Teneur des pruneaux en fibres alimentaires et en polysaccharides pariétaux
Dietary fibre content and cell wall polysaccharides in prunes
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 quantités significatives de pectines et d'hémicelluloses. Keywords prunes, plums, dietary fibre.
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 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 protection 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 harvest, 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 constipation. They are known to contain dietary fibre, sugars, sugar alcohols (sorbitol) and other substances having effects the intestinal function as laxatives (Somogyi, 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 solubleand insoluble-dietray fibre by their sugar composition. 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). Duplicates (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 thermostable -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 performed by the same procedure. IDF and SDF values were obtained after corrections 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 La 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 metahydroxybiphenyl method (Thibault, 1979).
a. FATIMI, M.-C. RALET, m.-j. CREPEAU, s. RASHIDI, J.-F. THIBAULT
prunes, plums, dietary fibre.