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Postharvest Biology and Technology 62 (2011) 50–58
Contents lists available at ScienceDirect
Postharvest Biology and Technology
journa l homepage: www.e lsev ier .com/ locate /postharvbio
oes ethylene degreening affect internal quality of citrus fruit?
ina Mayuonia, Zipora Tietela, Bhimanagouda S. Patil b, Ron Porata,∗
Department of Postharvest Science of Fresh Produce, ARO, the Volcani Center, P.O. Box 6, Bet-Dagan 50250, IsraelVegetable and Fruit Improvement Center, Department of Horticultural Sciences, Texas A&M University, College Station, TX 77845, USA
r t i c l e i n f o
rticle history:eceived 14 March 2011ccepted 16 April 2011
eywords:itrusegreeningthylenelavorutritional quality
a b s t r a c t
Citrus fruit are non-climacteric. However, exposure to exogenous ethylene, e.g., during ethylene degreen-ing, stimulates various ripening-related processes in the peel tissue, such as destruction of the greenchlorophyll pigments and accumulation of orange/yellow carotenoids. Nonetheless, it is not yet knownwhether exogenous ethylene affects internal ripening processes in citrus flesh. To address this question,we examined the possible effects of ethylene on taste, aroma, perceived flavor, and nutritional qualityof various citrus fruit, including ‘Navel’ oranges, ‘Star Ruby’ grapefruit and ‘Satsuma’ mandarins. Expo-sure to ethylene enhanced peel color break, and respiration and ethylene production rates in all citrusfruit tested. However, ethylene degreening had no effect on juice total soluble solids and acid contents,and had only minor effects on contents and composition of juice aroma volatiles. Moreover, sensoryanalysis tests revealed that ethylene degreening did not affect the flavor of oranges and grapefruit, but
marginally impaired sensory acceptability of mandarins; the latter change could be attributed, at leastpartially, to storage of the fruit for 5 days at 20 ◦C. Nevertheless, ethylene degreening did not enhanceoff-flavor perception or accumulation of off-flavor volatiles, nor had any effect on levels of health pro-moting compounds such as vitamin C, total phenols and flavonoids, or antioxidant-activity of citrus juice.We conclude that although ethylene affects peel color break, it is probably not involved in regulation ofinternal ripening processes in citrus fruit and, therefore, does not impair internal fruit quality.. Introduction
In climacteric fruit, ethylene plays a key role in governinghysiological and biochemical changes that occur during ripen-
ng, including color break, softening, and accumulation of sugars,cids, aroma volatiles, vitamins, etc. (Lelievre et al., 1997; Barry andiovanoni, 2007). In contrast, citrus fruit are non-climacteric, i.e.,
heir natural ripening is not accompanied by rises in respirationnd ethylene production rates (Eaks, 1970). However, exposure toxogenous ethylene has been shown to stimulate various ripening-elated processes, such as destruction of the green chlorophylligments and accumulation of orange/yellow carotenoids, in citruseel tissue (Stewart and Wheaton, 1972; Barmore, 1975; Purvisnd Barmore, 1981; Rodrigo and Zacarias, 2007). In the light ofhese observations, degreening practices involving exposure of theruit to ethylene at concentrations of 2–5 �L L−1 for about 72 h at0–30 ◦C were developed, in order to accelerate peel color change
nd to render the fruit more acceptable for marketing (Grierson andewhall, 1960; Cohen, 1978). In particular, commercial degreen-ng treatments are especially important for early varieties, in order
∗ Corresponding author. Tel.: +972 3 9683617; fax: +972 3 9683622.E-mail address: [email protected] (R. Porat).
925-5214/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.postharvbio.2011.04.005
© 2011 Elsevier B.V. All rights reserved.
to extend their marketing seasons, and for fruit grown in warm,tropical climates, such as those in Florida or India, where natu-ral color development is relatively weak (Wardowsky et al., 2006;Porat, 2008).
Nevertheless, despite widespread knowledge of the effect ofethylene on peel color development, it is not yet known whetherexogenous ethylene regulates other biochemical changes associ-ated with internal ripening of citrus fruit, as it does in climactericfruit (Goldschmidt, 1998). The common dogma is that, in contrast toits effects on peel color change, ethylene has only relatively minoreffects on ripening processes in citrus flesh, but this has neveryet been examined systematically. On the contrary, several linesof evidence suggest that ethylene may regulate various processesrelated to internal ripening. First, it is well known that exposureto ethylene accelerates respiration and ethylene-production ratesof citrus fruit, and these rates are indicators of activation of bio-chemical changes, such as breakdown of sugars and acids that serveas respiratory substrates (Aharoni, 1968; Vines et al., 1968; Eaks,1970). Second, previous studies have shown that ethylene degreen-ing affects various metabolic pathways in citrus flesh. For example,
ethylene degreening decreased acidity levels in ‘Mosambi’ oranges(Ladaniya and Singh, 2001), increased production of aroma volatilesin green lemons (Norman and Craft, 1968), and slightly affectedaccumulation and composition of carotenoid pigments in the flesh
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f ‘Satsuma’ mandarins (Matsumoto et al., 2009). Third, it has beeneported that presence of ethylene in storage rooms results inoss of desired flavor, and enhanced accumulation of off-flavorsn oranges, whereas removal of ethylene from storage roomsmproves overall fruit quality (McGlasson and Eaks, 1972; Testonit al., 1992). Fourth, we recently found that the expression patternsf 661 transcripts in mandarin flesh were significantly altered by aactor of at least 3, following exposure to ethylene at 4 �L L−1 for8 h, which suggests that this exposure might have affected variousetabolic and adaptation processes (Mayuoni et al., 2011).In summary, notwithstanding its advantages in improving fruit
isual appearance, the ethylene degreening process also has var-ous adverse effects on fruit quality and postharvest storability:t increases susceptibility to stem-end rots, enhances weight loss,nd accelerates rind and calyx senescence (Barmore and Brown,985; Carvalho et al., 2008; Porat, 2008). Therefore, the decisionn whether or not to degreen citrus fruit is not simple, and isnfluenced by various circumstances, such as market demands andstimated durations of storage and shelf life (Pool and Gray, 2002).
Over the last few years, because of increased competition inlobal markets and increasing public awareness of the nutri-ional benefits of horticultural produce, growers and consumersre attaching increasing importance to the flavor and nutritionaluality of fruit and vegetables (Kader, 2008; Patil et al., 2009). Inrder to evaluate the possible effects of ethylene degreening onegulation of the internal ripening processes and on the quality ofitrus fruit, we have systematically examined the effects of ethyleneegreening on taste, composition of aroma volatiles, perceived fla-or, and nutritional quality of the citrus fruit ‘Navel’ oranges, ‘Staruby’ grapefruit, and ‘Satsuma’ mandarins. Overall, we concludehat ethylene is probably not involved in regulation of internalipening processes in citrus and, therefore does not impair internalruit quality.
. Materials and methods
.1. Plant material
‘Navel’ oranges (Citrus sinensis [L]. Osbeck), ‘Star Ruby’ grape-ruit (Citrus paradisi Macf.), and ‘Satsuma’ mandarins (Citrus unshiuv. ‘Miho’) were purchased from commercial packinghouses duringeptember through November of the 2009 and 2010 growing sea-ons. In all cases, fruit were harvested at the beginning of naturalolor break, and were collected directly from the harvest bins athe packinghouse. For taste score evaluations, additional mandarinarieties were also tested, including ‘Michal’, ‘Odem’, ‘Or’, ‘Mor’, and
Mama’.
.2. Ethylene degreening
Fruit were selected for uniformity of size and color, and dividednto two lots, which were exposed, respectively, to air or to ethy-ene for 24, 48, or 72 h. They were exposed to ethylene by placinghem in 250-L airtight sealed plastic tanks, into which appropriatemounts of pure ethylene were injected, to achieve a final concen-ration of 4 �L L−1. Ethylene concentrations were verified by gashromatography according to Porat et al. (1999). The tanks wereushed daily to ensure that accumulated carbon dioxide levels didot exceed 0.2%. Control fruit were held in the same storage roomt 20 ◦C, but without ethylene.
.3. Juice soluble solids, titratable acidity, and ascorbic acid
ontentsTotal soluble solids (TSS) content in the juice was determinedith a Model PAL-1 digital refractometer (Atago, Tokyo, Japan), and
nd Technology 62 (2011) 50–58 51
acidity percentages were measured by titration to pH 8.3 with 0.1 MNaOH by means of a Model CH-9101 automatic titrator (Metrohm,Herisau, Switzerland). Each measurement comprised five replica-tions, each using juice collected from three different fruit, i.e., a totalof 15 fruit per measurement.
Ascorbic acid (vitamin C) contents in citrus juice were deter-mined by titration with 2,6-dichlorophenolindophenol accordingto Hiromi et al. (1980). Ascorbic acid levels were determined bycomparing the titration volumes of citrus juices with those of0.1% ascorbic acid (Sigma–Aldrich, St. Louis, MO), and results areexpressed as milligrams of ascorbic acid per 100 mL of juice.
2.4. Sensory evaluations
Fruit sensory quality was tested on the day of harvest and after 5days of exposure to air or ethylene. Fruit were peeled, and separatedsegments were cut into halves and placed into covered glass cups.Each treatment included a mixture of cut segments prepared fromfive different fruit. Fruit taste was evaluated by a trained panel con-sisting of 10 members, five males and five females, aged 25–62. Eachpanelist assessed the various attributes of three samples, accordingto an unstructured 100-mm scale, with anchor points ‘very weak’and ‘very strong’ for each attribute, and sensory data were recordedas distances (mm) from the origin. The samples were identified bymeans of randomly assigned three-digit codes. In addition, pan-elists were requested to rate overall fruit flavor preference on ascale of 1–5: 1 = very bad, 2 = bad, 3 = fair, 4 = good, and 5 = excellent.The sensory analysis scores of oranges and grapefruit presentedhere are means of three independent experiments, and those ofmandarins are means of six independent experiments.
2.5. Analysis of aroma volatiles
Aroma volatiles were extracted from homogenized segmentsaccording to Tietel et al. (2010a). Fruit were hand-peeled, weighed,and blended for 30 s with an equal amount of 30% NaCl (w/v),to inhibit enzymatic degradation. Aliquots (2 mL) were placed in10 mL glass vials, and 5 �L of 1-pentanol (Sigma–Aldrich, St. Louis,MO), diluted 1:1000 in water, were added as an internal standard.The vials were stored at −20 ◦C pending analysis.
Aroma volatiles were determined by three replicate measure-ments, each prepared from three different fruit, i.e., a total of ninefruit per time point. They were identified by gas chromatography(GC) coupled with mass spectrometry (MS). Prior to analysis, sam-ples were allowed to equilibrate for 5 min at 40 ◦C, after which theywere incubated at the same temperature for an additional 25 min.Volatiles were extracted by solid-phase microextraction (SPME).Stable-flex fibers, 1 cm in length, coated with a 50/30 �m layer ofdivinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS)(Supelco, Bellefonte, PA) were used to trap volatile compounds inthe vials’ headspaces. After incubation, the fibers were desorbedfor 5 min at 250 ◦C in the splitless inlet of a Model 7890A GC (Agi-lent, Palo Alto, CA) equipped with an HP-5 column (30 m × 0.25 mmID, 0.25 �m film thickness) (J&W Scientific, Folsom, CA). The ovenwas programmed to run at 50 ◦C for 1 min, to ramp up to 160 ◦C at5 ◦C min−1, then to ramp up to 260 ◦C at 20 ◦C min−1, and to remainat that temperature for 4 min. The helium carrier-gas flow was setat 0.8 mL min−1. The effluent was transferred to a Model 5975C MSdetector (Agilent, Palo Alto, CA) that was set to scan from mass 40to 206 at 7.72 scans/s, in the positive-ion mode, and mass spectra inthe electron impact (EI) mode were generated at 70 eV. Chromato-graphic peaks were identified by comparing the mass spectrum of
each component with the US National Institute of Standards andTechnology (NIST) library of mass spectra, 2006 version. Identi-fication of aroma volatiles was further confirmed by calculatingtheir linear retention indices (RIs) by comparison with a series of
52 L. Mayuoni et al. / Postharvest Biology and Technology 62 (2011) 50–58
Fig. 1. Visual appearance of ‘Navel’ oranges, ‘Star Ruby’ grapefruit and ‘Satsuma’ mandarins at harvest and after 24, 48 and 72 h of exposure to ethylene at 4 �L L−1 at 20 ◦C.
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-alkanes (C5–C20) and comparing their values with the publishedatabases of Adams (2001) and the University of Florida Cit-us Flavor Database (http://www.crec.ifas.ufl.edu/rouseff/). Peaksf interest were semi-quantified by comparison with addednternal standards, and are expressed as 1-pentanol equivalents.he identities of 11 compounds, including �-pinene, ethyl 2-ethylbutanoate, hexanal, �-myrcene, �-terpinene, limonene,
-terpinene, octyl acetate (E)-2-nonenal, linalool, and 1-terpinen--ol were further confirmed by running authentic chemicaltandards.
.6. Determination of total phenols and flavonoids
Phenols and flavonoids were extracted by stirring 1 mL juiceamples with 9 mL of 80% methanol for 30 min at room tem-
perature, followed by centrifugation at 10,000 × g for 10 min.Total phenolics were determined by the Folin–Ciocalteu method(Singleton et al., 1999). Briefly, the reaction mixtures comprised0.2 mL of juice methanol extracts, 0.2 mL of Folin–Ciocalteu reagent,and 7 mL of 7% Na2CO3. The reaction mixtures were incubated for90 min at room temperature, after which absorbance at 750 nm wasmeasured against a prepared blank with a spectrophotometer. Totalphenolic contents were expressed as gallic acid equivalent (GAE).
Total flavonoids were determined according to Shin et al. (2007).Briefly, the reaction mixtures comprised 1.0 mL of juice methanolextracts, 0.3 mL of 5% NaNO2 and 0.3 mL of 10% AlCl3. The reaction
mixtures were incubated for 10 min at room temperature, 2 mL of1 N NaOH were added to stop the reaction, and the total volumewas adjusted to 10 mL by adding double-distilled water. Flavonoidcontents were measured by comparing the absorbance at 510 nm
L. Mayuoni et al. / Postharvest Biology and Technology 62 (2011) 50–58 53
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ig. 2. Evaluation of respiration and ethylene production rates of ‘Navel’ oranges, ‘So air or to ethylene at 4 �L L−1 at 20 ◦C. Data are means ± SE of six replications.
ith that of a prepared blank, and results are expressed as catechinquivalent (CE).
.7. Antioxidant activity
Total antioxidant activity was determined by using the ABTS*+
adical cation assay (Miller and Rice-Evans, 1997). The reactionixture comprised 1 mL of 75 �M K2O8S2 and 150 �M 2,2′-
zinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS*+) dis-olved in acetate buffer, pH = 4.3, and a 10 �L juice sample. Theeaction mixtures were incubated for 15 min at room temperature.fterwards, total antioxidant activity of juice samples, as comparedith that of a 1 mM trolox solution, was measured by determining
he degree of disappearance of the blue color by comparing thebsorbance at 734 nm against that of a prepared blank. The Troloxquivalent (TE) was calculated according to the formula: TE valuemM) = (Abs sample − Abs blank)/(Abs standard − Abs blank). Theesults were expressed as Trolox Equivalent Antioxidant Capac-ty (TEAC), calculated as: TEAC (�M TE/g) = (TEV)/(1000M) in which= sample volume and M = sample weight.
.8. Statistical analysis
One-way analysis of variance (ANOVA) and Tukey’s HSD pair-ise comparison tests were applied by means of the SigmaStat
tatistical software (Jandel Scientific Software, San Rafael, CA), andicrosoft Office Excel programs.
by’ grapefruit and ‘Satsuma’ mandarins at harvest and after 24 and 48 h of exposure
3. Results
3.1. Effects of ethylene degreening on respiration and ethyleneproduction
We examined the stimulating effects of ethylene degreeningon ripening processes and on internal quality of ‘Navel’ oranges,‘Star Ruby’ grapefruit and ‘Satsuma’ mandarins. It can be seen thatin all the tested citrus species, exposure to ethylene for up to72 h gradually accelerated the change of peel color from green toorange/red/yellow (Fig. 1), whereas exposure to air alone barelyaffected peel color change (data not shown).
To evaluate the effects of exogenous ethylene on ripening-related processes and metabolic activity, we first examined itseffects on respiration and ethylene-production rates. In all thetested citrus fruit, oranges, grapefruit and mandarins, exposure toethylene temporarily enhanced respiration rates after 24 h, but therates then tended to revert to their initial time-zero levels (Fig. 2).In all cultivars, respiration rates after 24 h of exposure to ethylenewere more or less double those observed at time zero or in con-trol fruit held in air (Fig. 2). In addition, exposure to exogenousethylene also enhanced endogenous ethylene production rates inall fruit, from ∼0.2 �g kg−1 h−1 at time zero to 0.8–1.2 �g kg−1 h−1
after 24–48 h (Fig. 2). Thus, ethylene degreening of citrus fruit stim-ulated respiration and increased ethylene production rates, both ofwhich are fundamental parameters of climacteric fruit ripening,indicating increased overall metabolic activity in the fruit.
54 L. Mayuoni et al. / Postharvest Biology and Technology 62 (2011) 50–58
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ig. 3. Evaluation of TSS and acidity levels in juice of ‘Navel’ oranges, ‘Star Ruby’ grao ethylene at 4 �L L−1 at 20 ◦C. Data are means ± SE of five replications.
.2. Effects of ethylene degreening on taste and flavor
The taste of citrus fruit is principally governed by TSS and acidontents, and the ratio between them. In the present study, weound that ethylene degreening did not have any effect on juiceSS or acidity levels in any of the tested fruit (Fig. 3). Furthermore,etailed flavor evaluations with the aid of a trained panel revealedhat ethylene degreening had no effect on the flavor of orangesnd grapefruit, but marginally impaired the flavor of mandarinsFig. 4). The observed loss of mandarin flavor following exposureo ethylene was attributed to a slight decrease in the perception ofypical mandarin flavor (Fig. 4). At least part of the observed slightecrease in flavor acceptability could be attributed to storage of theruit for 5 days at 20 ◦C, even without ethylene (Fig. 4). In any case,thylene degreening did not enhance off-flavor sensations in anyf the tested fruit (Fig. 4).
.3. Effects of ethylene degreening on aroma volatile contents andomposition
Overall, our SPME/GC–MS analysis detected 54 volatiles inNavel’ orange juice, 62 volatiles in ‘Star Ruby’ grapefruit juice, and8 volatiles in ‘Satsuma’ mandarin juice (Supplementary Tables
1–S3). We found that exposure to ethylene at 4 �L L−1 for 72 ht 20 ◦C had only minor effects on volatile contents and compo-itions in citrus juices (Table 1). For example, in juice of ‘Navel’ranges, exposure to ethylene decreased the contents of two alde-it and ‘Satsuma’ mandarins, at harvest and after 24, 48, or 72 h of exposure to air or
hyde volatiles [(Z)-3-hexenal and citronellal], and increased thoseof three carvone-derived volatiles [(E)-carveol, (Z)-carveol and car-vone]; in ‘Star Ruby’ grapefruit, exposure to ethylene increasedthe contents of four aldehydes [pentanal, (E)-2-hexenal, 2-heptenaland (E)-2-octenal]; and in ‘Satsuma’ mandarins ethylene exposureincreased the contents of three terpene-derived alcohols [linalool,terpinene-4-ol and �-terpineol] (Table 1). In some cases, increasesor decreases in aroma volatile contents similar to those that fol-lowed degreening were observed also in juices of air-stored controlfruit, therefore, the changes in their levels could be attributed tostorage of the fruit at 20 ◦C for 3 days rather than to the directeffect of ethylene (Table 1). For example, in ‘Navel’ oranges, thelevels of citronellal decreased similarly in both ethylene-exposedand control fruit, and in ‘Satsuma’ mandarins, the levels of allthree terpene-derived alcohols were similar in both control andethylene-treated fruit (Table 1). Lastly, exposure to ethylene didnot enhance accumulation of off flavor volatiles, such as ethanol orethyl acetate, as might have been expected (Table 1).
3.4. Effects of ethylene degreening on nutritional quality
In order to evaluate the effects of ethylene degreening onfruit nutritional quality, we examined vitamin C, total phenol and
flavonoids contents, and total antioxidant activities of citrus juices.At time zero, the levels of vitamin C in juice of ‘Navel’ oranges,‘Star Ruby’ grapefruit, and ‘Satsuma’ mandarins were 48, 32, and21 mg/100 mL, respectively. In ‘Navel’ oranges, we detected a slight
L. Mayuoni et al. / Postharvest Biology and Technology 62 (2011) 50–58 55
Fig. 4. Evaluation of the sensory quality of ‘Navel’ oranges, ‘Star Ruby’ grapefruit, and ‘Satsuma’ mandarins at harvest and after 72 h of exposure to air or to ethylene at4 �L L−1 at 20 ◦C. Figures on the left indicate overall taste score evaluations, whereas figures on the right represent flavor profile analyses. Fruit flavor was evaluated by atrained taste panel. Data are means ± SE of 10 replications.
Table 1Aroma volatiles whose contents increased or decreased following ethylene degreening in homogenized segments of ‘Navel’ oranges, ‘Star Ruby’ grapefruit and ‘Satsuma’mandarins.
Concentration (�g L−1)
Compound RI At harvest Air (72 h) Ethylene (72 h)
‘Navel’ orange(Z)-3-Hexenal 800 697 354 –Citronellal 1152 471 – –(E)-Carveol 1201 94 108 527(Z)-Carveol 1219 134 1154 2316Carvone 1244 232 626 1461
‘Star Ruby’ grapefruitPentanal 700 – – 174(E)-2-Hexenal 851 108 126 4862-Heptenal 955 – – 135(E)-2-Octenal 1058 – – 115
‘Satsuma’ mandarinLinalool 1099 375 890 1075Terpinene-4-ol 1177 98 234 217�-Terpineol 1190 143 370 388
Data represent aroma volatiles whose contents increased or decreased significantly (P ≤ 0.05) and by factors of at least 2, following exposure to ethylene at 4 �L L−1 for 72 hat 20 ◦C. The presented data are 1-pentanol equivalents, and are means of three measurements, each of juice collected from three different fruit.
56 L. Mayuoni et al. / Postharvest Biology and Technology 62 (2011) 50–58
Fig. 5. Evaluation of total phenol and flavonoid levels in juice of ‘Navel’ oranges, ‘Star Ruby’ grapefruit, and ‘Satsuma’ mandarins, at harvest and after 24, 48 and 72 h ofe cation
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xposure to air or to ethylene at 4 �L L−1 at 20 ◦C. Data are means ± S.E. of five repli
ecrease in vitamin C levels in juice, from 48 mg/100 mL at timeero to about 40–42 mg/100 mL after 3 days at 20 ◦C. However, thisecrease was observed in both control and degreened fruit and,herefore, was not attributed to exposure to ethylene but rathero storage of the fruit. In juices of ‘Star Ruby’ grapefruit and ‘Sat-uma’ mandarins, we did not detect any notable changes in vitaminlevels either during the control or the degreening treatments.
With regard to evaluation of total phenol and flavonoids con-ents in citrus juices, we did not detect any notable changes inhe contents of total phenolic compounds in orange and grape-ruit juices, but did observe a slight decrease in total phenolicsn ‘Satsuma’ mandarin juices which occurred in both control andegreened fruit. In ‘Navel’ oranges, a slight decrease in totalavonoids content, from 4.2 mg per 100 g FW at time zero to 2.5 mger 100 g FW after 72 h at 20 ◦C was observed in both control andegreened fruit. In grapefruit juice, we observed some fluctuations
n total flavonoids levels, which ranged from 5.5 to 9.5 mg per00 g FW. In ‘Satsuma’ mandarin juice, total flavonoids levels weretable, at about 8.0–8.5 mg per 100 g FW, and were not affected byhe degreening treatment.
Finally, we examined whether ethylene degreening had anyffect on total antioxidant activities in citrus juices. It was found
hat the average levels of total antioxidant activities of ‘Navel’range, ‘Star Ruby’ grapefruit, and ‘Satsuma’ mandarin juices werepproximately 5, 4, and 2 �M TE g−1, respectively, and were notffected by exposure to ethylene.s.
4. Discussion
Citrus fruit are non-climacteric, but it is well known that ethy-lene, both endogenous and exogenous, is involved in regulation ofpeel color change (Purvis and Barmore, 1981; Goldschmidt et al.,1993; Rodrigo and Zacarias, 2007). However, it is not yet knownwhether ethylene is involved in regulation of internal ripening pro-cesses in the citrus flesh.
In light of the increased importance attached to maintenanceand improvement of internal fruit quality, and of the findingsof some previous studies that ethylene degreening might haveharmed overall fruit quality, the goal of the present study was touse three main citrus species, orange, grapefruit and mandarin,to evaluate systematically whether ethylene degreening affectedinternal ripening processes in citrus fruit, with especial emphasison determination of fruit flavor and nutritional quality aspects.
The present results clearly show that although ethylenedegreening for up to 3 days at 20 ◦C accelerated peel color change(Fig. 1) and temporarily increased respiration and ethylene pro-duction rates (Fig. 2), it did not have any major effects on variousinternal fruit-quality parameters, including TSS contents and acid-ity levels in juice (Fig. 3), flavor perception (Fig. 4), contents and
composition of aroma volatiles (Table 1, Supplementary TablesS1–S3), vitamin C content, total phenols and flavonoids contents(Fig. 5), and overall antioxidant activity. Thus we conclude thatethylene is probably not involved in regulation of internal ripening
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L. Mayuoni et al. / Postharvest Bio
rocesses in citrus flesh, and therefore, does not affect or impairnternal fruit quality attributes, including perceived flavor andutritional quality. Our current results are somewhat in contra-iction with our previous findings demonstrating that exposureo ethylene influenced the expression patterns of 661 transcriptsn mandarin flesh (Mayuoni et al., 2011). Nevertheless, most ofhe identified degreening-regulated genes in mandarin flesh tis-ue were involved rather in governing metabolic arrest (slow downf general metabolic activity) and activation of biotic and abiotictress tolerance; both factors are not directly involved in regulatingnternal fruit quality parameters. Moreover, the slight activation ofenes involved in metabolic processes was probably not sufficiento cause marked changes in fruit flavor or nutritional quality.
Regarding the evaluation of possible effects of ethylene onruit flavor perception, our results support the common dogmahat ethylene does not affect citrus fruit flavor perception. Thenly exception was observed with mandarin fruit, where flavorcceptability slightly decreased following degreening, but this phe-omenon could be attributed, at least partly, to storage of the
ruit at 20 ◦C for 5 days (Fig. 4). It is known that mandarins areore perishable than other citrus fruit, and especially suffer from
apid decline in sensory acceptability after harvest (Tietel et al.,011). Therefore, it is recommended not to keep mandarins athelf-life temperatures for long periods. Although we claim thatthylene degreening at moderate temperatures (∼20 ◦C) did notmpair fruit flavor, it should be noted that degreening citrus fruitt higher temperatures of ∼30 ◦C led to a decrease in juice acid-ty levels (Tietel et al., 2010b). This observation is consistent withhe detection of decreased acidity levels in ‘Mosambi’ oranges byadaniya and Singh (2001) following degreening at high tempera-ures of 27–29 ◦C. Similar to our present findings, Nishikawa et al.2002) also did not observe any changes in aroma volatile profilesn oils that were cold-pressed from degreened oranges. Further-
ore, in climacteric fruit, such as apple and banana, exposure tothylene enhances accumulation of ester volatiles via activation oflcohol acetyl transferases (AATs) (Golding et al., 1999; Schaffert al., 2007). Nevertheless, in the present study we did not detectny accumulation of ester volatiles after the degreening treatmentuggesting that aroma volatile production in citrus fruit is not gov-rned by ethylene. A similar conclusion that volatile emission wasot controlled by ethylene was recently reported in the case ofon-climacteric cut roses (Borda et al., 2011).
With regard to possible effects of ethylene on fruit nutritionaluality, our results clearly show that ethylene degreening did notffect the overall nutritional quality of citrus fruit. Previous studiesound that ethylene induced phenylalanine ammonia lyase (PAL)ene expression and phenylpropanoid metabolism in mandarineel (Cajuste and Lafuente, 2007). However, in the present studye did not detect any increase in total phenolic compounds. Possi-
le explanations for these differences are that phenylpropanoidsccount for just a minor part of the total phenolic compoundsresent in the juice cells, or, alternatively, that activation of PALnd phenylpropanoid metabolism might result in massive accumu-ation of the final biosynthetic product, lignin, but not of phenolicompounds per se. Otherwise, our results are consistent with thosef other studies in which no significant influences of ethylene onavonoid content in citrus flesh were observed (Nishikawa et al.,002).
Finally, our present findings also are in full agreement with thosef a recent study by Distefano et al. (2009), who evaluated peelolor change and internal characteristics of a normal Clementinend a late-ripening Clementine mutant defective in ethylene per-
eption; they found that the ‘Tardivo’ mutant, defective in ethyleneynthesis and perception, showed delayed color-break but, never-heless, the mutation did not affect internal ripening characteristicsuch as juice acidity and TSS levels (Distefano et al., 2009). Thus, asnd Technology 62 (2011) 50–58 57
in our present case, it was concluded that ethylene affected peelcolor-break but not internal fruit quality parameters.
Acknowledgments
This manuscript is contribution no. 606/11 from the AgriculturalResearch Organization, the Volcani Center, Bet Dagan, Israel. Thisresearch was supported by Research Grant No. TB-8056-08 fromTDA, Texas–Israel Exchange, and BARD, the United States IsraelBinational Agricultural Research and Development Fund.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.postharvbio.2011.04.005.
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