Added-value molecules recovery and biofuels production from spent coffee grounds

Highlights

• Spent Coffee Grounds (SCG) worldwide production is estimated at more than 6 M tons.

• Conventional applications of SCG are biogas, bioethanol and biodiesel production.

• Economically valuable molecules are extracted from SCG oil and used as additives.

• Coffee oil can be used as feedstock for bioplastic and biosurfactant production.

• A biorefinery model is proposed to optimize the SCG valorisation.

Abstract

Spent Coffee Grounds worldwide production is estimated at around 6 M tons only at industrial level. The abundance and the heterogeneity of this substrate make it an ideal substrate for a biorefinery approach based on the “cascade biorefinery hierarchy”. Currently, the major part of spent coffee grounds is sent to incineration and landfill disposal, options which should be avoided. Instead, they could be valorised through biofuels production. All the operational parameters leading to the highest biogas (350-400L_CH4/kg_TVS), bioethanol (3–4%v/v) and biodiesel (over 90% of Fatty Acid Methyl Esters concentration) yields from spent coffee grounds have been discussed in this review paper. They are rich in an oil phase containing different added-value molecules (tocopherols, cafestol, kahweol along with linoleic and palmitic acids), which can be extracted and used as additives for food, cosmetic and pharmaceutical applications. Solid/liquid extraction techniques of coffee oil from spent coffee grounds such as the most common Soxhlet technique and the more innovative fluids in supercritical conditions have been discussed, with coffee oil recovery of around 5–15%w/w and 15–20%w/w, respectively. The most recent applications of the extracted coffee oil have been also presented: the added-value molecules recovery and purification after micro/ultra and nano filtrations processes and the polyhydroxyalkanoates (0.84 g/g) and biosurfactants (3.5 g/L) production. Considering the whole information, an integrated biorefinery scheme, along with the respective mass balances were proposed. The novelty of this paper lies in the integration of the state-of-the-art data, in a biorefinery concept that would allow the production of both biofuels and value-added products.

Introduction

The increasing attention drawn upon the catastrophic effects of climate change is leading European Union (EU). In particular, EU has recently approved a second "circular economy package" which expressly sets out the transition from a linear economic system to a circular one. This latter is characterized by a three «r» principle: reduce, reuse and recycle (Com (2015) 614 final) [1]. This Circular Economy package includes the EU Directive 2018/851 [2], which contains some amendments to the waste Directive 2008/98 EC [3] and has introduced relevant changes regarding the waste management, and mainly their collection and valorisation. Compared to the previous Directive EU 2008/98, the most recent EU 2018/851 encourages the adoption of a fiscal system of taxation for the less green friendly waste management's practises and incentives for the most virtuous recovery and recycling of the materials. With references to organic wastes, it promotes the “cascade pyramidal biorefinery hierarchy”, where the recovery of valuable biomolecules for pharmaceutical, chemical, cosmetic, agronomic applications and the production of added-value compounds are prioritised. The European Commission, therefore, specifies that bioenergy production should be approached only after these processes, while waste disposal in landfills should be avoided.

In Directive 2008/98 EC, in fact, the waste hierarchy is functional not only to protect the environment and human health, but also to achieve a further goal, namely the strengthening of the competitiveness of EU. More precisely, it aims to ensure an efficient supply of raw materials, thus reducing EU dependence on their import. In this perspective, waste is not considered a negative output but becomes a resource to be re-inserted in the economic cycle, creating new valuable final products.

This review paper aims to guide through the possible processes which can be integrated into the "cascade pyramidal biorefinery hierarchy" approach, using one of the most abundant organic wastes in the world, the SCG.

Processing of coffee bean involves many steps such as wet or dry process, milling, roasting, grinding and brewing, in which solid residues such as SCG are obtained [4,5]. During wet technique, pulp is removed mechanically and dried, while during dry technique entire coffee cherries are dried without removing the pulp. This work focuses on the SCG that constitute the main residue after the brewing of coffee [[6], [7], [8], [9]]. SCG are massively generated during coffee brewing where just a few components of the coffee beans are extracted [10]. 65% of coffee beans ends up as SCG [7]. Moreover, for each kg of soluble coffee, the amount of SCG produced is 2 kg [11]. Thus, the total amount of SCG produced globally is immense. Almost half of this amount is produced from coffee shops and industrial plants, while the remaining amount is produced domestically [12,13]. At an industrial level, SCG produced are estimated to reach 6*10⁶ tonnes, presenting fluctuations among countries given the dispersity of the coffee production plants.

In contrast to most part of other organic substrates, coffee contains certain components with adverse effects to the environment given its tannin, polyphenol and caffeine contents rendering it toxic [7,9,10,12]. Therefore, SCG may not be valorised through composting or incineration and are mainly dumped in landfill sites, inducing severe environmental issues. Hence, alternative ways of treating, or even better valorising SCG are sought [10], since SCG should be managed in a more sustainable way [9]. Scopus data (Fig. 1) revealed that the interest for SCG had an exponential growth only since 2013.

In particular, in 2010 only 5 works investigated the exploitation of SCG, while this number increased to 83, 97 and 150 in the last three years (2017–2019). Going deep into the analysis of the topics of these works, it emerged that almost 50% of the papers published in 2017 involved biofuels production, essentially biogas and bioethanol, while the extraction of added-value compounds was considered by only 14%. Nevertheless, the entrance in force of the EU Directive 2018/851 had a double effect: it increased the SCG publications by 33% in just one year, promoting the investigation on the extraction and the synthesis of valuable molecules instead of SCG biofuels production. Specifically, reference related to biogas production dropped by 30% of the papers published in 2019.

In this context, a thorough literature review of SCG characterisation will be presented along with all the parameters that influence their composition. Following, the conventional valorisation techniques will be completed by the newly established and the techniques that are still in their infancy concluding at integrating all the knowledge by setting up a SCG biorefinery. Thus, different techniques will be presented along this review, analysing their performances, their advantages and weaknesses. To the authors’ knowledge, there exist no comprehensive research work that have studied and made a comparison of the various SCG valorisation pathways and management practices. Hence, this work evaluates the implications of using SCG as a resource for biofuels and value-added products. In addition, this study paper aspires to identify the most beneficial SCG valorisation hierarchy in an effort to enhance the consistency across the diverse methods.