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Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

Contents lists available at ScienceDirect

Food and Bioproducts Processing

j ourna l ho me page: www.elsev ier .com/ locate / fbp

onceptualization of a spent coffee groundsiorefinery: A review of existing valorisationpproaches

ackie Massayaa, André Prates Pereiraa, Ben Mills-Lampteyb,ack Benjaminb, Christopher J. Chucka,∗

Department of Chemical Engineering, University of Bath, Bath, BA2 7AY, UKBio-bean Ltd, Unit 4002, Alconbury Weald Enterprise Park, Alconbury, Huntingdon, PE28 4WX, UK

r t i c l e i n f o

rticle history:

eceived 28 January 2019

eceived in revised form 2 August

019

ccepted 13 August 2019

vailable online 20 September 2019

a b s t r a c t

The valorisation of food waste is an increasingly practical and sustainable solution to the

problem of a growing demand for chemicals, fuels and materials and the rising tonnage

of municipal waste sent to landfill. Spent coffee grounds (SCG) are the end product of the

coffee processing industry, generated after beverage preparation, and have been exploited

as a valuable source of polysaccharides, lipids, protein, minerals and bioactive secondary

metabolites including diterpenes, sterols, chlorogenic acids, flavonoids and caffeine. Within

the biorefinery paradigm, where renewable resources are converted into a range of high,

medium and low value products, in an analogous manner to fossil fuels in a petrochem-

ical refinery, SCG have been established as an amenable lignocellulosic feedstock through

numerous research efforts. In this critical review, we give an extensive overview for the first

time of the primary and secondary product suites that can be generated from SCG, along

with their potential applications. The handful of preliminary technoeconomic and lifecycle

assessment of using SCG for bioenergy is discussed, highlighting the economic limitations

of a single capability, phase one biorefinery operating under the current scale and logistics

of SCG collection. A concluding perspective towards future SCG-based biorefineries is pre-

sented, where isolation and production of higher value bioactive products is expected to be

integral to the economic feasibility of the process.

© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

. Introduction

lobal coffee production in 2017/18 totalled 160 million 60 kg bags, an

ncrease of 36% from 116.5 million bags that were produced in 2007/8

International Coffee Organization, 2011, 2018). This growth in cof-

ee trade reflects a rise in global coffee consumption, where coffee

as become an everyday commodity that is firmly entrenched in the

aily routines of people worldwide. With rising rates of consumption,

aste residues from the coffee industry (by-products from harvesting,

rocessing, roasting and brewing stages of coffee production and pro-

essing) represent a challenge to EU directives aims of reducing landfill

European Commission Publications Office, 1999). The inherent toxic-

ty of several constituents within coffee also present an environmental

∗ Corresponding author.E-mail address: c.chuck@bath.ac.uk (C.J. Chuck).

ttps://doi.org/10.1016/j.fbp.2019.08.010960-3085/© 2019 Institution of Chemical Engineers. Published by Elsev

contamination concern (Fernandes et al., 2017). Research efforts are

therefore focussed on valorising coffee industry residues within the

circular economy paradigm, reducing the tonnage sent to landfill by

exploiting the biomass as a potential feedstock (Mata et al., 2018).

In the last decade, a large body of research has been published to

this effect, suggestive of the suitability of using SCG as a feedstock

for bioprocessing. For example, in depth reviews have focussed on the

compositional analysis of spent coffee grounds (Campos-Vega et al.,

2015), the suitability of the feedstock for fermentation (Kovalcik et al.,

2018b), for bio-gas manufacture (Girotto et al., 2018), for bioenergy prod-

ucts (Karmee, 2018), and more recently using spent coffee grounds

as a basis for a biorefinery within the circular economy (Zabaniotou

and Kamaterou, 2019). Such biorefineries typically integrate chemical

and bioprocessing platforms yielding concentrates of saccharide and

antioxidant mixtures and ethanol (Burniol-Figols et al., 2016) or alter-

native further products are combined into this paradigm (Mata et al.,

2018). However, whilst reviews (McNutt and He, 2019; Stylianou et al.,

ier B.V. All rights reserved.

150 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

2018) and a number of further research articles (Nguyen et al., 2019)

have established some of the potential products available from SCG, the

pertinent issue of what can be completely extracted from SCG and the

economic and practical feasibility of a coffee based biorefinery remains

to be addressed.

Spent coffee grounds are an ideal substrate for the bioeconomy.

While coffee is one of the most highly consumed beverages worldwide,

only about 30% of the grounds will be solubilised in the coffee. This

creates a large waste stream that typically is disposed of into water

courses and landfills, leaching active biochemicals into the environ-

ment (Fernandes et al., 2017). However, with collection and valorisation

this waste stream can be used for the production of fine chemicals and

energy displacing a small fraction of current fossil feedstocks.

Accordingly, this paper aims to explore the potential of SCG as

a commercial feedstock for sustainable processes. We begin by out-

lining the main stages within the coffee process chain, highlighting

the physicochemical properties of the coffee cherry at each step. We

then review the literature on the composition of SCG (and therefore

the possible primary products), the secondary product systems that

have been developed to exploit the composition, and the various end-

product applications reported for SCG. Subsequently, we consider the

issues concerning the logistics and supply chain of a SCG biorefin-

ery, both of which are intrinsic components of process operations. We

then conclude with a critical evaluation of published technoeconomic

assessments and a perspective on the viability of a SCG based biorefin-

ery beyond the lab bench — is it truly possible to generate value on an

industrial scale?

2. Composition of the coffee bean

Coffee is the generic term (genus Coffea) for a group of flower-ing plants of the Rubiaceae family. The seeds of coffee plantsare referred to as coffee beans, which are cultivated and har-vested for human consumption. Coffee grows in a regionbetween the tropic of Cancer and Capricorn, taking up to fiveyears for a tree to bear its first crop (Murthy and MadhavaNaidu, 2012). Two species of the Coffea genus are importantin the international trade of coffee: Coffea Arabica L. and Cof-fea Canephora P. widely referenced as Arabica and Robusta,respectively.

The cherry is comprised of two main parts: the pericarp andthe seed. The pericarp includes the three outermost layers ofthe fruit, the skin (exocarp), mucilage (mesocarp) and parch-ment (endocarp). In coffee processing, the pulp often refers tothe mucilage and part of the skin and is a waste stream in thepost-harvest stage. The parchment is also known as the hull,a thin layer surrounding the coffee bean. There are two seedsat the heart of the coffee cherry. Each is comprised of a silverskin, endosperm and embryo. The endosperm is the tissueof the bean, containing holocellulose, sugars, lignin, proteins,oils, bioactives (including chlorogenic acids, trigonelline, nico-tinic acid, caffeine) alkaloids and minerals. Flavour and all itscomplex determinants (including taste and aroma) are derivedfrom the endosperm, whose composition varies according toenvironmental factors, such as origin, climate, altitude, fertil-izer use, processing techniques, degree of roasting, and finallybrewing method (Sunarharum et al., 2014). The silverskin sur-rounding the endosperm is lost from the bean during theroasting process as “chaff” — a waste residue (Mussatto et al.,2011c).

The aims of coffee harvesting are traditionally coupledwith the post-harvest process. Wet or semi-dry method post-harvesting maximises the quantity of ripe cherries, whilstdry method post-harvest processing prioritises harvest of all

cherries (ripe, over-ripe, unripe), minimising the proportionof unripe. Product quality and costs also heavily influence

the mode of harvest, which can be carried out manually ormechanically. Manual harvesting is conducted by selectivehandpicking, using the colour of the cherry as an indicator ofripeness, hence its readiness for harvest, or stripping whichindiscriminately removes all cherries from a branch. Mechan-ical harvesters are typically on wheels or hand-held, usingshaking and vibrations, respectively, to initiate cherry fall.

Beans with physical defects result from processing unripe,overripe or partially dried cherries along with fresh crop.Defective beans (such as “black-beans”) bestow unwantedqualities to the final beverage such as harshness, a ‘green’ oreven alcoholic taste. Therefore, a separation process usuallyprecedes post-harvest processing to minimise the presence ofunsuitable cherries (Brando, 2008).

Two main methods transform the harvested crop into agreen bean that is ready for storage and export: wet-method(or washed) and dry-method (unwashed). Related semi-dryand mechanical methods have emerged to resolve problems ofprocessing immature fruit or water scarcity (Alves et al., 2017).The objective of each method is to remove the pulp (mucilageand skin) surrounding the parchment, to enhance cup quality(and therefore market price of the green bean).

The beans are then stored for several months before roast-ing. Storage of green and roast coffee beans can significantlypromote growth of Ochratoxin A (OTA), a mycotoxin producedby Aspergillus and Penicillium species (Farah, 2012). OTA is aknown nephrotoxin and hepatoxin, with potential mutagenic,carcinogenic, teratogenic activity- and current EU regulationlimits its concentration at 5 and 10 �g/kg for roasted andinstant coffee beans respectively (European Union, 2005).

During roasting, thermal convection of hot gases flowingover a moving bed of green beans initiates pyrolytic reac-tions and the release of carbon dioxide. The process is initiallyexothermic from 190 to 210 ◦C, yet the release of volatilecompounds from the bean matrix transitions the processto be endothermic. After the loss of volatiles, the tempera-ture increases again, after which cold air or water is usedto quench the roasting process (Alves et al., 2017; Buffoand Cardelli-Freire, 2004). Roasting dramatically alters thechemical composition of coffee, determining the organolepticqualities and colour of the roasted bean.

Over 800 volatile compounds that are responsible for thearoma (flavour and fragrance) cup quality have been identifiedin roasted and ground coffee. These include acids, aldehydes,alcohols, sulfur compounds, phenolics, pyrazines, pyridines,thiophenes, pyrroles and furans (Yang et al., 2016b). Reactionpathways that lead to formation of volatile and non-volatilecompounds during roasting remain to be defined but areknown to include:

• Strecker Degradation: transforming alpha-amino acids intoaldehydes or ketone meat flavour related compounds andreleasing carbon dioxide.

• Maillard reactions: carbonyl groups of reducing sugarsand amino groups of proteins and amino acids react toform compounds of various molecular weight, includingmelanoidins, aldheydes, ketones, dicarbonyls, acryl amidesand hetrocyclic amine moieties and advanced glycation endproducts (AGEs). Maillard reaction products (MRPs) impartthe characteristic brown colour to coffee, exhibit antiox-idant activity and modest cytotoxicity and genotoxicity(Wang et al., 2011).

• Caramelization: a series of sugar decomposition reactionsreleasing brown-coloured volatile and non-volatile com-

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 151

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pounds. These compounds are associated with a sweet,nutty, caramel flavour (Kroh, 1994).

Important metabolites such as caffeine are notltered under roasting conditions, yet partial degrada-ion of trigonelline into nicotinic acid (vitamin B3) and-methylnicotinamide derivatives, and isomerization,pimerization, degradation and lactonization of chlorogeniccids (esters of hydroxycinnamic and quinic acids) occursFarah, 2012; Kucera et al., 2016; Sunarharum et al., 2014).eoformed toxic components of roasted coffee include acry-

amide (ACR) and hydroxymethylfurfural (HMF), with knownarcinogenic and cytotoxic properties (Tokimoto et al., 2005).

Duration and temperature of roasting (typically 8 to 15 min,80–240 ◦C, respectively) are therefore controlled to tune theomplex array of reactions that ultimately lead to desiredroma and colour characteristics of the bean (Sunarharumt al., 2014). Bean colour directly correlates with degree ofoast: darker beans are associated with longer and/or highemperature (dark) roasts, lighter beans are derived fromhorter and lower temperature roasts.

A major by-product of roasting is silverskin (thin layer sur-ounding the bean), which is lost as chaff. For approximately

tons of industrially roasted coffee, 30 kg of chaff is produced-hich is primarily sent to landfill. Silverskin is therefore aaste-stream of the coffee processing industry that is pro-uced in considerable quantities (Alves et al., 2017).

.1. Brewing

pon roasting, a beverage may be prepared (brewed) from hotater and roasted coffee beans that have been ground to a size

ppropriate for the extraction method. Typical brewing meth-ds involve pressure (espresso), infusion (“French-press”) orravity filtration (filter), with grind-size, water/solid ratio, tem-erature and extraction time additionally influencing the finalrew composition for a given technique.

Brewing only partially extracts components found in cof-ee, leaving many secondary metabolites, carbohydrates,ipids, proteins, minerals and other chemical compounds ofnterest in the residual solids, referred to as spent coffeerounds (SCG). SCG are thought to be produced globally at

rate of 6 million tonnes per year (Kovalcik et al., 2018b;ussatto et al., 2011c; Tokimoto et al., 2005). However, taking

nto account the growing consumption rates since the figurerst appeared in the literature (2005), it is most likely that SCGre generated in much larger quantities, presenting a majory-product of the coffee processing industry.

The instant/soluble coffee industry further processes roastrounds by aqueous extraction, obtaining a freeze-dried solu-le fraction after concentration and dehydration of the waterAlves et al., 2017). These grounds (sometimes referred to asxhausted coffee residues or SCG) are thought to be producedt a 1:2 ratio of wet SCG: soluble coffee, again presentingnother challenge for waste management (Pfluger, 1975).

. Primary product suite from spent coffeerounds

offee is a natural composite, comprised of crude fibrelignin, hemicellulose, cellulose, poly- oligo- and mono-

accharides), lipids (triacylglycerides, free fatty acids andterols), nitrogenous compounds (proteins, peptides free

amino acids, melanoidins) and minerals. Coffee also con-tains minor quantities of biologically active species namely,alkaloids (caffeine, trigonelline), diterpenes (cafestol andkahweol), polyphenols (chlorogenic acids (CGA), tannins,tocopherols and anthocyaninins), which are responsible forthe observed antioxidant, antimicrobial and anticarcinogenicactivity of coffee brews (Almeida et al., 2006; Cavin et al.,2002; Daglia et al., 2007; Farah et al., 2008; Shahidi andChandrasekara, 2010).

The chemical composition of coffee varies along its processchain — for example melanoidins formed during the roastingprocess are not present in green coffee (Buffo and Cardelli-Freire, 2004). In addition, factors associated with the harvest(climate, soil quality, species, variety, maturity at harvestetc.), post-harvest (depulping method, storage etc.), roasting(temperature, time etc.) and brewing method determine theidentity and quantity of components found within the matrixof coffee (Alves et al., 2017). Finally, extraction and characteri-sation methods used to isolate and identify the constituents ofcoffee are similarly influential to its chemical profile (Jenkinset al., 2014). Table 1 displays a compilation of reported val-ues for the components of green and roasted coffee, as wellas the major-by products of the coffee-processing industry:coffee husks, pulp, silverskin and spent coffee grounds (SCG).

SCG contain hemicellulose (32–42 % w/w), cellulose (7–13% w/w), lignin (0–26 % w/w), protein (10–18 % w/w), lipids(2–24 % w/w), CGAs (1–3 % w/w), caffeine (0–2 % w/w) andashes (1–2 % w/w), (Table 1). The inorganic fraction containsabundant microelements including potassium, magnesium,calcium, iron and phosphorous (Mussatto et al., 2011a). Withinthe biorefinery paradigm, SCG are a pertinent lignocellulosicfeedstock. The considerable quantities of bioactive chemi-cals in SCG are suitable for recovery and valorisation withinnutraceutical, pharmaceutical, food or fine chemical indus-tries. Their associated ecotoxicity, where coffee waste extractshave been demonstrated to be toxic to aquatic organisms andeven induce mutagenicity and genotoxicity, presents a furtherresearch motivation to prevent their accruement in landfill(Fernandes et al., 2017). However, it must be noted that therecovery of toxic sugar degradation products, such as fur-fural, which can accumulate in the wastewater and/or sludgeeffuents of biorefineries, remains imperative to minimisingthe environmental impact of operations.

With such a complex feedstock, research has tradition-ally focused on the characterisation of components presentin SCG- referred to in this paper as primary products. Underthe current trend of sustainability, transforming the biomassinto a plethora of value-added material, energy and chem-ical products along the process chain is at the forefront ofresearch efforts. The development of integrated technolo-gies to fractionate SCG, isolating valuable bioactives and/orplatform precursor molecules as well as demonstrating theefficacy of the resultant product and process is the typicalmode of study. With this in mind, it is useful to examinethe compound class present in SCG inherent properties andpotential to generate value through studies in the literature.

3.1. Carbohydrates and fibre: hemicellulose, celluloseand lignin

SCG contain cellulose and lignin, though the amount of ligninreported varies between studies and is heavily reliant on the

method of analysis. The hemicellulose fraction of SCG is com-prised of mannose, galactose and arabinose monomers. SCG

152 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

Table 1 – Collation of literature values reported for components (wt%) of green, roast coffee and by-products of coffeeprocessing.

Componenta Green coffee Coffee pulp Coffee husk Silverskin Roasted coffee Spent coffee grounds

Hemicellulose 3–10 2–66 4–10 4–22 b 32–42Cellulose 32– 43 1–18 35–51 12–24 b 7–13Lignin 1– 3 13–20 7–11 1–3 3 0–26Lipids 8–18 2–5 0.5–6 0.3–4 10–16 2–24Proteins 9–15 10–14 3–13 15–23 8–17 10–18Ash 3–5 2–15 b 5–8 4–6 1–2Caffeine 1– 3 0.5–3 0.5–2 0–1 1–3 0–0.4Chlorogenic Acids 1–12 1–6 2–3 3–4 2–9 1– 3Moisture 9–10 8–77 b 5–7 1 50– 60Pectins 2 5.5–7.5 0.5–3 1 2 0Total sugars b 14–15 48–68 7–17 b 7–14Total Amino Acids 7–11 b b b 5–6 b

Total dietary fibre b 58–64 18–30 56–63 47–50 21–59

a Values obtained from (Acevedo et al., 2013; Ballesteros et al., 2014; Braham and Bressani, 1979; Caetano et al., 2014; Casal et al., 2003; Chimtonget al., 2016; Costa et al., 2018; Dong et al., 2015; Farah, 2012; Jenkins et al., 2014; López-Barrera et al., 2016; Martinez-Saez et al., 2017; Murthyand Madhava Naidu, 2012; Mussatto et al., 2011b, c; Oliveira et al., 2006; Pandey et al., 2000; Rios et al., 2014; Ulloa Rojas et al., 2003; Vardonet al., 2013).

b Component not determined.

Fig. 1 – Range of FAME compositions of transesterified SCGoil. Values collated. from the literature (Efthymiopouloset al., 2017; Jenkins et al., 2014; Somnuk et al., 2017; Vardon

also contains minor quantities of sugar breakdown productsincluding 5-HMF, furfural, levulinic and acetic acids (Mortaset al., 2017; Mussatto et al., 2011b). Despite their classificationas platform molecules, 5-HMF and furfural have measuredgenotoxicity, cytotoxicity and carcinogenic activity in preclini-cal studies- yet in-vivo investigations have disputed this claim(Abraham et al., 2011; Morales, 2008; Pandey et al., 2000; Shaplaet al., 2018).

Galactomannans and type II arabinogalactans are the mainpolysaccharides of SCG. Galactomannans are high molecu-lar weight �-1-4, linked mannose residues with various C6-linked galactose side chains and are thought to be closelyassociated with cellulose within the cell wall matrix. Arabino-galactans have a �-1, 3 linked galactose backbone with sidechains of galactose and arabinose residues. The higher degreeof branching in arabinogalactans is thought to contribute to alower thermal stability than galactomannans during roasting,leading to lower recoveries of the corresponding monomersupon acid hydrolysis of SCG (Ballesteros et al., 2017b; Farah,2012; Oosterveld et al., 2003; Simões et al., 2014). Utilisationof the SCG polysaccharide fraction as antixodiant dietary fibreand a prebiotic is further discussed in Section 5.1.

3.2. Lipids from spent coffee grounds

A wide range of literature has examined the lipid profileand achievable yields from SCG (Table 2, Fig. 1). The oilderived from SCG consists of tri- di- and monoglycerides,free fatty acids (FFA) and unsaponifiable compounds: mainlyditerpenes, sterols and tocopherols. In general coffee oil con-tributes 10–15 % w/w of dry SCG, and contains up to 80–90 %of glycerides and FFA (Jenkins et al., 2014). However, multiplestudies noted variance in the oil yield and retention of FFArelated to extraction regime, brew technique and feedstocktype respectively (Efthymiopoulos et al., 2017; Jenkins et al.,2014).

Using n-hexane Soxhlet extractions, Efthymiopoulos et al.established the highest oil recovery (30.4% w/w) after 8 h. Thepresence of FFA in the lipid fraction was dependent on the

duration of extraction, with decreased FFA observed for longerextractions. The same study noted higher yields from indus-

et al., 2013).

trial (soluble) SCG versus retail grounds (30.4 vs 13.4% w/w)(Efthymiopoulos et al., 2017).

Al-Hamamare et al. similarly noted higher yields for n-hexane (15.83%) vs chloroform, pentane, toluene (8.6, 15.18,14.32%w/w respectively) and other polar solvents for Soxh-let extractions of the same duration (30 min). Time and oilyield were shown to have a non-linear relationship (e.g.14.57, 11.20 and 15.28% w/w at 20, 25 and 30 min for then-hexane regime), with fluctuations attributed to hydropho-bic interactions between moisture bound to the SCG matrixand non-polar solvents, impacting extraction efficiencies. Thepresence of water-soluble extractives was also suggested as apotential source of elevated yields. Though, the authors notedin general, increased extraction time led to slight increasedyields. Highest proportions of FFA were extracted using iso-propanol (6.4 g/100 g SCG oil), yet a loose trend of increased FFAyield with increased carbon chain length can be inferred fromtheir results (e.g. FFA yield = 3.55 and 3.65; 3.85 and 6.40 g/100 gSCG oil for pentane and hexane; ethanol and isopropanolrespectively) (Al-Hamamre et al., 2012).

Jenkins et al. demonstrated that there was no relation-ship between the origin, species and brew technique and

the quality of biodiesel produced from retail SCG. However,despite higher oil yields from fresh grounds (FCGs), lower con-

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 153

Table 2 – Published regimes for the extraction of coffee oil from SCG.

Feedstock Extractiontechnology(solvent)

Parametersa Product Yield Ref

Various SCG: retail andindustrial

Soxhlet(n-Hexane)

S/L = 22.5/100–200 g/mL SCG oil Retail = 13.4–14.8Industrial = 24.2–30.4%w/w

(Efthymiopouloset al., 2017)

Commercial and industrialSCG

Supercritical CO2

(ScO2)S/ CO2 = 1/35 kg/kgCO2, 250 bar, 1.5 h

SCG oil (triglyc-erides = 7.13,FFA = 1.6 5 w/w)

>12.6 %w/w (>90 %of total Soxhletextracted SCG oil)

(Cruz et al., 2014)

Commercial SCG Ultrasonication S/L = 10/30 g/mL60 min

SCG oil >75 % total Soxhletextracted oil

(Wu et al., 2016)

a S/L = solid-to-liquid ratio.

cboa(ielwctwff2

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entrations of unsaponifiable material in SCG gave similariodiesel quantities. Brew technique was also shown to impactil yield, with coffee oil collected from filter, espresso, cafetièrend aeropress SCG in decreasing order of mass percentage13.3–10.3%). This was rationalised by the use of pressuren aeropress and cafetière regimes, which led to higher oilxtraction efficiencies during brew preparation. Resultantower yields and a relatively higher number of unsaponifiablesere then observed for coffee oil derived from aeropress and

afetière SCG. The same study demonstrated higher oil quan-ities for Robusta SCG versus Arabica (11.0–14 vs. 9.5–13.2 %/w, respectively). Absence of caffeine in biodiesel prepared

rom SCG (and its presence in FCG biodiesel) was deemedavourable in reducing potential NOx emissions (Jenkins et al.,014).

.2.1. Sterols, tocopherols and diterpeneshe unsaponifiable fraction of SCG oil, usually isolated dur-

ng transesterification, contains diterpenes, phytosterols andocopherols. Diterpenes exist esterified with fatty acids, with

small amount present in free form. Due to the bioactivity ofhese minor components “green” extraction techniques suchs supercritical fluid extraction (SFE) using scCO2 and variouso-solvents are often employed, for compatibility with medic-nal, nutraceutical and cosmetic applications.

Diterpenes Kahweol, Cafestol and 16-O-methyl Cafestolonly present in the Robusta species) have physiological activ-ty with beneficial and adverse effects on human health.ahweol and Cafestol have been shown to increase serumholesterol levels in humans, yet favourably reduce the geno-oxicity of carcinogens (Cavin et al., 2003, 2002; Lam et al., 1987;rgert et al., 1995; Weusten-Van der Wouw et al., 1994). Diter-enes have been isolated directly from saponified SCG by etherxtraction and from saponification of SCG oil derived usingFE (40 ◦C, 98 bar and 80 ◦C, 379 bar), Soxhlet and solid–liquidxtraction. Acevedo obtained the highest yields for directaponification (Kahweol = 0.21% w/w, Cafestol = 0.47% w/w),et a subsequent optimization study obtained higher totalields of diterpenes using SFE (1.27%w/w, ScCO2, 140 bar, 55 ◦C)Acevedo et al., 2013; Barbosa et al., 2014).

The serum cholesterol lowering effects and antioxidantctivity of sterols and tocopherols, respectively, has moti-ated research into their isolation from SCG. A study by Akgünt al. found SCG to contain 7.57–15.60 % w/w sterols: stig-asterol (2.90–6.04 % w/w), campesterol (1.19–2.44 %w/w)

nd sitosterol (3.48–7.12 %w/w), with highest yields obtainedsing scCO2 (33.18 ◦C, 284 bar, 220.90 min) vs n-hexane and

ichloromethane Soxhlet extractions. ˛-tocopherol and ˇ-ocopherol were found to be present in 0.06–0.28 and 0.84–2.09

%w/w respectively, with the absence of � - and ı-tocopherolsattributed to losses during roasting, brewing or storage (Akgünet al., 2014).

3.3. Protein content

SCG contain 6.7–16.9 % w/w protein, a higher concentra-tion than green coffee due to non-extraction during brewing(Campos-Vega et al., 2015; Cruz et al., 2012; Lago R and Freitas,2001; Martinez-Saez et al., 2017). Commonly employed quanti-tative methods such as Kjeldahl and Dumas do not determinethe true protein content. Rather, by measuring the concen-tration of total nitrogen within a sample and converting tocrude protein content (using a conversion factor, typically 6.25for SCG) the quota includes contributions from non-proteinnitrogenous compounds. As SCG contains melanoidins, alka-loids (including caffeine and trigonelline), free amino acids,and peptides, literature values derived by these techniquesare presumably an overestimate.

Other methods to determine protein content are based oncolorimetric assays such as Bradford, Bichinoninic and Lowry.Interferants within the sample matrix (e.g. reducing sugars),incomplete extraction and/or exclusion of non-soluble pro-tein are sources of potential error in quantitation using thesemethods (Sapan et al., 1999).

3.3.1. Amino acidsStudies by Lago et al. and more recently Castillo et al.have characterised SCG amino acids (AA, Fig. 2), reveal-ing that 42–49 % of total AA are comprised of essentialAAs. Branch chained AA (BCAA = leucine, valine, isoleucine)and aromatic AA (AAA = phenylalanine and tyrosine) werepresent in high and low quantities (5.12–21 and 0.9–1.51 %total protein) respectively. The corresponding Fischer ratiorange (BCAA/AAA, 3.4–24.1) indicates the applicability of SCGderived protein in functional foods, where proteins with Fis-cher ratios >20 and AAA content <2% have been used to treatpatients with hepatic encephalopathy (Lago R and Freitas,2001; Martinez-Saez et al., 2017; Okita et al., 1985).

3.4. Bioactive components

Phenolic compounds of SCG are mainly comprised ofmelanoidins, CGA protocatechiuc acid (PCA) and tanninsincluding gallic acid. Along with nitrogenous compounds(such as caffeine and trigonelline), these minor compo-nents have great pharmacological value, due to positive

physiological effects such as antioxidant, antiaging, antimi-crobial, antidiabetic, anticancer, anti-fatigue anti-allergic,

154 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

s so

Fig. 2 – Amino acid range composition compiled from variou

anti-inflammatory and organo-protective activity (Badhaniet al., 2015; Daly et al., 1983; Kakkar and Bais, 2014; Moreiraet al., 2012; Yoo et al., 2018; Zhou et al., 2012). As for bioactiveswithin the lipid fraction of SCG, use of non-toxic solvents toisolate these compounds is imperative for their use in foodand medicinal applications.

3.4.1. Total phenolic content (TPC) and antioxidantactivityThere are several in-vitro colorimetric radical scaveng-ing assays used to determine the total phenolic content(TPC) and antioxidant activity of SCG extracts, based onhydrogen atom transfer (HAT) and/or single electron trans-fer (SET) mechanisms. HAT assays measure the abilityto quench a reactive oxygen species (e.g. peroxyl radical(ROO)) by hydrogen donation and include oxygen radicalabsorbance capacity (ORAC). SET assays measure the elec-tron transfer ability to reduce radicals, pro-oxidant metals(e.g. Fe2+) and carbonyls. SET assays commonly used inthe SCG literature include ferric reducing ability of plasma(FRAP), Folin–Ciocalteu’s phenol reducing reagent ability, andscavenging of sTable 1,1-diphenyl-2-picyrlhydrazyl (DPPH)and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic) acid(ABTS) radicals(Granato et al., 2018). TPC and antioxidantactivity are calibrated against a standard — Gallic Acid (TPC)and Trolox (Antioxidant Activity) by use of a standard curve,generated over a range of concentrations.

Whilst useful in gauging the antioxidant potential, the lackof specificity of these methods (any reducing agent presentin the sample matrix can give a positive response), absenceof standard protocols, and incomparability (due to differentmechanisms, redox potentials, susceptibility to pH, solventand sample matrix) of the assays limit their analytical value(Resat Apak et al., 2013). Furthermore, in-vitro activities maynot translate to in-vivo behaviour (e.g. low bioavailability, reac-tions with other species) and may be appropriated for productmarketing purposes (Harnly, 2017). A recent shift in the cred-ibility of these assays is evident- according to a recent FoodChemistry paper, manuscripts with data derived from colori-metric assays need additional quantitation techniques (such

as chromatography) as well as in-vitro biological tests and/orin-vivo studies to avoid rejection from the Journal. These sen-

urces (Lago R and Freitas, 2001; Martinez-Saez et al., 2017).

timents are also echoed in an editorial by the Journal of FoodComposition and Analysis (Granato et al., 2018; Harnly, 2017).

3.4.2. Chlorogenic acids (CGA)The major CGAs present in SCG are mono and di- acylquinic acids, substituted with either one ferulic acid moiety(ferulolylquinic, FQA) and up to two caffeic acid moieties ((di)-caffeoylquinic acids CQA). CGAs also contain minor quantitiesof p-coumaroylquinic and caffeoyl-feruloylquinic acids, withconcentrations dependant on the species, degree of roast,brewing, storage, extraction methods (Ramalakshmi et al.,2009). CQAs account for nearly 80% of total CGA content- ofwhich 3-CQA accounts for almost 60%, followed by 4- and5-CQA structural isomers. For this reason, 3-CQA is oftenreferred to as CGA, and is widely used as an analytical stan-dard for CGA quantitiation (Farah, 2012). CGAs are present inSCG up to 5 mg CQAE/g SCG (Acevedo et al., 2013).

4. Secondary product classes

Biotechnological, thermochemical and chemical transforma-tions of primary SCG products have resulted in the isolationof platform molecules, bioplastics, fuels and smart materials.Along with use of SCG for soil amendment and animal feed,this review classifies these energy, materials and chemicalproducts as secondary products. Such nomenclature signifiesboth the medium to low market value of these products andthe secondary conversion processes required for their isola-tion.

4.1. Bioprocessing products

4.1.1. Lactic acidA recent study by Hudeckova et al. has used reducing sugars inSCG hydrolysates (SCGH), derived from dilute acid hydrolysis,to culture lactic acid producing bacteria. The SCGH underwentcellulase hydrolysis before batch inoculation with bacterialstrains including Lactobacillus rhamnosus. It was found that,despite presence of antimicrobial 5-HMF, levulinic acid andpolyphenol components, 25.69 g/L of lactic acid was producedusing SCGH substrate (Yp/s = 0.98), a similar productivity to

other lignocellulosic biomass (Hudeckova et al., 2018). Lacticacid is a highly valuable platform molecule with its main appli-

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 155

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ation as a monomer in the production of polylactic acid (PLA) a biodegradable polymer in wide use (Dusselier et al., 2013).

.1.2. Poly-(3-hydroxybutryate) (PHBs)CG oil can also be used as a fermentation broth for theroduction of PHB, a biodegradable alternative to polyethy-

ene and polypropylene. Using fed-batch mode cultivation,bruca et al. observed high biomass and PHB yields (89.1% w/wnd 49.4 g/L, respectively) for PHB production by Cupriavidusectar using SCG oil substrate. Preliminary experiments estab-ished higher cultivation and PHB yields for SCG oil versusaste rapeseed, palm, sunflower and crude rapeseed oils. Theigher FFA content of SCG oil, undesirable in the productionf biodiesel, was beneficial in promoting accumulation of PHB

Obruca et al., 2014b). PHB has also been previously producedrom scCO2 derived SCG oil (Cruz et al., 2014).

.1.3. Anaerobic digestionCG were first utilized for anaerobic digestion (AD) in the980s, where gas yields of 0.54 m3 kg−1 (56–63 % methane)ere obtained in the co-digestion of activated municipal

ewage sludge and SCG at a rate of 3 kg/m3 day (Lane, 1983).owever, in recent batch biomethane potential (BMP) testssing waste activated sludge (WAS) and SCG as co-substrates,im et al. observed a deleterious effect in the yield and produc-

ion rate of methane with increasing percentage compositionf SCG (Kim et al., 2017b). As commonly used co-substrates,he observed antagonistic relationship questions the efficacyf inoculating SCG with WAS in anaerobic digestion systems.

reverse trend was observed in the co-digestion of SCG withood waste, Ulva and whey, where methane was produced atn enhanced reaction rate without significant loss in BMP.o-digestion of food waste and SCG at 75/25%w/w composi-

ion increased both the cumulative methane potential (0.355 LH4/g VSin) and BMP (0.344 L CH4/g VSin) by 13.1 and 13.5%ith respect to the AD of SCG alone.

SCG-hydrochar, derived from the hydrothermal carbonisa-ion (HTC) of SCG at 180–250 ◦C for 1 h, was recently inoculatedith cow manure for the AD production of biomethane. Theovel coupling of thermochemical and biochemical processesas found to increase biomethane energy conversion effi-

iency up to 32% for hydrochar produced at 180 ◦C (R 180),ver raw SCG. The hydrophilicity of R 180 led to maximumMP of 0.491 L CH4/gVSin, shorter lag phase (� = 11 days) andhe highest biomethane production rate (k = 46 mL CH4/gVSin)Codignole Luz et al., 2018).

.1.4. Triglyceride oils from oleaginous yeastleaginous yeasts have been used for microbial oil produc-

ion from defatted SCG. After dilute acid hydrolysis andnzymatic saccharification pre-treatment to release reducingugars (563 mg/g SCG), SCGHs were cultured with Lipomycestarkeyi, giving total consumption of sugars after 3 days, with-ut inhibition to microbial growth (Wang et al., 2016b). Total

ipid concentration after 7 days was 1.5–2 mg/mL — a con-ersion rate consistent with other lignocellulosic feedstocksHuang et al., 2014; Matsakas et al., 2014).

UV-C irradiated Yarrowia lipolytica, for the enhanced pro-uction of triglycerides and ammonia, were cultured withluyveromyces marxianus treated simulated coffee wasteedium (60% pulp, 40% mucilage). The fatty acid composition

f oil shifted towards linoleic acid in oil produced from the Y.

ipolytica fermentation of simulated coffee waste dosed with0% SCG (36.7% linoleic, 27.1% palmitic and 20% stearic) vs

simulated coffee waste alone (26.5, 28.5 and 30%, respectively).Such fatty acid compositions are amenable for the productionof biodiesel (Lindquist et al., 2015).

4.1.5. BioethanolSaccharomyces cerevisiae has been used for the bioconversionof SCG derived sugars into bioethanol. Inoculation of defat-ted and non-defatted SCGH with 10% (w/w) S. cerevisiae at30 ◦C for 40 h produced bioethanol yields of 0.43 and 0.46 g/gSCGH, from 50 and 58 g/L of total sugars, respectively (Kwonet al., 2013). GC–MS characterisation of volatile compoundspresent in beverages produced by the S. cerevisiae fermenta-tion of antioxidant phenolic extracts of SCG was conducted ina recent study. Higher alcohols (isobutanol, isoamylic alcohol),esters and minor quantities of volatile acids were identifiedand quantified in fermented and subsequently distilled bever-ages, imparting positive organoleptic qualities such as a fruityflavour and floral aroma (Machado et al., 2018). Possibilitiesfor increased productivity and bioethanol yield by augmentedfermentation of galactose have been highlighted in a recentreport of a bioengineered strain of S. Cerevisiae PE-2. Expres-sion of CEN.PK113-5D GAL 2 by the new strain led to increasedgalactose transport activity, resulting in the consumption ofall available sugars within the SCGH, promising for increasedconversion efficiencies (Domingues et al., 2018).

4.2. Biodiesel

Biodiesel is prepared from alkali or acid catalysed trans-esterification of fatty acid triglycerides with alcohols, mostcommonly methanol. The reaction produces 3 mol of biodiesel(fatty acid monoalkyl esters (FAAE)) and 1 mol of glycerol, aplatform molecule with chemical feedstock, feedstuff, andenergy applications (Yang et al., 2012). The FAAE compositionof biodiesel is identical to that of the feedstock from which thebiodiesel was produced (Moser, 2009).

Currently, the majority of biodiesel is derived from rape-seed (47% of EU, and 68% of world feedstock input In2016) and soy bean (67% of US feedstock input in 2016)oils, with animal fats, used cooking, sunflower and palmoil comprising the remaining feedstocks (Administration,2018; Kim et al., 2018). The physicochemical properties ofbiodiesel are influenced by the fatty acid methyl ester (FAME)composition(American Society for Testing Materials (ASTM),2008; Mishra and Goswami, 2018; Ramos et al., 2009). The fattyacids of coffee oil are mainly comprised of linoleic (40–50 %),palmitic (30–40 %), oleic (6–10 %), stearic (7–9 %), Arachidic (1–4%) and linolenic (0.5–2 %) acids, illustrating the direct applica-bility of SCG for the production of biodiesel (Table 3).

A major issue affecting the widespread use of biodieselis poor oxidative stability, arising from factors includingautoxidation of unsaturated fatty acids (UFA). The rate ofdegradation of UFA is influenced by the position and num-ber of double bonds and it can be generally noted that fuelswith a lower degree of polyunsaturated components (hence alow iodine number) display greater stability. Table 3 displaysselected properties of diesel and biodiesel derived from SCGand other biomass feedstocks, which are included for compar-ison. From the data, it is evident that all biodiesels failed toreach the EN 14112 test limit, requiring antioxidant additivesto improve performance. Suitable additives include BHT oreven SCG extracts — a recent study has improved the oxidative

stability of waste rapeseed oil by blending with an antioxidantrich — SCG-oil extract (Knothe, 2009; Todaka et al., 2018).

156 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

Tabl

e

3

–

Fuel

Prop

erti

es

of

die

sel a

nd

biod

iese

l der

ived

from

SC

G

and

sele

cted

biom

ass

feed

stoc

ks.

Fuel

pro

per

ty

Test

Met

hod

Lim

its

Die

sel

Rap

esee

d

Soyb

ean

Sun

flow

erse

edPa

lm

Jatr

oph

a

Alg

ae

SCG

Kin

emat

ic

visc

osit

y

EN

ISO

3104

3.5–

5.0

2.6

4.4

4.2

4.2

4.5

2.6

4.15

–

–Fl

ash

poi

nt

(◦C

)

EN

ISO

3679

>12

0

68

170

171

177

176

135

–

–

–C

etan

e

nu

mbe

r

–

>51

–

55

49

50

61

–

–

–

60.1

Oxi

dat

ive

stab

ilit

y,

110

◦ C

EN

1411

2

>6.

0

–

2

1.3

0.8

4

–

5.42

– 0.

2A

cid

valu

e

(mg

KO

H/g

)

EN

1410

4

<0.

50

–

0.16

0.14

0.15

0.12

0.4

0.52

1.85

0.11

Iod

ine

valu

e

gI2/1

00

g

EN

1411

1

<12

0

109

128

132

57

–

39.7

6

70

–H

igh

er

Hea

tin

g

valu

e

GB

/T

384–

81

>35

42

–

–

–

–

39.2

3

–

39.6

Ref

eren

ce

–

(Ku

mar

Tiw

ari e

t

al.,

2007

)

(Ram

os

et

al.,

2009

)( R

amos

et

al.,

2009

)( R

amos

et

al.,

2009

)( R

amos

et

al.,

2009

)( K

um

arT

iwar

i et

al.,

2007

)

(Ch

en

et

al.,

2012

)( C

aeta

no

et

al.,

2014

)( V

ard

on

et

al.,

2013

)

A higher performance paraffinic fuel has also been derivedfrom coffee oil through hydrogenation/deoxygenation. Stan-dard catalysts such as NiMo/�-Al2O3 and Pd/C producedsuitable fuels with a cetane number over 80 (Phimsen et al.,2016).

4.3. Fertiliser

When used as a soil amendment, SCG exhibit deleteriouseffects on the growth of horticultural plants (Cervera-Mataet al., 2018; Cruz and Marques dos Santos Cordovil, 2015;Hardgrove and Livesley, 2016). Despite initial mineralisation,the immobilisation of nitrogen and phosphorous in soils incu-bated with SCG has been attributed to reduced plant growth,for application volumes as low as 2.5% (v/v) (Hardgrove andLivesley, 2016). Phytotoxins present in SCG, such as caffeine,tannins and chlorogenic acid, are also inhibitive to yield, withcomposting and vermicomposting employed to reduce theirconcentrations relative to fresh SCG (Sanchez-Hernandez andDomínguez, 2017).

Beneficial attributes in crop grown from soil treated withcomposite SCG composts have been reported. Pepper plantsgrown in the presence of functional composts derived fromvarying compositions of SCG (78–85 %), tomato stalk basedbiochar (0–2.6%) and poultry manure (15–20 %), displayedenhanced DPPH radical scavenging activities (up to 14%), andincreased TPC by 68% with respect to the control (Emmanuelet al., 2017). Suppression of weed growth, enhanced organiccarbon, N, K, P and Na concentrations as well as increasedchlorophyll and carotenoids in SCG treated soils, additionallyhighlight the potential application of SCG as nutrient-rich fer-tilisers (Cervera-Mata et al., 2018; Cruz et al., 2015). Furtherresearch, however, is required to increase the bioavailabilityof micronutrients and improve the viability of SCG as a soilamendment.

4.4. Animal feed

Early reports of SCG as a ruminant feed (250–260 g dry mat-ter (DM) a day) observed low digestible energy (0.5–4.3 MJ/kgDM) and negative metabolisable energy content, leading tothe rejection of their use as a feed ingredient (Givens andBarber, 1986). Subsequent investigations involving a microbialfermentation pre-treatment of SCG for improved palatability,crude protein digestibility and utilisation of nitrogen in live-stock, relative to those fed non-fermented SCG have sincebeen reported. However, whilst leading to a reduction incosts, reduced digestibility of crude protein, acid detergentfiber, neutral detergent fiber, relative to the control diet, wasobserved with increasing concentrations of Lactobacillus spp.treated SCG in silage, hay, corn and soybean meal total mixratio (Choi et al., 2018; Seo et al., 2015; Xu et al., 2007). A max-imum limit of up to 10% (w/w DM) fermented SCG in feed hasbeen established, yet increased production costs of a fermen-tation pre-treatment negates the economic value of using SCGin animal feed (Seo et al., 2015).

4.5. Thermochemical conversion products

4.5.1. Pyrolysis derived bio-oilSCG have been investigated as a suitable feedstock for pyroly-sis — the thermal decomposition of biomass in the absence of

oxygen, under temperatures up to 500 ◦C. Pyrolysis of biomassproduces condensable liquids (pyrolytic or biocrude oil), non-

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 157

ctypaSistettpe

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ondensable gases and a residual biochar. Bio-oil yields of upo 55% can be achieved through high temperature flash pyrol-sis, with high char yields also observed (Bok et al., 2012). Theyrolysis of SCG has been demonstrated on the pilot scale in

screw conveyor reactor, yielding up to 60% bio-oil. As theCG was not defatted prior to use, the oils contained predom-

nantly fatty acids, esters as well as olefins and oxygenatedpecies from the polysaccharides (Kelkar et al., 2015). In fur-her studies, the energy efficiency of the pyrolysis has beenstimated between 77–85 % depending on the moisture con-ent of the coffee (Li et al., 2014). A similar study demonstratedhe suitability of spent coffee grounds as a feedstock in slowyrolysis producing a bio-oil and bio-char with similar prop-rties to other lignocellulose feedstocks (Vardon et al., 2013).

.5.2. Hydrothermal conversion products: Bio-oils andolid charsnergy expenditure on drying feedstocks, comparatively lowernergy efficiencies and higher tar yields are major barriersgainst the implementation of pyrolysis for wet biomass suchs algae (Elliott et al., 2015; Gollakota et al., 2018). To cir-umvent these issues, hydrothermal liquefaction (HTL), anmerging technology, has been employed, where biomass ishermally converted into a carbonaceous material using 5–35%olid loading in water. The reactions are heated to between80–350 ◦C, under pressures of up to 180 bar. The resultingiomass breaks down into a biocrude oil, an aqueous phase,olid residue (commonly referred to as hydrochar or biochar)nd gas, comprised mainly of CO2. Under rapid reaction times,he predominant product is the biocrude oil.

SCG, with a typical moisture content of 50–60 % (Table 1),re viable feedstocks for hydrothermal processes. For the con-ersion of SCG, just under 47% biocrude oil, with 31 MJ kg−1

HV, was produced at 275 ◦C. Similar to pyrolysis products,he HTL derived biocrude comprised of long chain acids andsters (Yang et al., 2016a). Lower temperatures (200–250 ◦C) andonger reaction times yield a carbon rich major solid producthrough a process termed hydrothermal carbonization (HTC).iochars have been reported in 60% yields from SCG. One studyeported the reduction of the O/C ratio from 0.64 in the SCGeedstock to 0.17 in the hydrochar product (HTC parameters:50 ◦C, 1 h), with the material obtained under optimum con-itions (210–240 ◦C, 1 h) exhibiting a HHV of 26–27 MJ kg−1 —imilar to coal (Kim et al., 2017a).

.6. Product applications and formulations

.6.1. Formulated applications of polysaccharidesublished extraction regimes for isolating polysaccharidesrom SCG are given in Table 4. Recent interest in theolysaccharide extracts of SCG has determined antimi-robial, antioxidant and thermal stability properties. Aftern alkali pre-treatment of defatted SCG Mussatto et al.easured a higher antioxidant activity in the DPPH radi-

al scavenging assay of the retained SCG polysaccharidesconcentration needed to inhibit 50% of radical activity,C50 = 0.7 mg/mL) than polysaccharide extracts from edible

ushrooms (4–7.2 mg/mL) (Mussatto et al., 2011a). Despiteroviding a source of carbon to promote microbial growth, theame study observed growth inhibition of fungal strains at lowoncentrations of the polysaccharide extracts (41.3 and 48.6%nhibition of P. violacea and C. cladosporioides, respectively at

.9 �g/mL of extract). Such behaviour was postulated to ariserom residual phenolics present in the extracts, corroborated

by the observed antioxidant activity and higher results in thetotal phenolics assay (230 mg GAE/g SCG) than other studieson conventional solid-liquid SCG extracts. The competitionbetween anti-microbial components and the polysaccharidecarbon source within the extracts was concentration depen-dent, with fungal growth favoured at higher concentrations(Ballesteros et al., 2015).

A valuable application of SCG polysaccharides then is asa source of antioxidant insoluble dietary fibre ingredient infunctional foods. A recent study by Castillo et al. incorpo-rated instant coffee SCG into biscuit formulations. The groupdemonstrated that at 4% w/w in biscuits, SCG did not affectthe food preparation or product quality. The extracts wherefound to contain essential amino acids (42% w/w total aminoacids), displayed thermal and gastrointestinal digestion resis-tance and analysis of the soluble fraction revealed it containednearly 18% of the total antioxidant activity of SCG. From thisit was concluded that the insoluble fraction could thereforepossess the majority of the antioxidant activity. The groupnoted that microbial regulations for SCG in foodstuffs areyet to be established, yet confirmed less than 10 CFU/g intotal aerobic, endospore-forming, mould and yeast microbialassays on thermally stabilized SCG (dried at 40 and 70 ◦C). Thefood safe conclusion was corroborated with the absence ofmoulds, preventing growth of OTA and low concentrations of5-HMF and ACR (61.3 mg/kg SCG and 37.2 �g/kg SCG, respec-tively), regulated contaminants with potential carcinogenicactivity in humans (Martinez-Saez et al., 2017). A similar studyby Hussein et al. found at 6% w/w SCG enrichment, biscuitspassed organoleptic (taste, colour, texture, flavour, appearanceand overall acceptability) tests with minimal modifications ofthe dough’s rheological properties. Both studies suggested asuitable market for these products: for patients with obesity-related diseases, diabetes or diet (reduced energy intake) goals(Hatem S. Ali et al., 2018).

SCG also contain a diverse array of oligosaccharides,shorter chain (n = 2–20) sugar residues. A recent study hasidentified mainly hexose (tentatively manno-oligosaccharides(MOS) and galacto-oligosaccharides) oligomers containing amixture of glucose, rhamnose, xylose, arabinose residuesin coffee brews and their respective SCG. The studydemonstrated variability in the abundance, structure andcomposition of obtained oligosaccharides according to brew-ing techniques, with their diverse composition suitablefor novel prebiotic and symbiotic applications (Tian et al.,2017).

A preceding study demonstrated an improvement ofdefecation behaviour and the dominant presence of Bifidobac-terium in the faeces of people ingesting 1 and 3 g/day ofMOS. Bifidobacterium produce lactic acid and short chain fattyacids, establishing a lower pH in the intestine that inhibitsgrowth of harmful bacteria (Asano et al., 2004). This beneficialgut activity has been corroborated in other studies, high-lighting the value of MOS and other sugar residues derivedfrom coffee as prebiotics (Jaquet et al., 2009; Jiménez-Zamoraet al., 2015; Umemura et al., 2004; Walton et al., 2010). MOSfrom coffee extracts was also demonstrated to have visceraland sub-cutaneous fat reducing properties and blood pres-sure suppression in Dahl-salt sensitive (hypertensive) rats(Hoshino-Takao et al., 2008; Salinardi et al., 2010; St-Onge et al.,2012; Takao et al., 2006). A commercial launch of food productscontaining thermally derived coffee MOS in Japan 2007, indi-

cates the lucrative potential of the carbohydrate fraction ofSCG (Fujii et al., 2007).

158 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

Table 4 – Published regimes for the isolation of polysaccharides from SCG.

Entry Feedstock Extractiontechnology(solvent)

Parametersb Product Yield Ref

1 DefattedindustrialSCG

Alkali Pre-treatment 4M NaOH + 0.02 MNaBH4, 25 ◦C,

Polysaccharides(Gal = 60.27,Ara = 19.93,Glu–15.37, Man = 4.43%mol)

Y1 = 6.05, Y2 = 2.38,Y3 = 4.57Total sugars = 39 %a

(Ballesteroset al., 2015)

2 CommercialEspresso SCG

Microwave assistedextraction (MAE)(water)

MAE1 = S/L = 1/10 g/mL,200 ◦C, 40 barMAE 2 = 18 bar

MAE 1 = MainlyarabinogalactansMAE 2 = Mainlygalactomannans

MAE 1 = 0.57MAE 2 = 0.34 g/batchMAE 1 + MAE 2 = 55 %sugar recovery

(Passos andCoimbra,2013)

3 CommercialandindustrialSCG

Autohydrolysis,(water)

S/L = 1/15 g/mL160 ◦C, 10 min

Polysaccharides(Gal = 47.74,Man = 31.88,Glu = 10.35,Ara = 10.02 %mol)

Y1 = 3.59, Y2 = 1.07,Y3 = 2.06Total sugars = 29.29%a

(Ballesteroset al., 2017b)

4 DefattedArabica SCG

Ultrasoundpre-treatmentfollowed by SWE(water)

S/L = 6/ 160 g/mL,179 ◦C, 20 bar,5 min

Polysaccharides(Gal = 64.13,Man = 29.82,Ara = 3.33, Glu = 2.33%mol)

Total sugars = 47.72%

(Getachewet al., 2018)

5 Dark roastArabica SCG

Solid–LiquidExtraction (SLE)(Hot water)

S/L = 1/20 g/mL,100 ◦C, 20 min

Oligosaccharides(Man = 54.08,Gal = 34.67,Ara = 5.73, Glu = 4.08,Xyl = 0.75, Rha = 0.69%mol)

0.05 % w/w (Tian et al.,2017)

6 CommercialEspresso SCG

Strong acidhydrolysis

S/L = 3/70g/mL, 100 ◦C, 16 h

SCG solidhydrolysate, with30 × antioxidantcapacity of originalSCG

31 % w/w (Panzellaet al., 2016,2017)

7 CommercialSCG

Dilute acidhydrolysis thenenzymatichydrolysis

S/L = 10.5/ 90 g/mL121 ◦C, 20 min

SCGH(Man + gal = 39.62,Ara = 3.39, Glu = 10.01% w/w)

61.9 % w/w (Hudeckovaet al., 2018)

a Y1 = total extraction yield (g/100 g SCG), Y2 = quantity of sugars extracted (g/100 g SCG), Y3 = quantity of sugars extracted/ total sugars in SCG(g/100 g total SCG sugars).

b S/L = solid-to-liquid ratio.

Similarly, recent studies by Panzella et al. have demon-strated antioxidant and prebiotic activity in solid SCGhydrolysates (SCGH) and digestion-fermentation treatedSCGHs (Panzella et al., 2016, 2017). Oxidatively stressed humanhepatocellular carcinoma HepG2 cells, with increased pres-ence of reductive oxygen species (ROS), were protected fromstress-induced death by SCGH ROS scavenging action. Theirfindings, also validated with the above reports, as SCGHincreased the production of Lactobacillum spp. and Bifidobac-terium spp. (and release of SCFA) after fermentation. The groupclaim the solid residues from acid hydrolysis are a multifunc-tional material with biomedical, industrial and technologicalapplications.

4.7. Antioxidant formulations

Recent research by Mussatto et al. and Riberio et al. exploredpreservation of antioxidant activity by encapsulation andincorporation of bioactive SCG into skin care products, usingsubcritical water (SWE) to derive phenolic SCG extracts.

Mussato et al. determined the optimum storage condi-tions for SCG phenolics, which are susceptible to degradation

under oxidising conditions commonly induced by light, oxy-gen, moisture. The study established that freeze drying with a

maltodextrin coating preserved 73–86 % of the original antiox-idant activity of the SCG extract as well as retention of 62 and73% of TPC and flavonoids respectively (Mussatto et al., 2011a).By highlighting storage as a potential driver in the valuechain of SCG, this work complements a considerable body ofresearch that has exhaustively demonstrated the bench iso-lation of these high value components (Table 5) (Ballesteroset al., 2017a).

Similarly, Ribeiro et al. demonstrated the application ofbioactives in cosmetic products. Extracts with EC50 of 21 �g/mLwere incorporated in hydrogels, and resultant inhibition ofelastase and tyrosinase (99 and 79% inhibition of enzymeactivity, respectively) conveyed their efficacy in antiaging andskin lightening topical applications (Ribeiro et al., 2018).

4.8. Biosorbents

SCG are natural adsorbents due to the chelating and coor-dinating properties of polyphenolic (Sánchez-Vioque et al.,2013), lactone and amino-containing components (Kyzas,2012), and the matrix itself can be further exploited for con-taminant uptake through thermochemical processing, leading

to increased porosity and surface area of the biochar prod-ucts (Azouaou et al., 2010; Jung et al., 2016; Kante et al., 2012;

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 159

Table 5 – Published “green”extraction regimes for the isolation of SCG bioactives.

Feedstock Extractiontechnology(solvent)

Parameters Products, Yield,Antioxidantactivity

Ref

Commercial SCG Deep Eutectic Solvents(DES)And Ultrasound (UAE)

S/L = 100/2.6 mg/mL,67.5 % DES w/w(DES = 1,6 Hexanedioland Choline Chloride,7:1 molar ratio),60 ◦C, 10 min

TPC = 17 mg GAE/g SCGDPPH = 26.2 TE/g SCGFRAP = 32.9 TE/g SCG(recovery of 93 % totalextracted compounds onSP-207 resin)

(Yoo et al., 2018)

Commercialespresso SCG

Solid Liquid Extraction(water)

S/L = 1/20 g/mL,∼100 ◦C, 5 min

Total CGA = 478.9 mg/100 gSCG3-CQA = 140.8 mg/100 g SCG

(Cruz et al., 2012)

SCG capsules SWE(water)

Semi-continuous batch:3 g SCG, 1 mL/min H2O,90 min, 0– 140 ◦C (S1)and 140–220 ◦C (S2)

S1: Yield = 15 % w/wTPC = 19 mg GAE/g SCG3-CQA = 1.9 mg/g SCGPCA = 0.7 mg/g SCGTrigonelline = 2.1 mg/g SCGCaffeine = 6.1 mg/g SCGDPPH: EC50 = 20.6 ug/mL

(Ribeiro et al., 2018)

Defatted Arabica SWE + UAE S/L = 6/ 160 g/mL, 179 ◦C, TPC = 2.2 % w/w (Getachew et al.,

LceoPe2prfbAs2

4

T(tfo(wlosbdpeaTpbaeacfi(i

SCG 20 bar, 5 min

aksaci et al., 2017; Ma and Ouyang, 2013). SCG and SCGomposites have been utilised for removal of dyes (Pavlovict al., 2014; Rattanapan et al., 2017; Wen et al., 2019), antibi-tics (Dai et al., 2019) and heavy metals (Macch et al., 1986;ujol et al., 2013) in the treatment of wastewater and similarffluent decontamination applications (Imessaoudene et al.,013). Recently SCG, chitosan and poly (vinyl alcohol) com-osite films were examined for their suitability in removing aange of pharmaceuticals and bioactive species including caf-eine. High recovery rates were feasible, and the material coulde reused with little drop in performance (Lessa et al., 2018).

similar study utilised SCG to remove Hg(II) from aqueousolutions at a maximum capacity of 32 mg/g (Alvarez et al.,018).

.9. Biodegradable films for food packaging

hrough incorporation of polysaccharide rich SCG extractsentries 1 and 3, Table 4), Mussatto et al. greatly enhancedhe light barrier of carboxymethyl cellulose (CMC) films forood packaging. Films were produced with up to 0.20% (w/v)f alkali pre-treatment derived (PA) or autohydrolysis derived

PB) extracts, where the greater opacity of PB-CMC filmsas attributed to the relatively higher concentration of Mail-

ard reaction products, formed under the high temperaturesf autohydrolysis. Physicochemical properties including ten-ile strength and surface hydrophobicity were also improvedy addition of PA and PB, whilst supplementary antioxi-ant and antimicrobial qualities (due to the presence ofolyphenolic components in the extracts) demonstrated thefficacy of SCG-extracts in improving the function of materi-ls for food packaging applications (Ballesteros et al., 2018).his work follows an earlier example of biocomposite SCGackaging, reported by Dufresne et al. where HTC derivediochar (250, 270 ◦C, 2 h) was extruded with poly(butylenedipate-co-terephthalate) (PBAT, 10–30 %w/w SCG) to similarlynhance the thermo-mechanical properties of the biodegrad-ble film (Moustafa et al., 2017). More recently, the continuousasting of SCG (5–20 %w/w SCG) with pectin also modi-ed the physicochemical properties of the pectin-based films

HDM). Increased colour, thermal stability and a reductionn the water vapour permeability rate was observed in the

2018)

SCG-pectin composites, yet adversely, the water vapour per-meability rate of the films increased from 1.8 g m−1 s−1 Pa−1 to3.0 g m−1 s−1 Pa−1 at 0 and 20%w/w SCG, respectively (Mendeset al., 2019).

4.10. Construction materials

SCG have been used to stabilize alkali activated fly ash andslag geopolymeric subgrade construction materials (Arulrajahet al., 2016, 2017; Arulrajah et al., 2014; Kua et al., 2016, 2017a;Kua et al., 2017b). Increasing proportions of SCG have beenreported to reduce thermal conductivity of bricks, bestowingsuperior insulating properties relative to the plaster compos-ite control (Vilão et al., 2014). Velasco et al. reduced thermalconductivity by up to 50% (0.74 to 0.36 W m−1 K−1) by firingclay with incremental additive concentrations (0–17 % w/wSCG respectively). Yet, increased water absorption limited therecommended incorporation at 11% w/w SCG, with higherconcentrations requiring coating to protect against weath-ering (Velasco et al., 2016). A recent study by Allouhi et al.demonstrated through the incorporation of 6% w/w SCGswith industrial plaster powder, a reduced thermal conductiv-ity (0.31 W m−1 K−1). Using a residential building in Morocco asa basis for simulation, utilisation of the 6% w/w SCG plastercomposites afforded energy savings of 24%, mitigating yearlyCO2 emissions by up to 1505.62 kg CO2 eq/y (Lachheb et al.,2019).

4.11. Energy storage devices

Recent preparation of a non-porous carbonaceous lithium-ion battery (LIB) using pyrolysed SCG (800 ◦C, 2 h) andpoly(vinylidene fluoride) (PVDF) binder, with carbon black Pconducting agent, resulted in a novel anode material witha high capacity retention. A considerable reversible capac-ity of 285 mA h g−1 at a current density of 0.1 A g−1 was alsomeasured, signalling the eminence of SCG in the develop-ment of high performing sustainable energy storage devices(Luna-Lama et al., 2019). Similar incorporation of pyrolysedSCG into sodium-ion batteries gave a reversible capacity of

154.2 mA h g−1 at a current density of 200 mA/g(Gao et al.,2018). SCG have also been utilised as electrode materials in

160 Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166

ential SCG based biorefinery.

Table 6 – Typical market prices for the products, fromconventional sources, listed in Fig. 3.

Cost [£/kg]

Biohydrogen 1.17Biodiesel 0.59Bioethanol 0.42Sterols 10-100Nutraceuticals 1–100Crude glycerol 0.5–0.20Caffeine 10–175PHA 3.9–4.7

Fig. 3 – Scheme for a pot

vanadium redox flow batteries (Krikstolaityte et al., 2018),fuel cells (Ramasahayam et al., 2015), lithium-sullfur batter-ies (Zhao et al., 2017) and supercapacitors (Park et al., 2016;Wang et al., 2016a).

5. SCG as a feedstock in biorefineries

Biorefineries, where biomass is converted into energy, chem-icals and materials through a series of integrated conversiontechnologies, aim to maximise the value generated fromthe biomass feedstock through the co-production and isola-tion of high, medium and/or low value products (Kamm andKamm, 2004, 2007; Martinez-Hernandez and Samsatli, 2017;Sadhukhan et al., 2015). For operations using waste streams asfeedstocks, the tenets of the circular economy can be realisedthrough the recovery and transformation of waste in this man-ner.

The concept of valorising SCG in a system of stepwise, inte-grated processes is still nascent in the literature. A number ofarticles have been published in the recent literature, in whichconversion technologies that have been applied to SCG are sur-veyed and integrated as potential biorefineries. Assessmentof technology readiness, economic viability, environmentalimpact and conceivability of processes on the large scale isthen made, giving credence to the SCG based biorefinery con-cept (Burniol-Figols et al., 2016; Girotto et al., 2018; Karmee,2018; Kookos, 2018b; Kovalcik et al., 2018b; Mata et al., 2018;Zabaniotou and Kamaterou, 2019).

Published examples of SCG biorefineries separate the start-ing material into two streams by aqueous/polar solventextractions of bioactives and/or carbohydrates and a non-polar lipid extraction (Fig. 3). Downstream processes along theaqueous stream include biotechnological conversion of thereducing sugars to bioethanol, lactic acid and other platformmolecules (Burniol-Figols et al., 2016; Obruca et al., 2014a). Thelipid stream uses the oil for the coproduction of biodiesel andglycerol, or undergoes biotechnological conversion into PHAs.Glycerol can be further converted to biohydrogen throughsteam methane reforming or purified to high value chemicals.The residual solids from both streams as well as the raw SCGcan undergo thermochemical conversions into solid, gaseousand liquid fuels (Haile, 2014; Vardon et al., 2013), composite

materials and energy, or be used as a substrate for mushroomcultivation, aerobic or anaerobic microbial digestion (Freitas

et al., 2018). Table 6 shows the market prices for some of thesepotential products.

5.1. Supply chain and techno-economic considerations

One of the key issues in industrial bioprocessing is ensur-ing the availability and continuous supply of the feedstock,and one of the key limitations of an SCG biorefinery will bethe upstream supply chain which entails the collection andtransportation of the SCG from individual coffee shops or aninstant coffee manufacturing site. Little research has beenconducted in this area and aspects like the collection points,quantities available for collection, seasonality, type and costsof transportation are parameters/variables that need to becomprehensively assessed.

The consumption of coffee has been increasing worldwideand is now estimated at just under 10 million tonnes perannum (USDA Foreign Agricultural Service, 2018). One majorpositive aspect of a SCG biorefinery is the lack of seasonalbehaviour from the feedstock and the ready availability allyear round. The higher consumption of coffee has led to alarge increase in the number of coffee shops, mainly in largemetropolitan areas. Hence, this presents a challenge to theSCG industry as there are a large amount of collection points,widely dispersed throughout a city. For transportation, trucksappear to be the only reasonable method for collection. Thereis a lack of literature data on the transportation costs of SCG,however, it can be assumed that its costs are similar to othertypes of biomass with similar density. The wet density ofSCG is approximately 720 kg m−3 and is therefore similar to

corn grits, where the transportation cost is approximately$10.25/metric tonne (USDA Agricultural Marketing Service,

Food and Bioproducts Processing 1 1 8 ( 2 0 1 9 ) 149–166 161

2sinpt

lilwn

plasaSi8HCs

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5-Hydroxymethylfurfural in food. Mol. Nutr. Food Res. 55,

018). Several arrangements for truck transportation are pos-ible, for example, a biorefinery can use the logistics alreadyn place by the major coffee companies for collection. Alter-atively, the company themselves could invest upstream, toerform the collection and transportation allowing more con-rol and increased competition over the feedstock pricing.

If a SCG biorefinery project is to be executed at a nationalevel, the optimal number and capacity of biorefineries to bemplemented would be reasonably small. A few biorefineriesocated near large metropolitan areas with larger capacitiesould seem to be the optimal. However, further research isecessary to determine this optimal design.

Few reports exist on the upstream supply chain or theotential techno-economics of a process, with Mata et al. high-

ighting the lack of literature and the need for this type ofnalysis (Mata et al., 2018). Giller et al. published a comprehen-ive TEA study of a coffee biorefinery to produce biodiesel and

solid fuel. The authors estimated that the capital costs for aCG 10,000 t y−1 facility, would be approximately $4 M, includ-

ng the requisite fleet of trucks to deliver the coffee from the75 coffee shops in the New Jersey area (Giller et al., 2017).owever, the authors found that despite the relatively lowAPEX, a biorefinery producing biodiesel and a solid fuel wasimply not economic at this scale.

Similarly, a recent techno-economic analysis of biodieselroduction from SCG in Europe highlighted that at the currentrice of biodiesel (∼0.8$ kg−1 in 2018), an operation produc-

ng 1000 t y−1 cannot be profitable and in fact the minimumelling price (MSP) required is 3.6 $ kg−1 at this scale (Kookos,018a). The authors established from modelling MSP as a func-ion of production capacity, that the current market price ofiodiesel requires production of 42 000 t y−1 for profitability.entralized collection of SCG could improve the economic

easibility, yet sufficient transport and logistical infrastruc-ures in countries currently valorising SCG are yet to be putn place(Kookos, 2018b). Considering these results and theurrent market prices, decentralized processing cannot beonsidered an option, as such would mean smaller scatterednits which would not have suitable economy of scale.

A potentially more feasible biorefinery model would there-ore need to be based around recovery of the bioactive

etabolites of SCG. The high market value (estimated at40 kg−1) of CGA is an economic driver that warrants itsxtraction prior to deteriorating heat and chemical processesBurniol-Figols et al., 2016; Kookos, 2018b). The latent toxicityf SCG phenolics could also inhibit microbial growth, reducingfficiencies of downstream fermentation processes, as well asimiting the application of the solid cake in animal feeds, soould potentially need to be extracted anyway prior to further

alorisation (Kovalcik et al., 2018a, b; Mata et al., 2018; Obrucat al., 2014a). The global market size of CGA in 2015 was $122.16illion and is expected to have an increase of 17% until 2021

o $143.50 million.Methods for minimising the cost of operations include effi-

ient solvent recovery and generation of combined heat andower (CHP) by combustion of SCG (HHV = 39.6 MJ/kg) (Vardont al., 2013). The thermal and electrical energy generated byHP can increase process sustainability, when used for dryinget feedstocks and providing electrical power to the plant,ith the potential of selling surplus electricity to the grid

Kookos, 2018b). However, centralised collection and sufficienteparation of SCG are major obstacles to profit margins androcess efficiencies. Ensuring sufficient quantities of feed-

tock for maximum output, with minimal contaminants (such

as plastics) and sources (e.g. industrial vs commercial) is inher-ent to the economic worth of future SCG Biorefineries (Kookos,2018b; Mata et al., 2018).

A handful of studies have investigated the potential envi-ronmental impact of products produced from SCG. Santoset al. showed that if SCG was used for compost alone thenwhile production of methane and NOx was observed this wastypically low compared to other fertilisers (Santos et al., 2017).

While Kookas et al. determined that the cost of manu-facture of biodiesel was extremely high, the environmentalaspects were acceptable and could be considered sustain-able. Overall the authors calculated that biodiesel productionfrom SCGs had a GHG production of −2.1 kg CO2-eq per kgof biodiesel produced, similar to other biodiesel processesfrom soybean and rapeseed. Though also suggested that fur-ther research was vital into higher value bioactive compoundsto add value and make the process economically attractive(Kookos, 2018b). Attempts have been made to increase theenvironmental impact of the SCG biodiesel process, includ-ing in removing hexane and producing the oil in situ withoutoil extraction (Tuntiwiwattanapun et al., 2017). However, thein situ process used more energy and created a greater GHGimpact than the traditional process. Though the authors alsoreasoned that the energy content of the defatted SCG wouldbe enough to process the biodiesel, without additional productrecovery.

6. Further necessary research anddevelopment

Rather than single instances of valorisation, research nowneeds to focus on qualifying possible scenarios for SCG biore-fineries. Limitations to process efficiencies from inherentcomponents within SCG (such as secondary metabolites) andco-contaminants within the feedstock should be addressedand accounted for in studies. Furthermore, there is a dearthof investigations into the reduction of nitrogen and sulphur aswell as the enrichment of O/C and H/C ratios along the processchain. Such work is critical for the development of SCG derivedsolid fuels - which need to be blended with other LCFs (suchas pine sawdust) in order to mitigate particulate matter, NOx

and SOx emissions from their combustion (Kim et al., 2017a;Limousy et al., 2013).

It seems likely however that economic SCG biorefineriescannot be solely based on energy production. Recent workhas suggested the limitations of processes designed to pro-duce predominantly fuels, especially the scale needed to beeconomic. However, as we have demonstrated in this reviewarticle, there are over 40 novel higher value components andalternative products that could be produced from coffee — allof which have existing markets that could be entered withfew regulatory issues. Establishing processing methods forthe extraction and isolation of antioxidant fractions, proteins,peptides and amino acids from the SCG matrix needs to bedeveloped further but it is this type of processing and fur-ther valorisation of these compounds that holds the key toan economic biorefinery from spent coffee grounds.

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