Dates are known to be a reasonably good source of many minerals. The mineral composition of date fruit is largely affected by the level of soil fertility as well as by the amount of chemical fertilizers and manures applied to the trees (Hussein et al., 1976).
The ash content of date fruits decreases from the kimri stage (3.5–3.9%) to the tamer stage (1.3–1.8%) of maturity (Al-Hooti et al., 1997f). The ash content of some common fruits like grapes, apples, plums, and oranges is also similar (1.9–3.1%) to that of date fruits (FAO, 1982). Obviously, date fruits are an equally good source of important minerals.
Although the mineral content decreases at the tamer stage of maturity, the change is small when compared with changes in other constituents such as sugars. The composition of date fruits from five major cultivars being grown in the UAE, in terms of important mineral contents, has been reported in detail by Al-Hooti et al.
(1997f). These date cultivars are rich in most of the macroelements but are poor in microelements. Like most other fruits, these cultivars are low in sodium (1.5–9.4 mg%) but high in potassium (402.8– 1668.6 mg%). Minerals, particularly potassium, accumulate in date fruits during ripening (Ragab et al., 1956). This low sodium–potassium ratio obviously makes the date fruit a desirable food for persons suffering from hypertension. Date fruits are considered a rich source of iron (Anwar-Shinwari, 1987).
Different varieties generally contain significantly varied amounts of iron per unit weight or per fruit, the variation is attributed to genetic differences.
Boron is another important mineral present in date fruits. The formation of a red complex between boron and the quinalizarin reagent can be used to determine the boron content in date fruits through a simple and sensitive spectrophotometric method. At 620 nm, the absorbance is linear (r = 0.999) over the 0.25–2.5 g/ml concentration range (Al-Warthan et al., 1993). This method can detect the prevailing wide variation in the mineral contents of Saudi Arabian date cultivars.
Tannins The tannins in date fruit play an important role not only in flavor perception but also in the development of color in the date fruits during ripening and storage.
The color of dates is primarily due to the pigments produced by browning reactions during ripening, processing, and storage. These brown pigments could be produced by three possible mechanisms: browning of sugars, enzymatic oxidation of polyphenols, and oxidative browning of tannins (Coggins and Knapp, 1969). Generally, the oxidative browning reactions occur more rapidly at elevated temperatures than at low refrigerated temperatures. Even at room temperature, the enzymatic browning of polyphenols and tannins is much faster than sugar browning reactions.
However, at temperatures above 38◦C, sugar browning predominates. As the date fruit matures, tannins decrease rapidly from the highest value of 1.8–2.5% at the kimri stage to about 0.4% at the tamer stage of maturity (Al-Hooti et al., 1997f). This trend of reduction in tannins with ripening was also observed earlier in other date cultivars (Hussein et al., 1976).
Kimri fruit is quite astringent and unpalatable but this decreases drastically at tamer stage when the level of tannins reaches a very low level, indicating some probable contribution of tannins in the flavor of date fruits.
The Lulu cultivar had the lowest tannin content at the khalal, rutab, and tamer stages of maturity, when compared with other cultivars. The major enzyme involved in the metabolism of tannins in date fruits, polyphenol oxidase (PPO), has been studied at different stages of ripening (Benjamin et al., 1979). Using catechol as a substrate, the optimum pH and temperature for its activity are 6.4 and 37◦C, respectively.
This enzyme has no monophenol oxidase activity and varies in specific activity toward several diphenols. Its heat inactivation follows firstorder reaction conditions. During the ripening of date fruit, the PPO activity is the highest at the kimri, followed by khalal and tamer stages. The PPO activity can be completely inhibited by 0.01 M of ascorbic acid, cysteine, sodium sulfite, and sodium dithiocarbamate.
Vitamins Compared with other fruits, the tamer date fruit is not considered a good source of vitamins, but khalalstage fruits contain appreciable amounts of ascorbic acid and -carotene (Watt and Merrill, 1963). Khalal dates are reported to contain 1.8–14.3 mg and tamer dates 1.1–6.1 mg of ascorbic acid per 100 g of fresh fruit pulp. Similarly, the -carotene content (Pro-vitamin A), expressed as IU of vitamin A, ranged between 20 and 1416 IU in khalal and 0–259 IU per 100 g fresh fruit pulp in tamer date fruits (Hussein et al., 1976). Date fruit is also a reasonably good source of thiamin, riboflavin, and niacin (Watt and Merrill, 1963).