Coffee Pulp: An Industrial By-product with Uses in Agriculture, Nutrition and Biotechnology

Abstract

The coffee shell or pulp is the first by-product obtained from the processing of coffee. It represents approximately 40 to 50% of the coffee berry’s weight. Currently, in much of the industry, it is a waste product with a major environmental impact on the water and soil, flora and fauna, and a problem to nearby populations in terms of odor and proliferation of insects and pathogenic microorganisms. This is a review that compiles alternative uses of coffee pulp in agriculture, food and nutrition, medicine and biotechnology. In food and agriculture, for example, the pulp can be used as organic fertilizer to improve degraded soils, in the biological control of plant pathogens, as food or substrate for microorganisms and worms, as feed for chickens, sheep, goats, fish and other animals, and in the productions of foods and beverages for human consumption. In biotechnology, coffee pulp can be used in the cultivation of edible fungi, production of enzymes, substrate for caffeine degrading microorganisms and for microorganisms that produce natural fungicides. Although many of these applications have been proposed and studied, there are also several novel uses that are in the early stages of development; for example, the use of pulp bioactive compounds to make food supplements, or to increase dietary fiber contents in foods and beverages, as well as for the production of biocontainers and biopackaging, alternatives to plastics and their serious environmental impact.

1. Introduction

The first by-product of coffee processing is the pulp (PC). To separate it from grain or bean, a wet or dry process (Pandey et al., 2000) can be applied. PC includes both the shell (skin, epidermis, exocarpius) and the pulp itself (mesocarp) (Molina et al., 1974; Bayne et al., 1976; Calzada et al., 1981, 1984b; a, 1986; Torres et al., 2019) The production of PC residue globally is estimated at 9.4 million tons/year. It is a source of environmental problems for coffee-producing countries (El Achaby et al., 2019); this polluting waste affects agricultural lands (Porres et al., 1993), as well as rivers and lakes located near coffee processing areas (Roussos et al., 1995). In rivers and lakes, PC lowers oxygen levels, lowers pH, and suspended PC solids block sunlight (Randhir and Genge, 2005; Cervantes et al., 2015b). PC in soils can affect populations of the soil microflora (acidophilic fungi) (Cervantes et al., 2015a); these effects can have a significant impact on flora and fauna. The emission of gases such as methane (CH₄) and nitrous oxide (N₂O) that result from the fermentation of PC is becoming a growing environmental problem (Corro et al., 2014). In addition to generating unpleasant odors, there is a proliferation of insects due to the decomposition of PC; and there are associated health risks (Chala et al., 2019). Therefore, the management of coffee waste remains a challenge in the coffee producing regions of the world.

Coffee pulp represents approximately 40 to 50% weight of the (Rolz et al., 1980; Hernandez et al., 2009; Torres et al., 2019) and its composition fluctuates depending on the coffee varieties; for example, wide ranges of macronutrients have been reported (per 100 g of dry matter): 4–12 g of protein, 1–2 g of lipids, and 45–89 g of carbohydrates (Oliveira and Franca, 2014; Salinas-Rios et al., 2014b), including pectin (Hasanah et al., 2019). Minerals such as P, K, Ca and Mg have been reported at 2.48, 25.13, 4.10 and 1.39 g/kg of coffee pulp, respectively, and those for Fe and Mn, at 77 and 46 mg/kg (Fierro-Cabrales et al., 2018). The approximate percentage of caffeine is between 0.12 to 0.26% (Arimurti et al., 2017; Fierro-Cabrales et al., 2018), pectin and tannins range from 1–9 g/100 g of dry matter, or about 6% of the weight (Oliveira and Franca, 2014; Rakitikul, 2017). Coffee pulp also contains caffeic acid at about 16 mg/g dry matter, gallic acid at about 3 mg/g dry matter, and chlorogenic acid at about 62 mg/g dry matter (Salinas-Rios et al., 2014b; da Silveira et al., 2020). These concentrations can vary according to the extraction and drying method used (Peñaloza et al., 1985; Villa-Montoya et al., 2019; Torres-Valenzuela et al., 2019).

The composition of coffee pulp makes it an interesting by-product for research and development of multiple uses, for example, as a biofertilizer, biological control agent, animal feed concentrate, nutritional product for human consumption, as well as other bioproducts, and bioenergy. Figure 1a shows that biotechnology has been the main field of application, followed by agriculture and energy, and finally, food products. In general, as represented by Figure 1b, there is a growing interest in the study of coffee pulp. This review summarizes the multiple uses of coffee pulp.

Figure 1: Number of scientific articles published from 1969 to 2020 on coffee pulp. (a) Information obtained from the database Scopus (search criteria: ARTICLE TITLE, ABSTRACT, KEYWORDS: “coffee pulp” AND ARTICLE TITLE, ABSTRACT, KEYWORDS: “each group of application”). (b) Information obtained from the database Scopus (search criteria: ARTICLE TITLE, ABSTRACT, KEYWORDS: “coffee pulp”).

2. Uses

2.1 Uses in agriculture Table 1

Table 1 summarizes the applications of coffee pulp in agriculture. One of its uses is as an organic fertilizer obtained by composting methods (Pierre et al., 2009; Paco et al., 2011; Berecha et al., 2011; Chong and Dumas, 2012; Escobar Escobar et al., 2013; Muñoz C et al., 2015; Ulsido and Li, 2016; Moriones-Ruiz and Montes-Rojas, 2017) either alone or with the inoculation of mycorrhizals (Quiñones-Aguilar et al., 2014; Ibarra-Puón et al., 2014), vermicompost (Orozco et al., 1996; Paco et al., 2011; Raphael and Velmourougane, 2011; Raphael et al., 2012; Campos Mota and Flores Sánchez, 2013) or bocashi (Ramírez-Builes and Naidu Duque, 2010). It has been used on its own (Posada and Sieverding, 2014) or combined with other agro-industrial wastes. As a biofertilizer, coffee pulp has been shown to improve growth (Ibarra-Puón et al., 2014) increases height (Berecha et al., 2011; Quiñones-Aguilar et al., 2014; Ulsido and Li, 2016) production(Escobar Escobar et al., 2013; Moriones-Ruiz and Montes-Rojas, 2017)and biomass (Berecha et al., 2011; Ulsido and Li, 2016) of various agricultural plants. It improves moisture (Campos Mota and Flores Sánchez, 2013), index of humification (Pierre et al., 2009), pH of the soil (Pierre et al., 2009; Paco et al., 2011; Muñoz C et al., 2015; Moriones-Ruiz and Montes-Rojas, 2017), and increases cation exchange capacity (CIC) (Ramírez-Builes and Naidu Duque, 2010; Muñoz C et al., 2015; Moriones-Ruiz and Montes-Rojas, 2017) both of macroelements (Pierre et al., 2009; Paco et al., 2011; Raphael and Velmourougane, 2011; Chong and Dumas, 2012; Raphael et al., 2012; Moriones-Ruiz and Montes-Rojas, 2017) (N, P, K, Ca and Mg) and microelements (Pierre et al., 2009) (Fe, Zn, Cu and Mn).

Earthworms and various microorganisms are used in the production of natural fertilizers. The use of coffee pulp as a substrate for earthworms has been studied, and found to improve the growth and reproduction of P. corethrurus, Amynthas corticis (Garcı́a and Fragoso, 2002). The use of coffee pulp as a substrate for phosphate solubilizing microorganisms (Cisneros-Rojas et al., 2016) [Kocuria sp, Bacillus subtilis (Cisneros-Rojas et al., 2017b)^,(Cisneros-Rojas et al., 2017a) and S. diversispora, P. ochrochloron(Cisneros-Rojas et al., 2017a)] has also been studied; and has been reported to improve the availability of phosphorus for the development of coffee seedlings (Cisneros-Rojas et al., 2016, 2017a; b). Studies on the effects of Bacillus subtilis (BS), Aspergillus niger (AN), or Trichoderma reesei (TR), and PEG 4,000, have also been reported, with PEG 4000 showing better performance in reducing sugar production than the pulp waste alone (Iswanto et al., 2019a). Another surfactant, Tween 80, has shown improvements (Widjaja et al., 2019).

Coffee pulp has been used to amend and improve soils by composting methods (Betancourt et al., 2016) and inoculating mycorrhizals (Osorio et al., 2002), management of soil treatment cycles (Pagan-Roig et al., 2016; Pagán-Roig et al., 2016) and phytoremediation (Wasilkowski et al., 2017). In these cases, increased growth (Senthilkumar et al., 2015), (Osorio et al., 2002) and production (Pagán-Roig et al., 2016), (Senthilkumar et al., 2015) of treated plants, increased biomass (Pagan-Roig et al., 2016; Pagán-Roig et al., 2016; Wasilkowski et al., 2017), increased humic acids and polymerization (Pagán-Roig et al., 2016) have been reported. There was overall improvement in soil fertility and quality (Cervantes et al., 2014, 2015a; b; Betancourt et al., 2016; Wasilkowski et al., 2017), as well as efficient (Pagán-Roig et al., 2016) immobilization of Zn(8)7–91%), Cd(70–83%), and Pb (33–50%) contaminated land (Wasilkowski et al., 2017).

Table 1: Agricultural applications of coffee pulp

Use	Technology/method	References
Organic fertilizer	Composting and inoculation with micorriza (Glomus sp. Zac-2 and Glomus aggregatum FS-39)	Quiñones-Aguilar et al., 2014
	Vermicomposting using exotic species (Eudrilus eugeniae) and native earthworm species (Perionyx ceylanesis)	Raphael and Velmourougane, 2011; Raphael et al., 2012
	Inoculation of mycorriza and nitrogen-fixing bacteria (Rhizophagus intraradices and Azospirillum brasilense) on a coffee pulp based substrate	Ibarra-Puón et al., 2014
	Biofertilization treatment of mycorriza and Phytomas-E in coffee pulp-based substrate	Barroso-Frómeta et al., 2015
	Fresh pulp added to the soil directly as compost	Posada and Sieverding, 2014
	Earthworm substrate	Garcı́a and Fragoso, 2002
	Substrate for phosphate solubilizing microorganisms (Kocuria sp., Bacillus subtilis, S. diversispora and P. ochrochloron)	Cisneros-Rojas et al., 2017a; b
Soil recovery	Inoculation of mycorriza in compost coffee pulp	Osorio et al., 2002
	Management of soil treatment cycles (addition of coffee pulp compost to the soil, sowing and addition of legume green manure mixtures, with the use of mycorriza and compost tea)	Pagán-Roig et al., 2016
	Fitoremediation, use of coffee pulp to minimize the environmental risk of Zn, Pb and Cd in the soil surrounding lead and zinc mines	Wasilkowski et al., 2017
	Heavy metal stabilizer (Zn, Cd and Pb)	Wasilkowski et al., 2017
	Adding coffee pulp to different soil types (ratio 1:3)	Cervantes et al., 2014, 2015a; b
	Waste-based soil enhancer: coffee pulp, banana stalks and guinea pig manure, biotechnological transformation by composting	Betancourt et al., 2016
Biological Control	Plant pathogen control	Cisneros-Rojas et al., 2016; Iswanto et al., 2019a
	Roya control using proanthocyanidin (Hemileia vastatrix, raza 2) of the coffee pulp	González de C et al., 1998
	Coffee pest control: use of coffee pulp vinegar with alcohols (methanol and ethanol) for control of Hypothenemus hampei (Ferrari; capture of > 400 adult insects/week/trap)	Fernández and Cordero, 2005

In addition, the application of an organic fertilizer based on coffee pulp improves the distribution of Trichoderma spp (T. koningii, T. harzidigestoanum and T. longibrachiatum) in soil. Trichoderma spp. are used as biological control agents for the management of plant pathogens in soil (Onsando and Waudo, 1992). Use of pulp fertilizer was also reported to reduce infestation by the nematode (Tylenchulus semipenetrans) (Senthilkumar et al., 2015). The benefits of fertilizer formulations with 15 isolations of indigenous bacteria on coffee seedlings have also been reported (Sutanto et al., 2019). Moreover, the proanthocyanins of coffee pulp have been used in the control of rust disease caused by Hemileia vastatrix (González de C et al., 1998) and in the control of the drill pest, Hypothenemus hampei (Ferrari) (Fernández and Cordero, 2005). These studies show the potential of coffee pulp in the development of low-cost organic pest control methods.

The main limitations in the use of this waste for large scale agriculture are mainly the lack of studies into possible long-term effects in real field-ecosystem situations, and possible consequences with the use of different micro-organism strains. Current studies have methodologies that involve mostly ideal conditions, or simulated conditions, with limited variable that do not reflect real ecosystems. Furthermore, current studies have reported the use of micro-organism strains that in a real field situation could involve much extra cost and work effort.

2.2 Uses as food Table 2

Animal feed

Animal foods have been made using fermented coffee pulp to improve their nutritional value (silage); for example, fermentation in solid state (SSF) with Streptomyces increases protein content (Orozco et al., 2008), and with Aspergillus niger increases total amino acid content. Supplemented with 10% of the fermented product demonstrated a feeding efficiency in chickens similar to the standard diet (Bressani and González, 1978). And the recommendation for these birds is coffee pulp addition up to 25 g/kg of feed (Donkoh et al., 1988).

Feeding studies for rodents have been carried out with Wistar rats: 32 experimental rations, 16 with fresh coffee pulp and 16 with silage by-product, distributed over four different protein levels (10, 15, 20 and 25%) and three levels of pulp (15, 30 and 45%) have been studied. The silage pulp had a higher nutritional value, lower toxicity and better digestibility than fresh pulp. The higher the protein level of the ration the greater the protection against negative effects of coffee pulp on animal yield (Gómez-Brenes et al., 1985). This diet was also evaluated for commercial guine pig (Cavia porcellus L) production: 25% coffee pulp powder was found to increase casing performance (Yoplac et al., 2017).

In rabbits, it is possible to use up to 85% silage coffee pulp with molasses, while in pigs it is possible to use 20% in the growth stage and 15% in the finishing stage; in both cases, the use of coffee pulp did not result in losses in terms of animal production parameters (Noriega et al., 2008). In cattle, such as Swiss-Zebu bulls, coffee, pulp between 25 and 50% can be incorporated in silage, achieving greater preference and speed of consumption (Pinto-Ruiz et al., 2017). When the feed of Holstein-Brown and Swiss-Zebu cross cows was supplemented with coffee pulp (up to 20%), no changes in body weight, yield and milk composition were observed; thus, with this strategy one can achieve a 20% reduction in the cost of concentrate (Pedraza-Beltrán et al., 2012). Another study concluded that coffee pulp between 15 and 20% can be used to reduce costs in dairy feed, and does not affect the consumption of star grass (Cynodon plectostachyus K. Schum.) (Estrada-López et al., 2014).

Table 2: Studies on the applications of coffee pulp in animal and human food

Use/Function	Technology/ method	References
Feeding of animals	Pure or mixed rations of fresh or dried pulp have been prepared to feed birds, rodents, pigs, goats, sheep and cattle	Noriega et al., 2008
	Fermentation processes (silage) to transform coffee pulp into a more nutritious product, through reductions of compounds such as caffeine and polyphenols	Porres et al., 1993; Ulloa et al., 2003
	Silage and use of microorganisms for digestion/ fermentation of coffee pulp	Yonatan et al., 2011
	Treatment with alkali (NaOH) and acid-alkali to reduce anti-nutritional properties of coffee pulp	Ulloa et al., 2002
	Pre-treatment of coffee pulp by aerobic decomposition	Ulloa et al., 2003
	Inoculation of aerobic bacteria in coffee pulp samples for pre-feeding treatment
	Coffee pulp use at various percentages (16% to 30%) and with different preparation methods (dryed, powdered, boiled)	Rasowo and Ochieng, 2005; Salinas-Rios et al., 2014a; Yoplac et al., 2017
	As a substrate for the growth of edible larvae (Ornidia obesa and Hermetia illucens)	Lardé, 1989, 1990
	Composting or biological decomposition of coffee pulp for pathogen control	Hernández et al., 2003
	Inoculation with mycelia of various fungi (especially Pleurotus auratus) and comparing coffee pulp with other agro-industrial wastes from rice, fig, palm	Huerta et al., 2009; Benavides et al., 2016
	Solid-state fermentation for biotransformation of lignocellulose agricultural by-products (coffee pulp, coconut shell, cocoa shell, sawdust) using Pleurotus spp.	Bermúdez-Savón et al., 2013
Food for humans	Solid-state fermentation by Streptomyces and Aspergillus niger to improve the usefulness of coffee for human consumption	Peñaloza et al., 1985; Orozco et al., 2008
	Cultivation of the fungus P. ostreatus, and analyses of unsaturated acids	Benavides et al., 2015
	Production of the Cascara drink, based on coffee pulp, and its analyses	Heeger et al., 2017
	Preparation of toffee-type sweets with coffee pulp	Manrique and Monteblanco, 2015

In fish, up to 18% silage coffee pulp incorporated into the diet of cachamay hybrid fry (Colossomax piaractus) resulted in better growth rate by weight (0.53 g/d, length (0.68 mm/d) and cost-benefit ratio (1.42) (Bautista et al., 2005). Nile Tilapia fry (Oreochromis niloticus) fed with boiled coffee pulp as a source of carbohydrates, replacing rice bran, gained weight gain, had increased proportional growth, and exhibited high survival (>98%) (Rasowo and Ochieng, 2005). In a study with 30% pulp supplementation, an increase in growth rate of Tilapia aurea and total production was reported, together with a substantial increased annual net financial gain (Bayne et al., 1976). In another study on the feeding of fry (Oreochromis aureus), coffee pulp treated with bacteria of the genus Bacillus sp. (BT-CoP) at a concentration of 6%, was used to replace wheat flour; no adverse effects were observed in terms of growth and food utilization parameters were reported with the coffee pulp (Ulloa and Verreth, 2002).

Coffee pulp silage (16%) with molasses (5%) was included in the blackbelly lamb diet, and resulted in an increased casing yield from 48 to 51%, and decreased fat content of the rumen and intestines from 3.4 to 2.5% (Salinas-Rios et al., 2014a), without alteration of the protein and fat content in the meat, and with normal antioxidant capacity during refrigerated storage (Salinas-Rios et al., 2014a). Another study reports that the inclusion of up to 28% of coffee pulp does not affect the production parameters of lambs, but there is a decrease in neutrophils, creatine, and increased urea (Hernández-Bautista et al., 2018). On the other hand, 25% of coffee pulp in the diet of Suffolk x Dorset cross sheep was associated with an increased antioxidant activity, but decrease in fertility (from 100 to 79%); lipid oxidation was unchanged. In these sheep there was also no effect of the pulp on the onset or response of sexual receptive capacity, or progesterone, or gestation (Salinas-Rios et al., 2016). In another study of sheep diet supplementation with 16% coffee pulp, it was observed that there were no changes in production parameters, and a decrease in oxidative stress parameters (Salinas-Rios et al., 2015). Similar results were observed with the inclusion of up to 10% coffee pulp in sheep diet during 16 days before breeding: there was an improvement in oxidative status without any adverse effects in pregnancy or prolificity (Gutiérrez-Prado et al., 2019).

In a study involving goats (Abate, 1989), carried out for 20 weeks (7 months of age and weights between 13 and 18 kg), diets containing coffee pulp and ad libitum hay resulted in metabolizable energy (EM) of 35.8%. In cows with permanent ruminal cannula, the in situ digestibility of a special coffee pulp preparation-stove-dried pulp with addition of sal-was studied; and the results indicated an effect on the process of ruminal colonization: this salt-pulp preparation exhibited a slower than normal degradation by ruminal microorganisms (Noguera and Posada, 2017). Coffee pulp has also been reported to be a good substrate for the growth of edible larvae, Obese ornidia and Hermetia illucens, two protein-rich sources used in animal production (Lardé, 1989, 1990).

Various studies with coffee pulp have indicated that anti-nutritional factors such as phenols, tannins and caffeine, should be reduced in order to optimize its use as a supplement in animal feed formulations (Ulloa et al., 2003; Gualtieri A et al., 2007). Reported treatments take advantage of the pulp's high antioxidant capacity (Salinas-Rios et al., 2014b): silage (Porres et al., 1993; Ulloa et al., 2003; Noriega et al., 2009; Pinto-Ruiz et al., 2017), aerobic decomposition by inoculation with Bacillus spp. (Peñaloza et al., 1985; Ulloa et al., 2003; Orozco et al., 2008), and fermentation in solid state (SSF) (Peñaloza et al., 1985; Orozco et al., 2008). Mixtures of sorghum and coffee pulps, for example, have been studied as a substrate in SSF with the fungus Rhizopus oryzae (MUCL 28168) and a caffeine demethylase activity of 18,762 U/g to reduce the content of caffeine (Peña-Lucio et al., 2020) and tannins (Londoño-Hernandez et al., 2020). Other reported treatments to reduce antinutrients include alkali (NaOH), and acid treatments (Ulloa et al., 2002).

Human foods

Production of some human foods can be impacted by the use of coffee pulp. For example, coffee pulp was used as a substrate of edible fungi (Pleurotus spp., Auricularia spp., Volvariella volvacea) increasing their biological efficiency (Salmones et al., 1996; Hernández et al., 2003; Huerta et al., 2009; Bermúdez-Savón et al., 2013; Benavides et al., 2015, 2016; Ríos-Ruiz et al., 2017) and unsaturated fatty acids (oleic acid and linoleic acid) (Benavides et al., 2015). Composting is one of the most economical and environmentally friendly processes for the production of edible fungi; however, a current limitation is the presence of unpleasant odors (Hernández et al., 2003; Hikichi et al., 2017; Calderón Lopez and Bhaktikul, 2018).

In Switzerland, a drink called Cascara has been produced from coffee pulp. Analysis of this drink shows that it contains 226 mg caffeine/L and 283 mg (galic acid equivalent) GAE/L of total polyphenols, while the antioxidant capacity amounted to about 9 mmol (trolox equivalents) TE/L (Heeger et al., 2017). Also, infusions have been developed with temperature and extraction controls to obtain optimal amounts of total polyphenols, total flavonoids, condensed tannins (Rosales Delgado et al., 2019). In addition, soft, toffee-type sweets have been produced with the inclusion of coffee pulp and honey (Manrique and Monteblanco, 2015).

The main limitation for the use of coffee pulp in animal feed is the presence of antinutritional factors (caffeine, tannins, polyphenols, chlorogenic acid, among others); these factors must be reduced, neutralized, or otherwise eliminated. Silage together with solid state fermentation through inoculation with Bacillus sp., Pleurotus spp., Streptomyces and Aspergillus niger has been shown increase the nutritional value of coffee pulp (Peñaloza et al., 1985; Ulloa et al.,2003; Orozco et al., 2008); however, care must be taken with the ensiling method due to the emission of gases generated in the anaerobic fermentation process.

2.3 Uses in biotechnology Table 3

Biotechnology is perhaps the field in which coffee pulp has the most applications, some examples include vinegar production, enzyme production, and caffeine degradation. As early as 1931, Freise (Freise, 1931) prepared vinegar by fermenting coffee pulp with Saccharomyces octoporus. Currently two main types of fermentation processes are in use, one based on solid-state fermentation (SSF) and the other on submerged fermentation (SmF), with SSF being more widely used (Pandey et al., 2000). Work has been done on process optimization methods for coffee pulp; for example, Pérez et al. (Pérez et al., 2017) developed a method of rapid, sensitive determination of fungal biomass by quantifying chitin. They found a strong correlation between the biomass concentration determined by gravitmetry and the estimated chitin concentration by spectrophotometry; thus, microbial growth could be efficiently tracked during standard fermentation processes.

Production of enzymes

Enzyme production is a field with potential use in many sectors such as food industry, pharmaceuticals, biofuels, agro-industry (Hikichi et al., 2017) Fungi and bacteria are most commonly used for enzyme production: e.g., cellulobiohydrolase (Velázquez-Cedeño et al., 2002), Mn-peroxidase (Velázquez-Cedeño et al., 2002), lactase (107–109), feruloyl esterasae (Pérez-Morales et al., 2011; Romero Borbón et al., 2018), endoglucase (23,107,112), pectinase (Antier et al., 1993; Oumer and Abate, 2018), endopectinase (Venugopal et al., 2007), s-amylase(Murthy et al., 2009), tanase (Bhoite and Murthy, 2015), cellulase (118,119), s-glucosidase (120,121), and proteases (122).

The fungi P. ostreatus and P. pulmonarius are also used for the production of some lignocellulolytic enzymes with hydrolase activity such as endoglycase (CMCasa) and cellulobiohydrohydrolase (CBH), and enzymes with oxidase activity such as lactase (LAC) and Mn-peroxidase (MnP) (107,108). Another producer of lactase is the Pleurotus sajor-caju; and addition of P. ostreatus var. Florida in the inoculum (i) and colonization (c) phases increases enzymatic activity (i, 5,214 x 0.059 U/g at 18 days; c, 4,512 x 0.479 U/g at 44 days) (García-Oduardo et al., 2016). The fungus Pycnoporus sanguineus is also a producer of lactase. The addition of coffee pulp to 10% resulted in increased enzymatic activity by 32% and fungal biomass by 16%; however, with the addition of 25% coffee pulp, both the activity of the lactase and cellulase enzymes decrease (González Bautista et al., 2019). In another study, it was found that of the 99 isolated actinomycetes grown on agar supplemented with 0.1% carboxymethylclelose (CMC) and xylan, 16 showed cellulase activity (cellulase index 2), 20 isolations showed xylase activity (xylase index 1), and 5 showed both activities (Putri et al., 2019).

Rolz et al. (Rolz et al., 1988a; b) studied changes in the lignocellulose polymer matrix and enzyme digestion using 26 fungi with lignocellulolytic potential. Degradation percentages of approximately 70, 55 and 47 were reported by these authors for total polyphenols, caffeine and lignin permanganate, respectively. They also noted that hemicelulose was degraded by most fungi, of which at least two strains degraded it by more than 80%; among these fungi Trametes versicolor showed greater degradation of holocellulose and lignin. In addition, white rot fungi such as Sporotrichum pulverulentum and Stropharia rugosoannulata showed a low lignin solid residue with the same enzymatic digestibility as that found for the original coffee pulp. Other studies report that alkaline hydrogen peroxide (AHP) treatment of coffee pulp residue results in a reduction of lignin content; and thus leads to increased digestibility of cellulose (Pudjiastuti et al., 2019).

Alpha-amylase has been produced using Neurospora crassa grown on a coffee pulp substrate. With the optimized growth parameters. an enzymatic activity of about 4900 U/g of dry substrate was obtained; and an higher yield was obtained if the pulp was pretreated with water vapor: 7084 U/g of dry substrate (Murthy et al., 2009). Feruloyl esterase was produced by Aspergillus tamarii, with an enzymatic activity of about 14 and 11 units per gram of dry matter, when using the methyl substrate and ferulated ethyl, respectively (Pérez-Morales et al., 2011). Production of the same enzyme using Aspergillus ochraceus isolated from coffee pulp resulted in a specific activity of about 58 U/mg towards methyl ferulate. Moreover, this type C enzyme had a fivefold higher rate of synthesis of butyl caffeate compared to the type B enzyme (Romero Borbón et al., 2018).

Tanase (Bhoite and Murthy, 2015) was isolated using Penicillium verrucosum, with an enzymatic activity of about 28 U/g, and about 4 fold higher with optimized production parameters (116 U/g). This enzyme can be used to improve the taste of some fruit juices, e.g., Emblica Officinalis and Punica granatum L. Also, by using Penicillium verrucosum with fermentation in solid state, extracellular glucosidase was produced using coffee pulp as the only source of carbon (Bhoite et al., 2013a). Pectinase was obtained from Aspergillus niger, with a maximum production of 138 U/g dried pulp at 72 h (Antier et al., 1993). Another study reported that Mycotypha sp reduced pectin content by up to 85%, and showed a maximum endopectinase activity of 5.4 U/mL and 4. 9 U/mL in aerated and stationary conditions, respectively (Venugopal et al., 2007). Oumer and Abate (Oumer and Abate, 2018) identified pectinase-producing microbes; approximately half of the colonies showed enzyme activity, and of those, about 70% were bacteria of the genus Bacillus.

Coffee pulp has been studied as a substrate of (a) Bacillus sp. for producing proteases, with a reported enzymatic activity of 920 U/mL after 60 h of incubation (Selvam et al., 2016); (b) Bacillus amyloliquefaciens for producing the enzyme CMCase, 780 U/mL at 72 h (112); (c) Bacillus subtilis with an activity of about 3 U/mL and a caffeine tolerance capacity of up to 0.4% (Arimurti et al., 2017); (d) Acinetobacter sp. to produce cellulase with 888 U/ml at 60 h of incubation (Selvam et al., 2014); and (e) Bacillus subtilis for production of s-glucosidase, 59 IU/mL at 24 h (Dias et al., 2015).

Caffeine degradation

Coffee pulp is used as a source and substrate of caffeine degrading microorganisms. Caffeine is sometimes considered to be an antinutrient (Mazzafera, 2002). In 1995 Roussos et al. (Roussos et al., 1995) found that strains of Aspergillus, Penicillium, Trichoderma, Fusarium and Humicola, typical of coffee pulp, possessed the ability to degrade caffeine. They reported that two strains of Penicillium and five strains of Aspergillus eliminated almost 100% of the caffeine in a liquid medium.

Other caffeine reducing fungi isolated from coffee pulp, are Chrysosporium keratinophilum, Gliocladium roseum, Fusarium solani and Aspergillus restrictus. The G. roseum strain has a maximum degradation of about 1.5 mg/ml at 72 h in a medium with the presence of nitrogen. C. keratinophilum has the highest rates of degradation without the presence of nitrogen (0.5 mg/ml) at 72 h. In another evaluation it was reported that nitroten in the medium, but not glucose, affects caffeine degradation (Nayak et al., 2013; Pai et al., 2013).

As for bacteria with caffeine degrading potential, a coffee pulp isolate of Pseudomonas monteilii has been studied, with a maximum degradation of 99 % at 24 h; and E. coli DH5a transformed with a plasmid isolated from Brevibacterium sp., degraded caffeine up to 2 g/L, and with a high tolerance to coffee (Nayak et al., 2012; Arimurti et al., 2018). Caffeine degrading soil bacteria have also been isolated and identified, including 5 gram-negative species, namely Pseudomonas japonica (4/12), Methylobacterium populi (5/12), Raoultella ornithinolytica (1/12), Klebsiella quasipneumoniae (1/12) and Stenotrophomonas chelatiphaga (1/12) with a potential to be used in various applications that require removal of this antinutrient (Iswanto et al., 2019b).

Other biotechnological uses

Trichoderma spp., used as a natural fungicide, show efficient growth rates in coffee pulp (16 and 22 g of carbon). T. koningii, T. harzianum and T. longibrachiatum, species appear to have the greatest potential for pathogen control (Onsando and Waudo, 1992). The production of gallic acid by Penicillium verrucosum has been reported at about 35 g of GA /g coffee pulp, and up to 76 g of GA/g of coffee pulp with optimization of growth conditions (Bhoite et al., 2013b). For the production of lactic acid, Bacillus coagulans achieved production rates of about 4 g/L/h, and higher rates using the downstream method (Pleissner et al., 2016). Finally, a mixture of three enzymes, xylanase, arabinofuranosidase and cellulase from Aspergillus sp. was shown to hydrolyze coffee pulp waste to simple sugars, with a hydrolysis grade of 79% (Muzakhar, 2019). A higher hydrolysis rate has been reported for coffee pulp subjected to solid state fermentation by Pestalotiosis sp. VM9 and Aspergillus sp. VTM5 (Ubaidillah and Muzakhar, 2019). The main limitations for these studies lie in the traceability of the processes that may vary between different laboratories or countries, as well as the lack of specialized centers for the safe handling of the microorganisms that are used.

Table 3: Uses of coffee pulp in biotechnology

Use		Microrganism	References
Vinegar		Saccharomyces octoporu.	Freise, 1931
Edible fungi		Pleurotus spp	Hernández et al., 2003; Hikichi et al., 2017; Calderón Lopez and Bhaktikul, 2018
		Auricularia spp
		Volvariella volvácea
Enzyme production	Cellulobiohydrolase	Pleurotus ostreatus	Velázquez-Cedeño et al., 2002
		Pleurotus pulmonarius
	Mn-peroxidase	Pleurotus ostreatus	Velázquez-Cedeño et al., 2002
		Pleurotus pulmonarius
	Lactase	Pleurotus ostreatus	Velázquez-Cedeño et al., 2002; García-Oduardo et al., 2016; González Bautista et al., 2019
		Pleurotus pulmonarius
		Pleurotus sajor-caju
		Pycnoporus sanguineus
	Feruloyl esterase	Aspergillus tamarii	Pérez-Morales et al., 2011; Romero Borbón et al., 2018
		Aspergillus ochraceus
	Endoglucanase	Bacillus subtilis	Velázquez-Cedeño et al., 2002; Selvankumar et al., 2011; Arimurti et al., 2017
		Pleurotus ostreatus
		Pleurotus pulmonarius
		Bacillus amyloliquefaciens
	Pectinase	Bacillus sp	Antier et al., 1993; Oumer and Abate, 2018
		Aspergillus niger
	Endopectinase	Mycotypha sp	Venugopal et al., 2007
	α -amylase	Neurospora crassa	Murthy et al., 2009
	Tanase	Penicillium verrucosum	Bhoite and Murthy, 2015
	Cellulase	Acinetobacter sp	Calzada et al., 1987; Selvam et al., 2014
	β -glucosidase	Bacillus subtilis	Bhoite et al., 2013b; Dias et al., 2015
		Penicillium verrucosum
	Proteases	Bacillus sp	Selvam et al., 2016
Degradation of caffeine		Chrysosporium keratinophilum	Nayak et al., 2012, 2013; Pai et al., 2013; Arimurti et al., 2018
		Gliocladium roseum
		Fusarium solani
		Aspergillus restrictus
		Pseudomonas monteilii
		Brevibacterium sp
Protein production		Pestalotiosis sp. VM9 and Aspergillus sp. VTM5	Ubaidillah and Muzakhar, 2019

3. Conclusion and future perspectives

Over the years, researchers have shown interest in developing coffee pulp for various applications, agriculture, food, medicine, biotechnology, power generation; and in each of these categories multiple specific applications have been studied and developed. Comprehensive actions are required with public policies, however, to ensure the quality of each process within a circular economy (UN, 2020), and with a vision to the future.

Figure 2: Uses of coffee pulp: a vision for the future

In relation to diet and health, for example, coffee pulp has hepatoprotective effect (Ontawong et al., 2019) of may be useful to lower rates of cardiovascular diseases (CVD), leading causes of death in the world. There is evidence that oxidative stress is involved in the pathology of some CVD. Coffee pulp with its high polyphenol content and antioxidant (Serna-Jiménez et al., 2018) may be helpful in this context. Moreover, coffee pulp as a dietary supplement could have additional beneficial effects for CVD as an anti-inflammatory agent. It may also decrease endothelial dysfunction because of its ability to modulate apoptotic processes in the vascular endothelium (Quiñones et al., 2012).

Because of its high iron content (approximately 77 mg/kg of coffee pulp), coffee pulp could become the basis for dietary products to combat malnutrition and anemia. It could go from being an undervalued by-product to valued, such as some flower species that are not only sought after for ornamental use but are required under the concept of floriphagia (Lara-Cortés et al., 2013), that is, the consumption of flowers for their nutritional properties. By optimizing growth, harvesting and processing conditions, coffee pulp rich in minerals such as Fe, P, K, Ca, Mn and Mg, and B vitamins such as folate could be obtained and used therapeutically (Arimurti et al., 2017; Fierro-Cabrales et al., 2018). Iron from coffee pulp, for example, could be used to meet the recommendations set out by the Food and Agriculture Organization of the United Nations (FAO) for children (Greenfield, 2006). Iron and folates are mandatory enrichment in food provided by the world's social programs in the fight against malnutrition and anemia, and both of these nutrients are abundant in coffee pulp.

Dietary fibre is another component of coffee pulp that can be used to promote a healthy diet. Previous studies reported that dried coffee pulp contains 351.7 g/kg of fibre (Donkoh et al., 1988), and coffee pulp powder from Amazonas Peru has a fibre concentration of 19.29%; these values are similar to or higher than the fibre content of yellow maize and quinoa, two high-consumption cereals of the world (Yoplac et al., 2017).

Dietary fiber represents a group of food components, mostly carbohydrates, that are resistant to digestion by enzymes of the small intestine, and are partially or totally fermented in the colon, with favorable health (Vilcanqui-Pérez and Vílchez-Perales, 2017). There are already commercial preparations of powdered coffee pulp for use in infusion and pastries. In general, for the preparation and presentation of consumer products from coffee pulp (drinks, mousse, biscuits and toffees, for example) one has to consider many other factors such as appearance, taste, texture, and age of the intended consumer. Moreover, the presence of antinutrients (Ulloa et al., 2002) requires identifying and applying procedures that ensure the safety of the coffee pulp-based food products.

Coffee pulp could be used as a low-cost substrate to generate energy through the action of microorganisms (Calzada et al., 1981; Calzada and Rolz, 1984) and also as the starting material for the production of biopolymers or biomaterials for biodegradable packaging. Although its starch content (13.92%) (Yoplac et al., 2017) is low for some of these uses in producing biocontainers, compared to the 60 and 75% (dry weight) present in cereal grains (Ulloa et al., 2003; Phuong et al., 2019), there is the possibility of incorporating starch from other local plant products such as banana stalks. Such bio-package production using fiber and starch waste would contribute to the protection of the environment, and reducing the use of plastic containers which typically have degradation times in the order of centuries (Gross and Kalra, 2002). As an additional environmental benefit, the lignocellulose composition and adsorption capacity of coffee pulp (Aguilar et al., 2019) could be used as a low-cost alternative method for the removal of heavy metals that often contaminate waste water.

In the pharmaceutical industry, pectin extracted from coffee pulp (Hasanah et al., 2019) could be used as biodegradable membrane matrices for controlled drug release. Pectin and other coffee pulp components are also being investigated as hepatoprotective agents (Ontawong et al., 2019) to lessen the harmful effects of exogenous or endogenous toxins in this high metabolic activity organ.

Further research is needed to identify novel applications of coffee pulp, which in turn would open up new opportunities and benefits for coffee producers. Social benefits in terms of environmental protection, sustainable production of energy and materials, as well as nutritional supplementation and health promotion are currently being established by researchers in this field.

Acknowledgments

This research was financially supported by the Project CONCYTEC-WORLD BANK. Contract 008-2018-FONDECYT-BM-IADT-MU

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