The Raw Coffee Bean

Green coffee beans
Photo by Coffee Geek on Unsplash

Raw coffee has a complex chemical compound. Coffea arabica (arabica) and coffea canephora (robusta) not only vary in mass but also in the chemical composition, both quantitatively and qualitatively. Arabica contains more lipids and trigonelline, while Robusta has more caffeine and Chlorogenic acids

The composition of raw coffee beans is shown in the table below as a % of dry matter:

Class

Arabica

Robusta

Caffeine

1.2

2.2

Lipids

16

10

Chlorogenic acids

6.5

10

Aliphatic acids

1.7

1.6

Total amino acids

10.3

10.3

*Free amino acids

0.5

0.8

Trigonelline

1.0

0.7

Glycosides

0.2

Traces

Mineral

4.2

4.4

Carbohydrates

58.9

60.8


Water Content

The water content of raw coffee beans influences water activity and stability during storage. For storage and transportation the water content (as measured by the moisture content) is proposed to be 12.5% by the European Coffee Federation. However, it is not suggested that a moisture level of 12.5% is for all coffee. Coffee such as Indian monsoon coffee naturally has higher moisture content and can tolerate it without being defected. Coffee that contains moisture above 12.5% has a significant risk of fungi growth. Coffee that has moisture content lower than 10% is susceptible to the decrease of germinating power, along with the drying and cracking of the coffee beans themselves. 

Carbohydrates

Carbohydrates make up about 50% of green coffee. The complex composition consists of poly-, oligo- and monosaccharides.

Sucrose is a carbohydrate that occurs naturally in coffee as a product of photosynthesis. Sucrose content in green coffee varies from 6.25% - 8.45% for arabica and 0.9% - 4.85% for robusta. Beside sucrose, small amounts of fructose, glucose, mannose, arabinose and rhamnose have been identified.

Sucrose content is increasing with the degree of ripening. This is evident in the defective beans such as immature-black and immature-green, the sucrose level is found to be ⅓ - ⅕ of the normal beans.

Glucose content has a negative correlation with aroma level and a positive correlation with cup sweetness, while (according to Illy and Viani) fructose content shows a negative correlation with sweetness.

Lipids

Arabica beans contain around 15% of lipids, while robusta contains around 10%. Most of the lipids (coffee oil) are mainly located in the endosperm. Its yield in the green coffee depends on the bean composition, while in brewing condition on the grind size and surface area, duration of extraction and water composition. In brewed coffee, lipids are responsible for the mouthfeel and the body.

Lipids in coffee are mainly composed of triglycerides with fatty acids in a similar proportion to the common edible vegetable oil. Further, it’s also in the form of coffee wax (2% - 3% of the total lipids)  which covers the surface of green coffee beans. 

A study conducted (Maier, 1981) shows the composition of the lipids in the green coffee as depicted in the table below:

Component

Percent

Triglycerides

75.2

Esters of diterpene alcohols and fatty acids

18.5

Diterpene alcohols

0.4

Esters of sterols and fatty acids

3.2

Sterols

2.2

Tocopherols

0.04 - 0.06

Phosphatides

0.1 - 0.5

Tryptamine derivatives

0.6 - 1.0


Chlorogenic acids

Chlorogenic acids (CGA) are a group of phenolic acids esterified to quinic acid, which accounts for 6 - 7% in arabica and up to 10% in robusta. CGA production is influenced by environmental conditions, plant stress and pest infestation. CGA production also correlates with caffeine.

During roasting, 50% of the original CGA is destroyed (medium roast) and the remainder is broken down to form caffeic and quinic acid as the roasting progresses into a darker roast.

In green coffee mono- and di- caffeoylquinic acids have been identified with substitution at the 3-, 4- and 5-position of quinic acids. The phenolic acid contains caffeoyl-,  p-coumaroyl- and feruloyl-acids.

These components are essentially mono- and diester. The monoester concentration increases at the ripe cherry and decreases slightly in the overripe cherry, as well as present more in the wet-processed coffee than natural processed.

Amino acids and proteins

Free amino acids amount to 0.5% in arabica and 0.8% in robusta. At 40 C, the total amino acids don’t show any changes, but the individual contents changes for some acids: glutamic acids increase about 50%, aspartic acids decrease and hydrophobic acids increase. Furthermore, in the higher temperature during roasting, free amino acids degrade non-enzymatic browning reactions (Maillard reaction). 

The protein content of mature beans are more than in immature beans. The proteins consist of approximately an equal amount of water-soluble (albumin) and a water-insoluble fraction.

Ash and Mineral

Coffee contains about 4% of minerals (arabica: 3.6% - 4.5%; Robusta 3.6% - 4.8%), with potassium amounting to 1.63 - 2% of the total. The mineral content may be affected by the use of fertiliser. Other minerals in green coffee are: Calcium (0.07% - 0.035%), Magnesium (0.16% - 0.31%), Phosphate (0.13% - 0.22%) and Sulphate (0.13%).

Alkaloids

Alkaloids in coffee consist of trigonelline and caffeine. Trigonelline amounts for 0.6% - 1.3% in arabica beans and 0.3% - 0.9% in robusta beans. Trigonelline is the precursor of many aroma compounds and coffee bitterness. It degrades to pyridines and nicotinic acids (niacin) during roasting which contributes to coffee’s anti-cavity effect.

The average content of caffeine in arabica beans is 1.2% - 1.3% and in robusta is 2.2% - 2.4%. Caffeine contributes to coffee bitterness and produces stimulant effects. It’s a natural defence mechanism for coffee trees against insects.

Carboxylic acids

Quinic acid, the aliphatic acid moiety in the CGS is also present in the free state. Its content is affected by the processing, fermentation and age (higher concentration in older beans). Besides Quinic, the major aliphatic acids present in green coffee are malic, citric and phosphoric acids.

Glycosides

Glycoside is a substance where sugar is bound to a hydroxy compound. Arabica beans contain significantly more of the glycosides that robusta. A study by Brandbury and Balzer (1999) showed that glycosides in green coffee contain two carboxyl groups at C-4.


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