Semi-commercial gamma-irradiation of mangoes for market access R Blakey, Z van Rooyen and J de Graaf Westfalia Technological Services PO Box 1103, Tzaneen 0850, South Africa E-mail: robert.blakey@westfalia.co.za ABSTRACT Gamma-irradiation is a mitigating treatment for phytosanitary insect pests that is required for access into certain markets. A dose mapping trial found the dose ratio for a half pallet (10 layers of cartons) to be 3.3, i.e. at a minimum dose of 400 Gy, the maximum dose received by the fruit would be 1 320 Gy higher than the permitted maximum of 1 000 Gy. However, during treatment and due to some modifications, the dose ratio was lower than this. After three years of experimentation, a semi-commercial trial for the irradiation of South African mangoes was conducted in 2011. Half pallets of Tommy Atkins, Shelly, Princess, Keitt and three of Kent were irradiated in Cape Town. Fruit were then stored for 28 days at 10 C and ripened at 20-23 C for three days before being evaluated externally. Equal amounts of non-irradiated fruit were used as untreated controls. A generic dose of 400 Gy was applied, according to the requirements of the USDA-APHIS. Post-harvest decay was high for all the cultivars after cold storage and ripening. As with previous seasons, lenticel damage and irradiation scalding were the main physiological disorders that caused external fruit quality loss. With these three factors, the percentage of export quality fruit was very low for all the cultivars: 19% for Tommy Atkins, 31% for Shelly, 24% for Princess, 4% for Keitt and 12%, 25% and 32% for the three consignments of Kent. INTRODUCTION The use of gamma-irradiation as a mitigating treatment for phytosanitary insect pests is well established globally. Mangoes produced in India and Pakistan are already certified for export to the USA after irradiation in the producing country (Green, 2009). In 2011 table grapes produced in South Africa were authorised for export to the USA, following a 400 Gy irradiation treatment. In South Africa, a gamma-irradiation treatment would be a mitigating treatment for mango seed weevil (Sternochetus mangiferae) and Ceratitis fruit fly species by sterilising any insects within the treated fruit. Research on the effect of gamma-irradiation and simulated shipping on South African mango fruit quality has been on-going since 2008, with important findings from each year. The cold chain break during irradiation needed to be minimised to reduce lenticel damage, and there was no significant difference in fruit quality, whether fruit were irradiated pre- or post-storage (Du Rand et al., 2009). There are cultivar differences in irradiation-tolerance: it was found that Tommy Atkins and Keitt were sensitive to irradiation, while Kent was fairly tolerant. The new cultivars Shelly, Princess and Honey Gold were less tolerant than Kent (De Graaf, 2008; Blakey et al., 2010). This trial continues on from this previous research. The aim of this trial was to test the commercial feasibility of the proposed treatment on a semi-commercial scale where South African mangoes would be irradiated at 400 Gy and stored at 10 C for 28 days to simulate shipping. MATERIALS AND METHODS Dose mapping The dosimetry was done by Hepro Cape (Pty) Ltd staff. One hundred and twenty cartons, i.e. 10 layers of a standard pallet, of factory grade Tommy Atkins fruit from Bavaria Pack House were irradiated on the 26 th of January 2011. GEX Radiochromic Dosestix dosi meters were used for measuring the irradiation doses. Specific dose trial Five consignments of fruit were sent for irradiation, a pallet per consignment (Table 1). Fruit were sourced from Bavaria Pack House, transported to Klapmuts at 10 C over two days, transferred to a refrigerated van (ca. 10-15 C) and taken to Hepro s irradiation facility in Cape Town, 40 km away. Fruit were irradiated for 16 minutes to achieve a 400 Gy minimum dose. Fruit were then transported in the refrigerated van to the ARC-Infruitec facility in Stellenbosch for storage and evaluation. Fruit were stored for 28 days at 10 C, thereafter transferred to 20-23 C for three days and the external evaluation undertaken. Fruit were evalu- SA MANGO KWEKERSVERENIGING NAVORSINGSJOERNAAL, VOL. 31, 2011 13
Table 1. Cultivar, count and irradiation date of fruit used in the semi-commercial trial (2011). Cultivar Count Irradiation date Tommy Atkins 9 28 January Shelly 8 18 February Kent 1 10 18 February Princess 10 28 February Kent 2 10 28 February Kent 3 8 7 March Keitt 10 22 March Table 2. Dose ratio, minimum dose and maximum dose for each cultivar used in the semi-commercial trial (2011). Cultivar Date Dose ratio Min dose Max dose Tommy Atkins 28 January 3.32 352 1170 Shelly 18 February 2.27 489 1111 Kent 1 18 February 2.60 427 1111 Princess 28 February 2.34 489 1142 Kent 2 28 February 2.49 458 1142 Kent 3 7 March 3.30 365 1204 Keitt 22 March 2.96 365 1079 Figure 1. Percentage export quality fruit at evaluation for each cultivar and date and four treatments from the semi-commercial trial. Fruit were irradiated at 400 Gy. Directly ripened fruit were evaluated after five days at 20-23 C, stored fruit were stored for 28 days at 10 C and then three days at 20-23 C. Figure 2. Percentage disease-free fruit at evaluation for each cultivar and date from the semi-commercial trial. Fruit were irradiated at 400 Gy. Directly ripened fruit were evaluated after five days at 20-23 C, stored fruit were stored for 28 days at 10 C and then three days at 20-23 C. The fungal diseases were stem-end rot, soft brown rot and anthracnose. ated by ARC-Infruitec staff. Twelve cartons per treatment per cultivar were evaluated for directly ripened fruit and 120 cartons per treatment per cultivar were evaluated for stored fruit. Due to the large number of fruit and findings from the previous season (Blakey et al., 2010) that only fruit that received 1 000 Gy had internal browning, fruit were only evaluated externally. External fruit appearance was rated on a scale of 0 4, where 0 = excellent, 1 = acceptable, 2 = poor, 3 = severely damaged, 4 = too decayed to evaluate. Fruit with a rating of 0 or 1 were considered export quality fruit. Fruit with an external appearance rating of 0 3 were then further evaluated on a scale of 0 3 for lenticel damage, irradiation scalding, latex burn, cold damage, stem-end rot and soft brown rot, where 0 = clean, 1 = slight damage, 2 = severe damage and 3 = unacceptable damage. Scale damage, mechanical injury, latex burn, cold damage and anthracnose were also noted. RESULTS AND DISCUSSION Dose mapping The dose ratio (DR) for a pallet with 10 layers of cartons, henceforth a mini pallet, on the 26 th of January 2011 was 3.25. This is the ratio between the maximum and minimum doses received. This indicates that when the fruit in the minimum position received the required 400 Gy dose, the fruit in the maximum position received 1 300 Gy. The recommended maximum is 1 000 Gy (DR = 2.5), but for South African mangoes, a maximum dose of less than 800 Gy (DR < 2.0) would be preferable. The maximum dose that the fruit received was found at two places: in the centre of the short side of the pallet, in the sixth layer from the top and the bottom layer. The minimum was found to be on the long side of the pallet, on the bottom layer of cartons, 14 cm from the edge. The dose ratio, minimum and maximum dose for each batch is given in Table 2. The dose ratio for the specific dose trial varied between 3.32 for Tommy Atkins and 2.27 for Shelly. Specific dose trial Although the dataset is incomplete due to some technical mishaps, it is apparent that cold storage at 10 C for 28 days has a more detrimental effect on fruit quality than the 400 Gy irradiation treatment (Figure 1). The incidence and severity of cold damage was low throughout the trial (data not presented), for all cultivars. The major cause of quality loss was fungal decay, lenticel damage and scalding. The percentage of disease-free fruit was cultivar dependent, with only Keitt and Shelly having more than 75% disease-free fruit when fruit were stored at 10 C for 28 days (Figure 2). At least 80% of the directly ripened fruit were disease-free for all cultivars. It is clear that cold storage at 10 C for 28 days increased the incidence of fungal rots, most noticeably for Tommy Atkins and Kent. On a cultivar basis, there was a high incidence of fungal decay in the stored Tommy Atkins, with 18% 14 SA MANGO GROWERS ASSOCIATION RESEARCH JOURNAL, VOL 31, 2011
of the non-irradiated and 34% of the irradiated fruit being disease-free. The directly ripened Tommy Atkins, however, were almost completely disease-free (Figure 2). The severity of lenticel damage between irradiated and non-irradiated fruit was similar, but the severity of lenticel damage increased during cold storage (Table 3). Scalding in the irradiated stored fruit was the most severe compared to the other cultivars (Table 4). The non-stored, irradiated Tommy Atkins fruit had the second highest percentage of export quality fruit (70%) after Kent treated on February 28 th and March 7 th. Depending on financial considerations, it may be feasible to irradiate and air freight Tommy Atkins, because the export percentage from 2010 and 2011 was between 65% and 85% for three consignments. Kent was generally the best performing cultivar, although the fruit irradiated on the 18 th of February had a high incidence of severe lenticel damage (Table 3), but it is not possible to say whether this was due to the irradiation, as no control fruit were included. The scalding of all the Kent fruit was low (Table 4) and it appears that Kent is the most irradiation tolerant of the cultivars tested, when previous data is also considered (Blakey et al., 2010). Even though the Kent fruit were the most tolerant to irradiation, the unevaluable percentage of stored fruit from the latter two Kent consignments was high (Table 3), which will challenge the financial feasibility of exporting irradiated fruit via sea freight. Shelly had the equal highest export percentage (31.4%) with the third Kent consignment (31.6%) for stored, irradiated fruit, which is partly due to the low disease incidence (Figure 2) and low incidence of slight to severe lenticel damage. However, this is a very low percentage of export quality fruit, considering that this excludes the percentage of fruit deemed not fit for export prior to packing fruit at the pack house. Of the Princess fruit that were irradiated and stored, 10.8% had severe or unacceptable scalding (Table 4) and 33.6% severe or unacceptable lenticel damage (Table 3), with 9.4% of the fruit being unevaluable. Hence, Princess has an intermediate and Table 3. Lenticel damage severity of the control fruit and fruit irradiated with 400 Gy. Rating 1, 2 Cultivar Treatment 3 0 1 2 3 Unevaluable Tommy Atkins Control direct 12.0 47.2 33.3 7.4 0.0 Control stored 51.2 25.1 4.0 0.9 18.8 Irradiated direct 14.0 44.9 35.5 5.6 0.0 Irradiated stored 2.4 22.2 37.7 19.2 18.5 Kent 1 Control direct - - - - - Irradiated direct 4.0 26.0 52.0 18.0 0.0 Irradiated stored 27.8 36.0 17.3 3.8 15.1 Shelly Control direct - - - - - Irradiated direct 2.5 35.0 56.3 6.3 0.0 Irradiated stored 8.4 42.5 39.0 7.0 3.0 Kent 2 Control direct - - - - - Irradiated direct 43.0 54.0 2.0 1.0 0.0 Irradiated stored 33.5 24.0 5.5 0.7 36.2 Princess Control direct - - - - - Irradiated direct 94.0 2.0 4.0 0.0 0.0 Irradiated stored 17.9 39.0 29.6 4.0 9.4 Kent 3 Control direct - - - - - Control stored 34.1 20.3 14.9 3.1 27.7 Irradiated direct 72.9 25.0 2.1 0.0 0.0 Irradiated stored 41.1 22.4 13.2 1.8 21.5 Keitt Control direct - - - - - Control stored 20.5 18.6 21.6 7.1 32.3 Irradiated direct 0.0 50.0 38.3 11.7 0.0 Irradiated stored 2.2 9.9 46.4 36.3 5.1 1 Four point rating scale where 0 = clean, 1 = slight, 2 = severe, 3 = unacceptable; Unevaluable indicates decay was too severe to evaluate fruit. 2 Treatments without values were not included in the experiment. 3 Direct directly ripened at 20-23 C for five days before evaluation; Stored stored at 10 C for 28 days and ripened at 20-23 C for three days before evaluation. SA MANGO KWEKERSVERENIGING NAVORSINGSJOERNAAL, VOL. 31, 2011 15
variable tolerance to irradiation and storage, when compared to the other cultivars in the trial. This variability in irradiation tolerance increases the risk of proceeding commercially with the irradiation of Princess. Although Keitt had a relatively high percentage of disease-free fruit, the export quality percentage was low, due to 14.9% having severe or unacceptable scalding, and 82.7% having severe or unacceptable lenticel damage. Keitt is the most irradiation-sensitive cultivar of those tested and should not be commercially irradiated at 400 Gy. Due to the gradients in the irradiation dose through the pallet (Table 2), differences in lenticel damage and scalding may be because of position of cartons in the mini-pallet. It is expected that fruit that received a higher dosage would have more severe scalding and lenticel damage. Although it appears that the irradiation increased the percentage of disease-free fruit in stored Tommy Atkins, Kent 3 and Keitt (Figure 2), this is most likely natural variation random error because it is thought that disease control only occurs at doses far greater than 1 000 Gy (McLauchlan et al., 1990). This experiment assumed a worst case scenario, i.e. 28 days cold storage and a 400 Gy dose. It is clear that South African mangoes should not be coldstored for 28 days, especially after being irradiated at a minimum of 400 Gy, when it is expected that there will be high disease pressure, and modifications to the protocol will be required to make irradiation and export a financially viable option. With regards to the irradiation process, recent alterations to the irradiation chamber at Hepro have reduced the dose ratio and should reduce the lenticel damage and irradiation scalding, but there is uncertainty to what extent. These modifications include installing a plastic attenuator in front of the Cobalt-60 source to absorb some of the irradiation, and thereby reduce the maximum dose, and installing new Cobalt-60 pencils to increase the homogeneity of the source. The most important change that could be made to the irradiation protocol is to reduce the required minimum dose from the generic 400 Gy dose to Table 4. Irradiation scalding severity of the control and fruit irradiated with 400 Gy. Rating 1,2 Cultivar Treatment 3 0 1 2 3 Unevaluable Tommy Atkins Control direct 97.2 0.9 1.9 0.0 0.0 28 January Control stored 74.0 5.3 1.3 0.6 18.8 Irradiated direct 100.0 0.0 0.0 0.0 0.0 Irradiated stored 32.1 22.5 18.7 8.2 18.5 Kent 1 Control direct - - - - - 18 February Irradiated direct 83.0 7.0 9.0 1.0 0.0 Irradiated stored 82.5 1.5 0.6 0.3 15.1 Shelly Control direct - - - - - 18 February Irradiated direct 75.0 10.0 13.8 1.3 0.0 Irradiated stored 92.0 3.6 1.1 0.3 3.0 Kent 2 Control direct - - - - - 28 February Irradiated direct 87.0 13.0 0.0 0.0 0.0 Irradiated stored 60.8 2.1 0.7 0.2 36.2 Princess Control direct - - - - - 28 February Irradiated direct 93.0 6.0 1.0 0.0 0.0 Irradiated stored 71.1 8.6 10.3 0.5 9.4 Kent 3 Control direct - - - - - 7 March Control stored 67.2 2.9 2.1 0.1 27.7 Irradiated direct 95.8 4.2 0.0 0.0 0.0 Irradiated stored 69.0 6.1 3.2 0.1 21.5 Keitt Control direct - - - - - 22 March Control stored 57.3 5.7 4.4 0.3 32.3 Irradiated direct 98.3 0.8 0.8 0.0 0.0 Irradiated stored 70.0 9.9 13.7 1.2 5.1 1 Four point rating scale where 0 = clean, 1 = slight, 2 = severe, 3 = unacceptable; Unevaluable indicates decay was too severe to evaluate fruit. 2 Treatments without values were not included in the experiment. 3 Direct directly ripened at 20-23 C for five days before evaluation; Stored stored at 10 C for 28 days and ripened at 20-23 C for three days before evaluation. 16 SA MANGO GROWERS ASSOCIATION RESEARCH JOURNAL, VOL 31, 2011
150 Gy for the Ceratitis fruit fly species (Green, 2009). This 150 Gy dose may be accepted if the USDA-APHIS accepts the systems approach for the control of mango seed weevil (including one thiamethoxam application, two fenthion applications, the physical removal of egg-infested fruit and orchard sanitation and possibly fruit bagging) (De Graaf, 2010). It is expected that a 150 Gy minimum dose and a reduced dose ratio would increase the fruit quality compared to a 400 Gy treatment and reduce within pallet variation with regards to irradiation damage. This would increase the feasibility of irradiation as a phytosanitary pest mitigation treatment for South African mangoes. Beyond modifications to the irradiation process, post-harvest decay has to be addressed before any shipments are possible (Figure 2). Mango exports from South Africa are already limited to less than 5% of the total crop, partly due to high levels of decay after sea freight. Air freight is an option to reduce the cold storage period, but it is costly compared to sea freight. Evidence suggests that post-harvest decay is minimal after a cold storage period of less than about 21 days 1. Unfortunately, the markets that applications have been made for market access (India and USA), and other potential markets, are all considerable distances from South Africa. Considering additional time required for transport from the pack house, irradiation, time at Cape Town port and distribution in the destination market, a cold storage period of less than 21 days is highly unlikely. CONCLUSIONS A 400 Gy irradiation treatment for Kent, and to a lesser extent, Shelly, Tommy Atkins and Princess, appears feasible, although not without risk. The postharvest decay of fruit stored for 28 days at 10 C is the limiting factor at present. A reduction in the required irradiation dose and cold storage period should ensure a higher percentage of export quality fruit. ACKNOWLEDGEMENTS Thanks are extended to: Vic van Wyk from Bavaria Pack House; Marius van der Merwe, Cornette van der Westhuizen and Calle Badenhorst from Westfalia Fruit Products; the ARC-Infruitec staff; and Cherin Balt and Karin Hussey from Hepro. Most importantly, thank you to SAMGA and Westfalia Fruit Estates for considerable financial support. LITERATURE CITED BLAKEY, R.J., DE GRAAF, J., BRUWER, T. & VAN ROOYEN, Z. 2010. Gamma-irradiation of mangoes. South Afican Mango Growers Association Research Journal, 30: 13-22. DE GRAAF, J. 2008. Phytosanitary Research - Irradiation of Mango. Internal Westfalia Technological Services Report. DE GRAAF, J. 2010. Developing a Systems Approach for Sternochetus mangiferae (Coleoptera: Curculionidae) in South Africa. Journal of Economic Entomology, 103(5): 1577-1585. DU RAND, N., VAN ROOYEN, Z. & DE GRAAF, J. 2009. The effect of gamma irradiation on mango quality. South African Mango Growers Association Research Journal, 29: 14-21. GREEN, A. 2009. USDA s operational experience in the growing use of irradiation as a plant quarantine treatment. Retrieved 01 August 2011, from http://www.senasica.gob.mx/includes/asp/download.asp?iddocumento=3 886&idurl=6802. MCLAUCHLAN, R.L., MITCHELL, G.E., JOHNSON, G.I. & WILLS, P.A. 1990. Irradiation of Kensington Pride mangoes. Acta Horticulturae, 269: 469-476. 1. Personal communication, Dr J. van Niekerk, Co-ordinator of Plant Pathology and Market Access Research, Westfalia Technological Services, Tzaneen, South Africa SA MANGO KWEKERSVERENIGING NAVORSINGSJOERNAAL, VOL. 31, 2011 17