Coffee berries (Fig. 1) are processed either by dry or wet processes (Fig. 2), the former being adopted mostly (ca. 95%) for C. arabica in Brazil, Ethiopia, Haiti, Paraguay, partially in India and Ecuador, almost totally for G. robusta everywhere. Irrespective of the species or cultivar, wet processing is used commonly in Costa Rica, Colombia, Guatemala, Peru, Bolivia and Ecuador. In the dry process the coffee cherries are dried to about 10-11% moisture content. Drying can be accomplished by either natural or artificial methods. Then the coffee beans are separated from the covering layers (skin, pulp and parchment) in a de-hulling machine. The solid residues generated by de-hulling are called coffee husk. In the wet process, drying is not required. The skin and pulp are mechanically removed, generating a solid residue called coffee pulp. Then beans are fermented to remove the layer of remaining pulp material or dried directly to 12% moisture content. Finally the beans are de-hulled to remove the parchment.

Fig. (1) Scheme of a coffee cherry.

Table 1
Yields of green coffee after dry and wet processing (modified from [4S. Franca, and L.S. Oliveira, "Alternative uses for coffee husks - a solid waste from green coffee production", In: Chemical, Biological and Environmental Engineering, Word Scientific Publishing Co. Pte. Ltd.: Singapore, 2009, pp. 21-24.
[http://dx.doi.org/10.1142/9789814295048_0005] ]).

During the primary stage of dry processing, the solid wastes generated are the coffee husks and pulps, and the low-quality or defective coffee beans. Secondary processing leads to the production of roasted coffee and soluble coffee. The major solid residue generated during this stage is represented by the spent coffee grounds from soluble coffee production. Due to the high amounts generated during the two stages, these solid residues (i.e. coffee husks, defective coffee beans and SCG), having different water content and bulk density, are problematic in terms of disposal, and pose environmental concerns and specific problems associated with the type of residue. Coffee husks, formed by dry outer skin, pulp and parchment, are probably the major residue from coffee berries processing [11A.S. Franca, and L.S. Oliveira, "Coffee processing solid wastes: current uses and future perspectives", In: Agricultural Wastes, Nova Science Publishers Inc. , 2009. ISBN 978-1-60741-305-9]: for every tonne of coffee beans produced, ca. one tonne of husks are generated during dry processing, ca. two tonnes during the wet processing, depending on the amount of water added. Defective beans represent over 50% of the coffee beans consumed, while SCG are produced at a proportion of 1.5 tonne (25% moisture) for each tonne of soluble coffee. This solid residue presents an additional disposal problem, since it can be used to adulterate roasted and ground coffee, being practically impossible to detect. The technical data of post-harvest processing are summarized in Table 1.

Fig. (2) Coffee processing and residues produced (adapted from Ghosh and Venkatachalapathy [6P. Ghosh, and N. Venkatachalapathy, "Processing and drying of coffee – a review", Int. J. Eng. Res. Technol., vol. 3, pp. 784-794, 2014.]).

According to Franca and Olivera [2United States Department of Agriculture, Coffee: World Markets and Trade, Available at: http://apps.fas.usda.gov/psdonline/circulars/coffee.pdf. 2015.] and Ghosh [4S. Franca, and L.S. Oliveira, "Alternative uses for coffee husks - a solid waste from green coffee production", In: Chemical, Biological and Environmental Engineering, Word Scientific Publishing Co. Pte. Ltd.: Singapore, 2009, pp. 21-24.
[http://dx.doi.org/10.1142/9789814295048_0005] ] green coffee beans are composed by carbohydrates (ca.60% w/w), namely insoluble polysaccharides like cellulose and hemicellulose, and soluble carbohydrates such as the monosaccharides fructose, glucose, galactose and arabinose, the oligosaccharides sucrose, raffinose and stachyose, and polymers of galactose, mannose, arabinose and glucose. In addition, there are non-volatile aliphatic acids (citric, malic and quinic acids) and volatile acids such as acetic, propanoic, butanoic, isovaleric and hexanoic acids. Oils and waxes account for 10-16% of the dry mass, along with proteins and free amino acids (10-11% w/w) and minerals (ca 4% w/w). The average composition of the coffee beans for the Grevillea arabica cv. Arabica and Robusta are shown in Table 2.

Table 2
Average chemical composition of coffee beans (g/100 g dry weight) (modified from [7S. Mussatto, I. Mussatto, E.M. Machado, S. Martins, and J.A. Teixeira, "Production, composition, and application of coffee and its industrial residues", Food Bioprocess Technol., vol. 4, pp. 661-672, 2011.
[http://dx.doi.org/10.1007/s11947-011-0565-z] ]) .

The chemical composition of coffee husks and pulp does not differ dramatically, from a quantitative point of view, from that of coffee beans (Table 3, [12L.S. Oliveira, and A.S. Franca, "An overview of the potential uses for coffee husks", In: Coffee in Health and Disease Prevention., Academic Press, 2015, pp. 283-291.
[http://dx.doi.org/10.1016/B978-0-12-409517-5.00031-0] ]), except that there is lower content of lipids, different ratios among the glucidic fractions, and a higher content of minerals. However, both husks and pulp still contain caffeine, lignins and tannins. The first component is a bioactive compound, the latter two being recalcitrant to bio-degradation. The possible uses of the solid residues and wastewaters are briefly summarized in Tables 3 and 4 [13K.G. Bouafou, B.A. Konan, V. Zannou-Tchoko, and S. Kati-Coulibally, "Potential food waste and by-products of coffee in animal feed", Electr. J. Biol., vol. 7, pp. 74-80, 2011.-21A.S. Wogderess, "Available information on the feeding value of coffee waste and ways to improve coffee waste for animal feed", Afr. J. Biol, vol. 3, pp. 243-257, 2016.].

Table 3
Chemical composition of coffee husks and pulp (g/100 g dry weight).

The coffee pulp can replace up to 20% of commercial concentrates in feeds for milking cows [22S. Roussos, C. Augur, I. Perraud-Gaime, D.L. Pyle, G. Saucedo-Castañeda, C.R. Soccol, A. Pandey, I. Ferrao, and M. Raimbault, "Development of bioprocesses for the conservation, detoxification and value-addition of coffee pulp and coffee husk", In: T. Sera, C.R. Soccol, A. Pandey, and S. Roussos, Eds., Coffee Biotechnology and Quality., Springer Netherlands Publ., 2000, pp. 377-392.
[http://dx.doi.org/10.1007/978-94-017-1068-8_35] , 23P. Pedraza-Beltrán, J.G. Estrada-Flores, A.R. Martínez-Campos, I. Estrada-López, A.A. Rayas-Amor, G. Yong-Angel, M. Figueroa-Medina, F.A. Nova, and O.A. Castelán-Ortega, "On-farm evaluation of the effect of coffee pulp supplementation on milk yield and dry matter intake of dairy cows grazing tropical grasses in central Mexico", Trop. Anim. Health Prod., vol. 44, no. 2, pp. 329-336, 2012.
[http://dx.doi.org/10.1007/s11250-011-0025-9] [PMID: 22119887] ]. For pigs, the replacement of cereals with dehydrated coffee pulp can reach up to 16% in the cereal feed. This, in turn, could spare for human consumption, or other uses, up to 50 kg of cereals at the end of one pig feeding cycle. In field trials in Brazil, the pigs fed with up to 15% ensiled coffee pulp and 5% of bagasse showed the same weight as those fed with commercial concentrates [21A.S. Wogderess, "Available information on the feeding value of coffee waste and ways to improve coffee waste for animal feed", Afr. J. Biol, vol. 3, pp. 243-257, 2016.]. Similar results were obtained when feeding fishes, chicken, lambs and rabbits. The pulp has also been proposed for silage to reduce the meat production costs. However, due to the presence of caffeine, pre-treatments are needed which include repeated washings and the use of commercial inoculants to speed-up the fermentation process, making this alternative poorly feasible economically. Thus, to reduce the costs, animal feeding without pre-treatment is the more frequently applied technology, and could represent a possible re-use.

Table 4
Use of liquid and solid residues from coffee processing.

The mucilages, and particularly the pulp, are also important for their content of polyphenols, antocyanins and bioflavonoids. The chemical extraction of these bioactive compounds has been proposed, to be commercialized as food “natural” supplement. Mucilages and pulp could be used as a source of pectin to be used as a substitute for fat in the preparation of emulsions such as those for salad dressing and mayonnaise, or to be used as soluble dietary fiber for humans. Raw unrefined pectins form thermo-soluble irreversible gels with distinct flavour [3R. Rathinavelu, and G. Graziosi, "Potential alternative use of coffee wastes and by-products”", ICS-UNIDO Project, Available at: http://www.ico.org/documents/ed1967e.pdf, 2005. [viewed on 11/09/2016]]. The pulp is also used as a source of substrate for fermentation to produce alcoholic beverages [24D.P. Navia, "Production and evaluation of ethanol from coffee processing by-products", Vitae, vol. 18, no. 2, pp. 287-294, 2011. ISSN 0121-4004 / ISSNe 2145-2660.]. The presence of caffeine is not considered an obstacle since this compound is often added to the beverages, and there are on the market liqueurs based on coffee, such as “Caffè Borghetti” in Italy or “Kahlua” in Mexico [3R. Rathinavelu, and G. Graziosi, "Potential alternative use of coffee wastes and by-products”", ICS-UNIDO Project, Available at: http://www.ico.org/documents/ed1967e.pdf, 2005. [viewed on 11/09/2016]]. The natural sugars of the coffee berry, after the de-pulping phase, have a distinct fruity flavour, which might be of interest for food industry, once the single chemical compounds will be analytically identified. However, the large annual production of coffee berries and the fact that 80% of the fruit becomes waste during processing [25M.R. Alfaro, and J.J. Rodríguez, "Impacto ambiental del procesamiento del café en Costa Rica", Agronomía Costarricense, vol. 18, pp. 217-225, 1994.] poses clearly problems of decontamination or requires the re-use of large, yearly amounts of residues. The re-use of coffee by-products for cosmetic purposes [26A. Barbulova, G. Colucci, and F. Apone, "New trends in cosmetics: by-products of plant origin and their potential use as cosmetic active ingredients", Cosmetics, vol. 2, pp. 82-92, 2015.
[http://dx.doi.org/10.3390/cosmetics2020082] ] has been recently evaluated extensively by different research groups [27F. Rodrigues, C. Gaspar, A. Palmeira-de-Oliveira, B. Sarmento, M. Helena Amaral, and M.B. P P Oliveira, "Application of coffee silverskin in cosmetic formulations: physical/antioxidant stability studies and cytotoxicity effects", Drug Dev. Ind. Pharm., vol. 42, no. 1, pp. 99-106, 2016.
[http://dx.doi.org/10.3109/03639045.2015.1035279] [PMID: 25996462] -32A. Iriondo-DeHond, P. Martorell, S. Genovés, D. Ramón, K. Stamatakis, M. Fresno, A. Molina, and M.D. Del Castillo, "Coffee silverskin extract protects against accelerated aging caused by oxidative agents", Molecules, vol. 21, no. 6, pp. 721-735, 2016.
[http://dx.doi.org/10.3390/molecules21060721] [PMID: 27258247] ]. The results clearly suggest that coffee silverskin aqueous extract may be used for other applications, such as antiaging cosmetics and dermaceutics, due to the presence of anti-oxydant compounds, phenols and other bioactive components. However these potential uses, for cosmetics or industrial niche commodities cannot, though brilliant, cope with the accumulation of million tonnes of solid residues and wastewaters every year. In addition, there are components in the residues which are poorly degradable such as the tannin and lignin fractions, and decontamination tools must be put in place to alleviate the overburden of environmental impacts. The advantages and disadvantages of the uses of coffee residues are summarized in Table 5.

Table 5
Advantages and disadvantages of the coffee husk and pulp utilization, and further developments.

A relatively few studies have focused on the use of coffee husk and coffee pulp as a substrate for mushrooms production, among them Pleurotus ostreatus, Lentinus edodes and Flammulina velutipes. In all studies, the biological efficiency of fungi increased 40 to 97% in 60 days and fructification was observed after 20-25 days of inoculation [33S-T. Chang, "World production of cultivated edible and medicinal mushrooms in 1997 with emphasis on Lentinus edodes (Berk.) Sing, in China", Int. J. Med. Mushrooms, vol. 1, pp. 291-300, 1999.
[http://dx.doi.org/10.1615/IntJMedMushr.v1.i4.10] -36L. Fan, "L., A. Pandey, R. Mohanand, C. R. Soccol, “Use of various coffee industry residues for the cultivation of Pleurotus ostreatus in solid state fermentation", Acta Biotechnol., vol. 20, pp. 41-52, 2000.
[http://dx.doi.org/10.1002/abio.370200108] ]. With Pleurotus florida, 220 g of mushrooms were obtained after four flushes per 100 g of substrate in optimal conditions [17P.S. Murthy, and H.K. Manonmani, "Bioconversion of coffee industry wastes with white-rot fungus Pleurotus florida", Res. J. Environ. Sci., vol. 2, pp. 145-150, 2008.
[http://dx.doi.org/10.3923/rjes.2008.145.150] ]. Caffeine and tannins concentrations of coffee husk decreased 60% and 70%, respectively. However, in some cases caffeine was found in the fruiting body indicating that it was not completely degraded. Nonetheless, the results obtained so far suggest the feasibility of using coffee residues as a substrate without pre-treatment for the cultivation of edible and medicinal fungi [30J. Marto, L.F. Gouveia, B.G. Chiari, A. Paiva, V. Isaac, P. Pinto, P. Simões, A.J. Almeida, and H.M. Ribeiro, "The green generation of sunscreens: Using coffee industrial sub-products", Ind. Crops Prod., vol. 80, pp. 93-100, 2016.
[http://dx.doi.org/10.1016/j.indcrop.2015.11.033] ]. Residues remaining after the fungal cultivation have been proposed to feed ruminants as several toxic compounds decrease while the protein concentration increases (~9%) [34L. Fan, A.T. Soccol, A. Pandey, and C.R. Soccol, "Cultivation of Pleurotus mushrooms on Brazilian coffee husk and effects of caffeine and tannic acid", Micol. Aplic. Int., vol. 15, pp. 15-21, 2003., 35F. Leifa, A. Pandey, and C.R. Soccol, "Production of Flammulina velutipes on coffee husk and coffee spent-ground", Braz. Arch. Biol. Technol., vol. 44, pp. 205-212, 2001.
[http://dx.doi.org/10.1590/S1516-89132001000200015] ]. There are no large-scale applications to date, probably because the elevated cost of sterilisation and inoculation in aseptic conditions [34L. Fan, A.T. Soccol, A. Pandey, and C.R. Soccol, "Cultivation of Pleurotus mushrooms on Brazilian coffee husk and effects of caffeine and tannic acid", Micol. Aplic. Int., vol. 15, pp. 15-21, 2003.].

Owing that some of the coffee residues have a high content of volatile solids and sugars, they can be considered potentially good feedstock for bio-digesters. However, methanogenesis can be hampered by the presence of toxic or non-biodegradable compounds such as saponins and lignins, thus requiring pre-treatments [37A.K. Kivaisi, "Pretreatment of robusta coffee hulls and co-digestion with cow-dung for enhanced anaerobic digestion", Tanz. J. Sci., vol. 28, pp. 1-10, 2002.]. The use of actinobacteria such as Streptomyces has been suggested as a pre-treatment to minimize the content in polyphenols, which are known to act as antimicrobial compounds [38A.L. Orozco, M.I. Perez, O. Guevara, J. Rodrıguez, M. Hernandez, F.J. Gonzalez-Vila, O. Polvillo, and M.E. Arias, "Biotechnological enhancement of coffee pulp residues by solid-state fermentation with Streptomyces. Py-GC/MS analysis", J. Anal. Appl. Pyrolysis, vol. 81, pp. 247-252, 2008.
[http://dx.doi.org/10.1016/j.jaap.2007.12.002] ]. Some studies carried out in Tanzania suggest high methane (CH₄) yields from coffee residues: 650 m³ CH₄/tonne of volatile solids for Grevillea robusta solid waste and 730 m³ CH₄/tonne of Coffea arabica solid waste. Case studies on the coffee processing factories indicate that exploitation of the residues for the production of electricity on site is feasible [39A.K. Kivaisi, and M.S. Rubindamayugi, "The potential of agro-industrial residues for production of biogas and electricity in Tanzania", Renew. Energy, vol. 9, pp. 917-921, 1996.
[http://dx.doi.org/10.1016/0960-1481(96)88429-1] ]. Although several attempts were carried out as early as in the 90’s, the technology of bio-gasification is far from being adopted on a large scale. Feasibility studies indicate that slurries containing 20% (w/v) coffee waste solids can be treated anaerobically in one and two-phase thermophilic methane fermentation systems at 53°C without discharging anything but the coffee waste residues [40K. Kida, and I. Yorikazu Sonoda, "Treatment of coffee waste by slurry-state anaerobic digestion", J. Ferment. Bioeng., vol. 73, pp. 390-395, 1992.
[http://dx.doi.org/10.1016/0922-338X(92)90285-3] ]. Biogas could be produced by the co-digestion of coffee-pulp and cow-dung mixture under solar radiation [41G. Corro, L. Paniagua, U. Pal, F. Bañuelos, and M. Rosas, "Generation of biogas from coffee-pulp and cow-dung co-digestion: Infrared studies of postcombustion emissions", Energy Convers. Manage., vol. 74, pp. 471-481, 2013.
[http://dx.doi.org/10.1016/j.enconman.2013.07.017] ]. From the first month of co-digestion at mesophylic conditions, methane content in the biogas attains 50% of the yield. This content increased up to 60% and remained almost constant for at least eight months of further digestion. Unfortunately combustion emission of the biogas contain several components like CH₄, C₃H₈, CO, SO₂, HI, and probably Br₂ which are strongly harmful to human and animal health (for the presence of isocyanic acid and bromomethane in the biogas). These results indicate that if the biogas is to be considered as a fuel, the conventional combustion technology has to be upgraded to prevent these hazardous emissions to the atmosphere.

Coffee tree is widely used as a firewood for domestic use, particularly in small coffee farms, or it is sold for firewood as whole trees and logs, providing a range of flexible uses and therefore yielding the highest returns per household. In Kenya there is a heavy reliance on Grevillea robusta to respond to market demands for firewood and timber [42C. Holding, S. Carsan, and P. Njuguna, "Smallholder timber and firewood marketing in the coffee and cotton/tobacco zones of eastern Mount Kenya", in Small-scale forestry and rural development: The intersection of ecosystems, economics and society, Proceedings of IUFRO 3.08 Conference, hosted by Galway-Mayo Institute of Technology, Galway, Ireland, 18-23 June 2006, pp.178-190, 2006. ISBN : 1902696506.]. Diversification of fast-growing species would enhance farmers’ product options and the sustainability of the landscape. However, in several Countries the farmers’ tree planting activities are limited by the lack of coordination of germplasm supply, leading to a limited diversity and quantity available at farm level. Though varying among countries, the yields of firewood per ha from annual prunings of coffee bushes can be significant: as an example, in Costa Rica [43M. Romijn, and E. Wilderink, "Fuelwood from coffee prunings in the Turrialba valley", In: Centro Agronomico Tropical de Investigacion y Enseaanza, CATIE, Programa de Recursos Naturales Renovables, Turrialba., Costarica, 1981, pp. 1-25.] the yield ranged from 121 kg to 1.6 tonnes/ha for plantations with less than 3.800 coffee bushes (low density) per hectare; for plantation with more than 3.800 coffee bushes/ha (high density) the range was wider and covered 288 kg to 2.9 tonnes/ha. The amount of firewood produced had no strong correlation with the plant density. lndications exist that coffee fuelwood yield is more strongly correlated with pruning intensity, which on its turn depends on the management practice adopted by the farmer.

Recently [3R. Rathinavelu, and G. Graziosi, "Potential alternative use of coffee wastes and by-products”", ICS-UNIDO Project, Available at: http://www.ico.org/documents/ed1967e.pdf, 2005. [viewed on 11/09/2016]] a multi-stage bio-refinery concept has been hypothesized, with the potential to convert waste produced at crop processing operations, such as coffee pulping stations, to valuable biofuels and bio-products using biochemical and thermochemical conversion technologies. The bioconversion stages use the yeast Kluyveromyces marxianus to produce bioethanol from sugars, Yarrowia lipolytica to produce bio-based ammonia for fertilizer, and peptides and free amino acids for animal feed. The lignocellulosic fraction would be treated to release sugars for fermentation in a third stage, while the residual protein and lignin solids can be jet cooked and passed to a fourth-stage fermenter where another yeast, Rhodotorula glutinis, would convert methane into isoprenoid intermediates. The mixed residues would be transferred into pyrocracking and hydroformylation reactions to convert ammonia, protein, isoprenes, lignins, and oils into renewable gas. The remaining waste could be thermo-converted to biochar as a humus soil enhancer. This interesting integrative vision of multiple technologies for the treatment of coffee waste has the potential to contribute to economic and environmental sustainability of the process, though its accomplishment would necessarily depend on the size of the coffee pulping installations, their localization in a country, their economic robustness and the variety of expertise involved. Moreover, most of the technologies proposed need to be fully developed yet at industry-level, assessed for safety-related aspects, and evaluated by using Life Cycle Management tools to uncover the end-of-use impacts and the overall real sustainability in coffee-producing countries. The requirement for a sustainable coffee industry, and the subsequent need for the development of methods for the utilization of coffee by-products has been stressed also by Cholakov [44G. Cholakov, V. Toteva, R. Nikolov, and Sn. Uzunova, "St. Yanev, “Extracts from coffee by-products as potential raw materials for fuel additives and carbon adsorbents", J. Chem. Technol. Metall., vol. 48, pp. 497-504, 2013.]. Among these tools, the preparation of renewable fuel additives (e.g. biodiesel) and carbon adsorbents is a promising alternative to the eventual dumping of biogenic substances, harmful to the environment. The actual commonly adopted use of coffee husks is the treatment in coarse ovens to dry out the coffee and obtain the parchment, in addition or in alternative to sun drying, or burning the partially dried husks in a gas generator coupled to a power converter. The residual heat may be used to dry out more coffee. The use of coffee pulp for the production of briquettes and pellets for heat generation has recently been proposed [14R. Cubero-Abarca, R. Moya, J. Valaret, and M. Tomazello Filho, "Use of coffee (Coffea arabica) pulp for the production of briquettes and pellets for heat generation, Ciênc. Agrotec", Lavras, vol. 38, pp. 461-470, 2014.]. The study included pulp drying (using air, solar and hot air methods); the production of briquettes and pellets; the evaluation of their energy, physical and mechanical properties; and the evaluation of pellet quality using X-ray densitometry. The results clearly show that hot air drying coffee pulps is the best option for coffee residues because of its low drying time. The physical properties of the pellets and briquettes comply with most of the standards established for these products. A negative aspect of the pellets is that their durability failed to comply with some requirements specified in the literature, because of the presence of cracks which may affect the pellet quality. In another recent study, briquettes made from fat dregs mixed with coffee residue were shown to be easy to ignite, left no stains on hands, burned for a long time, and had good heat output. These briquettes did not give off sparks and had less smoke and ash content than those of charcoal they normally used [45N. Tangmankongworakoon, and P. Preedasuriyachai, "The evaluation of fuel briquetts produced from municipal wastes", Int. J. Env. Sci. Dev., vol. 6, pp. 221-224, 2015.
[http://dx.doi.org/10.7763/IJESD.2015.V6.594] ].

Finally, the solid residues of coffee processing are very often disposed into arable land and surface water leading to environmental pollution, although their physical-chemical traits represent an ideal substrate for making excellent fertilizers. Moreover, when the bio-transformation process, namely composting, is carried out particularly at small-scale coffee farms, mostly is made in uncontrolled conditions (or just adding animal manure) giving rise to immature, non-humified and non-sanitized amendments. Bioconversion processes in controlled conditions, with the addition of plant material and/or microbial starters and/or chemical fertilizers, have been published [46H. Kassa, and T. Workayehu, "Evaluation of some additives on coffee residue (coffee husk and pulp) quality as compost, southern Ethiopia", Int. Inv. J. Agric. Soil Sci, vol. 2, pp. 14-21, 2014.-48N-A. Dzung, T.T. Dzung, and V-T. Phuong Khanh, "Evaluation of coffee husk compost for improving soil fertility and sustainable coffee production in rural central highland of Vietnam", Resour. Environ., vol. 3, pp. 77-82, 2013.] sorting beneficial effects [49D. Sekhar, P.B. Pradeep Kumar, and K. Tejeswara Rao, "Effect of coffee husk compost on growth and yield of paddy", J. Acad. Ind. Res., vol. 3, pp. 195-197, 2014.]. The polyphenolic compounds can be removed prior to composting in solid-state fermentations, if considered desirable or economically profitable [50E.M. Machado, R.M. Rodriguez-Jasso, J.A. Teixeira, and S.I. Mussatto, "Growth of fungal strains on coffee industry residues with removal of polyphenolic compounds", Biochem. Eng. J., vol. 60, pp. 87-90, 2012.
[http://dx.doi.org/10.1016/j.bej.2011.10.007] ]. Very similar experiences have been described for another agro-industrial commodity, i.e. the olive oil. To extract olive oil from olives (15-20% w/w) the Countries of Mediterranean basin, totalling 94% of the world production, which includes the three major producers, i.e. Spain, Italy, Greece (totalling 86% of the world production), face every year problems surprisingly similar to those of the coffee production and extraction: vast amount of residues, both solid (olive husks) and wastewaters strongly impacting the environment, recalcitrant to biodegradation. Also the possible, alternative uses of wastewaters and solid residues of olive oil extraction are very similar to those described for coffee, including [10A. Cossu, S. Degl’Innocenti, M. Agnolucci, C. Cristani, S. Bedini, and M. Nuti, "Assessment of the life cycle environmental impact of the olive oil extraction solid wastes in the european union", Open Waste Manag. J., vol. 6, pp. 6-14, 2013.
[http://dx.doi.org/10.2174/1876400220131014001] ] thermo-chemical processes, i.e. combustion, pyrolysis or gasification; anaerobic digestion; alcoholic fermentation; blending and chemical extraction; agronomic: direct application by land-spreading, usage as animal feed, composting and subsequent field usage as soil amendment, the re-use for the production of industrial niche commodities and green amendments. These similarities should allow to comparatively evaluate the impacts and the possible exit strategies for the disposal coffee residues, particularly taking into account and comparatively assess the impacts of the various transformation processes. In fact, the main uses of olive virgin husks and waste waters in the Mediterranean basin are power generation, domestic heating and composting, the latter process leading to a high-quality, deeply humified “green” fertilizer without the addition of animal manure, at both industrial [51M.C. Echeverria, R. Cardelli, S. Bedini, A. Colombini, L. Incrocci, A. Castagna, M. Agnolucci, C. Cristani, A. Ranieri, A. Saviozzi, and M. Nuti, "Microbially-enhanced composting of wet olive husks", Bioresour. Technol., vol. 104, pp. 509-517, 2012.
[http://dx.doi.org/10.1016/j.biortech.2011.11.042] [PMID: 22154749] ] and farm level [52M. Agnolucci, C. Cristani, F. Battini, M. Palla, R. Cardelli, A. Saviozzi, and M. Nuti, "Microbially-enhanced composting of olive mill solid waste (wet husk): bacterial and fungal community dynamics at industrial pilot and farm level", Bioresour. Technol., vol. 134, pp. 10-16, 2013.
[http://dx.doi.org/10.1016/j.biortech.2013.02.022] [PMID: 23500553] ]. The use of this amendment has an environmental impact 2-4 times lower than the use of the husks for the production of electricity and house-hold heating, generates beneficial effects on the quality of horticultural products, can substitute for turf in the preparation of cultivation substrates. The latter findings have been calculated using the tools of Life Cycle Impact Assessment [10A. Cossu, S. Degl’Innocenti, M. Agnolucci, C. Cristani, S. Bedini, and M. Nuti, "Assessment of the life cycle environmental impact of the olive oil extraction solid wastes in the european union", Open Waste Manag. J., vol. 6, pp. 6-14, 2013.
[http://dx.doi.org/10.2174/1876400220131014001] ]. If the correct composting strategy of the coffee residues could be implemented at a larger scale also in small farms, it could help minimize the eutrophication effects of nutrient leaching in soil, avoid lagooning of large volumes of wastewaters, and provide the ecosystem with more stable, humified organic carbon. The latter aspect is particularly relevant in those fields where biodiversity is a conservation goal and where soils need to restore their fertility through the maintenance or increase of organic matter above the threshold of 3.5%, which is the minimum to maintain the soil functional biodiversity.