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Industrial Crops & Products 205 (2023) 117504
0926-6690/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Review of potential and prospective strategies for the valorization of coffee
grounds within the framework of a sustainable and circular bioeconomy
Ana Arias
a
,
*
, Soa María Ioannidou
b
, Nikos Giannakis
b
, Gumersindo Feijoo
a
,
Maria Teresa Moreira
a
, Apostolis Koutinas
b
a
CRETUS, Department of Chemical Engineering, School of Engineering, Universidade de Santiago de Compostela, 15705 Santiago de Compostela, Spain
b
Department of Food Science and Human Nutrition, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece
ARTICLE INFO
Keywords:
Spent coffee grounds
Fermentation
Lignin
Lipids
Phenolics
Life cycle assessment
Techno-economic assessment
Biorenery
Sustainability
ABSTRACT
Moving from the linear production model to the circular economy approach is the main concern of the EU
Circular Economy Action Plan. Population growth coupled with high demand for goods has led to a signicant
increase in solid waste, especially food waste, triggering the depletion of natural resources and the environ-
mental burdens associated to their disposal. However, their physical, chemical and biological characteristics
make them by-products with a high potential to be valorized and therefore used as resources for other industrial
production models. This is the case of spent coffee grounds (SCGs), which are produced in large quantities on a
daily basis. Therefore, valorization models under the approach of the biorenery concept can be envisaged with
the aims of recovering potential bioactive compounds and renewable energy. This has been the approach
developed in this critical review, in which SCG recovery alternatives have been studied to obtain lignin, lipids,
biofuels and phenolic compounds. In addition, a critical analysis of the outcomes of techno-economic and
environmental evaluations available in the literature is included, in order to identify those indicators that pro-
vide information on the feasibility of their valorization.
1. Introduction
The search for an adequate management of all by-products produced
in industrial facilities has become one of the main drivers of research
and development in the context of the circular economy. However, a
huge percentage of the total waste generated daily is managed in
landlls or, depending on its composition, its energetic valorization is
considered. However, it should be taken into account that, in general
terms, the waste produced is suitable for its chemical composition, ac-
cording to various routes to recover products with a higher added value
in the market, as is the case of antioxidants, antimicrobials or bioactive
compounds. Therefore, a biorenery approach should be proposed, with
a cascade strategy to produce several products, which greatly increases
the value of this waste stream, which could now be dened as a "feed-
stock" rather than a waste product.
It is in this context that spent coffee grounds (SCG) could be
included. This is the main waste associated with the coffee industry, as it
is obtained by mixing coffee powder with water to obtain soluble coffee.
Although it is possible to manage them as waste in composting and
biomethanization stages, it is possible to valorize this type of waste in a
biorenery strategy, which is considered the objective of this critical
review. This manuscript identies the different valorization routes for
SCG, focusing mainly on the extraction of lignin, lipids and phenolics.
For this purpose, an in-depth evaluation of the research reports that have
been published on this topic has been addressed. In addition, since
economic and environmental issues must be considered as feasibility
indicators for the processes under development, techno-economic
assessment (TEA) and life cycle assessment (LCA) reports have also
been taken into account. In addition, software tools for reference man-
agement have been used: Mendeley Desktop® and VosViewer®, mainly,
which allows the compilation of research reports and the visualization of
the most important data. Critical analysis of this information can pro-
vide researchers and stakeholders with the key operational conditions
and potential for biorenery development using SCGs as raw material.
* Corresponding author.
E-mail address: anaarias.calvo@usc.es (A. Arias).
Contents lists available at ScienceDirect
Industrial Crops & Products
journal homepage: www.elsevier.com/locate/indcrop
https://doi.org/10.1016/j.indcrop.2023.117504
Received 1 August 2023; Received in revised form 11 September 2023; Accepted 11 September 2023
Industrial Crops & Products 205 (2023) 117504
2
2. Preliminary analysis of available literature on SCGs
valorization
In order to develop the literature review of published reports based
on the valorization of SCGs, the time frame of the last 12 years, from
2011 to September 2023, was used as a search lter. In addition, the
logical operators AND, to include the SCGs together with the words
“lignin”/“lipids”/“phenolics”/“oil”/“LCA”/“TEA”/“polymer”, and also
* to consider both singular and plural forms (i.e., “lipid*”) were used.
As a database for the research, Scopus® has been used, as it is
considered one of the largest peer-reviewed scientic literature data-
bases in the world, so that its use guarantees an in-depth overview on the
current state of the art of SCG valorization. A large amount of research
articles has been found, with a wide range of keywords, most of them
referring to the same topic, but with other denominations (Figure 1SM).
For example, this is the case for the keyword SCGs, as it has been found
to vary as: “Spent Coffee Ground”, “spent coffee grounds”, “Coffee
Grounds”, “SCG”, “SCGs”, but all these terms refer to the same material.
To have a better overview of the evaluated topics, the selected keywords
of the authors have been standardized according to the topics developed
in this review article (Figures 2SM-5SM).
Four main groups can be distinguished: spent coffee grounds, coffee,
phenolic compounds and fermentation. In addition, the keyword
"extraction" is one of the most used by the authors, which is presented as
the main focus of the research developed for spent coffee grounds, the
recovery of active compounds, mostly phenolics with antioxidant
properties, with high added value in the market. On the other hand, the
keyword "biorenery" is not as widespread as expected, which is a signal
of the early stage of development of valorization routes for SCGs.
Moreover, considering the year of publication of the research articles
based on SCGs recovery, the interest on this topic has been gaining
weight over the last year, with the year 2021 presenting the highest
number of research manuscripts on the topic (Fig. 1). However, it is
believed that in the present year, the number of manuscripts will be even
higher, since the number of research papers published until mid-2023 is
more than half compared to last year. To this end, it can be concluded
that the interest of researchers in the valorization of SCGs is growing and
it is very likely that SCG biorenery approaches will be considered at an
industrial level in the near future, given their compositional potential,
their content in bioactive compounds, their possible use as a source of
fermentable sugars for biofuels and their heat capacity as an alternative
to fossil fuels.
The most relevant journals that have published articles and reviews
evaluating the recovery of SCG for the extraction of lipids, lignin, phe-
nolics or others, based on the TEA and LCA analysis, are Bioresource
Technology (with 28 manuscripts), followed by Waste and Biomass
Valorization (with 16 research articles), Chemical Engineering Trans-
actions (with 12 reports), Fuels (with 10 manuscripts) and Journal of
Fig. 1. Column chart including the number of references published per year in the period 2011–2023.
Table 1
Top-authors on the scientic productivity, according to Scopus database, on
SCGs recovery strategies and valorization topics.
Author Reports
on SCGs
Institution h-
Index
Citations References
Coimbra,
M.A.
9 University of
Aveiro
56 10763 (Oliveira et al.,
2021; Cl´
audia P.
Passos et al.,
2019; Passos
et al., 2015;
Sim˜
oes et al.,
2013)
Mussatto,
S.I.
9 Technical
University of
Denmark
53 11579 (Ballesteros
et al., 2015; Lina
F. Ballesteros
et al., 2017a,
2017b;Conde
and Mussatto,
2016;Machado
et al., 2018)
Chuck, C.
J.
8 University of
Bath
25 2100 (Jenkins et al.,
2017; Massaya
et al., 2021b,
2021a, 2019;
Pereira et al.,
2021)
M´
arov`
a, I. 8 Brno
University of
Technology
29 2158 (Hudeckova
et al., 2018;
Kovalcik et al.,
2018; Obruca
et al., 2015; S.
Obruca et al.,
2014; Stanislav
Obruca et al.,
2014; Petrik
et al., 2014)
Teixeira,
J.A.
8 University of
Minho
83 30787 (Lina F.
Ballesteros et al.,
2017a, 2017b;
Mussatto et al.,
2012; Sampaio
et al., 2013)
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
3
Cleaner Production (with 9 research reports). The top-authors in terms
of scientic productivity, according to the Scopus database, are shown
in Table 1.
3. Composition of spent coffee grounds
When thinking about the valorization of SCGs and the recovery of the
high value-added components available in their molecular structure, the
rst step is to consider their composition. However, it should be borne in
mind that, depending on the coffee variety, the composition of certain
components, especially in terms of phenolic, antioxidant and avonoid
content, can vary signicantly. Therefore, before considering a cascade
valorization of SCG, it is important to analyze the composition of the
SCG in terms on lignin, phenols and lipids in order to decide which
cascade biorenery process is the most convenient, considering eco-
nomic, environmental and technological aspects. Bearing this in mind,
Table 2 and Table 3 are depicted below, including the chemical
composition of SCGs in a range percentages according to dry matter
content, as can be found on literature (Table 2) and the content on
phenolics, avonoids and antioxidant compounds in
μ
g/g, also in dry
matter basis (Table 3). On the other hand, it is worth mentioning that the
ranged values included in both Tables 2 and 3 are the result of the
analysis of different research reports that have taken into account how
the composition of SCGs could be inuenced by external factors (i.e.
cultivation conditions, geographical area, climate conditions, etc.).
4. Valorization routes: paving the way for circular economy
The different techniques of valorization of SCGs are described in the
following sections, focusing on the recovery of lignin, lipids, phenolics
and oil. The huge variety of applications of these products in different
sectors, such as the food, pharmaceutical, medical and energy, makes
them potential products of great interest.
4.1. Lignin extraction alternatives for valorizing SCGs
Lignocellulosic biomass could represent an important renewable
resource due to the different alternative routes that could be developed
in the search for the substitution of fossil resources. In particular, the
recovery and valorization of lignin is one of the most interesting options,
due to the various products that could be obtained, both energetic and
bio-chemicals. However, due to its molecular structure and physico-
chemical properties, it is also considered as a challenging feedstock, as it
is necessary to break strong molecular bonds for its useful valorization.
It is true that there are not many research articles analyzing lignin
removal as a main objective, but it is a pretreatment step aiming at
higher productivity for the production of fermentable sugars from
lignocellulosic biomass (Sugebo, 2022). To this end, this section of the
manuscript discusses the different techniques used for lignin removal.
There are novel techniques that have been used for the recovery/
removal of lignin from SCGs, such as the use of ionic liquids. Tolesa et al.
(2018) have considered the use of ammonium-based ionic liquid for the
extraction of lignin using mild conditions, up to 71.2% after 4 h of re-
action time at 120 ºC (Tolesa et al., 2018).
Nevertheless, conventional procedures are the most common, due to
their simplicity, mostly based on physical and chemical treatments.
Those are based on the chemical modication of the lignin content of
SCGs, by phenolation and acetylation, which not only implies low costs
and shorter reaction time, but also allows the reuse of lignin for other
applications, since it is not degraded (Taleb et al., 2020). One of the most
innovative and recent research for lignin applications is its use in the
production of aluminum-air batteries, where it is used as an electrolyte
additive. The addition of lignin implies enhanced corrosion inhibition
and improved battery performance, given the chemisorption properties
of lignin molecules, which involves an electrostatic-based interaction
between the battery surface and the hydroxyl groups of lignin (Lee et al.,
2021).
To achieve high-purity and high-quality lignin from SCG, organosolv
pretreatment has proven to be a viable alternative. In this case, an
organic solvent and catalyst in combination of high temperatures are
required. (Ravindran et al., 2018) have studied the optimization in the
organosolv pretreatment of SCG, considering both the maximum
amount of lignin removal, the largest phenolic extraction yield and the
highest amount of reduced sugars extracted. To this end, the optimized
process was obtained when using ethanol (68%) as organic solvent,
1.5% H
2
SO
4
as catalyst at 51ºC during 45 min. In addition, the authors
also concluded that the requirements of this pretreatment process can be
scaled up in a biorenery approach, given the ease, cost-effectiveness
and yield values obtained (Ravindran et al., 2018).
On the other hand, it is also important to consider the downstream
process required to obtain lignin with the highest purity possible. A
sequential separation procedure based on centrifugation for the liquid
Table 2
Chemical composition of SCGs (*Unit: % dry matter).
Component Amount* Reference
Cellulose 8.6-12.4 (Arya et al., 2022; Ballesteros et al., 2014; Kwon
et al., 2013; Lavecchia et al., 2016; L´
opez-Barrera
et al., 2016; Mussatto et al., 2011)
Hemicellulose 19.0-
39.1
(Ballesteros et al., 2014; Kelkar et al., 2015; Mussatto
et al., 2011)
Arabinose 1.9-3.6 (Ballesteros et al., 2014; Kwon et al., 2013;
L´
opez-Barrera et al., 2016)
Mannose 13.2-
19.1
(Ballesteros et al., 2014; Kwon et al., 2013;
L´
opez-Barrera et al., 2016)
Galactose 16.4-
26.0
(L´
opez-Barrera et al., 2016; Passos et al., 2019)
Lignin 23.9-
33.6
(Ballesteros et al., 2014; Caetano et al., 2014; Kelkar
et al., 2015)
Glucan 8.6-13.8 (Caetano et al., 2014; Mussatto et al., 2011)
Ashes 1.2-2.3 (Hernandez-Arriaga et al., 2017; Kelkar et al., 2015)
Protein 10.0-
17.4
(Caetano et al., 2014; Lerda, 2016)
Carbohydrates 45.0-
68.4
(Hernandez-Arriaga et al., 2017; Lerda, 2016;
Martinez-Saez et al., 2017)
Lipids 15.1-27-
0
(Hernandez-Arriaga et al., 2017; Lerda, 2016)
Total ber 57.1-
60.5
(Ballesteros et al., 2014; De Cosio-Barron et al., 2020;
Martinez-Saez et al., 2017)
Insoluble ber 50.8-
57.1
(Ballesteros et al., 2014; De Cosio-Barron et al., 2020;
Martinez-Saez et al., 2017)
Soluble ber 1.6-9.7 (Ballesteros et al., 2014; De Cosio-Barron et al., 2020;
Martinez-Saez et al., 2017)
Table 3
Main composition on phenolics/avonoids/antioxidant compounds on SCGs
(*Unit:
μ
g/g dry matter).
Component Amount* Reference
Caffeine 209-439 (Badr et al., 2022; Ho et al., 2020)
Caffeic aid 7.2-41.4 (Badr et al., 2022; Ho et al., 2020)
Catechin 16.9-24.0 (Badr et al., 2022; Ho et al., 2020)
Chlorogenic
acid
7.4-24.0 (Badr et al., 2022; Okur et al., 2021)
Gallic acid 3.1-18.2 (Badr et al., 2022; Ho et al., 2020)
Ferulic acid 21.1-119 (Angeloni et al., 2021; Ho et al., 2020)
p-cumaric acid 0.2-18.3 (Badr et al., 2022; Ho et al., 2020)
Quercetine 1.4-3.96 (Angeloni et al., 2021; Ram´
on-Gonçalves et al.,
2019)
Rutin 4.9-10.11 (Angeloni et al., 2021; Badr et al., 2022)
Syringic acid 64.1-
78.63
(Angeloni et al., 2021; Badr et al., 2022)
Vanillic acid 0.5-54.3 (Badr et al., 2022; Ho et al., 2020)
Sinapic acid 10.1-17.1 (Badr et al., 2022; Hussein et al., 2022)
Salicylic acid 7.61-12.7 (Badr et al., 2022; Hussein et al., 2022)
Epicatechin 37.2-53.8 (Badr et al., 2022; Ho et al., 2020)
Naringin 0.40-0.62 (Angeloni et al., 2021)
Kaempferol 0.4-3.2 (Badr et al., 2022; Kr´
ol et al., 2020)
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
4
separation, and a precipitation stage with methanol was considered (Lee
et al., 2019). Another way to recover lignin with a higher level of purity
is to perform precipitation stages, using 60% ethanol with the addition
of HCl to maintain the pH at a value of 2. Within the precipitation stages,
centrifugation is required, as the supernatant is used for precipitation,
while the insoluble part requires additional dissolution stages, at high
temperature (150 ºC) during 70 min with ethanol/water, to allow the
subsequent precipitation stage (Du et al., 2021).
Alkaline methods have also proven to be an effective procedure for
lignin recovery/removal, as they lead to an increase in internal surface
area and the breaking of lignin-carbohydrates bonds. However, the
highest yields of this pretreatment method are obtained when the lignin
content of the lignocellulosic biomass is low (Amin et al., 2017). When
medium to high lignin content is available, as it is the case of SCGs, the
organosolv pretreatment method is more adequate, as it has a higher
capacity to break the internal bonds of hemicelluloses and lignin (Loh
et al., 2019).
Thermal pretreatments, such as steam explosion, have been used for
many lignocellulose-based industries as it leads to higher yields when
the breakdown of lignin structures is desired. But, when developing a
biorenery process, this thermal pretreatment could lead to degradation
of the available sugars in the lignocellulosic feedstock, if the tempera-
ture requirement of the process is too high (Xia et al., 2020). In addition,
the high energy demand leads to signicant environmental damage due
to the use of fossil fuels for the production of energy requirements
(Prasad et al., 2016).
Currently, there is a strong tendency to try to use biological pre-
treatment methods, as they are considered low demanding in chemical
and energetic terms, more environmentally friendly and with reduced
costs (Khir and Pan, 2019). In this case, the degradation of the bonds and
recalcitrant cell wall structures of the lignocellulosic feedstock is per-
formed by microorganisms. But, despite the high yields and efciency of
the procedure, biological treatments are time demanding and require
enormous control over microbial growth, leading to more difculties in
applying this method in industrial facilities considering the economic
protability of the process (Joshi et al., 2021).
But what about the emerging uses of lignin for the extraction of high
value-added compounds or for the production of biocomposites? The
high molecular weight of lignin makes it a potential source for bio-
composites production, as it could be used as a coupling agent, resulting
in a high-strength matrix that could be used as an adhesive, i.e., in wood
panels. Then, to take advantage of this, a pretreatment stage of the lignin
is required, to increase the availability of the phenolic hydroxyl groups
that are available in its structures. There are different ways to "activate"
lignin, which implies a breakdown of its structure to turn it into a more
accessible molecule, the most common ones are based on phenolation,
demethylation (Zhao et al., 2022), oxidation (Azadfar et al., 2015),
depolymerization (Gao et al., 2021) and/or glyoxalation (El Mansouri
et al., 2007; Younesi-Kordkheili et al., 2016).
Looking for more sustainable process, Arias et al. (2022) have carried
out an environmental assessment on the use of lignin as a renewable
resource for the production of wood bioadhesives (Arias et al., 2022).
The production of bioadhesives has been based on a rst lignin func-
tionalization step based on a carbonation reaction with dimethyl car-
bonate followed by a crosslinking step with hexamethylene diamine.
The feasibility of the process and the environmental prole have been
found to be adequate to consider the use of lignin as an alternative
resource to formaldehyde in the production of wood adhesives (Arias
et al., 2022). But, on the other hand, a previous study by the same au-
thors has shown that the pretreatment of lignin for activation is a crucial
step, since if a glyoxalation reaction occurs, the amount of glyoxal
needed as a crosslinking agent, together with the raw amount of energy
needs towards the process, turns the lignin-based bioadhesive into a not
so good process alternative (Arias et al., 2020).
In addition, another option that is gaining attention in the valori-
zation of lignin is the recovery of high value products such as phenolic
compounds, vanillin, aromatic diacids and quinones, among others.
Faustino et al. (2010) have used ethyl acetate as an extraction agent for
the recovery of 17 phenolic compounds from lignin representing
1009 mg GAE/g and an antioxidant index of 11.4 (Faustino et al., 2010).
Seeking to reduce the amount of chemicals used, V´
azquez-Olivo et al.
(2019) have performed an acid hydrolysis, giving a total amount of
1421 mg GAE/100 g of lignin residues and an antioxidant capacity of
11.75 mmol ET/g (Vazquez-Olivo et al., 2019). In addition, renewable
aromatic chemicals could also be obtained using lignin as a renewable
feedstock. Mycroft et al. (2015) have studied the microbial degradation
of lignin into aromatic chemicals, using Rhodococcus jostii as a strain
(Mycroft et al., 2015). This procedure has also been recently evaluated
for Pseudomonas putida to obtain pyridine dicarboxylic acid, as a pre-
cursor of bioplastics (G´
omez-´
Alvarez et al., 2022).
Fenton oxidation has also been used for lignin depolymerization.
Cronin et al. (2017) have used sodium percarbonate to enable depoly-
merization and subsequent extraction of dicarboxylic acids from lignin
(Cronin et al., 2017). The alkaline reaction medium provided by the use
of this chemical agent allows to reduce the amount of waste produced, as
thermal degradation is prevented, which has been the main drawback
detected by other authors when using Fenton oxidation procedure (Ma
et al., 2014).
This opens the scope of potential uses of lignin. Its valorization
beyond energy production has been gaining importance and is one of the
main current research topics. In fact, some industries have also focused
on marketable products from lignin, as is the case of Stora Enso (“Stora
Enso,” n.d.), which has developed different lignin-based products:
Lineo®, lignin to replace phenol in resins and to manufacture biode-
gradable polymers, Lignode®, a lignin hard carbon battery to replace
lithium-ion batteries, Neoligno®, a bio-based binder with uses in the
manufacture of particleboard and insulation products and Neober®, a
renewable carbon ber composed of cellulose and lignin. METGEN is
another industry that has developed METNIN
TH
, which is a lignin
rening technology based on converting lignin into a more accessible
molecule to take advantage of its molecular compounds. And, lignin
biopolymers are also produced by Borregaard, located in Norway and
one of the most advanced sustainable bioreneries based on wood ma-
terials (“Borregaard,” n.d.).
4.2. Lipid extraction alternatives for valorizing SCGs
The lipid composition of SCGs (around 16% w/w) makes this bio-
based by-product a potential feedstock for biodiesel production (Go
et al., 2020; Loyao et al., 2018), given its availability, its affordable price
and the high-quality of the biodiesel produced (Muharam and Ram-
adhany, 2021).
To obtain them, both mechanical extraction and solvent extraction
could be developed, the latter being the most efcient in terms of yield,
due to the fact that it is more convenient when the lipid content is lower
than 20% w/w, as is the case of SCGs (Koubaa et al., 2016). Different
solvents has been used by authors: hydrocarbons, such as n-octane
(Caetano et al., 2013), n-hexane (Efthymiopoulos et al., 2019) or toluene
(Al-Hamamre et al., 2012), alcohols, such as ethanol and isopropanol
(Battista et al., 2020; Son et al., 2018), esters, with ethyl acetate being
the most common (Go et al., 2020; Loyao et al., 2018; Supang et al.,
2022), and mixed solvents, based on the combination of hexane
(Ahangari and Sargolzaei, 2013) with isopropanol or methanol (Chol-
akov et al., 2013; Efthymiopoulos, 2018). The co-occurrence of key-
words used by authors in the research articles based on the recovery of
lipids from SCG is depicted in Figure 6SM.
Regarding n-hexane, Efthymiopoulos et al. (2019) has investigated
which are the most convenient extraction conditions to allow higher
extraction yield, leading to optimized productivity of the recovery
process (Efthymiopoulos et al., 2019). Large scale extraction has been
evaluated, to have more accurate values closer to those of an industrial
level approach. According to the report, the main ndings were the
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
5
extraction duration, which should be less than 10 h to avoid the
decrease of the extraction yield, being 8 h the optimal value, for the
laboratory level case. This time is reduced to 2 h when pilot scale ap-
proaches are developed, reaching results similar to those of the labo-
ratory level. On the other hand, it has been concluded that the moisture
content of SCGs directly affects the overall productivity of the recovery
process, with a value of 10% moisture leading to the highest yield
(Efthymiopoulos et al., 2019).
In search of more environmentally friendly options for lipid extrac-
tion from SCGs, solvent alternatives have been evaluated. This is the
case of aqueous 2-methyloxolane, a bio-based and safe solvent (Claux
et al., 2021; Gharby et al., 2020), used in a conventional Soxhlet system,
requiring a solid-to-solvent ratio of 1:10 and a process time of 6 h. This
alternative has been compared to hexane, and a higher yield has been
achieved using 2-methyloxolane, probably due to the higher polarity
and solubilization for polar lipids, as is the case for phospholipids.
Among them, it has been identied that triacyl glycerides enabled the
highest extraction yield, reaching 94% (Chemat et al., 2022). The sol-
vent 2-methyltetrahydrofuran produced from renewable lignocellulosic
resources has also been used for lipid extraction (Pace et al., 2012). Its
use leads to a higher lipid extraction yield, when compared to hexane,
almost 10 points higher, making it a potential alternative in lipid re-
covery from SCGs (Mkhonto and Chetty, 2021).
Not only greener solvent extractants were used, but also emerging
extraction technologies, such as supercritical CO
2
extraction (Muharam
and Ramadhany, 2021). It has been reported that, as expected, extrac-
tion conditions directly affect lipid yield, with increased pressure, at
constant pressure, reduced particle size and higher solvent owrate
(Couto et al., 2009; Muangrat and Pongsirikul, 2019; Muharam and
Ramadhany, 2021).
4.3. Phenolics extraction alternatives for valorizing SCGs
The polyphenol content of SCGs makes them a potential source of
bioactive compounds, although there is not much research on this topic,
at least on a large scale, both from an economic and technological point
of view (Gąsecka et al., 2020). However, recent research articles have
been focused on the recovery of phenolic compounds from SCGs, as
these products could give signicant and raw value for biorenery
development. The market for phenols has a high added value, with a
multitude of applications in sectors such as cosmetics and medicine
(Badr et al., 2022; Bondam et al., 2022).
Chlorogenic acids (CGAs) are the main components of the phenolic
fraction of SCGs, and usually the total content of phenolic compounds is
expressed as milligrams of gallic acid equivalent (mg GAE). (Panusa
et al., 2013) characterized the SCG extracts in terms of their composition
in total phenolic content and antioxidant activity, and also evaluate the
possible changes that could occur in terms of composition and antioxi-
dant activity by replacing aqueous ethanol with pure water as extraction
solvent. These authors reported that SCG is a rich source of natural
phenolic antioxidants, as it contains a high percentage of residual CGAs.
Furthermore, although the use of ethanol and water can dissolve a wider
range of phenolic compounds and avonoids than the case where only
pure water is used as solvent, not all types of CGAs are affected by the
type of solvent. Thus, the alternative of using pure water to produce
extracts rich in specic CGAs is preferable and desirable, since the
reduction in the amount of chemicals leads to both lower costs and
environmental impacts.
The evaluation of different parameters for the extraction of phenolic
compounds using solvents, as well as the use of alternative techniques
that are considered environmentally friendly for the efcient extraction
of phenolic compounds, are common topics in the literature. Solid-liquid
extraction with organic solvents, ultrasound-assisted extraction,
microwave-assisted extraction, supercritical uid extraction and high-
pressure processes are some of the alternative techniques. (Solomakou
et al., 2022) presented the conventional methodology for the extraction
of phenolic compounds, as well as three different alternatives, namely
ultrasound-assisted extraction, microwave-assisted extraction, and sol-
vent extraction using β-cyclodextrin as solvent. The factors evaluated
were temperature, solvent concentration, liquid/solid ratio and power.
The optimum extraction yield (31.79 ±0.25 mg GAE/g SCG) was ach-
ieved using microwave-assisted extraction, whereas the lowest yields
were obtained with β-cyclodextrin as solvent. (Solange I. SolangeI.
Solange I. SolangeI. Mussatto et al., 2011; S.I. Mussatto et al., 2011) also
reported that the extraction of phenolic compounds was affected by the
methanol concentration, solvent/solid ratio and extraction time used.
The maximum value of phenolic compounds extracted from SCG was
18 mg GAE/g SCG, which was obtained using 50% methanol at a ratio of
25 ml per g SCG, for 90 min
Further processing of the extracted phenolic compounds has been
evaluated to preserve their properties for a longer time. Encapsulation of
these compounds is an important strategy as the phenolic compounds
could be protected from oxidation as the coating material acts as a
barrier against oxygen and water. Typical encapsulation techniques are
usually based on spray drying, uidized bed drying, uid bed coating
and freeze drying, due to the liquid nature of the extracts containing the
bioactive compounds (Ballesteros et al., 2017).
The bioactive and phenolic content of SCGs, with anti-inammatory,
neuroprotective, antimicrobial, and anticancer properties (Dorsey &
Jones, 2017), suggest their use as food supplements and ingredients, as
well as in cosmetic products. There is a wide variety of products on the
market that contain formulations based on coffee extracts or its by-
products, as shown in Table 4. For example, the company Pectcof
developed a product called Dutch Gum that has emulsifying and stabi-
lizing properties and is formulated from coffee pulp; this is sold as an
ingredient to the food and beverage industry (Pectcof, 2020). In addi-
tion, the company Aqia Nutrition has developed a line of products called
AQIA coffee (AQIA, 2020), based on green coffee and cherry coffee.
Among the products marketed are green coffee and cherry coffee oils,
extracted by cold pressing coffee seeds (Bondam et al., 2022).
4.4. Fermentative production of chemicals and polymers by valorizing
SCGs
Butanediol is a chemical compound with many applications in in-
dustry, as it is used in polyesters, cosmetics, pharmaceuticals, food ad-
ditives and fertilizers, among others. Its usual production route is based
on acid fermentation, but various co-products are obtained, such as
ethanol, lactate, acetone, etc., depending on the type of microorganism
and the fermentation conditions. The microbial production of
Table 4
Some examples of commercialization and application of SCGs in the cosmetic,
pharmaceutical and food products.
Commercial
product
Bioactive
compound
Application Reference
Cosmetic Caffeine and
chlorogenic acid
Anti-photoaging
agent
(Choi and Koh,
2017)
Sunscreen Coffee oil Skin treatment and
protection
(Kanlayavattanakul
et al., 2021)
Pharmaceutical 5-caffeoylquinic
acid
Anti-inammatory (Marto et al., 2016)
Cosmetic Chlorogenic
acid
Anti-wrinkle effects (Cho et al., 2017)
Food ingredient 5-caffeoylquinic
acid
Higher
nutraceutical value
(Bertolino et al.,
2019)
Cosmetic Phenolics Skin antiaging and
lightening effect
(Ribeiro et al., 2018)
Food ingredient Rosmarinic and
syringic acids
Antifungal, anti-
mycotoxigenic and
anti-cytotoxic
effect
(Badr et al., 2022)
Cosmetic Coffee silver
skin
Phyto cosmetic
effects
(Rodrigues et al.,
2016)
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
6
butanediol from Cellulosimicrobium cellulans (Ribeiro et al., 2020) could
be developed by considering a sumerged coffee fermentation in a
controlled pH medium and addition of pectinolytic enzymes, but also
using Rhizopus oligosporus (Lee et al., 2016) with a solid-state fermen-
tation. This butanediol could also be used to produce polymers, as it is a
precursor of polyurethanes, and other products, such as methyl ethyl
ketone, a fuel additive with many applications to produce additives,
resins or other solvents (Hazeena et al., 2020; Tinˆ
oco et al., 2021).
Another microorganism used for the valorization of SCGs within a
solid-state fermentation (SSF) process is Aspergillus sp. for the biotech-
nological production of polyphenols such as chlorogenic, quinic and
caffeic acids, compounds recognized for their antioxidant and neuro-
protective properties. After a SSF process with hydroalcoholic extrac-
tion, an increase of 2.3 times g GAE/kg is obtained compared to the use
of the simple and conventional hydroalcoholic extraction with ethanol
(Arancibia-Díaz et al., 2022), in addition to the lower amount of solvent
associated with the SSF process.
In the nutraceuticals section, SCGs could be used as a source of
prebiotics, after a process of acid hydrolysis and incubation with lactic
acid bacteria (Prasanna and Rastall, 2017; Varzakas et al., 2018). The
results obtained showed that SCG extracts have a more effective prebi-
otic capacity compared to that of inulin, an established and commonly
used commercial prebiotic (Sarghini et al., 2021).
Another alternative for the valorization of SCGs is the production of
biopolymers, potential substitutes for petroleum-derived polymers such
as plastics. Biopolymers are biocompatible and degradable with physi-
cochemical, thermal and mechanical properties very analogous to those
of petro-based origin (Saratale et al., 2020; Saratale and Oh, 2015).
Stanislav et al. (2014) explored that the production of PHBs [poly
(3–hydroxybutyrates)] by Cupriavidus necator H16 in culture media
containing SCGs (Stanislav et al., 2014).
Aerobic fed/discontinuous aerobic fermentation was performed with
an initial coffee oil concentration of 30 g/L, at a controlled neutral pH
medium and at room temperature. In the case of a fed-batch fermenta-
tion, in the feed stage, an additional amount of 20 g/L coffee oil and 3 g/
L ammonium sulfate (as a nitrogen source) need to be added to increase
the yield and productivity of the process (Stanislav et al., 2014).
Furthermore, one of the advantages of using oil from SCGs as a bio-
resource for PHB production is based on the fact that the weight per-
centage of the polymer in the biomass produced is around 90%, which
facilitates its isolation and the subsequent steps required. C. necator has
also been used to produce PHA [polyhydroxyalkanoate], in a mineral
medium supplemented with 20 g/L of SCG oil, previously extracted by a
semi-continuous supercritical extraction. After 48-h incubation time, the
polymer must be extracted, using lyophilized cells and chloroform as
solvent, followed by a ltration step under vacuum conditions. With this
biotechnological route and operating conditions, a polymer content of
78% w/w and a yield of 0.77 kg PHA/kg SCG oil was obtained, with a
molecular weight of 2.34⋅10
5
and a low polydispersity index (Cruz et al.,
2014).
5. Assessing the environmental loads and economic protability
of SCGs valorization
5.1. Life cycle assessment for the environmental evaluation of SCGs
When it comes to assessing the sustainability of a new production
process under development, it is important to establish what environ-
mental benets it brings and what the advantages are compared to
existing production schemes at the industrial production level. The new
biorenery concept is usually developed on the basis of the utilization of
unusable waste resources, the direct management of which is mostly
based on landll disposal. This type of management, although one of the
most widespread, is not the most interesting from an environmental
point of view, since the emission of particles, as well as the production of
gases, mainly methane and carbon dioxide, and the damage to the
landscape, give rise to signicant environmental impacts. For this
reason, more and more efforts are being made to valorize these by-
products as feedstock for the development of bioreneries, i.e. various
cascading products. In most cases, energy valorization appears as the
most developed option, but given the richness of the biochemical
composition of SCGs, a wide variety of high value-added products can be
obtained, as introduced in the previous sections of this critical review.
But, in order to select which of them is the most suitable, or to determine
which one contributes more positively to the concepts of circular
economy and sustainability, the use of the Life Cycle Assessment
methodology is essential. This methodology is based on the evaluation
of the potential environmental burdens that may result from the
development of a given production process or the manufacture of one or
more products from SCGs. However, given the lack of development of
biorenery processes based on SCGs on a large scale, life cycle analysis
studies are not extensive in the literature, given the lack of necessary
inventory data.
Several scenarios were evaluated in which different SCG manage-
ment strategies were combined, including biodiesel production, anaer-
obic digestion, composting, direct application to cropland, incineration
and thermal energy generation, and landlling with recovery of the
generated biogas for electricity generation (Schmidt Rivera et al., 2020).
Among all the options evaluated, it was found that anaerobic digestion
and direct application of the SCGs as fertilizer were the most environ-
mentally sustainable options (Schmidt Rivera et al., 2020). On the other
hand, different routes for biodiesel production have also been evaluated,
although the conventional one is based on a rst solvent extraction, to
continue with a 2-step transesterication with an acid pretreatment and
using NaOH as catalyst, an attempt has been made to develop a new
process based on transesterication (Tuntiwiwattanapun et al., 2017).
In this case, the aim would be to reduce the number of process steps, as
well as the use of different types of solvents, such as the catalyst or the
rst extractive solvent. However, it was concluded that the energy
consumption of the conventional process was signicantly lower than
that of the transesterication one, resulting in a lower environmental
impact, as the energy requirement contributes to the consumption of
fossil resources, which implies a high environmental burden. (Yang
et al., 2021) have compared two thermochemical valorization routes,
one focused on a rst biodiesel extraction followed by a hydrothermal
liquefaction of the defatted SCGs to produce biocrude, and the other
based on the production of biocrude directly from the HTL of the raw
SCGs. The lower yield of the route 1, compared to that of direct HTL, led
to a greenhouse gas emission value of almost 3 times higher, being
297.6 g CO
2
eq /MJ for route 1 and 103.3 g CO
2
eq/MJ for the direct
conversion (Yang et al., 2021).
On the other hand, it is also important to evaluate which stages of the
SCGs valorization process for the production of biodiesel have the
greatest environmental impact and, therefore, required a greater degree
of improvement or optimization in order to reduce the environmental
damage of the process. Although the drying of the SCGs gives rise to a
large energy requirement, this is not the stage with the greatest envi-
ronmental contribution, the oil extraction stage being more than 10
times higher, according to the (Bui et al., 2021) research report, given
the enormous amount of solvent required for extraction, in addition to
the energy requirements, both electrical and caloric.
5.2. Techno-economic assessments for considering the protability of
SCGs valorization
Usually, the development of an environmental analysis using the
LCA methodology is combined with a techno-economic evaluation
(TEA), as it is also important to consider the protability of the process.
No matter how environmentally friendly a process may be, if it is not
adequate in economic terms, i.e., if it does not generate sufcient ben-
ets to offset all costs and, in addition, generate income, the biorenery
approach developed will not be implemented at an industrial production
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
7
level. This has been the concern in the studied developed by (I. K.
Kookos, 2018), as in his research it has been evaluated the environ-
mental and techno-economic analysis of the production of biodiesel,
glycerol and electricity using SCGs as raw materials. It has been
conrmed the fact that the environmental loads of the process are
comparable to the BATs (Best Available Techniques) in the production of
biodiesel, but the large-scale production of this biorenery is only
economically affordable when a centralized manufacture is developed.
(Banu et al., 2021) have considered different biorenery pathways
on the valorization of SCGs under a cascade approach. Given the
chemical and physical properties of SCGs, there is a wide range of pos-
sibilities in its recovery, from the production of biofuels, bioplastics and
biopolymers to the manufacture of polyurethane foams and bioactive
compounds, a carotenoids and antioxidants. According to the most
recent research articles, the most protable SCGs bioreneries scenarios
are the ones based on the production of biomolecules, derived from the
saponication and neutralization of the free fatty acids (FFA) obtained
from the extracted oil of SCGs (De Melo et al., 2014).
Another common route of valorization relies on the use of SCGs for
bioethanol production, following an ABE fermentation procedure. It also
requires a pretreatment stage, based on a milling procedure, to increase
the productivity of the following stages (as it leads to an increase on the
surface area allowing a higher available contact surface) and an acid
hydrolyzation with sulfuric acid. Afterwards, an enzymatic sacchari-
cation is carried on, using a cellulase enzyme, with the aim of obtaining
the fermentable sugars for the ABE fermentation, in a free-form, as the
solid residue containing the lignin is separated by ltration (Carmo-
na-Garcia et al., 2019). This valorization route also gives economic
protability, with revenues that amounts to $ 6.84 M/y.
Electricity production has also been considered as an alternative
valorization route for SCGs, the extraction of oil is also required, but
afterwards it is needed to develop a transesterication reaction using
methanol and an acid catalyst, for obtaining the bio-glycerol, which will
be used for the power production by its combustion. Even though this
valorization route is not taking advantage of the biomolecules and
polyphenols of the composition of SCGs, the protability of this alter-
native, which revenues could amount to $ 1.461 M/y (I. K. Kookos,
2018).
In the eld of biodiesel production (Thoppil and Zein, 2021) and
despite the fact that it could be considered as a sustainable approach,
since a renewable biofuel is produced from waste, it does not reach
economic protability (Thoppil and Zein, 2021). The main reason is the
low selling price, which leads to a revenue value that is not sufcient to
offset the cost associated with the purchase of equipment, utilities,
operation, labor and other direct/indirect costs. A similar conclusion
was raised from a TEA analysis (I. K. Kookos, 2018), which reported that
the economic viability of SCGs for biodiesel production is difcult to
achieve when considering production capacities below 42 t/year. The
availability of SCGs could be considered for this purpose as a bottleneck
in their valorization pathways. Likewise, the market price of the product
and feedstock transportation could be listed as key aspects to ensure the
protability of bioreneries (Crist´
obal et al., 2018).
6. Sustainability and circular economy with SCGs
The EU Circular Economy Action Plan aims to move from linear to
circular production. To this end, the utilization of waste streams, and co-
products, as resources for other facilities has turned out into a real
possibility. The increasing population and high demand for coffee
beverage has led to the production of large quantities of coffee waste,
specically SCGs, making them potential sources for developing bio-
renery processes. But, seeking to gain an advantage in using these
biomass-based renewable resources, it has been reported that such
bioreneries should be developed on a large scale and centralized,
meaning that they should be placed next to the coffee factory (Yeoh and
Ng, 2022). Furthermore, SCGs bioreneries have been shown to be
suitable for meeting biodiesel demand, with lower GHG emissions and
similar production costs, making them a potential alternative to fossil
fuels (Mayson and Williams, 2021; Yeoh and Ng, 2022). This conclusion
is based on the fact that, although the energy potential of SCGs is lower
compared to fossil fuels, their energy content is higher than that of other
conventional biomasses used as biological resources for energy pro-
duction. In fact, SCGs have a disadvantage, when assessing their envi-
ronmental suitability, as there is a high concentration of nitrogen in
their composition, leading to the production of NOx emissions (Mayson
and Williams, 2021).
Furthermore, taking into account circular economy and cascading
production approaches, it has been assessed that the use of SCG only for
energy production is not cost-effective. The need for recovery of
bioactive compounds, with many applications in pharmaceuticals, cos-
metics and medical products, has become a necessity to ensure both
environmental and economic suitability of SCGs bioreneries (Bijla
et al., 2022; Massaya et al., 2019). To this end, the development of novel
technologies for the extraction and recovery of bioactive compounds
should be the main focus of future research, seeking to achieve new
biorenery approaches that can be categorized within the 12 Principles
of Green Chemistry.
7. Conclusions
The availability of spent coffee grounds, and their suitability for use
as a raw material in biorenery processes, have made them potential
candidates for obtaining bio-based and bioactive compounds. The
different recovery routes that could be developed are benecial for
circular economy approaches and the preservation of fossil resources, as
waste becomes a raw material input, also favoring the industrial sym-
biosis strategy: "your waste, my feed". This review has focused on the
evaluation of SCG valorization alternatives to obtain lignin, lipids,
polymers and phenolics, key products for the development of other
production routes. From lignin, production of bioadhesives for wood,
from lipids, bio-oil, bioplastics obtained from SCG polymers and phe-
nolics, with a multitude of applications in cosmetics, food and phar-
maceutical industries. The LCA and techno-economic literature reports
available have also been evaluated, as environmental awareness and
economic protability are essential aspects to develop large-scale pro-
duction strategies. To this end, it is hoped that this review article can be
used as a reference for researchers, decision-makers and stakeholders to
decide where to focus on SCG valorization under a biorenery approach.
CRediT authorship contribution statement
A.A.: Methodology, Formal analysis, Investigation, Writing – orig-
inal draft. S.M.I.: Investigation, Writing – original draft. N.G.: Formal
analysis, Investigation. G.F.: Supervision, Writing – review & editing.
M.T.M: Supervision, Writing – review & editing. A.K.: Conceptualiza-
tion, Formal analysis, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interest or personal relationships that could have appeared to inuence
the work reported in this paper.
Data Availability
No data was used for the research described in the article.
Acknowledgements
This research has been nancially supported by the European Com-
mission HORIZON-CL6–2021-ZEROPOLLUTION-01 (Grant Agreement
101060684 and 101060588). AA, GF and MT belong to the Galician
A. Arias et al.
Industrial Crops & Products 205 (2023) 117504
8
Competitive Research Group (GRC ED431C 2017/29) and to the Cross-
disciplinary Research in Environmental Technologies (CRETUS
Research Center, ED431E 2018/01).
Appendix A. Supporting information
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.indcrop.2023.117504.
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