is the difference in flavor profile between a single Forastero variety produced originally in Ghana and now grown in Malaysia (4), developing possibly through geographic, climatic conditions as well as duration and/or method of fermentation.

Table 2. Origin, Cocoa Variety, and Fermentation Duration Effects on Flavor Charactera
Origin Cocoa type Duration (days) Special flavor character
  1. a

    From Ref. 3.

Ecuador Nacional (Arriba) 2 short aromatic, floral, spicy, green
Ecuador Criollo (CCN51) 2 acidic, harsh, low cocoa
Ceylon Trinitario 1.5 floral, fruity, acidic
Venezuela Trinitario 2 low cocoa, acidic
Venezuela Criollo 2 fruity, nutty
Zanzibar Criollo 6 medium floral, fruity
Venezuela Forastero 5 fruity, raisin, caramel
Ghana Forastero 5 strong basic cocoa, fruity notes
Malaysia Forastero/Trinitario 6 acidic, phenolic
Trinidad Trinitario 7–8 Long winy, raisin, molasses
Grenada Trinitario 8–10 acidic, fruity, molasses
Congo Criollo/Forastero 7–10 acidic, strong cocoa
Papua New      
Guinea Trinitario 7–8 fruity, acidic

Bulk cocoas typically show strong flavor characteristics, whereas fine cocoas are perceived as aromatic or smoother (40). Clapperton (4) noted consistent differences in flavor attribute, specifically overall cocoa flavor intensity, acidity, sourness, bitterness, and astringency. Bean origins include the West African Amelonado variety (AML), four Upper Amazon clones [Iquitos Mixed Calabacillo 67 (IMC67), Nanay 33 (NA33), Parinari 7 (PA7), and Scavina 12 (SCA12], and Unidentified Trinatario (UIT1) grown in Sabah, Malaysia. The flavor characteristics in UIT1 differed from West African Amelonado, characterized by intense bitterness and astringency associated with caffeine and polyphenol contents. Fermented beans from Southeast Asia and the South Pacific are characterized by a higher acidity (more lactic and acetic acids) than West African beans (4) as a result of varietal differences, box fermentation, and rapid artificial drying.

Cocoa liquors differ in sensory character. The West African group (Ghana, Ivory Coast, and Nigeria) are generally considered sources of standard (benchmark) cocoa flavor with a balanced but pronounced cocoa character with subtle-to-moderate nutty undertones. Cameroon liquors are renowned for bitterness, whereas those from Ecuador for floral-spicy notes. American and West Indian varieties range from aromatic and winy notes from Trinidad cocoa to the floral or raisin-fruity notes of Ecuadorian stocks, making unique contributions to blends. Asian and Oceanian beans exhibit a range of flavor profiles ranging from subtle cocoa and nutty/sweet notes in Java beans to the intense acid and phenolic notes of Malaysian (41). Counet and co-workers (13) reported that fine varieties with short fermentation processes had high contents of procyanidins, while Trinatario from New Guinea and Forastero beans were specifically higher in total aroma. Aroma compounds formed during roasting were found to vary quantitatively and directly with fermentation time and inversely with the procyanidin content of cocoa liquors.

High concentrations of phenol, guaiacol, 2-phenylbutenal, and γ-butyrolactone characterize Bahia beans known for typical smoked notes. Also reported are higher contents of 2-methylpropanal and 3-methylbutanal in Caracas (Venezuela) and Trinidad dried fermented beans (1). Of Maillard products, Reineccius (6) reported that roasting yields higher levels of pyrazines in well-fermented beans (Ghana, Bahia) than in less-fermented (Arriba) or unfermented from Sanchez (Dominican Republic) or Tabasco (Mexico). Lower in astringency and bitterness imparted by polyphenols, Criollo beans, in which anthocyanins are absent, are often less fermented than Forastero (4).

5 Post-Harvest Treatments of Cocoa

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

5.1 Fermentation Processes—Changes in Physical, Biochemical, Microbial Succession, and Enzymatic Activities

Fermentation is essential for development of appropriate flavors from precursors present in cocoa beans. After pod harvest, beans and adhering pulp are transferred to heaps, boxes, or baskets for fermentations lasting from 5 to 6 days for Forastero beans but for Criollo only 1 to 3 days. Figure 3 shows a heap of cocoa beans gathered to undergo heap fermentation at a Ghanaian cocoa farm. During fermentation, microbial activity on the cocoa pulp generates heat and produces ethanol as well as acetic and lactic acids that kill the bean. Until the pods are split, the beans are microbiologically sterile. Once the pod is split, the beans and pulp are exposed to numerous sources of microorganisms, including the farmer's hands and implements, the pod's exterior, and the insect activity on the farms. The immediate effect of this exposure is the initiation of the microbiological attack of the sugar-rich acidic pulp. At the initial stages of the fermentation process, also known as the anaerobic hydrolytic phase, the pulp condition is anaerobic and anaerobic yeast flourish.

original image

Figure 3. Cocoa beans to undergo heap fermentation system in Ghana, West Africa.

The yeasts quickly generate an alcoholic fermentation and the sugars in the pulp are converted to alcohol and carbon dioxide. The citric acid is used in the metabolism of the yeasts. This initiates a slow rise in the pH of the pulp material. The yeasts dominate the first 24–36 hours of the fermentation process, after which the rising pH creates a self-limiting factor on further proliferation. In addition, enzymes released by the yeasts attack the pectin constituents of the cell walls of the pulp mass. The subsequent release of the fluid cell contents runs off the fermenting pulp as what is referred to as “sweatings.” Examples of yeasts isolated during cocoa fermentation include Saccharomyces cerevisiae, Kluyveromyces marxianus, Saccharomyces exiguous, Candida castelli, Candida saitoana, Candida guilliermondii, Schizosaccharomyces pombe, Pichia farinose, and Torulopsis spp. (16).

The continuous breakdown of the pulp and its liquefaction result in the formation of voids between the cells in the pulp. The loss of fluids through the sweating process increases the rate of acid depletion as it is carried away in the runoff. These voids increase in size and allow air to percolate through the pulpy mass. The combination of this change from anaerobic to aerobic conditions in the substrate, the rise in pH as the citric acid is consumed and loss through sweating and increasing alcohol content being generated by the fermentation of the sugars leads to the eventual inhibition of yeast activity. This signals the end of the anaerobic phase of the process.

The second phase known as the oxidative condensation phase occurs under aerobic conditions and is initially dominated by lactic acid bacteria. Lactic acid bacteria increases in numbers when part of the pulp and “sweatings” have largely drained away, and the yeast population is declining. Yeast metabolism favors the growth of acidoduric lactic acid bacteria. Of the lactic acid bacteria isolated from cocoa fermentations Acetobacter lovaniensis, A. rancens, A. xylinum, Gluconobacter oxydans, Lactobacillus fermentum, Lb. plantarum, Leuconostoc mesenteroides, and Lactococcus (Streptococcus) lactis were the most abundant species in the first 24 h of fermentation (16). As the microbial activity increases, the temperature of the bean mass also begin to increase until it reaches approximately 45°C (113°F). The conditions at this temperature are more favorable for the promotion of the growth of acetic acid forming bacteria replacing lactic acid formers as the dominant microflora.

After the decline in the populations of yeasts and lactic acid bacteria, the fermenting mass becomes more aerated. This creates conditions suitable for the development of acetic acid bacteria. These bacteria are responsible for the oxidation of ethanol to acetic acid and further oxidation of the latter to carbon dioxide and water. The acidulation of cocoa beans and the high temperature in the fermenting mass, which causes diffusion and hydrolysis of proteins in the cotyledons, has been attributed to the metabolism of these organisms. Thus, the acetic acid bacteria play a key role in the formation of the precursors of chocolate flavor. In general, the members of genus Acetobacter have been found to be more frequent than those of Gluconobacter. Species of Acetobacter aceti and Acetobacter pasteurianus have been isolated in most cocoa beans (16). The acetic acid formers go on to become approximately 80–90% of the microbial population and their activities (heat and the acidity) eventually leads to the death of the seeds. This result in the breakdown of cellular components and a variety of reactions are initiated.

The increased aeration, increased pH value (3.5 to 5.0) of cocoa pulp, and a rise in temperature to approximately 45°C in the cocoa mass in the later stages of fermentation are associated with the development of aerobic spore-forming bacteria of the genus Bacillus. Many Bacillus spp. are thermo-tolerant, and others grow well at elevated temperatures. B. stearothemophilus, B. coagulans, and B. circulans were isolated from cocoa beans that had been subjected to drying and roasting (150°C) temperatures. Aerobic spore-forming bacteria produce a variety of chemical compounds under fermentative conditions. These contribute to the acidity and perhaps at times to the off-flavors of fermented cocoa beans. Indeed it has been suggested that C3–C5 free fatty acids found during the aerobic phase of fermentation and considered to be responsible for off-flavors of chocolate are produced by B. subtilis, B. cereus, and B. megaterium. Other substances such as acetic and lactic acids, and 2,3-butanediol, all of which are deleterious to the flavor of chocolate, are also produced by Bacillus spp (1, 16). Pulp fermentation products penetrate slowly into beans causing swelling and stimulating enzymic reactions that yield flavor precursors, and on roasting characteristic flavor and aroma notes. Fresh beans with low contents of flavor precursors will have limited commercial usage and activities in fermentation will be unable to rectify this shortfall (31, 37). Appropriate amounts and ratio of precursors are essential for optimal flavor volatiles production in roasting.

Subcellular changes in the cotyledons release key enzymes effecting reactions between substrates preexisting in unfermented beans (28). Enzymes exhibit different stabilities during fermentation and may be inactivated by heat, acids, polyphenols, and proteases. Aminopeptidase, cotyledon invertase, pulp invertase, and polyphenol oxidase are significantly inactivated, carboxypeptidase partly are inactivated, and endoprotease and glycosidases remain active during fermentation (28). During the anaerobic phase, the complex pigment components are attacked by glycosidases and are converted by hydrolysis to sugars and cyanidins. Also, sucrose is converted to glucose and fructose by invertase, the conversion of proteins to peptides and amino acids by proteinase, and the conversion of polyphenols to quinines by polyphenol oxidase. During these processes, the color of the coteledons slowly changes and in the case of Forastero varieties, the deep purple tissue is converted to a red-brown color. As the anaerobic phase nears its termination, the products of the enzymatic actions remain to be further converted in subsequent reactions.

In the aerobic phase, cyaniding and protein-phenolic complexes undergo oxidative reactions that are eventually expressed as the final spread of brown color across the cotyledon surfaces as the re-purple pigments react. Quinone, generated by the actions of the polyphenols-oxidase, now reacts with hydrogen-bearing compounds. These, in turn, form complexes with amines, amino acids, and sulphur-bearing compounds, leading to the lessening of astringency and bitterness during subsequent roasting of the nibs. Clearly, the changes that occur within the bean during fermentation are very complicated and that the hydrolytic and subsequent oxidative reactions generate numerous biochemical complexes that serve as flavor precursors during the roasting process. The genetic makeup of the bean is also certainly crucial to this process. Differences in enzyme activities can be partly explained by pod variation and genotype, but in general, activities present in unfermented beans seem not a limiting factor for optimal flavor precursor formation in fermentation (28). Significant fermentation effects may relate to factors such as storage protein sequence and accessibility, destruction of cell compartmentalization, enzyme mobilization, and pulp and testa changes.

Proteases affect multiple cellular processes in plants, such as protein maturation and degradations associated with tissue restructuring and cell maintenance. Key aspartic proteinases (EC 3.4.23) have been characterized in several Theobroma cacao gymnosperms and activity in seeds of Theobroma cacao extensively studied (32). Partially purified aspartic proteinase had activity optima at 55°C and pH 3.5. Subsequently, Theobroma cacao seed aspartic proteinase was purified into a heterodimer of 29 and 13 kDa polypeptides that efficiently hydrolysed Theobroma cacao seed vicilin and (less effectively) trypsin inhibitor into peptides (30).

Two cDNA species, TcAP1 and TcAP2, respectively, encoding different polypeptides of the plant aspartic proteinase gene family, have been cloned and characterized (42). Both genes are induced early in seed development and show significantly decreased expression as the seeds reach maturity. However, TcAP2 expression is induced to higher levels, suggesting the gene encodes the primary aspartic proteinase in the mature seed. It should also be noted that T. cacao seeds have unusually high levels of such aspartic proteinase activity (31). Physical and biochemical properties of the active T. cacao seed TcAP2 aspartic proteinase complex are novel, suggesting the highly expressed gene product may represent a previously uncharacterized activity. Purified TcAP2 gene product efficiently degrades cocoa seed vicilin into low-molecular products, including di- and tripeptides, implying that this gene product may play an important role during fermentation (32).

A processing sequence is required to produce cocoa beans with good flavor. Pulp sugar fermentation should yield high levels of acids, particularly acetic acid (31). As seed pH decreases, the cell structure is disrupted that triggers mobilization and/or activation of the primary aspartic proteinase activity with massive degradation of cellular protein (32). Fermentation proteinase and peptidase activities seem critical for good flavor quality (33, 42).

Significant differences in enzyme activities exist between cocoa genotypes, but simple and general relationships have not been established between genotype flavor potential and key enzyme activities in unfermented beans. Therefore, how enzymatic processes are regulated, how substrates and products relate to desirable flavors, and the limiting factors for the enzymatic contribution to fermentation processes remain unclear.

Important flavor-active components produced during fermentation include ethyl-2-methylbutanoate, tetramethylpyrazine, and certain pyrazines. Bitter notes are evoked by theobromine and caffeine, together with diketopiperizines formed from roasting through thermal decompositions of proteins. Other flavor precursor compounds derived from amino acids released during fermentations include 3-methylbutanal, phenylacetaldehyde, 2-methyl-3-(methyldithio)furan, 2-ethyl-3,5-dimethyl-, and 2,3-diethyl-5-methylpyrazine (39). Immature and unfermented beans develop a little chocolate flavor when roasted, and excessive fermentation yields unwanted hammy and putrid flavors (6).

5.2 Drying

Flavor development from cocoa bean precursors continues during drying with development of the characteristic brown color. After fermentation, the beans are removed from the heaps or boxes and dried in the sun on raised platforms covered with mats (Fig. 4) or on the ground until fully dried within a sunny 7–8 days. During the process, major polyphenol oxidizing reactions are catalyzed by polyphenol oxidases, giving rise to new flavor components, and loss of membrane integrity, inducing brown color formation. Use of artificial drying can increase cotyledon temperatures causing case hardening. Dimick and Hoskin (43) reported that case hardening restricts loss of volatile acids, with detrimental effects on the final chocolate flavor.

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Figure 4. Drying of cocoa beans on raised platforms.

After fermentation and drying, the target for cocoa beans is ca 6–8% moisture contents. For storage and transport, moisture contents should be <8% or mold growth is possible (5, 22). Indicators of well-dried, quality beans are a good brown color and low astringency and bitterness and an absence of off-flavors such as smoky notes and excessive acidity. Sensory assessment of cocoa beans dried using different strategies, ie, sun drying, air-blowing, shade drying, and oven drying, suggested sun-dried beans were rated higher in chocolate development with fewer off-notes (15). Table 3 summarizes key odorants in cocoa mass after the fermentation and drying stages.

Table 3. Dominant Odor-active Volatiles in Cocoa Massa
Compound Odor quality Flavor dilution factor
  1. a

    From Refs. 1 and 3.

2- and 3-methylbutanoic acidb sweaty 2048
3-methylbutanala,b malty 1024
ethyl 2-methylbutanoatea,b fruity 1024
hexanala,b green 512
unknowna fruity, waxy 512
2-methoxy-3-isopropyrazinea,b peasy, earthy 512
(E)-2-octanala,b fatty, waxy 512
unknowna tallowy 512
2-methyl-3-(methyldithio)furana,b cooked meat-like 512
2-ethyl-3,5-dimethylpyrazinea,b earthy roasty 256
2,3-diethyl-5-methylpyrazinea earthy roasty 256
(E)-2-nonenala,b tallowy green 256
unknowna,b pungent, grassy 128
unknowna,b sweet, waxy 128
phenylacetaldehydea,b honey-like 64
(Z)-4-heptanal a,b biscuit-like 64
δ-octenolactonea,b sweet, coconut-like 64
γ-decalactone b sweet, peach-like 64

Similar flavor compounds in cocoa powder were identified using molecular sensory correlations (12). Off-notes from incomplete drying or rain soaking may result in high levels of water activity and mold contamination, producing high concentrations of strongly flavored carbonyls, leading to alterations in bean flavor, producing hammy off-flavors, which is also correlated with over-fermentation (12, 43).

6 Flavor Development During Cocoa Processing

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

6.1 Effect of Roasting

Roasting of cocoa is an essential step to developing further chocolate flavor from the precursors formed during fermentation and drying. Whole bean roasting loosens the shell, which is then readily removed in winnowing. Prior to roasting, cocoa beans have bitter, acidic, astringent, and nutty flavors. Roasting further diminishes acidity reducing concentrations of volatile acids such as acetic acid (18) but not nonvolatiles such as oxalic, citric, tartaric, succinic, and lactic acids (43). The degree of cocoa roast shows a time/temperature-dependent relationship, over periods of 5 to 120 min and in the range 120–150°C. Low-temperature roasts are employed for milk and certain dark chocolates. An alternative practice is nib roasting, where whole beans are preheated, at just below 100°C, to loosen the shells, which are then removed. The thermal operations to loosen the shell include hot air shock, steam, or infrared heating (5, 9). The nibs are then treated (eg, alkalized) and roasted. The Maillard reaction is central to cocoa flavor development, and it does occur during roasting by reactions of free amino acids, peptides, and reducing sugars (26). Hydrophobic amino acids—leucine, alanine, phenylalanine, and tyrosine—that are released by proteinase activities in fermentation are important contributors (30, 37), so are reducing sugars—fructose and glucose derived from sucrose hydrolysis (38).

6.2 Effects of Alkalization

Alkalization (treatment of cocoa nibs or liquor with solutions of alkali) is carried out primarily to change color but also to influence the flavor of cocoa powder. Alkalizing is common for cocoa products such as drinks to enhance solubility or in baking or coatings (1). Cocoa nibs from Malaysia and Brazil are characterized by high acidity and a low chocolate flavor, limiting possible character developments in processing, and they showed that improvements in quality of cocoa nibs and liquors from these origins could be achieved by alkali treatments reducing acidity before nib roasting or thin-film processing (43). Alkalizing Malaysian cocoa nibs to pH 6.0 did not change flavor relative to a control, but chocolates from nibs alkalized to pH of 7.2 and 8.1 were significantly different and dark chocolate prepared from Ivory Coast, Malaysian, and Brazilian cocoa had sour, bitter, fruity, and moldy notes significantly changed by alkali treatment. The conclusion was that chocolates from alkalized and thin-film processed cocoa liquor had better flavors than nonalkalized nib-roasted chocolate. Alkalization reduces acidity as well as astringency with aspects like typical cocoa and bouquet enhanced and intensified. Reductions in astringency are effected by further polymerizations of flavonoids during alkali treatments (43).

7 Flavor Development During Chocolate Manufacture

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

7.1 Conching

Conching is regarded as essential for final flavor development and appropriate texture. This is the final stage in chocolate manufacture, whether dark or milk. Residual volatile acids and moisture are removed, angular sugar crystals and viscosity are modified, and the color is changed as a result of emulsification and oxidation of tannins (3, 6). Generally a two-stage process, the first stage converts flake or powder into a paste by mechanical or heat energy, driving off moisture and undesirable volatiles, and affects oxidations and distributes lipids through a continuous fat phase. Fowler (22) suggested oxidations modify precursors developed in fermentation and roasting processes to achieve final cooked flavor and eliminate undesirable astringent and acidic notes. The second stage converts the thick paste into a free flowing liquid through addition of cocoa butter and lecithin.

Conching conditions show interactions between time and temperature so that higher temperatures reduce processing time. Conching conditions for crumb milk chocolate are 10–16 h at 49–52°C but 16–24 h at 60°C for milk powder chocolates; temperatures above 70°C lead to changes in cooked flavors. Dark chocolates are typically conched at higher temperatures, 70°C or up to 82°C. Conditions may be modified (generally shortened) by pretreatment of chocolate liquors as thin films at temperatures >100°C (1, 9). The air spaces surrounding a conche in operation have an odor of acetic acid, suggesting an initial loss of short-chained volatile fatty acids, such as acetic acid, the end products of fermentation. This was confirmed by quantitative studies (43). Volatile phenols show 80% reductions in headspace concentrations within a few hours of conching (44). Hoskin and Dimick (45) reported that phenols decreased from 21.3 µg/100 g to 10.9 µg/100 g after 44 h in low roast chocolate and from 10.3 µg/100 g to 6.0 µg/100 g after 24 h in high roast in conching. In a later paper, Dimick and Hoskin (43) concluded that polyphenols, through oxidation and enzymatic mechanisms, form complexes with amino acids, peptides, and proteins. The outcome is withdrawal of flavor-active volatiles from headspaces and reductions in perceptions of astringency through irreversible phenol interactions, as well as more “mellow” final flavors. Typical milk and dark chocolate products are as shown in Figure 5.

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Figure 5. Chocolate products.

During conching of dark chocolate, amino acid concentrations do not fall as temperature and/or the concentrations of amino acids and sugars are below thermal thresholds for Maillard reactions. The Amadori compounds formed in drying and roasting decrease during conching A consensus is that chocolates show marked decreases in overall off-flavors after conching (13, 43-45). Counet and his co-workers (13) concluded that key dark chocolate odorants were present prior to conching, during which Strecker aldehydes were partially lost through evaporation and/or chemical reactions. On the other hand, 2-phenyl-5-methyl-2-hexenal content was increased through aldol condensation of phenylacetaldehyde and 3-methylbutanal followed by dehydration (13). Also, furaneol and maltol were generated during conching. Of heterocycles, only concentrations of the least volatile compounds were increased, notably polysubstituted ethyl-, isobutyl-, and isopentylpyrazines, tri- or tetramethylpyrazine, furans, and acetylpyrrole.

8 Conclusion

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography

Chocolate flavor resides in not only a volatile aromatic fraction of flavor-active components but also in nonvolatile compounds influencing taste perception. Its complex composition depends on the cocoa bean genotype, specifically on the contents of bean storage proteins, polysaccharides, and polyphenols. The inheritance and regulation of such flavor origins remain an area for advanced research. Enzymic and microbial fermentations after harvest induce physical and chemical changes in beans over 5 to 7 days with key browning reactions of polyphenol with proteins (ca 12–15% total) and peptides giving colors characteristic of cocoa. Drying limits mold growth during transportation and storage, reducing bean moisture content from 60% to 8%. Sun drying is favored for flavor development and can be carried out above or on hard surfaces, with differences in air flow and final moisture content. Beans are transported under controlled storage conditions to chocolate manufacturing sites, or they are processed in the origin country to add value with requirements for traceability in quality assurance. Following critical review of the entire process, a summary of the parameters important for chocolate flavor generation has been developed (Fig. 6). An appropriate starting composition can be converted through controlled post-harvest treatments and subsequent processing technologies to a high-quality flavor character. Cocoa bean fermentation is crucial not only to the formation of key volatile fractions (alcohols, esters, and fatty acids) but also to provision of flavor precursors (amino acids and reducing sugars) for important notes contributing to chocolate characteristics. Drying reduces levels of acidity and astringency in cocoa nibs by decreasing the volatile acids and total polyphenols.

original image

Figure 6. Mechanism of chocolate flavor formation and development from raw cocoa to processed chocolate.

Maillard reactions in roasting convert flavor precursors formed during fermentation into two main classes of flavor-active component: pyrazines and aldehydes. Although no new key odorants are synthesized during conching, the levels of 2-phenyl-5-methyl-2-hexenal, furaneol, and branched pyrazines significantly increase and form key odorants in both milk and dark chocolates, while Strecker aldehydes are lost by evaporation. These processes suggest an important role of conching in chocolate manufacture in determining final flavor characteristics. Direct relationships are thus observed between the initial composition and post-harvest treatments (fermentation and drying) of cocoa beans and the subsequent processing (roasting and conching) and technological effects on the flavor formation, development, and character in chocolate. However, comparison of flavor characteristics in chocolate is complicated by variations caused by different genotypes, geographical origin, pod differences, fermentation and drying methods, and subsequent processing (roasting, alkalization, and conching). Although this review suggests major causes of variations, it is still premature to conclude that it is fully understood. There have been few systematic studies and difficulties in comparing cocoa varieties grown under different conditions with various fermentation methods. Therefore, conclusions are difficult to make on how enzymatic processes are regulated, which enzyme substrates/products are related to good flavor character and limiting factors for the enzymatic processes (enzyme activity, substrate or enzyme availability, cocoa genetic background, growth conditions, or the post-harvest treatments), and finally the process controls in chocolate manufacture. To understand the variations in chocolate character fully, further research is required to optimize the post-harvest treatments (pod storage, pulp preconditioning, depulping, fermentation, and drying) of cocoa beans differing in genotype, subsequent manufacturing processes (roasting, alkalization and conching) during chocolate manufacture, as well the sensory strategy in evaluation of final flavor character in chocolate.

Bibliography

  1. Top of page
  2. Introduction
  3. Cocoa Cultivation and Practices
  4. Chemical and Biochemical Composition of Cocoa Beans
  5. Effect of Genotype on Cocoa Bean Flavors
  6. Post-Harvest Treatments of Cocoa
  7. Flavor Development During Cocoa Processing
  8. Flavor Development During Chocolate Manufacture
  9. Conclusion
  10. Related Articles
  11. Bibliography