FOOD CHEMISTRY

Module 13. Beverages

Lesson 29 B
CHEMICAL CHANGES DURING PROCESSING – COFFEE

29.B.1 Introduction

Coffee or coffee beans are the seeds of fruits from which various layers surrounding the beans are successively removed during its processing. The seeds are used as raw/roasted, whole/ground and should be obtained from the botanical genus Coffea. The beverage prepared from such seeds is called as coffee. Coffee drink is a variable and complex beverage.

The two main species of commercial interest in the genus Coffea are Coffea arabica and Coffea canephora var. robusta. They are also known as Arabica and Robusta, respectively.

Coffee is grown in countries situated between the Tropics of Capricorn and Cancer. Its native is African continent – mainly Ethiopia. Brazil is the largest coffee producing country in the world. Arabia, Java and Venezuela are also producing coffee with special grade of their choice.

The widely cultivated species of the coffee are:

1. 75% of the total world’s coffee production. Major coffee producing states in India are Tamil Nadu, Karnataka, Kerala and Orissa.
2. Coffea canephora - provides about 25% of the world’s coffee production.
3. Coffea liberica –provides less than1% of the world’s coffee production.

All varieties of Coffea canephora are marketed under the common name “robusta”. (Fig. 29.B.1:The coffee berry in a cross- sectional diagram)

29.B.2 Methods of Processing of Green Coffee

The processing of coffee involves removal of various layers. The outermost red skin, also known as epicarp, is completely removed. The pulp of coffee berry is then removed which is followed by removal of mucilage. These two layers are collectively known as mesocarp. Then Parchment layer (endocarp) is removed. Finally, the silver skin (endosperm) surrounding the green coffee berry is removed. Two methods are employed for coffee processing.

1. Dry method - natural method
2. Wet method - washing method

1. Dry method - natural method

This method is widely used in Brazil. It involves sun drying until the beans get separated by shrinking from the surrounding parchment layer. This is followed by dehulling and polishing of the beans. Sometimes, the fresh beans are piled up, allowed to ferment the fruity pulp of beans for 3-4 days under their own heat. Then the beans are dried to remove the husks and parchment from the dried berries, and as much as possible of silver skin. The dried and dehulled beans are then polished. The dehulled and cleaned coffee beans are then classified according to size/shape and packed in bags.

2. Wet method - washing method

In the wet process, the uniformly ripe and freshly harvested coffee berries are brought to a pulper, wherein the fruits are rubbed between a rotating cylinder and a slotted plate. The passage of fruit produces a rubbing action, which facilitates the removal of the skin and pulp from the beans without any damage to the seeds. The beans obtained from pulper have the silver skin, the parchment layer and a mucilage layer. Coffee beans are then carried into water fermentation tanks and beans are left to ferment for 12-48 hours. During fermentation, the mucilaginous layer is hydrolyzed by enzymes of the coffee and other enzymes produced by the microorganisms. The mucilage is degraded to large extent and can be readily removed by washing with water. The beans may be then sun-dried or with the help of mechanical dryers (hot air 65-85 °C). Beans are still covered by parchment shell which is removed by dehulling process. Finally, beans are polished to a smooth, glossy surface.
The wet (washing) method of coffee processing is more sophisticated than dry method, and produce better quality coffee. Almost half of the world coffee is processed by the wet method.

29.B.3 Roasting of Green Coffee Beans

Green berries, after the removal of various layers, smell green-earthy. To bring about the delightful aroma of coffee, they must be heat treated in a process called roasting. The roasting process is characterized by a decrease in old compound and generating new compounds. Green coffee contains normally 12% moisture. Most of the moisture is lost during early stages of roasting process, followed by a loss of up to 8% of the dry beans. The style of roasting process is a matter of personal taste and liking of flavour of finally roasted beans. Some Arabica coffees become bitter if high a temperature of roasting is employed. Hot gases at about 200-250°C are used to roast the beans in a perforated steel rotating drum. Beans are heated in small quantities for a total of only 5 min. When the beans have reached to the desirable internal temperature and colour, they are moved away from the oven. The roasted beans are immediately cooled, with fine water spray, to prevent the further pyrolytic changes in the bean.

In general, the changes taking place during roasting of coffee beans are:

• The beans increase in volume (50-80%)
• The colour changes from green to brown
• 13-20% loss in weight of beans
• Build-up of the typical roasted flavour of the beans
• Specific gravity of beans falls drastically from 1.126-1.272 to 0.570-0.694
• The structure of the beans changes. The horny, tough and difficult to crack beans become brittle and mellow after roasting

29.B.4 Roasting Process is Distinguished into four Major Phases

1. Drying
2. Development
3. Decomposition
4. Full roasting

The initial changes during roasting process occur at or above 50°C when the protein present in the tissue cells gets denatured and evaporation of water takes place. At above 100°C temperature, browning reactions take place due to pyrolysis of organic compounds. Volatile products (water, CO, CO₂) are release at about 150°C resulting in an increase in bean volume. Part of the CO₂ produced gets absorbed within the texture of the roasted beans.

During decomposition phase, at 150-200°C, beans are being forced to pop and burst with formation of bluish smoke and subsequent release of coffee aroma. During roasting, pressure develops in the beans which hold the initial breakdown products together. When the proper stage of roasting is reached, these breakdown products react with each other to produce coffee flavour. Finally, the full roasting phase is achieved, in which the moisture level of the beans drops to the final level of 1.5 - 3.5%.

29.B.5 Changes in Individual Constituents During Coffee Processing

1. Proteins: Extensive changes taking place in protein of coffee beans during roasting. This happens in the presence of carbohydrates. A shift in the amino acid composition of coffee protein acid hydrolysates is observed before and after bean roasting. A drop of about 30% in the total amino acid content of the hydrolysate take place because of considerable degradation during roasting.
2. Carbohydrates: During roasting, a high amount of the carbohydrates, most of the sucrose and monosaccharides are degraded and decomposed. The carbohydrates are also involved in the caramelization process.
3. Lipids: A little change observed in the lipid fraction. It remains stable and survives the roasting process with some minor changes. The diterpene glycosides (Cafestol and kahweol) are degraded during roasting and their residues get entry into the soluble coffee powder and subsequently in coffee beverage to contribute to the bitter taste of coffee beverage.
4. Acids: Chlorogenic acids are the most abundant acids of coffee. Formic and acetic acids predominate among the volatile acids. The non-volatile acids are lactic, tartaric, pyruvic and citric acids. During roasting process, these acids are decomposed by about 30 -70%.
5. Caffeine: The caffeine level in beans is only slightly decreased during roasting. It is mildly bitter in taste with a threshold value in water is 0.8–1.2 mmole/l. It is the best known N-compound having physiological effects like stimulation of the central nervous system, increased blood circulation and respiration, etc.
6. Trigonelline and nicotinic acid: It is present in green coffee upto 0.6%. About half quantity of these compounds are decomposed during roasting. The degradation products include nicotinic acid, pyridine, their methyl esters, etc.
7. Aroma Compounds: The aroma of coffee is not stable. The aroma profile of coffee is of sweet/caramel-like, earthy, sulfurous/roasty and smoky/phenolic. The volatile compounds of roasted coffee have very complex composition.

• The group of aliphatic compounds includes hydrocarbons, alcohols, and above all, carbonyl compounds, which are derived during roasting from carbohydrate fragmentation. Also numerous alicyclic compounds are found.

• Phenols are predominant among the aromatic compounds and are most probably derived from thermal decomposition of chlorogenic acids; Phenol ethers, carbonyl esters and polycyclic compounds are also present.

• There is a large number of heterocyclic compounds, among which are many 2- and 2,5- substituted furans, probably derived from the pyrolysis of sucrose and other sugars.

8. Other Constituents: Brown pigments (melanoidins) are derived from Maillard reactions or from carbohydrate caramelization. Apparently, cholrogenic acid is also involved in such browning reactions. Secondary products of the degradation of mixtures of carbohydrates and proteins are probably involved in the formation of the bitter flavour of roasted coffee.

29.B.6 Grinding and Storage of Roasted Coffee

The grinding of roasted beans is generally done in industrial grinders like roll and breaker bar systems. The roasted beans are rolled past the toothed rollers, where they are cut. A series of these rollers produces a successively finer grind.

Other systems used on a large scale include hammer mills with cutting blades. The roasted beans should be cool, hard, and brittle. Dark roasted beans grind the most readily. Large amounts of carbon dioxide are released from roasted coffee, along with other volatile compounds, during this process. This provides temporary protection to the freshly ground coffee from the atmospheric oxygen and moisture in the air. Ground coffee is usually packed within 8 h of grinding. Storage in evacuated and sealed container at -20°C prevents from staling.