unpleasant tasting


This article discusses the coffee plant; for information on the beverage, see coffee.

Coffea (coffee) is a large genus (containing more than 90 species) of flowering plants in the family Rubiaceae. They are shrubs or small trees, native to subtropical Africa and southern Asia. Seeds of several species are the source of the beverage coffee. The seeds are called "beans" in the trade. Coffee beans are widely cultivated in tropical countries in plantations for both local consumption and export to temperate countries. Coffee ranks as one of the world's major commodity crops and is the major export product of some countries.


When grown in the tropics coffee is a vigorous bush or small tree easily grown to a height of 3–3.5 m (10–12 feet). It is capable of withstanding severe pruning. It cannot be grown where there is a winter frost. Bushes grow best at high elevations. To produce a maximum yield of coffee berries (800-1400 kg per hectare), the plants need substantial amounts of water and fertilizer. Calcium carbonate and other lime minerals are sometimes used to reduce acidity in the soil, which can occur due to run off of minerals from the soil in mountainous areas. The caffeine content in coffee "beans" is a natural defense, the toxic substance repelling many creatures that would otherwise eat the seeds, as with nicotine in tobacco leaves.

There are several species of Coffee that may be grown for the beans, but Coffea arabica is considered to have the best quality. The other species (especially Coffea canephora (var. robusta)) are grown on land unsuitable for Coffea arabica. The tree produces red or purple fruits (drupes), which contain two seeds (the "coffee beans", although not true beans). In about 5-10% of any crop of coffee cherries, the cherry will contain only a single bean, rather than the two usually found. This is called a 'peaberry', which is smaller and rounder than a normal coffee bean. Some claim that peaberries have a different flavor profile in the cup, while others dispute this. Either way, it is often removed from the yield and either sold separately (such as in New Guinea Peaberry), or discarded.

The coffee tree will grow fruits after 3–5 years, for about 50–60 years (although up to 100 years is possible). The blossom of the coffee tree is similar to jasmine in color and smell. The fruit takes about nine months to ripen. Worldwide, an estimate of 15 billion coffee trees are growing on 100,000 km² of land.

Coffee is used as a food plant by the larvae of some Lepidoptera (butterfly and moth) species including Dalcera abrasa, Turnip Moth and some members of the genus Endoclita including E. damor and E. malabaricus.

Shade grown coffee

In its natural environment, coffea grows under the shade. Most coffee is produced on full-sun plantations, some of which were prepared through deforestation. Shade grown coffee naturally mulches its environment, lives twice as long as sun grown varieties, and depletes less of the soil's resources. Shade grown coffee is also believed by some to be of higher quality than sun grown varieties, as the cherries produced by the coffea plants under the shade are not as large as commercial varieties. Some believe that this smaller cherry concentrates the flavors of the cherry into the seed (bean) itself. Shade grown coffee is also associated with environmentally friendly ecosystems that provide a wider variety and number of migratory birds than those of sun grown coffea farms.

Chemistry: Green-Coffee-Bean

In this paragraph the expression “green coffee bean” refers to both mature and immature coffee beans, having been processed by wet or dry method for removing the pulp and mucilage, with an intact wax layer on the outer surface, a green (immature) and brown to yellow colour, and a weight of 300 to 330 mg per dried coffee bean. Non-volatile and volatile compounds of green coffee beans are responsible for their attraction of insects but also distraction of animals preventing them from eating the coffee fruit and beans. Further, both non-volatile and volatile compounds are contributing to the flavor of the roasted coffee bean. Among the non-volatile compounds nitrogenous compounds together with carbohydrates are of major importance for the full aroma of roasted coffee and its biological action. The nitrogenous compound are consisting of alkaloids, trigonelline, proteins and free amino acids.

Non-volatile Alkaloids

Caffeine (1,3,7-trimethyl-xanthine) is the most important molecule of the alkaloids being present in green and roasted coffee beans. Dried green coffee beans have a weight of 300 mg to 330 mg. The content of caffeine is between 1% (w/w) and 2,5% (w/w of dry green coffee). The content of caffeine does not change during maturation of green coffee beans . Lower concentrations of theophylline, theobromine, paraxanthine, liberine, and methylliberine can be found. The concentration of theophylline, a major alkaloid of green tea, is gradually reduced along the roasting time of green coffee (15 min, 230 degrees Celsius) whereas the other alkalois are not changed . The solubility of caffeine in water increases with temperature and with the addition of chlorogenic acids, citric acid, tartic acid, which all are present in green coffee beans; e.g. 1 g caffeine dissolves in 46 mL of water at room temperature, and 5,5 mL at 80 degrees Celsius . The xanthine alkaloides are odorless but have a bitter taste in water, which however is masked by organic acids being present in green coffee. Trigonelline (N-methyl-nicotinate) is a derivative of vitamine B6 with a low bitter taste compared to caffeine. In green coffee beans the content is between 0,6% (w/w) and 1% (w/w). At roasting temperature (230 degrees Celsius) trigonelline is degraded to nicotinic acid by 85% leaving little amounts of the unchanged molecule in the roasted beans. In green coffee beans, trigonelline is synthetised from nicotinic acid (pyridinium-3-carboxylic acid) by methylation from methionine, a sulfur containing amino acid . Mutagenic activity of trigonelline has been reported .

Non-volatile Proteins

Proteins are accounting for 8% (w/w) to 12% (w/w) of dried green coffee beans, presenting a valuable source of amino acids. Major part of the proteins are consisting of 11-S-storage proteins (alpha - component of 32 kDa, beta – component of 22 kDa), which are degraded during maturation of green coffee beans to free amino acids. Further, 11-S-storage proteins are degraded to the individual amino acids under roasting temperature and thus being an additional source of bitter components due to generation of maillard products . Water boiling temperature, oxygene and acid pH 2.0 to pH 4.5 are degrading 11-S-storage–proteins of geen coffee beans to low molecular weight peptides and amino acids. The degradation is accelerated in the presence of organic acids such as chlorogenic acids and their derivatives. Other proteins are consisting of enzymes such as catalase, and polyphenoloxydase which are important for the maturation of green coffee beans. Mature coffee contains free amino acids (4.0 mg amino acid / g robusta coffee and up to 4 .5 mg amino acid /g arabica coffee). In arabica, alanine is the amino acid with the highest concentration, i.e. 1.2 mg / g followed by asparagine of 0.66 mg/g, whereas in in robusta alanine is present with 0.8 mg/g and asparagine with 0.36 mg/g ; . Free hydrophobic amino acids being present in fresh green coffee beans are contributing to the unpleasant bitter vomiting organoleptic taste making it impossible to prepare a beverage with such compounds. The concentrations of free hydrophobic amino acids in fresh geen coffee from Peru have been determined as follows: isoleucine 81 mg /kg, leucine 100 mg/kg, valine 93 mg/kg, tyrosine 81 mg/kg, phenylalanine 133 mg /kg. The concentration of gamma-amino-butyric-acid, a neurotransmitter in the CNS, has been determined between 143 mg/ kg and 703 mg/kg in green coffee beans from Tanzania . Roasted coffee beans literally do not contain any free amino acid. Amino acids from geen coffee beans are degraded under roasting temperature to Maillard products, i.e. reaction products between the aldehyde group of sugar (aldo-hexose) and the alpha-amino-group of the amino acids. Further, diketopiperazines, e.g. cyclo(proline-proline), cyclo(proline-leucine), cyclo(proline-isoleucine), are generated from the corresponding amino acids, being the major source of bitter taste of roasted coffee . The bitter perception of diketopiperazines starts around 20 mg/ 1 liter water. The content of diketopiperazines in espresso is about 20 mg to 30 mg which is responsible for the bitter taste perception of an espresso.

Non-volatile Carbohydrates

Green coffee beans are a valuable source of carbohydrates which make up about 50% of the dry weight of green coffee beans. The carbohydrate fraction of green coffee is dominated by polysaccharides such as arabinogalactan, galactomanan, and cellulose contributing to the tasteless flavor of green coffee. Arabinogalactan is making up to 17% of dry weight of green coffee beans with a molecular weight of 90 kDa to 200 kDa. It is composed of beta-1-3-linked galactan main chain with frequent members of arabinose (pentose) and galactose (hexose) residues at the side chains comprising immunomodulating properties by stimulating the celluar defence system (Th-1 response) of the body. Mature brown to yellow coffee beans contain less residues of galactose and arabinose at the side chain of the polysaccharides, making the green coffee bean more resistant against physical brake down and less soluble in water .The molecular weight of arabiniogalactan is higher than from other plants, making an improvement of the cellular defence system of the digestive tract compared to arabinogalactan with lower molecular weight ).Free monosaccharides are present in mature brown to yellow yellow green coffee beans. The free part of monosaccharides contains sucrose (gluco-fructose) up to 9000 mg/ 100g of arabica green coffee bean, a lower amount in robustas, i.e. 4500 mg/100g. In arabica green coffee beans the content of free glucose was 30 mg to 38 mg / 100 g, free fructose 23 mg to 30 mg/ 100 g; free galactose 35 mg/ 100g and mannitol 50 mg/100g dried coffee beans, respectively. Mannitol is a powerful scavenger for hydroxy radicals which are generated during the peroxydation of lipids of biological membranes .

Non-volatile Lipids

Lipids are non-polar, hydrophobic molecules and not soluble in water. Lipids are comprising linoleic acid, plamitic acid, oleic acid, stearic acid, arachidic acid, diterpenes, triglycerides, unsaturated long-chain fatty acids, esters between the hydroxy group of triglycerol and the carboxyl group of fatty acids and amides. The total content of lipids from dried green coffee is between 11,7 g to 14 g / 100 g . Lipids are present on the surface and in the interior matrix of green coffee beans. On the surface they include derivatives of carboxylic acid-5-hydroxytryptamides with an amid bond to fatty acids (unsaturated C6 to C24) making up to 3% (w/w) of total lipid content or 1200 to 1400 microgram / g dried green coffee bean. Such compounds are building up a wax cover on the surface of the coffee bean (200 mg to 300 mg lipids / 100 g dried green coffee beans) protecting the interior matrix against oxydation and immigration of insects and diffusion of low molecular compounds generated during the wet processing fermentation from the fermentation brew into the tissue of the green coffee bean. Further, such molecules are generating antioxydative activity due to their chemical structure . Lipids of the interior tissue are triglycerids, linoleic acid (46% of total free lipids), palmitic acid (30% to 35% of total free lipids), and esters. Arabica have a higher content of lipids (13,5 g to 17,4 g lipids/100 g dried green coffee beans) than robustas (9,8 g to 10,7 g lipids / 100 g dried green coffee beans). The content of diterpenes is about 20% of the lipid fraction. Diterpenes are comprising e.g. cafestol, kahweol, 16-O-methyl-kafestol, cafestal, kahweal. Diterpenes are known for their in-vitro protection of liver tissue against chemical oxydation .In coffee oil from green coffee beans the diterpenes are ersterfied with saturated long chain fatty acids.

Non-volatine Chlorogenic acids

Chlorogenic acids belong to a group consisting of compounds which are known as polyphenols which are antioxydants. The content of chlorogeneic acid in dried green coffee beans of robusta is 65 mg/ g and of arabica 140 mg / g, depending on time of harvesting. At roasting temperature chlorogenic acids are destroyed by more than 70% or leaving a residue of less than 30 mg/g roasted coffee bean. In contrast to green coffee, green tea contains an average of 85 mg polyphenols/g. Therefor chlorogenic acids could be a valuable inexpensive source of antioxydants. Chlorogenic acids are homologous compounds comprising caffeic acid, ferulic acid and 3,4 dimethoxycinnamic acid which are connected by an ester-bond to the hydroxy groups of quinic acid (1alpha, 3R, 4 alpha, 5R-tetrahydroxy-cyclohexane carboxylic acid) The anti-oxydation capacity of Chlorogenic acid is more potent than of ascorbic acid (vitamine-C) or mannitol, which is a selective hydroxy-radical scavenger . Chlorgenic acids have a bitter taste in low concentrations such as 50 mg / 1 L water. At higher concentrations of 1 g/ 1 L water they appear with a sour taste. Chlorogenic acid increase the solubility of caffeine and are imoprtant modulaters of taste.

Volatile compounds of Green Coffee

Volatile compounds of green coffee beans are comprising short chain fatty acids, aldehydes, and nitrogen containing aromatic molecules such as derivatives of pyrazines green-herbeaceous-earthy odor. Briefly, such volatile compounds are responsible for the unpleasant odor and taste of green coffee being capable of causing nausea and vomiting on inspiration of the odor of ground green coffee beans or ingestion of a beverage made up by pulverised green coffee beans. Due to this nauseating odor green coffee beens have never been used first hand for the preparation of a refreshing beverage, which in fact would cause vomiting, although green coffee beans are containing the same amount of caffeine as rosted coffee. On roasting of green coffee beans other molecules with the typical pleasant aroma of coffee are generated, which is not present in fresh green coffee. Other tried to neutralise or transform the nauseating odor-molecules of green coffee beans into innovative flavor by fermentation of the intact mature brown to yellow coffee bean similar to the wet processing of the mature coffee fruit . However such fermented green coffee beans must be further roasted in order to obtain an organoleptic accepted beverage based on coffee. On roasting major part of the unpleasant tasting volatile compounds are neutralised. Unfortunately other important molecules such as antioxydants and vitamines being present in green coffee are destroyed. Such volatile compounds with nauseating odor for humans which have been identified are e.g. acetic acid (pungent, unpleasant odor); propionic acid (odor of sour milk, or butter); butanoic acid (odor of rancid butter, present in green coffee with 2 mg/100 g coffee beans); pentanoic acid (unpleasant fruity flavor, present in green coffee with 40 mg/100 g coffee beans); hexanoic acid (fatty-rancid odor), heptanoic acid (fatty odor); octanoic acid (repulsive oily rancid odor); nonanoic acid (mild nut-like fatty odor); decanoic acid (sour repulsive odor); and derivatives of such fatty acids. 3-methyl-valericacid (sour, green-herbaceous, unpleasant odor); acetaldehyde (pungent-nauseating odor, even in high dilutions; present in dried green coffee beans with 5 mg/1 kg); propanal (chocking effect on respiratory system, penetrating-nauseating), butanal (nauseating effect; present in dried green coffee beans with 2 to 7 mg /1 kg); pentanal or valerianic aldehyde very repulsive nauseating effect .

Health properties of Green Coffee

Green coffee beans are a rich source of antioxydants such as polyphenols and mannitol with a high protecting effect against chemical oxydation. The high content of arabinogalactans can stimulate the immune system (e.g. the macrophages) of the gastrointestinal tract and might help to overcome problems of colon irritabile or inflammable bowl diseases. Extracts of green coffee have been shown to improve vasoactivity in humans . Green coffee can only be consumed by humans in capsules because of the nauseating odor of the volatile compounds of the green coffee beans.

New coffee-species

Recently, two new coffee-species have been discovered in Cameroon; namely the Coffea charrieriana and the Coffea anthonyi. Through the field of bionics, these two new species could introduce radical changes as they offer two new improvements: respectively the caffeine-less coffeebeans and self-pollination. Through crossing the new species with other known coffees (eg the arabica/robusta-varieties), the new coffees might allow these new improvements at the regular coffee-plantations (eg in Kenya, as the arabica/robusta remains accustumed to these growing conditions).

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