Radio Frequency Heating for Postharvest Insect Control…

Radio Frequency Heating for Postharvest Insect Control… January 2005 Elizabeth Mitcham, Maria Monzon and Bill Biasi Department of Plant Sciences, University of California, Davis Tropical and subtropical fruit research From October 2003 – December 2004 we conducted preliminary tests and replication tests of RF-heating persimmon, guava, passion fruit, and longan fruit to target temperatures, 48, 50 and 52°C, to determine the tolerance of the fruit The temperatures and holding times included those that should be effective for the Mexican fruit fly based on heating block work conducted by Guy Hallman The treatments were based on the thermal-death-time (TDT) curve for third-instar Mexican fruit fly, as was available in October 2003 (Table 1) Table Target temperature and holding times for RF treatments for control of Mexican fruit fly third instar larvae on persimmon, guava, passion fruit, and longan fruit designed to provide 100% mortality (TDT), Probit mortality, and beyond Probit mortality (extra timing) to test the upper limit of fruit tolerance Untreated fruit and fruit treated in water at 24°C for were included as controls Target Temperature (°C) TDTz Estimated Probit 9y Extra timing 48 50 52 1.5 0.5 12 18 4.5 Holding times (minutes) z Thermal-death-time for 100% insect mortality of third instar Mexican fruit fly larvae y Estimated time to reach 99.9968 % mortality of third instar Mexican fruit fly larvae **Probit was achieved after 6.6, 2.1 and 0.8 minutes at 48, 50 and 52C, respectively, in heating block studies conducted in 2004 after these studies began ‘Fuyu’ persimmon research Materials and methods Persimmon fruit were harvested November and 12, 2003 from a commercial orchard in Winters, CA The fruit were transported to the Laboratory the same day of harvest and sorted for size (150 – 200g) and defects The fruit were stored in metal tanks that were ventilated with humidified, ethylene-free air at 20°C for 24 or 48 hrs prior to being treated For each treatment and replicate, persimmon fruit were placed into the fruit mover (designed at Washington State University, Pullman by Birla, Shaojin and Tang) and covered with an acrylic top plate to ensure fruit submersion in the solution and proper fruit rotation Water electroconductivity was adjusted to 560 µS/cm with added NaCl (0.035% NaCl) to generate an even heating rate between the saline solution and the fruit Persimmon fruit had a very low temperature variation (~2°C variation among fruits) Fruit temperature was controlled by monitoring the heating rate of the saline solution using a fibre-optic probe inserted in the container cm below the water level Immediately after RF treatment, fruit were hydrocooled by pumping 0°C water into the container until the water temperature decreased to 30°C Fruit were then transferred to a 10°C water bath for 20 and finally fan-dried to avoid surface moisture (Figure 1) Fruit were stored in jars with a uniform air flow of 150 ml min-1 with 90-95% relative humidity for 12 days at 20°C to determine effects of heat treatment on fruit ripening and condition Respiration and ethylene production rates were measured to determine fruit stress during fruit ripening at 20°C Temperature (°C) 'Fuyu' persimmon Holding time 50 Drain hot water and add iced water 40 RF heating 30 Transferred to 10°C water bath 20 10 Minutes Figure Water temperature during RF heating and hydrocooling of ‘Fuyu’ persimmon fruit Fruit temperature was similar to water temperature after treatment ‘Fuyu’ persimmon heated at 6°C/min Water and fruit temperature were heated from 20 to 50 ± 0.5°C, held at 50°C for min, and hydrocooled to 30°C prior to transfer to a 10°C water bath Fruit quality was evaluated when controls were ripe, following storage at 20°C for 12 days Persimmon fruit were evaluated for skin color, firmness, soluble solids and titratable acidity Hedonic scales were used to evaluate external and internal appearance of the fruit Results Persimmon fruit were RF-heated to °C/min with a gap of 200mm For all treatments, the RF-heating time was of approximately plus the additional holding time in the RF-heated water (Fig 1) During storage at 20°C the respiration rates of RF treated fruit were higher than the control fruit and the respiration rates were highest in fruit heated to 52°C The higher respiration rates indicate fruit stress and correlates with the internal damage observed for the same treatments RF treatments had no commercial affect on fruit firmness, soluble solids, titratable acidity, or weight loss and there was no skin browning Persimmon fruit were affected by RF-heating to the highest temperature, 52°C, compared to RF-heating the fruit to 50 and 48°C The quality attributes that affected fruit quality were internal browning of the flesh, calyx browning, and the fruit attained an intense orange to red color RF treatments at 50 and 52°C resulted in slight to moderate internal browning, while none was observed at 48°C (Fig 2) Calyx browning increased slightly in all RF treatments and was highest in the longer treatments at each temperature (Fig 3) The long treatments at 48°C resulted in an increase in orange-red color; this increase in orange- red color was also seen in fruit heated to 52°C for Internal browning Phytosanitary control Estimated Probit Extra timing Severe Moderate a b b b Slight cc Very slight cd cd d d d Control 24 None 48 50 52 (°C) Treatment Figure Internal browning of the flesh following RF treatment and 12 days of storage at 20°C for ‘Fuyu’ persimmon fruit Values beyond were considered commercially unacceptable Calyx browning Phytosanitary control Estimated Probit Extra timing Severe Moderate a ab bb b cc Slight bb Very slight d d None Control 24 48 50 52 (°C) Treatment Figure Calyx browning following RF treatment and 12 days of storage at 20°C for ‘Fuyu’ persimmon fruit Values beyond were considered commercially unacceptable Guava fruit research Materials and methods ‘Mexican Cream’ and ‘Bangkok’ guavas were harvested from a commercial orchard in Escondido, CA Fruit were harvested twice in January or March 2004 for ‘Mexican Cream’ or ‘Bangkok’ cultivars, respectively, at different maturity levels to compare treatment effects After harvest, fruit were shipped overnight to the Postharvest Laboratory at the University of California at Davis where they were sorted for color uniformity, surface defects and size Fruit size ranged from 60 - 90 g for ‘Mexican Cream’ and 150 – 200 g for ‘Bangkok’ cultivars Fruit were treated 48 h after harvest, which included the overnight shipment and 24 h storage at 20°C with 95% relative humidity prior to treatment For each treatment and replicate we used 15 ‘Mexican Cream’ and ‘Bangkok’ guava fruit The fruit were placed into the fruit mover (designed at Washington State University, Pullman by Birla, Shaojin and Tang) and covered with an acrylic top plate to ensure fruit submersion in the solution and proper fruit rotation Water electroconductivity was adjusted to 510 ± µS/cm (0.032% NaCl) and 540 ± µS/cm (0.033% NaCl) for ‘Mexican Cream’ and ‘Bangkok’ guava, respectively, to generate an even heating rate between the saline solution and the fruit with a gap of 200 mm The given EC for each variety generated an approximate water heating rate of ± 0.1°C/min and ± 0.1°C/min for ‘Mexican Cream’ and ‘Bangkok’ guavas, respectively (Fig 4), creating a very low temperature variation of approximately 1.5°C among fruits Water temperature after RF treatments was about to 2°C higher than the average fruit temperature, depending on the target temperature Fruit temperature was controlled by monitoring the heating rate of the saline solution using a fibre-optic probe inserted in the container cm below the water level Temperature (°C) 'Bangkok' 'Mexican Cream' Hold time 50 Drain hot water 40 and add iced water RF heating 30 Transferred to 10°C water bath 20 10 Minutes Figure Water temperature during RF heating and hydrocooling of ‘Mexican Cream’ and ‘Bangkok’ guava fruit Fruit temperature was similar to water temperature and checked after treatment ‘Mexican Cream’ guavas heated at 5°C/min while ‘Bangkok’ guavas heated at 6°C/min Water and fruit temperature were heated from 20 to 50 ± 0.5°C, held at 50°C for min, and hydrocooled to 30°C prior to transfer to 10°C water bath Immediately after RF treatment, fruit were hydrocooled by pumping 0°C water into the container until the water temperature decreased to 30°C Fruit were then transferred to a 10°C water bath for 20 and finally fan-dried to avoid surface moisture (Fig 4) Fruit were stored in jars with a uniform humidified air flow of 350 and 100 ml min-1 for ‘Mexican Cream’ and ‘Bangkok’ guavas, respectively ‘Mexican Cream’ and ‘Bangkok’ guavas were stored in jars at 20°C for up to and 11 days, respectively CO2 and ethylene production rates were measured to determine fruit stress during subsequent fruit ripening at 20°C Fruit quality was evaluated when controls were ripe, following storage at 20°C for and 11 days for ‘Mexican Cream’ and ‘Bangkok’ guavas, respectively Guava fruit were evaluated for skin color, firmness, soluble solids and titratable acidity Hedonic scales were used to evaluate external and internal appearance of the fruit Results ‘Mexican Cream’ guava The respiration rate of ‘Mexican Cream’ guava fruit was higher at harvest than at harvest and fruit from harvest ripened in days compared to fruit from harvest that took days to ripen (Table 2) The higher respiration rate and the faster ripening of the fruit in harvest were likely due to the riper stage at harvest of these fruit For harvest the RF treated fruit to 50 and 52°C had higher respiration rates compared to control fruit indicating that RF had an affect on the fruit The ethylene production of ‘Mexican Cream’ guava fruit was also higher in the more mature harvest fruit than the less mature harvest fruit Regardless of harvest, C 2H4 production increased on day when fruit had been treated with RF, this transient increase in ethylene production in both harvests indicates a short term stress response rather than a general stimulation of ripening Table Mean values for ‘Mexican Cream’ guava fruit respiration rate following RF treatment and 1, 5, and days of storage at 20°C Respiration (mg CO2 kg-1h-1) Treatment Day Day Day Temperature Time Harvest Harvest Harvest Harvest Harvest Harvest (°C) (min) 2 z 122 ab 45 d 145 ab 56 b 59 c 24 94 ab 48 cd 144 ab 55 b 72 bc 48 107 ab 55 bcd 126 ab 81 ab 114 abc 12 88 bc 49 cd 116 ab 73 ab 110 abc 18 80 c 52 cd 106 b 85 ab 122 abc 50 1.5 124 a 70 a 159 a 109 a 157 a 105 ab 69 a 141 ab 81 ab 116 abc 4.5 108 ab 59 abc 122 ab 91 ab 139 ab 52 0.5 98 ab 55 bcd 114 ab 61 b 66 c 98 ab 67 ab 105 b 66 b 97 abc 115 ab 59 abc 130 ab 81 ab 121 abc z Different letters indicate significant differences (α = 0.05) within columns RF-heating ‘Mexican Cream’ guava had no commercial affect on the percent weight loss, soluble solid content and titratable acidity RF energy had no commercial affect on the internal quality, but slightly affected the external quality of the fruit ‘Mexican Cream’ guava fruit exhibited circular brown spots on the skin that were more severe when fruit were RF heated (Fig 5) Fruit heated to 48°C and held for 6, 12 or 18 (the longest holding times) and to 52°C (the highest temperature) and held for or had the most severe brown spots The ‘Mexican Cream’ guava fruit were significantly less firm when heated with RF compared to the control fruit (Fig 6) Fruit from harvest that were RF heated to 48°C for or 12 were firmer than fruit heated to 50 or 52°C, but significantly softer than control fruit Area affected with spots (%) Harvest 15 Phytosanitary control Estimated Probit Extra timing Harvest 15 10 10 5 Control 24 48 50 52 Control 24 48 50 52 °C Treatment Figure Effect of harvest and treatment on mean percent fruit area affected with circular brown spots for RF treated ‘Mexican Cream’ guava fruit Fruit were held at target temperature for the length of time required to achieve phytosanitary control or estimated Probit control based on the thermal death time curve developed for 100% mortality of third instar Mexican fruit fly larvae, plus an extra timing to test fruit tolerance Hold times were 48°C: 6, 12, 18 min; 50°C: 1.5, 3, 4.5 min; and 52°C: 0.5, 1, Firmness (N) 60 Phytosanitary control Estimated Probit Extra timing Harvest Harvest 60 50 50 40 40 30 30 20 20 10 10 Control 24 48 50 52 Control 24 48 50 52 °C Treatment Figure Effect of harvest and treatment on mean firmness values for RF treated ‘Mexican Cream’ guava fruit Fruit were held at target temperature for the length of time required to achieve phytosanitary control or estimated Probit control based on the thermal death time curve developed for 100% mortality of third instar Mexican fruit fly larvae, plus an extra timing to test fruit tolerance Hold times were 48°C: 6, 12, 18 min; 50°C: 1.5, 3, 4.5 min; and 52°C: 0.5, 1, ‘Asian’ guava ‘Bangkok’ guava fruit had similar respiration rates at harvest and 2, and there were no statistical differences between RF-heated and control fruit Ethylene production was less than 0.25 µl C2H4 kg-1h-1 at harvest and (data not shown) There was no treatment or harvest effect on the skin color, firmness, and percent weight loss, and there was no commercial affect on the SS and TA RF energy had no affect on the internal quality, but slightly affected the external quality of the fruit (Table 3) The external quality of ‘Bangkok’ guava fruit was slightly affected by skin browning and shrivel symptoms Skin browning was very slight and was observed as small circular brown spots Although skin browning was very slight, there was significantly more browning in fruit heated to 52°C for minutes compared to the control fruit Shrivel was Table Mean values for skin browning, shrivel, decay, and internal browning of the flesh of ‘Bangkok’guava following RF treatment and 11 days of storage at 20°C Skin Internal Shrivelz browningz browningz Harvest 0.1 by 1.1 a 0.2 a 0.3 a 0.3 b 0.1 b Treatment °C 24 0.0 b 0.0 b 0.1 d 0.7 abc 0.0 b 0.0 b 48 12 18 0.3 ab 0.3 ab 0.3 ab 0.7 abc 1.2 a 0.9 ab 0.1 ab 0.3 ab 0.4 a 50 1.5 4.5 0.1 ab 0.2 ab 0.2 ab 0.7 bc 0.7 abc 1.0 ab 0.2 ab 0.1 ab 0.1 ab 52 0.5 0.1 ab 0.3 ab 0.4 a 0.3 cd 0.6 bcd 0.6 bcd 0.0 ab 0.2 ab 0.1 ab z Shrivel, skin browning, internal browning: = none, 1=very slight, 2=slight, 3=moderate, 4=severe y Different letters indicate significant differences (α = 0.05) within harvest and treatment increased by RF treatment, showing very slight symptoms Independent of the temperature, exposing the fruit to a circulating saline solution increased shrivel, even at 24°C (control), and the longer the time in the circulating water, the more shrivel damage Harvest time also had a significant effect on shrivel, with significantly more shrivel on fruit from harvest The additional shrivel on fruit from harvest may be due to treating immature-green fruit RF treatment had no affect on fruit decay Passion fruit research Materials and methods Passion fruit were bought from a commercial distributor in August - September, 2004 Fruit were sent to the Postharvest Laboratory and sorted for defects and size The fruit were stored at 20°C with 90-95 % relative humidity for 12 h prior to being treated For all treatments and replicates, 10 passion fruit were placed into the fruit container (designed at the University of California, Davis) and covered with a lid to ensure fruit submersion in the saline solution Initial tests showed that a water EC of 150 µS/cm with a gap of 250 mm would produce an even heating rate between the saline solution and the fruit This EC generated an approximate water heating rate of 8°C/min (Fig 7) Fruit temperature was controlled by monitoring the heating rate of the saline solution sung a fibre-optic probe inserted half way the height of the container Fruit temperature was measured immediately after RF-heating and the fruit had a temperature variation of – 5°C Immediately after RF treatment, fruit were hydrocooled by pumping 20°C water into the container until the water temperature decreased to 35°C (Fig 7) Fruit were then cooled with air at 10°C for 20 and finally fan-dried to avoid surface moisture Fruit were stored for 15 days at 20°C with 90 – 95 % relative humidity to determine effects of heat treatment on fruit ripening and condition External fruit quality was evaluated on day and 15 of storage at 20°C and internal quality was evaluated on day 15 of storage at 20°C Passion fruit were evaluated for skin color, firmness, soluble solids and titratable acidity Hedonic scales were used to evaluate external and internal appearance of the fruit Passion fruit Temperature (°C) Holding time 50 RF heating 40 Drain hot water and add 20°C water 30 20 Air dried at 10°C 10 Minutes Figure Water temperature during RF heating and hydrocooling of passion fruit Fruit heated at 8°C/min Water and fruit temperature were heated from 20 to 50 ± 0.5°C, held at 50°C for min, and hydrocooled to 35°C prior to being transferred and cooled in air at 10°C for 20 Results Fruit quality is being analyzed Results indicate that RF energy affected the external quality of the fruit with water soaked and sunken areas and the fruit failed to wrinkle in the typical manner as compared to control fruit On average, the water soaked areas covered from 50 – 75% of the fruit RF also affected the internal appearance of the arils resulting in fair to poor quality There seems to be no commercial affect on the TA and SS content of the arils Longan fruit research Materials and Methods Longan fruit were harvest in November - December, 2004 from a commercial orchard in Southern California Fruit were sent to the Postharvest Laboratory and sorted for defects and size The fruit were stored at 10°C with 90-95 % relative humidity for 24 h prior to being treated For all treatments and replicates, 50 passion fruit were placed into the fruit container (designed at the University of California, Davis) and covered with a lid to ensure fruit submersion in the saline solution Initial tests showed that a water EC of 255 µS/cm with a gap of 250 mm would produce an even heating rate between the saline solution and the fruit This EC generated an approximate water heating rate of 11°C/min (Fig 8) Fruit temperature was controlled by monitoring the heating rate of the saline solution using a fibre-optic probe inserted half-way up the side from the bottom of the 10 container Fruit temperature was measured immediately after RF-heating and the fruit had a temperature variation of 0.5 – °C Immediately after RF treatment, fruit were hydrocooled by pumping 20°C water into the container until the water temperature decreased to 35°C (Fig 8) Fruit were then cooled with air at 7°C for 20 and finally fan-dried to avoid surface moisture Fruit were stored for days at 20°C with 90 – 95 % relative humidity to determine effects of heat treatment on fruit ripening and condition Fruit quality was evaluated days after of storage at 20°C with 90-95 % relative humidity Longan fruit were evaluated for skin color, firmness, soluble solids and titratable acidity Hedonic scales were used to evaluate external appearance and decay Longan heating rate Temperature (°C) Holding time 50 RF heating 40 Drain hot w ater and add 20°C water 30 20 Air dried at 7°C Minutes Figure Water temperature during RF heating and hydrocooling of longan fruit Fruit temperature was similar to water temperature after treatment Longan fruit heated at 11°C/min Water and fruit temperature were heated from 20 to 50 ± 0.5°C, held at 50°C for min, and hydrocooled to 35°C prior to being transferred and cooled in air at 7°C for 20 Results Longan fruit quality is being analyzed Results indicate that RF-heated fruit were more firm and had more decay compared to control fruit Conclusions RF-heating persimmon fruit to 48°C then holding for 12 or to 50°C for or 4.5 showed the least damage and could be a potential quarantine treatment 11 Heating ‘Mexican Cream’ and ‘Bangkok’ guava fruit with RF to 50°C and holding for showed the least damage and could be a potential quarantine treatment Passion fruit were externally affected by RF Despite the internal appearance of the fruit, the organoleptic properties of the arils seem to show no differences compared to the control fruit Regardless, there does not seem to be a lot of potential for RF to be used for passion fruit Longan fruit were affected by RF energy and showed more decay than control fruit Recent analysis (Shaojin and Juming) indicate that the required holding times to achieve Probit security for Mexican fruit fly are shorter (6.6, 2.1, and 0.8 after RF-heating to 48, 50, and 52°C, respectively) and therefore heating with RF could be a potential quarantine treatment RF is a promising physical heat treatment for persimmon and guava fruit if the initial fruit quality and maturity is at optimum levels The shorter holding times required for Probit security could result in better quality of the fruit and a combination with cold storage after RF treatment could extend the shelf-life of persimmon and guava fruit and alleviate immediate stress from the heat by lowering fruit metabolism 12 ... University of California at Davis where they were sorted for color uniformity, surface defects and size Fruit size ranged from 60 - 90 g for ‘Mexican Cream’ and 150 – 200 g for ‘Bangkok’ cultivars... Southern California Fruit were sent to the Postharvest Laboratory and sorted for defects and size The fruit were stored at 10°C with 90-95 % relative humidity for 24 h prior to being treated For all... Fruit were sent to the Postharvest Laboratory and sorted for defects and size The fruit were stored at 20°C with 90-95 % relative humidity for 12 h prior to being treated For all treatments and