Effectiveness of 1-MCP treatments on ‘Bartlett’ pears as influenced by the cooling method and the bin material

Postharvest Biology and Technology 51 (2009) 49–55

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Effectiveness of 1-MCP treatments on ‘Bartlett’ pears as influenced by the cooling method and the bin material Gabriela Calvo a , Gabriel O. Sozzi b,c,∗ a

Instituto Nacional de Tecnología Agropecuaria, EEA Alto Valle de Río Negro, C.C. 782, R8332 General Roca, Provincia de Río Negro, Argentina Cátedra de Fruticultura, Facultad de Agronomía, Universidad de Buenos Aires, Avda. San Martín 4453, C1417DSE Buenos Aires, Argentina c Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina b

a r t i c l e

i n f o

Article history: Received 24 January 2008 Accepted 19 June 2008 Keywords: 1-Methylcyclopropene Bin-stored European pears Cooling method Ripening Ethylene Firmness

a b s t r a c t Wooden bin-stored ‘Bartlett’ pears (Pyrus communis L.) were hydrocooled (HC) or forced-air cooled (FAC) and immediately treated or not with 1-methylcyclopropene (1-MCP) for 24 h. 1-MCP gas concentrations used were 0, 0.3 or 0.6 ␮L L−1 (called 0, 0.3 and 0.6, respectively). Fruit were subsequently kept at 20 ◦ C for 20 d or stored at −0.5 ◦ C and 95% RH for 60, 90, 120 or 150 d. After cold storage, fruit were kept at 20 ◦ C for up to 16 d for further ripening. In another experiment, pears stored in wooden bins (W) or plastic bins (P) were all hydrocooled, treated or not with 0.5 ␮L L−1 1-MCP (called 0.5 and 0, respectively), stored at −0.5 ◦ C and 95% RH for 0, 30, 60, 90 or 120 d, and transferred to 20 ◦ C for further ripening. In FAC pears, increasing 1-MCP concentrations usually resulted in delayed increases in ethylene production and lower ethylene production rates, as well as delayed softening. In contrast, HC-0.3 pear firmness did not differ from that of HC-0 fruit after cold storage. Generally, HC-0.3 pears displayed higher ethylene production and lower firmness values than FAC-0.3 pears after a 7-d exposure to 20 ◦ C, regardless the length of cold storage. FAC-0.6 pears always showed lower ethylene production rates and higher flesh firmness values than HC-0.6 fruit. Soluble solids concentration was not consistently affected by 1-MCP. FAC-0.3 and HC0.6 fruit showed higher titratable acidity values than HC-0 fruit after 0, 60, 120 and 150 d of cold storage plus 7 d at 20 ◦ C. Effectiveness of 1-MCP treatments on HC pears was influenced by the bin material; P-0.5 pears were firmer than W-0.5 pears after 7 d at 20 ◦ C, regardless the length of the cold storage. HC-0.5 fruit exposed to −0.5 ◦ C for 90 d reached eating quality (firmness ≤23 N) by day 7 if placed in W, and by day 21 when stored in P. Results and previous evidence suggest that wet wooden bin material may represent a major though unpredictable source of 1-MCP sorption that could bind a significant percentage of the 1-MCP applied. When used at relatively low doses 1-MCP partial removal by wet wooden bins can compromise the application effectiveness for controlling ethylene action. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Effective postharvest protection of pears (Pyrus communis L.) typically involves utilization of cooling methods, good temperature and relative humidity maintenance throughout storage and distribution, and the use of supplements to proper temperature management (Mitcham and Elkins, 2007). In the Alto Valle de Río Negro (Argentina) and in other growing areas, ‘Bartlett’ pears are frequently harvested into large bins and stored for weeks to months in cold storage prior to packing. Because the respiration rate of ‘Bartlett’ pears is high (increasing CO2 production from

∗ Corresponding author at: Cátedra de Fruticultura, Facultad de Agronomía, Universidad de Buenos Aires, Avda. San Martín 4453, C1417DSE Buenos Aires, Argentina. Tel.: +54 11 4524 8055; fax: +54 11 4514 8739. E-mail address: [email protected] (G.O. Sozzi). 0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.06.011

1.1–1.6 ␮g kg−1 s−1 at 0 ◦ C to 8.2–19 ␮g kg−1 s−1 at 20 ◦ C; Mitcham et al., 2008), they are usually forced-air cooled or hydrocooled to remove field heat within 24 h of harvest, particularly when they are bound for long-term storage or long distance transport. As a result of the higher surface heat transfer coefficient of produce-to-water in comparison to that of produce-to-air (ASHRAE, 2002), hydrocooling provides a faster temperature reduction of the produce than forced-air cooling, with lower moisture loss (less than 0.5%) and higher energy use efficiency (Thompson et al., 2000). In spite of some disadvantages (e.g. increase in contamination potential, requirement of water sanitization, need of water-resistant containers), hydrocooling reduces the length of exposure to high temperature and is an useful cooling method for products such as pears which tolerate water contact and chlorine solutions. Control of fruit ripening by 1-methylcyclopropene (1-MCP) is a supplement to proper temperature that continues to be explored

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as a potential commercial application in summer (Calvo and Sozzi, 2004; Ekman et al., 2004; Trinchero et al., 2004; Bai et al., 2006), autumn and winter pears (Argenta et al., 2003; Kubo et al., 2003; Chen and Spotts, 2005; Rizzolo et al., 2005; Bai et al., 2006; Gapper et al., 2006; Isidoro and Almeida, 2006; MacLean et al., 2007). Recently, Vallejo and Beaudry (2006) demonstrated that commercial chamber walls, bin or packing box materials may absorb/adsorb a significant portion of the 1-MCP during the treatment period. When the sorptive capacity of different materials was tested, major differences in the absorption of 1-MCP were detected. Wooden bin material immobilized significant quantities of 1-MCP while plastic bin material absorbed little to no 1-MCP. Moreover, wetting of the wood and cardboard test samples greatly increased 1-MCP depletion (Vallejo and Beaudry, 2006). They suggested that wet wooden bin material may reduce 1-MCP concentrations to nearly minimal effective levels within necessary treatment periods. Additional work with ‘Jonagold’ apple fruit indicated that the 1-MCP response to dose is reduced significantly by wetted wooden bin material at concentrations at or below 0.4 ␮L L−1 , although commercially applied levels (approximately 1 ␮L L−1 ) are still fully efficacious (Randolph Beaudry, unpublished data). In contrast to apples, uncontrolled concentrations of 1-MCP in a storage room may limit its commercial use in European pears, as high concentrations can prevent or cause excessive delay in ripening while very low concentrations may have no effect at all (Calvo and Sozzi, 2004; Ekman et al., 2004; Chen and Spotts, 2005; Bai et al., 2006). The objective of this work was to assess and compare the keeping quality of ‘Bartlett’ pears whether forced-air cooled or hydrocooled in either wooden or plastic bins followed by treatment with different concentrations of 1-MCP. 2. Materials and methods 2.1. Effect of 1-MCP treatments on ‘Bartlett’ pears placed in wooden bins, as influenced by the cooling method 2.1.1. Plant material ‘Bartlett’ pears (mean fruit weight = 174.2 g) were harvested from a commercial orchard in Cipolletti (38◦ 57 S, 67◦ 59 W), Alto Valle de Río Negro, Argentina, on 31 January 2006 (late harvest for this cultivar). The 15-year-old trees, grown on seedling rootstocks, were planted and trained to a high-density free-standing central-leader system with trees spaced 1.0 m by 4.0 m. Cultural management practices such as pruning, fruit thinning, fertilization, irrigation and pest control were carried out according to the EUREPGAP® protocols (GLOBALGAP, 2007). Twelve weathered wooden bins, made of solid poplar (Populus sp.) wood and bearing 470 kg of ‘Bartlett’ pears, were used. Pears were sorted for uniformity, appearance and absence of physical defects. Thirty fruits were used to analyze different maturity indices (firmness, soluble solids concentration and starch degradation) at the beginning of the experiment. Fruit treatments and storage were performed using commercial cooling facilities (Kleppe S.A., Cipolletti, Río Negro, Argentina). Six bins were immediately hydrocooled from 20 to 5 ◦ C in 20 min by means of a shower-type cooler while the other six bins were forced-air cooled from 20 to 5 ◦ C (core temperature) in 8 h. After cooling, all bins were immediately placed in cold (0 ◦ C) rooms overnight. 2.1.2. 1-MCP application and fruit storage The refrigerated wooden bins were placed in 4 m3 air-tight reinforced vinyl chambers (two bins per chamber) inside a cold room. Fruit and room temperatures at the beginning of the treatments were 0.2 and 0 ◦ C, respectively. The current commercial

minimum dose of 1-MCP for ‘Bartlett’ pears indicated by the manufacturer is 0.3 ␮L L−1 (Bai et al., 2006; Rohm & Haas Argentina, pers. commun.). Thus, fruit were treated with 0, 0.3 or 0.6 ␮L L−1 1-MCP for 24 h. 1-MCP was released from tablets (SmartFreshTM SmartTabs, 0.63% active ingredient; Rohm & Haas) containing the SmartFreshTM cyclodextrin powder, sized to develop the appropriate treatment concentrations. SmartTabs were comprised of three components presented in a kit, containing 1-MCP tablets, an activator solution containing citric acid, and activator tablets containing sodium bicarbonate. Treatments were performed according to the manufacturer’s recommendations. Untreated fruit were held in a similar vinyl chamber with no 1-MCP. Thus, two bins per cooling method (hydrocooling and forced-air cooling) and 1-MCP concentration (0, 0.3 or 0.6 ␮L L−1 ) were used. Following 1-MCP treatment, fruit were either kept at 20 ◦ C for 20 d or stored at −0.5 ◦ C and 95% RH for 60, 90, 120 or 150 d. After cold storage, fruit were placed at 20 ◦ C for up to 16 d for further ripening. Fruit firmness, skin hue angle, soluble solids concentration (SSC), and titratable acidity (TA) were analyzed on days 1 and 7 at 20 ◦ C using 20 pears per cooling method (hydrocooling and forced-air cooling) × treatment (0, 0.3 or 0.6 ␮L L−1 1-MCP) × cold storage duration (0, 60, 90, 120 or 150 d) × ripening period (1 or 7 d). Another three 3-fruit groups per cooling method × treatment × cold storage duration × ripening period were used to determine ethylene production throughout the experiment. 2.2. Effect of 1-MCP treatments on hydrocooled ‘Bartlett’ pears, as influenced by the bin material 2.2.1. Plant material ‘Bartlett’ pears (mean fruit weight = 174.1 g) were harvested from a commercial orchard in Cipolletti, Alto Valle de Río Negro, Argentina, on 27 January 2007 (late harvest for this cultivar). Two plastic and four wooden bins with 470 kg of pears each were used in the experiment. Pears were sorted for uniformity, appearance and absence of physical defects. Thirty fruit were used to analyze different maturity indices (firmness, soluble solids concentration and starch degradation) at the beginning of the experiment. On the same date, all fruit were hydrocooled in a shower-type cooler from 23 to 5 ◦ C (core temperature) in 20 min and immediately placed in cold (0 ◦ C) rooms overnight. 2.2.2. 1-MCP application and fruit storage Fruit were exposed to 1-MCP (SmartFresh® , AgroFresh, Philadelphia, USA) for 24 h. 1-MCP treatments were carried out in 4 m3 vinyl chambers, as previously described. Treatments were as follows—T1: fruit in wooden bins treated with 1-MCP 0 ␮L L−1 (W0); T2: fruit in wooden bins treated with 1-MCP 0.5 ␮L L−1 (W-0.5); T3: fruit in plastic bins treated with 1-MCP 0.5 ␮L L−1 (P-0.5). In previous experiments, no differences were detected in the pear performance between fruit not treated with 1-MCP stored in wooden or in plastic bins (data not shown). Two bins per each bin material and 1-MCP concentration were used. Fruit were immediately kept at 20 ◦ C or stored at −0.5 ◦ C and 95% RH for 30, 60, 90 or 120 d. After cold storage, fruit were transferred to a room at 20 ◦ C for 1 or 7 d for further ripening and fruit firmness was assessed. Additionally, fruit stored at −0.5 ◦ C and 95% RH for 90 d and subsequently exposed to ambient temperature were evaluated for firmness until they reached eating quality (17–23 N). 2.3. Fruit quality assessment The starch–iodine test was performed at harvest on 30 fruits, taking a slice from the equatorial region of the fruit and dipping it in a solution of iodine crystals plus potassium iodide (Kingston,

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1992). Then, each sample was rated using a chart for comparison (Le Lezec and Belouin, 1994). Ethylene production was assessed by placing three pears in a 3L glass container which was in turn tightly sealed with a lid with a silicon septum. One milliliter of the head-space gas was extracted after 30 min and ethylene was quantified on a Shimadzu GC14-A gas chromatograph (Shimadzu Seisakusho Co., Kyoto, Japan) fitted with a FID and a stainless-steel Porapak N column (3.2 mm × 2 m; 80/100 mesh). The injector, oven and detector temperatures were 110, 90, and 240 ◦ C, respectively. Helium was used as the carrier gas at a flow rate of 0.37 mL s−1 (linear gas velocity = 4.5 cm s−1 ). Results were expressed as nanograms of ethylene produced per kilogram of fruit in 1 s. Three independent samples per cooling method, 1MCP treatment, cold storage duration and ripening period were analyzed. Fruit firmness was determined by measuring the force required to penetrate each pear, with the skin removed, using an EPT-1 (Lake City Technical Products, Kelowna, British Columbia, Canada) (Argenta et al., 2003; Calvo and Sozzi, 2004). Each fruit was placed on a stationary steel plate. Two spots located on opposite sides of the equatorial region of each fruit were punctured to a depth of 10 mm using a 7.9-mm round-surfaced cylindrical probe. The average of those two measurements was considered as one replicate. Twenty pears per cooling method, 1-MCP treatment, cold storage duration and ripening period were evaluated. Hue angle (h◦ ) values were measured on the equatorial region of intact fruit with a Minolta chromameter (model CR-300; Osaka, Japan) using CIE illuminant C lighting conditions and an 8-mm diameter measuring area (Trinchero et al., 2004). The chromameter was calibrated to a white calibration plate (CR-A43). Two spots on opposite sides of the fruit were measured and the mean of the two measurements was considered as one replicate. Twenty pears per cooling method, 1-MCP treatment, cold storage duration and ripening period were evaluated. A longitudinal wedge was removed from each fruit. Its juice was extracted using a juicer and analyzed for SSC with a handheld temperature-compensated refractometer (Atago Co., Tokyo, Japan). Twenty pears per cooling method, 1-MCP treatment, cold storage duration and ripening period were evaluated. Each sample was measured twice and the average of those two measurements was considered as one replicate. TA was determined by potentiometric titration of a 10-mL juice sample with 0.1 mol L−1 NaOH to an endpoint of pH 8.2, and was calculated as mmol H+ per liter of juice (Trinchero et al., 2004). Twenty pears per cooling method, 1-MCP treatment, cold storage duration and ripening period were evaluated. Each sample was measured twice and the average of the two measurements was considered as one replicate. 2.4. Statistical analysis Experiments were performed according to a factorial design. Data were analyzed by means of a one-way ANOVA, using the PCSAS software package (SAS Institute Inc., Cary, NC, USA). In the case of a significant F-value, data were subjected to the Tukey’s test for comparison of treatments within each date. 3. Results and discussion 3.1. Effect of 1-MCP treatments on ‘Bartlett’ pears placed in wooden bins, as influenced by the cooling method 3.1.1. Ethylene production Both hydrocooled and forced-air cooled fruit not treated with 1-MCP (HC-0 and FAC-0, respectively) followed a typical climac-

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teric pattern for ethylene biosynthesis when immediately exposed to 20 ◦ C. The maximum ethylene production occurred on day 12, with an ensuing decline (Fig. 1a). Forced-air cooled fruit treated with 0.3 or 0.6 ␮L L−1 1-MCP (FAC-0.3 and FAC-0.6, respectively) did not show a peak in ethylene production but only a gradual rise towards the end of the experimental period (Fig. 1a), as previously described (Trinchero et al., 2004). Increasing 1-MCP concentrations resulted in delayed increases in ethylene production, and lower ethylene production rates. In contrast, hydrocooled pears treated with 0.3 ␮L L−1 1-MCP (HC-0.3) did not show differences in ethylene synthesis in comparison to HC-0 or FAC-0 for 16 d. Hydrocooled 1-MCP-treated fruit produced less ethylene than the untreated controls during ripening when the concentration applied was 0.6 ␮L L−1 (HC-0.6; Fig. 1a). Upon transfer to 20 ◦ C, hydrocooled and forced-air cooled untreated fruit previously exposed to cold storage for 60–150 d showed higher initial rates of ethylene production (Fig. 1b–e) than fruit not stored under cooling conditions (Fig. 1a). This agrees with a previous report that ‘Bartlett’ pears not treated with 1MCP and stored for 6 weeks at 0 ◦ C produced approximately twice as much ethylene during ripening as fruit immediately ripened at 20 ◦ C (Ekman et al., 2004). Also, the climactericassociated peak of untreated fruit previously exposed to long-term (60–150 d) cold storage reached maximum values within 3–5 d regardless the cooling method applied (Fig. 1b–e), as previously observed (Ekman et al., 2004; Trinchero et al., 2004). Exposure to cold conditions hastens the onset of the climactericassociated rise of ethylene production in ‘Bartlett’ pears, either attached to or detached from the tree (Wang et al., 1971; Looney, 1972). In pears stored under cooling conditions for 90–150 d, HC-0.3 fruit produced ethylene at levels similar to those of HC-0 and FAC0 fruit (Fig. 1c–e). 1-MCP-treated forced-air cooled fruit displayed far lower ethylene production than hydrocooled fruit treated with the same 1-MCP concentration. After 60, 120 or 150 d of cold storage, FAC-0.3 pears showed ethylene production during ripening similar to that of HC-0.6 pears (Fig. 1b, d and e). FAC-0.6 fruit displayed lower ethylene production rates in all cases. These results provide evidence of different ethylene climacteric behaviors of 1MCP-treated ‘Bartlett’ pears stored in wooden bins, depending not only on the concentration of the ethylene blocker applied but also on the cooling method utilized. 3.1.2. Firmness and color Indicators of fruit maturity at the beginning of the experiment were the following (mean ± S.D.): firmness 88.1 ± 0.5 N; SSC: 11.5 ± 0.5%; starch degradation: 58.5 ± 5.5%. Softening was markedly reduced by 1-MCP application in fruit immediately kept at 20 ◦ C for 1 d (data not shown). Also, FAC-0.6 treatment tended to delay softening of pears exposed to 120 and 150 d of cold storage plus 1 d at 20 ◦ C, though differences with untreated fruit were only significant (P < 0.05) in the latter (data not shown). In contrast, large differences in firmness values were observed after 7 d of fruit ripening at room temperature (Table 1). Both duration and magnitude of the effects of 1-MCP were related not only to the concentrations applied but also to the cooling method. Increasing 1-MCP concentrations brought about progressively greater effects on firmness retention of forced-air cooled fruit (Table 1). FAC-0.3 fruit were firmer than FAC-0 fruit after 0 or 60 d of cold storage plus 7 d at 20 ◦ C. Also, FAC-0.6 fruit were firmer than FAC0.3 fruit after 60, 90, 120 or 150 d of cold storage plus 7 d at 20 ◦ C (Table 1). On the other hand, HC-0.3 and HC-0.6 fruit were not firmer than HC-0 fruit after 60, 90 or 150 d of cold storage plus 7 d at 20 ◦ C (Table 1). In general, hydrocooled fruit treated with 1-MCP

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Fig. 1. Effect of the cooling method (hydrocooling or forced-air cooling) and 1-MCP concentration (0, 0.3 or 0.6 ␮L L−1 ) on ‘Bartlett’ pears placed in wooden bins. After cooling and 1-MCP treatment, fruit were held at 20 ◦ C (a) or stored at −0.5 ◦ C for 60 (b), 90 (c), 120 (d) or 150 d (e). After cold storage, fruit were kept at 20 ◦ C for further ripening. Each value represents the mean of three 3-pear replicates. Means with a different letter are significantly different within a single date, according to the Tukey’s test. The legend indicates the cooling method (letters “HC” stand for hydrocooling and “FAC” for forced-air cooling) followed by a number that specifies the concentration (␮L L−1 ) of 1-MCP used in each treatment.

softened at a higher rate than forced-air cooled fruit submitted to an identical 1-MCP protocol. In general, fruit softening showed a similar though inverse pattern to that of ethylene production. Nevertheless, HC-0.6 pears that displayed lower ethylene production rates than HC-0 pears after 90 d of cold storage did not show differences (P < 0.01) in firmness values during the ripening period at 20 ◦ C (Fig. 1c and

Table 1). A similar situation was detected between FAC-0.3 and FAC-0 pears after 90 d of cold storage. Ethylene not only triggers the onset of pear softening but also determines its progression, even after ripening has started (Hiwasa et al., 2003). These results suggest that a partial reduction of ethylene production in ‘Bartlett’ pears may not suffice to slow down their softening rate.

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Table 1 Effect of the cooling method (hydrocooling –HC– or forced-air cooling –FAC–) and 1-MCP concentration (0, 0.3 or 0.6 ␮L L−1 ) on ‘Bartlett’ pears placed in wooden bins Cold storage duration (d)

Cooling method

1-MCP concentration (␮L L−1 )

Firmness (N)

Hue angle (h◦ )

Soluble solids concentration (%)

Titratable acidity (mmol L−1 )

0

HC FAC HC FAC HC FAC

0 0 0.3 0.3 0.6 0.6

19.1 d 24.1 c 27.5 c 81.6 a 67.5 b 88.8 a

98.7 c 101.1 bc 104.2 b 112.2 a 111.4 a 113.1 a

12.5 13.5 13.2 12.2 12.3 12.3

43.3 c 46.3 bc 49.3 abc 54.7 a 53.0 a 52.3 ab

***

***

NS

*

22.3 c 23.9 c 27.0 c 56.1 b 29.6 c 80.6 a

94.4 bc 93.0 c 94.7 bc 97.0 bc 97.6 b 108.3 a

13.1 13.3 12.9 13.1 13.1 13.0

43.6 c 47.0 bc 41.7 c 57.0 a 54.0 ab 41.7 c

***

***

NS

***

11.5 b 11.1 b 13.1 b 28.4 b 30.3 b 65.5 a

94.7 b 94.8 b 96.0 b 98.1 b 95.4 b 104.5 a

12.3 13.1 12.7 13.2 12.9 12.9

45.0 50.7 52.7 51.3 53.3 54.3

**

**

NS

NS

4.6 c –a –a 35.4 b 31.2 b 67.7 a

90.6 b 89.5 b 90.2 b 90.9 b 90.8 b 99.1 a

12.2 –a –a 12.9 13.3 12.6

15.7 b –a –a 50.0 a 48.3 a 44.3 ab

**

*

NS

***

4.0 c –a 14.7 bc 23.4 b 16.8 bc 72.4 a

90.3 90.2 94.1 91.6 92.1 94.8

11.0 b –a 12.3 a 12.5 a 12.8 a 13.0 a

11.7 b –a 40.3 a 48.3 a 43.0 a 48.0 a

***

NS

*

*

Probability 60

HC FAC HC FAC HC FAC

0 0 0.3 0.3 0.6 0.6

Probability 90

HC FAC HC FAC HC FAC

0 0 0.3 0.3 0.6 0.6

Probability 120

HC FAC HC FAC HC FAC

0 0 0.3 0.3 0.6 0.6

Probability 150

HC FAC HC FAC HC FAC

0 0 0.3 0.3 0.6 0.6

Probability

Levels of statistical significance are *P < 0.05, **P < 0.01 and ***P < 0.001. After cooling and 1-MCP treatment, fruit were stored at −0.5 ◦ C and 95% RH for 0, 60, 90, 120 or 150 d and subsequently kept at 20 ◦ C for 7 d for further ripening. Means with a different letter are significantly different within a single cold storage duration, according to the Tukey’s test. a After 7 d at 20 ◦ C, firmness, titratable acidity and soluble solids concentration could not be recorded due to the high incidence of internal physiological disorders (watery breakdown and/or core breakdown).

Under the same cooling technique, the h◦ (90◦ = yellow; 115◦ = pale green) was lower for untreated fruit and higher for 1MCP-treated fruit immediately kept at 20 ◦ C (Table 1). FAC-0.3 and FAC-0.6 fruit showed similar h◦ values but were greener than FAC-0 fruit. In contrast, HC-0.3 fruit displayed lower h◦ values than FAC-0.3 and HC-0.6 fruit after a 7-d ripening period (Table 1). As cold storage duration increased, h◦ declined in all treatments (Table 1). In general, no differences in h◦ were detected between fruit treated with 0.3 ␮L L−1 1-MCP and untreated fruit regardless the cooling method applied, the cold storage duration and the length of the ripening period (Table 1). In contrast, after 60, 90 or 120 d of cold storage plus 7 d at 20 ◦ C, FAC-0.6 pears remained greener than those undergoing other treatments, including HC-0.6 fruit (Table 1). Also, dissociation of the skin color change and flesh softening was apparent in 1-MCP-treated fruit after a long-term (120–150 d) cold storage plus a 7-d ripening period. The effect was more pronounced in FAC-0.6 pears stored at −0.5 ◦ C for 150 d and ripened for 7 d; yellow color development (h◦ ∼ 90) preceded softening, as previously described (Ekman et al., 2004).

3.1.3. Soluble solids concentration and titratable acidity Generally, no differences in SSC between treatments were detected within the same storage and ripening period (Table 1). Pears kept at 20 ◦ C for 1 d were an exception (data not shown). Also, 1-MCP-untreated hydrocooled fruit stored at −0.5 ◦ C for 150 d and allowed to ripen at 20 ◦ C for 7 d displayed lower SSC than fruit exposed to other treatments (Table 1). Chen and Spotts (2005) reported that SSC in 1-MCP-treated ‘d’Anjou’ pears was slightly higher than that in untreated fruit. On the other hand, Calvo and Sozzi (2004) did not detect differences in SSC between 1-MCPtreated and untreated ‘Red Clapp’s’ pears, either after cold storage or during subsequent ripening at 20 ◦ C. 1-MCP treatments did not influence TA after 1 d at 20 ◦ C, regardless of the cooling method or cold storage duration (data not shown). In contrast, FAC-0.3 and HC-0.6 fruit showed higher TA values than HC-0 fruit after 0, 60, 120 and 150 d of cold storage plus 7 d at 20 ◦ C (Table 1). In most cases, 1-MCP treatments did not consistently influence the magnitude of TA loss of ‘Red Clapp’s’ pears during ripening at 20 ◦ C after 60 d of cold storage (Calvo

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Table 2 Effect of 1-MCP on softening of hydrocooled ‘Bartlett’ pears, as affected by the bin material and the concentration of 1-MCP applied Ripening period at 20 ◦ C (d)

Bin material

1-MCP concentration (␮L L−1 )

1

Wood Wood Plastic

0 0.5 0.5

Probability 7

Wood Wood Plastic

0 0.5 0.5

Probability

Storage at −0.5 ◦ C (d), firmness (N) 0

30

60

90

120

74.7 b 77.0 ab 81.6 a

71.3 b 75.9 ab 80.9 a

69.1 b 75.3 ab 81.1 a

68.8 73.5 77.9

65.9 b 75.0 a 76.0 a

*

**

***

NS

**

22.9 c 67.8 b 81.6 a

8.5 c 47.8 b 81.1 a

5.7 c 28.7 b 78.9 a

9.0 c 21.1 b 79.1 a

12.3 c 16.2 b 74.4 a

***

***

***

***

***

Levels of statistical significance are *P < 0.05, **P < 0.01 and ***P < 0.001. After hydrocooling and 1-MCP treatment, pears were stored at −0.5 ◦ C for 0, 30, 60, 90 or 120 d and subsequently kept at 20 ◦ C for 1 or 7 d for further ripening. Means with a different letter are significantly different within a single cold storage duration and ripening date, according with the Tukey’s test.

and Sozzi, 2004). Conversely, 1-MCP-treated ‘d’Anjou’ pears maintained higher TA than untreated fruit during 150 d storage (Chen and Spotts, 2005).

3.2. Effect of 1-MCP treatments on hydrocooled ‘Bartlett’ pears, as influenced by the bin material Indicators of fruit maturity at the beginning of the experiment were the following (mean ± S.D.): firmness: 85.1 ± 1.5 N; SSC: 11.4 ± 0.6%; starch degradation: 60.5 ± 5.5%. Fruit in wooden bins, not exposed to 1-MCP and held at 20 ◦ C reached a buttery and juicy texture (flesh firmness ≤23 N) in 7 d (Table 2). In contrast, W-0.5 and P-0.5 fruit failed to ripen to eating quality. P-0.5 pears maintained the initial firmness values and were firmer than W-0.5 pears after 7 d. Generally, P-0.5 fruit, unlike W-0.5 fruit, were firmer than W0 fruit 1 d after cold storage, regardless of the storage duration (Table 2). As an exception, no differences were detected in fruit cold-stored for 90 d. After a 7-d ripening period, W-0.5 fruit were firmer than W-0 fruit and softer than P-0.5 fruit, regardless of the storage duration at −0.5 ◦ C (Table 2). W-0 pears softened to an overripe condition (firmness ≤14 N) in all cases. P-0.5 did not soften at all, displaying firmness values ≥74 N satisfactory for shipping and distribution but not for retail or consumption. Interestingly, W-0.5 fruit reached firmness values adequate for consumption after 90 or 120 d cold storage plus 7 d at 20 ◦ C (Table 2), with low incidence of internal breakdown (data not shown). To further evaluate possible differences in the effectiveness of 1-MCP treatments on hydrocooled ‘Bartlett’ pears as influenced by the bin material, fruit were stored under cooling conditions for 90 d and subsequently kept at 20 ◦ C until softened to firmness between 14 and 23 N (Fig. 2). W-0 pears did not maintain suitable eating quality after 6 d (firmness ≈10 N). W-0.5 fruit ripened by day 7 (firmness ≈21 N), while P-0.5 reached firmness values ≤20 N by day 21. Pears are more sensitive to relatively low concentrations of 1-MCP than other fruit species and, in addition, many pre- and postharvest factors can affect that sensibility (Sozzi and Beaudry, 2007). The use of saturating levels of 1-MCP on summer pears is not recommended, since it can excessively delay/alter ripening and prevent the gain of a buttery and juicy texture, with the resulting reduction of their quality (Calvo and Sozzi, 2004; Ekman et al., 2004). While ‘Bartlett’ pears should have a flesh firmness over 45 N upon entry into the shipment and distribution chain to avoid impact bruising and other mechanical damage (Mitcham and Thompson, 2004), optimum pear firmness for preferred juiciness ranges between 17.9 and 22.4 N (Kappel et al., 1995). Chen et al. (2003) pointed out that

‘d’Anjou’ pears reach the ideal eating quality with a flesh firmness of 14–23 N. Consequently, 1-MCP appropriate dose calibration is relatively difficult to manage with this fruit species. Although application of 1-MCP to ‘Bartlett’ pears can provide more flexibility to different commercial operations (grading, packaging, shipping and expenditure) requiring fruit resistant to physical damage (Trinchero et al., 2004), the exposure to unsuitable or uncontrolled concentrations may either bring about an excessive postponement of ripening or have no effect at all (Table 1). Plant material in general (Nanthachai et al., 2007) and wet wood in particular (Vallejo and Beaudry, 2006) deplete the environment of the treatment chamber of 1-MCP such that initial concentrations are rapidly altered. Wet wood may provide a huge undetermined number of non-specific sorption sites for 1-MCP which facilitates its rapid uncontrolled removal from the chamber. The mechanism for the increased sorption of 1-MCP by wet wood is not known (Vallejo and Beaudry, 2006), but evidence has been obtained suggesting that hydratation of matrix polysaccharides affects cell wall porosity (Zwieniecki et al., 2001); in turn, this could increase the retentiveness of 1-MCP by non-specific sorption sites in the cell wall. Together, our results confirm and demonstrate that effectiveness of 1-MCP on ‘Bartlett’ pears under commercial conditions can

Fig. 2. Combined effect of the bin material and 1-MCP application on hydrocooled ‘Bartlett’ pear firmness. Pears were hydrocooled in wooden (W) or plastic (P) bins and subsequently exposed or not to 0.5 ␮L L−1 1-MCP (0.5 and 0, respectively). Fruit were then stored at −0.5 ◦ C for 90 d. After cold storage, pears were kept at 20 ◦ C until they reached eating quality (17–23 N). Each firmness value represents the mean of 20 replicates. Means with a different letter are significantly different within a single storage duration, according to the Tukey’s test. No differences were detected in the pear performance between 1-MCP-untreated fruit hydrocooled in wooden and in plastic bins in previous experiments.

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be compromised by the presence of wooden bins, particularly if these are wet and 1-MCP concentration is relatively low (HC-0.3, Table 1). According to Vallejo and Beaudry (2006), dry extensively weathered oak can absorb more than 75% of initial 1-MCP in the storage environment after 24 h. Thus, 1-MCP partial depletion by wooden bins has probably occurred even when pears were forced-air cooled (Table 1). Not only wooden bins or boxes, but also cardboard materials are usually found in pear storage facilities and constitute non-physiological binding targets that compete with fruit for 1-MCP (Vallejo and Beaudry, 2006). This fact should be considered when evaluating or comparing the results obtained in previous laboratory research since pears have been exposed to 1MCP under different situations: unpacked (Calvo and Sozzi, 2004; Ekman et al., 2004), placed in bushel boxes (MacLean et al., 2007) or packed, either in cardboard or in wooden boxes (Trinchero et al., 2004; Chen and Spotts, 2005; Bai et al., 2006; Gapper et al., 2006). Similar precautions should be taken into account when scaling-up experimental information for industrial use. Moreover, if water evaporation from wet wooden bins after hydrocooling differs among chambers, dissimilar responses of ‘Bartlett’ pears to low 1-MCP doses can be expected, since 1-MCP potential binding on wooden materials evidence a significant degree of variation, depending on their level of moistening upon 1-MCP application (Vallejo and Beaudry, 2006; Sozzi, unpublished results). If available, and in order to avoid a major yet unpredictable source of 1-MCP sorption, it is highly advisable to use plastic bins followed by an accurate dose calibration when hydrocooling and 1-MCP techniques are to be combined. If pears are to be hydrocooled and stored in wooden bins, 1-MCP concentrations of up to 0.4–0.5 ␮L L−1 may be necessary to achieve a response (Table 2) depending on the length of the cold storage period. In conclusion, this research demonstrates that wet wooden bin material can compromise 1-MCP effectiveness in controlling ethylene action in ‘Bartlett’ pears. This fact should be taken into account when combining different technologies for European pear longterm storage. Acknowledgements The authors thank Jorge Aragón (Kleppe S.A.) for kindly providing materials and facilities for the experiments, and Rohm & Haas for technical and financial support. G.O.S. acknowledges the Consejo Nacional de Investigaciones Científicas y Técnicas, the Universidad de Buenos Aires (UBACyT Program) and the Agencia Nacional de Promoción Científica y Tecnológica for financial support. References Argenta, L.C., Fan, X., Mattheis, J.P., 2003. Influence of 1-methylcyclopropene on ripening, storage life, and volatile production by d’Anjou cv. pear fruit. J. Agric. Food Chem. 51, 3858–3864. ASHRAE, 2002. Methods of precooling fruits, vegetables, and cut flowers. In: Refrigeration Handbook, vol. 14. American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., Atlanta, GA, USA, pp. 3–4.

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