Thermal stability of spent coffee ground polysaccharides: Galactomannans and arabinogalactans
Introduction
Roasted coffee bean polysaccharides are mainly galactomannans, type II arabinogalactans, and cellulose. During the preparation of coffee brew, a part of these polysaccharides, mainly galactomannans and also arabinogalactans, are extracted (Nunes & Coimbra, 2001). However, the majority of the galactomannans, as well as the arabinogalactans, remains associated to the cellulose-rich cell wall matrix. The galactomannans are the main components of spent coffee grounds (SCG) (Simões et al., 2009). These polysaccharides are composed by a linear (1→4)-linked mannopyranose residues backbone substituted at O-6 with single residues of galactopyranose, as well as single arabinose residues (Simões, Nunes, Domingues, & Coimbra, 2010). The galactomannans present in coffee infusions contain also β-(1→4)-d-glucopyranose residues interspersed in the main backbone (Nunes, Domingues, & Coimbra, 2005), a structural feature that may also occur in all other coffee galactomannans, as inferred by the occurrence of small amounts of (1→4)-linked glucose residues and by the presence of oligosaccharides derived from galactomannans resistant to endo-β-(1→4)-d-mannanase (Simões et al., 2010). Coffee arabinogalactans are branched polysaccharides composed by a main backbone of β-(1→3)-linked galactose residues with side chains of β-(1→6)-linked galactose oligosaccharides attached to the O-6 position of the main chain. The β-(1→6)-linked side chains are substituted at O-3 with arabinose and arabinose oligosaccharide residues presenting a rhamnose residue as non-reducing terminal (Nunes, Reis, Silva, Domingues, & Coimbra, 2008). Some populations of arabinogalactans also contain single glucuronic acid residues in the terminal position of the β-(1→6)-linked side chains (Redgwell, Curti, Fischer, Nicola, & Fay, 2002).
Coffee arabinogalactans are the most vulnerable of the coffee polysaccharides to degradation during roasting, especially the more labile arabinose residues (Moreira et al., 2013, Nunes and Coimbra, 2002a, Nunes and Coimbra, 2002b, Redgwell et al., 2002b) present as side chains. Also, the arabinogalactans depolymerize during roasting and a huge decrease in molecular weight is observed even at after a light roast (Redgwell, Trovato, et al., 2002). The debranching of the arabinose side chains seems to occur more rapidly than the hydrolysis of the galactan backbone (Oosterveld, Harmsen, Voragen, & Schols, 2003). Although coffee bean galactomannans are known to be more resistant than arabinogalactans (Oosterveld et al., 2003), it has been shown that during the roasting of the coffee beans, the galactomannans can undergo depolymerisation, debranching, Maillard reactions, caramelization, isomerisation, oxidation, decarboxylation, and melanoidins formation (Nunes, Reis, Domingues, & Coimbra, 2006). These changes promote the increase in coffee galactomannans extractability to the coffee brew (Nunes & Coimbra, 2001). Also, the higher the degree of roast, the higher the amount of galactomannans present in the coffee brews of both Coffea arabica (Nunes & Coimbra, 2002a) and Coffea canefora (robusta) (Nunes & Coimbra, 2002b). Also, the roasting of the coffee bean promotes the easy of extraction of cell wall polysaccharides with NaOH aqueous solutions (Oosterveld et al., 2003). Using (1→4)-linked manno-oligosaccharide model compounds, Moreira, Coimbra, Nunes, Simões, and Domingues (2011) showed that the roasting can also promote polymerisation reactions by transglycosilation, giving rise to molecules with (1→2)- and (1→6)-linked mannose residues, isomerisation reactions by the presence of (1→4)-linked glucose residues, and the occurrence of anhydrohexoses by the identification of mono- and tridehydrated derivatives. In order to better understand the thermal stability of SCG galactomannans and arabinogalactans and the reactions that can occur upon roasting, the thermal study of these SCG polysaccharides was performed. The thermograms from 20 to 600 °C at a heating rate of 10 °C/min were obtained and compared with other polysaccharides, namely cellulose, locust bean gum, and Gum Arabic. The coffee polysaccharides were also submitted to an isothermal heating at different temperatures (160, 180, 200, 220, and 240 °C) with long time of exposure, up to 3 h. The activation energies of thermal degradation were determined using both methods. The resultant products of thermal heating were analysed according to the sugars and linkage composition and also by electrospray mass spectrometry.
Section snippets
Samples
Galactomannans (GM) and arabinogalactans (AG) were isolated from espresso spent coffee grounds (SCG) obtained after a commercial espresso coffee preparation (Simões, Nunes, Domingues, & Coimbra, 2013). SCG carbohydrates were polymeric mannose (46%), galactose (27%), glucose (20%) and arabinose (7%). The arabinogalactan-rich sample was obtained by SCG extraction with a 4 M NaOH solution. This fraction remained soluble upon neutralisation of the extract and was mainly composed by galactose (67%),
Polysaccharides thermal behaviour
Fig. 1a shows the plot of the thermogravimetic analysis, performed at 10 °C/min until 600 °C, of spent coffee grounds (SCG) obtained from espresso coffee preparation. According to the first derivative plot (Fig. 1b), the major weight losses occurred at 309 °C (56%), 439 °C (19%), and 497 °C (17%). The total sample mass was lost at 510 °C, allowing to infer that the SCG did not contain significant amount of minerals. When the same experiment was performed using a coffee galactomannan-rich fraction
Conclusions
The thermogravimetric study performed showed that the roasting at 200 °C up to 3 h promotes structural changes of the coffee residue galactomannans with no apparent weight loss. Compared with the coffee residue arabinogalactans, the galactomannans are more resistant to weight loss at this temperature. However, at 180 °C the arabinogalactans are also thermally stable up to 3 h. This different thermal stability is in accordance with their activation of energy for thermal degradation, which was
Acknowledgments
The authors thank the financial support provided to project PTDC/QUI-QUI/100044/2008, QOPNA (project PEst-C/QUI/UI0062/2013) and RNEM by the Foundation for Science and Technology (FCT), Compete and FEDER. Joana Simões thanks FCT for the PhD grant SFRH/BD/28572/2006.
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