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Table 7.4 Distribution of nitrogen compounds in dry beans.

Bean type Total nitrogen Protein nitrogen Methylxanthine nitrogen

Table 7.4 Distribution of nitrogen compounds in dry beans.

Bean type Total nitrogen Protein nitrogen Methylxanthine nitrogen

(g/100 g)

(g/100 g)

(g/100 g)

(% of total nitrogen)

Theobromine

Caffeine

Criollo (Trinidad)

2.086

1.636

78.4

0.312

0.138

Forastero (Trinidad)

2.282

1.852

81.1

0.367

0.063

Forastero (West African)

2.280

1.720

75.4

0.530

0.030

relative proportion vary with the bean variety as presented in Table 7.4 (22).

The large quantity of non protein nitrogen makes difficult the in vivo nutritional evaluation of cocoa proteins, as a large proportion of methylxanthines are absorbed and excreted in the urine, which interferes with the nitrogen balance parameters digestibility and biological value.

As shown by Zak and Keeney (23), cocoa proteins of fresh beans are distributed in different protein classes: albumin, globulin, prolamine and glutenin. Their proportion changes with the bean variety; in the non-pigmented variety Criollo, the percentage distribution of these protein classes is 32, 25, 12 and 31% respectively, while in the pigmented variety Nacional the percentage distribution is 51, 25, 12 and 12% respectively. These differences in the protein classes influence the protein solubility, which is about 30% in the non-pigmented varieties and more than twice as much in the pigmented varieties. Tanning reactions of polyphenols most likely contribute to these differences.

A certain amount of protein nitrogen is present as free amino acids. According to Marvalhas (24) only 3% of the amino acids of cocoa beans are free. After fermentation, this level increases to higher levels (Table 7.5).

Amino Acid Composition

The amino acid composition of cocoa beans appears to be relatively constant from one variety to another. Zak and Keeney (23) did not find discernible differences among three non-pigmented and three pigmented varieties and Offem (25) did not find any significant differences in amino acid composition of three varieties of cocoa beans from south-eastern Nigeria.

The fermentation step does not significantly change the total amino acid composition as measured after acid hydrolysis. However, during fermentation, some enzymatic hydrolysis occurs, liberating peptides and free amino acids. The total amino acid content remains the same, but the free amino acids are multiplied by a factor of three (Table 7.5). During roasting, these free amino acids are partly destroyed through thermolysis and Maillard-type reactions.

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Table 7.5 Total and free amino acids in unfermented and fermented Bahia defatted cocoa beans.

Amino acid

Lysine

Histidine

Arginine

Aspartic acid

Threonine

Serine

Glutamic acid

Proline

Glycine

Alanine

Valine

Methionine

Isoleucine

Leucine

Tyrosine

Phenylalanine

Total

Unfermented (g/100 g)

Total

2.34 0.49 1.62 2.60 0.66 0.60 3.60 1.13 1.00 0.90 1.28 0.21 0.71 1.50 0.61 1.18 20.43

Free 0.027

0.034

0.024

0.189

0.009

0.035

0.122

0.045

0.032

0.042

0.008

0.012

0.018

0.018

0.025

0.024

0.664

Fermented (g/100 g)

Total

2.36 0.47 1.48 2.71 0.63 0.62 3.59 1.14 1.00 0.88 1.32 0.21 0.74 1.48 0.65 1.17 20.45

Free 0.204

0.026

0.134

0.325

0.074

0.101

0.190

0.092

0.074

0.154

0.083

0.026

0.067

0.307

0.116

0.228

2.201

Protein Modifications during Cocoa Processing

From the fresh beans to cocoa powder, cocoa proteins are chemically modified mainly by polyphenols, which affect their functional and nutritional properties.

The fermentation process modifies proteins and polyphenols. Proteins are partially hydrolysed into peptides and free amino acids, which are much more sensitive to chemical modifications than protein-bound amino acids. In the anaerobic step of fermentation, polyphenols are hydrolysed by b-galactosidases, which liberate the aglycones (mainly anthocyanes) and sugars (galactose, arabinose). In the aerobic phase of fermentation, anthocyanes polymerize into tannins, which can bind proteins through hydrogen bridges. In addition, orthodiphenols, the most sensitive polyphenols to oxidative reactions (like epicatechin), oxidize through the action of polyphenol oxidases into quinones. Quinones are able to covalently bind the free amino groups of every free amino acid and the epsilon amino group of lysine and the sulphydryl group (SH) of cysteine, bound to proteins (26). As a conclusion, during fermentation, three types of proteinpolyphenol combinations occur:

  • Reversible hydrogen bridges between the proton of the phenolic OH and the oxygen of the peptide bonds.
  • Irreversible hydrogen bonds between the peptide bonds and leucoanthocyans, procyanidins or polymers of flavanols (tanning effect).
  • Irreversible covalent bonds between amino acids and quinones.

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All these reactions contribute to reduce the solubility of the cocoa proteins, which decreases from 170 to 65 mg/g fat-free beans during fermentation (27). The protein quality is therefore expected to decrease similarly.

During roasting, the Maillard reaction takes place between the amino groups of the small peptides and of the free amino acids liberated during fermentation and the free reducing sugars, some of them also being liberated during fermentation. This Maillard reaction is responsible for the development of the cocoa aroma at the expense of the amino acids involved, and to a minor extent, of the nutritional quality.

Nutritional Value of Cocoa Powder Proteins

The composition of the essential amino acids of the Criollo variety is presented in Table 7.6 and compared with the essential amino acid profile proposed as a reference for a good protein quality according to FAO/WHO (28).

Table 7.6 Amino acid composition of fresh, fermented and roasted cocoa beans. Chemical score calculated from the proposed FAO/ WHO amino acid pattern (28).

Amino acid

Criollo variety (g/16 g N)

FAO/WHO pattern (g/16 g N)

Chemical score (%)

Histidine

4.3

(1.9)

>100

Isoleucine

4.1

2.8

>100

Leucine

6.8

6.6

>100

Lycine

4.3

5.8

74

Methionine + cysteine

2.0

2.5

80

Phenylalanine + tyrosine

6.2

6.3

98

Threonine

5.2

3.4

>100

Tryptophan

?

1.1

Valine

7.7

3.5

>100

Cocoa proteins are limiting in lysine and methionine with a chemical score of 74, which can be considered as being good for a protein of plant origin. The value for tryptophan is never mentioned and might also be limiting. However, due to multiple modifications during processing, the protein quality is reduced and in vivo tests are necessary for its absolute evaluation. This evaluation has been made on cocoa powder under very specific conditions due to the nature of this food ingredient.

The protein quality of a food is generally evaluated in young rats fed a standard diet containing 10% protein either in a growth test to measure the protein efficiency ratio (PER) or in a nitrogen balance test to measure the nitrogen digestibility and the biological value. In these tests casein is taken as the reference protein. However, because of its palatability, no more than 25% cocoa powder can be given in a rat diet, which limits the protein level to around 5% in the diet.

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An additional problem is that about 20% of cocoa powder nitrogen is composed of non-protein nitrogen coming from methylxanthines.

To overcome these limitations, Shahkhalili et al. (29) compared the nitrogen digestibility of a diet containing 25% cocoa powder with the digestibility of a diet containing the same levels of casein and of methylxanthines. They found that the true digestibility of total cocoa nitrogen was 2830%. When this value was corrected by the digestibility of methylxanthines, the digestibility of cocoa proteins was reduced to 1617%. This value is very low as compared to the digestibility of animal proteins (95100%) and that of plant proteins (7285%) (28). We can therefore consider that the proteins of cocoa powder have little practical value.

Cocoa Powder As a Milk Modifier

Cocoa powder is currently used to give a pleasant taste to raw milk. This contributes to increase milk consumption in populations, like adolescents and the elderly, who need an extra supply of good quality proteins. A glass of 200 ml milk contributes significantly to the daily requirement of proteins (about 13%) and also of calcium (about 30%).

Milk Chocolates

Chocolate bars are very often associated with milk to change or to improve their palatability. The proportion of milk can vary significantly according to the recipe. Table 7.7 gives an indication of the (available) proteins and calcium present in some chocolate bars expressed in quantity (g) and percentage of their respective daily requirement (RDA).

In conclusion, cocoa powder cannot be considered as a source of protein because its proteins are not digested due to their reactions with cocoa polyphenols. However, cocoa is often associated with milk, as a milk modifier or as

Table 7.7 Protein and calcium contents in 100 g milk chocolate compared to black chocolate bars.

White chocolate Milk chocolate Dark chocolate

Calories (kcal)

Cocoa powder 357

Protein (g)

19.8

Available protein

8.58.8 1617

Calcium (mg)

301 37

214 27

114 14

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milk chocolate. It is in these forms, having improved taste over cocoa powder alone, that cocoa provides much pleasure to people, and also results in increased consumption of milk proteins.

Carbohydrates are naturally present in cocoa as sugars, starches and fibres. Fermentation and processing usually causes their concentrations to change as chemical reactions occur. Cocoa powder contains appreciable concentrations of dietary fibre, although in chocolate, these levels are substantially reduced after blending with cocoa butter and adding sugar. Consequently, the greatest contribution of carbohydrates found in chocolate is as added sugars, usually sucrose, which is present to offset the bitterness of cocoa, thereby enhancing its palatability; and as lactose from milk solids used to make milk chocolate. It is technically possible to replace sucrose by polyols, but it tends to be at the expense of various desirable characteristics valued in quality chocolate. This practice is permissible in Europe, but not in North America. In any event, there are few examples to be found on the European market, suggesting little real success with the concept.

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  9. Geilinger, I., Amado, R., Neukom, H., Kleinert, J. and Mikle, H. (1984) Einfluss verschiedener Verfahren zur Schocoladeherstellung auf Kakao-Inhalstoffe, speziell Kakaostärke. II. Chemische und physikalische Analysen der Cacao massen. Lebensm.-Wiss. u.-Technol. 17, 201204.
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