Assessment of the Boiling Process and Enriched- Co2 Atmosphere as Methods of Artificial Ripening of Helali and Khesab Date Fruits

There are many astringent date cultivars at maturity (Full coloration) around the world. These fruits could not be consumed at this stage due to the presence of soluble tannins. Growers have to wait until about 50% of these bunches reach to the rutab stage (ripening) which represents a major source of fruit loss due to abscission, birds attack and pathogens. Furthermore, late-maturing cultivars in the Gulf region such as Helali suffer from the lack of conversion to the rutab stage because of the drop in temperature at this time of the year. Thus, there is a great need to induce rutab development in many date fruits. Two separate studies were conducted during 2001 and 2002 seasons. In the first one, Helali and Khesab fruits at the Khalal stage were used to assess the boiling process as a method of artificial ripening especially its adverse effects on fruit quality. Fruits were boiled for 20, 30, 40, 50, and 60 mins by using a water bath provided with thermostat in addition to the control. In the second study, strands of Helali and Khesab were sealed in thick plastic bags for 1, 2, 3, 4, 5 or 9 days in Helali experiment and for 8, 16, 24, 32 hrs or 6 days in Khesab experiment in addition to the control in both cultivars (open, no sealing). Boiling at all durations caused full rutab development in both cultivars. However, such treatment caused a significant increase in electrolyte leakage and in fruit acidity, a significant reduction in vitamin C and the J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 2 TSS to acidity ratio while the TSS significantly increased as compared with the control. The assessment was done after five days on the shelf following the treatments. In the second study, sealing the Helali and Khesab fruits in thick plastic bags led to enriched-CO2 atmosphere which resulted in a significant increase in rutab Helali dates especially after 5 and 9 days while in Khesab sealing for hours was not effective in that mater but a significant increase in rutab development was obtained after 6 days of sealing. Moreover, the effective sealing periods in inducing rutab development led to the reduction in weight loss without an adverse effects on electrolyte leakage, TSS, acidity, TSS to acidity or vitamin C in the fruits of both cultivars. These studies provided evidences that boiling as an artificial method of Khalal dates causes a major loss in nutritional value due to the significant increase in electrolyte leakage and results in adverse effects on fruit quality. On the other hand, enrichedCO2 atmosphere induced rutab development while reduced water loss and maintained fruit quality. INTRODUCTION Date palm is a very important economic crop especially in the Gulf region. It has been providing food, shelter and even medicine to many people around the world. Astringent fruits at the Khesab stage could not be consumed due to the presence of soluble tannins. In spite of the large number of cultivars and clones of date palm, most of them are not edible at the full coloration stage (Bisr or Khalal). Date growers suffer from the prolonged period of rutab development especially when ripening is not uniform in the bunch which exposes fruits to abscission. The initiation of rutab development has been defined as the initiation of ripening in date fruits (Farag, 2005). Date growers are forced to wait until at least about 50% of the bunch is ripe. Moreover, a huge amount of dates must be harvested in a short time. Thus, there is a great need for method to induce rutab development especially in astringent cultivars at the Khalal stage. Use of the ethylene-releasing compound namely Ethrel, before or after harvest was not effective in inducing rutab development in astringent cultivars even at high concentrations. This was attributed to the J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 3 presence of soluble tannins that bind to cell proteins preventing the action of ethylene (Farag and Konaissi, 2001). Traditional methods of ripening induction in dates involved boiling fruits at the Khalal stage (date cooking) or dipping in acetic acid and salt. Although the boiling method has been widely used in the Gulf region, no assessment was done to evaluate the adverse effects of such method on fruit quality. The use of acetic acid plus sodium chloride after harvest was found to change the taste of the resulting rutab fruits (Kaira et al., 1977). Furthermore, fruits produce higher levels of ethanol and acetic acid especially at the initiation of ripening (Reuveni, 1986). EnrichedCO2 atmosphere around the fruit after harvest might be able to induce ripening of dates as was the case in persimmon (Pesis et al., 1986). The objective of this study were to assess the boiling process as a method of rutab development in terms of its effect on fruit quality and to discover the possibility of using another safe, simple and feasible mean of ripening induction by enriching CO2 atmosphere around the date fruits. MATERIAL AND METHODS This study was conducted during the two successive seasons 2001 and 2002 by using date fruits of two astringent cultivars at the Khalal stage, namely Helali and Khesab. Two separate studies were conducted during that period. The first study aimed at assessing the boiling process as a method of artificial ripening while the second aimed at investigating the effect of enriched-CO2 atmosphere on rutab development. Both types of studies used fruits of Helali and Khesab cultivars. I. The Assessment Study: Helali and Khesab fruits at the Khalal stage were used in this experiment. Fruits were harvested from Al-Kwaitat Experimental Station located at Al-Ain City, United Arab Emirates. Fruits were cleaned in tap water, surface sterilized for 3 mins in sodium hypochlorite (NaOcl) at 0.5%, V/V of 5% stock solution, then washed again in distilled water and left for air drying. A water bath provided with thermostat was used to boil fruits for various durations. Four replications were used with each treatment in a completely J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 4 randomized design and 12 fruits per replications. After reaching to boiling, fruits were dipped in the water bath and time was monitored from the beginning of boiling again. Treatments included: control (on shelf at ambient temperature of 22 2°C), boiling for 20 min, boiling for either 30, 40, 50, or 60 mins. After treatments, fruits were left on the bench in foam trays at 22 2°C for 5 days then the following parameters were taken. Electrolyte leakage of fruit tissue by using a conductively method (following the treatment and 6 days on shelf then killing the fruit tissues by deep freezing and measuring electrolyte leakage again). Percentage of electrolyte leakage before to after killing was calculated (Inab and Crandall, 1988). Percentage of rutab fruits was also determined in addition to total soluble solids (TSS) by using a hand refractometer, titratable acidity by titration against NaOH, TSS to acidity ratio and vitamin C content by using the endophenol method (A. O. A. C., 1984). Statistical analyses were performed by using Costat computer software while the means comparisons were done by the least significant difference (at 0.05 level). II. The Enriched-CO2 Atmosphere Study: Helali and Khesab bunches were harvested from the same experimental Station mentioned in the above study. Strands were separated from bunches, cleaned, surface sterilized as mentioned before. Strands were divided into groups. Each group contained 4 strands as one replication. Strands of each replication were placed in thick plastic bags and sealed by electric sealer. Control strands were kept open in the same plastic bags (0.8 mm thickness and 100 ml volume). Treatments included the control (in open bags), sealing for 1, 2, 3, 4, 5 or 9 days in Helali experiment while in Khesab experiments the treatments included: the control (in open bags), sealing for 8 hr, 16 hrs, 24 hrs, 32 hrs and 6 days. Plastic bags of each treatment were opened after the indicated time and strands were kept inside the opened bags until the end of the experiment. Measurements included: percentage of water loss from the initial weight before the treatment and the final weight after the treatment, percentage of rutab fruits, total soluble solids (TSS) by using a hand refractometer, titratable acidity against NaOH, TSS to acidity ratio, vitamin C content by using the endophenol method, and percentage of electrolyte leakage. All J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 5 procedures were the same as mentioned under the first study. The experiment was completely randomized with 4 replications per treatment. Analysis of variance and least significant difference (LSD) at 0.5% level were obtained by using Costat compute software. RESULTS AND DISCUSSION I. The Assessment Study: 1. Helali Experiments: The data in Table 1 indicated to rutab development and some quality parameters of Helali date fruits as influenced by the boiling process. The data showed that boiling for all used durations was effective in inducing full rutab development in Helali dates in both seasons. In the control, no rutab fruits were obtained in the first season and significantly much lower percentage than the treatments in the second season. No significant difference in rutab development was found among used boiling durations. With regard to the percentage of electrolyte leakage of fruit tissues, the data revealed that all boiling durations caused a significant electrolyte leakage as compared with the control. This trend was consistent in both seasons. Such leakage was much higher in boiling treatments for 50 and 60 mins than that obtained with other durations in the first season. However, this trend was not consistent in the second season even though leakage values of all boiling durations were higher than found with 20 mins boiling (Table 1). Total soluble solids of Helali dates after five days of the treatment were also affected by various treatments. Boiling fruits for 30, 40, 50 and 60 mins caused a significant increase in TSS in both seasons as compared with the control. This increase might be due to the concentration of soluble solids in fruit tissues after 5 days on the shelf due to water loss or the breakdown of more pectic materials in the cell wall as a result of boiling. Even boiling for 10 mins, tended to cause an increase in total soluble solids especially in the first season. Moreover, the long boiling durations, for example 40, 50 and 60 mins, did not significantly vary in TSS in both seasons in Helali dates. J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 6 Fruit acidity was also affected by the boiling treatments. It was evident that boiling caused an increase in juice acidity when compared with the control in both seasons. A considerable increase in fruit acidity occurred due to just 20 mins of boiling. The breakdown in cell wall integrity could be a main reason for the release of more acids from its structure as a result of the boiling process. Another possibility could be the conversion of some sugars, such as glucose, to acids through the process of gluconeogenesis. Changes in the TSS to acidity ratio were shown in Table 1. The data indicated to significantly higher TSS / acidity ratio in the control fruits than that found in all boiling durations in both seasons. Meanwhile, no significant difference was found among all boiling durations. The reduction in the ratios of TSS to acidity in Helali dates, in this study, could be mainly attributed to the increase in acidity that was previously reported. Regarding vitamin C content in the juice of Helali fruits, it was found that all boiling durations led to the breakdown of vitamin C causing a significant reduction in such vitamin as compared with the control in both seasons. Even when boiling was done for a relatively short period such as 20 mins, vitamin C was significantly lower than that of the control. This period seemed to be enough to cause the breakdown of available vitamin C in the tissue since other boiling durations did not cause further reduction in such vitamin (Table 1). 2. Khesab Experiments: The data in Table 2 showed the effect of boiling for various durations on the percentage of rutab fruits and some quality parameters of Khesab dates. Boiling Khesab fruits for various durations resulted in full development to the rutab stage (Table 2). Even boiling for 20 mins only led to rutab development. The difference was significant between rutab development in the control and that of the treatments. This trend was consistent in both seasons. It did not make a difference to boil Khesab fruits for 20, 30 or even 1 hr in terms of the ability to induce rutab development. With regard to electrolyte leakage of the fruits, the data indicated that boiling for 40, 50 and 60 mins resulted in a significant increase in electrolyte leakage when compared with the control in both seasons. The least leakage occurred in the control fruits. Electrolyte leakage of the fruits exposed to 30 mins boiling was higher than that of the control. The maximum leakage of electrolytes occurred J.Agric.&Env.Sci.Alex.Univ.,Egypt Vol.5 (1)2006 7 especially in the second season. After 50 mins boiling then levelled off since the difference in electrolyte leakage was not significant when comparing leakage at 50 and 60 mins of boiling. Boiling for various durations also reflected on the percentage of total soluble solids in Khesab fruits (Table 2). After 5 days on the shelf following boiling, TSS of all fruits treated for various durations was significantly higher than that of the control in both seasons. Even boiling for 20 mins resulted in a significant increase in TSS as compared with the control. The concentrations of solutes after boiling and leaving fruits at ambient temperature for 5 days might be the reason for such increase in TSS. Concerning the changes in fruit acidity as a result of boiling it was found that such treatment caused a significant increase in fruit acidity as compared with the control even after short period of boiling for 20 mins in both seasons. Boiling for longer duration did not result in a proportional increase in fruit acidity. The difference in fruit acidity caused by either 20, 30 or 40 mins boiling was not significant. This trend was consistent in both seasons (Table 2). The ratio of TSS to acidity was also influenced by the boiling treatments. It was evident that such ratio for the control was higher than that of boiling for used durations in both seasons. The increase in fruit acidity due to boiling could be the reason behind the reduction in the TSS to acid ratio. These trends coincided with what was found in Helali study with regard to change in vitamin C content in the fruit as a result of boiling, the data indicated that there was a significant reduction in such vitamin caused by all boiling durations when compared with the control. Even boiling for a relatively short period, for example 20 mins was able to breakdown vitamin C in fruit tissue in a consistent manner in both seasons. Moreover, boiling for 60 min did not further reduce vitamin C as compared with 20 or 30 mins