Postharvest Biology and Technology 20 (2000) 303–308

Short communication

Effect of fruit maturity on efficiency of1-methylcyclopropene to delay the ripening of bananas

D.R. Harris a,b,*, J.A. Seberry b, R.B.H. Wills a, L.J. Spohr b

a Centre for Food Industry Research and De6elopment, Faculty of the Central Coast, Uni6ersity of Newcastle, PO Box 127,Ourimbah, NSW 2258, Australia

b Horticultural Research and Ad6isory Station, NSW Agriculture, Locked Bag 26, Gosford, NSW 2250, Australia

Received 7 March 2000; accepted 25 July 2000

Abstract

Three bunches of unripe ‘Williams’ banana fruit of different maturity, 173, 156 and 71 days from bunch emergence,were harvested. Fruit from the top, bottom and middle hands from each bunch were fumigated for 24 h with1-methylcyclopropene (1-MCP) at 0, 5, 50 or 500 nl l−1 at 20oC. All fruit were then stored at 20oC in air containing0.1 ml l−1 ethylene and the time taken for each fruit to ripen (green life) was noted. The green life of fruit treated with500 nl l−1 1-MCP varied with fruit maturity. In the two most mature bunches it was 27.992.3 days, 4-fold longerthan fruit fumigated with 0 nl l−1 1-MCP (6.790.6 days). In the least mature bunch, green life was 39.793.0 days,1.5-fold longer than fruit fumigated with 0 nl l−1 1-MCP (25.792.5 days). Most fruit treated with 500 nl l−1 1-MCPshowed an unacceptable uneven skin colouration when ripe. There was no significant effect on green life of 1-MCPat 50 nl l−1 and 5 nl l−1. Other fruit from these bunches were not exposed to 1-MCP and were held in ethylene-freeair until ripe. In the two most mature bunches, these fruit had a significantly shorter green life (11.295.6 days inhand 1; 18.994.1 days in hands 4 and 6) than fruit that were fumigated with 500 nl l−1 1-MCP. In the least maturebunch, however, these fruit had a significantly longer green life (56.095.9 days) than 1-MCP treated fruit. Since theeffectiveness of 1-MCP varied with fruit maturity and in any commercial consignment there is a mixture of fruitmaturity, it is concluded that 1-MCP has limited commercial potential for the storage of unripe ‘Williams’ bananas.© 2000 Elsevier Science B.V. All rights reserved.

Keywords: Banana; Ripening; 1-methylcyclopropene; Maturity; Ethylene

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1. Introduction

1-Methylcyclopropene (1-MCP) has been re-ported to delay or reduce ethylene induced effectson senescence in a variety of potted flowering

* Corresponding author. Tel.: +61-2-43481946; fax: +61-2-43481910.

E-mail address: david.r.harris@agric.nsw.gov.au (D.R. Har-ris).

0925-5214/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

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D.R. Harris et al. / Posthar6est Biology and Technology 20 (2000) 303–308304

plants and cut flowers (Serek et al., 1994, 1995a,1996; Porat et al., 1995; Sisler et al., 1996a; Heyesand Johnston, 1998; Newman et al., 1998). Effectsof 1-MCP on fruit and vegetables include inhibit-ing the ripening of tomatoes (Serek et al., 1995b;Sisler et al., 1996b), delaying senescence of straw-berries (Ku et al., 1999) and broccoli (Ku andWills, 1999) and inhibiting the degreening of or-anges while not suppressing other ethylene-in-duced effects such as chilling injury (Porat et al.,1999).

While 1-MCP has been shown to delay theripening of bananas, the effect of 1-MCP is quitevariable among the reported studies. Sisler et al.(1996b) showed that 24 h fumigation with 10 nll−1 1-MCP was sufficient to protect fruit for11–12 days at 25oC against 18 h exposure to 1000ml l−1 ethylene, while bananas treated with 0.7 nll−1 1-MCP did not respond for 7 days and 0.4 nll−1 1-MCP did not give any protection. Goldinget al. (1998) reported that 6 h fumigation with 450ml l−1 1-MCP extended the green life (time toripen) of bananas held in air from 20–30 days,and from 2–20 days when held continuously in500 ml l−1 propylene at 20oC. They also reportedthat 1 h exposure to 45 ml l−1 1-MCP extendedthe green life of bananas when applied 6 or 12 hafter the start of exposure to 500 ml l−1 propyl-ene, but not after 24 h exposure. Macnish et al.(1998) demonstrated a 4.4-fold extension in shelflife of ‘Cavendish’ fruit at 20oC, treated with 15 mll−1 1-MCP for 12 h and then exposed to 100 mll−1 ethylene for 12 h, compared with fruit nottreated with 1-MCP but similarly exposed to eth-ylene. Jiang et al. (1999a) demonstrated that a 24h exposure to either 500 or 1000 nl l−1 1-MCP at20oC extended the green life of ‘Cavendish’ ba-nanas from 16–31 days in the absence of ethylene,compared with untreated controls. Jiang et al.(1999b) showed that a 1 h exposure to 1000 nl l−1

1-MCP at 20oC eliminated the effects of a 24 hexposure to 100 ml l−1 ethylene for at least 5 days(the duration of the experiment) in ‘Cavendish’fruit and that a 12 h exposure to 50 nl l−1 1-MCPwas just as effective. In these studies no referencewas made to fruit maturity which could have aninteractive effect on ripening, since sensitivity toethylene increases with increasing physiologicalage of bananas (Liu, 1976).

This study examined the effectiveness of 1-MCPto protect preclimacteric bananas of different ma-turity from continuous exposure to levels of ethyl-ene, commonly encountered in the market placeprior to gas ripening. Ethylene at 0.1 ml l−1 wasused as the background concentration as Wills etal. (1999) found it to be present at that levelaround preclimacteric bananas in commercial sit-uations. Our hypothesis was that there was asignificant interaction between the effectiveness of1-MCP and the maturity of the banana fruit.

2. Materials and methods

Three bunches of banana fruit (Musa sp., AAAgroup, Cavendish subgroup, cultivar ‘Williams’)were collected from a commercial grower in theMacksville district of New South Wales, Australiaon 28 January 1999. The bunches (designated asA, B and C) at harvest were estimated from thegrower’s records to be 173, 156 and 71 days,respectively, from bunch emergence. Hands werenumbered from 1 (largest and most proximalhand) to 6 or 7 (most distal) and represent fruitage. Finger diameter was measured for each fingeron each hand. Callipers were placed two-thirdsdown the finger towards the blossom end, and theminimum diameter obtainable was recorded.Hands 1 (oldest), 4 and 6 (youngest) were selectedfrom bunches B and C; and hands 1, 4 and amixture of 6 and 7 were selected from bunch A.The mixture of hands 6 and 7 were then treated asone hand. Wing fruit were removed from thehands. The remaining fruit were then cut intosingles and dipped in a solution containing 400mg l−1 a.i. of the fungicide thiabendazole (‘Tecto90’, Merck, Sharp and Dohme, Granville, NSW,Australia) for 2 min and then dried with a papertowel. Fourteen fingers from each hand were se-lected for uniformity. Three fruit from each handwere fumigated at 20oC in sealed 10 l plasticbuckets for 24 h with air containing 0, 5, 50 or500 nl l−1 1-MCP. Fruit were then placed singlyinto a 1.8 l glass jar and ventilated at 60 ml min−1

with humidified air containing 0.1 ml l−1 ethyleneat 20oC. The remaining two fruit from each handwere not treated with 1-MCP, but placed directly

D.R. Harris et al. / Posthar6est Biology and Technology 20 (2000) 303–308 305

into 1.8 l glass jars and ventilated at 60 ml min−1

with humidified ethylene-free air.The source of 1-MCP was Ethyblock® (Floralife,

Burr Ridge, Illinois). One gram of Ethyblockpowder releases 1.83 ml 1-MCP gas at 20°C whendissolved in 20 ml of aqueous 2% KOH (Floralife,Ethyblock product specification sheet). Calculatedamounts of powder were weighed into 20 ml vialsand the vials placed in 10 l buckets. KOH solution(15 ml) was added to the vials and the buckets wereimmediately sealed. Ethylene-free air was obtainedby passing dried, compressed air through a series of4×5 l chambers filled with Purafil® (Purafil, At-lanta, Georgia).

Air streams containing 0.1 ml l−1 ethylene weregenerated by mixing metered flows of a standardcontaining 100 ml l−1 ethylene in nitrogen (BOCGases, Sydney) with air. Ethylene was monitored atthe inlet ports of representative glass jars using gaschromatography as previously described in Wills etal. (1999). Both air and the ethylene mixture werehumidified at 20oC by passing through 25 l plasticdrums 3

4 filled with water.The carbon dioxide level in the effluent gas

stream from each jar was measured daily by passingthrough an Infra Red Gas Analyser (Horiba,Kyoto). Calibrations were performed at regularintervals throughout the experiment using a stan-dard carbon dioxide gas mixture (BOC Gases,Sydney) and carbon dioxide-free air. Respirationrates were calculated from the carbon dioxideemissions, and the end of green life was determinedfor each banana by attainment of the respiratoryclimacteric. Fruit were then rated on a subjectivescale for colour development. The peel colour offruit was scored 1 (normal yellow colour), 2 (persis-tent green tip) and 3 (blotchy green and yellow).

The influence of fruit maturity and 1-MCPtreatment on green life was modelled using a mixedlinear regression approach (Searle, 1971) whichallowed the separation of variance components intofixed and random effects. Natural logarithms ofgreen life were taken to normalise the data. Analysisof loge (green life) was conducted using the REMLdirective in Genstat 5, Release 4.1, 3rd edition.

In a preliminary analysis, no difference wasfound between the green life of fruit from the

bunches with maturity levels of 156 and 173 days.Data from these two bunches were combined, sothat there were now two replications of a ‘mostmature’ maturity level, as well as the original ‘leastmature’ 71 days maturity.

The blocking strata are given by: bunch/hand/ba-nana, where bunch, hand and banana are factorswith 3, 3 and 14 levels respectively. The ANOVAdecomposition of the strata are as follows: bunch— maturity, residual; bunch.hand — age, residual;bunch.hand.banana — treatment, treatment.maturity, treatment.age, treatment.maturity.age,residual.

The model was given by

loge (green life)

=maturity+A+maturity.A+maturity.A.hand

+bunch+bunch.hand,

where the italicised terms were included in themodel as random effects and A=1-MCP treat-ment. Treatment effects were examined for signifi-cance using Wald tests (Rao, 1973), while treatmentmeans were compared using the least significantdifference (LSD) technique at the 5% level and thenback-transformed into original units.

The effect of bunch on the relationship betweenfinger diameter and hand number was modelledusing linear regression of grouped data also usingGenstat.

3. Results and discussion

All the terms fitted as fixed effects in the model(Maturity, 1-MCP treatment, Maturity×1-MCPtreatment and Maturity×1-MCP treatment×Hand) were found to be significant (PB0.001)(Table 1). Green life was affected by bunch matu-rity as well as 1-MCP treatment (Table 2). Fruitthat were not exposed to 1-MCP, and held contin-uously in ethylene-free air, took longer to ripen inthe ‘least mature’ bunch compared with the ‘mostmature’ bunches. Also, within the ‘most mature’bunches, the less mature hands lower on thebunches had a significantly longer green life thanthe ‘No. 1’ hands. Apart from this one case, there

D.R. Harris et al. / Posthar6est Biology and Technology 20 (2000) 303–308306

Table 1Wald tests for fixed effects of fruit maturity and 1-MCPconcentration on green life of ‘Williams’ bananas at 20oC

d.f.Fixed term x2 probabilityWald statistic

Maturity 1647.4 ***41372.4 ***1-MCP treatment

225.0 4Maturity.1-MCP ***treatment

52.0Maturity.1-MCP 20 ***treatment.Hand

1-MCP gave a 4-fold increase in green life, com-pared with the respective control fruit held in 0.1ml l−1 ethylene. In the ‘least mature’ bunch, theincrease in green life was in the order of 1.5-foldonly. This is a much higher level than 0.7 nl l−1

1-MCP for 24 h claimed to be effective by Sisler etal. (1996b). Comparison of fruit exposed to 500 nll−1 1-MCP and low ethylene with fruit held inethylene-free air showed that in the ‘most mature’bunches, the 1-MCP treatment was more effectivein extending green life than removing ethylenefrom the atmosphere (Table 2). The 1-MCP treat-ment was, however, not as effective as ethyleneremoval in the ‘least mature’ bunch where thegreen life was 1.5-fold longer in the fruit held inethylene-free air. The results demonstrate that inour study, the effectiveness of 1-MCP to extendthe postharvest life of bananas varied significantlyaccording to the maturity of the fruit. As fruitmatures and green life decreases, 1-MCP becomesrelatively more effective in delaying ripening al-though the absolute time to ripen still decreases.

1-MCP at 500 nl l−1 had a deleterious effect onfruit appearance, with uneven colouring of theskin of ripe fruit at a level considered unaccept-able for marketing (score\2) (Table 3). Onlythree of the nine hands of fruit were acceptableafter treatment with 500 nl l−1 1-MCP and subse-quent exposure to 0.1 ml l−1 ethylene. While theuse of 500 nl l−1 1-MCP was effective in prolong-

was no effect of hand number within any matu-rity×1-MCP treatment combination.

The relationship between finger diameter andhand number was dependent on fruit maturity.The following linear regression model accountedfor 60% of the variation in diameter: y=42.53090.337−0.99590.084x for bunches Aand B ; y=35.48790.498−0.42990.134x forbunch C, where y=finger diameter (mm) andx=hand number (1–7).

Green life was affected by 1-MCP only at thehighest concentration tested (Table 2). Exposureto 5 and 50 nl l−1 1-MCP had no significanteffect on the green life of any fruit from the samehand not treated with 1-MCP. There was a signifi-cant increase in green life due to 500 nl l−1

1-MCP. In the ‘most mature’ bunches, 500 nl l−1

Table 2Effect of fruit maturity and 1-MCP concentration on green life (Days) of ‘Williams’ bananas at 20oC1

Hand no. Ethylene free 2 After fumigation with 1-MCP (nl l−1)3Bunch maturity

0 5 50 500

‘Least Mature’ 1 50.4 jk4 23.0 ef 24.6 ef 26.3 ef 37.3 ghi39.3 hij27.3 f24.6 ef26.3 ef58.3 k4

58.9 k 27.6 f 28.0 f 29.6 fgh6 42.3 ij1 11.2 c 6.7 a‘Most Mature’ 6.5 a 7.1 ab 26.7 ef4 6.8 ab6.5 a16.7 d 27.3 f7.2 ab

6.8 ab20.5 de6 7.0 ab 8.9 bc 29.3 fg

1 Analysis performed on loge (Green life) and back transformed into original units (days).2 Not fumigated with 1-MCP and held continuously in ethylene-free air.3 1-MCP fumigation for 24 h followed by continuous exposure to 0.1 ml l−1 ethylene.4 Means with the same letter are not significantly different according to LSD at the 5% level. LSD=0.2832 of loge (Green life)

data.

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Table 3Blotchiness index of ripe banana fruit fumigated with 1-MCP and held in 0.1 ml l−1 ethylene until ripe

Blotchiness index1,2Fruit

Bunch Maturity3 Hand 0 5 50 5004

1 1.0A 1.0173 days 1.0 2.390.64 1.0 1.0 1.0 1.790.66 and 7 1.0 1.0 1.0 2.0

156 daysB 1 2.091.4 1.0 1.791.2 2.790.64 1.0 1.0 1.0 2.590.76 1.0 1.0 2.0 2.0

71 daysC 1 1.0 1.0 1.390.6 2.590.74 1.390.6 1.0 2.0 2.790.66 1.0 1.0 1.790.6 3.0

1 1, normal, yellow colour; 2, persistent green tip, 3, persistent green throughout the fruit.2 Values are mean9standard deviation. Where no standard deviations are shown, all data in the set were normal.3 Days after bunch emergence.4 1-MCP concentration (nl l−1).

ing green life of fruit of both levels of maturity,when the fruit eventually ripened its colour wasgenerally uneven and blotchy in appearance. Un-even ripening of ‘Williams’ bananas treated with1-MCP was also observed by Golding et al. (1998)when they used 45 ml l−1 1-MCP for 1 h. 1-MCPwould, thus, seem to have limited potential as acommercial treatment to delay the ripening of‘Williams’ bananas. This is not only due to un-even colour development, but also because in anycommercial consignment of fruit, there is a mix-ture of fruit maturity. Since the effectiveness of1-MCP to protect bananas from the effects ofexogenous ethylene depends on the maturity ofthe fruit, it is highly likely that a consignment of1-MCP-treated fruit in a ripening room wouldripen at different times, resulting in ‘mixed ripe’fruit.

It is known that in tomatoes, 1-MCP preventsthe accumulation of a number of mRNAs codingfor the expression of ACC synthase, ACC oxidaseand the ethylene receptor involved in positivefeedback regulation of autocatalytic (System 2)ethylene production (Nakatsuka et al., 1998). Thelonger ripening time of the ‘most mature’ fruitwhen treated with 500 nl l−1 1-MCP, comparedto fruit stored in ethylene-free air, suggests thatthe accumulation of mRNAs required for thetransition from system 1 to system 2 ethylene

production was blocked or retarded by 1-MCPand the fruit did not respond to the low level ofexogenous ethylene in this treatment. Fruit in theethylene-free control, however, had sufficientquantities of these mRNAs that even the very lowlevels of ethylene in this treatment were enough toquickly initiate ripening. Even so, there were sig-nificant differences in green life among hands inthe most mature bunches, consistent with thefindings of Liu (1976). The opposite effect in the‘least mature’ bananas, that is, a shorter ripeningtime when treated with 500 nl l−1 1-MCP com-pared to fruit stored in ethylene-free air, suggeststhat although both treatments were initially lesssensitive to ethylene than the ‘most mature’ fruit,the mRNAs required for the transition from sys-tem 1 to system 2 ethylene production were pro-duced by this fruit after 1-MCP treatment.Presumably a continuous exposure to exogenousethylene would stimulate the production of thesemRNAs, or prevent the accumulation of mRNAsinvolved in negative feedback regulation, thusreducing the effectiveness of this treatment com-pared with holding fruit in ethylene-free air.Golding et al. (1998) have suggested that whennew ethylene receptors are synthesised or are ca-pable of ethylene reception after 1-MCP treat-ment, a continuous presence of propyleneactivates the ethylene receptors.

D.R. Harris et al. / Posthar6est Biology and Technology 20 (2000) 303–308308

These findings demonstrate the need for regardto fruit maturity when quantifying the benefits of1-MCP treatment on bananas.

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