FOOD CHEMISTRY

Module 13. Beverages

Lesson 29 C
CHEMICAL CHANGES DURING PROCESSING – COCOA

29.C.1 Introduction

Cocoa is indigenous to South America and is believed to have originated from the Amazonvalley and Orinoco valley. Cocoa beans are the seeds of the tropical cocoa tree. The obroma cocoa family sterculiaceace. Cocoa drink is different from tea or coffee as it is consumed as a suspension and not in the form of an aqueous extract. Cocoa and its products contain stimulating alkaloids, ex. the obromine; with substantial amounts of fats, carbohydrates and proteins. Unlike coffee and tea, cocoa drinks are to be consumed in large quantity to have a stimulating effect.

29.C.2 Classification

1. Criollo (means native): bears highly aromatic beans, hence their commercial name “flavor beans”. It was probably cultivated first in the region from southern Mexico.

2. Forastero (means foreign): less flavorful beans. Most important commercial type of cacao and accounts for the bulk of world cacao production. It has its origins in the upper Amazon basin area of northern Brazil, eastern Venezuela and Colombia.

3. Trinitario (“of Trinidad”) is a cross between Criollo and Forastero which probably occurred naturally.

4. Nacional (or Arriba) is scientifically a Forastero, but is classified separately due to its distinctive aromatic floral nature.

29.C.3 Processing of Cocoa

On farm processing of cocoa consists of fermentation and drying. The bulk of the harvest is fermented before drying.

Fermentation of cocoa

The cocoa beans along with the pulpare fermented for 2-8 days. Cocoa fermentation is necessary for:

1. Formation for the flavour precursors
2. Killing the beans to prevent cocoa butter degradation.
3. Degradation of the fruit pulp to make bean drying easier.

Fig. 29.C.1 The major changes during fermentation of cocoa

The fermentation of cacao beans is accomplished by two processes

1. External microbial processes

2. Internal autolytic processes

The microbial processes are characterized by the production of the ethanol and acetic acid. The autolytic process involves changes due to cocoa beans enzymes. The group of microorganisms involve in the fermentation are mainly yeast, lactic acid bacteria, acetic acid bacteria and spore forming microorganisms.

1. External microbial process

• The pulp sugar is fermented by yeast to alcohol and carbon dioxide on the first day.
• Pectolytic enzymes and other glycosidases cause degradation of the polysaccharides. This results in liquefaction of the fruit pulp, which is drained away.
• As a result aeration of the beans improves. This causes oxidation of alcohol to acetic acid by the acetic acid bacteria during second to fourth day. These processes cause the temperature of the cacao mass to rise to about 45-500C. At the same time the pH of the pulp rises from an initial value of 3.5-4.0 to around a pH of 5.0. Simultaneously, the pH of the cotyledons drops from about 6.5 to a value of 4.5.
• The cell walls become permeable and the cacao seed is killed. The causing the death of beans has been identified as acetic acid. After the bean death, the oxidative processes take over the entire mass.

2. Internal autolytic process

It occurs during 5^thto 7^th day of fermentation. The enzymatic changes in cocoa beans canbe divided into two phases: Anaerobic hydrolytic phase and Aerobic oxidative phase. The changes occur during processing are:

1. The glycosidase splits the anthocyanins to cause bleaching of the violet colors of the tissues under anaerobic conditions.

2. Certain proteases which are activeduring anaerobic hydrolytic stage of the fermentation cause changes in the proportion of the proteins and amino acids of the fermented cocoa beans.

3. Polyphenol oxidase present causes oxidation of the epicatechins to quinone. Thus quinone is capable of complexing with itself or other quinones to produce a brown substance. The oxidized epicatechins also cause irreversible tanning of the proteins in cocoa beans.

4. Reaction of the polyphenols decreases the soluble phenol content and thereby, mellows the original harsh and astringent cocoa flavours.

29.C.4 Changes Taking Place in specific constituents

1. Carbohydrates

Total sugar content decreases throughout the fermentation. During fermentation, sucrose hydrolysis provides the reducing sugar pool, which is important for aroma formation during the roasting process.

2. Proteins

Proteolysis begins soon after initiation of fermentation and continues throughout the fermentation period. A definite decrease in the protein nitrogen content along with an increase in the peptides and free amino acid occurs. Tanning of enzymes of cocoa beans has also been found during fermentation.

3. Lipids

The fat does not change appreciably during fermentation.

4. Acids

Organic acids in cocoa beans are formed mainly during cocoa fermentation and consist mainly of acetic acid(a flavour constituent), citric acid and oxalic acid. Of total acids (1.2-1.64%),0.79-1.25% is volatile acids and 0.19-0.7% is acetic acid. Acetic acid and to a lesser extent the volatile acids and non-volatile acids contribute to final chocolate flavour.

29.C.5 Changes taking Place during Drying Process

The browning of the cotyledons is considered to be most important change during drying. The polyphenolic brown pigments are formed as a result of polyphenol oxidation. By the end of drying,oxidation reaction are terminated and moisture content of the beans drops to about 8%.

29.C.6 Changes During Roasting Process

Roasting is the processing step generally done by the consuming countries. Generally roasting temperature should not exceed 150⁰C.The major changes taking place during roasting are:

1. Reduction in the moisture content to about 3%
2. Contribution to the further oxidation of phenolic compounds.
3. Causes removal of the acetic acid, volatile esters and undesirable aroma compounds.
4. Enhanced aroma of the beans: The typical aroma of the roasted nuts is derived from pyrazine; the bitter taste is derived from purines, the obromine and caffeine.
5. The seed hardens and becomes more brittle. The shell is loosened and easily removed.
6. The color of the beans deepens.
7. The reducing sugars and amino acids in cocoa beans react via maillard’s reaction and strecker’s degradation reaction to form many of the components found in chocolate flavour.
8. The ammonia liberated during roasting can react with reducing sugar or other compounds to produce compounds important to the flavour of the chocolate.
9. The level of the monocarbonyl compounds changes.
10. The overall loses induced by roasting are 5-8%