Effect of Gamma Radiation on Tomato Quality during Storage and Processing

Fruits of two tomato cultivars Amani and Beto86 were exposed to gamma rays doses of 0.25, 0.50 and 1.00 KGy at mature green stage during 2010\2011 season to delay their ripening period and hence extend their shelf life. Tomato fruits were stored at 15±1°C (85-90% R.H) and examined for physiological and physicochemical changes during storage period. Organoleptic qualities were made for Beto86 tomato paste and fresh slices prepared from Amani tomato cultivar. Irradiation treatments doubled the shelf life of tomato fruits in both cultivars. Gamma radiation treatment at all doses has decreased significantly (p≤0.05) the weight loss, respiration rate and delay the softening of tomato fruits in both cultivars. The maximum level of ascorbic acid, total soluble solids and total sugars was reached in more time with irradiated fruits compared to untreated fruits. No significant difference was observed in tomato paste made from Beto 86 and fresh tomato slices prepared from Amani fruits among the treated fruits in terms of color, texture, taste and flavor and over all acceptability.


INTRODUCTION
Tomato (Lycopersicon esculentum) is one of the most important vegetable crops in Sudan and is grown commercially in every state in the country. The production of tomato in Sudan has been increasing steadily up to 145909 ha (FAO, 2009). Tomato is a winter crop in Sudan and is harvested during summer (March-April) which is characterized by high temperature and low relative humidity. These conditions generally result in excessive water loss, faster ripening and greater deterioration of the produce.
This study was carried out to evaluate the effect of gamma irradiation on quality of fresh and processed tomato fruits.

MATERIALS AND METHODS
Mature green fruits of Amani and Beto 86 tomato cultivars were harvested from Demonstration farm of Agricultural Research Station, Khartoum North, Shambat (15°40´N, 32°22´E) during 2010\2011 season. Fruits were selected for uniformity of size, color and freedom from blemishes. Fruits were washed with tap water to remove latex and dust, dried and transported in carton boxes. The fruits of each cultivar were distributed among the three treatments in complete randomized design with three replications. The tomato fruits were exposed to three doses of gamma rays which included 0.25 KGy for 5 min 48 sec; 0.5 KGy for 11 min 38 sec and 1.0 KGy for 23 min 17 sec. Irradiation source was cobalt 60 (gamma cell 220 excel). The doses were calculated according to the International Atomic Energy Agency (IAEA) absorbed dose was measured as 0.25; 0.50 and 1.00 KGy and calculated in j/KGy IAEA (1982).
The irradiation treatments were carried out at the laboratory of Ali Kaila radiation processing lab, Chemistry and Nuclear Physics Institute, in Sudan Atomic Energy Commission (SAEC), Soba. All irradiated and non irradiated (control) tomato fruits were packed in carton boxes and stored in cold room (15±1°C and 85-90% RH) at Food Research center (Khartoum North-Shambat).
The physiological and physicochemical tests were run immediately after irradiation treatment and during storage at suitable intervals.
Physiological analysis: Loss weight (%) in fruits was determined directly every 3 days according to the equation: W1= [(W0-Wt)]/100%; where W1is the percentage weight Loss, W0 is the initial weight of fruits at harvested, Wt is the weight of fruits at the designated time. Respiration Rate (mgCO2/kg fruit/hour) was determined every 3 days in 14 fruits from each replicate using the total absorption method of Charlimers (1956). Fruit flesh firmness was measured by the Magness and Taylor firmness tester (D. Ballanf Meg. Co.) equipped with an 8mm diameter plunger tip. Flesh Firmness was expressed in Kilograms per square centimeter. Two readings were taken from the opposite sides of each fruit.
Physiochemical analysis: Total soluble solids was measured directly from the fruit juice using Krus hand refractometer (model HRN.32) at 20°C and expressed as percent or degree Brix (AOAC, 1990). Titratable acidity was determined according to the method described by Board (1988) and expressed as percent citric acid. Ascorbic acid (mg/100) was determined using the 2, 6-dichlorophenol-endophenol titration method of Ruck (1963). Total Sugar was determined, in the pulp extract according to the Anthrone method of Yem and Willis (1954). Total Sugars were expressed as percent of fresh weight.
Organoleptic analysis: After the ripening of tomatoes, fruits were selected for processing paste from Beto 86 cultivar, tomato were washed and pulped then put in a boiler to reach the same soluble solids concentration of tomato paste and cold homogenized. After that paste was analyzed for organoliptic quality. Tomato fruits were assessed organoleptically following the ranking test according to the procedure described by Ihekononye and Ngoddy (1985). The panelists were asked to evaluate color, flavor, taste, texture and recorded overall acceptability preference.
Statistical Analysis System (SAS) followed by least significant difference with a significance level of (p≤0.05) were performed on all data.

RESULTS AND DISCUSSION
Physiological parameters: Weight loss: Compared to the control (non treated tomato fruits), irradiation has significantly (p≤0.05) reduced weight loss in both tomato cultivars. After twelve days of ripening period, the control fruits in both cultivars were discarded due to complete rotten whereas irradiated fruits continued to kept well up to 24 days. There were no significant differences (p≤0.05) in loss of weight among fruits irradiated using the three doses of 0.25, 0.50 and 1.00 KGy (Table 1). Weight loss in tomato decreased with increasing irradiation dose as was found by Sparks and Iritani (1964).

Respiration rate:
The untreated fruits of both cultivars reached the maximum climacteric peak after nine days (47.43 and 56.19 mgCO2/kg fruit/hour) for Amani and Beto86 respectively (Table 2). There were no significant differences (p≤0.05) among the respiration rates of the fruits treated with different irradiation doses (0.25, 0.50 and 1.00 KGy) and kept for 24 days. This would suggest that gamma radiation treatment at all doses has decreased significantly (p≤0.05) the respiration rate of tomato fruits in both cultivars. Larrigaudiere et al. (1990) reported that, early climacteric in cherry tomatoes following irradiation at 1.00 KGy is due to the stimulation of translation of pre existing mRNAS enzymes whereas at higher doses        irradiation greatly inhibited the activity of ethylene forming enzyme. Irradiation increased fruit respiration immediately after treatment, but delayed the time to attain the climacteric respiratory peak and reduced the magnitude of this peak rate (Dharkar et al., 1966).

Fruit flesh firmness:
During ripening, tomato cultivars in all treatments had shown a continuous decline in tomato fruit flesh firmness. The untreated fruits reached an undesirable soft stage (kg/cm 2 shear resistance) after 12 days (Table 3), compared with fruits treated with different doses (0.25, 0.50 and 1.00 KGy) which reached final soft stage after 24 days of ripening period. There were no significant differences (p≤0.05) in flesh firmness among the irradiated tomato fruits at 24 days of storage. UV radiation was found to delay the softening of whole tomato fruits significantly during storage (Liu et al., 1993;Maharaj et al., 1999).

PHYSIOCHEMICAL ANALYSIS
Ascorbic acid: During ripening the level of ascorbic acid in tomato fruits increased and eventually decreased with all treatments. Worth mentioning that the maximum level of vitamin C is reached in more time with irradiated fruits compared to untreat ones ( Table 4). The loss in ascorbic acid content beyond the climacteric stage during storage could be attributed to the increase in as corbate oxidase activity (Cordello, 1998). Destruction of vitamin C is a consequence of alteration of fruits metabolic oxidation pathways by radiation, which can convert vitamin C into dehydro-ascorbic acid, which can still be metabolized as vitamin C (Snauwart, 1973).

Total Soluble Solids (T.S.S): Showed continuous
increase during ripening in all tested fruits. The untreated fruits of Amani tomato cultivar recorded 5.67% T.S.S in 12 days of storage, whereas similar values of T.S.S were reached in 21days (6.0; 5.67; and 6.0% T.S.S) in fruits treated with 0.25; 0.50; and 1.00 kGy dose, respectively. On the other hand the untreated fruits of Beto 86 tomato cultivar reached maximum T.S.S value (6.33%) in 12 days of storage. The same T.S.S value recorded in tomato fruits treated with 1.00 kGy dose (6.33%) in 21 days of storage (Table 5) which means that maximum level of T.S.S is reached in more storage time with irradiated fruits. Total soluble solids are predominantly influenced by the amount of sugars in the fruits (Saltviet, 2005).
Titratable acidity: During ripening tomato fruits had shown an increase in titratble acidity in all treatments shortly after the breaker stage and progressively decreased afterwards (Table 6). Charles et al. (2005) observed a lower titratable acidity and higher pH in UV treated tomatoes.

CONCLUSION
Gamma irradiation treatments were found suitable for delay of ripening and extension of shelf life of tomato fruits. Slices from Amani and paste prepared from Beto 86 tomato fruits treated with three doses of gamma rays (0.25-0.50 and 1.00 kGy) were found acceptable in color, texture, taste and flavor.