The influence of processing method on coffee flavor

CFRR- In nature, the coffee berry is a fruit of the berry group that is green as young but as ripe, the berry is red, or yellow (depending on the genotype), taste sweet.

It is a fruit of the genus Coffea in the family Rubiaceae (Madder family) and may belong to many different genera, commonly C. arabica and C.canephora (robusta). In international coffee transactions, most of them are in the form of green coffee beans (coffee beans that have been processed).

Structure of coffee berry

With the outer skin covering the pulp (mesocarp) containing fibre and sweetness, followed by a thin, viscous layer of mucilage (called pectin), then a light-yellow parchment (husk), and a silver shell (silk skin) covering each hemisphere of the coffee bean (Belitz, 2009; Berbert and coworkers, 2001). 

1. Exocarp likewise described as the peel, is the outermost layer of the coffee fruit.

The color of the exocarp at the start of fruit advancement is green due to the presence of chloroplasts which then vanish as the fruit develops. Color upon maturation relies on the coffee range, however, is most typically red or yellow. Red skin color originates from anthocyanin pigments while yellow skin color is credited to luteolin. (Marin and coworkers, 2003).

The exocarp is formed by a single layer of compact parenchyma cells with thin main walls which contain chloroplasts and can absorb water. (De castro and coworkers, 2006; Wintgens and coworkers, 2008). There are 1% tannins, 5% caffeine, trigonelline, organic acids and enzymes per calculation unit.

2. Mesocarp likewise described as the mucilage, is the flesh of the coffee berry. 

With maturation, pectolytic enzymes break down pectic chains, leading to an insoluble hydrogel that is rich in sugars and pectins located beneath the mucous membrane making a soft, succulent structure of high viscosity, about 0.5-2mm in thickness (Ouguerram, 1999), the percentage of mucus increases as the elevation increases.

For freshly harvested coffee berries, mucus consists of 84.2% water, 8.9% protein, 0.91% pectat, 27% fructose, 21% glucose, 6-9% sucrose and 7.3% organic acid, of which malic acid, quinic acid and gluconic acid are the most abundant (SCA, 2017).

3. Endocarp: Parchment/ Husk

The hard layer that surrounds the coffee beans, consists of three to seven layers of sclerenchyama cells about 150 micrometers in size (Mendes, 1942). The husk is the outermost layer of the bean, the main ingredient is Cellulose 40-49%, rough and in direct contact with the pulp, containing a little caffeine about 0.4%, hemicellulose 25-32%, lignin 33-35% (Bekalo and coworkers, 2010)

The cells of the endocarp harden throughout coffee fruit maturation.

In coffee processing, the typical weight of the parchment is around one kilogram of same coffee beans weight.

4. Silverskin – Silver Case / Outermost Skin

The silverskin is the outermost layer surrounding the bean, formed from sclerotic cells which is silvery-white after drying, about 70 micrometers in thick, so it is also known as the silver skin (Dedecca, 1957), containing about 18% protein, and small amounts of fats and carbohydrates. This silvery peel can be removed from the beans during roasting.

5. Coffee bean:

Coffee beans are included exoskeleton, endosperm, and embryo, and the size of the coffee bean will vary depending on the coffee variety average bean can be 10mm long, and 6mm wide. As usual, a coffee fruit will have two kernels symmetrically. The coffee mutants will have three or one culi.

Exoskeleton will have the main task of accumulating nutrients for embryo germination, contributing to the physical characteristics of coffee beans. (Geromel and coworkers, 2008; Geromel and coworkers, 2006) From the outside, the ectoplasm must have a uniform amber yellow, no spots, no bruises, and no signs and symptoms of disease… The ectoplasm also has a protective effect on the endosperm and the embryo inside.

Endosperm is the tissue produced inside the bean where almost all the nutrients of the coffee bean are stored, and the highest caffeine content is located here, but the water-soluble compounds are caffeine, trigonelline, nicotinic acid, chlorogenic acid, disaccharides, proteins, minerals, and carboxylic acids. The water-insoluble components are cellulose, polysaccharides, lignin, hemicellulose, several proteins, minerals, vitamins and lipids, glutamic acid, aspartic acid, leucine, and valine (De Castro and coworkers, 2006; Wintgens and coworkers, 2008)

Embryo is inside the endosperm consisting of an axis and two cotyledons. The embryo will become a plant by a pulley that will pull it out of the ground.

Chemical composition in coffee bean

1. Water

Water content accounts for 70-80% of fresh fruit through the processing, the water content in the seeds remains about 10-12%, and higher water content will easily cause the beans to be moldy and microorganisms to grow. Strongly, the effects aroma and flavor quality too low a coffee bean color will often fade, creating cracks in the beans, reducing germination, and having a hay smell. After roasting, the amount of water will remain from 1-2% depending on the degree of roasting (Farah and coworkers, 2004).

2. Mineral

Mineral content accounts for 3-5%, mainly: Potassium, Magnesium, Chlorine, Phosphorus, Nitrogen, Aluminum, Iron, Copper. Among these minerals, magnesium will have a significant difference among several nuts. (Clarke and coworkers, 2003). One can also rely on the composition of minerals to distinguish the nutrient composition of the soil (Costa and coworkers, 2010).

3. Carbohydrate

Constituting more than 50% of the weight and is also the main component of coffee (Trugo and coworkers, 1985), polysaccharides account for 44-47%, and sucrose accounts for 6-9% of the dry weight of the beans that decomposed in the process. Roasting to form anhydrous, sugars and glyoxal compounds (Flament and coworkers, 1968; Kolling and coworkers, 2005; Clifford, 1985; De Maria and coworkers, 1994), carbohydrates that are precursors for the Maillard reaction, sucrose will be hydrolyzed under the action of organic acids and hydrolytic enzymes in the Caramelization reaction to produce acetic acid. The sugar content in the beans will depend on the ripeness of the coffee cherries.

Glucid is only involved in the brown color and caramel taste in coffee. But its reaction products after roasting contribute to the flavor of the coffee.

4. Protein, peptide and free amino acid

The protein content of about 9-11% plays a role in shaping the taste of coffee. In Protein there are amino acids such as Cysteine, Alanine, Phenylalanine, Histidine, Leucine, and Lysine … these amino acids and reducing sugars involved in the Maillard reaction are released to the outside or act on the aromas and flavors in coffee (Maillard, 1912).

Substances that contribute to the scent and retain the scent of coffee beans have the effect of reducing oxidation for aromatic substances in this group such as Methionine, Proline …

5. Lipid

Lipid content accounts for about 15% in Arabica seeds or about 10% in Robusta beans (Mazzafera and coworkers, 1998), the main lipid composition is triacylglycerol about 75%, free fatty acids 1%, sterol 2, 2%, tocopherol 0.05%, oil, and wax 8-18% dry weight (Trugo and coworkers., 1984; Folstar and coworkers, 1985; Arya and coworkers, 2007; Belitz and coworkers, 2009), under the effect of heat will create aroma, retain aroma thanks to unsaturated fatty acids (Toci and coworkers, 2008), and create palatability for coffee, while the amount is not changed by heat. solvents for aromatics.

During extraction, only a small amount of lipid is dissolved and most of it remains on the residue. Lipids are easily oxidized and rancid during the storage of coffee beans after roasting. 

6. Alkaloid

Alkaloids, which are responsible for the bitter taste in coffee, play an important and well-studied role, such as caffeine and trigonulin.

• Caffein accounts for 1-4% depending on the type of seed and cultivar (Belitz and coworkers, 2009; Mazzafera and coworkers, 2010). In Arabica beans, caffeine is from 0.9-1.7%, while in robusta coffee beans, the caffeine content is doubled, accounting for 1.8-4%. Caffeine contributes about 10% of the bitter taste and is a central nervous system stimulant, increases blood circulation, and is well known for its effects on alertness (Farah and coworkers, 2006, Belitz and coworkers, 2009). Caffeine also acts as a plant protectant against harmful diseases. The caffeine content of the pods is two to ten times lower than that of the seeds (Koshiro and coworkers, 2006), the variation of which depends on the genotype of each type and the stage of fruit development.
• Trigonellin (N-methylnicotinic acid C7H7NO2) up to 0.8% in green coffee beans, melting point is 218 degrees Celsius. Under the effect of heat Trigonellin will be hydrolyzed 50% to form Nicotinic acid (precursor of vitamin PP with cholesterol-lowering effect), pyridine, 3-methyl pyridine, methyl ester… In the chemical composition, there will be no nicotinic acid, it is only formed in the hydrolysis reaction of Trigonellin (Lang and coworkers, 2008).

7. Gas and aromatic compounds

Green coffee beans contain more than 200 volatiles and have little aroma. Roasting produces more than 800 aromatic compounds (compounds responsible for the flavor in coffee), but only about 40 compounds contribute to aroma (Belitz and coworkers, 2009). Aromatic substances include many constituent molecules such as: Acid, Adehide, Acetone, Ethylic, Phenol, Este… Aromatic substances are volatile substances that, without good preservation of coffee beans, will lose their smell very quickly.

8. Organic Acids

The main organic acids, Chlorogenic Acid (CGA), can account for up to 12% in green coffee and about 4% in the beans after roasting, contributing to the sour, bitter taste of coffee (Ginz and coworkers, 2001), and antioxidant properties of roasted coffee (Sato and coworkers, 2011).

Volatile acids include formic acid, and acetic acid. Non-volatile acids such as lactic, tartaric, pyruvic and citric acids. Additional components such as malonic acid, glutaric acid, malic acid.

Common processing methods

The preliminary processing of green coffee beans takes place after harvesting quickly to avoid damage and mold growth. What type of processing will affect the roasting method and the quality of the coffee cup when preparing? There are three commonly used methods in the preliminary process of green coffee beans: Dry preliminary, wet preliminary, and semi-wet preliminary.

1. Dry/ Natural/ Un-Washed method

Here is a commonly adopted method from Yemen, Ethiopia, and some regions in Brazil. Currently, in many coffee-growing areas of Vietnam, farmers still mostly use this preliminary processing method. The coffee berries, after harvesting, are dried in the sun, then put into a peeling machine to remove the outer shell, mucus, husk, and retain the silk shell (Belitz and coworkers, 2009), which is a simple processing technique, but difficult to achieve high coffee quality.

Natural pre-processing method (Dry/ Natural/ Un-Washed)

Step 1: Harvest the coffee berries.

Step 2: Remove impurities (wood, stone, leaves, dry branches) and wash. The fruit is placed in a bucket of water, the green, young, squishy, and pests will be removed in 24 hours.

Step 3: Dry the coffee berries on the rig. During the first three days, a lot of sun is needed to dry the crust surface and a humidity lower than 35% should be achieved. Continue drying until the humidity reaches about 15%. This period can last from 25-30 days.

Step 4: After being dried, the coffee berries will be milled by machine to remove the outer skin and husk, dried again until the moisture content reaches about 12%, green coffee beans will be harvested.

Step 5: Store coffee beans in sacks, cool, place in a dry place.

Advantages: being simple, saving economical, the environment less polluting, using less machinery and water.

Disadvantages: depending on the weather, need for sunshine, long time, and the quality is not uniform.

Tastes: because when drying the seeds are still in the fruit, when there is an impact from the sun, the chemicals absorbed into the beans, especially sugar, will give the beans a high sweet taste, less sour, and a dark, slightly bitter body, less fragrant than wet processing (Tianyu Pan and coworkers, 2021). After processing, coffee beans also contain about 35% carbohydrates, 5.2% protein, 30.8% fiber, and 10.7% minerals (Brand and coworkers, 2001)

2. Wet processing method (Full Washed/ Washed/ Wet)

Popular in Latin America, Africa, and worldwide. During the initial washing after harvest, it is possible to remove the damaged or unripe fruit that floats to the top, separate them from the ripe fruit that sinks to the bottom (Belitz et al., 2009), and then use mechanically to separate the fruit pulp, but still some stick to the husk.

Wet processing

Step 1: Harvest the coffee berries

Step 2: Remove impurities (wood, stone, leaves, dry branches) and wash. Fruits putting in a bucket of water. Therefore, the cherries, which are green, young, squishy, and some pests cover the cherries, had to remove in 24 hours.

Step 3: Put in the peeler to remove the coffee skin, meat skin, and mucus.

Step 4: Pour the coffee beans into a large bucket of water, and remove the floating beans again. Then start the fermentation process with natural enzymes, which lasts about 36-72 hours depending on the temperature (hotter areas will ferment faster), climate, altitude, enzyme concentration, and thickness of the substance, mucus, equipment, and manufacturer’s needs.

Step 5: After the fermentation stage, the coffee washed to remove all the mucus surrounding the beans and brought to the drying rack to reach a moisture content of about 12%, which will take 8-10 days or dried by machine in areas with insufficient sunlight and high humidity.

Step 6: Store coffee beans in sacks, cool place, and put in a dry place.

After processing, the coffee grounds (including the outer shell and mesoderm) removed, which are about 29% of the dry weight of the fruit. The content of chlorogenic acid (CGA) and trigonelline is higher than in dry processing about of sucrose sugar is about 5.07%, the organic acids are about 4.96%, proteins from 13-15%, free amino acids from 0.8-1.4%, 5-caffeoylquinic acid is about 67.1% (Duarte and coworkers, 2010; Gustavo and coworkers, 2020)

Advantages: uniform quality should be widely used globally, control the processing process, and minimize risks due to human factors or weather.

Disadvantages: to need a lot of specialized machinery and equipment, requires a lot of water for processing, and there must be a hygienic waste treatment process for the environment.

Taste: Washing the coffee makes the beans show the flavor characteristics of the coffee growing area, the sugar content in the beans decreases, and the amino acids increase, contributing to an unicque aroma of the coffee beans, purity, high, creative sour taste into high overall quality (Tianyu Pan and coworkers, 2021), wet processing is suitable for Pour Over method.

3. Semi-wet processing method (Honey/ Semi Washed/ Pulped natural)

The method is used commonly in Central American countries such as Costa Rica, EI Salvador… and Brazil. This method is kike the combination of dry and wet processing, or so-called honey processing, because mucus will be dried along with the seeds to create a sweet, honey-like aroma or sugar after being dried.

Semi-wet processing process

Step 1: Harvest ripe coffee berries.

Step 2: Remove impurities (wood, stones, leaves, dry twigs) and rinse. The fruits are put into a water bucket, which is green, young, and squishy and all of the pests will be removed in 24 hours.

Step 3: After washing, the fruit is put into the pod rubbing machine

Step 4: Preliminary processing of the remaining mucus on the seeds. The ratio of mucus and beans, and an impact on the color, sweetness, variety, and complexity of taste in coffee beans. Based on the percentage of mucus can be divided into four types as follows:

• White Honey: mucus accounts for 10-15% of the husk when dried
• Yellow Honey: mucus accounts for 15-50% of the husk when dried
• Red Honey: mucus accounts for 50-90% of the husk when dried
• Black Honey: mucus accounts for 90-100% of the husk when dried.

The higher the percentage of mucus, the longer the drying time, and the more complex the variety and aroma will be.

Step 5: After being prepared with mucus which accord with the manufacturer’s demand, the beans will be dried on trellises and thinly layered in the natural sun for about 8 to 14 days to achieve a humidity of about 12-12.5% or dried by drying method.

Step 6: Store coffee beans in sacks, cool and put in a dry place.

Carbohydrates account for about 21-32%, protein 7.5-15%, fat 2-7% (Ulloa-Roas and coworkers, 2003), trigonelline lower than dry and wet processes, and higher sucrose content (Schwan and coworkers, 2012).

Advantages: Environmental protection with little water use, variety of flavors, sweet and sour tastes.

Disadvantages: depend on the skill of the preprocessor or the weather being difficult to control the quality.

Taste: Brings a mellow taste, mild sourness, natural sweetness, and herbal aroma (Xie Jijian and coworkers, 2011). Fresh grass and sweet cane aromas are also described for this semi-wet preparation, accompanied by a medium to the thick body (Cortez and coworkers, 2000).

Chemical changes in processing

• During dry processing, the substances in the fruit continue to exchange, germination is inhibited when the coffee beans are covered with flesh, and the germination inhibitor is abscisic acid (Valio and coworkers, 1980). The isocitrate lysine (ICL) enzyme content is low at the initial processing stage and gradually increases 6 days after the start of processing, the aroma quality in coffee lies in this metabolism (Selmar and coworkers, 2006).
• Due to the high humidity in the mucus, fermentation will be intensive (Idago and coworkers, 2015), and the content of free sugars such as glucose, fructose, and galactose is more due to being absorbed from the mucus, resulting in coffee beans will have a high sweetness (Bytof and coworkers, 2005)
• Wet processing has washed away the outer crust and mucus, germination continues to grow, and the lysine isocitrate enzyme with the highest content in the head process gradually decreases with drying is an enzyme specific for germination. (Valio and coworkers, 1980; Bewley and coworkers, 1997).
• This germination process causes the metabolism in the coffee bean to continue, and amino acids become flavor precursors in the beans (Valio and coworkers, 1980), the scents that characterize the soil of that bean. In parallel, free amino acids contribute to the protein breakdown reaction that creates a substrate for germination (Bytof and coworkers, 2005; Selmar and coworkers, 2008).
• Fermentation is also an important step in wet processing method, that rich microbiomes consume nutrients in mucus promoting metabolites and organic acids that affect the coffee quality (Massawe, 2010; Silva and coworkers, 2013). This process contributes to superior aromatic quality in coffee beans.
• Metabolism during germination may cause a large amount of acid to increase the sugar content used to metabolize substances in decreases (Knopp and coworkers, 2005).
• Honey processing removes the crust, leaving the mucus that envelops the seeds and the mucus composition consists of carbohydrates that produce sugary odors, a pronounced sweet taste from honey (Avallone and coworkers, 2001). The bitter-related compounds, which are chlorogenic acid and trigonelline, carried lower levels than dry or wet processing, with sweeter-associated sucrose content found to be (Schwan and coworkers, 2012).
• The fermentation process creates differences in taste, aroma, and physicality in the seeds. This process needs to be thoroughly controlled, when fermenting for too long. It produces more acetic acid and phenolic compounds that make the coffee bitter or sour (Jackels and coworkers, 2005).

Conclusion:

In addition to factors such as plant variety, type of beans, roast level, or the preparation, processing methods have a strong impact on coffee beans’ flavor. Therefore, people will choose a specific way to create the flavors they are aiming for or suitable for the physicality of the coffee beans and the conditions of that production area.

Reference:

1. Arya, M., & Rao, L. J. M. (2007). An impression of coffee carbohydrates. Critical Reviews in Food Science and Nutrition, 47, 51–67.

2. Avallone, S.; Guiraud, J.-P.; Guyot, B.; Olguin, E.; Brillouet, J.-M. Fate of Mucilage Cell Wall Polysaccharides during Coffee Fermentation. J. Agric. Food Chem. 2001, 49, 5556–5559 

3. Berbert, P. A., Queiroz, D. M., Sousa, E. F., Molina, M. B., Melo, E. C., & Faroni, L. R. D. (2001). Dielectric properties of parchment coffee. Journal of Agricultural Engineering Research, 80, 65–80.

4. Bekalo, S. A., & Reinhardt, H. -W. (2010). Fibers of coffee husk and hulls for the production of particleboard. Materials and Structures, 43, 1049–1060.

5. Belitz, H. -D., Grosch, W., & Schieberle, P. (2009). Food chemistry (4th ed.). Heidelberg: Springer (Chapter 21).

6. Bewley, J.D. Seed Germination and Dormancy. Plant Cell 1997, 9, 1055–1066 

7. Bytof, G.; Knopp, S.E.; Schieberle, P.; Teutsch, I.; Selmar, D. Influence of processing on the generation of γ-aminobutyric acid in green coffee beans. Eur. Food Res. Technol. 2005, 220, 245–250 

8. Brand, D., Pandey, A., Rodríguez-León, J. A., Roussos, S., Brand, I., & Soccol, C. R. (2001). Packed bed column fermenter and kinetic modeling for upgrading the nutritional quality of coffee husk in solid-state fermentation. Biotechnology Progress, 17, 1065–1070. 

9. Clarke, R. J. Coffee: green coffee/roast and ground. In: Encyclopedia of Food Science and Nutrition, 2nd edition, Caballero, B., Trugo, L. C., Finglas, P., eds. Oxford: Academic Press; 2003, Vol. 3. 

10. Clifford MN (1985a) Chemical and physical aspects of green coffee and coffee products. In: Clifford MN and Wilson KC (Eds) Coffee: botany, biochemistry and production of beans and beverage, pp.305-375. Croom Helm, London.

11. Costa, L. L., Toci, A. T., Silveira, C. L. P., Herszkowicz, N., M., Pinto, A., Farah, A. Discrimination of Brazilian C. Canephora by location using mineral composition. Proc. 23rd Int. Conf. Coffee Sci. ASIC, 2010. Bali, Indonesia

12. Coffeechemistry.com – by Joseph A. Rivera served as the former Director of Science & Technology for the Specialty Coffee Association of America

13. Cortez,J.G.andMenezez,H.C.,2000.RecentDevelopments in Brazilian Coffee Quality: New Processing Systems, Beverage Characteristics and Consumer Preferences. In Coffee Biotechnology and Quality (pp. 339-346). Springer, Dordrecht 

14. De Castro, R.D.; Marraccini, P. Cytology, biochemistry and molecular changes during coffee fruit development. Braz. J. Plant Physiol. 2006, 18, 175–199.

15. Dedecca, 1957. Anatomia e desenvolvimento ontogennético de Coffea arabica L. var. Typiaca Cramer. Bragantia, 16 (1957)

16. De Maria CAB, Trugo LC, Moreira RFA, Werneck CC (1994) Composition of green coffee fractions and their contribution to the volatile profile formed during roasting. Food Chem. 50: 141-145.

17. Diego, E.R.; Flavio, M.B.; Marcelo, A.C.; Mariele, V.B.P.; Vany, P.F.; Helena, M.R.A.; Jose, H.D.S.T. Interaction of genotype, environment and processing in the chemical composition expression and sensorial quality of Arabica coffee. Afr. J. Agric. Res. 2016, 11, 2412–2422. 

18. Duarte, G. S., Pereira, A. A., & Farah, A. (2010). Chlorogenic acids and other relevant compounds in Brazilian coffees processed by semi-dry and wet post-harvesting methods. Food Chemistry, 118, 851–855. 

19. Farah, A., & Donangelo, C. M. (2006). Phenolic compounds in coffee. Brazilian Journal of Plant Physiology, 18, 23–26. 

20. Farah, A. Distribuic ̧a ̃o nos gra ̃os, influeˆncia sobre a qualidade da bebida e biodisponibilidade dos a ́cidos clorogeˆnicos do cafe ́. Instituto de Qu ́ımica, Universidade Federal do Rio de Janeiro, RJ, Brasil, Doctorate Thesis, 2004. 

21. Februadi Bastian, Talha Bin Emran , Olly Sanny Hutabarat , Andi Dirpan , Firzan Nainu , Harapan Harapan , and Jesus Simal-Gandara, 2021. From Plantation to Cup: Changes in Bioactive Compounds during Coffee Processing . Foods 2021, 10, 2827. https://doi.org/10.3390/foods10112827 

22. Flament, I., Gautschi, F., Winter, M., Willhalm, B., Stoll, M. Les composants furanniques de l’aroˆme cafe ́: quelques aspects chimiques et spectroscopiques. Proc. 3rd Coll. Int. Coffee Sci. ASIC, 197–215. 1968. Paris

23. Folstar, P. Lipids. In: Coffee, Clarke, R. J., Macrae, R., eds. London: Elsevier Applied Science; 1985, Vol. 1: Chemistry, pp. 203–222. 

24. Geromel, C.; Ferreira, L.P.; Davrieux, F.; Guyot, B.; Ribeyre, F.; Scholz, M.B.D.S.; Pereira, L.F.P.; Vaast, P.; Pot, D.; Leroy, T.; et al. Effects of shade on the development and sugar metabolism of coffee (Coffea arabica L.) fruits. Plant Physiol. Biochem. 2008, 46, 569–579.

25. Geromel, C.; Ferreira, L.P.; Guerreiro, S.M.C.; Cavalari, A.A.; Pot, D.; Pereira, L.F.P.; Leroy, T.; Vieira, L.G.E.; Mazzafera, P.; Marraccini, P. Biochemical and genomic analysis of sucrose metabolism during coffee (Coffea arabica) fruit development. J. Exp. Bot. 2006, 57, 3243–3258. 

26. Ginz, M., & Engelhardt, U. (2001). Analysis of bitter fractions of roasted coffee by LC– ESI–MS-new chlorogenic acid derivatives. : Association Scientifique Internationale du Café (ASIC).

27. Gustavo A.Figueroa Campos, Sorel Tchewonpi Sagu, Pedro Saravia Celis and Harshadrai M. Rawel, 2020. Comparison of Batch and Continuous Wet- Processing of Coffee: Changes in the Main Compounds in Beans, By-Products and Wastewater. Foods 2020,9, 1135; doi:10.3390/foods9081135 

28. Idago, R. G. and Cruz, R. S. D., 2015. Value Chain Improvement of Robusta and Liberica Coffee. 

29. Jackels, S. C., & Jackels, C. F. (2005a) U.S. Patent No. US20060204620. Washington, DC: U.S. Patent and Trademark Office. 

30. Jackels, S. C., & Jackels, C. F. (2005b). Characterization of the coffee mucilage fermentation process using chemical indicators: A field study in Nicaragua. Journal of Food Science, 70(5), C321–C325. 

31. Knopp, S.; Bytof, G.; Selmar, D. Influence of processing on the content of sugars in green Arabica coffee beans. Eur. Food Res. Technol. 2006, 223, 195–201

32. Knopp, S., Bytof, G., & Selmar, D. (2005). Influence of processing on the content of sugars in green Arabica coffee beans. European Food Research and Technology, 223(2), 195–201 

33. Ko ̈lling-Speer, L., Speer, K. The Raw Seed composition. In: Espresso Coffee, the Science of Quality. Illy, A., Viani, R., eds. Italy: Elsevier Academic Press; 2005, pp. 148–178. 

34. Koshiro, Y., Zheng, X. -Q., Wang, M. -L., Nagai, C., & Ashihara, H. (2006). Changes in content and biosynthetic activity of caffeine and trigonelline during growth and ripening of Coffea arabica and Coffea canephora fruits. Plant Science, 171, 242–250 

35. Lang, R.; Yagar, E.F.; Eggers, R.; Hofmann, T. Quantitative investigation of trigonelline, nicotinic acid,and nicotinamide in foods, urine, and plasma by means of LC-MS/MS and stable isotope dilution analysis.J. Agric. Food Chem. 2008,56, 11114–11121

36. Maillard LC (1912) Action des acides aminés sur les sucres: formation des mélanoïdines par voie méthodique. Compt. R. Acad. Sci. Paris 154: 66-68.

37. Massawe GA, Lifa SJ. Yeasts and lactic acid bacteria coffee fermentation starter cultures. International Journal of Postharvest Technology and Innovation. 2010;2(1):41-82. DOI: 10.1504/IJPTI.2010.038187

38. Mazzafera, P., & Silvarolla, M. B. (2010). Caffeine content variation in single green Arabica coffee seeds. Seed Science Research, 20, 163–167.

39. Mendes AJT (1942) Desenvolvimento do embrião e do endosperma em Coffea arabica L. Bragantia 2: 115-128

40. Marín-López SM, Arcila-Pulgarin J, Montoya-Restrepo EC, Olivero-Tascón CE (2003) Cambios físicos y químicos durante la maduración del fruto de café (Coffea arabica L. var. Colombia). Cenicafé 54: 208-225.

41. Ouguerram A (1999) Contribution à l’étude des parois cellulaires de deux fruits inductriels: lolive et le café. Paris VI University, France. PhD Thesis.

42. Sato, Y., Itagaki, S., Kurokawa, T., Ogura, J., Kobayashi, M., Hirano, T., et al. (2011). In vitro and in vivo antioxidant properties of chlorogenic acid and caffeic acid. International Journal of Pharmaceutics, 403, 136–138.

43. Schwan, R.; Silva, C.; Batista, L. Coffee Fermentation. In Handbook of Plant-Based Fermented Food and Beverage Technology, 2nd ed.; CRC Press: Boca Raton, FL, USA, 2012; pp. 677–690. 

44. Selmar, D.; Bytof, G.; Knopp, S.-E. The Storage of Green Coffee (Coffea arabica): Decrease of Viability and Changes of Potential Aroma Precursors. Ann. Bot. 2008, 101, 31–38 

45. Selmar, D., Bytof, G., Knopp, S. E., & Breitenstein, B. (2006). Germination of coffee seeds and its significance for coffee quality. Plant Biol (Stuttg), 8(2), 260–264. 

46. Silva CF, Vilela DM, de Souza Cordeiro C, Duarte WF, Dias DR, Schwan RF. Evaluation of a potential starter culture for enhance quality of coffee fermentation. World Journal of Microbiology and Biotechnology. 2013;29(2):235-247. DOI: 10.1007/s11274-012-1175-2

47. Tianyu Pan, Yuxin Nian, Rong Xiang, Ru Jia, Liping Liu, Ruifang Wang, 2021. Quality Analysis of Coffee Bean Treated by Sunning and Water Washing processing.

48. Toci, A. T., Neto, V. J. F. M., Torres, A. G., Calado, V., Farah, A. Tryacylglicerols changes during the storage of roasted coffee. Proc. 22nd Int. Conf. Coffee Sci. ASIC. 504–507. 2008. Campinas, SP, Brazil. 

49. Trugo, L. C. Carbohydrates. In: Coffee, 1st edition, Clarke R. J., Macrae, R., eds. Essex: Elsevier Applied Science Publishers; 1985, Vol 1: Chemistry, p. 83. 

50. Trugo, L. C., Macrae R. A study of the effect of roasting on the chlorogenic acid composition of coffee using HPLC. Food Chem. 1984, 15, 219–227. 

51. Ulloa-Rojas, J. B., Verreth, J. A. J., Amato, S., & Huisman, E. A. (2003). Biological treatments affect the chemical composition of coffee pulp. Bioresource Technology, 89, 267–274.

52. Valio, I.F.M. Inhibition of Germination of Coffee Seeds (Coffea arabica L. cv Mundo Novo) by the Endocarp. J. Seed Technol. 1980, 5, 32–39. 

53. Xie Jijian, Liu Chengping. Fuzzy Mathematical Method and its Application (4th Edition) [M].10 Wuhan: Huazhong University of Science and Technology Press,2011 Wintgens, J.N. Coffee: Growing, Processing, Sustainable Production: A Guidebook for Growers, Processors, Traders, and Researchers. In Coffee: Growing, Processing, Sustainable Production: A Guidebook for Growers, Processors, Traders, and Researchers; Wiley: Weinheim, Germany, 2008; pp. 1–603.