Cocoa

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The starting material for cocoa and chocolate is the seed of Theobroma cacao which was first cultivated by the Aztec and Mayan civilisations more than 2500 years ago and imported by the Spanish in 1528. Processing is in the tropics where the cocoa is grown, with ensuing manufacturing in the countries where the end products are consumed.

There are two major types of T. cacao. Criollo affords cocoas that have a refined flavour but low yield. Forastero affords much higher yields and is therefore the predominant type used, accounting for approximately 95% of the cocoa beans used in the manufacture of chocolate and cocoa products.

Cocoa pods (Fig. 16.1) develop on the trunks and branches of the tree and are harvested throughout the year. They comprise an embryo and shell. There are between 35 and 45 seeds (or beans or cotyledons) encased in a mucilaginous pulp known as the endocarp and composed of sugars (mainly sucrose), pectins, polysaccharides, proteins, organic acids and salts (Table 16.1). The plant contains alkaloids, notably the methylxanthines theobromine (1-2% of the dry weight) and caffeine (0-2%) (Fig. 16.2) The former affords bitterness to cocoa. The embryo of the seed comprises two folded cotyledons that are covered with a rudimentary endosperm. It is these cotyledons that are used for making cocoa and chocolate (Fig. 16.3).

The ripe pods are harvested and their husks broken using sharp objects or wooden billets. The wet beans are removed from the husk and heaped (50-80 cm deep) on the ground or in boxes (100 cm deep) to allow 'sweatings' to drain from the bottom. The beans are covered mainly with banana leaves, and left for 5-7 days with one or more turnings to allow for a more even fermentation. The temperature will rise to around 50°C and must be maintained below 60°C to avoid over-fermentation and excessive growth of fungi.

During fermentation, the pulp becomes infected with diverse microorganisms from the environment. At the start of fermentation, the low pH and high sugar in the surrounding pulp favour anaerobic fermentation by yeasts and also the growth of lactic acid bacteria. The ethanol produced represents a substrate for the acetic acid bacteria, which predominate when the sugars are exhausted. Pectinolytic activity is supplied by Kluyveromyces marxianus, but Saccharomyces, Torulopsis and Candida are other yeasts that have significant roles to play. The pectinolysis leads to the draining of the pulp off the beans as 'sweatings'. This allows air into the spaces between the beans and so, late in fermentation, aerophiles develop, including Bacillus, as well as filamentous fungi, such as Aspergillus fumigatus, Penicillium and Mucor spp.

Fig. 16.1 The cocoa pod. Photograph supplied by Dave Zuber of Mars, Incorporated. Table 16.1 The composition of the cocoa cotyledon.

Component

Percentage by weight

Water

32-39

Cocoa butter (lipid)

30-32

Protein

8-10

Polyphenols

5-6

Starch

4-6

Pentosans

4-6

Cellulose

2-3

Theobromine

2-3

Salts

2-3

Sucrose

2-3

Caffeine

1

Acids

Theobromine

Caffeine

Fig. 16.2 Methylxanthines in cocoa.

Cocoa beans

Cocoa beans

Fig. 16.3 An overview of cocoa processing.

The increasing concentration of ethanol and acetic acid, together with a rise in heat, eventually leads to the death of the bean. Once this occurs, the biological barriers within the cotyledon are broken down, permitting the release of several types of enzymes.

Initially the anaerobic conditions inside the cotyledon favour hydrolytic enzymatic reactions but, later, aerobic conditions prevail, which favour oxidative reactions, especially of the polyphenols.

Invertase hydrolyses sucrose to the reducing sugars, glucose and fructose. These will later combine with peptides and amino acids. During roasting of the beans (discussed later), these compounds enter into the Maillard reaction, and the resultant flavoursome substances are highly significant for the flavour of chocolate.

Glycosidases release polyphenols from their attachment to sugars. The anthocyanidins released polymerise to leucocyanidins, which in turn complex with some of the protein, lessening their astringency and bitterness, as well as reducing the levels of unpleasant flavours and odours sometimes associated with roasted proteins.

After fermentation, the beans are exposed to drying, either by sun or by a forced hot-air source. Drying is an important continuation of the fermentation process and, consequently, flavour-precursor development. During drying, aerobic conditions prevail, favouring oxidative reactions, especially of the polyphenols through the action of PPOs. Since fermentation is a gradual process spread over a 5-7-day period, the action of PPO commences towards the end of the anaerobic phase of fermentation. Quinones are also formed by the oxidative changes brought by the action of the PPO on the polyphenols. These complex with free amino and imino groups of proteins, the tanning of the protein leading to a colour change in the beans and a reduction of astringency.

There appears to be a fine balance between the fermentation and drying that must be adhered to if a consistent flavour is to be achieved in the bean. It is barely credible that the crude and sometimes haphazard methods employed allow this balance to be maintained. Care must be taken not to dry the beans too rapidly, which can lead to case hardening of the bean, thus entrapping more of the unwanted volatile acetic acid.

Whichever method is used, it is essential that the beans are dried down to 5-7% moisture to inhibit the development of mould during storage. The ensuing mouldy taste in the chocolate is almost impossible to eradicate by further processing.

The extent to which the biochemical changes have progressed during fermentation and drying is assessed from the colour change of the cotyledons, resulting from the oxidation of the polyphenolic constituents. A brown colour in the bean is indicative of complete fermentation, purple/brown suggests partial fermentation, purple signifies under-fermented and slate-colouration indicates that the bean has not been fermented. Chocolate made from slate-coloured beans is bitter, astringent and almost devoid of chocolate flavour.

Acetic acid is a by-product of the fermentation of the sugars occurring in the surrounding pulp and significant diffusion into the cotyledon during fermentation causes a decrease in the pH of the beans. For some types of Forastero beans, pH is used as a secondary measurement of the degree of fermentation.

Levels of theobromine and caffeine decline during fermentation, as is also the case for the lipid component of the bean, cocoa butter.

Cocoa butter is fully saturated, hence it is one of the most stable fats in nature and resistant to oxidation. Depending on its polymorph, cocoa butter has a melting temperature of approximately 34.5°C, some 2.5°C lower than normal body temperature. Its melting profile is sharp, so that the chocolate made from it melts cleanly in the mouth with no residual, waxy aftertaste. However, sufficient unmelted solids remain to give body to the chocolate at regular distribution temperatures.

The melt temperature of cocoa butter varies according to the genetics and geographical source of the cocoa. Malaysian cocoa butter has the highest melt temperature and is the hardest in texture. Depending on the season, Brazilian cocoa butter, produced from the winter crop, is the softest and has the lowest melting temperature.

The starch remains virtually chemically unchanged during the fermentation process.

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