Article pp.423-429 du Vol.27 n°6 (2007)

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doi:10.3166/sda.27.423-430 SCIENCES DES ALIMENTS, 27(2007) 423-429

SHORT COMMUNICATION

Dietary fibre content and cell wall polysaccharides in prunes

A. Fatimi¹, M.-C. Ralet², M.-J. Crepeau², S. Rashidi¹, J.-F. Thibault^2*

RÉSUMÉ

Teneur des pruneaux en fibres alimentaires et en polysaccharides pariétaux

Les teneurs en fibres alimentaires des pruneaux ont été mesurées par la méthode de l’AOAC (Association of Official Analytical Chemists) qui permet de quantifier les fibres solubles, les fibres insolubles et les fibres totales (somme des solubles et des insolubles). Les pruneaux sont riches en fibres puisque leur teneur est de 6-7% par rapport à la matière fraîche ; les fibres solubles représentent environ 57% et les insolubles 43%. Les fibres solubles sont particulièrement riches en substances pectiques alors que les fibres insolubles contiennent principalement de la cellulose avec des quanti- tés significatives de pectines et d’hémicelluloses.

Keywords

prunes, plums, dietary fibre.

SUMMARY

The dietary fibre in the prunes was prepared using the Association of Official Analytical Chemists (AOAC) method for the soluble and insoluble dietary fibres. Prunes are very rich in dietary fibre (6-7% of the fresh fruit) with approximatively 57% of soluble dietary fibre and 43% of insoluble dietary fibre. The soluble dietary fibre is rich in pectins whereas the insoluble dietary fibre contains mainly cellulose, with significant amounts of pectins and hemicelluloses.

1. Bureau National Interprofessionnel du Pruneau – 2, rue des Magnolias – BP 130 – 47303 Villeneuve-sur- Lot – France.

2. Unité de Recherche Biopolymères, Interactions, Assemblages – INRA – Centre de Recherche Agro-Ali- mentaire – Rue de la Géraudière – BP 71627 – 44316 Nantes Cedex – France.

* Correspondence: thibault@nantes.inra.fr SDA27_6.book Page 423 Lundi, 18. août 2008 4:27 16

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424 Sci. Aliments 27(6), 2007 A. Fatimi et al.

1 – ABBREVIATIONS

AIS: alcohol insoluble solids DF: dietary fibre

SDF: soluble dietary fibre IDF: insoluble dietary fibre TDF: total dietary fibre

2 – INTRODUCTION

Many western countries have recommended increased consumption of foods rich in dietary fibre because of their potential health benefit such as pro- tection against cardiovascular diseases, diabetes, colon cancer, as well as regulation of the transit time (Dreher, 2001; Gibney, 2001).

Prunes are recognized as a healthy food (Stacewitz et al., 2001; Tinker et al., 1991). They are dried plums, the fruits of Prunus domestica cv d’Ente. After har- vest, the fruits are dried for 16-24h at 75˚C to reduce the moisture content down to 21-23% ; the prunes are then stored and rehydrated to 33-35% before marketing (Somogyi, 1987). They may be consumed directly in the diet or more rarely after processing to juices or purees for exemple (Stacewitz et al., 2001; Tinker et al., 1991).

Prunes have long been used to treat intestinal disorders such as constipa- tion. They are known to contain dietary fibre, sugars, sugar alcohols (sorbitol) and other substances having effects the intestinal function as laxatives (Somo- gyi, 1987; Stacewitz et al., 2001; Tinker et al., 1991). Prunes are also a good source of energy, phenolic acids, minerals or vitamins (Dikeman et al., 2004;

Donavan et al., 1998; Kayano et al., 2003; Piga et al., 2003; Somogyi, 1987;

Stacewitz et al., 2001; Tinker et al., 1991).

The literature is very scarce on the constitutive dietary fibres and their polysaccharides. A value of ~11 g/100 g dry matter was found for total dietary fibre (Piga et al., 2003); this value is in the range (6-16g/100 g fresh matter) given by Stacewitz et al. (2001). Some food table compositions (Englyst et al., 1988;

Fang et al., 2002) gave values of 8-9 g/100 g dry matter for total dietary fibre with 55-67% of soluble dietary fibre.

This paper deals with the determination of dietary fibres in various prune samples and the analysis of their constitutive polysaccharides. We have measured in this work the dietary fibre by the conventional enzymatic-gravimetric method and characterised the soluble- and insoluble-dietray fibre by their sugar composition.

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Dietary fibre in prunes 425

3 – MATERIALS AND METHODS

3.1 Plant material

The prunes were sampled in region of Villeneuve-sur-Lot (F). The 11 samples used in this study varied by their storage conditions, their moisture contents and their sizes; some were stoned.

3.2 Preparation of alcohol insoluble solids (AIS)

The prunes were first stoned and cut into small pieces (d ~ 5 mm). The samples were grinded in boiling ethanol 80% (12 mL ethanol/g of sample) : after boiling for 20 min, the samples were filtered on a Büchner (G3) funnel. The insoluble material was washed with ethanol 80% until a sugar-free extract was obtained (phenol-sulfuric acid test). The residue (AIS) was washed successively with ethanol 96%, and acetone, then air-dried overnight at 40˚C and weighed.

3.3 Determination of dietary fibre (DF)

The dietary fibre was measured gravimetrically after removing the non fibre constituents by enzymatic means as described by Prosky et al. (1988). Duplica- tes (2 x 1 g) in a 0.08 M phosphate buffer pH 6.0 were analysed for soluble (SDF) and insoluble (IDF) dietary fibres. Samples were treated with a thermosta- ble α-amylase from Aspergillus niger (Termamyl 120 L, Novozymes, Denmark) and then digested with a protease from Bacillus licheniformis (P5380, Sigma Aldrich, France) and an amyloglucosidase from Aspergillus niger (A-3042, Sigma). SDF and IDF were separated by filtration on a G4 funnel. SDF was recovered from the filtrate by precipitation with 4 volumes of ethanol 96%, then dried by solvent exchange and under vacuum overnight at 40˚C. IDF was dried by solvent exchange and under vacuum overnight at 40˚C. A blank was perfor- med by the same procedure. IDF and SDF values were obtained after correc- tions from the blank, and residual ash and proteins. Total dietary fibre (TDF) was calculated as the sum of IDF and SDF.

Preliminary assays (data not shown) evidenced that very similar values of DF were obtained directly from the prunes and from AIS. Therefore, for easiness reasons, the starting material for the determination of DF was the AIS and not the entire fruit

3.4 Determination of the uronic acids

SDF were first deesterified with 0.05 M NaOH for 30 min and neutralised with 0.1M HCl, whereas the AIS and IDF (~ 25 mg), after grinding in liquid nitrogen, were prehydrolysed with 13 M H₂SO₄ for 1h at 25˚C (Saeman et al., 1954).

The uronic acids (as galacturonic acid) were determined by the automatic meta- hydroxybiphenyl method (Thibault, 1979).

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3.5 Determination of the neutral sugars

The AIS and IDF (~ 25 mg), after grinding in liquid nitrogen, were prehydrolysed with 13 M H₂SO₄ for 1h at 25˚C followed by dilution to 1M and hydrolysis for 2h at 100˚C (Saeman et al., 1954). SDF were hydrolysed by 1M H₂SO₄ at 100˚C for 2 h, and 6 h for rhamnose quantification (Quemener and Thibault, 1990). The neutral sugars were then determined as alditol acetates by glc (Englyst and Cummings, 1988).

3.6 Moisture

The moisture of the prunes was determined by the AOAC official method for prunes and raisins (Anonym, 1995).

3.7 Protein and ash

Nitrogen was determined by the semi-automatic Kjeldal method and the protein content was estimated as N x 6.25.

Ash was determined as the weight of the sample leaving after incineration overnight at 550˚C and 1h at 900˚C.

4 – RESULTS AND DISCUSSION

AIS was first obtained from the prunes. It contains all the macromolecules which are insoluble in alcohol, including cell wall polysaccharides, starch, proteins and lignin. In the case of fruits poor in proteins and devoid of starch, this value may be very close to the TDF value (Thibault et al., 1994).

4.1 AIS content and composition

The content of AIS of the prunes (table 1) varies in the range 6.4-8 g/fresh matter or 9.8-12.1 g/100 g dry matter; in average, the values are 7.1 g/100 g and 10.9 g/100 g, respectively. These values are close to the values published previously (Raynal et al., 1991).

The composition of the AIS in sugars is indicated in table 2. They amounted to ~ 66% and the prunes also contained proteins (~ 9%) and ashs (~ 3%).

Sugars were mainly galacturonic acid (~24%), glucose (~14%), galactose (~14%), arabinose (~8%) with lower amount (0.5-1.9%) of rhamnose, fucose, xylose and mannose. This composition indicated the major presence of pectins with some cellulose and minor amount of hemicelluloses.

4.2 DF content and composition

Table 1 summarizes the content in soluble (SDF), insoluble (IDF) and total dietary fibre (TDF = SDF + IDF). TDF represented an average of 9.2 g/100 g dry

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matter (range 8.3- 9.9) and 5.9 g/100 g fresh matter (range 5.4-6.5). This value is in agreement with values reported for californian prunes (Stacewitz et al., 2001) or in composition tables (Englyst et al., 1988; Fang et al., 2002).

Table 1

Content of prunes in AIS and dietary fibre (values in g/100 g;

values in parentheses are standard deviations).

Teneur (g/100 g) des pruneaux en matériel insoluble dans l’acool et en fibres alimentaires (les valeurs entre parenthèses sont les déviations-standard).

Table 2

Composition in sugars of the prune AIS and DF (values in g/100 g dry weight;

values in parentheses are standard deviations).

Composition (g/100 g poids sec) en oses neutres et en acides galacturoniques des matériels insolubles dans l’alcool et des fibres alimentaires des pruneaux

(les valeurs entre parenthèses sont les déviations standard).

The prunes are richer in SDF (average 5.3 g/100 g dry matter; range 4.8-6.1) than in IDF (average 3.9 g/100 g dry matter) ; this proportion is close to the value given by Fang et al. (2002).

The amount of TDF is slightly lower than the values of AIS due to the presence in AIS of non dietary fibre substances such as starch, proteins,… which are not removed by enzymic degradation in the enzymatic-gravimetric method of dietary fiber measurement.

The compositions of SDF, IDF and TDF in sugars are indicated in table 2.

The different sugars amounted to 70.2% for SDF and 57.0% for IDF. Other

% Fresh fruit % Dry fruit

Alcohol insoluble solids 7.1(0.5) 11.0 (0.8)

Soluble Dietary Fibre 3.4 (0.3) 5.3 (0.4)

Insoluble Dietary Fibre 2.5 (0.3) 3.9 (0.5)

Total Dietary Fibre 5.9 (0.4) 9.2 (0.7)

AIS SDF IDF

Rhamnose 1.1 (0.1) 1.9 (0.2) 0.8 (0.1)

Fucose 0.5 (0.0) 0.3 (0.0) 0.7 (0.0)

Arabinose 7.7 (0.4) 6.9 (0.5) 6.3 (0.5)

Xylose 1.9 (0.1) 0.5 (0.1) 3.4 (0.2)

Mannose 1.5 (0.2) 1.8 (0.3) 2.4 (0.2)

Galactose 14.7 (0.5) 16.0 (1.0) 10.2 (0.8)

Glucose 14.4 (1.1) 1.1 (0.1) 26.2 (1.7)

Galacturonic acid 24.0 (1.1) 41.9 (3.6) 7.1 (0.5)

total 65.8 (0.8) 70.2 (4.2) 57.0 (1.6)

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components could be wall proteins, subsituents of the pectins (acetic acid, methanol…) and non sugar cell wall constituents such as polyphenols or lignin.

However, comparaison of glucose amount in AIS and TDF showed that starch was a minor component of the prunes.

The main sugars of SDF are galacturonic acid, galactose and arabinose with minor amount of rhamnose, fucose, xylose, mannose, and glucose. This is a typical composition of pectins. The IDF is very rich in glucose while galacturonic acid, galactose, arabinose are still present with xylose and mannose. This composition reveals the presence of cellulose mainly, with lower amounts of pectins (presence of galacturonic acid, rhamnose,...) and hemicelluloses (xylose, mannose,…). The high proportion of galacturonic acid in SDF is indicative of the predominant presence of pectins.

CONCLUSION

This work shows clearly that prunes are very rich in dietary fibre; they consti- tute approximatively 6-7% of the fresh fruit. Among dried fruits, which are often claimed to contain high amounts of dietary fibre, prunes and apricots have likely the highest amount (Stacewitz et al., 2001).

The prune fibre contains approximatively 57% of soluble dietary fibre and 43 of insoluble dietary fibre. This is a good balance, prune having possibly the advantages of both types of fibre. Together with the other components, prunes may be considered as interesting functional food.

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