Depending on the cultivars as well as the stage of maturity, date fruit varies its shape, size, and color. Besides changes in physical characteristics, date fruit varies greatly in chem- ical composition. As the fruit matures, it undergoes major morphological and chemical changes that subsequently de- termine the overall quality and acceptability of this fruit. A lot of information on the physicochemical changes occurring in date fruits during different stages of maturity is now available from all the major date-fruit growing countries of the world (Salem and Hegazi 1971; Shabana et al. 1981; Mohammed et al. 1983; Al-Hooti et al. 1997f).
Date fruits may have round, oval, oblong, or cylindrical shapes depending on the cultivar. Al-Hooti et al. (1997f) reported physical measurements and colors of five major date-fruit cultivars grown in the UAE. All the cultivars were green in color at thekimristage but at thekhalalstage, the color varied among the cultivars. At thekhalalstage,Shahla
and Bushibal were red, Gash Gafaar and Lulu were yel- low, whereasGash Habashfruits were yellow-scarlet (Figs.
34.2A–G). At the last stage of maturity (i.e., tamer), the fruits of all the cultivars turned dark brown and shriveled considerably. The fruit weight, pulp–seed ratio, and physical measurements at the various stages of maturity of these five cultivars have been described in detail elsewhere (Sidhu and Al-Hooti 2005). At thekimristage, theBushibalandGash Gafaarfruits are cylindrical in shape, whereas theLulufruits are nearly round. These shapes are more or less retained by all cultivars throughout the various stages of fruit development.
The lengths of Bushibal, Gash Gafaar, Gash Habash,and Shahlaare 26.5, 24.8, 23.7, and 27.7 mm, respectively, with the corresponding values for width being 15.4, 15.5, 17.7, and 18.6 mm. In contrast, theLulucultivar has a length and width of 19.3 and 18.6 mm, respectively, indicating its nearly round appearance. Fruit weights are generally the highest at thekhalalstage (6.1–10.0 g) and decrease subsequently to- ward the tamer stage (4.9 g). The pulp percentage varies from 83% to 90%, and the seed percentage varies from 10%
to 17% among these five cultivars.
The physicochemical characteristics of 55 Saudi cultivars atkhalal andtamer stages of maturity are quite similar to those of the UAE cultivars reported by Al-Hooti et al. (1997f).
Some of Saudi cultivars had a bigger fruit size (25.6–26.8 g) at thekhalalstage, but it was reduced to 13.7–14.1 g at the tamer stage. The fruit weights ranged from 5.8 to 26.8 g (with an average of 13.5 g) at thekhalalstage and from 4.8 to 18.3 g (with an average of 9.8 g) at thetamerstage. The weight of seeds ranged from 0.7 to 1.8 g atkhalalstage and 0.6–1.3 g at thetamerstage of maturity of these cultivars. The pulp percentage of these cultivars was reported to be in the range of 86–96% and was slightly higher than that reported by Al-Hooti et al. (1997f), although some of the smaller fruit cultivars studied by them had similar fruit size and pulp per- centages. Sourial et al. (1986) evaluated four local cultivars, namely,Sofr-Eldomain, Kabooshy, Sergy,andHomr-Baker, in relation to their control cultivar,Hayany, grown in Egypt for their physicochemical characteristics. The fruit lengths for these five cultivars ranged from 4.9 to 5.6 cm and fruit weights ranged from 15.79 to 25.30 g, seed weights ranged from 1.87 to 2.38 g, and pulp percentage ranged from 88.15%
to 90.70%. In another study, Nour et al. (1986) reported the physical characteristics of nine dry palm cultivars (i.e., Balady, Bartamuda, Degna, Garguda, Gondalia, Kolma, Malkabi, Sakkoti,andShamia) grown in Aswan, Egypt. The fruit weights, fruit lengths, fruit widths, and seed weights ranged from 6.5 to 16.9 g, from 3.89 to 5.40 cm, from 1.75 to 3.32 cm, and from 1.0 to 1.63 g, respectively. The cultivar Malkabihad the highest fruit weight (16.9 g),Shamiawas the longest (5.4 cm), andBartmudahad the smallest seed (1.0 g only). Physicochemical characteristics and sensory quality of two date varieties have been investigated under commer- cial and industrial storage conditions by Ismail et al. (2008).
Khalas variety maintained the best quality for only 2 months
at−3◦C, whereas Barhee variety was good for 1 year at this temperature.
Carbohydrates
The major chemical constituents of date fruit are carbohy- drates, 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 cul- tivars 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 thekhalal stage (42.58%) but subsequently decreased to a nondetectable level at thetamerstage 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 thekimri to thetamer stage, the fructose content increased approxi- mately threefold, which accounts for the characteristic sweet taste of tamerdate fruits? The total sugar contents, which were 32.99–38.20% at thekimristage, reached nearly 80%
by thetamerstage 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 to- tal sugars and invert sugars increased with ripening, reaching a maximum level by the later stages of development (Vinson 1924).
The total reducing sugar contents are related to the cultivar as well as to the stage of maturity. In the semidry varieties of Egyptiantamerdates, both the sucrose and reducing sug- ars 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 varied from 61% to 80% (Hussein et al. 1976). The sucrose content is usually the highest (10–30%) at thekhalalstage in most of the cultivars, but it declines to 0–2% at thetamer stage. In contrast, the reducing sugars generally increase with fruit development, reaching 29–85% at thetamerstage of maturity. Reducing sugars are mainly the predominant sugars attamerstage in most of the cultivars, with the exception of two cultivars, namely,SukkariandSukkarat Al-Shark,which contain more sucrose at thetamerthan at thekhalalstage 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). The majority of Saudi Arabian cultivars (Hussein et al. 1976) having higher concentrations of reduc- ing sugars at the tamerstage, are the soft-type date fruits.
The five cultivars from the UAE 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 thetamer date fruits extreme resistance to fungal spoilage during storage.
As the texture and color of dates are the important at- tributes 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 influ- encing 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 inDeglet Noor date fruits during maturation and ripening have been stud- ied 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 melez- itose 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 qual- ity. 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 concen- tration, temperature, and the time of treatment are impor- tant 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 (Zehdivar.), and for both enzymes 45◦C is an optimum temperature for activity (Marouf and Zeki 1982). The optimum pH ranges for soluble and insolu- ble invertase are 3.6–4.8 and 3.6–4.2, withKmvalues of 3.12
×10−3and 4.35×10−3mM, respectively. The specific ac- tivity of soluble invertase is 40.2μM/mg protein per minute, while the specific activity of insoluble invertase is 1.1μM/mg protein per minute. Sodium dodecyl sulfate inhibits both the enzymes.
Proteins
In addition to the major constituent carbohydrates, date fruits contain significant amounts of protein, crude fiber, pectin, tannins, minerals, and vitamins. Al-Hooti et al. (1997f) ana- lyzed five important date cultivars from the UAE at different stages of maturity. The crude protein content in these culti- vars is highest at thekimristage (5.5–6.4%) and gradually decreases to 2.0–2.5% as the fruit reaches thetamerstage of maturity. Although the protein content is not high intamer date fruits, the essential amino acid content of these proteins is quite good. Similar protein content in other cultivars has been reported by various other workers (Hussein et al. 1976).
Fat
The date fruit is low in crude fat, which usually ranges from 0.5% at thekimristage to 0.1% at thetamerstage of maturity (Ragab et al. 1956; Al-Hooti et al. 1997f).
Crude Fiber
The crude fiber content of date fruits at thekimristage is substantially higher (6.2–13.2%) than that at thetamerstage (2.1–3.0%) of maturity (El-Kassas 1986). The crude fiber content of date fruits is not a good indication of their di- etary 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 ofd-galactose, d-glucose,l-arabinose, d-galacturonic acid, and l-rhamnose. Glucomannan is an- other 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 phos- phate 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 thetamerstage. 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 Fib- ertec 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 di- etary fiber content measured by the enzymatic method (Lund et al. 1983) has been reported to be 9.2% (6.9% as insoluble and 2.3% as soluble fiber). The total fiber content in some of the dates from Saudi Arabian, Egyptian, Iraqi, and Irani culti- vars, 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 in- take of dietary fiber (20–25 g daily) lowers the incidence of colon cancer, heart diseases, diabetes, and other diseases. Ob- viously, 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 thekimristage to 3.6% at thetamer 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 thekimri, khalal,andrutabstages) rather than by consuming the fully maturetamerfruits. 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 bifidobac- teria in the lower gastrointestinal tract (Topping and Clifton 2001). Chemical composition and characteristics of the di- etary fiber obtained from two date palm cultivars grown in Tunisia have been reported by Elleuch et al. (2008). They obtained a concentrate from date fruits that contained a total dietary fiber content ranging from 88% to 92.4% and that exhibited a water-holding capacity of approximately 15.5 g water/g sample.
Pectins
The pectic substances (considered a part of the soluble di- etary 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 prod- ucts 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, polygalacturonase, and pectin methyl esterase (Coggins et al. 1968; Al-Jasim and Al-Delaimy 1972). 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 thekhalaland rutab stages of maturity (Al-Jasim and Al-Delaimy 1972).
Cellulase activity, which is absent in kimri-stage fruit, in- creases as the fruit develops, reaching its peak at the laterutab stage and remains constant during thetamerstage (Hasegawa and Smolensky 1971). As the date fruit ripens, the pectin content increases, reaching a maximum level at thekhalal 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.
Minerals
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 thekimristage (3.5–3.9%) to thetamerstage
(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 thetamerstage 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 macroele- ments 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, high potassium makes this fruit a desirable option for persons suffering from hypertension.
Date fruits are considered a rich source of iron (Anwar- Shinwari 1987). Different varieties generally contain signif- icantly varied amounts of iron per unit weight or per fruit, the variation is attributed to genetic differences. Boron is an- other 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 μg/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 development of color during ripening and storage. The color of dates is primarily due to the pigments produced by browning reactions during ripen- ing, processing, and storage. These brown pigments could be produced by three possible mechanisms: browning of sugars, enzymatic oxidation of polyphenols, and oxidative brown- ing 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 pre- dominates. As the date fruit matures, tannins decrease rapidly from the highest value of 1.8–2.5% at thekimristage to about 0.4% at thetamerstage 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).
Kimrifruit is quite astringent and unpalatable but this de- creases drastically attamerstage when the level of tannins reaches a very low level, indicating some probable contribu- tion of tannins in the flavor of date fruits. TheLulucultivar had the lowest tannin content at thekhalal, rutab,andtamer
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 differ- ent stages of ripening (Benjamin et al. 1979). Using catechol as a substrate, the optimum pH and temperature for its ac- tivity 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 first- order reaction conditions. During the ripening of date fruit, the PPO activity is the highest at thekimri, followed bykhalal andtamerstages. The PPO activity can be completely inhib- ited by 0.01 M of ascorbic acid, cysteine, sodium sulfite, and sodium dithiocarbamate.