Identification of fine aroma components in two genotypes from the cocoa tree

Scientists have for some time been investigating the effects of different fermentation times on the development of fine aroma in cocoa. In addition, the identification of fine aroma components in two genotypes of the cocoa tree can facilitate better differentiation between fine cocoa and consumer cocoa. The scientific essay by Dr. Daniel Kadow from the research team at the University of Hamburg sheds light on the formation of the multidimensional flavour properties in post-harvest processing. An article by Daniel Kadow, Jörg Bohlmann, Wilberth Phillips, Sascha Rohn and Reinhard Lieberei.

Scientists have for some time been investigating the effects of different fermentation times on the development of fine aroma in cocoa. In addition, the identification of fine aroma components in two genotypes of the cocoa tree can facilitate better differentiation between fine cocoa and consumer cocoa. The scientific essay by Dr. Daniel Kadow from the research team at the University of Hamburg sheds light on the formation of the multidimensional flavour properties in post-harvest processing. An article by Daniel Kadow, Jörg Bohlmann, Wilberth Phillips, Sascha Rohn and Reinhard Lieberei.

The cocoa tree (Theobroma cacao L.) originates from the tropical rainforests on the eastern slopes of the Andes (Bartley, 2005). Today it is cultivated in tropical regions throughout the world and is an important, stabile source of income for millions of small private farmers. The cocoa tree’s seeds, often referred to as cocoa beans, are the key raw material for the manufacture of chocolate (Motamayor et al., 2008). The cocoa tree is distinguished by a high level of genetic diversity (among others, Motamayor et al., 2008) reflected for example by its broad range of fruit colours and forms (Ill. 1) and olfactory and taste properties (“flavour”). Fundamentally, cocoa flavour has a variety of facets. Bitter and acid notes accompany the characteristic chocolate note. In addition, its astringency is one of cocoa’s centrally important tactile properties. Flowery, fruity and nutty notes are additionally present. Cocoa’s flavour properties can therefore be described as multidimensional (Ill. 2). Flowery, fruity and nutty notes are also characterised as fine aromas. They do not appear in every cocoa, so dealers decide between consumer cocoa and fine cocoa depending on the variety. Due to its special flavour properties, the latter variation is often used for dark chocolate and is sold for a higher price than consumer cocoa (Donovan, 2006).

Not all of the flavour properties of cocoa are already developed when cocoa is harvested. Instead, fresh, unprocessed cocoa seeds primarily feature heavy bitterness and a dominant astringent characteristic. It does not develop a chocolate flavour during roasting, because the preliminary stages required for this are missing and are only formed later in post-harvest processing (e.g. Ziegleder and Biehl, 1988) that the cocoa seeds undergo in the countries where they are cultivated. This post-harvest processing consists of fermentation and drying. The fermentation causes the fruit pulp encasing the seeds to be decayed by microorganisms. This leads to the formation of organic acids that permeate the cocoa seed tissue and generate a significant temperature increase in the fermentation mass. The effect of the acid and heat result in the cocoa seed tissue dying. As a result, the formation of the preliminary stages of the chocolate flavour begins with a corresponding reduction of bitterness and astringency (e.g. Schwan and Wheals, 2004; Afoakwa et al., 2008). At the same time, a transformation and subsequent loss of the fruity and flowery fine aroma components also begins. In contrast, nutty fine aroma components are apparently formed at the start of the fermentation process, but diminish as the process nears its end (Ill. 3). As such, the fermentation duration and the variety are centrally important for the flavour properties of raw cocoa. Long fermentation times (e.g. 7 days) significantly enhance the chocolate flavour with little bitterness and little astringency, causing a weak acid note and the almost complete loss of the fine aroma properties. In contrast, a medium fermentation duration (e.g. 4 days) produces a comparatively flat chocolate flavour, stronger bitterness, stronger astringency and a distinct acid note with simultaneous retention of the fine aroma properties. An insufficient fermentation time generates flavour properties like those of fresh seeds with a distinct astringency and bitterness as well as no chocolate flavour (Ill. 3).

As regards most of the flavour characteristics, the substances or preliminary stages largely responsible for these are already known. The preliminary stages of the chocolate flavour consist of short-chained peptides, free amino acids and reducing sugar formed as part of the storage substance breakdown in the cocoa seeds (among others, Ziegleder and Biehl, 1988). The breakdown is a result of enzymes inherent to the seeds. The two alkaloids theobromine and caffeine are important bitter components, and reaction products resulting from theobromine with diketopiperazines also apparently make an important contribution to the bitter taste. The diketopiperazines, which themselves have a bitter taste, and portions of the chocolate flavour preliminary stages emerge from the storage proteins of the cocoa seeds (Schwarz, 2011). The astringency is primarily attributable to phenolic substances (e.g. catechins and their oligomers), which make up some 15% of the dry weight of cocoa seeds (Kim and Keeney, 1984). In contrast to these components formed in the seeds, the acid note is generally based primarily on the acetic acid and lactic acid formed in the fruit pulp during fermentation. As regards the fine aroma, nutty notes are evidently attributable to larger oligopeptides which are abundant, above all with shorter fermentation times (Voigt et al., 2013). In contrast to this, little is known about the substances responsible for the flowery and fruity fine aromas. It has been reported that the monoterpene linalool may be associated with the flowery fine aroma (among others, Ziegleder, 1990). The authors demonstrate that fermented and dried fine cocoa contains larger quantities of linalool then consumer cocoa. The flowery and fruity fine aroma components apparently come from the fruit pulp (among others, Eskes et al., 2007). The authors examined the sensory properties of the fruit pulp of different cocoa genotypes, including EET62, a fine cocoa from Ecuador and CCN51, a consumer cocoa with no fine aroma. The sensory characteristics of the fruit pulps corresponded with fine aroma properties of the respective raw cocoa, leading Eskes et al. (2007) to the conclusion that the responsible components originate in the fruit pulp and migrate into the cocoa seed tissue during fermentation in the same way as acetic acid and lactic acid. This hypothesis is underscored by the finding that cocoa seeds fermented in the presence of Annona muricata fruit pulp ultimately display flavour properties from these fruits. Interestingly, Quijano et al. (2010) was able to establish the presence of linalool in the fruit pulp of fresh, unprocessed cocoa seeds, equally supporting the hypothesis from Eskes et al. (2007) that fine aroma components originate from the fruit pulp. These findings aside, little is known nevertheless about the substances and the corresponding genotypic differences responsible for the fine aroma properties.

The volatile fruit pulp components of cocoa genotypes EET62, SCA6 and CCN51 were examined within the framework of the work presented here. EET62 is a fine cocoa whose fruit pulp and raw cocoa displays primarily fruity, but also flowery fine aromas. SCA6 is known for its flowery notes in both the fruit pulp and the raw cocoa. CCN51, a consumer cocoa, has no fine aroma whatsoever (Eskes et al., 2007).

The same applies for 2-Heptanon (Mosciano, 1991a). 2-Heptanol is described as fruity and flowery; 2-Nonanon as sweet and fruity (Mosciano, 1991b). In contrast to this, the sensory properties of ?-cis-Ocimene and ?-Linalool are described primarily as flowery, while the sensory properties of ?-Myrcene and ?-trans-Ocimene are described as lemony and sweet (Mosciano, 1996 and 2000). Table 1 outlines the sensory properties of the volatile compounds characteristic of EET62 and SCA6. These concur with the sensory descriptions for the pulp and raw cocoa of the two genotypes. It can therefore be concluded that 2-Heptanol acetate, 2-Heptanol, 2-Heptanone and 2-Nonanone are important components of the fine aroma of EET62 and ?-Myrcene, while ?-cis-Ocimene, ?-trans-Ocimene and ?-Linalool are important components of the fine aroma of SCA6. However, since the amount of these present in a compound does not guarantee that they are also relevant in sensory terms, corresponding olfactometric provisions are necessary. The corresponding examinations are currently being conducted on the 3 genotypes.

These findings could be applied in the fields of quality assurance and product development. Up until now the classification “fine cocoa” has been made by the trade sector primarily on the basis of the cocoa’s origin. However, one can now observe a change on the plantations in many countries in favour of the actual plant material. This also coincides with changes in the aroma potential. Of even greater significance is the fact that, depending on the post-harvest processing protocol, the fine aroma can be weakened or even lost completely. Detailed information on the compounds responsible for the fine aroma compounds make it possible to establish analysis procedures or quick methods with which the fine aroma potential of a raw cocoa batch can be quantified.

In addition, the post-harvest processing and chocolate manufacture can also be adapted based on this knowledge in the interest of retaining and perhaps even emphasising the genotypic fine aroma. The change in the contents of the compounds responsible for the fine aroma can be examined in this regard following every processing step.

There are also application possibilities in the field of cultivation. Previous projects have focused primarily on resistance and yield properties (e.g. Schnell et al., 2007). There are advanced procedural methods for this, in particular as regards support by molecular markers. In contrast, no molecular markers have been established up to now for fine aroma properties. A corresponding development in this regard will be greatly simplified by information on the substance-related background of fine aroma. This would enable fine aroma properties to play a greater future role in cultivation.

Ultimately, this methodology can also be used for examining the fine aroma potential of additional genotypes. As such, the variability so characteristic of the cocoa tree could be used to achieve greater product diversity in raw cocoa, similar to how this is observed in wine production.

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