OTHER STORAGE PROBLEMS

date palm
date palm

All varieties of date fruits are commonly attacked by a number of insect pests and fungi during storage. Most of the insects belong to the Lepidoptera and Coleoptera orders. Some species belonging to the order Acarina can also cause considerable damage to date fruits. The commonly reported insects found in stored date fruits are Oryzaephilus surinamensis, O. mercator, Tribolium confusum, Plodia interpunctella, Cadra cautella, C.

calidella, and C. figulilella (Abdelmonem et al., 1986b; Carpenter and Elmer, 1978). The commonly isolated fungi from date fruits are Aspergillus sp., Alternaria sp., Fusrieum sp., Penicillium sp., Rhizopus sp., and Saccharomyces sp. (Carpenter and Klotz, 1966; Chohan, 1972).

Most important is the fact that any insect (at any stage of their growth) and/or insect fragments, sharply reduce the market value of such fruits. Moreover, the presence of these insects in date fruit products create serious quarantine problems in the international trade (Abdelmonem et al., 1986a). Apart from insects, mycotoxins (low molecular weight chemicals) can be produced by many common filamentous fungi found on various food and feed products.

Aflatoxins are among the most potent carcinogenic, mutagenic, and teratogenic chemicals that can be produced by Aspergillus flavus and A. parasiticus, under conditions suitable for growth. Four major aflatoxins, namely,B1, B2, G1,andG2, are commonly found in foods in tropical and subtropical climates. Date fruit under conditions of high humidity and moderate temperature can be contaminated with aflatoxins.

Some varieties of date fruit stored under simulated conditions of 98% relative humidity and 30◦C were found to contain significant levels of aflatoxin B1 or B2 ranging from 35 to 11,610 g/kg (Shenasi et al., 2002).

The presence of A. flavus and A. parasiticus on date fruits has been reported from a number of countries (Abu-Zinada and Ali, 1982; Emam et al., 1994; Ahmed et al., 1997; Ahmed and Robinson, 1999; Ahmed and Robinson, 1997; Salik et al., 1979). Two of the isolates from the date fruits treated with methyl bromide (MB) and stored in polyethylene bags for 8 months at 60–75% relative humidity and 20–25◦C, produced aflatoxins B1, G1, and G2 in synthetic medium and on date fruits (Emam et al., 1994).

The presence of A. flavus and A. parasticus in some of the date fruits imported to the United Kingdom has been observed (Ahmed et al., 1997).

The mold A. parasiticus is able to penetrate the intact date fruit tissue and can produce aflatoxin in 10 days at 28◦C in all stages of maturity except at the tamer stage, which do not support mold growth (Ahmed and Robinson, 1999), but the extracts from all four stages of date fruit were able to support growth of this mold for aflatoxin production (Ahmed and Robinson, 1997), and the amounts of aflatoxin produced increased with ripeness.

Generally, the pattern of aflatoxin production appears to be broadly in line with the changes in the sugar content and chemical composition of maturing date fruits. Therefore, maximum care should be taken during the processing and handling of date fruits to avoid aflatoxigenesis.

Detection of aflatoxins in date fruits requires an accurate method for extraction and derivatization. The Romer minicolumn method is able to detect aflatoxins in contaminated date fruits. However, using high-performance liquid chromatography and postcolumn derivatization, the contaminants branch (CB) method gives average recoveries of 75.7% and 83.5% for the Lulu and Naghal date varieties, respectively (Ahmed and Robinson, 1998).

The recovery of total aflatoxins by the best food (extraction and purification procedure) is about 35% less than that with the CBmethod. The available Association of Official Analytical Chemists methods, with slight modifications, give better recoveries of aflatoxins from date fruits.

Date fruits are a vital component of the diet in most of the Arabian countries. Besides insect infestation, it is not known whether or not the dates available in the market are also contaminated with aflatoxins.

Consequently, a number of control measures have been suggested to overcome these problems of insect infestation and the growth of undesirable mycotoxinproducing microorganisms on date fruits during marketing and storage. The commercially packed Zahdi cultivar treated with ionizing radiation or with MBfumigation can provide an insect-free product for about 25 days (Ahmed et al., 1982) but with longer storage, re-infestation takes place.

The major fungi, Botrytis cinerea and Penicillium expansum, causing postharvest decay of soft-type date fruits of the Zagloul var. can be drastically reduced without causing any shrinkage of the fruits when irradiated with up to 200 krad gamma ray dosages (El-Sayed, 1978). Additionally, the reduction of the tannin content with irradiation leads to reduced astringency and improved sensory quality.

Fortunately, irradiation does not produce any changes in the amino acids, sugars, and protein contents of these treated date fruits during their extended storage life.

Irradiation of date fruits can be used to control the growth of undesirable microorganisms without adversely affecting the sensory quality of the fruit. To reach a decimal reduction value (D10 value), a dosage of 1.4 kGy is sufficient for total plate counts, 1.2 kGy for yeasts, and 0.9 kGy for bacterial spores present in some of the date fruit varieties being grown in Saudi Arabia.

A dosage of 4–5 kGy is required to reduce the microorganisms to an undetectable level without adversely affecting the sensory quality of date fruit (Grecz et al., 1986). Emam et al. (1994) found that irradiation (1.5 and 3.0 kGy) is more effective in preventing insect infestation than MB fumigation of Egyptian semidried date fruits of the El-Seidi cultivar during storage for 8 months at room temperature.

Neither irradiation nor MB caused any significant changes in moisture content, pH, or titratable acidity, but both produced significant changes in browning, total, reducing, and nonreducing sugars; and in the sugar–acid ratio. An irradiation dosage of 3.0 kGy was more effective than MB fumigation in inhibiting fungal growth and aflatoxin production, which is recommended for maintaining date fruit quality during long-term storage.

Fumigation and aeration of mature, dry date fruits of the Zahdi cultivar with phosphine gas at various levels from 0 to 56.7 mg/l for 72 h did not produce significant differences in the sugar and protein contents, but the threonine and methionine contents decreased significantly (Al-Hakkak et al., 1986).

A method exists to determine if a date fruit has been irradiated or not. An unirradiated date stone contains a radical with a single line g = 2.0045, feature A. Irradiation up to a dosage of 2.0 kGy (the recommended dosage for irradiation of fruits in the United Kingdom) induces the formation of additional radicals with signals g = 1.9895 and 2.0159, feature C. The single line having g = 2.0045 decays both in the irradiated and control date fruit samples, whereas the additional signals g = 1.9895 and 2.0159 remain almost unaltered for 15 months of storage at room temperature and at 4◦C (Ghelawi et al., 1996).

Three fumigants, namely, phostoxin, MB, and hydrogen cyanide; dry heat treatment (55 ± 2◦C); and cold storage (at −15◦C and 5◦C) have been tested against the most common insects present in stored date fruits (Abdelmonem et al., 1986a). The three fumigants caused 100% mortality of all insect species (Cadera cautella, Oryzaephilus surinamensis, and Tribolium confusum) after 24 h of exposure.

Dry heat treatment also produced similar results after 1.5 h of exposure against all insects. Chilling temperature could achieve only up to a maximum of 84% mortality in 30 h, whereas −15◦C, treatment produced 100% mortality in just 6 h. These fumigants are also effective in reducing the fungi commonly present on date fruits, withMBbeing the most effective (Hegazi et al., 1986a).

The use of a 100% carbon dioxide environment to control the insects and fungi commonly present in stored date fruits has been tried, but the success rate is variable against these organisms (Mohammed et al., 1980). Carbon dioxide gas in saturated chambers produced 100% mortality of all insects after 48 h but the fungi and bacteria survived in the stored date fruits.