Method of Washing Citrus Influences the Rate of Subsequent Degreening

REFEREED MANUSCRIPT Proc. Fla. State Hort. Soc. 122:325 329. 2009. Method of Washing Citrus Influences the Rate of Subsequent Degreening MARK A. RITENOUR * University of Florida, IFAS, Indian River Research and Education Center, 2199 S. Rock Road, Ft. Pierce, FL 34945 ADDITIONAL INDEX WORDS. Flame grapefruit, navel orange, Orlando tangelo, peel color, high-pressure wash, ethylene, fruit decay Harvested Flame grapefruits, navel oranges, and Orlando tangelos were exposed to different washing treatments to determine their effects on color development during subsequent degreening. fruit were not washed before being degreened. Washing treatments included brush washing on a small research line, and washing on a commercial brush bed, high-pressure washer (HPW), or a combination of brush plus HPW wash. After washing, the fruit were exposed to 5 ppm ethylene at or F with 95% relative humidity to stimulate color development characteristic of each type of citrus. Grapefruit color development was significantly inhibited compared to the control after washing on the research packingline for as little as 1 minute. fruit could take more than twice as long to degreen as unwashed fruit. In tests conducted on a commercial packingline, fruit washed on both the brush bed plus HPW exhibited the greatest inhibition in color development compared to unwashed fruit. The HPW also significantly delayed fruit color development compared to the control, but the inhibition of degreening was the least of all the washing treatments. Treatments that most inhibited coloration also promoted the greatest water loss, suggesting that washing methods that disrupt the cuticle least, also inhibit degreening the least. While 90% of Florida s citrus is grown for juice, over 40 million cartons (4/5 bushel) of citrus were sold fresh during the 2007 08 season (USDA, 2008). Of this, fresh grapefruit represented over half of all the fresh citrus shipments. Figure 1 illustrates how fresh Florida citrus is handled after harvest. While unmarketable fruit should be removed from the handling chain as soon as possible to minimize the resources (fungicides, degreening space, etc.) wasted on them, Florida citrus is usually not graded until after the fruit is dumped onto the packingline and cleaned. This is because dirt and sooty mold on the fruit must be removed before the peel can be evaluated for diseases and defects such as canker, windscar, and melanose. Fig. 1. Postharvest fresh citrus handling practices in Florida. Footnotes: 1Recommended only when fungicide treatment on the packingline will be delayed more than 24 h. 2 Only when sufficient natural color has not developed. 3Only on poorly colored oranges. *Corresponding author; email: ritenour@ufl.edu; phone: (772) 468-3922 ext. 167 The 1995 discovery of citrus canker (Xanthomonas axonopodis pv. citri) in Florida, the end of the canker eradication program in Jan. 2006, the Aug. 2006 interim canker rule, and the Nov. 2007 final rule (USDA, 2007) radically changed the way Florida s $300 million fresh citrus industry grows, packs, and ships fruit. Regulations relied on postharvest fruit decontamination treatments and inspection of packed product that greatly increased the demands on graders to remove all fruit with canker symptoms. Recently, a new canker rule was enacted that allows even fruit with small canker lesions to be shipped to all domestic states and territories (USDA, 2009). However, shipments to export markets are governed by the receiving country regulations and many of these still require citrus to be inspected and found free of canker lesions before shipment. For example, ~24% of Florida s exported fresh citrus were shipped to countries of the EU during the 2005 06 season (Florida Dept. of Agriculture and Consumer Services, 2006) and all of these countries require fruit to be inspected for canker before shipping. If even a single canker lesion is found during inspection, the fruit is disqualified for EU shipment. Such fruit can be re-graded and re-inspected for shipment to a less restrictive market. These requirements add substantial packing costs. As the incidence of canker increases in Florida, the likelihood of canker-infected fruit arriving at the packinghouse increases. Depending on the incidence of canker within a load of fruit and the size of the lesions, it may be very difficult for graders to remove all symptomatic fruit. In Argentina, packinghouse managers realized there is a limit to the amount of symptomatic fruit that can be removed and generally do not pack loads with more than 2% infection (Ritenour et al., 2007). In some cases the fruit is washed and pre-graded to remove much of the unmarketable and canker-infected fruit before degreening or dumping onto the main packingline. The use of pre-grade facilities could remove unmarketable fruit before costly fungicides or precious degreen- 325

ing space is expended on them. Early knowledge of the severity of canker infection within a load would also allow packinghouse managers to adjust resources (e.g., human graders) and line speeds accordingly to make sure symptomatic fruit that are not removed by the pre-grade, are removed on the main packingline. However, the ability to wash and pre-grade Florida citrus before degreening is unclear. The most important drawback is that washing inhibits citrus degreening (Grierson and Newhall, 1960). Degreening is often required after harvest in tropical and subtropical environments where relatively warm temperatures (especially nighttime temperatures) result in fruit with inadequate external color development. During degreening, fruit are exposed to low levels (3 5 ppm) of a natural plant hormone (ethylene) that stimulates the breakdown of green chlorophyll and unmasks the characteristic orange and yellow colors (carotenoids) of the peel (Ritenour et al., 2003). In Florida, the optimum temperature for chlorophyll breakdown is F, but little carotenoid development occurs at this temperature (Stewart and Wheaton, 1972). The internal quality of the fruit is usually not significantly affected (Porat et al., 1999). In Florida, a pre-grade facility would be most useful for canker detection early in the season when canker lesions are relatively small and most difficult to detect, but could be used throughout the season to remove unmarketable fruit before degreening. To begin evaluating the overall feasibility of pre-grading citrus immediately after harvest, the current studies were conducted to evaluate the effects of different washing methods on fruit color development during subsequent degreening of early season citrus fruit. Materials and Methods Unless otherwise stated, citrus fruit were harvested, evaluated for initial color, and exposed to different washing treatments within a day of harvest. Washing treatments included brush washing on a small research line located at the Indian River Research and Education Center (IRREC), Fort Pierce, FL; or on a commercial brush bed, high-pressure washer (HPW) system (JBT Corp., Lakeland, FL), or a combination of the two at a commercial packinghouse in Vero Beach, FL. fruit were not washed before degreening. An alkali surfactant solution (Fruit Cleaner 395, JBT Corp.) was used during all wash treatments. Brush rotation on the research line was about 120 rpm and the fruit were washed and then rinsed on another brush bed for the indicated times. Brush speeds on the commercial line were also about 120 rpm, with fruit on the brush bed for about 45 s, and on the HPW for about 15 s. The HPW system subjects fruit to sprays of water under about 200 to 400 psi as they pass across a brush bed. Fruit were degreened at F with 95% relative humidity (RH) and 5 ppm ethylene and were not coated with a wax. Fruit were evaluated almost daily for color development and weight loss during degreening, and then weekly for decay during storage. Fruit color measurements were at three evenly spaced locations around the fruit using a Minolta chromameter (model CR-300, Minolta Camera Corp., Ramsey, NJ). Color was reported as a*/b* ratios where a* measures green (negative) to red (positive) and b* measures blue (negative) to yellow (positive). As fruit lose their green color, a*/b* values increase from negative to positive. The hue angle describes color and is expressed from 0 to 360. Fruit peel color changes from green to orange as hue angle moves from higher to lower values. EFFECT OF WASHING AND DEGREENING TEMPERATURE ON FLAME GRAPEFRUIT COLOR DEVELOPMENT. Flame grapefruit were harvested on 4 Oct. 2007, and the next day, half the fruit were washed for 1 min on the small citrus line with a 1-min, 200 ppm chlorine rinse. Both washed and unwashed fruit were then degreened at or F with 5 ppm ethylene. There were 3 replicates of each washing and temperature treatment and each replicate consisted of 30 fruit. The RH was set at 95% in both degreening rooms, but the system failed in the F room, with RH dropping to around 50% throughout the experiment. fruit at both temperatures were washed after 3 d of degreening and then returned to their respective storage conditions. Degreening conditions were maintained throughout the storage period. EFFECT OF WASH DURATION ON FLAME GRAPEFRUIT COLOR DEVELOPMENT DURING DEGREENING. Flame grapefruit were harvested 17 Oct. 2007, transferred to the IRREC postharvest facility, and different washing treatments administered on the same day. Fruit received either a short wash that included 1 min of brush washing followed by a 5-s rinse, or a full wash that included 2 min of brush washing followed by a 2 min rinse and another 2 min on drying brushes. To simulate canker fruit treatments, 2% sodium-o-phenylphenate (SOPP; Freshgard 5, JBT Corp.) was applied during the wash and the rinse water included 200 ppm chlorine. fruit were never washed. The fruit were then held continuously under standard degreening conditions. These experiments were repeated with navel oranges and Orlando tangelos harvested 14 Nov. 2007. The fruit were not treated with SOPP or chlorine and, because of the greater natural color development with the later harvest, the fruit were degreening for only 2 d before being transferred to 55 F for storage. fruit were washed just before being placed into storage. EFFECT OF COMMERCIAL WASHING METHODS ON FLAME GRAPEFRUIT, NAVEL ORANGE, AND ORLANDO TANGELO COLOR DEVELOPMENT OF DURING DEGREENING. Flame Grapefruit, navel oranges, and Olrando tangelos were harvested 31 Oct. 2007 and washed the next day on a commercial citrus line. Fruit were either washed only on the brush bed, only on the HPW, or run over both the brush bed and HPW. Because tangelos are more delicate than grapefruit or oranges, half of the HPW water nozzles were shut off as the fruit passed through. Two percent SOPP was included on the washing line as part of the normal procedures at the packinghouse. After washing, all fruit were degreened for 4 d, then transferred to 55 F and stored for an additional 27 d. fruit were never washed. These experiments were repeated on Flame Grapefruit and navel oranges that were harvested 6 Nov. 2007 and washed the next day. The fruit were degreening for 5 d before being transferred to 55 F for storage. All experiments were arranged in a completely random design with each treatment consisting of 3 replicates of 30 fruit each. Percentage data were transformed to arcsine values and all data was analyzed by analysis of variance using SAS (PROC GLM) for PC (SAS Institute Inc, Cary, NC). When differences were significant (P 0.05), individual treatment means were separated using Duncan s multiple range tests (P = 0.05). Means presented are untransformed values. Results and Discussion EFFECT OF WASHING AND DEGREENING TEMPERATURE ON FLAME GRAPEFRUIT COLOR DEVELOPMENT. After 5 d, Flame grapefruit washed before degreening developed significantly less color than unwashed fruit, and color development was significantly slower in fruit degreening at F compared to F (Fig. 2; Table 1). At harvest, fruit were green and had a*/b* and hue values of 0.36 326

Fig. 2. Effect of washing before the start of degreening and degreening temperature on Flame grapefruit color development. Both washed and control fruit were degreened continually at or F with 95% RH and 5 ppm ethylene. fruit were washed after 3 d of degreening. Table 1. Effect of washing before the start of degreening and degreening temperature on Flame grapefruit color development. Fruit were exposed to 5 ppm ethylene at or F with 95% RH for the entire storage duration. fruit were washed after 3 d. Days after Degreening washing Treatment temp ( F) a*/b* Hue (%) 5 0.03 bz 88.06 c 1.07 c 0.14 d 97. a 1.41 c 0.13 a 82.75 d 4.10 b 0.02 c 91.34 b 5.46 a Significance 11 0.14 a 82.06 b 2.05 c 0.05 b 86.91 a 2.67 c 0.16 a 81.31 b 7.91 b 0.08 b.31 a 10.17 a Significance Values within each column followed by unlike letters are significantly Significant at P 0.001. z and 109, respectively. Washing treatments had a much greater inhibitory effect on color development than did the degreening temperature. An additional 6 d of degreening time was needed for washed fruit to reach about the same color stage as control fruit (Table 1). The effect of degreening temperature became insignificant after 11 d, while washed fruit were still much greener than the control. Carotenoid biosynthesis has been shown to be greater as degreening temperatures are reduced from to 60 F (Stewart and Wheaton, 1971). Thus, even though chlorophyll degradation is inhibited by degreening at vs. F (Grierson and Newhall, 1960), increased carotenoid biosynthesis at F may eventually compensate for the slower chlorophyll destruction. Moisture loss from the fruit was most affected by degreening temperature. The warmer degreening temperatures resulted in a 3-fold increase in fruit water loss, depending on degreening duration, whereas washing effects increased water loss only a fraction and were nonsignificant when the fruit were degreening at F. EFFECT OF WASH DURATION ON FLAME GRAPEFRUIT COLOR DEVELOPMENT DURING DEGREENING. Commercially, the total time that citrus fruit spend on washing brushes can span from two to several minutes, depending on the size and speed of the packinghouse. To test if the delay in postharvest color development could be mitigated by shortening the brushing time, grapefruit either were washed for 1 min and then rinsed for 5 s, or washed, rinsed, and dried on brushes for a total of 6 min. While fruit exposed to the short washing treatment tended to develop better coloration than fruit receiving the full wash, there were no significant differences between the washing methods up to 13 d after washing (Table 2). Fruit coloration from both washing treatments was always inferior to the control. After 13 d of continuous ethylene exposure, washed fruit still had not reached the color stage that control fruit reached after 5 d. This again more than doubled the time needed to degreen the fruit. Grierson and Newhall (1960) reported that washing increased the required degreening time for grapefruit by between 23% and 42%, depending on the variety. However, green fruit are inhibited much more by washing treatments than are fruit that are naturally advanced in their coloration (Wardowski et al., 2006). The fruit evaluated in these first two experiments had very little natural color development when harvested. Interestingly, fruit used in these experiments lost less water than fruit used in the first experiment and there was no significant difference between wash treatments. The reason(s) for this are unclear. 327

Table 2. Effect of wash duration on Flame grapefruit color development during degreening. Fruit received either a short wash that included 1 min of brush washing followed by a 5-s rinse, or a full wash that included 2 min of brush washing followed by a 2-min rinse and 2 min on a drying brush bed. All fruit were degreened at F with 95% RH and 5 ppm ethylene. Days after washing Treatment a*/b* Hue (%) 1 0.23 a z 102.87 b 0.69, Short 0.42 b 112.43 a 0., Full 0.40 b 111.91 a 0.63 5 0.07 a 86.20 b 2.48, Short 0.08 b 94.55 a 2.33, Full 0.12 b 96.79 a 2.51 8 0.12 a 83.28 b 3.35, Short 0.01 b 90.78 a 3.12, Full 0.06 b 93.57 a 3.39 Significance ** NS 13 0.19 a 79.28 b 4.43, Short 0.02 b 88.74 a 4.10, Full 0.03 b 91.44 a 4.47 NS, **, Nonsignificant or significant at P 0.01 or 0.001, respectively. EFFECT OF COMMERCIAL WASHING METHODS ON FLAME GRAPE- FRUIT, NAVEL ORANGE, AND ORLANDO TANGELO COLOR DEVELOP- MENT OF DURING DEGREENING. Grapefruit, oranges, and tangelos washed using any of the tested commercial methods significantly inhibited color development compared to the control after 4 d of degreening (Table 3). However, subjecting grapefruit or tangelos to only the HPW resulted in significantly better color development compared to using only brush washing, or a combination of brush washing and HPW. There was no significant difference in color development between the washing methods used on oranges. The inhibitory effect of the washing methods decreased during 34 d of storage so that only oranges receiving the combination brush and HPW still had significantly poorer coloration compared to the control (data not shown). This agrees with the suggestion of Wardowski et al. (2006) that fruit with greater natural color at harvest will experience less degreening inhibition after washing than fruit that are harvested fully green. When these experiments were repeated with grapefruit and oranges, subsequent color development tended to be most inhibited on fruit receiving the combination brush plus HPW wash (Table 4). Washing with HPW alone resulted in the best coloration among the washing treatments and sometimes was not significantly different from the control. Peel color of grapefruit that was washed only on the brush bed was not significantly different from the combined brush plus HPW treatment, but with oranges, it was not significantly different from the HPW treatment. Even 28 d after washing, all grapefruit treatments still had significantly poorer coloration than the control, with the HPW treatment affecting coloration the least. In oranges, most treatments colored up similar to the control, with only the combination brush plus HPW treatment having significantly poorer color after 28 d. Compared to control fruit, all three citrus cultivars experienced significantly greater water loss from the brush-only, and Table 3. Effect of commercial washing method on color development of Flame grapefruit, navel orange, and Orlando tangelo during degreening. Fruit were either washed on a brush bed, on a high-pressure washer (HPW), or run over both a brush bed and HPW before being degreened for 4 d at F with 95% RH and 5 ppm ethylene. After degreening, fruit were stored at 55 F. Citrus species Treatment a*/b* Hue (%) Grapefruit 0.09 a z 84.77 b 1.61 b HPW 0.06 b 86.30 b 1.91 b Brush 0.03 c 88.21 a 2.81 a Brush + HPW 0.02 c 88.91 a 2.93 a Significance ** Orange 0.03 a 91.88 b 2.55 b HPW 0.07 b 94.01 a 2.86 b Brush 0.08 b 94.46 a 3.50 a Brush + HPW 0.08 b 94.77 a 3.98 a Significance ** ** ** Tangelo 0.21 a 77.94 c 3.16 b HPW 0.14 b 82.29 b 3.88 a Brush 0.07 c 86.09 a 4.39 a Brush + HPW 0.06 c 86.59 a 4.43 a Significance ** NS, **, Nonsignificant or significant at P 0.01 or 0.001, respectively. Table 4. Effect of commercial washing method on color development of Flame grapefruit and navel orange during degreening. Fruit were either washed on a brush bed, on a high-pressure washer (HPW), or run over both the brush bed and HPW before being degreened for 5 d at F with 95% RH and 5 ppm ethylene. After degreening, fruit were stored at 55 F. Days after Grapefruit Orange washing Treatment a*/b* Hue a*/b* Hue 3 0.07 a.94 b 0.02 a 88.86 c HPW 0.02 a 88.63 b 0.04 b 92.42 b Brush 0.03 b 91.44 a 0.04 b 92.51 b Brush + HPW 0.03 b 91.78 a 0.08 c 94.45 a Significance 28 0.22 a 77. c 0.19 a 79.30 b HPW 0.18 b 80.05 b 0.20 a 78.69 b Brush 0.16 bc 80.94 ab 0.21 a 78.22 b Brush + HPW 0.14 c 82.05 a 0.16 b 81.17 a Significance Significant at P 0.001. combination brush plus HPW treatments (Table 3). Water loss was not significantly different between control and HPW treated grapefruit or oranges. With tangelos, the HPW-washed fruit lost as much water as fruit washed with the other two methods. There were no significant differences in fruit decay or the development of physiological disorders between washing treatments or the control after fruit were degreened and stored at 50 F for 77 d (grapefruit and oranges) or 27 d (tangelos) (data not shown). It is likely that the addition of SOPP to the washing systems on the commercial line reduced postharvest decay compared to the 328

Table 5. Effect of commercial washing method on Flame grapefruit and navel orange fruit decay after 5 d of degreening at F plus 66 d at 55 F. Fruit were either washed on a brush bed, on a highpressure washer (HPW), or on both the brushes and HPW before degreening. Total decay Grapefruit Orange 23.49 ab z 46.55 b HPW 15.75 b 37.46 b Brush 28.64 ab 37.01 b Brush + HPW 33.89 a 73.33 a Significance * ** *, **Significant at P 0.05 or 0.01, respectively. control, which did not receive any fungicide. When the experiments were repeated on grapefruit and oranges and the fruit stored for up to 71 d, fruit washed with the HPW system alone developed significantly less decay than fruit receiving the combined brush plus HPW wash (Table 5). Oranges washed only on the brushes also developed significantly less decay than fruit washed with the combined brush plus HPW wash. With grapefruit, decay was not significantly different between these two treatments. Decay of both grapefruit and orange controls were not significantly different from the brush or HPW treatments. However, a direct comparison cannot be made between the control and wash treatments because control fruit were not treated with SOPP. This study demonstrates that degreening of grapefruit, oranges, and tangelos can be significantly inhibited if first washed with HPW for as little as 15 s. Washing fruit with brushes for longer periods of time usually resulted in substantially greater degreening inhibition. While the inhibitory effects of washing tended to diminish over time, they were still often significant even after 28 d of storage. Washing treatments with longer brushing durations resulted in the poorest color development and also tended to cause the greatest fruit water loss. Because none of the fruit in these experiments were waxed, this suggests that treatments disrupting the cuticle most also inhibit color development most. Others have reported that degreening is inhibited more as brushing duration increases (Grierson and Newhall, 1960). The HPW treatment inhibited degreening least of the washing methods tested and consisted of the least brushing (~15 s). It is not clear how much, if any, inhibition in degreening was due to the high-pressure spray itself. Tests are needed to evaluate if it is possible to adequately clean early season fruit using only the high-pressure water spray alone, as the fruit pass over rollers instead of brushes. Fruit degreening is most important early in the season when field temperatures have not yet fallen enough to stimulate sufficient natural coloration of the fruit peel. Rapid degreening of early season fruit is critical to maintain maximum fruit quality. However, the fact that early season fruit are relatively clean when harvested may mean that only minimal, if any, fruit cleaning may be sufficient to allow grading of canker and other blemishes before the fruit is degreened. As the season progresses, shorter degreening times would be required for sufficient external color, while more thorough cleaning would be required as sooty mold and other debris accumulates on the fruit surface. Studies are now underway to address these questions. Literature Cited Florida Department of Agriculture and Consumer Services. 2006. 2005 06 Season annual report. Division of Fruit and Vegetables, Fla. Dept. Agr. Consumer Serv., Tallahassee. Florida Department of Citrus. 2006. Economic and Market Research Department (FDOC EMRD). Grierson, W. and W.F. Newhall. 1960. Degreening of Florida citrus fruits. Fla. Agr. Expt. Sta. Bul. 620. Porat, R., B. Weiss, L. Cohen, A. Davis, R. Goren, and S. Droby. 1999. Effects of ethylene and 1-metacyclopropene on the postharvest qualities of Shamouti oranges. Postharvest Biol. Technol. 15:155 163. Ritenour, M., J. Graham, R. Muraro, and H. Browning. 2007. Florida citrus growers, packers visit Argentina s fresh fruit operations. Citrus Ind. 88(11):10 12. Ritenour, M.A., W.M. Miller, and W.F. Wardowski. 2003. Recommendations for degreening Florida fresh citrus fruits. IFAS Fact Sheet HS-195. <http://edis.ifas.ufl.edu/hs195>. Stewart, I. and T.A. Wheaton. 1971. Effects of ethylene and temperature on carotenoid pigmentation of citrus peel. Proc. Fla. State. Hort. Soc. 84:264 266. Stewart, I. and T.A. Wheaton. 1972. Carotenoids in citrus, their accumulation induced by ethylene. J. Agr. Food Chem. 20:448 449. USDA. 2007. Citrus canker; movement of fruit from quarantined areas. 7 CFR Part 301. Federal Register Vol. 72(222):65172 65204. USDA. 2008. Citrus summary 2007 08. <http://www.nass.usda.gov/ Statistics_by_State/Florida/Publications/Citrus/index.asp>. USDA. 2009. Citrus canker; movement of fruit from quarantined areas. 7 CFR Part 301. Federal Register Vol. 74(203):54431 54445. Wardowsky, W.F., W.M. Miller, and W. Grierson. 2006. Degreening, p. 277 298. In: W.F. Wardowsky, W.H. Miller, D.J. Hall, and G. Grierson (eds.). Fresh citrus fruits, 2nd ed. Florida Science Source, Longboat Key, FL. 329