112

IntroductionEthylene is one of the simplest plant growth

regulators and is known to play a role in manyphysiological processes in plants. Seed germinationand growth, abscission, fruit ripening and senes-cence can all be affected by ethylene (1–4). It isbelieved that the ancient Egyptians and Chinesewere aware of some of these effects and attemptedto artificially control the ripening of figs and pears.More recently, in the nineteenth century, it wasobserved that street lighting (fuelled by town gaswhich contained low levels of ethylene) was caus-ing stunting and other changes to nearby plantgrowth (2). Today, although it is well documentedthat almost all plants produce ethylene in varyingamounts, understanding how ethylene interactswith each plant type and the effect this has onplant development is a very active area of scientific research.

The ability to control ripening is very importantto the sale of many fresh produce types. Whilstconsumers often want to purchase food that canbe eaten straight away, suppliers may prefer to

market fruit in need of further ripening, to avoidlosses due to over-ripening in transit or storage.Therefore, the control of the ambient ethyleneconcentration is key to prolonging the shelf-life ofmany horticultural products.

Various methods of ethylene control areoffered commercially, including several based onethylene adsorption/oxidation. However, thesetechnologies have seen limited uptake, especially inretail packaging and fresh produce transportation.In the present paper, we report on the developmentof a new palladium (Pd)-based ethylene adsorber,which works effectively both in synthetic gasstreams and in laboratory-based trials using realfruit samples. We are now in a position to under-take ‘real-world’ testing.

Ethylene Production by Fresh ProduceFresh fruit and vegetables are generally classi-

fied in one of two ways, depending upon themechanism of ripening (5) and their capacity toproduce ethylene (Table I). Climacteric itemsrelease a burst of ethylene during ripening, accom-

Platinum Metals Rev., 2009, 53, (3), 112–122

A New Palladium-Based EthyleneScavenger to Control Ethylene-InducedRipening of Climacteric Fruit By Andrew W. J. Smith*, Stephen Poulston and Liz Rowsell Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.; *E-mail: smithawj@matthey.com

Leon A. TerryPlant Science Laboratory, Cranfield University, Cranfield, Bedfordshire MK43 0AL, U.K.

and James A. AndersonSurface Chemistry and Catalysis Group, Department of Chemistry, University of Aberdeen, Aberdeen B24 3UE, U.K.

A novel palladium-promoted zeolite material with a significant ethylene adsorption capacityat room temperature is described. It was characterised by diffuse reflectance infrared Fouriertransform spectroscopy (DRIFTS) and transmission electron microscopy (TEM) to showpalladium particles dispersed over the support. Initial measurements of the ethylene adsorptioncapacity were conducted with a synthetic gas stream at a higher ethylene concentrationthan would normally be encountered in fruit/vegetable storage, in order to obtain an acceleratedtesting protocol. Further laboratory-based trials on fruit samples show that the palladium-promoted zeolite material can be effective as an ethylene scavenger to prolong the shelf-lifeof fresh fruits.

DOI: 10.1595/147106709X462742

panied by an increase in respiration, whereas non-climacteric produce do not vary their rate ofethylene production in this fashion. Exogenousethylene (typically < 0.1–1.0 μl l–1) can initiateripening in many climacteric fruit which then canlead to autocatalytic production of ethylene by thefruit. Climacteric fruits, which include bananas,avocados, nectarines and pears, ripen after harvest,typically by softening, changing colour and becom-ing sweeter (6). The latter class, includingstrawberries, grapes and pineapples do not ripen asdramatically after harvest, but rather senesce, lead-ing to discolouration, unpleasant odour, shrinkageand general rotting. In such cases, the challenge tomaintain product quality is paramount. Other freshproduce types are sensitive to ethylene eventhough their ethylene production is very low forexample, potato tubers, bulb onions, broccoli andspring cabbage, along with some cut flowers.

To artificially reproduce the natural ripeningprocess, ethylene can be introduced during storage.This process is used in the fruit industry on freshproduce such as bananas, avocadoes and mangoes(7). As well as such deliberate exogenous introduc-tion, the level of ethylene can rise due to ethyleneemission from stored fresh produce or from acci-dental sources such as forklift truck exhaustemissions. For this reason, the industry prefers touse electric vehicles in fresh food storage areas tominimise the risk of the fresh produce coming intocontact with ethylene gas.

Excessive or uncontrolled levels of ethylene canresult in a number of problems. For example, thepremature ripening of fruits and vegetables, thefading and wilting of cut flowers and the loss ofgreen colour and increase in bitterness of vegeta-

bles are common problems when ethylene levelshave not been properly managed.

Ethylene-Sensitive Plants Plants, flowers and buds are also sensitive to

ethylene (see Table II for examples) and somerelease ethylene gas when they are cut or damaged.Some of the most detrimental effects of ethyleneon plants are:– Partial or incomplete flower abortion;– Retarded plant growth;– Growth abnormalities such as excessive leafi-

ness or the stimulated growth of daughterbulbs;

– Shortened vase lifespan of cut flowers (abscis-sion of leaves and flower petals);

– Inhibited development of immature (unopened)flower buds;

– Accelerated senescence of all types of plants;– Susceptibility to disease.

Ethylene Removal TechnologiesIn many instances, ethylene concentrations can

be controlled by ventilation of the area containing

Platinum Metals Rev., 2009, 53, (3) 113

Table I

Ethylene Production from Different Fresh Produce Types

Low Moderate High Very high (< 1.0 ml kg–1 h–1) (1–10 ml kg–1 h–1) (10–100 ml kg–1 h–1) (> 100 ml kg–1 h–1)

Pineapple, artichoke, Banana, mango, Apricot, nectarine, Apple, avocado,cauliflower, broccoli, plum, tomato pear, peach cherimoya,date, orange, rhubarb, passion fruitspinach, beetroot,green asparagus, celery, lemon, onion

Table II

Different Degrees of Ethylene Sensitivity amongstCut Flower Species

Low Moderate High Sensitivity Sensitivity Sensitivity

Tulip, Lily, freesia, Carnation,daffodil agapanthus, Geraldton

alstroemeria, waxfloweranemone,dahlia

the fresh produce. This can be aided by containerdesigns that allow for air circulation. However,ventilation is not always appropriate; for example,it cannot be used in sealed environments, such ascontrolled atmosphere storage or retail packaging.There are a number of ethylene removal technolo-gies available:– Catalysts. Often based on platinum/alumina,

these operate at elevated temperature(> 200ºC) and catalytically oxidise ethylene tocarbon dioxide (CO2) and water (8). There arealso reports of the use of photocatalytic oxida-tion of ethylene using titanium dioxide (TiO2),which can occur at room temperature (9).

– Stoichiometric oxidising agents. Mostly based onpotassium permanganate (KMnO4), whichagain oxidises ethylene and is itself reduced.Though some CO2 and water is produced,some partially oxidised species such as car-boxylic acids may also be formed.

– Sorbents. These materials work by sorption ofthe ethylene and are often based on high sur-face area materials, including activated carbon,clays and zeolites.

Ethylene Blocking TechnologiesA different approach is to inhibit ethylene

action in the produce itself, which can in turnreduce the amount of ethylene released by the pro-duce into the container or storage area. Severalchemicals have been shown to act as ethyleneinhibitors, including both volatile and aqueoustreatments:–– 1-Methylcyclopropene (1-MCP). 1-MCP is the most

widely used commercial volatile ethyleneinhibitor, which blocks ethylene binding sites.It is applied exogenously as a gas and has beenwidely applied to fruit (particularly apples) andflowers. 1-MCP is sold commercially asSmartFreshSM (10) into the fresh produceindustry and as EthylBlockTM into the floralindustry.

–– Silver thiosulfate (STS). The use of this material islargely restricted to cut flowers and it is soldcommercially under the trade name ChrysalAVB®. It is applied by putting the cut flowerstems in a solution containing the STS.

–– Aminoethoxyvinylglycine (AVG). This material issold commercially as ReTain® and acts as aplant growth regulator by blocking the produc-tion of ethylene in the plant tissue. It is sprayedonto the fruit, usually 1 to 3 weeks prior to harvesting.

New Palladium-Based EthyleneScavenging Technology

In this paper, we report on the discovery byJohnson Matthey scientists of a novel palladium-promoted material with a significant ethyleneadsorption capacity at room temperature (11). Awide range of materials were synthesised andscreened for activity. Pd gave by far the best per-formance of the promoter metals tested. Thematerial is a palladium-impregnated zeolite givingfinely dispersed palladium particles.

Initial testing was conducted with a syntheticgas stream at a higher ethylene concentration thanwould normally be encountered in fruit/vegetablestorage, in order to obtain an accelerated testingprotocol for measuring ethylene adsorptioncapacity. Ethylene adsorption capacity measure-ments were carried out at room temperature(21ºC) in a plug flow reactor using 0.1 g of activePd-based material with a gas composition of 200μl l–1 ethylene, 10% (v/v) oxygen balanced withhelium, at a flow rate of 50 ml min–1, with andwithout ca. 100% relative humidity (RH). Reactoroutlet gas concentrations were analysed using aThermo Onix ProLab mass spectrometer(Thermo Onix, Houston, Texas, U.S.A.). Amass:charge ratio of either 26 or 27 was used forethylene, as the use of nitrogen as a diluent gasleads to the presence of a large peak at m/z = 28.A value of m/z = 44 was used for CO2. Ethyleneuptake capacity was measured using a simple‘breakthrough’ measurement, in which the totalintegrated ethylene removal was determined afterthe outlet ethylene concentration from the reactorhad reached the inlet ethylene concentration,showing that the adsorber was saturated with ethylene (12).

The Pd-based material typically removed allmeasurable ethylene until breakthrough occurred.A typical example of an ethylene breakthrough

Platinum Metals Rev., 2009, 53, (3) 114

measurement under humid conditions is shown inFigure 1. Under these conditions the Pd-promotedmaterial was found to have an ethylene adsorptionof 4162 μl g–1 under ca. 100 % RH. This perfor-mance was increased to 45,600 μl g–1 under dryconditions.

Further experiments were carried out at roomtemperature in a sealed, unstirred batch reactor(0.86 l) with 0.1 g active Pd-based material and aninitial gas composition of 550 μl l–1 ethylene, 40%(v/v) air balanced with argon. Selected gas concen-trations were measured at hourly intervals with aVarian CP-4900 Micro-GC (Varian, Inc, Palo Alto,California, U.S.A.). Gas samples (40 ms duration)were taken via an automated recirculating samplingsystem. Column and injector temperatures were setat 60ºC and 70ºC, respectively. The 0.15 mm diameter, 10 m long column was packed withPoraPLOT (porous layer open tubular) Q station-ary phase. Ethylene and CO2 were calibratedagainst 10 μl l–1 ethylene balanced with air and 5%(v/v) CO2 balanced with Ar (Air Products Europe,Surrey, U.K.). A thermal conductivity detector wasused with He carrier gas at 276 kPa inlet pressure.Peak integration was carried out within the VarianSTAR software.

Under these conditions, CO2 and ethane pro-duction were observed, as plotted in Figure 2.The ethane is likely to be produced by the hydro-genation of adsorbed ethylene, the hydrogen beinggenerated by the partial dissociation of ethylene.

The selectivity to ethane varies with experimentalconditions but is typically no more than the maxi-mum value of ~ 10% observed under theconditions tested here. Ethane can be produced byplants in response to stress (13) and has not previ-ously been reported to be detrimental to plants inthe concentration range reported here. It is clear,however, that the palladium-based material is act-ing largely as an adsorber rather than as a catalyst.The mechanism of reaction is discussed further inthis article by interpretation of diffuse reflectanceinfrared Fourier transform spectroscopy (DRIFTS)data.

Characterisation of the palladium-based mater-ial was carried out to determine the Pddistribution through the support. Transmissionelectron microscopy (TEM) analysis (Figure 3)showed that the palladium particles (bright parti-cles) are dispersed over the support material. Theaverage size of the palladium particles in Figure 3was calculated to be 1.7 nm. Scanning electronmicroscopy (SEM) analysis (not shown) also iden-tified some larger palladium particles withdiameters around 20 to 40 nm. These results cor-respond well with CO chemisorption data whichgave a metal dispersion of ~ 15 %. From this dis-persion value, a slightly larger average size of thepalladium particles would be expected. This isconsistent with the observation of many small andsome larger particles by TEM (Figures 3 and 4)and SEM.

Platinum Metals Rev., 2009, 53, (3) 115

200

150

100

50

0 500 1000 1500 2000 2500 3000 3500

Eth

yle

ne c

oncentr

ation,

μl l–

1

Time, s

Fig. 1 Ethylenebreakthrough graph of thePd-based material in humidgas conditions. Ethylene wasmonitored using the mass 26signal on a massspectrometer (mass 26 and27 gave very similarbreakthrough profiles)

Ethylene–Metal Interaction viaDRIFTS Analysis

In order to gain further information regardingthe processes involved in ethylene removal, avibrational spectroscopic study was performed to probe the interactions between the ethyleneand the Pd-promoted scavenger. DRIFTS wasused to characterise the species adsorbed on theethylene scavenger after exposure to ethylene, inthe presence and absence of oxygen and watervapour.

DRIFTS allows spectra to be obtained of pow-dered samples in the presence of gaseousatmospheres. It produces infrared spectra of the

diffusely scattered radiation in the reflectancemode (Figure 5), which are analogous to those col-lected in the more conventional transmittancemode. Disadvantages of this method include theappearance of artefacts resulting from the collec-tion of specularly reflected radiation. It is alsodifficult to perform quantitative studies, since bothscattering and absorption coefficients must beconsidered. By contrast, only the latter is requiredto quantify adsorbed species when spectra are col-lected in the transmittance mode and converted toabsorbance (14). The key advantages of thismethod are that no sample preparation is neces-sary and that the cells can be operated in plug flowmode with the reactant gases being forced to trav-el through the bed of powdered scavengermaterial.

Commercial DRIFTS cells are readily availablewhich allow the collection of spectra of powderedsamples at ambient or elevated temperatures,while controlling the composition of the gaseousatmosphere. In the study performed here, the out-let port of the DRIFTS cell was coupled to aquadropole mass spectrometer (QMS) via a heated,glass-lined capillary. This permitted continualmonitoring of ethane (m/z = 30), carbon dioxide(m/z = 44) and ethylene (a value of m/z = 27 wasselected for this experiment) during exposure toethylene and during subsequent temperature-pro-grammed desorption (TPD) measurements.Experiments were performed by exposing samples

Platinum Metals Rev., 2009, 53, (3) 116

Time, h

Conce

ntr

ation o

f C

O2

and e

thyle

ne,

μl l–

1

Concentra

tion o

f eth

ane, μ

l l–

1

4 8 120

0

100

200

300

400

500 100

80

60

40

20

0

Ethylene (Pd scavenger)

Carbon dioxide

Ethane

Fig. 2 Gasconcentrationsin a batchreactor initiallycontaining 550 μl l–1

ethylene*,along with 0.1g of thePd-promotedethylenescavenger

*Note: Someethylene hasbeen removedby thescavengerprior to thefirst injection

10 nm

Fig. 3 TEM image of the Pd-promoted zeolite materialshowing nanometre size palladium particles (brightareas) on the zeolite support

with and without Pd, at ambient temperature, to aflow of ethylene in nitrogen with the addition ofeither no other gases, air or air and water. After aperiod of exposure to these gaseous atmospheres,the ethylene stream was switched off and a TPDmeasurement was performed in the presence ofthe remaining gases. Spectra were also obtained ofsamples that had been exposed to ripened fruit.

The Fourier transform infrared (FTIR) spec-trum of ethylene in nitrogen flowing through thecell, in the absence of scavenger, gave main fea-tures at 3112 cm–1 (ν9, νCH asym.), 2992 cm–1

(ν11, νCH sym.), 2048 cm–1 (2 × ν10, γCH2 rock-ing), 1889 cm–1 (2 × ν7, δCH2 out-of-plane) and1446 cm–1 (ν12, δCH2 in-plane). The absence ofbands for the IR inactive (totally symmetrical)modes at 1623 cm–1 (ν2, νC=C) and 1343 cm–1

(ν3, δCH2 in-plane) should be noted, although it isexpected that any interaction with the adsorbentmight permit these modes to be detected as thesymmetry is lifted. These interactions might alsobe expected to shift the frequencies away from thegas phase values listed above.

FTIR spectra of a sample exposed to fruit(Figure 6(a)) showed features at 1467, 1460, 1434and 1382 cm–1. FTIR spectra of the Pd-free zeoliteshowed only very weak features, including a bandat 1439 cm–1 which could be assigned to the pres-ence of adsorbates resulting from exposure toethylene (Figure 6(c)). However, these featureswere absent following heating to 50ºC in air. Thenarrow shape of the feature at 1439 cm–1, and theappearance of a single rather than a double compo-nent (as in the gas phase), confirms that thisfeature can be assigned to an adsorbed state,although the limited shift (Δν = 7 cm–1) withrespect to the gas phase feature (due to the ν12,δCH2 in-plane) would suggest a very weakly boundstate. This was confirmed by the disappearance ofbands at this frequency when the sample was heat-ed in air to 50ºC. Note that this ease of removalwas not the case for the sample containing Pd(Figure 6(b)). This, along with the detection ofother infrared bands (at 1467 and 1382 cm–1) whenPd was present, would indicate that the metalplayed a significant role in the retention of theadsorbate and in the dominant adsorbed state. Therelative intensities of the features indicate that the

Platinum Metals Rev., 2009, 53, (3) 117

Num

ber

of P

d p

art

icle

s

Pd particle size, nm

0.5 1 1.5 2 3.52.5 3 4 4.5 5 >50

40

60

80

20

100

120

140

Fig. 4 Particle size distribution ofpalladium in the palladium-promotedzeolite material, as determined from theTEM image in Figure 3

Incident beamDiffuse reflectance

Sample

Fig. 5 Scheme showing the scattering of light from apowdered sample and the collection of the diffuselyscattered component for DRIFTS analysis

presence of Pd was essential in order to achievethe 45,600 μl g–1 adsorption capacity obtainedunder dry conditions. The similarities between thefeatures resulting from exposing the sample tofruit (Figure 6(a)) and to exogenous ethylene, inparticular the features at 1467 and 1382 cm–1, arestrong evidence that similar adsorbed species existin both cases. Under these conditions, the C–Hstretching region gave two dominant bands at2969 and 2865 cm–1, which are consistent withexpectations for CH3 species (Figure 7).

The transformation from C2H4 to adsorbedspecies containing CH3 would be consistent withthe development of ethylidyne (CCH3) species (15,16). In these species, the carbon forms three bondsto the surface metal atoms (Figure 8(c)), probablyon three-fold hollow sites of Pd(111) type facets(17). Such an assignment would be consistent withthe detection of the 1467 and 1382 cm–1 band pair(Figure 6) arising from the corresponding CH3

deformation modes. The formation of a hydrogenatom for each ethylidyne species generated would

Platinum Metals Rev., 2009, 53, (3) 118

180 0 1 600 1400 1200

Refle

cta

nce/%

W a venu mb er/cm-1

20%

(a)

(b)

(b)

(a )(c)

(c)

Wavenumber, cm–1

Reflecta

nce, %

20 %

1800 1600 1400 1200

(a)

(b)

(c)

(a)

(b)

(c)

Fig. 6 DRIFT spectra of: (a) the Pd-promoted zeolitematerial following exposure tofruit, (b) the Pd-promoted zeolitematerial exposed to a flow ofethylene/nitrogen at 25ºC, (c) Pd-free zeolite exposed toethylene in nitrogen at 25ºC

3200 3 150 310 0 3050 30 00 2950 2900 28 50 2800 2 750 2700

Reflecta

nce/%

W ave num be r/cm-1

(a )

(b)

(c)

1 0%

Wavenumber, cm–1

Reflecta

nce, %

10 %

(a)

(b)

(c)

3200 3150 3100 3050 3000 2950 2900 2850 2800 2750 2700

Fig. 7 DRIFT spectra of thePd-promoted ethylene scavenger: (a) before exposure to ethylene,(b) after exposure to a flow ofethylene in nitrogen at 25ºC, (c) after exposure to a flow ofethylene in air at 25ºC

explain the formation of ethane following initialexposure of the scavenger to ethylene (Figure 2),which was observed in the mass spectrometry (MS)trace (m/z = 30) recorded during the FTIR mea-surements. The surface selection rule states thatvibrations parallel to a metal surface should not bedetected, due to the creation of an opposing imagedipole on the metal. This means that the νCH3

asymmetric and δCH3 asymmetric modes at 2969and 1467 cm–1 should be absent from the spec-trum. However, this rule will be relaxed for verysmall metal crystallites of the order of 2 nm andbelow and where extended flat facets are absentdue to the presence of steps and edges.

Such a description of the morphology is consis-tent with the TEM image presented (Figures 3 and4). This assignment cannot be confirmed from theexpected absorption at around 1130 cm–1 (νC–C),due to absorption by the support in this region.Bands due to ethylidyne were diminished in spec-tra recorded after heating the sample in nitrogen to150ºC. If air was present during this thermal treat-ment, then CO2 was detected by MS as a completeoxidation product. Below this temperature, FTIRevidence for partial oxidation included the detec-tion of carboxylate type species and a band at 2125cm–1 due to CO adsorbed at surface oxidised Pdsites. The latter was detected in spectra of the sam-ple recorded at 100ºC, indicating that the onset ofoxidation took place well below the temperature ofcomplete oxidation.

An MS trace was recorded during the collectionof spectra with exposure to ethylene. This showeda breakthrough shape which was strongly depen-dent on the composition of the gas phase, with thegreatest removal of ethylene occurring during treat-ment in nitrogen. The presence of air or air and

water reduced the adsorption capacity.Additionally, the presence of air or air and watermodified the predominant modes of adsorption,with four bands now appearing in the C–H stretch-ing region. In addition to the pair at 2957 and 2865cm–1 which have already been assigned to the CH3

modes of ethylidyne in the absence of air or water,a further pair at 2934 and 2853 cm–1 were detected,although these were very weak when water waspresent. These features are consistent with expec-tations for CH2 groups, although the vibrationalfrequencies are relatively low for vinyl species. Di-σ species of adsorbed ethylene (Figure 8(b)) givelower frequency CH modes than π-C2H4 (18)(Figure 8(a)), although the former are less favouredin the presence of adsorbed oxygen (18). The pres-ence of oxygen in the system also led to theappearance of a species giving a maximum at 1514cm–1, which was absent for the air-free system.

A similar feature was found at 1510 cm–1 inelectron energy loss spectroscopy (EELS) spectraof ethylene on oxygen covered Pd(100). This wasassigned to δCH2, so it would be tempting toassign the surface species to vinyl intermediatessuch as HCCH2 (17). However, an alternativeassignment, consistent with the known stepwisedehydrogenation of ethylene (19), is that the addi-tional maxima at 2934 and 2853 cm–1 representvibrations due to the CH3 groups of ethylidene(CHCH3), where the expected full conversion toethylidyne, observed in the absence of air andwater (18), is hindered due to the presence of theco-adsorbates which limit the activation and dissociation of the C–H bond. The lesser extent ofC–H dissociation at room temperature, and subse-quently the lesser population of the surface byadsorbed hydrogen, would explain the reduced

Platinum Metals Rev., 2009, 53, (3) 119

(a) (e)(b) (c) (d) (f)

Pd Pd Pd Pd Pd Pd

Fig. 8 Potential adsorbed species following exposure of the Pd-containing scavenger to ethylene: (a) π-bonded, (b) di-σ bonded, (c) ethylidyne, (d) ethylidene, (e) vinylidene and (f) vinyl

ability of the surface to liberate ethane resultingfrom ethylene hydrogenation during the initialexposure stages.

In summary, the surface speciation is some-what dependent on the gas mix and the moisturecontent. However, the DRIFTS study has identi-fied the key role of the palladium–ethyleneinteraction and the benefit that this has on bindingethylene to the zeolite surface.

Fresh Produce ResearchThe use of platinum group metals including pal-

ladium in different forms and on different supportsfor ethylene removal from fruit and vegetables hasbeen investigated by other authors in the past (20,21). Bailén et al. at University Miguel Hernándezhave recently published results on delaying tomatodecay using a combination of controlled atmos-phere packaging and a granular-activated carbon(GAC) or GAC impregnated with a palladium-based catalyst (22). Martínez-Romero et al. at thesame University have also reported the use of a car-tridge heater device (optimally running at 175ºC)joined to activated carbon containing palladium forethylene removal above room temperature (23).

In contrast, the new material described hereconsists of a specific combination of a preciousmetal with a zeolite and removes significantamounts of ethylene at low temperature (5ºC) androom temperature. In order to test the materialunder realistic conditions, the active material hasbeen evaluated in collaboration with the PlantScience Laboratory at Cranfield University, U.K.Bananas were included in the initial studies, as therole of ethylene in initiating ripening in these fruits

is well documented (24, 25). Initial findings, nowpublished (2), have demonstrated for the first timethat the presence of a palladium-based scavengerwas effective at removing ethylene to below phys-iologically active levels for preclimacteric greenbananas and green avocado fruits.

Reduced CO2 production and control of thecolour change from green to yellow was observedfor the preclimacteric bananas (Figure 9). The palladium-promoted ethylene scavenger was alsofound to be far superior to a KMnO4-based ethyl-ene adsorber when used in low amounts at highrelative humidity. No adverse effects on fruit qual-ity or subsequent ripening were observed afterremoval of the ethylene scavenger material.

Similar experiments were also conducted onavocados. Results showed that exogenous andendogenous ethylene concentration was reducedsignificantly with increasing amounts of the Pd-promoted material. In the presence ofPd-promoted material, ethylene was removed tobelow physiologically active levels. The effect ofethylene on the colour of avocado cv. Hass fruitsin the presence or absence of the palladium mate-rial is shown in Figure 10. Fruit held in thepresence of 100 mg or 1000 mg of the Pd-promot-ed material for three days were generally greener,and thus less ripe, than control fruit after seven toten days (Figure 10).

Furthermore, when avocados were treated withethylene and then subsequently held in the pres-ence of the Pd-promoted material (1000 mg) afterday 1, ethylene was removed to below physiologi-cally active levels. Despite the climacteric phasehaving been initiated for these fruits, the

Platinum Metals Rev., 2009, 53, (3) 120

Fig. 9 Colour of a five-day-old banana cv. Cavendish fruit previously held for three days at 16ºC in 3 l sealed jarscontaining the Pd-promoted ethylene scavenger material (0–50 mg) and previously treated with (+E) or without (–E)100 μl l–1 ethylene when at the preclimacteric stage (i.e. green) at day 0

–E

+E

0 mg 50 mg 40 mg 30 mg 20 mg 10 mg

subsequent total removal of ethylene resulted inbetter maintenance of fruit firmness as comparedto controls. This suggests that the normal andexpected climacteric respiratory rise has been dis-rupted. Therefore, for the first time an ethylenescavenger has been shown to be capable of extend-ing shelf-life even when the climacteric respiratoryrise has already been initiated.

Current work is focused on comparing the effi-cacy of the Pd-promoted ethylene scavenger to1-MCP for the control of ripening in avocado fruit,and the resultant effect on non-structural carbohy-drates and fatty acid methyl esters.

ConclusionsThe results from this study demonstrate that

the Pd-promoted ethylene scavenger described

here is effective for the control of ethylene to pro-long the shelf-life of climacteric fresh producesuch as bananas and avocados. The material hasthe potential to be used commercially, as an alter-native and/or supplemental treatment to 1-MCP.The technology does not require elevated temper-ature to remove ethylene, and the choice of zeolitesupport material makes it suitable for most freshproduce and floral applications under conditionsof high humidity and low or room temperature.Future research will elucidate further uses of thePd-promoted ethylene scavenger.

AcknowledgementsThe authors would like to thank Anglo

Platinum for sponsoring the research and develop-ment in this area.

Platinum Metals Rev., 2009, 53, (3) 121

+E

–E

0 mg 100 mg 1000 mg*1000 mg

Fig. 10 Colour of a seven-day-oldavocado cv. Hass fruit previouslyheld for three days at 12ºC in 3 lsealed jars containing thePd-promoted ethylene scavengermaterial (0, 100 or 1000 mg) andpreviously treated with (+E) orwithout (–E) 100 μl l–1 ethylenewhen at the preclimacteric stage (i.e. green) at day 0* The Pd-promoted material (1000 mg) was put into the jars after day 1, following treatment ofthe fruit with or without 100 μl l–1

ethylene

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The Authors

Dr Stephen Poulston is a PrincipalScientist at JMTC. His main interests liein heterogeneous catalysts andsorbents.

Dr Elizabeth Rowsell is a ResearchManager at JMTC. She is responsiblefor managing the Ethylene Scavengerresearch project and developing newapplications for platinum group metals.

Dr Andrew W. J. Smith is a PrincipalScientist at the Johnson MattheyTechnology Centre (JMTC), SonningCommon, U.K. He is a syntheticinorganic chemist with experience inmaterials, glass technology and catalystpreparation. He is interested indeveloping new materials and theirapplications in heterogeneous catalysis.

Dr Leon Terry is a Senior Lecturer inPlant Science and Head of the PlantScience Laboratory at CranfieldUniversity, U.K. The Plant ScienceLaboratory is one of the largestuniversity-based groups in the EUdedicated to research, consultancy andeducation in post-harvest technology offresh produce.

Professor Jim Anderson is Head of Chemistry (Research) at theUniversity of Aberdeen, U.K. His main interests lie in the areas ofsupported metal catalysis and mixed oxide-based acid catalysis andin the use of infrared spectroscopy for site quantification andinterpretation of selectivity effects in catalysis.