Depletion of 1-MCP by ‘non-target’ materialsfrom fruit storage facilities

Fernando Vallejo, Randolph Beaudry ∗

Postharvest Technology and Physiology Laboratory, Department of Horticulture, Michigan State University, East Lansing, MI 48824, United States

Abstract

The sorption of 1-methylcyclopropene (1-MCP) by a number of ‘non-target’ materials found in apple (Malus × domestica Borkh.) and pear(Pyrus communis L.) fruit storages was measured as a function of time, temperature, and moisture content. Materials included bin constructionmaterials [high density polyethylene (HDPE), polypropylene (PP), extensively- and slightly-weathered oak (Quercus sp.), weathered fir (Abiessp.) plywood, and cardboard] and wall surface materials (polyurethane foam and cellulose-based fire retardant). Bin construction materialstest pieces had an external surface area of 180 cm2. We placed ‘non-target’ materials in 1-l glass jars and added 1-MCP gas to the headspaceat an initial concentration of approximately 30 �l l−1. Gas concentrations were measured after 2, 4, 6, 8, 10, and 24 h. The concentration of1H2rbbal

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-MCP in empty jars was stable for the 24-h holding period. There was little to no sorption by HDPE, PP, polyurethane foam, or fire retardant.owever, plywood, cardboard, slightly-weathered oak, and extensively-weathered oak absorbed 16, 18, 55, and 75% of the 1-MCP after4 h. Moistening the test material increased the rate of sorption of 1-MCP for cardboard, plywood, weathered oak, and non-weathered oak,esulting in a depletion of approximately 98, 70, 98, and 98%, respectively, in 24 h. For oak bin material, the rate of sorption was not impactedy temperature and increasing the surface area by approximately 100% only marginally increased the rate of sorption. When moistened oakin material was included with apple fruit in a proportion similar to that found in fruit storage, 90% depletion occurred in 6 h compared topproximately 80% in 24 h for fruit alone. The data suggest that 1-MCP levels can be compromised by wooden and cardboard bin and bininer materials, but not by plastic bin materials or wall surface materials commonly used in Michigan.

eywords: 1-Methylcyclopropene; Apple; Pear; Sorption; Growth regulator; Ethylene

1-MCP is a gaseous, four-carbon cyclic olefin that inhibitsthylene action at the nl l−1 level (Blankenship and Dole,003). 1-MCP is presumed to tightly bind to ethylene bind-ng sites of ethylene receptors in target plant material, therebyreventing ethylene action (Ciardi and Klee, 2001). 1-MCPs used internationally in the apple industry to improve reten-ion of textural and taste quality attributes and is currentlyeing evaluated for its commercial potential for a number ofther crop species. 1-MCP is released as a gas from a formu-ated cyclodextrin powder through aqueous dissolution of theyclodextrin and release of the contained gas (Blankenshipnd Dole, 2003).

Jayanty et al. (2004) determined that the minimumconcentration resulting in near saturation of the ethyleneinhibitory response for ‘Redchief Delicious’ apples to beapproximately 0.1 �l l−1 from 0 to 20 ◦C when doses wereapplied on a weekly basis. DeEll et al. (2002) and Dauny andJoyce (2002) determined that approximately 6 h exposure areneeded following a single initial dose of 1 �l l−1 to saturatethe ethylene inhibitory response in apple.

The importance of maintaining labeled rates of 1-MCPduring the exposure period is not fully understood. Interest-ingly, full label rates differ from country to country. In theUnited States and Canada, the labeled treatment dosage forapple is 1 and 0.6 �l l−1, respectively. These dosages wereproscribed, in part, to supply 1-MCP at a concentration suf-ficient to saturate the response of the plant material (PestManagement Regulatory Agency, Health Canada, 2004).

Despite the numerous studies describing plant responsesto applied dosages (Blankenship and Dole, 2003), actualtreatment concentrations over the course of the treatmentduration following a single pulse of 1-MCP have not beenpublished. It is possible that applied concentrations may notbe stable and, in fact, may be altered by the plant materialand/or the storage environment itself.

The responsiveness of apple to 1-MCP varies with cultivar.‘Gala’, ‘Ginger Gold’ and ‘McIntosh’, for instance, differ inthe degree and duration of the response to 1-MCP (Fan etal., 1999; Watkins et al., 2000). The beneficial textural andphysiochemical responses of ‘McIntosh’ are brief relative toother apple cultivars and sometimes absent. The reason forthis is unknown, but may be related to barrier properties ofthe skin, elevated internal ethylene concentrations, or dif-ferences in the physiology and ethylene biology relative tomore responsive cultivars. Jayanty et al. (2004) found therange of effective, but sub-saturating 1-MCP doses for ‘Red-chief Delicious’ apple is between 0.01 �l l−1, which providedno observable textural benefit, and 0.1 �l l−1, which yields anear-saturating response. The extent to which minimum satu-rating doses differ between apple cultivars is not fully known.

There are situations in which saturating the response to1-MCP is not desirable, but rather, it would be preferableto obtain short-term or only partial responses (Calvo andSozzi, 2004; Ekman et al., 2004). This is more importantfprraep

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MCP during the 24-h treatment period currently proscribedby the SmartFresh label.

1. Materials and methods

1.1. Gas analysis

1-MCP, ethylene, and 1-butene measurement was by gaschromatography. For each olefin, a 1-ml sample was injectedinto a gas chromatograph (Carle Series 100 AGC) equippedwith a flame ionization detector. The stainless steel col-umn (2 mm i.d. × 2 m) was packed with 60/80 ChromosorbOV-103 (Alltech Associates Inc., Deerfield, IL) and held at140 ◦C; the flow rates for the carrier gas (He), H2, and air wereapproximately 50, 50 and 200 ml min−1, respectively. Thelower detection limit for 1-MCP was approximately 12 nl l−1.Ethylene concentration was determined by comparison to acertified standard (Matheson Gas Products, Chicago, IL). Agas standard of 1-butene was used to calculate the concen-tration of 1-MCP. To create a 10 �l l−1 1-butene standard,43-�l of pure 1-butene (Matheson Gas Products, Chicago,IL), was injected into a 4.3-l specially made glass chamberfitted with a Mininert valve (Supelco, Bellefonte, PA) as pre-viously described (Jayanty et al., 2004). It was assumed thatthe response factor for 1-MCP and 1-butene were similar.

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or pear than for apple since saturating levels of 1-MCP onear can cause excessive delay of ripening and alter ripening-elated developmental processes sufficiently to significantlyeduce product quality. Ekman et al. (2004) demonstrated thatconcentration of 0.5 �l l−1 on ‘Bartlet’ pear delayed soft-

ning and color development, while exposure to 1.0 �l l−1

revented normal softening even after 24 weeks in storage.For application situations in which the full label rate for

-MCP is not significantly above the minimal saturating con-entration for a plant material or for applications within theariable dose–response range, loses of 1-MCP during thexposure period could compromise the response to 1-MCP.

hile losses due to leaking of the treatment room atmospherean be addressed, little is known of the potential for lossesf 1-MCP due to absorption or adsorption at non-specific,on-target sites in the fruit itself or in the treatment rooms.ignificant sorption of 1-MCP by non-target materials has

he potential to compete with the ethylene receptors in theruit and thereby marginalize 1-MCP effectiveness. There haseen no previous report published about sorption of 1-MCPy the materials that accompany apples and pears inside theefrigerated or controlled atmosphere facilities typically useds treatment rooms.

We tested the sorptive capacity of a number of materialsound in apple storage rooms (bins and walls) and evaluatedhe impact of temperature and moisture on the rate of sorption.he capacity of oak to absorb the non-cyclic olefins ethylenend 1-butene was also tested. Our objective was to determinehe extent to which various materials absorbed 1-MCP andheir potential to compromise treatment concentrations of 1-

.2. ‘Non-target’ materials

Bin materials tested included plastic [high densityolyethylene (HDPE) and polypropylene (PP)] and woodslightly-weathered and extensively-weathered oak andeathered fir plywood] bin construction materials. The

lightly- and extensively-weathered oak pieces were taken,espectively, from newer, light brown-colored, non-fissurednd older, silver/gray-colored, highly fissured slats of binstored out-of-doors. The fir plywood was weathered andighly fissured and taken from a plywood bin sidewallxposed to the weather. The bin liner material evaluated wasraft corrugated cardboard, which is sometimes used for pear

ruit storage in pre-packed boxes and as a bin liner. Wall sur-ace materials included polyurethane foam (GVF industries,nc., Grand Rapids, MI) and cellulose-based fire retardantInternational Cellulose Corp., Houston, TX). For compar-son of the sorptive capacity of bin and box construction

aterials, the surface area of each piece was approximately80 cm2 with the exception of oak, which was also cut intomaller pieces to increase the surface area to volume rationd evaluate the effect of surface area (Table 1). The surfacerea of wall construction material pieces was approximately0 cm2.

.3. Gas exposure

Materials were placed singly into 1-l glass Mason jarsith metal lids fitted with rubber septa (Fisher Scientific,

Table 1Dimensions and mass of materials tested for 1-MCP sorption capacity and weight gain due to wetting

Material Width (cm) Length (cm) Thickness (cm) Surface area (cm2) Mass (g) Weight gain (%)

Oak slat 5.4 11.0 1.8 178 70.8 6.4Oak slat (five strips) 0.9–1.2 11.0 1.8 314 70.5 7.0Fir plywood 6.2 11.2 1.2 179 35.0 10.0LDPE 7.2 11.4 0.5 183 38.0 0PP 7.2 11.4 0.5 183 38.0 0Polyurethane foam 3.5 5.0 3.0 52 2.2 20.0Fire retardant 3.5 5.0 3.0 52 3.7 41.1Cardboard 7.5 11.5 0.2 184 6.3 17.2

Springfield, NJ) and held at a constant temperature of 20 ◦C,unless otherwise noted. 1-MCP gas was added to the treat-ment jar headspace from a concentrated stock gas source. Theconcentrated 1-MCP source was created by adding 10 ml ofdistilled water to a 1.0-l glass jar containing 1 g of Smart-Fresh (AgroFresh, Springhouse, PA), which had an activeingredient concentration of 0.14%. The concentrated 1-MCPsource container was connected by plastic tubing to a waterreservoir such that as a gas volume was removed from the 1-MCP source container, the same volume was replaced withwater from the reservoir, thereby preventing dilution of the1-MCP source or the creation of a pressure deficit. The 1-MCP concentration of the concentrated stock was determinedby gas chromatography and the needed volume removedand injected into the treatment jars to obtain target gas con-centrations of 30 and 300 �l l−1. Gas concentrations in theheadspace of the treatment jars were measured after 0, 2, 4,6, 8, 10 and 24 h. In addition to test material, treatment vari-ables included moistening of the test material, temperature,initial 1-MCP concentration, surface area and competitionwith apple fruit. In addition, to test the sorptive capacity ofoak to other olefins, dampened, extensively-weathered oaktest pieces were exposed to approximately 30 �l l−1 1-buteneand ethylene and the gas concentrations in the jar headspacemeasured over time as previously described.

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concentration of approximately 30 �l l−1 was tracked for24 h. Three replicate samples were tested.

1.6. Effect of surface area

The effect of surface area on the rate of 1-MCP absorptionwas tested for slightly- and extensively-weathered oak binmaterials and for dry and moistened extensively-weatheredoak pieces held at 20 ◦C. Surface area was adjusted byplacing one large piece (180 cm2) or five small pieces(314 cm2) in the treatment jars, but keeping mass (approx-imately 70 g) unchanged. Three replicate samples of eachtreatment/surface area combination were tested.

1.7. Effect of initial 1-MCP concentration

The effect of initial 1-MCP concentration was determinedby establishing low (30 �l l−1) and high (300 �l l−1) 1-MCPlevels. The depletion of the initial concentration was trackedfor 24 h. The experiment was performed for extensively-weathered oak pieces with low (180 cm2) or high surface(314 cm2) surface area at 20 ◦C. Three replicate samples ofeach concentration/surface area combination were tested.

1.8. Competition with apple fruit

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.4. Effect of moisture

Absorption of 1-MCP by bin materials, cardboard,olyurethane insulation, and cellulose fire retardant wasested for dry and moistened samples having a surface area ofpproximately 180 cm2. The test temperature was 20 ◦C. Toet the samples, they were submerged in deionized water for0 min (bin materials) or misted with water using an atomizero the point of runoff (cardboard, polyurethane, and fire retar-ant). The increase in weight from wetting was recorded andhe wt.% increase calculated. Three replicate samples wereested.

.5. Effect of temperature

The effect of temperature on the rate and extent of sorptionas tested by placing bin materials (slightly- and extensively-eathered oak) at 0 and 20 ◦C. The depletion of an initial

The impact of including slightly- and extensively-eathered oak bin material with a single apple fruit in the

ar was evaluated. The ratio of the weight of bin material toruit weight was maintained between 1:7 and 1:8, which isoughly similar to that of filled oak bins used in Michigan.he oak was pre-wetted in order to simulate the applicationf diphenylamine (DPA) to apples via bins by drenching. Thexperiment was conducted at 20 ◦C. Three replicate samplesere tested.

.9. Statistical analysis

Regression analysis was performed on the percentage of-MCP absorption with time for each sample using a simplexponential model: y = a + b · exp(−x/c), where x was timeh), a was the asymptotic minimum percentage, a + b was theaximum percentage, and c was the initial slope (Table 2).n some occasions the best fit-curve was linear: y = a + bx.

Table 2Values of constants for exponential and linear depletion curves fit to the declining 1-MCP concentration (y) in the headspace of chambers containing theindicated non-target materials

Material Non-moistened materials Moistened materials

a b c R2 a b c R2

Weathered oak 20.0 76.0 7.5 0.97 1.3 98.1 4.0 0.99Non-weathered oak 37.4 61.5 8.8 1.0 3.5 94.3 4.9 0.98Fir plywood 82.7 17.4 9.6 0.97 14.2 84.8 14.5 0.99LDPE 91.8 8.6 8.0 0.91 90.2 10.5 12.3 0.89PP 92.3 8.3 10.4 0.86 90.9 9.4 18.7 0.86Polyurethane 100.2 −0.3 0.99 90.3 10.2 5.5 0.96Fire retardant 77.3 22.7 47.9 0.97 100.1 −0.4 0.96Cardboard 101.8 −0.7 0.94 −6.1 108.6 8.8 1.0

Where a, b, and c are reported, a was the asymptotic minimum percentage, a + b was the maximum percentage, and c was exponential constant for the expressiony = a + b · exp(−x/c). Where only a and b are reported, the best-fit curve was linear: y = a + bx. R2 is the coefficient of determination for the fitted curve.

To compare rates of sorption, the time required for 1-MCP todecline by half (t1/2) was determined from the above equation.Analysis of variance (ANOVA) was performed on the t1/2data. Mean comparisons were carried out by LSD multiplerange test.

2. Results

There were marked differences in the sorption of 1-MCP between wooden and plastic bin construction materials(Fig. 1). The percentage of 1-MCP remaining after 24 h fordry HDPE and PP was 92.5 and 93.5%, respectively (Fig. 1).The remaining 1-MCP for plywood, cardboard, and slightly-and extensively-weathered oak declined to 84.1, 82.7, 41.1,and 21.1% of initial, respectively. Moistening the test mate-rials did not affect 1-MCP depletion by plastic materials, butmarkedly increased depletion by wood and cardboard. After24 h, the remaining 1-MCP concentration for plywood, card-board, slightly-weathered oak, and extensively-weatheredoak were 29.8, 1.9, 2.8, and 1.3%, respectively (Fig. 1). Wallinsulation and fire proofing had a very low 1-MCP absorptionrate during the 24 h of the test period. The remaining 1-MCPfor dry polyurethane insulation and cellulose fire retardantwas 92.5 and 91.0%, respectively. When these materials weremoistened, the percentage of 1-MCP remaining decreasedsa

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Temperature (0 versus 20 ◦C) had no significant impacton the t1/2 or final 1-MCP concentration for slightly- orextensively-weathered oak or for oak pieces differing in sur-face area (data not shown). The t1/2 was 11.5 and 18.5 h at0 ◦C for extensively-weathered and slightly-weathered oak,respectively, while the respective values for t1/2 were 13 and20 h at 20 ◦C.

Fig. 1. Effect of material on the sorption of 1-MCP at 20 ◦C for dry (A) andwetted (B) test samples of ‘non-target’ materials found in apple and pearfruit storages as a function of time. The initial 1-MCP concentration wasapproximately 30 �l l−1. Each data point is an average of three replicates;vertical bars are the standard error.

lightly to 90.6 and 90.4%, respectively. Wetted oak did notbsorb ethylene or 1-butene (data not shown).

Increasing surface area of dry oak bin material increased-MCP absorption after 24 h; the effect of surface area wasignificant only after 10 h (data not shown). On the contrary,hen wet, there were no differences between large and smallieces during the 24 h time-course. Also, the time to 50%oss of initial 1-MCP concentration (t1/2) differed betweenet and dry oak for the high and low surface area test pieces.he t1/2 was less than 3 h when wet and 12.5 h when dry.

The rate of 1-MCP absorption increased with increas-ng initial 1-MCP concentration (Table 3). At the higheroncentration, the t1/2 was 6 and 11 h for extensively- andlightly-weathered oak, respectively, but at the lower con-entration was, respectively, 13 and 20 h.

Table 3Effect of 1-MCP concentration, exposure, and surface area on the t1/2 (h) for1-MCP absorption for dry oak bin construction material

Exposure Surface area(cm2)

1-MCP Concentration(�l l−1)

30 300

Extensively-weathered 180 13 Baa 6 BbExtensively-weathered 314 13 Ba 5 BbSlightly-weathered 180 20 Aa 11 AbSlightly-weathered 314 19 Aa 10 Ab

Data are the average of three sample replicates. Mean separation withincolumns (upper case letters) for comparisons between exposure and surfacearea and within rows (lower case letters) for comparisons between 1-MCPconcentrations.

a Means followed by the same letter are not different according to LSDtest (P < 0.001).

Fig. 2. Effect of inclusion of wetted oak bin material with apple fruit on thetime-dependent depletion of 1-MCP at 20 ◦C. Each data point is an averageof three replicates; vertical bars are the standard error.

The 1-MCP absorption percentage by apples alone was of79.2% after 24 h. The t1/2 was 12 h. However, the percentageof 1-MCP absorbed decreased when oak was included withthe apple fruit. The percentage of 1-MCP absorption for thecombined apple and oak (wetted) were 1.1 and 1.2% after 24 hfor both slightly- and extensively-weathered oak, respectively(Fig. 2). The t1/2 for both was 2 h.

3. Discussion

It is recognized that, as a gas, 1-MCP has the potential tocontact ‘non-target’ materials and living organisms as a resultof venting of fruit treatment facilities (EPA, 2002; EFSA,2005). However, little is known of the fate of 1-MCP in thestorage environment. Given the loss of 1-MCP to woodenbin material, it is possible that environmental exposure mightbe less of an environment concern than previously thought(EPA, 2002; EFSA, 2005). More importantly, on the basisof these results, the depletion of 1-MCP might compromise1-MCP activity under some situations. It is interesting tonote that experimental studies to evaluate responses to 1-

MCP application often employ plastic and metal containers.Dose–response curves generated using inert experimentalcontainers may not always accurately reflect responses undercommercial conditions with the same initial concentrationsof 1-MCP.

The mechanism for the sorption of 1-MCP by oak andcardboard is not known. Given the lack of sorption of either1-butene or ethylene by the wood, the binding of 1-MCP isprobably not due to an olefin-specific binding system suchas copper binding (Sisler, 1991). It is possible, however,that 1-MCP absorption may be by glucose-based structuresin plant cell walls. The �-1,4 glycosidic structure of cellu-lose microfibrils has a cavity roughly similar in size to thecyclodextrin used in the SmartFresh formulation (Carpita andMcCann, 2000).

For apple alone, 50% depletion occurred in approximately13 h (Fig. 2). However, the addition of wet oak resulted in a90% depletion of the initial 1-MCP concentration in only6 h (Fig. 1). Assuming a similar rate of loss, in the case of aninitial concentration of 1 �l l−1 there would be only 0.1 �l l−1

available after 6 h (Fig. 2). DeEll et al. (2002) determinedapproximately 6 h are needed to saturate ethylene inhibitoryresponse. Furthermore, Jayanty et al. (2004) demonstratedthat a minimum concentration of 0.1 �l l−1 is near the lowerlimit to obtain a response to 1-MCP by ‘Redchief Delicious’apple fruit. Therefore, wet wooden bins material may reduce1n

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-MCP concentrations to near minimal effective levels withinecessary treatment periods.

Based on the often-observed effectiveness of 1-MCP inommercial settings, the loss of 1-MCP to non-target mate-ials commonly encountered in CA and RA storage roomsor apple and pears is likely not of serious concern mostituations when 1-MCP levels are near the maximum rate rec-mmended by AgroFresh. However, under some situations asn the case of leaking treatment rooms, greater absorbance bys yet untested materials, significant sorption by fruit, use ofub-saturating 1-MCP levels, or inclusion of additional woodr cardboard packaging barriers, 1-MCP efficacy might beompromised.

cknowledgements

The authors are grateful for the financial assistance pro-ided by Macro Plastics. SmartFresh was kindly donated bygroFresh. F. Vallejo is indebted to both Ministerio de Edu-

acion y Ciencia and Comision Fulbright Espana-EEUU foris postdoctoral fellowship.

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