Like most other fruits, dates are a rich source of carbohydrates, mainly fructose and glucose, but smaller quantities of vitamins, minerals, and other minor constituents are also present. The major chemical constituents of date fruit are carbohydrates, mainly reducing sugars, such as glucose and fructose, and also a nonreducing sugar, sucrose.
Carbohydrates (i.e., sugars) are, therefore, the most widely studied constituents of date fruits. The chemical composition of five major cultivars grown in the UAE at various stages of maturity has been described by Al-Hooti et al. (1997f). In a majority of the cultivars, the sucrose content increased rapidly as the date fruit matured, reaching the highest level at the khalal stage (42.58%) but subsequently decreased to a nondetectable level at the tamer stage of maturity.
As the date fruits matured, the glucose and fructose sugars increased rapidly to reach a level of 38.47–40.04%. When the date fruit matured from the kimri to the tamer stage, the fructose content increased approximately threefold, which accounts for the characteristic sweet taste of tamer date fruits. The total sugar contents, which were 32.99–38.20% at the kimri stage, reached nearly 80% by the tamer stage of maturity. The presence of equal amounts of glucose and fructose in soft-type cultivars is responsible for their enhanced levels of sweetness. On the other hand, some of the semidry and dry cultivars are reported to retain higher levels of glucose than fructose or the unhydrolyzed sucrose.
One of the earliest studies also reported that the total sugars and invert sugars increased with ripening, reaching a maximum level by the later stages of development (Vinson, 1924). Soft dates with almost no sucrose, semidry cultivars like Deglet Noor with higher levels of sucrose, and dry dates with equimolar concentrations of sucrose and reducing sugars are also available (Sawaya et al., 1982). The total reducing sugar contents are related to the cultivar as well as to the stage of maturity. In the semidry varieties of Egyptian tamer dates, both the sucrose and reducing sugars are about 35–40% each. The total sugar concentration at this stage reaches between 80 and 90% of the dry weight. During the curing stage, the sucrose content of soft varieties disappears completely (Ragab et al., 1956).
The total sugar contents of 39 Saudi Arabian cultivars falls between 61 and 80%, while the total sugar contents of three cultivars amounted to 80–90% (Hussein et al., 1976). The sucrose content is usually the highest (10–30%) at the khalal stage in most of the cultivars, but it declines to lowlevels of 0–2% at the tamer stage.
In contrast, the reducing sugars generally increase with fruit development, reaching 29–85% at the tamer stage of maturity. Reducing sugars are mainly the predominant sugars at tamer stage in most of the cultivars, with the exception of two cultivars, namely, Sukkari and Sukkarat Al-Shark, which contain more sucrose at the tamer than at the khalal stage of maturity. In a given cultivar, the sucrose and reducing sugar contents are related to the quality and texture of the date fruit (Coggins and Knapp, 1969).
Therefore, the majority of these Saudi Arabian cultivars (Hussein et al., 1976) having higher concentrations of reducing sugars at the tamer stage, are of the soft-type date fruits. The five cultivars from theUAE reported by Al-Hooti et al. (1997f) with nondetectable sucrose contents at the tamer stage also belonged to the soft-type date fruits. The declining moisture content coupled with the rapid increase in glucose and fructose contents render the tamer date fruits extreme resistance to fungal spoilage during storage.
As the texture and color of dates are the important attributes affecting fruit quality and acceptability, most of the biochemical and enzyme studies have been limited to these aspects of date fruit physiology. The higher activity of the sucrose-hydrolyzing enzyme invertase present in soft-type date fruit cultivars is the most important enzyme influencing the date fruit quality and is considered to be mainly responsible for the highest levels of reducing sugars present at the tamer stage of maturity (Vinson, 1911; Vandercook et al., 1980). The changes in invertase activity in Deglet Noor date fruits during maturation and ripening have been studied by Hasegawa and Smolensky (1970).
Soluble invertase increases dramatically when the date fruit matures from the green stage to the early red stage. The insoluble form of this enzyme is present in substantial amounts during the green stage, when it decreases to 50% of its original activity and then remains fairly constant later on.
Both the insoluble and soluble invertase hydrolyzes sucrose, raffinose, and melezitose in a similar manner. Among the four grades of dates evaluated in their study, soft, good quality dates had a higher activity of this enzyme than the tougher dates of inferior quality. Invertase can be used to improve the quality and market value of date cultivars, which have crystalline sucrose present in their tissues (Smolensky et al., 1975).
The enzyme concentration, temperature, and the time of treatment are important to bring the ratio of sucrose– reducing sugars to a level low enough to prevent sucrose crystallization later on during storage.
Soluble invertase and insoluble invertase have been isolated from date fruits (Zehdi var.), and for both enzymes 45◦C is an optimum temperature for activity (Marouf and Zeki, 1982). The optimum pH ranges for soluble and insoluble invertase are 3.6–4.8 and 3.6–4.2, with Km values of 3.12 × 10−3and 4.35 × 10−3mM, respectively. The specific activity of soluble invertase is 40.2 mol/mg protein/min, while the specific activity of insoluble invertase is 1.1mol/mg protein/min. Sodium Dodecyl Sulfate inhibits both the enzymes Proteins In addition to the major constituent carbohydrates, date fruits also contain significant amounts of protein, crude fiber, pectin, tannins, minerals, and vitamins.
Al-Hooti et al. (1997f) have analyzed five important date cultivars from the UAE at different stages of maturity. The crude protein content in these cultivars is highest at the kimri stage (5.5–6.4%) and gradually decreases to 2.0–2.5% as the fruit reaches the tamer stage of maturity.
Although the protein content is not high in tamer date fruits, the essential amino acid content of these proteins is quite good. Similar protein content in other cultivars have been reported by various other workers (Hussein et al., 1976; Sawaya et al., 1986b). Fat The date fruit is quite low in crude fat, which usually ranges from 0.5% at the kimri stage to 0.1% at the tamer stage of maturity (Al-Hooti et al., 1997f). Evidently, date fruit, like most other fruits, cannot be considered as an important source of fat or fatty acids in our diet. These values for crude fat content are similar to those of some of the cultivars reported by others (Sawaya et al., 1982, 1986b; Ragab et al., 1956).