Postharvest Biology and Technology 88 (2014) 3439

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Postharvest Biology and Technology

journa l h om epa ge : www.elsev ier .com/ l

Fast cu quhot clim

Dani Esh SaaVictor RoDepartment of

a r t i c l

Article history:Received 3 JulAccepted 15 S

Keywords:CuringHeat treatmentOnion skinPostharvest storage

landor up y outst qubove

bulbs, resulting in a sturdy tunic beneath the muddy outer skin. We applied fast curing (FC) at 30 C and98% RH for up to 9 days postharvest: the onion neck became 52% narrower after 6 days, similar to theeffect of 5 months of cold storage. FC also changed the color of the treated onion bulbs outer skin to adarker reddish brown. FC of onions harvested with a long neck and stored for 290 days reduced weightloss and rot by 30% and 80%, respectively, as compared to non-FC onions. The better onion quality induced

1. Introdu

Onion (Aand harvesstored for uat 02 C. Donions decrBlack Neck humid conBotrytis alliiespecially ibasal rot ca(Galvn et a

The agrileaves afterwith their rtop of the

CorresponE-mail add

0925-5214/$ http://dx.doi.oby FC was accompanied by an increased number of onion skin layers (from an average of 1.8 to 4) and ahigher force needed to tear the tunic (average 4.8 N as compared to 3.5 N in the control). FC compressedthe effects of 5 months of cold storage into a few days, since most of the bulb neck and tunic changes mea-sured during FC occurred only after long cold storage without FC. Although emission of the lachrymatoryfactor was not affected by FC, it markedly increased after 5 months of storage. Histological observationshowed that FC keeps the onion tunic and inner eshy scales intact and ensures postharvest quality, evenafter 8 months of cold storage. The high temperature used for FC can be reached in hot-climate storageareas with minimal energy investment.

2013 Elsevier B.V. All rights reserved.

ction

llium cepa) is grown in Israel during the winterspringted during the hot summer. Postharvest onions arep to 3 months without cooling and up to 8 monthsuring storage, they start to sprout and root and theease in weight. In most cases, rots appear as well, mainlyRot caused by the fungus Aspergillus niger in warm andditions (Abd-Alla et al., 2006), Botrytis rot caused by, which rst infects the sprout and spreads to the neck,n wounded green necks (Li et al., 2011), and fusariumused by Fusarium oxysporum f. sp. cepae in long storagel., 2008).cultural practice in Israel is to remove (top) the aerial

they are completely dry. Because the onions are leftoots in the ground until the leaves are totally dry, thebulb is exposed to direct solar radiation, which then

ding author. Tel.: +972 3 9683621; fax: +972 3 9683622.ress: dani@agri.gov.il (D. Eshel).

causes the tunic to crack and fall off during storage and market-ing. The tensile strength of the papery scales, expressed as force,varies considerably among cultivars and growing sites and is posi-tively correlated with thickness, all of which play an important rolein the cracking and sloughing resistance of onion bulbs (Komochi,1990).

Harvesting the onions while the leaves are still green requiresa heat-drying process with high energy cost to minimize microbialrots that favor the high moisture produced in commercial storage(Wright and Triggs, 2005; Schroeder and du Toit, 2010). In the UK,due to the relatively wet climatic conditions, most onions are arti-cially cured for a few weeks to reduce rot risk and losses during eldcuring. Long articial curing forms a tunic which prevents exces-sive water loss from the bulb and reduces disease incidence. Inaddition, articial curing dries the muddy outer skin, which canthen be easily removed, leaving a cleaner nish skin, and improv-ing the appearance of brown onions (Gubb and MacTavish, 2002;Downes et al., 2009). Chope and Terry (2010) suggested method-ological alterations that would better suit todays onion cultivars,such as reducing the temperature and duration of the curing anddrying periods to deliver benets in the form of energy savings and

see front matter 2013 Elsevier B.V. All rights reserved.rg/10.1016/j.postharvbio.2013.09.002ring: A method to improve postharvestate harvest

el , Paula Teper-Bamnolker, Yakov Vinokur, Inbaldov

Postharvest Science, The Volcani Center, ARO, Bet-Dagan 50250, Israel

e i n f o

y 2013eptember 2013

a b s t r a c t

Most of the onions (Allium cepa cv. Orbefore storage, and are cold-stored fotheless, in most cases the complete drstorage. To improve onion postharvedown), leaving about 10 cm of neck aocate /postharvbio

ality of onions in

d, Yohanan Zutahy,

) grown in southern Israel are treated with maleic hydrazideto 8 months with minimal losses to rots or sprouting. Never-er skin (tunic) cracks and loosens, and tends to fall off duringality, bulbs were harvested at 80100% green leaf drop (top-

the bulb. The early harvest reduced skin cracks in 93% of the

D. Eshel et al. / Postharvest Biology and Technology 88 (2014) 3439 35

reduced carbon emissions, while still producing a bulb of satisfac-tory quality.

In the southern Israeli summer, as in all hot desert countries,ambient temperature and solar radiation are high and can be usedto advantagwas to takeadapt a spetemperaturvest of onioin high humand reduce

2. Materia

2.1. Plant m

Onions (northwesteE, elevationsprayed wiagriculturalat total leafand the leacm or 1-cmin each plasFC room.

2.2. Bulb cu

FC was pfogger (SMDthe onions w

2.3. Bulb-qu

Skin crathe muddy surface. Skincracks.

Bulb necdigital calipthe bulb nepeeled tuni

The coloa Minolta CJapan) and performed per treatme

Weight ltaining 40 oof the longestorage.

Skin numskin layers, after remov

Skin-tea(Chatillon, sliced (5 cmneeded to mmetal probe

Severityonions per tneck to thewith fungal

2.4. Onion lachrymatory factor

Lachrymatory factor (thiopropanal-S-oxide) was measuredby gas chromatography/mass spectrometry (GC/MS) using

hasep e

frousingn ex

he we coken s repeane

in methserteing is inlA) em lmraturC mig at tas setor thitive enernrmed , 197

istolo

tologtionthod. Tistic atratiistoc

on min fo

in tc, Lcatied w

atisti

isticversimentn Neariabted.

lts

Israelially d

as aand mn), le

ith tuneasu

6 anddays e in energy saving during onion curing. Our objective advantage of the excessive heat of the product, andcic fast-curing (FC) process that is practiced at highe and high humidity. We hypothesized that early har-ns with long necks followed by high-temperature curingidity would reduce skin cracks, increase skin stability

storage rots.

ls and methods

aterial

A. cepa cv. Orlando) were grown in sandy soil in thern Negev desert, in the south of Israel (3125 N, 3429

50150 m) in the years 20082012. Onions were notth maleic hydrazide before leaf drop as per common

practice. At 80100% fallen leaves (top-down) or later, dryness in the eld, onions were harvested manuallyves were removed with a sharp knife, leaving a ca. 10-

long neck above the bulb. About 40 bulbs were placedtic net box and transferred on the day of harvest to the

ring and storage

erformed at 30 C in 98% RH created by an ultrasonic Technology, Rehovot, Israel) for 0, 3, 6 and 9 days, thenere transferred to 2 C and 70% RH for up to 10 months.

ality measurements

cks were detected visually in the tunic after removingouter skin, which is easily removed after drying the bulb

was categorized as cracked in the case of one or more

k diameter and skin thickness were measured using aer. For the former, the caliper was placed at the base ofck where it emerges from the bulb; for the latter, thec was measured.r of the tunic was measured by diffuse reectance withR-300 Chroma Meter (Konica Minolta Sensing, Osaka,expressed in hue angle (H). Four measurements werewith each onion around its widest diameter, 30 onionsnt.oss was analyzed by weighing four boxes of onions con-nions each for every treatment, before curing, at the endst curing (9 days), and after 140 and 290 days of cold

ber represents the number of intact, dry and brownwhich were counted by skinning 40 bulbs per treatmenting the muddy outer skin.ring strength was measured using a digital force gaugeDRC 200N, Florida, USA). The tunic of each onion was

5 cm) and attached to the force gauge base. The forceake a hole in the onion skin with a at 1-cm diameter

was measured. of bulb rot was assessed by cutting three replicates of 40reatment, each bulb being cut down the center from the

basal plate, and determining the percentage of onions rot symptoms.

solid-pJrvenexcised180 g) betweefrom tthat thwas taused awas clplacedpolydiwas inFollowsplitlesCA, US0.25 tempeat 20

holdinrate wdetectin poswere gwas copublishPascale

2.5. H

Hissue secthe mecations5% aceconcenwith hspreadSafrandratedScientimagniequipp

2.6. St

Statware (experiwesterlittle vpresen

3. Resu

The have totprobablystorage (top-dowbulbs w

We mfor 0, 3,up to 9 microextraction (SPME) as previously described byt al. (1998) with a few modications. Flesh plugs werem medium-size peeled bulbs (ca. 6 cm in diameter, ca.

an 8-mm cork-borer. To neutralize possible variationsternal and internal scales, the plugs were evacuatedhole bulb thickness in its widest (equator) region, sork-borer passed the bulbs central core. Only one plugfrom each bulb, and four uniform healthy bulbs werelicates for each measurement point. The cork-borerd after each sampling. The plugs were immediately20 mL sampling vials incubated at 25 C. A 100 mylsiloxane SPME ber (Supelco, Bellefonte, PA, USA)d into the vial immediately after its closure for 1 min.ncubation, the bers were desorbed at 100 C in theet of an Agilent 7890A gas chromatograph (Palo Alto,quipped with an HP-5 column (30 m 0.25 mm i.d.,

thickness; J&W Scientic, Folsom, CA, USA). The ovene program was as follows: increase from 40 C to 150 Cn1 and further to 220 C at 5 C min1 and nally,hat temperature for 1 min. The helium carrier gas ow

to 0.8 mL min1. The Agilent 5975C mass spectrometerat was set to scan from m/z 40 to 206 at 7.72 scans s1

ion mode, and mass spectra in electron impact modeated at 70 eV. The identity of the lachrymatory factored by comparing the mass spectra to the previously

spectra obtained from Allium samples (Brodnitz and1; Ohsumi et al., 1993).

gical analysis of curing and cold storage effect

ical analyses were performed on 10 m thick bulb tis-s cut by microtome. Samples were handled according to

described by Kamenetsky (1994) with minor modi-sues were xed overnight in FAA (10% formaldehyde,cid, 50% ethanol, v/v), then dehydrated in increasingons of ethanol (25%, 50%, 75%, 95%, and 100%), clearedlear, and embedded in parafn. Prepared sections wereicroscope slides, rehydrated, and stained with 1% (w/v)

llowed by 0.2% (w/v) Fast Green. Sections were dehy-he ethanol series and mounted in Permount (Fishereicestershire, UK). Slides were examined at 1640on under bright-eld using a Leica light microscopeith a camera.

cal analysis

al analysis of the data was performed with JMP-in soft-on 3 for Windows; SAS Institute, Cary, NC, USA). Each

was conducted at least twice in two areas of the north-gev in three consecutive growing seasons. There wasility between seasons and representative results are

agricultural practice of harvesting onion bulbs after the aerial leavesried (Fig. 1A) was found to promote tunic cracks in 93% of the bulbs,

result of sunburn damage; the skin then tended to peel off duringarketing (Fig. 1C). Harvesting the onions at 80100% green leaf dropaving about 10 cm of the green neck (Fig. 1B), reduced the number ofic cracks to 15%, leading to a more sturdy tunic (Fig. 1D).red the neck diameter of onions that were exposed to FC after harvest

9 days and then transferred to 2 C storage. Application of FC forinduced fast drying of the onion neck, as measured by a reduction

36 D. Eshel et al. / Postharvest Biology and Technology 88 (2014) 3439

Fig. 1. Onions harvested at total leaf dryness (A) and after leaf drop (B), and effects on the bulb tunic (C and D, respectively).

in its diameter (Fig. 2A and C). The average neck diameter of treated onions wasreduced by 52% after 6 days of FC and did not change in the subsequent 3 days oftreatment (Fig. 2A). The neck diameter of bulbs after 6 days of FC was similar tothat of non-FC (time 0) bulbs after 5 months of storage. Onions stored after FCcontinued to s(Fig. 2A and D

The skin color of FC onions changed from light white-brown to dark red-brownwith duration of curing. The hue angle (H) of the bulb tunic decreased graduallyfrom 84 to 50 after 9 days of FC (Fig. 2B and C). After 5 months of storage, the tunicwas a dark red-brown, with a H value similar to that after 3 days of FC (H = 74);

bjectee (Fig

Fig. 2. Effect owere performehow an up to 80% narrower neck diameter, reecting a dryer neck).

bulbs suof storag0 3 6 9

f duration of postharvest onion bulb curing on bulb neck diameter (A) and bulb tunic cod after 9 days (circles and C, left to right) and after an additional 5 months of cold storagd to 9 days of FC showed no additional color change after 5 months. 2B and D).lor (B). Onions were cured for 0, 3, 6 and 9 days and measurementse (circles and D, left to right). Error bars indicate standard error.

D. Eshel et al. / Postharvest Biology and Technology 88 (2014) 3439 37

0

3

Fig. 3. Effect odence of rottenecks except fand cured for (P 0.05) numted onion harv

Long-neckcured counterpweight duringaverage 78% wFC was relatedonions (Fig. 3A

Although lrots in onion, of storage. Mobefore storagewas topped clofound rotted (

Since FC dsized that thischaracteristicsand brown skbrown skin laycold storage oSurprisingly, wwas about 3.6 days (Fig. 4B).an average 4.8

Although sumed for thefoods. FC did nthe level of la6

90 3 6 9

f onion neck length and curing duration on weight loss (A) and inci-d onions in storage (B and C). Onion bulbs were harvested with longor treatment 6s in which bulbs were harvested with short necks6 days. Different lowercase letters represent signicantly differentbers of rotted onions. Error bars indicate standard deviation. (C) Rot-ested with short neck (left) as compared to long neck (right).

onions stored after FC lost less weight during storage than their non-arts (Fig. 3A). Whereas non-cured onions lost an average 10% of their

290 days of storage, 69 days of FC before storage resulted in aneight loss (Fig. 3A). The dramatic decrease in onion weight during the

to fast drying of the neck, as evidenced by comparison to short-neck).ow temperature is considered a major tool for reducing postharvestwe found an average of about 5% soft-rotted onions after 10 monthsst of the rots were covered by Botrytis spp. mycelia. Application of FC

reduced onion rot to an average 0.5% (Fig. 3B). When the onion neckse to the bulb, FC was less efcient, with an average 4% of the onions

Fig. 3B and C).id not have any effect on tunic thickness (not shown), we hypothe-

treatment induces a parallel change in dry skin number and physical of the skin that results in reduced cracking. We measured the dryins and found that 9 days of FC induces the formation of up to fourers, compared to 1.8 in non-treated onions (Fig. 4A). Five months of

f FC onions eliminated the differences between the curing durations.e found that the force needed to tear the outer skin prior to curing

N and increased to an average 5.5 N as curing duration extended to 9 After 5 months of storage, the tearing strength of the skin exceeded

N and FC onions retained the same trend of higher values, up to 6.4 N.they possess signicant nutritional value, onions are primarily con-ir unique avor and for their ability to enhance the taste of otherot appear to have any effect on the onions pungency, evaluated aschrymatory factor (Fig. 4C). In both non-treated and cured onions,

Fig. 4. Effect oto tear the comwere cured foadditional 5 m

the emission o(Fig. 4C).

Histologicpapery scales outer scales omis and collendeterioration producing an bulb presenteparenchyma c

4. Discussi

Growingthe Israeli Ntion of outeby Wright (some of thedown (greebe further eharvested e

Topping(2001) whoscales whenof harvestinthe onionssuggested beffect of topneck rot, as(Fig. 3).0 3 6 9

f curing duration on number of brown skin layers (A), force neededplete dry outer skin (B) and lachrymatory factor (C) in onions. Bulbsr 0, 3, 6 and 9 days and measured after 9 days (circles) and after anonths in cold storage (circles). Error bars indicate standard error.

f lachrymatory factor markedly increased after 5 months of storage

al observation showed that FC induces drying of the outer pigmented,of the bulb, producing 34 dry scales in 9 days (Fig. 5). Drying of theccurred through deterioration of the parenchyma, while the epider-chyma remained intact (Fig. 5). Eight months of cold storage inducedof the lower epidermis and parenchyma of the eshy scales as well,air space between them (Fig. 5). The eshy scales of the FC-treatedd a compact structure, with the lower epidermis and most of theells remaining intact (Fig. 5).

on

and harvesting onion bulbs in the hot dry summers ofegev region present a challenge with respect to preven-r scale cracking and instability. Field curing, as described1997), is not an option in this extreme weather, although

farmers still practice it. Early harvest, at 80100% top-n leaf drop), minimizes outer scale cracks but shouldxamined for its effect on nal yield, since the bulbs arearlier.

was performed before curing, based on Wright et al. found no difference in the mean number of intact bulb

onions were topped before or after curing. The methodg bulbs with a long, green neck improves intactness of

outer scales and did not induce more microbial rot, asy Wright and Triggs (2005). We assume that the majorped neck size is on curing efciency and prevention of

shown by comparison to storage of short-necked bulbs

38 D. Eshel et al. / Postharvest Biology and Technology 88 (2014) 3439

Fig. 5. Effect o Uppertunic; f, outer ymato

FC inducthe rst 3 douter scalecuring has allii by seawas efcienwith long-nlonger necktration. Unyam that arditions (Wais not knowin blocking

The mecbrowning htion of phenglucosides (as major pthe inner et al., 1998age, and thfresh, dryinglucoside csuggesting ceeds along(Takahamacellular comcells, enzytive substracells during1955).

Up to fFC. Wrightrity increasvalue was Red Baron total anthoet al., 2010)anthocyani

en onrenceoundet ala anpera009)

did of FCthouat higult inende

expr. Thursof fast curing (FC) on dry scale formation in onion bulbs before and after storage. eshy scales. Lower pictures: uep, upper epidermis; col, collenchyma; pp, parench

ed rapid drying of the green neck, as clearly seen afterays of curing (Fig. 2). Nevertheless, FC did not induce

cracks, probably due to the high humidity. Forced airbeen shown to reduce neck rot disease caused by B.ling the bulb neck (Langston, 2001). In our study, FCt at preventing neck rot only when it was combinedeck harvest (Fig. 3B). This is probably related to thes better ability to seal itself against pathogen pene-like tropical roots such as sweet potato, cassava ande cured immediately after harvest under similar con-lter and Schadel, 1983; Eshel, 2011), wound peridermn in onion and the dry outer skin plays the major role

pathogen penetration.hanism and physiological signicance of onion bulbave been associated with peroxidase-dependent oxida-olics (Takahama, 2004). Onion scales contain quercetin

betwea diffetions fTerry Sherping temet al., 2

Weresult that alcured not res

Extgency,cuttingof precquercetin 4-glucoside and quercetin 3,4-diglucoside)henolics, with their concentration increasing from

to outer scales (Tsushida and Suzuki, 1995; Hirota). The outer scales of onion bulbs turn brown withe browning scale can be divided into three regions:g and dried brown. In dried brown scales, quercetinoncentration is low but that of quercetin is high,that deglucosidation of the quercetin glucosides pro-

the border between drying and dried brown areas and Oniki, 2000). This might be a result of loss ofpartmentalization due to cell death, since in living

mes which catalyze glucosidation and the respec-tes may be compartmentalized. Death of onion scale

browning has been reported (Walker and Stahmann,

our brown outer skin layers were formed during et al. (2001) found that increasing harvest matu-es the mean skin color score. The onion skins H

found to be higher in the red-skinned onion cultivarcured at 28 C, and to be negatively correlated withcyanins and total avonols (Downes et al., 2009; Terry. The negative correlation between total avonols/totalns and H value suggested that differences in skin color

(Lancaster The hydrolypropanol Spyruvic acifactor is pralliinase unUddin and lier studiesof pungenceffect of thtrials.

Storage arrived at benough toto cause cdeterioratioeshy scalepact structthe heat trsion.

We beliestorage of quality with pictures: the dotted line shows where the onions were sampled; t,us pith; lep, lower epidermis cells. Bars = 500 m.

ions cured at different temperatures might be due to in the avonol and anthocyanin glucoside concentra-

in the skin tissue of red onions (Downes et al., 2009;., 2010). However, darkening of the brown cultivarsd Wellingto was not correlated with the effect of cur-ture on avonol concentration in the bulb skin (Downes.not detect early sprouting of cv. Orlando onions as a

(not shown). Similarly, Chope and Terry (2010) foundgh ABA concentration was lower in Red Baron onionsher (24 or 28 C) vs. lower (20 C) temperatures, it did

earlier sprout growth.d storage results in a signicant increase in onion pun-essed as amount of lachrymatory factor released aftere pungent avor of onions is produced by hydrolysisr compounds such as S-alk(en)yl-l-cysteine sulfoxides

et al., 2000) when the cells are mechanically ruptured.sis reaction is catalyzed by allinase and produces thio-

-oxide (a lachrymator), ammonia, sulfur volatiles andd (Block, 1992). The dramatic increase in lachrymatoryobably connected to the elevated specic activity ofder cold storage in a non-modied atmosphere (MiarMacTavish, 2003). A similar trend was observed in ear-

(Freeman and Whenham, 1976); however, reductiony was later associated with the onset of sprouting. Noe curing procedure on pungency was detected in our

temperatures of root, stem and bulb vegetables arey compromising between temperatures that are low

inhibit the sprouting process but not so low ashilling injury. Eight months of cold storage inducesn of the lower epidermis and parenchyma of thes, probably as a result of chilling injury. The com-ure of the eshy scale induced by FC suggests thateatment induces strengthening of intercellular adhe-

ve that this method can be adapted to the harvest andonions in hot climates, to dramatically improve bulb

minimal energy investment.

D. Eshel et al. / Postharvest Biology and Technology 88 (2014) 3439 39

Acknowledgment

This research was funded by Grant number 430-0416-12 fromthe Chief Scientist of the Ministry of Agriculture and Rural Devel-opment of Israel. The manuscript is a contribution from theAgricultural Research Organization, the Volcani Center, Bet Dagan,Israel, No. XXX.

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Li, C., Schmidt, N.E., Gitaitis, R., 2011. Detection of onion postharvest diseases byanalyses of headspace volatiles using a gas sensor array and GCMS. LWT Food Sci. Technol. 44, 10191025.

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Fast curing: A method to improve postharvest quality of onions in hot climate harvest1 Introduction2 Materials and methods2.1 Plant material2.2 Bulb curing and storage2.3 Bulb-quality measurements2.4 Onion lachrymatory factor2.5 Histological analysis of curing and cold storage effect2.6 Statistical analysis

3 Results4 DiscussionAcknowledgmentReferences