Written by: Martina Vavrusova Hedegaard, Ph.D. / Technologist / SiccaDania
The history of coffee in brief
- The wild Arabica coffee, Coffea arabica, is indigenous to Ethiopia. A wide-spread legend has it that an Ethiopian goatherd called Kaldi noticed that his goats started to jump more energetically after having chewed some red berries (coffee) from a bush. Kaldi also tried the berries and enjoyed their stimulating effect.
- Coffee made its successful journey from Ethiopia to Arabia, the part today known as Yemen. The popularity of coffee created a lucrative trade for Arabians, who guarded the coffee production jealously for many years. The raw beans were steeped in boiling water or heated to destroy their germination potential before they were allowed out of the country.
- However, the first seeds were smuggled to India by a pilgrim. Later, coffee estates were developed by the British and also the Dutch saw a future in coffee for the coffee-house trade of Europe. The Dutch therefore began expanding the first coffee farms in Sri Lanka and the Dutch East Indies (today known as Indonesia). The Arabica coffee was on its way to other parts of the world with the efforts of the British, the Dutch, later also the French, and other nationalities around the world.
- The coffee tree belongs to the Rubiaceae family, genus Coffea, and today more than 80 coffee species have been identified around the world.
- Coffea canephora known as Robusta coffee grows wild in the equatorial forests from West Africa to Lake Victoria. Here the berries of Robusta coffee were also chewed and used for ceremonial purposes. From Africa, the Robusta coffee made its journey to other parts of the world. Today, Indonesia is the largest producer of Robusta coffee in the world and this coffee represents 90% of the country´s production.
- Today, Coffea arabica is of the most economic importance, as 80% of the world’s coffee production is made from Coffea arabica. The remaining 20% is made from Coffea canephora (Robusta coffee).
- In general, Brazil is the world´s largest coffee producer followed by Vietnam which produces a fantastic signature drink where the coffee is mixed with sweetened condensed milk. The third-largest coffee producer in the world is Columbia.
Green coffee beans
- The coffee plant produces red cherry-like fruits containing two seeds. The seeds, after being separated from the fruit pulp, are known as green coffee and they are also referred to as green coffee beans. So, although they are called coffee beans, they are actually the seeds of the cherries of the coffee plant.
- In general, coffee beans are typically harvested by manual picking and stripping, or they are harvested mechanically. After the harvest, the pulp can be extracted by means of either a dry or a wet method. After the seeds are dried, the coffee is sized, graded, and sorted to eliminate any defective seeds.
- The green coffee beans can at this stage be decaffeinated, steam-treated (to make the coffee less irritating to the stomach) or stored before roasting.
- Worth mentioning, the so-called monsoon coffee refers to a speciality coffee in which dry green Arabica and Robusta seeds of good quality are exposed to the moist monsoon winds for 3-4 months at the west coast of Southern India. During this process, the beans soak up moisture, swell in size, change colour, and lose their acidity. This coffee then has a pleasant aroma and a smooth earthy taste in the cup.
- The basic chemical composition of green coffee beans depends on the genetic aspects and the degree of maturation. In addition to these intrinsic factors, extrinsic factors such as soil composition, climate, agricultural practices, and storage conditions to some extent affect the chemical composition of the seeds.
- The Arabica coffee and the Robusta coffee species differ in many ways. Robusta coffee trees are more robust, hence the name, more resistant to diseases, insects, and less demanding with regard to climate and UV radiation than the Arabica coffee trees.
- Robusta coffee contains more soluble solids. Therefore, it is used for instant coffee production and it also adds body to the final beverage. It is characterised by a more aggressive flavour compared to the Arabica coffee which provides a more delicate taste and superior quality.
- Most of the green coffee is composed of carbohydrates which represent ⁓60% of the dry matter of the coffee, including soluble and insoluble polysaccharides (cellulose, arabinogalactan, and galactomannan), oligosaccharides (stachyose and raffinose), disaccharides (sucrose), and monosaccharides (glucose, galactose, fructose, arabinose, mannose, mannitol, xylose, and ribose). The lipids in the green coffee represent 8-18% of the dry matter of the coffee. The major lipid fraction is represented by 75% triglycerides. The other lipids are sterols (stigmasterol, sitosterol), fatty acids (linoleic, linolenic, oleic, palmitic, stearic, arachidic acid etc.), pentacyclic diterpenes (discussed below), and fatty acyl tryptamides in the coffee wax. Proteins, peptides, and free amino acids represent 9-16% of the weight of dry green coffee. Green coffee also contains several N-containing compounds like for example theobromine and theophylline in addition to caffeine and trigonelline (discussed below). A variety of phenolic compounds are present in green coffee with chlorogenic acids representing the major compounds.
- Below, the main bioactive compounds in green coffee beans will be discussed in brief:
- Caffeine is a naturally occurring alkaloid (1, 3, 7-trimethylxanthine) with bitter characteristics, but interestingly, it is responsible for no more than 10% of the perceived bitterness of coffee.
- Caffeine is heat stable and its concentration in Robusta coffee is about twice the concentration found in Arabica coffee.
- Caffeine is rapidly and completely absorbed and eliminated with an average half-life of 5 h. Once it is absorbed, it stimulates the central nervous system as an adenosine receptor antagonist. This means that since caffeine has a similar molecular structure as adenosine, it has the potential to occupy adenosine receptor sites. After caffeine connects to those receptors, an adenosine blockage is formed. This blockage stops the sleep-promoting effects of adenosine, resulting in the neurons speeding up instead of slowing down. Therefore, caffeine is known for its ability to increase the power of concentration and decrease the sensation of fatigue and enhance mood, alertness, attention, reaction time etc.
- Trigonelline is a pyridine alkaloid that also contributes to the bitterness of coffee. There are slightly higher values of trigonelline found in Arabica coffee than in Robusta coffee. During the roasting process, trigonelline is partially degraded to nicotinic acid (known as B3-vitamin or niacin), and several pyridine derivates.
- It has been shown that trigonelline has the potential as a hypoglycemic and neuroprotective agent and it also has anticarcinogenic effects.
- Chlorogenic acids comprise a major class of phenolic compounds which are often referred to as powerful antioxidants in vitro. The content of chlorogenic acids in Robusta coffee is generally higher than in Arabica coffee.
- Chlorogenic acids contribute to the astringency, bitterness, and acidity of coffee. High amounts of Chlorogenic acids in green coffee beans may produce undesirable flavours due to oxidation and degradation products formed before the roasting.
Cafestol and kahweol
- Cafestol and kahweol compounds are pentacyclic diterpene alcohols. These bioactive compounds and their derivates occur as esters or salts of saturated and unsaturated fatty acids. High concentrations of these diterpenes are present in unfiltered coffee since they are poorly soluble in water. Therefore, they are almost completely removed when the coffee brew is filtered either immediately prior to consumption or during the manufacture of soluble coffee powders.
- Although coffee diterpenes have been associated with the cholesterol-rising effect of coffee, they have also been shown to have chemo-preventive potential in vitro.
- Green coffee may also contain minor compounds (incidental non-volatile compounds) which are considered as microbial by-products which occur due to inappropriate harvesting, weather conditions or improper storage. Other undesirable compounds in terms of health concerns can be formed by high roasting temperatures. Therefore, to obtain a functional coffee, every aspect of the coffee production should be carefully considered, including high-quality beans and appropriate roasting conditions.
- The aroma of green coffee is described as a mild, bean-like aroma. This desirable and so typical coffee aroma develops during roasting.
- The chemistry related to flavour development is highly complex and still not well understood.
- The coffee aroma consists of a complex mixture of volatile compounds, whereas non-volatile compounds contribute to flavours such as sourness, bitterness, and astringency.
- More than 950 volatile compounds have, after roasting, been identified as compounds contributing to the coffee aroma. These volatile compounds (sulphur compounds, pyrazines, pyridines, pyrroles, oxazoles, furans, aldehydes, ketones, phenols, etc.) result from different reaction routes. The final composition of the volatile fraction of roasted coffee depends on the genetics, agricultural conditions, climate, degree of maturation, and also roasting degree and roasting conditions.
- Since the beans of the Arabica coffee and the Robusta coffee have different non-volatile compositions, they also exhibit different compositions of volatile compounds resulting in markedly different aromas.
- The common roasters used for home and industrial coffee roasting are drum roasters, in which the seeds are in direct contact with fire and/or a hot surface. Another type of roaster is the fluid bed roasters, which are preferred for industrial roasting where the seeds are in contact with hot air/gases. The fluid bed roasters are faster and allow better control of the air temperature and the speed inside the roasting chamber. The fluid bed roasters also produce a more homogeneous colour than other roasters and they allow a maximum temperature from 210°C to 240°C on an industrial scale.
- In the initial phase (endothermic process) of roasting, moisture is eliminated. The smell of the bean changes from green-bean-like to bread-like and the colour changes from greenish to yellowish. A series of endothermic and exothermic reactions take place at temperatures higher than 160°C. The beans become light brown and their volume increases. During this process, a large amount of carbon dioxide is produced which contributes to an increase of the internal pressure in the coffee beans. The beans change colour to dark brown at approximately 190°C and many compounds which are responsible for the aroma and the taste are formed. In the final phase of the roasting process, the beans are rapidly cooled by air or water.
- It is important to remember that the roasting conditions like for instance the time, the temperature, and the air-flow speed at which the seeds reach the desired colour affect the physicochemical and chemical properties and consequently the flavour and bioactivity of the compounds in the final coffee beverage. The final composition of the roasted coffee also varies according to the raw material and the roasting method.
- Coffees roasted at higher temperatures for shorter periods of time tend to have a higher acidity, more soluble solids and a different volatile profile than coffees roasted for longer periods of time at lower temperatures.
- The moisture content of roasted coffee is ⁓5-5% and can vary depending on the roasting degree.
- During the roasting, many chemical changes take place which leads to the degradation of some compounds and to the formation of new compounds. For example, a portion of proteins is degraded, and some amino acids react with reducing sugars (via the Maillard reaction) and form low-molecular-weight compounds and melanoidins.
- Changes of some bioactive compounds during roasting will be mentioned herein brief:
- Melanoidins are products of the Maillard reaction and they are responsible for the brown colour of the roasted coffee. Some studies indicate that melanoidins have antioxidant and antibacterial potential. Therefore, melanoidins may be considered bioactive compounds.
- Carbohydrates such as sucrose are involved in caramelisation and the Maillard reaction. They also contribute to the acidity of the coffee after roasting as the level of acids such as acetic, formic, glycolic, and lactic increases as a result of the degradation processes.
- Chlorogenic acids suffer great losses during the roasting process and undergo many chemical changes like example isomerisation, epimerisation, lactonisation, degradation, and incorporation into melanoidins, thus contributing to the development of the colour and the flavour of the coffee. For example, chlorogenic acid lactones contribute to coffee bitterness.
- Roasting contributes to the degradation of trigonelline and the production of many compounds including nicotinic acid known as niacin or vitamin B3 involved in the therapy of dyslipidemia (an abnormal amount of lipids in the blood) and pellagra (a disease caused by low levels of niacin).
- Only small losses of caffeine may occur during roasting.
- After roasting, the beans are ground and marked as ground roast coffee or used for instant coffee production.
- The coffee aroma is best preserved in the beans. The coffee flavour deterioration is known as staling which is faster in ground coffee because of the increased surface. Therefore, it is recommended to grind the beans just before brewing.
- Flat and conical burr grinders are used for home and industrial grinding. The burr grinder uses two rings to crush the beans. Roller mills are also used for coffee grinding on an industrial scale.
- Actual crushing is a process where the beans are crushed into smaller parts, smaller parts are crushed into rough particles and the rough particles are crushed into fine particles.
- The formation of small particles with a large surface area is important for the rapid liberation of carbon dioxide generated during the roasting process. The large surface area is also essential for the reduction of the diffusion distance for soluble substances during extraction and for the improved transfer of colloidal substances to the liquid phase.
- When ground, the coffee is packed in hermetic containers. Before packing, efficient degassing must be applied (within 2-8 h). Therefore, gas valves can be placed in the wall of the hermetic plastic bags to release carbon dioxide.
Coffee brewing (extraction)
- Generally, coffee brewing is a solid-liquid extraction in which the roasted coffee gets in contact with water. The water acts as a solvent extracting the water-soluble compounds. Depending on the extraction technique used, insoluble compounds may be present in the extraction water as dissolved or suspended solids.
- Factors affecting the composition of the coffee brew depend on the brewing method, the coffee to water ratio, the extraction time, the size of the ground coffee particles, the temperature of the water, and the hardness of the water. Interestingly, it has been reported that the temperature of the hot water used in the brewing of coffee should not exceed 90-95°C (home brewing).
- The most common extraction methods are the methods used for brewing espresso and filtered coffee. The extraction of water-soluble compounds such as chlorogenic acids, vitamin B3, caffeine, melanoidins, and hydrophilic volatile compounds is better at higher temperatures and pressures. A hydrophobic part represented by lipids can be found in coffee brews as a result of the high temperature and the high pressure used when brewing espresso coffee or due to the lack of filters made of lipophilic materials. Therefore, unfiltered coffee and espresso coffee brews contain higher amounts of these compounds together with bioactive diterpenes and sterols.
- Worth mentioning is the method of cold extraction which is a relatively new method in the market. The cold extraction is carried out at room temperature (20-25°C) or even colder over a long period of time (steeping time ranging from 8 to 24 hours) compared to the traditional hot brewing methods. This method produces a final coffee with different physicochemical and sensory properties.
- Another type of special extraction is the production of instant (soluble) coffee which is very popular due to its convenience. Instant coffee production requires extraction of the soluble coffee solids and the volatile compounds which are then dried into a powder or granules.
Instant (soluble) coffee production
- Interestingly, instant coffee production has a long history dating back to the 18th An enormous popularity of instant coffee was gained by the American soldiers during World War II.
- Robusta coffee is often used at a high percentage or even alone for the production of instant coffee. Robusta beans contain a higher concentration of soluble solids than Arabica beans. Beans used for instant coffee production are roasted more or less in the same way as the beans used for home brewing, typically at temperatures above 180°C. The roasting temperatures also depend on the desired quality of the final instant coffee as well as the downstream process.
- Since aroma compounds are lost during the initial phases of instant coffee manufacturing, these compounds can be recovered during several later stages of the manufacturing process. The gases released during the roasting and grinding processes can be collected. Sometimes the volatile aroma compounds can be removed from the ground coffee during the so-called pre-stripping process. Usually, the steam is passed through a bed of coarsely ground roasted coffee to release the aroma compounds. The aroma compounds and the steam are then condensed into a mixture of water and volatile compounds.
- The Aroma and the flavour are also preserved by removing oxygen from the coffee extract by foaming other gases (nitrogen or carbon dioxide) through the liquid before the dehydration process.
- The actual extraction of the soluble coffee solids is performed by means of water in highly efficient extraction equipment. The water passes through a series of five or even more columns of ground coffee. The processing temperature profile in the columns varies from 100-220°C depending on the desired extraction procedure. Last, the extract fractions often pass through a heat exchanger in which the extract is cooled.
- The brewed coffee is filtered/clarified and further concentrated to make a final extract with ⁓30-50% solids. This can be done by processing the coffee extract in centrifuges and evaporators. Another technique is to remove the water by means of freeze concentration.
- The coffee extract can be dried in a spray dryer at high temperatures or in a freeze dryer at very low temperatures.
- Freeze drying leads to the best product quality in relation to aroma recovery and good solubility. The final coffee powder contains < 5% moisture. The coffee extract is cooled to a slushy consistency of ⁓0°C. At this stage, the gases are added to adjust the bulk density and to secure the colour of the instant coffee. This product is further cooled to a temperature of -45°C. The cooling rate affects the size of the ice crystals i.e. the colour of the coffee and in general the whole freeze-drying process. As an example, fast cooling (30-120 s) results in smaller ice crystals and lighter coloured frozen coffee while slow cooling (10-180 min) generates larger ice crystals and darker frozen coffee. The frozen slab of coffee is broken into pieces and ground into granules of the proper size. These granules are freeze-dried in a multiple-step freeze-drying process.
- Volatile aroma compounds which have been recovered from earlier manufacturing steps can be sprayed onto the dry coffee particles.
- Since the instant coffee particles are hygroscopic, they must be packaged under low humidity conditions in a moisture-proof container to keep the instant coffee dry.
- Coffee, the black beverage packed with aromas, has been consumed for over 1000 years. For many of us, it has become an integral part of our lives. Quite recent research indicates that coffee may provide some interesting functional benefits when consumed in appropriate quantities.
- The whole process of making coffee is alchemy where attention must be paid to very fine details.
- The gentle technology of freeze-drying is a very frequent method used to produce instant coffee as it provides an excellent aroma profile. As indicated by Corso et al. (2016), the freeze-drying technology may also assist in the production of freeze-dried extracts from green coffee and roasted coffee for the development of specially infused instant coffee formulations. The freeze-drying technology preserves thermolabile compounds such as chlorogenic acids and improves the sensory quality resulting in interesting instant coffee products with antioxidant potential.
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