Supramolecular solvent extraction of bioactives from coffee cherry pulp

Highlights

• SUPRAS were applied for the valorization of coffee cherry pulp.

• Caffeine and protocatechuic acid were the major bioactives in SUPRAS extracts.

• SUPRAS offered better extraction efficiency compared to conventional organic solvents.

• SUPRAS extracts exhibited good antioxidant capacity (DPPH, ABTS).

Abstract

The potential of supramolecular solvents (SUPRAS) is investigated for the extraction of bioactive compounds from coffee cherry pulp, one of the major by-products generated in the coffee industry. SUPRAS made up of hexagonal inverted aggregates of octanoic acid in ethanol:water mixtures provided good extraction yields for bioactives (3.6 ± 0.3 mg caffeine g⁻¹ and 0.9 ± 0.1 mg protocatechuic acid g⁻¹) at a low solvent:sample ratio of 4:1 v/w and under mild operations conditions (5 min extraction at room temperature). SUPRAS-based extraction was optimized and extracts were analyzed to identify the main phenolic and alkaloid compounds. A variety of bioactives were present and extracts showed high antioxidant capacity by different assays (45% for DPPH and 91% for ABTS). Extraction efficiencies with SUPRAS were clearly superior than those obtained with organic solvents commonly used for valorization of coffee residues.

Introduction

Coffee is the most popular beverage with a production of over 9 billion kg of beans per year and it is cultivated in around 70 countries (International Coffee Organization, 2017). Coffee berries contain the beans surrounded by different layers: first the silverskin, then the parchment, the mucilage, the pulp and finally the skin (Esquivel and Jiménez, 2012). During the dry process, coffee cherries are sun-dried and then they are dehusked to remove the skin, the pulp, the mucilage, the parchment and part of the silverskin (Esquivel and Jiménez, 2012). These by-products are known as coffee husks. In the wet and semi-wet processes, after separating ripened from unripe berries with water, fruits are de-pulped to remove the skin and the pulp. The by-product or waste that is generated at this step is known as coffee cherry pulp or coffee pulp (Pandey et al., 2000). In the wet process, the beans are further fermented to remove the mucilage and the remaining pulp, then dehusked and finally dried. After any of these three processes, the beans are roasted and generate as by-product the silverskin. Considering that each 100 kg of mature fruits are composed by around 39 kg of pulp, 22 kg of mucilage and 39 kg of parchment coffee, it is easily concluded that the amount of residues generated is extremely high (Alves et al., 2017). Finally, spent-coffee grounds are generated during the production of instant coffee and coffee brewing (Kovalcik et al., 2018).

In coffee producing countries, the unsafe disposal of the corresponding wastes has a negative impact on the environment due to their high concentration in caffeine, polyphenols and tannins and high acidity. The large-scale management of coffee waste is still challenging. A very attractive strategy is its valorization to obtain benefits as compost, fuel, animal feed, bio-solvents or bioactive compounds, among others. Bioactive compounds obtained from coffee by-products are mainly alkaloids, melanoidins and polyphenolic compounds that exert beneficial antioxidant, anti-bacterial or anti-fungal effects of interest for the food, pharmaceutical and cosmetic industries (Belščak-Cvitanović and Komes, 2017; Esquivel and Jiménez, 2012; Galanakis, 2015; Janissen and Huynh, 2018; Rodrigues et al., 2017).

Extraction of bioactives from coffee by-products has been investigated using different solvents and techniques. Moderate polar solvents are usually employed, such as methanol, ethanol or isopropanol, sometimes mixed with water (up to 40% v/v) under typical sample to solvent ratios in the range 1:10–1:100 v/w. Supercritical fluids (Andrade et al., 2012), subcritical water (Getachew and Chun, 2017), and deep eutectic solvents (Yoo et al., 2018) have been also used. Techniques include conventional solid-liquid extraction (Mussatto et al., 2011; Zuorro and Lavecchia, 2012), Soxhlet extraction (Murthy and Naidu, 2012), supercritical fluid extraction (SFE) with and without co-solvent (Andrade et al., 2012), ultrasound (USAE) (Andrade et al., 2012; Getachew and Chun, 2017; Yoo et al., 2018) and microwave assisted extraction (MAE) (Getachew and Chun, 2017; Pavlović et al., 2013). Spent coffee grounds have been the most investigated coffee waste for the extraction of bioactives (Kovalcik et al., 2018) and in a lesser extent coffee husks (Andrade et al., 2012) and coffee silverskin (Narita and Inouye, 2014). As mentioned above, coffee cherry pulp is one of the main by-products of the wet processing of coffee (~40% of the coffee is wet processed, Garde et al., 2017). However, it is still hardly investigated for the extraction of bioactives despite its good antioxidant properties (Heeger et al., 2017, Murthy et al., 2012).

In this study, we investigate the suitability of supramolecular solvents (SUPRAS) for the extraction of bioactives from coffee cherry pulp that was obtained by a wet process. SUPRAS are nanostructured liquids produced spontaneously from colloidal suspensions of amphiphiles by self-assembly processes (Ballesteros-Gómez et al., 2018; Caballo et al., 2017). SUPRAS production involves the application of an external stimuli (pH or temperature change, addition of salt or addition of a poor solvent for the amphiphile) to the colloidal suspension where the amphiphiles arrange as three-dimensional aggregates, which are usually normal or inverted micelles or vesicles (Ballesteros-Gómez et al., 2010). The application of an external stimulus diminishes the repulsion among the polar groups of the amphiphilic molecules, which causes the growth of the aggregates that finally separate as a new liquid phase named coacervate or SUPRAS (Ballesteros-Gómez et al., 2018). The organized structures in the supramolecular phase are held together by intermolecular interactions, such as ion–ion, ion–dipole, dipole–dipole, hydrogen bonding, π–π and cation–π. Although these interactions are weaker than covalent bonds they can produce very stable assemblies and provide multiple biding forces for extraction, which makes them very efficient extractants (Caballo et al., 2017; Steed et al., 2007).

SUPRAS are tunable solvents since by changing the environmental conditions and/or the amphiphile functional group/s is possible to tailor their composition and structure (Ballesteros-Gómez et al., 2018). Thus, SUPRAS have been designed to exclude proteins and carbohydrates from extraction by chemical and physical mechanisms, respectively (Ballesteros-Gómez and Rubio, 2012). These versatile and efficient extraction materials have proved successful for the recovery of a variety of compounds for analytical purposes (e.g. PAHs, mycotoxins, perfluorinated compounds, drugs, dyes, etc.) (Ballesteros-Gómez et al., 2010; Caballo et al., 2017). However, their application to the extraction of bioactives from biomass or waste is still limited (Salatti-Dorado et al., 2019; Torres-Valenzuela et al., 2019).

Here, we investigate the suitability of SUPRAS produced by the addition of water to colloidal suspensions of decanoic or octanoic acid in ethanol (Ruiz et al., 2007) for the extraction of bioactives from coffee cherry pulp. SUPRAS components were selected from Generally Recognized As Safe (GRAS) chemicals in order to produce a green and biocompatible solvent for further application in the development of cosmetics, nutraceuticals or functional foods. SUPRAS extraction was optimized on the basis of the extraction yields for caffeine and protocatechuic acid, the two most abundant bioactives in this by-product (Heeger et al., 2017). SUPRAS extracts were further screened to elucidate the profile of phenolic and alkaloid compounds and to measure their antioxidant activity.