At the tamer stage of maturity, date fruit has good storage stability mainly because of low moisture and high sugar contents.
Due to its good shelf life during storage, date fruit is also known as a self-preserving fruit. As discussed earlier, most of date fruits are consumed at the tamer stage of maturity but because of higher nutritional value, substantial amounts are also consumed in the perishable, immature khalal and rutab stages.
Date fruits at these immature stages are rich in dietary fiber, ascorbic acid, and -carotene; hence they are traditionally quite popular in dategrowing regions (Al-Hooti et al., 1997a). Date fruits of some cultivars at the khalal and rutab stages of maturity are preferred by consumers (Al-Mulhim and Osman, 1986). Khalas, Shahl, and Khenaizi in Saudi Arabia have been the preferred fresh dates among consumers there. Over 84% of the consumers of fresh dates are reported to prefer Khalas dates and 73% of the consumers purchase fresh dates packaged in baskets. But unfortunately, date fruits at these immature stages of maturity not only have higher moisture contents and are susceptible to microbial spoilage but are also available only for a very limited period during the season. Among the potential pathogenic bacteria, Escherichia coli, Staphylococcus aureus, and Bacillus cereus have been identified in date fruits together with lactic acid bacteria, yeasts, Aspergillus flavus, and A. parasiticus. Date fruits at the khalal and rutab stages being high in moisture are the most heavily contaminated (Aidoo et al., 1996). Development of any postharvest techniques to extend the shelf life of date fruits at the khalal and rutab stages would enhance the commercial and economic value of this crop, on the one hand, and would also provide consumers with a greater choice of delicious products with desirable nutritional contributions to their diet over an extended period of time. One approach to achieving this objective of shelf life extension is the use of antifungal agents such as potassium sorbate (Al-Hooti et al., 1997a). Compared with the control, the shelf life of khalal stage fruits of Bushibal and Lulu cultivars treated with 0.05% potassium sorbate when stored at 4◦C, remained acceptable for an additional 8 and 10 weeks, respectively.
The microbial load on these date fruits stayed within the acceptable limits. When these treated fruits are stored at −2◦C and −20◦C, no coliforms or enterobacteriaceae were detected, while the aerobes and mold counts stayed within the acceptable limits.
These subzero temperatures retard the growth of naturally occurring microflora present on these fruits. The fruits of both the cultivars matured from the khalal stage to the rutab stage during the storage period, thus making the fruits more acceptable to consumers. The frozen storage of date fruits at the rutab stage also leads to changes in various physicochemical characteristics.
During 6 months of storage at −18±2◦C, rutab date fruits increased in moisture content, reducing sugars, and pH but decreased in tannins (Mikki and Al-Taisan, 1993). The fruits developed an acceptable sweet taste with the disappearance of astringency. At the end of storage, the thawed product became soft in texture and darker in color.
Similar findings on the chemical composition of Egyptian dates during frozen storage have also been reported (Goneum et al., 1993). Some of the Saudi cultivars at the rutab stage of maturity are suitable for preservation by refrigeration and frozen storage (Yousif and Abou-Ali, 1993; Al-Mashadi et al., 1993). The Haynai var. of date fruits, when picked at the red stage, can be stored in frozen conditions at −18◦C. On thawing, it has a relatively short shelf life when stored at varying temperatures (6◦C, 15◦C, 20◦C, or 26±0.2◦C). Predominantly, the presence of yeasts and lactobacilli is responsible for the relatively short shelf life, and storage temperature is the major influencing factor in the rate of spoilage. Considering the initial microbial load, and time– temperature conditions in storage, the organoleptic shelf life of soft dates can be predicted during transport, handling, and retailing.
Date cultivation poses another unique problem in countries like India and Pakistan, where the crop ripening period coincides with the rainy season (Chatha et al., 1986), which leads to extensive spoilage of the date crop.
However, to circumvent these circumstances, date fruits are harvested at the khalal stage and then cured using various techniques such as the use of sodium chloride, sodium hydroxide, acetic acid, 2,4-dichlorophenoxy acetic acid, etc. Even using these methods, the shelf life of khalal-stage fruits cannot be extended beyond 2 months, after which the fruits turn black in color.
Antimicrobial agents and refrigeration are used for the preservation of high-moisture dates (43–56%) (Hassan et al., 1979). Date fruits can be immersed for 1 min in 0.25%, 0.5%, 1%, 2%, 5%, and 10% solutions of calcium propionate, sodium benzoate, potassium metabisulfite (K2S2O5), potassium sorbate, dehydroacetic acid, and combinations of sodium benzoate with calcium propionate or with SO2, inoculated with molds and yeast spores, and stored at 18–25◦C or 2–5◦C. Treatment with 10% calcium propionate or 5% potassium metabisulfite did not provide adequate protection during 13 weeks of storage at any of these temperatures.
Potassium sorbate provided 2 weeks protection at 0.5%, 7 weeks at 2%, and 14 weeks at 5% at low temperatures. At low temperatures, 5% benzoate-treated dates stayed acceptable for 4 weeks. Use of 1% sodium benzoate with 2% potassium metabisulfite or calcium propionate protected these date fruits against microbial spoilage for 9 months with refrigerated storage. The khalal stage fruit can also be used for the preparation of Chhuhara (khalal matbukh ) by cooking for 10, 15, 20, or 25 min and then drying in a solar drier (Gupta and Siddiqui, 1986). Chhuhara (dried dates) prepared from Khadrawi cultivar yields better quality in terms of pulp content, sugars, proteins, and sensory characteristics, than those prepared from Shamran cv.
The quality of the chhuhara from Khadrawi dates improved with prolonged boiling, whereas the quality deteriorated with prolonged boiling for Shamran cultivar. The ripening of date fruits shows a small peak in ethylene production initially, which increases as the fruit matures, thus suggesting that dates could be considered a climacteric fruit, and the plant hormone ethylene is responsible for changes in its color, fruit texture, soluble solids, and acidity. Fruit firmness decreases during various stages of ripening.
The greatest loss of fruit firmness correlates with the greatest increases in both the polygalacturonase and the -galactosidase activity (Serrano et al., 2001).