Elsevier

Carbohydrate Polymers

Volume 40, Issue 1, September 1999, Pages 71-81
Carbohydrate Polymers

Polysaccharides from hot water extracts of roasted Coffea arabica beans: isolation and characterization

https://doi.org/10.1016/S0144-8617(99)00032-6Get rights and content

Abstract

A polysaccharide fraction was obtained from the hot water extract of defatted ground dark roasted coffee (pure Coffea arabica blend for espresso brewing technique) by means of classical precipitation methods. This high molecular weight (〈Mw〉=10 900 Da) product was shown to be composed of mannose, galactose, arabinose and traces of proteinaceous material. Further fractionation yielded two different carbohydrate polymers, which were structurally characterized. One polysaccharide was identified as a β-d-(1-4) mannan containing small amounts of galactose and arabinose. The second polysaccharide, obtained in low yield after removal of the mannose-containing polymeric material, was identified as an arabinogalactan. The starting fraction was found to be composed of about 80% (mol basis) of mannan and of about 20% of arabinogalactan. 13C-NMR spectra revealed that the arabinogalactan has a backbone chain of β-(1-3)-d-galactopyranose units. Some of these units were substituted with either terminal β-d-galactose or terminal α-l-arabinofuranose side chains mainly in C-6 position. Both the mannan and the arabinogalactan isolated are structurally related to the polysaccharides originally present in the green coffee beans.

Introduction

Coffee is one of the world's most widely consumed beverages. The soluble material that can be extracted from the roasted coffee is of particular importance to the quality of coffee brews. Under conditions normally used in the espresso brewing technique, relatively high quantities of soluble material can be extracted, particularly polysaccharides. These polysaccharides play an important role in the retention of volatile substances; they also contribute to the coffee brew viscosity and thus to the creamy sensation perceived in mouth, known as “body” (Illy & Viani, 1995).

Green coffee bean (Coffea arabica) polysaccharides have been the subject of several investigations in the 1960s (Wolfrom et al., 1961, Wolfrom and Patin, 1964, Wolfrom and Patin, 1965, Thaler and Arneth, 1967). The most recent structural study is that of Bradbury and Halliday (1990) who identified an arabinogalactan, a mannan and cellulose with only minor amounts of other polysaccharides in green coffee beans. In the same study, the major polysaccharides arabinogalactan and mannan, have been characterized in detail. The arabinogalactan on hydrolysis gave arabinose (19.8% w/w), galactose (48.2% w/w), mannose (0.8% w/w) and rhamnose (1.1% w/w). From these data it is clear that 30.1% (w/w) has not been defined. In spite of this, the polymer was described to be “principally a β1→3-linked galactan chain with frequent short side chains linked at C6 to galactose residues 1→3-linked to terminal arabinose residues. The polymer also contains some nonterminal arabinose residues (linked at C2 and C5) as well as a small proportion of 1→6-linked galactose residues”. All the arabinose residues have been found to be in the furanoside form. The possibility of covalent linkages between the arabinogalactan and proteins has also been suggested. The mannan on hydrolysis gave mannose (94% w/w) galactose (3.3% w/w) and glucose (1.7% w/w) but the authors defined it essentially as a linear β1→4-linked mannan “with only about 1 one-residue galactose stub at C6 per 100 mannose residues” (Bradbury & Halliday, 1990). However, the comparison between the results of the methylation analysis of the whole-bean polysaccharides and those of the isolated mannan reveals some discrepancies. In particular in the whole-bean analysis the mannan appeared to be more branched and polymerized than the isolated one.

The value of optical rotation of the isolated mannan (in 1 N NaOH) was found to be similar to that of the ivory nut mannan (Bradbury & Halliday, 1990). In our opinion the internal inconsistencies and the discrepancies are mainly due to the drastic conditions used to isolate the two polysaccharides. Therefore it is reasonable that the mannan and the arabinogalactan described by Bradbury and Halliday (1990) probably represent a simplified picture of the real situation in the green coffee beans.

Roasting strongly alters and degrades the carbohydrate polymers inducing depolymerization, structural modifications and formation of condensation complexes with proteins, protein fragments and other breakdown products. For this reason analytical study of carbohydrate polymers from processed Coffea arabica involves considerable difficulties in isolation and characterization. However, only a limited number of investigations have been devoted to the study of the polysaccharides or their conversion products in roasted coffee, coffee brews, extracts and instant coffee (Trugo, 1985, Viani, 1993). Wolfrom and Anderson (1967) were able to isolate from an instant coffee powder, of unknown source, arabinogalactan (only galactose and arabinose on acid hydrolysis) and mannan (94% w/w mannose and traces of galactose on acid hydrolysis); however, structural evidence for the two polymers was not produced. The value of optical rotation of the mannan was found to be similar to that of ivory nut mannan, a typical water insoluble polysaccharide, thus casting some doubts on the real water solubility of the starting coffee powder. In a more recent article, the water soluble residue obtained after a series of extractions with different solvents of instant coffee, was found to be composed of mannose, galactose and arabinose with some minerals, proteins and traces of chlorogenic acids (Pictet, 1975), but no information was provided about the possible arrangement of the sugar components.

Maier and Buttle (1973) reported the isolation of a fraction composed of polymerized material (MW 5000–50 000). This material was shown to be composed of mannose, galactose and arabinose, and approximately from 6 to 12 aminoacid residues were found per molecule, probably arranged as peptides. In contrast to Wolfrom and Anderson, 1967, Maier and Buttle, 1973 suggested that the sugar components are arranged as an arabinogalactomannan in which galactose and arabinose side chains are linked to a backbone of β-d-(1-4) mannose units. According to the authors, the arabinogalactomannan is linked to peptides or single aminoacids to form a complex. Unfortunately, in this study the term arabinogalactomannan is inferred from constituent monosaccharides only.

The isolation of a polysaccharide fraction containing mannose, galactose, arabinose and traces of proteinaceous material, from the hot (90°C) water extracts prepared from defatted dark roasted Coffea arabica beans, largely stimulated the present investigation. As a matter of fact, in spite of the difference of source, extraction and isolation procedures, the composition of this fraction resembles those reported in the literature. Accordingly, the study of this fraction gives the opportunity to provide further information on the fate of polysaccharides, originally present in green coffee beans, after roasting. In order to characterize the starting fraction and the derived sub-fractions, NMR spectroscopy has been chosen thanks to the several advantages offered by this technique in comparison with other classical methods. Moreover, no NMR data have been reported so far on roasted coffee polysaccharide conversion products.

Section snippets

Materials

Extraction and isolation of fraction A. Ground dark roasted coffee (pure Coffea arabica blend for espresso brewing technique), ICS grade, produced by ILLYCAFFE’ S.p.A., Trieste, Italy, was used. 25 g of coffee powder was heated under reflux with n-hexane 16 h in a Soxhlet apparatus. The defatted dried powder was extracted twice for 1 h with Milli-Q water (200 ml) at 90°C. The solid residue was removed by filtration, ammonium sulfate was added to give a saturated solution and the mixture was

Methods

Total carbohydrate content in fraction A was determined by phenol-sulfuric acid colorimetric method (Dubois, Gilles, Hamilton, Rebers, & Smith, 1956). Protein content of 6.9% w/w was estimated on the basis of elemental analysis (%N×6.25). Sugar composition was determined by gas-chromatography of the alditol acetate derivatives of the hydrolized samples (TFA 2N, 100°C, 16 h). A Hewlett–Packard model 5890A equipped with a flame ionization detector was used. The column was a 30 m×0.25 mm SP-2330

Fraction A

The extraction and isolation procedures followed to obtain fraction A were performed under very mild conditions in order to prevent chemical changes. The yield of fraction A was about 2.2% (w/w) of the starting coffee powder.

In agreement with the chemical analysis, the FTIR spectrum of fraction A, reported in Fig. 2, clearly shows the typical signal pattern expected for a carbohydrate moiety. In particular, the spectrum is dominated by signals in the region 3600–2800 cm−1 due to stretching

Conclusions

In spite of the different sources, extraction and isolation procedures, the sugar composition of the polysaccharide fraction isolated from hot water extract of roasted Coffea arabica beans presently investigated (fraction A), is in substantial agreement with previous reported data (Wolfrom and Anderson, 1967, Maier and Buttle, 1973). However two different interpretations were proposed as far as the arrangement of the sugar components is concerned. In one case two different polysaccharides

Acknowledgements

This work has been carried out within a project supported by ILLYCAFFE’ S.p.A. (Trieste, Italy) to POLY-tech s.c.a.r.l. (Trieste, Italy). We also thank E. Illy and M. Petracco for valuable scientific support.

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