There is a commonality in terms of enzyme systems present and raw material composition among many commercially important plants. This is reflected by the relative ubiquity of a number of metabolic pathways as exemplified by the terpenoid and phenylpropanoid complexes with their associated enzymes and substrates [1].
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There is a commonality in terms of enzyme systems present and raw material composition among many commercially important plants. This is reflected by the relative ubiquity of a number of metabolic pathways as exemplified by the terpenoid and phenylpropanoid complexes with their associated enzymes and substrates [1].
A similar situation prevails for processing where many similar operations are conducted. There are however some differences in treatment details between different raw materials; these being specific for selected starting materials. Transfer of technologies from one raw material to another presents potential routes for novelty in the quality and flavor of final products. This was exemplified by comparison of the processing of tea leaves and the curing of vanilla beans [2]. Similarities and differences exist between cocoa and vanilla bean processing and their final flavor compositions. This paper examines these two raw materials and considers opportunities in vanilla bean curing based on a knowledge of cocoa bean processing.
Cocoa Beans and Cocoa Bean Processing
There are three major genotypes of cocoa (Theobroma cacao) namely:
- Forestero originating from the Amazon basin and representing ca.70% of world production. Major growing areas are West Africa, South America and Asia; these origins produce beans with the strongest flavor. The smooth yellow pods contain pale purple beans.
- Criollo: a native of Central America accounting for 5-10% of global production. Grown in Indonesia and Central and South America, Criollo has a mild or weak chocolate flavor. Criollo trees produce softer red pods containing 20-30 white ivory or very pale purple beans.
- Trinitario: a cultivated hybrid of Forestero and Criollo. The crop is grown mainly in the Carribean, but also in the Cameroons and Papua, New Guinea. The pods contain 30 or more beans of variable color, though white beans are rare.
Cocoa trees begin to bear fruit when they are three to four years old, where the pods grow out of the trunk and main branches. Only a small proportion of the flowers develop into fruit over a maturation period of about five months. Each tree yields 20-30 pods per year. It takes the whole year’s crop from one tree to make 450gm of chocolate. The cocoa tree bears two harvests of cocoa pods per year. Around 20cm in length and 500gm in weight, the pods ripen to a color characteristic of the genotype. Within each pod there are 20-40 colored, 2cm long, cocoa beans covered in a sweet white pulp.
The harvesting of cocoa pods is very labor intensive. Ripe pods are gathered every few weeks during the peak season. The collected pods are split open by hand, resulting in beans covered in a sweet white pulp, which are then removed for the fermentation and drying processes. These processes prepare the beans for market and represent the first stage in the development of chocolate flavor [3].
The final flavor of the cocoa bean will depend on the genotype, growing conditions, the bean maturity and the processing conditions employed. Processing cocoa beans involves a number of key steps, such as fermentation, drying, winnowing and roasting [3].
Processing Cocoa Beans
Fermentation
The pods are broken open and the combined beans and pulp are removed. During fermentation the cocoa pulp, clinging to the beans, gradually liquefies and drains away. The fermentation method employs two basic procedures, namely heaps and sweating boxes.
The heap method traditionally involves piling wet cocoa beans, surrounded by the pulp, on banana leaves that are spread out on the ground. The heap is covered with more leaves and left for five to six days, with regular turning to ensure even fermentation.
In large plantations in the West Indies, Latin America and Malaysia, wooden boxes with drainage holes in the base are used, allowing ingress of air and drainage of liquid. This process takes six to eight days at unregulated temperatures, though usually in the range of 43o-49o C. During this time, the beans are frequently mixed to achieve process uniformity.
During fermentation, the chocolate aroma precursors begins to develop. Classical fermentation of cocoa beans involves an initial anaerobic phase, of ca. 48 hour duration, resulting in the production of ethanol and liberation of carbon dioxide. This is followed by an aerobic stage which generates lactic acid and additionally transforms ethanol into acetic acid. There is an overlapping microbial sequence involved that is dominated initially by yeasts; this is followed by lactic acid and acetic acid bacteria [4,5]. It is during this latter phase that the formation of chocolate aroma precursors is reported to occur.
A patent by Mars Incorporated provided methods for processing cocoa beans without the requirement of microbial fermentation. Beans were treated, with or without adhering pulp, to a solution of ethanol/water of a volume sufficient to cover the mass, while maintaining the solution at set temperatures and times [6].
The range of operating conditions reported in this patent included:
- Ethanol concentrations in the range of 7-16% v/v optimally containing up to 12mg of citric acid or 1-5mg acetic acid per g of ethanol solution. These conditions were such that microbial fermentation was inhibited.
- The beans, generally free from pulp, were incubated intact or following mechanical damage by cutting, exposure to freeze/thaw or treated to elevated temperatures.
- Incubations were carried under vacuum, or pressure, at 45o-51oC from 24-96 hours.
It would appear that ethanol and perhaps the acidic environment, provided by acetic or citric acid, has a part to play in this substitute process to classical fermentation. It seems that the activity of microorganisms in the classical fermentation process serves only for the anaerobic production of ethanol. Ethanol may additionally participate in ester formation or other reactions that contributes to the subsequent flavor generation step during roasting. Ethanol at the concentrations employed is reported to inhibit fermentation. It is known that ethanol also perturbs the permeability of phospholipid bilayer double membrane of cells and organelles. In the cocoa bean, this probably facilitates loss of compartmentation and cellular integrity resulting in extensive hydrolytic activity [7]. Cellular integrity is additionally compromised by mechanical disruption or freeze /thaw treatment.
Lima et al (2011) reported that in the fermenting cocoa bean, the loss of membrane integrity resulted in a cascade of reactions leading to the formation of cocoa flavor precursors. The principal precursors included reducing sugars, amino acids and peptides. The enzymes involved in these transformations include invertase, aspartic endoprotease and carboxypeptidase, respectively. Invertase present in the shell and cotyledons converts sucrose to the reducing sugars glucose and fructose [9].
In vitro studies by Voigt et al (1994) using the above proteolytic enzymes, showed that peptides and hydrophobic amino acids produced by enzymatic hydrolysis were correlated with the formation of cocoa aroma and that these precursors were themselves derived from vicilin-like globular proteins. The products from the amino acids/peptides and reducing sugars reacted during roasting, mainly by Maillard and Strecker degradation reactions, have formed key Strecker aldehydes and pyrazines [11].
At the end of the ethanol incubation period, beans were dried, winnowed, roasted and milled in the traditional way to produce good quality—in terms of final product flavor—cocoa powder and cocoa butter. These products were comparable in flavor quality to material generated by traditional fermentation procedures.
Drying
When the traditional fermentation process was employed, the wet mass of beans were dried either by spreading in the sun on mats or by using special drying equipment. Slow drying reduced the moisture content in the bean from around 60% to about 7%. It appears that the drying stage also contributes to the formation of flavor precursors. Dried beans were packed into sacks for transportation prior to further processing.
Winnowing
This is the process whereby the dried beans are cracked and a stream of air is used to separate the shell from the nib; these latter small pieces, rich in triglycerides, are used to make chocolate. The winnowing process may take place either before or after roasting depending on the end use of the beans.a
Roasting
The separated nibs are roasted in special ovens at temperatures between 105o-120oC. During this process great care is taken to prevent the nibs from under-or over-heating. The roasting conditions employed depend on the type of beans and their end use in cocoa or chocolate production. During roasting, the cocoa nibs darken to a rich brown color and acquire their characteristic chocolate flavor and aroma.
Grinding
Once roasted, the nibs are ground in stone mills which reduces them to a thick chocolate-colored liquid (mass). This mass contains 53-58% cocoa butter, which solidifies on cooling. This cocoa butter-rich material is the basis of all chocolate and cocoa product manufacture.
Pressing
The solid cocoa butter-rich mass is pressed to extract the cocoa butter for chocolate application. The solid blocks of compressed cocoa (press cake) that remains after extraction are pulverized into a fine powder. The resultant high-grade cocoa powder is used in the manufacture of tobacco products, cookies, seasonings, beverages or other food products. The flow chart for processing of cocoa beans is outlined in F-1.
The Aroma of Roasted Cocoa Beans
Ripe cocoa beans contain on average 60% moisture, and are rich in triglycerides (50-55% on a water free basis). Other components present in excess of 4% include phenolic compounds (5-10%), starch (8-10%) and protein (8-10%) [3].
About 500 volatile compounds have been identified in the aroma of cocoa [3], with aldehydes and pyrazines providing the major aromatic components of the roasted bean. The fermentation process hydrolyzes the endogenous vicilin-like proteins, while releasing the amino acids alanine, glycine, valine, leucine, isoleucine, threonine, phenylalanine, tyrosine and methionine. During roasting, these amino acids undergo Strecker degradation reactions in the presence of Strecker catalysts including α-dicarbonyl compounds, α-hydroxy carbonyls or α-unsaturated carbonyls. These Strecker catalysts include pyruvaldehyde, diacetyl, acetoin and (E)-2-hexenal [12].
Some of these Strecker reagents exist naturally in plants or are generated in response to tissue wounding. Others are derived thermally by scission of Maillard intermediates. From a typical amino acid, the Strecker reaction can lead to Cn-1 aldehydes or pyrazines. For the former, the reaction involves interaction of an α-amino acid with a di-carbonyl or other Strecker catalyst in an aqueous solution or suspension to produce carbon dioxide and an aldehyde or ketone containing one less carbon atom than the original amino acid.
Many of the Strecker aldehydes are odor potent compounds. These include phenylacetaldehyde derived from phenylalanine and 3-methylbutanal originating from leucine.
Aldehydes produced by Strecker reaction can react further through aldol condensation followed by a crotonisation reaction to form a group of α-substituted, α, β-unsaturated aldehydes. Some of these latter compounds are odor potent and exhibit a characteristic cocoa flavor character. Examples of these include the aldehyde products from the condensations between phenylacetaldehyde and acetaldehyde or 3-methylbutanal. The reaction between phenylacetaldehyde and acetaldehyde to form the so-called cocoa aldehyde 2-phenyl-2-butenal is shown in F-2.
Frauendorfer and Schieberle [13] examined the changes in key aroma compounds of Criollo cocoa beans during roasting. Comparative aroma extraction dilution analysis of the beans revealed 42 aroma compounds in the flavor dilution (FD) range of 1-4096 for fermented/ unroasted versus 4-8192 for fermented/roasted beans. The same compounds were present in both untreated and roasted beans though they differed in relative intensities. Highest FD factors for unroasted were found for 2- and 3-methylbutanoic acids (rancid) and acetic acid (sour), while for the roasted beans the highest FD values were observed for 3-methylbutanal (malty), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like) and 2- and 3-methylbutanoic acids (sweaty).
Determined concentrations and odor activity values revealed 22 compounds in unroasted and 27 in roasted with concentrations above their odor thresholds. Particularly strong increases in roasted beans were observed for the Strecker aldehydes 3-methylbutanal and phenylacetaldehyde, as well as 4-hydroxy-2,5-dimethyl-3(2H)-furanone. This suggests that these compounds contribute most to the changes in total aroma as a consequence of roasting.
Other compounds, including 3-methylbutanoic acid, were present in the unfermented unroasted beans and showed no increase on roasting.
The fermentation process in cocoa beans displays a spectrum of aroma compounds dominated by the methylbutanoic and acetic acid. The roasting stage for cocoa beans produces significant increases in 4-hydroxy-2,5-dimethyl-3(2H)-furanone, as well as the Strecker aldehydes 3-methylbutanal and phenylacetaldehyde. Further reaction between phenylacetaldehyde and acetaldehyde or 3-methylbutanal can form the odor potent 2-phenyl-2-butenal and 2-phenyl-5-methyl-2-hexenal, respectively – the so-called cocoa aldehydes. There is no comparable heating stage in vanilla curing akin to the conditions of 105o to 120oC for 1-2 hours which is employed in cocoa bean roasting.
Cured vanilla aroma
At least 200 aroma compounds have been identified in solvent extracts of cured Vanilla planifolia beans [14].
Identification of the key compounds and their quantification by GC-MS in conjunction with determination of flavor dilution (FD) factors was conducted on extracts of MV red Madagascan V. planifolia whole cured beans. Aroma extraction dilution analysis of the sample detected 15 odor active compounds with FD factors of >125. The seven most important of these compounds included vanillin, guaiacol, ethyl-(E)-cinnamate, 2- and 3-methylbutanoate, β-damascenone and p-cresol [15]. The flavor contribution of all 15 compounds was confirmed by reconstitution experiments. The flavor formulated on the combination of these compounds showed strong similarity to the original MV red whole bean extract; this is based on sensory evaluation of seven key vanilla aroma attributes.
Earlier work by Perez-Silva et al (2006) identified a total of 65 aroma compounds in solvent extracts of cured Mexican V. planifolia beans. Of the total, some 23 were identified and considered to be the compounds accounting for most of the aroma of the cured beans.
Of these organics there were twelve phenols, five aliphatic acids, two C4 alcohols and ketones and four mono-and di-unsaturated aldehydes. Reconstitution experiments based on the above compounds, at their measured concentrations in the cured beans, were sensorially similar to a Mexican vanilla extract [17].
The biochemical and/or chemical origin of most of these aroma compounds has not been established, though it is clear that virtually all are released or generated during curing. Vanillin and most of the other identified phenols were present in the green beans as their β-D-glucosides are liberated by hydrolysis primarily during the fermentation stage of curing [18].
Further transformation of some of the liberated phenols may occur to realize additional functionalized phenols. Non-phenolics may arise from polyunsaturated fatty acids by β-oxidation or lipoxygenase-type reactions and/or fatty acid synthesis. These latter reactions realize short chain, mainly unsaturated aldehydes, that contribute to the overall final flavor impact of cured vanilla beans [19, 20]. Amino acids by Strecker type-degradation or enzymatic transformation can realize Strecker aldehydes and the C4 hydroxyl and keto compounds acetoin and diacetyl, respectively [21]. Both diacetyl and acetoin are Strecker catalysts.
In the vanilla bean, the primary reaction involves release of the free phenols by hydrolysis of their β-D-glucosides. Secondary reactions involving further transformation of the liberated mono-phenols may occur under mild drying conditions employed. Of the other key compounds in cured vanilla beans, unsaturated fatty acids are likely to be involved in their formation. In cocoa bean roasting, the major important aroma compounds appear as a result of thermal processing. These include 4-hydroxy-2,5-dimethyl-3(2H)-furanone and the Strecker aldehydes 3-methylbutanal and phenylacetaldehyde. The major difference between the two raw materials in terms of flavor generation may be a reflection of more extensive hydrolysis of selected proteins and subsequent thermal treatment during cocoa bean processing.
Similarities in Vanilla and Cocoa Bean Processing
There are, however, some parallels between curing of vanilla and cocoa bean processing in terms of the common stages of fermentation and drying. Traditionally, both of these appear to be important. It is significant, however, that the Mars Incorporated patent [6] describes a process where incubating the cocoa beans with ethanol/water solutions appears to provide beans—that on subsequent drying, winnowing, roasting and milling in the traditional way— produced good quality final product flavor cocoa powder and cocoa butter that is comparable to products generated by traditional fermentation procedures. Clearly the incubation of cocoa beans with ethanol/water mixtures, or as the result of mechanical or freeze damage, must facilitate the release of free amino acids and reduction of sugars, since these compounds are responsible for the main aroma active compounds formed during roasting.
Classical alcoholic fermentation does not occur in vanilla sweating, nor is there the significant elevation of temperature associated with cocoa processing. The fermentation stage during curing of vanilla beans is primarily important in the liberation of the phenols, including vanillin, by enzymatic hydrolysis of their corresponding β-D-glucosides, though a number of other flavor forming reactions also occur [18].
There is no roasting stage employed in vanilla curing comparable to that employed in cocoa.On this basis, it is reasonable that there is an absence of Strecker aldehydes or related compounds in the flavor of cured vanilla beans.
Opportunities for novelty in vanilla curing can be evaluated using processing regimes based on cocoa treatments by subjecting the cured vanilla beans, post fermentation, to elevated temperatures over extended time periods. Such products would be expected to have high content of the characteristic phenols with associated caramellic and other notes due to Strecker type reaction products such as 3-methylbutanal and other thermal process notes including 4-hydroxy-2,5-dimethyl-3(2H)-furanone.
Summary
Although superficially the curing of vanilla beans and the processing of cocoa beans appear similar, overall they are quite different in terms of the biochemistry and chemistry of flavor development. Vanilla flavor is dominated by phenols and their transformation products mostly derived from enzymatic hydrolysis of the phenyl-β-D-glucosides. Other key aroma compounds are non-phenolic in origin and appear to be linked to transformation products of unsaturated fatty acids and of threonine through 2-acetolactate to the linear C4 hydroxy and keto compounds, including acetoin and diacetyl. Cocoa bean flavor is dominated by Strecker aldehydes and pyrazines and other products generated during the roasting process of the beans.
Fermentation in vanilla beans is essential for liberation of the phenols from their precursor glucosides, whereas in cocoa the key reaction appears to be the release of free amino acids from their protein precursors and reducing sugars from sucrose. The role of ethanol/water incubation of cocoa beans as an alternative to traditional fermentation is not fully understood. It seems likely however that ethanol disrupts cell and organelle membrane integrity in the cocoa bean cotyledons resulting in enzymatic hydrolysis of sucrose to glucose and fructose along with proteolysis of vicilin-like proteins to peptides and amino acids.
Similar membrane disruption also seems to be initiated by mechanical damage or freeze/thaw pre-treatment. The final products from these interventions are the precursors of the Maillard and Strecker generated flavor compounds produced during cocoa bean roasting. Surprisingly, in this roasting process there appears to be an absence of key aroma compounds related to transformation of the resident cocoa bean triglycerides; though these glycero-lipids are relatively low in polyunsaturated fatty acids and are thus more stable to oxidative and other transformations.
Incorporation of a significant heating stage during vanilla curing could provide opportunities for the production of novel flavor notes in cured vanilla beans.
Acknowledgements
To the growers and harvesters of cocoa beans and the scientists who have developed our understanding of the biochemistry and chemistry leading to the unique flavor that is cocoa.
Note
For further information please contact Patrick Dunphy at: [email protected]
