sustainability Review Monomers, Materials and Energy from Coffee By-Products: A Review Laura Sisti 1 , Annamaria Celli 1,* , Grazia Totaro 1 , Patrizia Cinelli 2, Francesca Signori 2 , Andrea Lazzeri 2 , Maria Bikaki 3, Philippe Corvini 3, Maura Ferri 4 , Annalisa Tassoni 4 and Luciano Navarini 5 1 Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Università di Bologna, Via Terracini 28, 40131 Bologna, Italy; laura.sisti@unibo.it (L.S.); grazia.totaro@unibo.it (G.T.) 2 Dipartimento di Ingegneria Civile ed Industriale, Università di Pisa, Largo Lucio Lazzarino 1, 56126 Pisa, Italy; patrizia.cinelli@unipi.it (P.C.); francesca.signori@unipi.it (F.S.); andrea.lazzeri@unipi.it (A.L.) 3 FHNW, School of Life Sciences, Institute for Ecopreneurship, Hofackerstrasse 30, 4132 Muttenz, Switzerland; maria.bikaki@fhnw.ch (M.B.); philippe.corvini@fhnw.ch (P.C.) 4 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Via Irnerio 42, 40126 Bologna, Italy; maura.ferri@unibo.it (M.F.); annalisa.tassoni2@unibo.it (A.T.) 5 Illy Caffè S.p.A. Via Flavia 110, 34147 Trieste, Italy; luciano.navarini@illy.it * Correspondence: annamaria.celli@unibo.it; Tel.: +39-051-209-0349 Abstract: In recent years, the circular economy and sustainability have gained attention in the food industry aimed at recycling food industrial waste and residues. For example, several plant-based materials are nowadays used in packaging and biofuel production. Among them, by-products and waste from coffee processing constitute a largely available, low cost, good quality resource. Coffee production includes many steps, in which by-products are generated including coffee pulp,   coffee husks, silver skin and spent coffee. This review aims to analyze the reasons why coffee waste can be considered as a valuable source in recycling strategies for the sustainable production of Citation: Sisti, L.; Celli, A.; Totaro, bio-based chemicals, materials and fuels. It addresses the most recent advances in monomer, polymer G.; Cinelli, P.; Signori, F.; Lazzeri, A.; Bikaki, M.; Corvini, P.; Ferri, M.; and plastic filler productions and applications based on the development of viable biorefinery Tassoni, A.; et al. Monomers, technologies. The exploration of strategies to unlock the potential of this biomass for fuel productions Materials and Energy from Coffee is also revised. Coffee by-products valorization is a clear example of waste biorefinery. Future By-Products: A Review. Sustainability applications in areas such as biomedicine, food packaging and material technology should be taken 2021, 13, 6921. https://doi.org/ into consideration. However, further efforts in techno-economic analysis and the assessment of the 10.3390/su13126921 feasibility of valorization processes on an industrial scale are needed. Academic Editor: Changhyun Roh Keywords: circular economy; food industrial waste; coffee waste; biotechnological conversion; biofuels; polymerisation; material production; zero waste Received: 28 May 2021 Accepted: 16 June 2021 Published: 19 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in While the world experiences climate change and every year authorities adopt more sus- published maps and institutional affil- tainable policies, the scientific community is investigating more environmentally friendly iations. solutions for the exploitation of food industrial residues. Waste management encompasses reuse, repair and recycling. Food waste, either derived from the agri-food industry or produced at the household level, is a good source of renewable raw materials. A large amount of food waste is generated in the EU, an estimate of approximately 100 million tons per year, nearly 30% of which comes from the agri-food Copyright: © 2021 by the authors. supply chain [1]. Such a production of waste and by-products induces high environmental Licensee MDPI, Basel, Switzerland. This article is an open access article impacts in terms of land use and a high carbon footprint and blue water footprint [2]. The distributed under the terms and global production of food waste is estimated to increase over 200 million tons by 2050 [3]. conditions of the Creative Commons Therefore, an efficient waste management is necessary. Attribution (CC BY) license (https:// In parallel, it is known that today most commercially available organic chemicals, creativecommons.org/licenses/by/ synthetic polymers and fuels are derived from oil resources. The use of fossil sources brings 4.0/). about a challenge in resource availability, cost fluctuation and very high impact in terms of Sustainability 2021, 13, 6921. https://doi.org/10.3390/su13126921 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 6921 2 of 19 carbon dioxide footprint and environmental consequences. For these reasons, chemical industries seek sustainable and innovative strategies in order to replace the traditional petroleum-based polymers and resins with new products that can still perform like their synthetic counterparts [4]. Indeed, the bio-based market keeps growing despite the fact that bio-sourced plastic represents only 1% of the 368 million tons of plastic produced annually. However, the total bioplastic production capacity is predicted to increase from 2.11 million tons in 2020 to 2.87 million tons in 2025 [5]. The most valuable alternative to fossil-based resources for the production of monomers and polymers is renewable biomass from vegetal and animal origin, preferably derived from agro-food waste and by-products, in order to avoid competition on the land use for the production of food and feed [2]. The search for sustainable alternatives to petro-sources has become increasingly inter- esting also in obtaining fuels from bioresources. The conversion of biomass to biofuels can be achieved via two major pathways, i.e., biochemical and thermochemical treatments [6]. A suitable biomass for biofuels production is characterised by a high content in sugar, starch or oils. Bioethanol and biodiesel constitute the main products, mostly meant for transport purposes. In this context, the energy produced from non-food/feed biomass would allow for a neutral carbon dioxide balance as natural ecosystems are currently unable to absorb CO2 and other greenhouse gas emissions in the natural cycle. In the European Union, the political targets for climate and energy by 2030 are: a 40% reduction in greenhouse gas emissions, 27% renewable energy installed capacity for the entire EU energy supply and a 27% improvement in energy efficiency [7,8]. It is clear that terms such as reuse, recycling and environmental sustainability are outlined as priorities of the European Union. In addition, citizens pay increasing attention to the ethical production of innovative and sustainable materials. These important new trends translate into a growing interest for education and training promoting the urgent need for sustainability and also by the increasing number of publications in international journals specialized in the circular economy and bioeconomy. Companies and stakeholders have realized the market value of sustainable materials and have turned their interest in such a direction. For example, many natural fibers and fillers have been tested for valorisation in biocomposites for a wide range of applications ranging from commodities such as packaging or agricultural items to more demanding applications such as automotive or construction panels, as well as energy production [4,6]. The increasing number of published studies on the use of plant-derived natural extracts or agricultural waste products addresses our need to both reduce waste and find new carbon resources. The low toxicity of plant-based materials favors their potential applications in food packaging and biomedicine [9–11]. Polysaccharide-based materials have been well investigated [12] and other renewable resources such as proteins and poly(lactic acid) (PLA) have also been examined [13]. Formulated composites with PLA as matrix and wastes deriving from the wine industry have shown improved mechanical properties [14], thus suggesting further applications and uses of wine waste [15,16]. The coffee plant is one of the agricultural resources for increased plant-based materials and its use includes a cycle from crop cultivation and processing to coffee beverage consumption. Considering that coffee is the second largest trade commodity with a global manufac- ture of 105 million tons per year worldwide, its industry generates enormous amounts of wastes during its processing from fruit to cup [17]. The annual coffee waste is estimated to exceed 23 million tons per year. By-products mainly arise from the removal of shells and mucilaginous parts from the fruits and depend on the processing method: wet or dry, roasting and brewing. Solid residues include coffee pulp, coffee husks, silver skin and spent coffee [18] (Figure 1). From a chemical point of view, coffee waste is an inexpensive raw material that contains fatty acids, which could be used as a sustainable carbon source, and it also represents an interesting source of bioactive compounds and fibers [18]. Indeed, coffee residues were proven to be an excellent resource for the production of high-value compounds [19,20]. The spent filter coffee (SFC) and the spent coffee grounds (SCG) [21], Sustainability 2021, 13, x FOR PEER REVIEW 3 of 20 Sustainability 2021, 13, 6921 3 of 19 and it also represents an interesting source of bioactive compounds and fibers [18]. In- deed, coffee residues were proven to be an excellent resource for the production of high- value compounds [19,20]. The spent filter coffee (SFC) and the spent coffee grounds (SCG) both widely pr[o2d1]u, cbeodtha twhidoemlye porowduocrekd, aast whoemllea sort hweocrokf, faese whuelslk as(C thHe) caonffdeec ohfufseke (pCuHlp) and coffee (CP), that is thepsuollpid (CfrPa)c, ttihoant wis athstee ssodliedr ifvraecdtifornom wacsotfefse edeinridvuesdt rfrieosm, c coonffsetiet uintdeucostfrfieees,w coanstsetitute coffee rich in polyphewnaoslste, craicrho tienn pooidlyspahnednoolrsg, caanrioctaenciodids s[ 2a2n–d2 5o]r.gTanhiec macaidins c[2o2m–2p5o].s iTtihoen ms oaifnt hcoempositions various fractionosf tahree vreaprioorutse dfrainctFioignus raere1 r.eported in Figure 1. Packaging proPdauccktaiogningfr opmrodcuocffteioenw fraosmte coorfgfeaen iwc aasctied osrigsaaninc oavceidl,s qisu iat enpovroeml, qisuinitge promising research area, arimeseeadrcaht parroead,u aciimngedb iaot -pbraosdedu,cilnogw btioox-bicaistyedp, ololywm teorxsic[i2t6y] .pIonlyamdedristi [o2n6,].c oInff aededition, cof- waste has beenfeseu wccaesstes fhualsly beuesne dsufcocrestshfeulplyr oudseudc tfioorn thoef pbrioodcuocmtipono soift ebsio[c2o7m–3p0o]s.itFeus r[2th7e–r3-0]. Further- more, biotechnmoloorgei,c bailopterochcensosleosgihcaavl perboeceenssdese vhealvoep beedenfo drebvieolfoupeeldp froord buioctfiuoenl pfrroomduccotifofene from coffee waste [31,32]. waste [31,32]. Figure 1F.igSucrhee 1m. aStcihcermepartiecs erenptaretisoentoafticoonf foefe cbofyf-eper boyd-upcrtosdauncdts tahnedir thmeairi nmcaoinm cpoomnpenontse.nts. 2. Challenges a2n. 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Thsueefsa catreth satitllt huencdlaeatar. cTohllee cfatecdt tshhaot wthes odmatea icnoclolencsteisdt esnhcoywa snodmteh aintcdoinffseisrteenntcy and that methods were duisfefedreinntt mheetvhaordios uwsesrteu udsieeds min athkee vitamrioourse sctoumdipesli cmaateket oit amssoerses ctohmepvlaicraioteu tso assess the biorefinery convcaerpiotsu.sI bnioardedfiintieorny ,ckoenycespttask. eInh oadlddeirtisofnr,o kmeyd sitfafkeerehnotlddeirssc fiprolimn edsifnfeereedntt odigsceitplines need together to deptloo ygemt utoltgiedtihsecirp tloin daerpylokyn omwulletiddgisec.iTphlienanroyv kenltoywinletdhgies.a Trehae mnoavkeelstyf oinr athniost ahreera makes for important gap ainoatphperr aimisipnogrtcaonftf egeapb iionr eafipnperariys.ing coffee biorefinery. The price of pTohlyem perricsei sofg proaldyumaellrys iisn cgreaadsuianlgly, aincdrepaslainsgti,c acnodn psulamstpict icoonseuxmcepetdioend exceeded 5 5 million tons minilGlioenrm toannsy ina GndermIt alnyyi ann2d0 I1ta7l.y Tinh e2s0e17t.w Tohecsoeu tnwtor iceosuwnterriesr wanekree dranfikrestd ifnirst in terms terms of coffeeofc oconfsfueem cpotniosunminpt2io0n1 8in[ 3240]1.8 I[n34t]h. eInir tchaesire ,cafosell,o fwolilnogwiangc iarc cuilracruleacro encomnoymy concept concept seems psereamctsic parbalcet.icCaobflfee.e Cwofafsetee wcoaustled ceoauslidly eabseileym bpe leomyepdlofyoerdp folry pmoelyrmpreor dpuroctdiounct,ion, apply- applying the ‘loincga lthpero ‘ldoucactl ipornofdourcltoiocna lfocor nloscuaml cpotniosunm’ cpotniocnep’ ct.oncept. Furthermore, EFurothpeeramnosrter,a Eteugroiepsesaunc shtrastetghies‘ EsuUchS tarsa tehgey ‘EfUor SPtlrastetigcys fionra PClaisrtciucsl ainr a Circular Economy’ andE‘Pcolansotmicsy’2 a0n3d0 ’‘Pailmastticosc 2h0a3n0g’ eai‘mth teow chaaynpglea ‘stthice pwraoyd pulcatstiacr perdoedsuigctnse adr,e pdreos-igned, pro- duced, used anddurcecdy, culseeddi nantdh ereEcUyc’l.ePdl ains ttihcem EaUn’u. fPalcatsutirce mrsaanruefacoctmurmerist taerde tcoomenmsuitrtedt htoa tensure that 60% of their pla6s0t%ic soff othrepira cpklaasgtiincsg fwori lplabcekareg-iunsge wdialln bde rreec-yucsleedd abnyd 2r0ec3y0c. led by 2030. Biofuel production is also an opportunity to explore. Indeed, advanced biofuels can be produced from lignocellulosic and non-lignocellulosic sources (forestry and food or agro- derived residues) by means of thermochemical conversion, mainly based on gasification or pyrolysis [6]. 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Two mTTTawwinooo mmclaasiiiinnnse cccsllllaaaosssfsssmeeesssa oooteffff rmmiaaaalttttceeearrriiniiaaallllb ccceaaapnnn rbbboeeed upppcrrreooodduuusccctaeeedddrt isssnttttgaaarrrfttttriiiinnnogggm ffffrrroooemmne wrrreeeannnbeeelwwe aaabbbbiolllleeem bbbaiiiioososmm. aaaOssssssn..... eOOnnneee iiiisss is relarrrteeelllldaaattttteeeodddt tthttoooe ttttshhhyeeen ssstyyyhnnnettttshhhiseeesssoiiiifsss tooorffffa tdtttrrriaaatdddioiiiinttttiiiiaooolnnnpaaallll a pppslllltaaaicssssttttiiiicccssssu csssuuuhcccahhhs aaabsssi obbb-iiiipooo----oppplyooo(llllyyeyt((((heeettttyhhhlyeyynlllleeeennn)eee())))b ((((ibbboiiii-oooP----EPPP)EEEa)))) naaadnnnddd bbbiiiiooo---- bio-poppplyooo(llllyyyet((((heeettttyhhhleyyynlllleeeennnteee rtttteeeprrreeehpppthhatttthhhlaaaatlllleaaa)tttteee(b)))) i((((obbb-iiiiPoooE----PPPTEEE),TTTw)))),,,,, hwwichhhhiiiicccahhhr eaaarrraeeelm aaallllommstooonssstttto nnooo-bnnni----obbbdiiiioooedgddreeeagggdrrraabdddlaaaebbb, llllbeeeu,,,,, bbbt uuuhtttte lhhhpeeellllppp rrreee---- replacpppellllfaaaocccseees ifffflooo-bssssssaiiiislllle----bbbdaaabssseeeudddil dbbbiuuuniiiiglllldddbiiiilnnnogggc kbbbsllllooowccckkkitsssh wwidiiiietttthhhn tiiiiidddcaeeelnnnmttttiiiicccoaaallllle cmmuooolellllesee.cccuuuTllllheeeesss.....o TTTthheeer ooocttttlhhhaseeesrrr rcccellllalaaassstssse srrreeetlllloaaattteeehssse ttttooo tttthhheee produpppctrrriooodddnuuuocccfttttiiiiboooinnno d oooeffffg bbbraiiiioooddddaeeebggglerrraaamdddaaatbbbellllreeei ammlsaaa, ttttieeenrrrciiiilaaaulllldsss,,,,,i niiiinnngccctllllhuuuedddriiiimnnngggo ptttthhhlaeeesrrrtmmicooosppptallllraaacssshttttiiii,ccca ssslittttpaaahrrrcccahhhti,,,,,c aaaplllliiiioppplhhhyaeaasttttiiiitccce rpppsooollllyyyeeesss---- like potttteeelyrrrsss( lalllliiiickkkteeeic pppaoooclllliyyyd(((()llllaaa(PcccttttLiiiicccA aaa),cccpiiiidddo))))l y((((PPPhLLLyAAdr))))o,,,,, xpppyoooalllllyyykhhhayyyndddorrraoootxxxeysyyaaa(PllllkkkHaaannnAooosaaa)ttttaeeensss d((((PPPpHHolAAysss(b)))) uaaatnnnydddle pppnoooellllsyyyu((((bbbccuuuittttnyyyallllteeeennn)eee sssuuuccc---- (PBS).cccIiiiinnnaaaattttdeee))))d ((((iPPPtiBBBoSSSn)))),..... nIIIInnno taaadddaldddliiiibttttiiiiioooonnnd,,,,,e nnngooortttta aaadllllallll bbbliiiieooodddbeeeiogggprrraaaladddsaaatbbbicllllseee abbbriiiioooeppprlllleaaaasssdttttiiiiiccclysss aaadrrreeeg rrrreeeaaaadddaiiiillllbyyyl eddd.eeeIgggnrrraaafdaddcaaatbbb, llllteeeh..... eIIIInnn ffffaaaccctttt,,,,, procesttttshhhoeee fpppbrrriooocdcceeeesssgsssr aooodffff abbbtiiiiiooondddeeedgggerrrpaaaedddnaaadttttsiiiioooonnnn dddtheeeepppeeesunnnrdddrssso uooonnnd ttittnhhhgeee esssnuuuvrrrrirrroooouuunnnnmdddeiiiinnngggta leeecnnnovvvniiiirrrdoooitnnnimomneeesnnn(tttteaaa.llllg .ccc,ooopnnnHdddiiiittttiiiiooonnnsss or tem((((eeep.....egggr.....a,,,,, tpppuHHre o)oo,rrro ttttneeemmthpppeeeemrrraaaattttuuuterrrreeei))))a,,,,,l ooomnnno ttttrhhhpeeeh mmolaaaottttgeeeyrrriiiiaaalllln mmd ooornrrpppthhhoooelllloooagggpyyyp laaaicnnnadddt iooonnn (ttttehhh.eeeg .aaa,pppbpppiolllliiiimcccaaaettttdiiiioooicnnna ((((leeeo.....gggr .....,,,,, bbbiiiiooo---- agricummltueeedddraiiiicccl)aaall[ll 3ooo5rrr] .aaagggrrriiiicccuuullllttttuuurrraaallll)))) [[[[333555]]]]..... Table 1TTT.aaaCbbbolllleeem 111m..... CCCerooocmmiamml peeeorrrrlcccyiiiiiaaamlllll eppprooosllllloyyybmmtaeeerrirrnssss eooodbbbtttteaaaniiiiitnnniereeedddl yeeennnottttriiiiirrrrpeeealllllyyyrt ioooarrrrl lpppyaaafrrrrrttttoiiiiiaaamllllllllllyyyre ffffnrrrroooewmma rrrrbeeelnnneeeerwwesaaaobbbullllleeerc rrrreeeesssss.ooouuurrrrccceeessss..... PPPooollllPyyyommlyeeemrrr er CCChhhheeeemmiiiiicccaalll SStttttrrrruuccccttutttuuurerrreee BBBBiiiioiooo-----ppoolllllyy(((eettttthhyyyylelllleeennnneeee) ))))( b((((ibbboiiii-oooP----EPPP)EEE)))) BBBBiiiioiooo-----pppooolllllyyy((((ppprrrrooooppppyyyylelllleeennnne)eee())))b ((((ibbboiiii-oooP----PPPP)PPP)))) BBBBiiiioiooo-----pppooollllyyy((((eeettttthhhyyyyllellleeennnneeeet ettttreeeerrrpeeehpppthhhttttahhhlaaaatlllleaaa)tttteee)))) (((((bbbbiiiiioooo-----PPEETT)))) BBBBiiiioiooo-----pppooollllyyy((((ttttrrriiiiimmeeeethtttthhhyyyylellllneeeennneeete rephthalate) tttte(eebrrrieeeoppp-PhhhTtttthhhTaaa) llllaaatttteee)))) ((((bbbiiiiooo----PPPTTTTTT)))) PPPollPoooll yyy((((ely(ee tttthethhh y yyy llene llleeennneee2 222,,,5,,, 5 -55 -- f--fu and urandicarb1oxylate) (PEF) 1 fffuurrraannddiiiicccaaarrrbbboooxxxyyyllllaaatttteee)))) ((((PPPEEEFFF)))) 111 BBBBiiiioiooo-----pppooollllyyyaaammiiiiiddddeeeessss Non-Biodegradable NNNNNNoooooonnnnnn----BB--BBBBiiiiooioioooddddddeeeeegegggggrrrrraaraaaaddddddaaaaaabbbbbblllleelelee e Sustainability 2021, 13, 6921 5 of 19 Sussttaiinabiilliitty 2021,, 13,, x FOR PEER REVIIEW 5 off 20 Table 1. Cont. Polymer Chemical Structure SustainabilityS 2St0at2a1r,r c1c3,h x FbOlRe nPEdEsRs R EVIEW 5 of 20 PPoolllyy((hydroxxyyaalkllkaanSnotaaortcaeht) besl)e(snP d(HsP AH)A) Poly(hydroxyalkanoate)s (PHA) PPoolllyy((lllactiic acciiidd))( P(PLLAA) ) Poly(lactic acid) (PLA) PPoolllyy((butyllenees suuccicniinataet)e()P (BPSB) S) Poly(butylene succ inate) (PBS) Poly(butylene adipPaotley-(bcuot-ytleernep ahditphaatel-actoe-) P(oPllByA(bTu) tyllene adiitperaepteh-thcaola-te) (PBAT) terephthallate) (PBAT) 1 1 currently in development, predicted to be available in commercial scale in 2023. currently in development, predicted to be available in commercial scale in 2023. 11 currenttlly iin devellopmeTnottd,, apyr,e bdioipcltToday, bioplastics are mostly priot aesdti ctso a rbee m aovstalyi duc te i ll aprbolldeu iicned c sotmartminge rfrcoiimall csacrabolleh yiidnr a2t0e-2r3ic..h plants such as corn or sugar cane, thde ssot-acarltleidn gfoofdr ocrmopsc oarr fbirosth-gyenderrattieon-r fiecehdstpoclka.n Ftirssts-guecnhera-s corn or sugar cane, the soti-ocna fleleeddstofcoko isd cucrrroenptlsy tohre bfiiormsta-sgs ethnaet prarotdioucnesf tehee highest yields of bioplastics [5], Today,, biiopllasetviiecns i are mostlly dstock. First-generationf th compe itio np wriothd fuoocde adn ds tfeaerdt piinrogd ufcrtoiomn i sc eavridbeonth. ydrate-riich pllants such feedstaosc kcoirsnc uorrr seungtlayr tchaenbei,, o Ttmhheea bssioosp-tlcahasatllictllsep dinr dofuodsoturdyc eicss raotlshpo esd ehoviregl ofhpiireinssgtt -tgyheei enulseder soafot inifoobnn-i ffoeopdel dacrssottpiocs cs(kse[..c5 oF]n,iidre savtn-edgn enera- if the competition with fothoirdd gaennderafteioend feepdrsotodckus)c. tIniononvative tectiion feedstock iis currentlly the biiomass that ipsreovdiud henonlotgies are focusing on non-edible by-prod- ucts from the production of food crops, wcheicsh . tihneev ihtaiibglyh egesnt yields of bioplaThe bioplastics industry is also developing the use of non-food cropse r(aisteeslc loarn gde a amnioduntthls o sft iics [5],, even iif the competiiwtiiaostne. wThiietrhe a froe soigdn iafincadnt fpeoetednt ipalr voodluumcetsii foorn u siise ien vbiiiodteecnhnto.. logical processes to cr ier-d generationTfheee dbs i toopclkass)t.icIasnt eni pnolavtfi l i i Tdhu aotrmiv cehetmecichalns foolro ingdiuesstrialr peufropocsues siuncgh aos nthen pornod-uecdtiiobn loef bioyp-lpasrtiocsd. ucts from the production of food cer svoatrploysri siiasti oanll soof b dy-epvroedlluoctps iianngd wthasete uflsoew so afs nraown m-faoteoridals cinr oap bsio r(esfienceoryn d and thiird generatiion feedstocks). I , provides a gr.e an w t andohvicahtivinevitablyvantaige ean tde crehdnucoeslo generates large amounts of waste. l pgreiisessu rae roen faoracbules liandg [o36n,3 7n].o Tnh-ee udsei bofl efo body -prod-Thereuacrtes sfirgonmifi tchaen tpproodtweuanscttteii acoalnn v booe fal u sfomoluoetdiso nf cfoorror tphuess ,de ewvienhloipbcmhioe ntiten och ef vsnuistol taabiogic i i l i nlayb ilig atyel bnpye rmroaecteeteinssg s leeansrvgitroeon cmarmeenatoatle platform chemicals for indu unts of and rsetsroiuar lcep cu harlpleongse ss. such, as tihe piroduictioln of bioplast ilcs. Twheasvtea..l oTrhiesareti aorne osiifgbnyiif-iIipnc raroencdte n upt coytetsaernsa ctnoiiadfflle ewv boayll-suptrmeodfleuscot sfw oresrc euaivsseedr iiamnwu bchiim omtaoertceeh artnitaeonllsltoioignn iiacsaa all b p piroormoreicsfieinsngs ereers-y to cre- providaetes pallgatfo newable feedstock fo reartma d cvhaenmtaiicgaellsa fnodr iirnedduuscter rii avlla rpiouurs pproosceesss e ss and for ts pr ssure on aurcahb laes htehire c ponrvoedrsuiocn tiiinoton h oigfh bviaiolue-added products thr land [36,37]. The use o pfllafsotoiicds.. waste can bTehea svoallluotriiiosantiifoonr toThafe k bidnyge - opurgoh dbiuorcetfisn earyn tdec hwniques. ivnteol ocopnsmideern attioonf t hseu psartseasteienn cfaell boofiw ldiitsfy f earbseny tr acmhweem emitcianalst gienre ciinoafvllfesie r iibonyn -pamr obediniuocttrase lfiinery and repsroouvriicdeecsh aa lglerenagte as.dsuvcahn atsa hgyed raoxnyd a cridesd, iut cise ws oprtrhe mssenutiroen iongn t haarta sbomllee llmaonledcu [le3s6 s,,u3c7h] a.. sT chafefe iuc saceid o f food wast represent interesting building blocks for the preparation of bio-based polymers such as In recee ncat ny ebaer sa scoollfufpetoeiiloyebnsy t ef-rosp,r r ptoohdlyeua mdcitedsveser elaloncdpe impvoeelydn (ta mnohfuy dscruhidsetm aeiisonteraresb). iaillTtiihttueysn ,b tviyao l onmrizeaiensgti iancogpff ereeon mrvesiiirdsouinensgm entall renewaanbdle rfeeseodusrtcoec kchfaollrllevonpagernieoss uu..p s npewro pcoesssisbielistieasn fodr af owridteh sepeirctrcuomn ovf earpspiliocantioinnst o[38h].i gh value-added In recent years coBfifoe-eac tbivye -mporloecduluesc dtse r riveecde fiirvome dco fmfeeu bcyh-p rmodourcets caatnt ealnsot iiboen v aalosr iaze dp irno poly-products through biorefinery miisiing re- newable feedstock mfoerr for tmecuhlantioinq uine osr.der to impart specific properties to the polymeric m Taking iln to conside arnattioi xovindaarttiiihvoeu ospr brpieorcsoiedcanel cpsersoeopsef ratdienisfd. fT e hfreoe crno ntthtecenhtii rein m cvoailncuavblesler imsniioolcenco ufiilnfeest esou bchhyi agtrhix vsuaclhu as ia-sp car fofedinlue coetr- sadded such apsrohdyudcrtosx tyhraocuidgsh, bictiihoilosrreowfgiieonnriectr hayci mdte cecannh tnbieiio qenxupienlosgit..e dth bay tussiongm coeffmee owlaestceus ales sfillseursc inh baioscocmapfofesiitecs a[20]. Takiing iinto The following sections highlight the recycling potential of coffee by-products for mon coi-d represent interesting bucoimlndesrii inadngedr bpaoltoliiyocmknes r tphfrooerd putchrteieosnpe, nmrecaptee raoiarfla fdtoiriiomfnfuelaroteifonnbt ia ocs h-wbeealml saeiisc dfaorllp sbo iiiolnfyu meclo aefnrfdes besi oubencyeh-rpgayr soducts polyesstuecrsh, paso lhyyamdriodxeys aancpidirdodpsu,,o citilityo nii(s. a nwhoyrdthri dmeeensttiieorns)ii.nTgh tuhsa,tv saolmoreiz imngollceocfufelleesr esusicdhu eass ocpafefnesiic aciid up newrepproessseinbti l iiitniteesrfeosrtiiangw bidueiillsdpiinecgt rbullomckosf afoprp tlhicea tpiorenpsa[r3a8t]ii.on of biio-based polymers such as 3.1. Monomer Production l Bpioo-llaycetsitveersm,, oploellcyualmesiiddeeAsr invaunemddbe fr proofo millnyte cg(oraaftnfeedh ecyobdnvyreii-rdpsieor no edtescuthecnrtosslo)cg.. iaeTns hfoaurl tssh,o,e tvbraaenllsovforariilmzoaiirntiiogzne odcfo cifonfffeepee o brylye- s-iidues mer foorpmenusl autpio nneiwn oprodsespiirbrotiidolluiitciiitmes scpa fnao rrret saus lptw ieniic dtihefie csrepcpoervcoetrpryu eomrf tci h eoesmf aitcopal ptbhluliiiecldapintiiogo lbnylosmc k[es3 rt8hi]ca..t mcana btre iuxsesdu focrh thae s antioxidatiBviieoo-arcbtiiivoec i mdaolllppercrooudupllceetsiro tndi eoefsr m.iivToenhdoem fecrrooicnm utne icntso tsfuifncehe va bsa llyac-tpicr aocdidu, scutcsc inciac nac iadl,s loev ublein ivcand polyols. uable mole cule s lsuc h a s aaccllaiodfr,f iaiezlicenodheo loiisn r polly- chloromgeenr i cfoarcmiducllaantiiobne eiinxp olorditeerd tboy iiumspinagrtc osfpfeeceiiwfiica spterospaserfitilileesr stoin thbieo cpoomllypmoesirtiiecs m[2a0t]r.iiTx hseuch as followaintgiiosxeiicdtiaotniivseh oigrh bliiogchiitdtahlle prreocpyecrlitniiegs..p Tohteen ctoianltoefnct o iinff eveablluya-pblrleo dmuoclltescfuollrems souncohm aesr canffdeiine or polymcehrlloprrogdeunciitcio anc,iidm acatenr ibael feoxrpmllouiiltaetdio bnya suswiinelgl acos fffoere b wioafsutels aansd fiibllllieoresn iienrg byiiopcromdupcotsioiitne.s [20].. The follllowiing sectiions hiighlliight the recyclliing potentiiall of coffee by-products for mono- 3.1. Mmoneorm aenrdP proodlluycmtieonr productiion,, materiiall formullatiion as wellll as for biiofuell and biioenergy Aprnoudmubcteiirono.f. integrated conversion technologies for the transformation of coffee by- products can result in the recovery of chemical building blocks that can be used for the produ3c..t1i..o Mn onfomoern oPmroedruictiiuoni ts such as lactic acid, succinic acid, levulinic acid, alcohols and polyolAs. number of iintegrated conversiion technollogiies for the transformatiion of coffee by- products can resullt iin the recovery of chemiicall buiilldiing bllocks that can be used for the productiion of monomeriic uniits such as llactiic aciid,, succiiniic aciid,, llevulliiniic aciid,, allcoholls and pollyolls.. Biodegradable BBBBiiiiooooddddeeeeggggrrrraaaaddddaaaabbbblllleeee Biodegradable Sustainability 2021, 13, 6921 6 of 19 3.1.1. Lactic Acid Lactic acid (LA) is the simplest hydroxyl acid with an asymmetric carbon atom and two optically active configurations, namely the L and D isomers (Figure 2). LA is a well- established bio-based chemical and its global annual production amounts to approximately 270 kilo tons per year [39]. It finds applications in several sectors ranging from the food industry to cosmetics and pharmaceuticals, and it is also employed for the synthesis of PLA, which is by far the most commercially developed bio-based plastic that is mainly applied in packaging and in pharmaceutical industries. LA can be produced through a petrochemical route or biotechnologically by exploiting renewable feedstocks, mainly corn [40], but also food wastes and by-product streams [41–43]. Currently, the industrial production of LA is mostly carried out by microbial fermentation of carbohydrates, since higher purity products can be obtained. The majority of the fermentation processes use species of Lactobacilli, which give high yields of LA. Some organisms predominantly produce the L isomer, such as Lactobacilli amylophilus, L. bavaricus, L. casei and L. maltaromicus, whereas L. delbrueckii, L. jensenii or L. acidophilus produce the D isomer or a mixture of L and D [44]. Recently, it was reported that wastes from the coffee-processing industry such as coffee mucilage, CP and SCG were converted into LA trough Bacillus coagulans. B. coagulans is a very interesting L-lactic acid producer due to its ability to ferment pentoses. Furthermore, its thermotolerant nature (50–55 ◦C) minimises the need for sterile conditions. Hudeckova et al. [45] described the use of SCG as a promising raw material substrate for LA production. SCG was first hydrolyzed and then used as substrate for culturing several lactic acid bacteria and LA producing B. coagulans. Among them, Lactobacillus rhamnosus CCM 1825 was identified as the most promising producer of LA on SCG hydrolysate, giving a yield of 98% in LA. The same substrate was also investigated by Breton-Toral et al. [46] and compared with other food waste carbon sources such as potato peel and almond shells. They used an undefined mixed culture isolated from coffee mucilage and also studied the application of several biomass pretreatment and calcium carbonate as a buffer to increase the release of fermentable carbohydrates from the wastes. Neu et al. [47] investigated the use of coffee mucilage as carbon source in the produc- tion of L-lactic acid with B. coagulans and yeast extract as an additional source of nitrogen. The raw material in this case was a liquid suspension containing several free sugars. The experiments resulted in the production of pure (99.8%) and highly concentrated (930 g/L L-LA) lactic acid formulation. CP hydrolysate was investigated by Pleissner et al. [48] for the lab production of LA and in pilot scales using B. coagulans. The raw material, with a high content of lignocellulose was first hydrolysed by means of thermochemical treatments and a subsequent enzymatic digestion. The fermentation was performed in presence of yeast extract and gave rise, at both scales, to a highly concentrated (937 g/L) and pure (99.7%) L-lactic acid. An engineered Saccharomices cerevisiae strain was used in a preliminary study as an alternative to B. coagulans for the fermentation of acid-pretreated SCG by Kim et al. [49]. The results proved the feasibility of using SCG for the selective production of LA, being the LA produced 0.11 g per g of SCG after 24 h. SSuussttaaiinnaabbiilliittyy 22002211,, 1133,, 6x9 F2O1 R PEER REVIEW 77 ooff 1290 Fiigurre 2.. Monomerriic uniittss ffrrom coffffee by--prroducttss.. 33..11..22.. SSuucccciinniicc AAcciidd SSuucccciinniicc aacciidd ((SSAA)) ((FFiigguurree 22)),, aann iinntteerrmmeeddiiaattee iinn sseevveerraall cchheemmiiccaall pprroocceesssseess,, hhaass eemmeerrggeedd aass oonnee ooff ththee mmoosts tcocmompepteittiivtiev ebiboi-ob-absaesde dchcehmemicaiclsa.l sT.heT hmeajmora jdorrivderrisv eforsr tfhoer gthroewgrthow oft hthoisf mthaisrkmeta rakree tthaere inthcreeianscinrega aspinpgliacaptpiolincsa tuiosinnsgu SsAin agnSdA thaen tdretnhde otrfe tnhde cohfetmhe- icchael minidcaulstirnyd tuos itnrycretoasiincreasingindustrial applications nbgalsye dseoanrc lhy fsoera brch for bio-this buiiold-binagsebdl osu based sustainable chemicals. Amon ckst,abiuntaabnlee dchioelm, PicBaSlsa. nAdmitosncgo ipnodluymstrei g rasl aapnpdlpicoaltyiounrse tbhaasneeds o(nP Uthsi)sh bauvieldtihneg lbalrogceks,t bsuhtaarneeodfitohl,e PmBaSr aknedt. iTtsh ceocpoonlvyemnetrios naanldp prooclyeuss- rfoetrhSaAnepsr (oPdUusc)t ihoanvies tchaer rliaerdgeosutt sfhraorme opfe tthroec mheamrkiceat.l Trahwe cmonatveerniatlisonstaalr ptirnogcefrsos mforn S-bAu tparnoe- dorubctuiotand iise ncea.rrPieetdro ocuhte mfroicmal pperotrdoucchteiomnichaals rraewm aminaetedrisatalsb sletafrotrinyge afrrso,mbu nt-rbeucteanntea dorv abnucteas- diniefneerm. Peenttraotciohnemfriocmal dpirfofedruecnttiognlu hcoasse rseomuaricneesd, i nstcalubldei nfogrm yiexaerds,f oboudt rwecaesntet, aadgvriacnuclteusr ianl wfearmsteenatnadtiotenx tfirleomw adstieff[e5r0e–n5t 2g],lusucoccseee sdoeudricnesm, ainkcinlugdbiinog-b masiexdedS Afoeocdo nwomasitcea, llaygartictrualcttuivrael, wanadstneo awndth teexfetirlme wenatsatteiv [e50S–A52p]r, osduucccetieodnehda isno mutackoimngp ebtieod-biatssecdo nSvAe netcioonnoalmpircoadlluyc atitotrna.c- tive, Iatnwd ansowre ctehnet flyermreepnotratteidve[ 2S3A] tphraotdcuocftfieoen hhuassk o,uit.ec.o,mapwetaesdte itosb ctoaninveedntfiroonmal pprroocdeuscs-- tiinogn.c offee cherries, could be used as substrate for bio-succinic acid production with A. succinIto gweanss .rMecoernetlsyp reecpifiocratelldy, [t2h3r]e tehparte c-otrfefeaetm heunstkm, i.eet.h, oad ws awseter eocbotaminpeadre fdro(mth eprrmoacle,stshinerg- cmooffcehee cmhiecrarlieasn,d cofuunldg ableh uydserdol yassi ss)u.bTshtreatthe efromr obciho-esmuciccainl ipcr ea-ctirde aptmroednutcwtioans pwroitvhe nA.t osubce- cthineomgeonsst. eMffoerceti vspeeacnifdiclaeldlyt,o tthhreeeh ipgrhee-tsrteSaAtmyeienltd m(0e.t9h5ogdsS Awpereer cgoomf preadreudci n(tghesurmgaarls, )thanerd- mproocdhuecmtiviciatyl a(n0.d5 4fugnLg−a1l hh−yd1)r.olysis). The thermochemical pre-treatment was proven to be the mTohset eufsfeecotifvCe SaSndw laesdi ntov tehseti ghaigtehdesatt SlAab ysiecladle (0b.y95N gi gSlAio peetra gl. o[f5 3re]dfourcitnhge spurgoadrusc) taionnd pofroSdAuactnidvibtyu t(a0n.5o4l gbLy−1f ehr−m1).e ntative process. CSS was first subjected to alkaline hydrolysis pre-trTehaetm uesen toaf nCdSSen wzyasm iantviceshtyigdartoeldy saits ltaobm scaaxliem biyze Nsiuggliaor erte laela. s[e5.3]T fhoer mtheed piuromduobcttiaoinne odf wSAas asnudb jbecutteadntool fbeyrm feernmtaetinotnatwivieth pCrolocsetrsisd. iuCmSSa cwetaobs uftiyrslitc usmubtjoecotbetda itno aamlkiaxltiunree hoyf dacreotloynsies, pburet-atnreoal tamndenett haanndo eln(AzyBmE)atinic thyedmrollyasrirsa ttoio moafx2i:m5:1iz,er essupgeacrt irveelleya,saey. Tiehlde mofe7d.3iu%min obutatianneodl wpearsg suofbjseucgteadrs taon fderampernotdatuicotniv wityithin CAloBstEriodfiu0m.0 6acgeLto−b1uthy−li1c.uTmh teos oambteaimn ead miuimxt,usrue bojef catceed- tonfer, mbuetnatnaotilo annwd ieththAan. soul c(cAinBoEge) nins, tghaev me aolSaAr rpartoiod uofc t2i:o5n:1y, ireelsdpoefct8i4v%elyp,e ar ygieolfds uogf a7r.3s%an idn bauptraondoul cptievri tgy of 0su.1g5agrsL −a1ndh −a1 .productivity in ABE of 0.06 gL−1 h−1. The same medium, subjected to fermentation with A. succinogens, gave a SA production yield of 84% per g of s3u.1g.3a.rsL aenvudl ain picroAdcuidctivity of 0.15 gL−1 h−1. Levulinic acid (Figure 2) is considered as a promising platform chemical due to its 3h.i1g.h3. rLeeavctuivliintyic cAoncifde rred by the presence of carboxylic acid and ketone functional groups. LevuLlienvicualicnidic iascaidv e(rFsiagtuilreeb 2u)i lids icnognbsliodcekrefdor atsh ea pprroodmuicstiinogn opflapthfoarrm accheeumticicaal,l pdluaset itcoiz ietrs hanigdhs reevaecrtaivl icthye cmonicfaelrrdeedr ibvya ttihvee spsreuscehnacse SoAf c,aγr-bvoaxleyrloicl aacctiodn aenadn kde1to,4n-ep efunntacntieodniaoll g[2r4o,u5p4]s.. LevuUlinpict oacnido wis, ale vveurlsiantiiclea bciudildisinegx cblluoscikv efloyr pthroe dpurcoedducftrioomn opfe ptrhoalremumacesuotuicracel,s p. laTshtie- ccioznevr eanntdio nseavleprraol dcuhcetmioincaol fdleervivualitnivicesa csuidchis absa SsAed, γo-nvaclheermolaiccatol nceo navnedr s1io,4n-peemntpalnoeydiniogl [m24in,5e4r]a. l acids or heterogeneous catalysts like zeolites. In recent years, several biomass wasteUsp, s tuoc hnoaws r,i lceevsutrlianwic, caocrind sitso evxecrl,umsiivcreolyal pgaroe,deutcc.e,d[5 f5r–o5m7] pweterroelueusemd sfooruirtcseps.r oTdhuec ctioonn-. vInenptaiorntiaclu plarro,dTuucktiaocns oeft laevl.u[l2i4n]icr aecpiodr itse dbatsheed uosne cohfemSCicGal acnondvceorosikoend emteaplloeyaivnegs mfoinr etrhael apcrioddsu ocrt ihoenteorof gleevnueoliunsic caatcaidly,svtsa lliokrei szienoglitthese.i rInl irgenceoncet lyluealorss,i csecvoenrtaeln bti.omThaessy wstausdteise,d suthche ainsfl ruiceen scteraowf w, caotrenr sptroevseern, cme iacnrodamlgiacer,o ewtca.v, [e5p5–re5-7t]r ewatemree unst.edIn ftoerr eiststi pnrgolyd,uthcteiyond. iIsnc opvaerrteicd- uthlaart, tThuekdarcys ientg apl. r[o2c4e]s rseopforbtieodm tahses ucsaen obf eSCavGo aidnedd c,oaonkdedth teeam leicarvoews afovre ttheec hpnroiqduuectciaonn Sustainability 2021, 13, 6921 8 of 19 drastically reduce reaction times while enabling the same conversion yields. In particular, in the case of a commercial arabica-based SCG, the yield in levulinic acid, calculated on the average cellulose content of biomass waste, was 12.9%, while in the case of robusta-based SCG, the yield was 14.6%. SCG was also used as a starting raw material by Kim et al. [58] to produce levulinic acid and formic acid by using a catalyst-free hydrothermal biphasic system, thus avoiding corrosion and undesirable side reactions. A single and a double steps conversion consist- ing in a lipid separation, followed by the conversion of levulinic and formic acids were investigated. In addition, several other parameters (pre-treatment conditions, temperature, reaction time, solvent amount and moisture content) affecting reaction yields were investi- gated and optimized. Thus, the yields of the two acids were maximized when SCG was used with water in a molar ratio of 1:8.33 at 180 ◦C for 3 h and a maximum yield of 47 and 29 wt.% (based on convertible monosaccharides in raw SCG) in levulinic and formic acid, respectively, was obtained. 3.1.4. Quinic Acid Quinic acid is a polyhydroxy molecule (Figure 2) with interesting biological activity (growth promoting property) and chirality that is useful in pharmaceutical applications. It can be extracted from several plant sources and from coffee beans [59]. Besset et al. [9] reported for the first time the use of quinic acid as a monomer for the synthesis of polycarbonates from renewable sources as an alternative to polycarbonates obtained using bisphenol A, which is a highly toxic and hormonally active compound. 3.1.5. n-Butanol, Isopropanol and Polyols Today, alcohols and polyols are produced almost exclusively from petrochemicals, but the industrial production of these chemicals by fermentation from renewable and sustainable resources has recently increased worldwide. n-Butanol can find broad appli- cations as an intermediate in pharmaceuticals and polymers production, as food grade solvent in the food and flavor industry and, recently, as fuel [60]. Isopropanol is one of the main short alcohols for industrial application. In addition to its use as a solvent, it is also employed as a chemical intermediate and a fuel additive. Polyols instead became increasingly interesting for the preparation of polyurethane, and several researchers are exploring different biomass residues to obtain bio-based polyols [61]. Thus far, Clostridium beijerinckii is the most widely used strain for fermentation to produce n-butanol and iso- propanol from biomass [62]. Procentese et al. [32] investigated for the first time the use of alkali pre-treated and enzymatically hydrolysed coffee silverskin as a carbon source for Clostridium beijerinckii fermentation for the production of n-butanol and isopropanol. Low yields were obtained for both alcohols in aqueous medium: 0.86 g and 1.66 g per 100 g of dry CSS for isopropanol and butanol, respectively. Spent coffee grounds were used to obtain liquefied polyols via sulfuric acid treatment using polyhydric solvents (polyethylene glycol and glycerol) at moderate temperatures [37]. Three key parameters were optimized to increase conversion, namely temperature, sulfuric acid concentration and reaction time. From an industrial point of view, the established process proved to be very interesting, as it only generated 8 wt.% of residue per total mass of polyols with 70 wt.% of conversion. Moreover, the obtained polyols showed adequate characteristics for the substitution of existing commercial polyols (produced from petrochemicals) in the synthesis of polyurethane foams. To this aim, isocyanates were reacted with the polyols obtained from spent coffee grounds [63]. In Table 2, the main examples discussed in Section 3.1 are summarized. Sustainability 2021, 13, 6921 9 of 19 Table 2. Example of monomers from coffee by-products. Monomer Coffee By-Product Organism Yield Ref. Saccharomices cerevisiae 0.11 g/gSCG [49] Spent coffee ground Bacillus coagulans 98.0% [45] Lactic acid Coffee mucilage Bacillus coagulans 99.8% [47] Coffee pulp Bacillus coagulans 99.7% [48] Coffee husk Actinobacillus succinogens 0.95 Succinic acid g/g [23] reducing sugars Coffee Silver skin Actinobacillus succinogens 84% [53] / 13–15% [24] Levulinic acid Spent coffee ground / 47% [58] n-Butanol Coffee Silver skin Clostridium beijerinckii 0.0086 g/gCSS [32] Isopropanol Coffee Silver skin Clostridium beijerinckii 0.0066 g/gCSS [32] Polyols Spent coffee ground / 70% [37] 3.2. Polymer Production As stated above, bio-based polymers have become increasingly attractive in recent decades thanks to the significant development of biorefineries, which allow the production of a wide variety of bio-based building blocks or the exploitation of naturally produced biopolymers. The production of polyhydroxyalkanoates (PHAs) directly from coffee by- products is described here as a main example of polymer production. PHAs are a family of biopolyesters synthesised as intracellular products by various microorganisms. These microorganisms use PHA as a form of carbon and energy storage. They are very versatile polymers, mostly used in the packaging industry. Due to their biocompatibility and biodegradability, they are also employed in the medical field and as carrier materials in agriculture, food technology or pharmaceutical sectors. PHAs can be produced from natural resources, classically from highly nutritional substrates such as glucose, starch or edible oils. Several researchers recently approached the possibility of using a broad range of industrial- and agro-waste as well as wastewater. The description of conversion of SCG or oils extracted from SCG into PHAs is well documented in the scientific literature. Kovalcik et al. [64] investigated the potential of Halomonas halophile to produce PHA from three different acid-treated SCG hydrolysates: unmodified, defatted and defatted without phenols. It was shown that all the different samples gave rise to high carbohydrate releases. However, the presence of phenols negatively affected the cultivation of the bacte- rial strain. The purified sample, without phenolic contaminants, enabled the production of high molecular weight poly(3-hydroxybutyrate) (27 wt.% of dry cell weight). Obruca et al. [25] studied several strains for the production of PHAs from SCG: Cupri- avidus necator H16, Bacillus megaterium or Burkholderia cepacia. In particular, oil extracted from SCG proved to be the best substrate for the poly(hydroxybutyrate) homopolymer production among the waste oils tested. C. necator H16. B. megaterium or B. cepacia were tested for their capacity to use SCG hydrolysates. B. cepacia demonstrated higher PHAs yields and production coefficients. In addition, this strain was able to accumulate the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate), which possesses better mechan- ical properties as compared to the homopolymer produced by B. megaterium. Probably, the presence of levulinic acid in the substrate induced the biosynthesis of valerate units by B. cepacia. The same authors also investigated various detoxification methods prior to hydrolysis for the removal of microbial inhibitors such as phenols present in SCG in order to improve PHAs production yield. Bathia et al. [65] studied the use of coffee waste oil as a source of fatty acid for the production of (3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer by genetically engineered Ralstonia eutropha without additional precursor feeding. Several parameters, Sustainability 2021, 13, 6921 10 of 19 such as the solvent for coffee extraction, concentration of coffee oil and concentration of nitrogen source and time, were optimized. The final production of PHA copolymer obtained was 69 wt.% based on cell dry weight. The use of oil extracts from SCG for the production of poly(3-hydroxybutyrate) by wild type Ralstonia eutropha was previously investigated by Obruca et al. [66]. The authors reported that copolymers could be obtained with the wild strain only by feeding additional precursors, such as propionate. Finally, Cruz et al. [67] reported the application of supercritical carbon dioxide ex- traction to improve oil extraction from SCG to be used as a substrate for C. necator strain culture for the production of poly(3-hydroxybutyrate). The final yield was 77% of PHA based on SCG oil weight. 3.3. Composite Materials Production Coffee silver skin (CSS), also called spent coffee chaff, is the tegument of green coffee beans. CSS is obtained as a main by-product from the roasting process [68] and consists principally of lignin (29 wt.%), polysaccharides such as cellulose (24 wt.%) and hemicellu- lose (17 wt.%) [69]. Silver skin may be valorised as poultry feed and/or as a raw material for paper production. None of these methods used for the valorisation of CSS is an ideal solution in terms of value addition, especially if one considers the large availability of such residue. Therefore, to date, most CSS is simply disposed in landfills as industrial waste [28]. A potential low-cost and largely available alternative for the sustainable exploitation of this residue is its use as a filler in polymeric matrices. Actually, silver skin turns out to be a suitable starting material for obtaining high value polysaccharide derivatives such as cellulose nanocrystals (CNCs), which have attracted a lot of attention given their interesting and remarkable mechanical properties such as high specific resistance (10 GPa) and relatively high elastic modulus (150 GPa) [70]. Furthermore, their low cost, availability, renewable nature, ease of chemical and mechanical modification and high aspect ratio have led to the use of CNCs as reinforcing fillers for polymeric composites. In the work of Sung et al. [28] bio-nanocomposite films based on a PLA matrix reinforced with CNCs were produced by using a twin-screw extruder. These CNCs were extracted from coffee silver skin by alkali treatment followed by sulfuric acid hydrolysis. The nanocomposites were prepared at different concentrations (1%, 3% and 5% of CNCs), resulting in increasing tensile strength, Young’s modulus and barrier to oxygen, while barrier to water was lowered. Some examples of biocomposites with silver skin fibers are reported. Zarrinbakhsh et al. [71] prepared composites with silver skin and spent coffee ground in a polypropylene (PP) polymeric matrix, resulting in non-biodegradable composites due to their polyolefin content. The morphology, mechanical and thermal properties of such composites were investigated with up to a content of 25% by weight of filler in a homo-polymeric matrix. The analysis of the biocomposite’s properties showed that coffee silver skin is a better rein- forcing agent than spent coffee ground given its denser fibrous structure, lower fatty acid content and higher thermal stability. Poor interfacial adhesion between both coffee silver skin and spent coffee ground with polypropylene was observed due to the hydrophilic na- ture of coffee by-products and hydrophobic nature of PP. Thus, the use of a compatibilizer, such as maleic anhydride, was recommended to improve the final properties. Considering the possibility to achieve production of PP from renewable resources, these biocomposites have potential for a high bio-based content. In the work of Essabir et al. [27] PP and spent ground coffee (SCG) were prepared by extrusion compounding and injection molding using different SCG contents (0, 5, 10, 15 and 20 wt.%). The authors investigated the effect of particle loading on the thermal, rheological and mechanical properties. In this study, the effect of the use of compatibilizers such as silane and styrene-ethylene-butene-styrene- graft-maleic anhydride on the properties of the biocomposite prepared at 15 wt.% was also Sustainability 2021, 13, 6921 11 of 19 examined. As expected, bleaching and the use of a coupling agent improved the adhesion between the matrix and fibers resulting in improved tensile strength. In the work of Dominici et al. [72], the use of BioPE-based composites containing CSS was evaluated. To improve the adhesion properties, a grafted PE was used as a compatibiliser. Moreover, a hydrophobic treatment of CSS with palmitoyl chloride showed a positive effect in promoting the interfacial adhesion, consequently improving mechanical properties and in particular increasing the strain at break values. The amount of CSS was up to 20 wt.%, which resulted in an adequate compromise between tensile strength and Young’s modulus. When the polymeric matrix is compostable, the use of coffee by-products allows for the production of potentially biodegradable composites. The biodegradable and petro- derived polymer, poly(butylene adipate-co-terephthalate) (PBAT), was blended with coffee by-products and the resulting composites were adapted for rigid food packaging appli- cations [26]. In this work, the torrefaction process (a mild thermal treatment occurring at 200−300 ◦C under nitrogen atmosphere) was optimised in order to increase the hy- drophobicity of the coffee grounds (CG), thus improving adhesion with PBAT matrix. The torrefaction process highlighted a significant enhancement in thermo-mechanical proper- ties for PBAT/torrefied coffee grounds as compared to PBAT/coffee grounds composites. In the PBAT-based composites with untreated coffee grounds, a consistent decrease in tensile strength values was observed. In contrast, the addition of 10 wt.% of torrefied coffee grounds to PBAT conferred better tensile properties as compared to neat PBAT, while the strain at break was slightly decreased. Beyond 10 wt.%, the tensile strength and strain at break began to decrease gradually but their values were still better than the untreated CG/PBAT composites. The water contact angle values were clearly increased proportionally to the torrefied CG content in the matrix, resulting in highly hydropho- bic biocomposites. Considering the high biodegradability of PBAT, such composites are valuable for the production of biodegradable rigid items usable in the packaging or agri- cultural sectors. Of note, Sarasini et al. [30] studied the effect of coffee silver skin size, variety, dis- tribution and content on the processability, thermal and mechanical properties of bio- composites based on a biodegradable blend of PBAT and poly(3-hydroxybutyrate-co-3- hydroxyvalerate) (PHB-HV). The authors showed the feasible use of coffee silver skin in the production of biodegradable composites with improved stiffness and tensile strength com- pared to the blend without coffee silver skin. Recently, coffee capsules, based on PHB-HV copolymers and CSS, have been produced by injection molding; the migration properties in contact with simulants are very promising for applications of these composites as plastic materials for food contact [73]. Composite materials based on PLA and torrefied SCG (TSCG) were also reported in the literature [74]. In order to improve the compatibility, maleic anhydride-grafted polylactide (PLA-g-MA) and cross-linked spent coffee grounds were used. Compatibilised materials exhibited improved mechanical properties. This effect was attributed to great compatibility of the grafted polymer and the natural filler. The dispersion of spent coffee grounds in the PLA-g-MA matrix was homogeneous due to ester formation and resulted in branched and cross-linked macromolecules. Moreover, the PLA-g-MA/TSCG composites exhibited low melt viscosities and were therefore easy to process. The water resistance of the PLA-g-MA/TSCG composite was greater than that of PLA/SCG and the mass losses following burial in soil compost indicated that both materials were able to disintegrate, especially at high levels of coffee ground content. Biocomposites containing PBS have also been reported [75]. Their production from renewable resources is possible with up to 30% by weight of silver skin. In PBS-based composites, silver skin increased tensile strength and Young’s modulus and had negligible effect on crystallinity. Table 3 presents some examples of biocomposites containing coffee by-products. Sustainability 2021, 13, 6921 12 of 19 Table 3. Example of biocomposites with coffee by-products as filler. Polymer Matrix Coffee By-Product Bio-Based Compostable/ Biodegradable Ref. PP Coffee Silver skin/ No/Yesspent coffee ground Possible with bioPP No [71] PP No/YesSpent coffee ground 1 Possible with bioPP No [27] bioPE Coffee Silver skin Yes No [72] Grafted- 2 -bioPE Coffee Silver skin Yes No [72] PBAT Coffee grounds No/Yes Yes [26] PBAT/PHBV Coffee Silver skin Partly (PBAT No,PHBV and CSS Yes) Yes [30,73] PLA Spent coffee ground 3 Yes Yes [74] PLA Nano cellulose produced fromcoffee silver skin Yes Yes [28] PLA/PBS Coffee Silver skin Yes Yes [75] 1 compatibilizer silane and styrene-ethylene-butene-styrene graft-maleic anhydride; 2 compatibilizer palmitoyl chloride; 3 compatibilizer maleic anhydride. 4. Biofuels Production from Coffee Waste According to the Food and Agricultural Organisation (FAO), biofuels are classified in wood fuels and agro-fuels. The latter are fuels obtained as a product of agricultural biomass and by-products at farming level, and/or the industrial processing of raw material (agro-industries). The term covers mainly biomass that is derived directly from fuel crops (see Table 4 for examples) and agricultural, agro-industrial and animal by-products [76]. Table 4. Fuel crops and agriculture derived main product. Biomass Type Derived from Plants Type Crops mainly devoted to the production of ethanol. Fuel mainly used in Sugar/Starch transport (alone or blended with gasoline). Produced by fermentation ofglucose derived from sugar-containing plants (e.g., sugarcane) or starchy materials after hydrolysis. Oleaginous plants (e.g., sunflower and rape) planted for direct energy Oil use of vegetable oil extracted, or as raw material for further conversion into a diesel substitute, using transesterification processes. Plants and specialised crops more recently considered for energy use, Other such as: elephant grass (Miscanthus), cordgrass and galingale (Spartinaspp. and Cyperus longus), giant reed (Arundo donax) and reed canary grass (Phalaris arundinacea). Thus, advanced biofuels can be produced from lignocellulosic and non-lignocellulosic sources (forestry and food or agro-derived residues) by means of thermochemical conver- sion, mainly based on gasification or pyrolysis plus a potential catalytic fuel production stage [77]. Biorefinery schemes based on this approach may vary, as well as their efficiency as per the configuration of the process itself and the final products. Spent coffee grounds after a thermal water extraction still possess more than 700 volatile compounds, insoluble and un-extractable, with high quality, organic and energy content. Moreover, spent coffee grounds contain tannins, polyphenol and caffeine that can result toxic if disposed into the environment [78]. Even spent coffee waste can be used for bioethanol, biodiesel, bio-oil, biochar, re- newable diesel or biogas production by anaerobic digestion, hydrogenation, esterification- transesterification, fermentation or pyrolytic reactions [79]. There are several references Sustainability 2021, 13, x FOR PEER REVIEW 13 of 20 Spent coffee grounds after a thermal water extraction still possess more than 700 vol- atile compounds, insoluble and un-extractable, with high quality, organic and energy con- Sustainability 2021, 13, 6921 tent. Moreover, spent coffee grounds contain tannins, polyphenol and c1a3foffe1in9e that can result toxic if disposed into the environment [78]. Even spent coffee waste can be used for bioethanol, biodiesel, bio-oil, biochar, renew- able diesel or biogas production by anaerobic digestion, hydrogenation, esterification- addressing thetravnasloesrtiesaritfiiocantioofns, pfernmt ecnotfafetieong roru pnydrsofloytricth reapcrtoiodnusc [t7io9]n. oTfhedrieff eare nstevtyepraels references of biofuels suacdhdarsesbsionegt hthaen voal,lobriosadtieosne ol,f fsupeelnpt eclolfeftese, gbriouhnydsr ofogre nth,eb pioro-odiul,ctbiioong oafs dainffderent types hydrocarbon foufe blsio[f8u0e,l8s1 s]u. ch as bioethanol, biodiesel, fuel pellets, biohydrogen, bio-oil, biogas and hy- Table 5 redproorctsarsboomne fuexelasm [8p0l,e8s1o].f coffee by-products valorisation in energy production, the methods used aTnadblteh 5e rseepcorntsd asorympe reoxdaumcptsleosb otfa icnoeffde.e by-products valorisation in energy produc- tion, the methods used and the secondary products obtained. Table 5. Some examples of coffee by-products valorisation in energy production. Table 5. Some examples of coffee by-products valorisation in energy production. Coffee By-Product Methods Fuel Secondary Products Coffee By-Product Methods Fuel Secondary Products Hydrolysis/fermentation Bioethanol Fuel pelletsF(usoeill paemendment)Spent coffee ground (SCG) Hydrolysis/fermentation Bioethanol llets (soil amend- Spent coffee ground (SCPGyr)o lysis Bio-oil Biochar, synmgeans t) Chemical convePrysiroonl/yseinsz ymatic/ Biodiesel BiGo-loycile rin toBbiiooc-hhyadr,r soygnengas Spent coffee grounds oil (SCGO) in situ Spent coffee grounds oil (SCGO) Chemical conversion/enzymatic/in situ Biodiesel Glycerin to bio-hydrogen Enzymatic convEenrsziyomn atic conversion Biodiesel BiGodlyiceesreiln toGbliyo-cheyridnr otog ebnio-hydrogen Defatted spent cDofeffeaettgerdo usnpdesnt coffee grHoyudnrdosly sis/fermHeyndtraotiloynsis/fermentationB ioethanol BiFoueethl apnelolel tsFuel pellets (DSCG) (DSCG) Pyrolysis Pyrolysis Bio-oil BiBoi-oocihl ar Biochar As summariseAdsi snuFmigmuarreis3edan ind Fdiegsucrreib 3e adnidn dtehsecrniebxetdp inar tahger naepxhts p, asreavgerraaplhasp, pserovaecrahle aspproaches can be used tocvanal boer iusesecdo ftfoe veablyor-pisreo cdouffcetes bfoyr-pthroedpurcotsd ufocrt itohne opfrofuduelcstiaonnd oef nfueerglsy a. nd energy. FigureFi3g.uSrceh 3e.m Scahtiecmeaxtaicm epxlaems polfecso offf eceofbfyee-p bryo-dpurocdtsuvctasl ovrailzoartiizoantioinn einn eerngeyrgpyr opdroudcuticotnio.n. 4.1. Biodiesel 4.1. Biodiesel In the literatuIrne ,thse vleitrearlatsutured, iseesvreerpalo srtuhdoiews rceopfoferet hcorowp scocfafene rcersouplst icnanm roerseulot iilnp merore oil per unit of land aruenait hoaf nlaontdh earetara tdhiatnio ontahlebr itordadieitsieolncarlo bpisodwieitshel ec.rgo.,p3s 8w6ikthg /e.hga., c3o8m6 kpga/rhead ctompared to 375 kg/ha for3s7o5y kbgea/hna[ f8o1r, 8s2o]y.bean [81,82]. For SCG, an oFiloyr iSeCldGo, fan21 o.5il% yiiesldre opfo 2r1t.e5d%t ios brepcoorntevde rttoe bdei ncotonvbeirotdedie sinetlow biitohdaiense8l2 w%ith an 82% yield [83]. yield [83]. Kondamudi etKaoln. d[8a4m] usudgi gete astl.e [d84to] seuxgtrgaecsttetdh etoo eilxftrroacmt tshpee onitl fcroofmfe espgernotu cnodffseaen gdrotuhnedns and then convert it to spceonntvceortf fiet etog srpoeunntd csofofielem gerothuynldess oteilr m(beitohdyile essetle)r, w(bhioidleiegsleylc),e wrinhiclea nglybceefruinrt chaenr be further processed intopbroiochesysderdo ignetno .bTiohheysdpreongtenco. Tffheee sgpreonutn cdosffceaen grboeurnedpsr ecsaenn bteed reapsretrsieonleteind awsi ttrhiolein with formula C57Hf1o0r4mOu6,law Ch5i7cHh10c4aOn6, bwehtircahn sceasnt ebreifi terdanusessintegrimfieedt huasninogl tmo efothramnoml etoth fyolrmol emateethyl oleate (C19H36O2) and glycerol. Being low in caffeine (C8H10N4O2), spent ground coffee has limited NOx production. Another process [83] converts spent coffee ground oils into fatty acid methyl ester or fatty acid ethyl ester via a non-catalytic transesterification process. Most recent studies have focused on oil extraction from SCG with different types of non-polar (hexane, toluene, n-pentane, etc.) or polar solvents (alcohols, acetone). In- novative technologies such as two-phase oil extraction [85], ultrasound-assisted solvent extraction [86] and supercritical fluid extraction using CO2 [87] achieved about 0.61–0.81 of n-hexane soxhlet extraction value. Sustainability 2021, 13, 6921 14 of 19 Transesterification can be carried out with catalytic alkaline-based transesterification or acid-catalysed esterification using methanol/carbon dioxide mixtures or with enzymatic catalysis; with a catalytic two-step reaction of acid-catalysed esterification followed by alkaline-based transesterification; by non-catalytic esterification; or by in situ transesterifi- cation [88]. In the last process, the production of biodiesel from spent coffee ground oils can be carried out by reacting directly spent coffee ground (in-situ) with no need for the solvent extraction process, followed by pyrolysis with biochar production [89]. 4.2. Bioethanol The direct conversion of SCG to bioethanol (without oil extraction) was reported as unfeasible due to the existence of triglyceride and fatty acids, which inhibits the activity of the enzymatic saccharification process [90]. In the work of Go et al. [91] the SCG was first hydrolysed with sulfuric acid extracting sugars, that can be then fermented to bioethanol. Lipids remained intact even after hy- drolysis and were successfully recovered from the hydrolysed SCG known as spent coffee grounds oil (SCGO) to produce biodiesel and glycerin. The solid residues after hydrolysis were ~68% of the initial biomass having a moisture content of ~65%, thus reducing the total amount of moisture to be removed as well as the energy required in the drying step prior to lipid extraction [92]. The obtained bioethanol can be converted to bioesters for the EU gasoline market. The defatted spent coffee grounds (DSCG) can be valorised as fuel pellets or soil amendment agents. In the work of Haile et al. [93], oil was extracted from dry SCG up to 19.73% w/w and further used to produce biodiesel with a yield of about 73.4% w/w. The solid remaining after extraction was used to produce bioethanol through acid hydrolysis (H2SO4) and fermentation (Saccharomyces cerevisiae), which resulted in a 8.3% v/v yield. The biomass remaining after bioethanol production can be used for the production of compost and solid fuel pellets with a 21.9:1 carbon to nitrogen ratio (C/N) and a 20.8 MJ/kg heating value, respectively. 4.3. Bio-Oil Spent coffee grounds and defatted SCG are a potential source for bio-oil and biochar obtained by means of pyrolysis. The process of pyrolysis is based on thermal decomposition of biomass under inert atmosphere (nitrogen), with limited oxygen and at relatively high temperatures in the range of 500 to 1000 ◦C. The biomass degradation produces bio-oil, water phase, biochar and syngas. These bio-oils are used as fuels or chemicals [94] while biochar can be used as energy source or soil amendment [95]. The co-pyrolysis process was investigated and led to improved oil yield and crude oil quality. The best performing conditions were set at 250 ◦C and a mixing ratio of 1:1 of SCG. The co-pyrolysis (co-liquefaction) of SCG with PP [96], paper filter, corn stalk and white pine has been reported to lead to interesting results [97]. 4.4. Biogas SCG is a valuable material for fermentation in both mesophilic and thermophilic processes due to its suitable elemental composition, C/N ratio and chemical composition (content in polysaccharides, proteins and minerals). Several papers report the production of biogas from SCG by anaerobic digestion. For example, in the study of Vítez et al. [98], methane production ranged from 0.271–0.325 m3/kg dry organic matter under mesophilic conditions. To improve the methane recovery, an alkaline pre-treatment was applied to SCG through anaerobic digestion. The highest concentration of NaOH (8% w/w) led to the best anaerobic digestion performances (392 mL CH4/gVS, calculated by dividing total volume of methane by measured total Volatile Solids) as a consequence of the slightly higher lignin degradation, which was 24% higher than that of the untreated substrate, and of the higher dissolved organic carbon concentration [99]. Sustainability 2021, 13, 6921 15 of 19 Integrated biological and physico-chemical process was studied by Lee et al. [100] for the production of fatty acid methyl ester, lignin and biogas from SCG. A maximum recovery of 62.4% and 55.5% of fatty acid methyl ester and lignin was obtained. The solid remaining after extraction was anaerobically digested and had a maximal methane yield of 36.0 mL CH4/gVS. Due to their reduced O/C and H/C ratios, torrefied coffee grounds are an interesting feedstock for the production of syngas. Torrefaction is actually a good process to enhance the efficiency of biomass and waste uses for renewable energy applications [101]. 5. Conclusions and Future Perspectives This review summarizes the most recent studies concerning monomer, material and fuel production from coffee waste. Through a series of integrated conversion technologies, chemical building blocks can be recovered from coffee by-products and used for the production of monomers such as lactic acid, succinic acid, levulinic acid, alcohols, etc., obtaining especially high yields for lactic acid. Polymers such as polyhydroxyalkanoates can also be directly produced from coffee waste with adequate yields. In parallel, coffee by- products can be used as fillers in composite materials leading to an improvement of the final properties if a good interfacial adhesion is achieved, and, in any case, composite materials with very high bio-based content can be obtained. Provided these materials contain the appropriate co-polymeric matrix, they are potentially compostable or degradable in soil. Coffee by-products can even find a very valuable exploitation in energy production. Indeed, biomass derived from spent ground coffee can be used for the production of biodiesel, by transesterification of the extracted lipids. Then, the defatted SCG can be con- verted in bioethanol by means of fermentation or in bio-oil and biochar through pyrolysis with no waste material. SCG can also be used directly for the production of biogas by means of anaerobic digestion. Future applications might involve the development of new materials for application in biomedicine, active food packaging and smart materials. However, research must also focus on a techno-economic analysis and feasibility of industrial scale production. Author Contributions: Conceptualization, L.N.; writing—original draft preparation, P.C. (Philippe Corvini), M.B., L.S. and A.C.; writing—review and editing, L.S., A.C., P.C. (Patrizia Cinelli)., F.S., A.L., L.N., P.C. (Philippe Corvini), A.T. and M.B.; supervision, G.T. and M.F.; funding acquisition, all authors. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the PROLIFIC project (“Integrated cascades of processes for the extraction and valorization of protein and bioactive molecules from legumes, fungi and coffee agro-industrial side streams”), which receives funding from the Bio Based Industries Joint Undertaking under the European Union’s Horizon 2020 research and innovation program, under grant agreement No 790157. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations ABE mixture of acetone, butanol and ethanol CH coffee husk CG coffee grounds CNCs cellulose nanocrystals CP coffee pulp CSS coffee silver skin DSCG defatted spent coffee grounds LA lactic acid PBAT poly(butylene adipate-co-terephthalate) PBS poly(butylene succinate) PE poly(ethylene) Sustainability 2021, 13, 6921 16 of 19 PEF poly(ethylene 2,5-furandicarboxylate) PET poly(ethylene terephthalate) PHA polyhydroxyalkanoates PHB-HV poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PLA poly(lactic acid) PLA-g-MA maleic anhydride-grafted-polylactide PP poly(propylene) PTT poly(trimethylene terephthalate) PUs polyurethanes SA succinic acid SCGO spent coffee grounds oil SFC spent filter coffee SCG spent coffee grounds TSCG torrefied spent coffee grounds VS volatile solid References 1. Gustavsson, J.; Cederberg, C.; Sonesson, U.; van Ottedijk, R.; Meybeck, A. Global food losses and food waste: Extent, causes and prevention. 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