The crude fiber content of date fruits at the kimri stage is substantially higher (6.2–13.2%) than that at the tamer stage (2.1–3.0%) of maturity (El-Kassas, 1986). The crude fiber content of date fruits is not a good indication of their dietary fiber content (Yousif et al., 1982).
The total dietary fiber content (comprised of pectin, hemicellulose, cellulose, gums, mucilages, resistant starch, and lignin) depends on the stage of maturity of the date fruits (El-Zoghbi, 1994).
Xylan has been identified as one of the components of date fruit fiber (Hag and Gomes, 1977). Alcohol-extractable material from date fruit, when further treated with water, dilute acid, and aqueous alkali yields polysaccharide, which contains varying proportions of D-galactose, D-glucose, L-arabinose, D-galacturonic acid, and L-rhamnose.
Glucomannan is another polysaccharide found in date fruits. The structure of glucomannan isolated from the seeds of Libyan dates has been elucidated (Ishrud et al., 2001).
This polysaccharide is extracted with 80% hot ethanol (Fraction-I) and 0.1 M phosphate solution (Fraction-II), fractionated and purified by ion exchange and gel filtration chromatography. According to methylation and hydrolysis analysis, the main chains of FI and FII consist of 1–4, linked glucomannan with only traces of branched sugar residues. The total fiber decreases as the date fruits lose their firm texture and become soft at the tamer stage.
A large variation in the total dietary fiber content of date fruits comes from the type of method employed in its determination.
The Southgate method does not determine resistant starch, whereas the Fibertec and Englyst methods do (Kirk and Sawyer, 1991). So, to obtain comparable results, internationally accepted methods of analysis for dietary fiber must be employed. The total dietary fiber content measured by the enzymatic method has been reported to be 9.2%, with 6.9% as insoluble and 2.3% as soluble fiber (Lund et al., 1983).
The total fiber content in some of the dates from Saudi Arabian, Egyptian, Iraqi, and Irani cultivars, determined by the Fibertec system, ranged from 8.1% to 12.7% (Al-Shahib and Marshall, 2002). Research conducted during the past three decades has shown that an adequate intake of dietary fiber (20–25 g daily) lowers the incidence of colon cancer, heart diseases, diabetes, and other diseases.
Obviously, the consumption of 100 g of date fruit (six to seven dates) would provide us with about 50% of the recommended daily amount of dietary fiber.
The total dietary fiber of dates decreases from 13.7% at the kimri stage to 3.6% at the tamer stage of maturity (Ishrud et al., 2001). The decrease in the pectin, hemicellulose, cellulose, and lignin contents during date-fruit ripening range from 1.6 to 0.5, from 5.3 to 1.3, from 3.4 to 1.4, and from 3.5 to 0.3%, respectively. This shows that maximum benefit can be obtained by consuming fresh dates (i.e., those at the kimri, khalal, and rutab stages) rather than by consuming the fully mature tamer fruits. The presence of resistant starch in the fresh dates will provide an additional advantage as it may be prebiotic, promoting conducive conditions for the growth of desirable bifidobacteria in the lower gastrointestinal tract (Topping and Clifton, 2001). Pectins The pectic substances (considered a part of the soluble dietary fiber) are a complex mixture of polysaccharides that are important constituents of plant cell wall structures.
The pectin contributes to the adhesion between cells and also plays an important role in some of the processed fruit products such as jams, jellies, and preserves (Jarvis, 1984). As the date fruit ripens, protopectin is converted into water-soluble pectin through the combined action of two pectolytic enzymes, i.e., polygalacturonase and pectin methyl esterase (Al-Jasim and Al-Delaimy, 1972; Coggins et al., 1968).
There is a close relationship between polygalacturonase activity and fruit softening during ripening (Hasegawa et al., 1969). Like invertase, polygalacturonase activity is relatively higher in soft dates than in the tough dates. Unlike cellulase and polygalacturonase, the activity of pectin esterase increases as the fruit grows reaching a maximum during the khalal and rutab stages of maturity (Al-Jasim and Al-Delaimy, 1972).
Cellulase activity, which is absent in kimri- stage fruit, increases as the fruit develops, reaching its peak at the late rutab stage and remains constant during the tamer stage (Hasegawa and Smolensky, 1971). As the date fruit ripens, the pectin content increases, reaching a maximum level at the khalal stage (7.0–14.3%), and then decreases at the tamer stage (1.3–1.9%) of maturity. Graces- Medina (1968) estimated the pectin contents of banana, mango, pineapple, and mountain apple to be 0.62%, 0.38%, 0.13%, and 0.47%, respectively. The date fruit, therefore, can be a better source of soluble dietary fiber in our diet than some of the more common fruits such as banana, mango, pineapple, and apple.