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Postharvest physiology, storage quality and physiological disorders of ‘Gem’ pear (Pyrus communis L.) treated with 1-methylcyclopropene
Scientia Horticulturae 240 (2018) 631–637
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Postharvest physiology, storage quality and physiological disorders of ‘Gem’ pear (Pyrus communis L.) treated with 1-methylcyclopropene Yu Donga,b, Yan Wanga, Todd C. Einhorna,c,
T
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a
Department of Horticulture, Oregon State University, Mid-Columbia Agricultural Research and Extension Center, 3005 Experiment Station Drive, Hood River, OR 97031, USA b College of Food and Bioengineering, Zhengzhou University of Light Industry, No. 166 Kexue Avenue, Zhengzhou, Henan 450002, PR China c Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ripening Ethylene production Fruit respiration Crisp Texture Eating quality
‘Gem’ is a crisp, juicy European pear (Pyrus communis L.) that can be consumed immediately at harvest or directly from cold storage. Alternatively, ‘Gem’ pears can ripen (5 d of 20 °C) to a soft, buttery, juicy texture once fruit accumulate 30 to 60 d of low temperature chill. In either condition, ‘Gem’ has a relatively short postharvest storage life of 5 months in regular air (RA). The purpose of this work was to evaluate two concentrations of 1methylcyclopropene (1-MCP), 0.15 and 0.3 μL L−1, to maintain crisp, juicy textural properties and extend the postharvest storage life of ‘Gem’. Fruit were treated with 1-MCP, held in −1.1 °C RA and evaluated monthly (+1 d at 20 °C) for 7 months. Only minor differences were observed between 0.15 and 0.3 μL L−1 1-MCP for any of the response factors assessed. The respiration (Rs) and ethylene production rates of non-treated fruit increased ∼2- and 30-fold, respectively, between 2 and 7 months. Fruit firmness (FF), peel chlorophyll content (IAD), and titratable acidity (TA) all decreased linearly over the 7-month storage period. Treatment with 1-MCP completely inhibited internal ethylene production for the first four months. Ethylene production increased linearly between 5 and 7 months to a maximum value ∼15% of non-treated fruit. 1-MCP similarly suppressed Rs. FF, IAD, and TA were all significantly higher for 1-MCP-treated fruit than non-treated fruit. 1-MCP maintained the crisp and juicy textural properties of non-ripened fruit throughout the entire 7-month experiment by inhibiting ripening, despite a five-day 20 °C ripening treatment. In contrast, non-treated ‘Gem’ ripened after 2 months; however, the eating quality of non-treated fruit decreased after 5 months. Poor eating quality was associated with mealiness and insufficient softening after ripening. Internal browning and scald were first observed in non-treated fruit following five months of RA and reached levels of 26% and 85%, respectively, after seven months. The development of scald was closely associated with the accumulation of α-farnesene and conjugated trienols (CTols) in the fruit skin. 1-MCP significantly reduced the incidence of internal browning and completely inhibited the development of scald. Overall, 0.15 μL L−1 1-MCP maintained texture and fruit quality for 7 months RA and reduced the incidence of physiological disorders.
1. Introduction
rendering ‘Gem’ unique among European pear cultivars. ‘Gem’ can also ripen to a soft, buttery and juicy dessert quality but requires a minimum of 30 d low temperature conditioning to attain ripening competency (Einhorn and Wang, 2016). To optimize fruit quality, the recommended harvest maturity of ‘Gem’, as indicated by flesh pressure, is 42 to 47 N. While other European pear cultivars are harvested at markedly higher FF, scuffing or peel damage of ‘Gem’ was not observed during commercial postharvest operations when harvested at these pressures (Einhorn and Wang, 2016). One drawback associated with a delayed harvest, however, is a short postharvest storage life. Postharvest RA storage of ‘Gem’ is four to five months with additional time in storage
‘Gem’ pear is a new, fire-blight resistant European pear (Pyrus communis L.) with a smooth, russet-free finish and red blush (Bell et al., 2014). ‘Gem’ is crisp and juicy at harvest and can be consumed directly from the tree (i.e., without ripening). While crispness is a desirable sensory attribute of apple (Daillant-Spinnler et al., 1996) a small, but significant population of pear consumers prefer crisp pears (Jaeger et al., 2003). The textural sensory attributes crispness, hardness and fracturability were strongly correlated for several European pear cultivars, but juiciness and crispness were not (Chauvin et al., 2010),
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Corresponding author. Present address: Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA. E-mail address: [email protected] (T.C. Einhorn).
https://doi.org/10.1016/j.scienta.2018.06.073 Received 14 May 2018; Accepted 26 June 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Ethylene production (A) and respiration rates (B) of ‘Gem’ pears influenced by 0.15 and 0.3 μL L−1 1-methylcyclopropene (1-MCP) following 7 months storage at –1.1 °C plus 1 day at 20 °C. Vertical bars represent standard error (SE). Means were separated among treatments by Fisher’s protected least significant difference test (LSD) (P < 0.05).
Fig. 2. Fruit firmness (A), peel chlorophyll content (B), soluble solids concentration. (SSC) (C), and titratable acidity (TA) (D) of ‘Gem’ pears as influenced by 0.15 and 0.3 μL L−1 1-MCP following 7 months storage at –1.1 °C plus 24 h at 20 °C. Vertical bars represent SE. Means were separated among treatments by Fisher’s protected LSD (P < 0.05).
and winter pears, ‘d’Anjou’ (Wang, 2016) and ‘Comice’ (Wang and Sugar, 2013) to 1-MCP has been previously examined. Generally, summer pears require two-fold 1-MCP concentration than winter pears (i.e, 0.3 μL L−1 versus 0.15 μL L−1 for summer and winter pears, respectively (Wang, 2016; Xie et al., 2015). 1-MCP increased storage life of pears, in part by reducing skin scald, internal browning and impact bruising but concomitantly inhibited fruit ripening capacity, which has limited its utilization (Bai et al., 2006; Chen and Spotts, 2005; Xie et al., 2014). ‘Gem’ pear, with its distinct crisp, juicy texture may prove an ideal candidate for 1-MCP given that the benefits of minimizing physiological disorders and extending the postharvest storage life would not come at the cost of softening inhibition. The objectives of this study, therefore, were to investigate the effect of 1-MCP on physiology, storage quality, eating quality and physiological disorder development of ‘Gem’ pears following 7 months of RA storage plus 5 days at 20 °C. Given that the physiology of ‘Gem’ cannot be easily characterized as either a summer or winter pear, two rates of 1-MCP (0.15 and 0.3 μL L−1) were evaluated.
resulting in a loss of ripening capacity (Einhorn and Wang, 2016) and the occurrence of storage disorders (Wang, unpublished data). Developing ‘Gem’-specific postharvest protocols to increase consistency of non-ripened fruit quality and storage life is an area in need of research attention. European pears are climacteric fruit and are susceptible to postharvest disorders during long-term RA storage due to high rates of internal ethylene production (Villalobos-Acuña and Mitcham, 2008). The use of 1-methylcyclopropene (1-MCP) to bind ethylene receptors has been successfully applied by commercial postharvest operations to inhibit ethylene action and maintain higher quality of climacteric fruits such as apple, pear and kiwifruit (Watkins, 2006). Several factors such as genotype, 1-MCP concentration, duration of exposure, and storage temperature can all modulate a fruit’s response to 1-MCP (DeEll and Ehsani-Moghaddam, 2011; Wang and Sugar, 2015; Xie et al., 2014). For pear, cultivars can be grouped broadly into two categories, summer and winter pears, based on differences in their development, storability, ripening and response to ethylene. The response of summer pears, ‘Bartlett’ (Wang and Sugar, 2015) and ‘Starkrimson’ (Xie et al., 2015) 632
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Fig. 3. Effect of 0.15 and 0.3 μL L−1 1-MCP and duration of RA storage after 5 days at 20 °C on ripening capacity (expressed as fruit firmness) of ‘Gem’ pears. Vertical bars represented SE. Means were separated among treatments by Fisher’s protected LSD (P < 0.05).
2. Materials and methods 2.1. Plant material and experimental design Twelve-year-old ‘Gem’ trees on Old Home × Farmingdale 97 rootstock were selected for uniformity at Oregon State University’s MidColumbia Agriculture Research and Extension Center in Hood River, Oregon (latitude 45.7 °N, longitude 121.5 °W, elevation 150 m, average annual rainfall ∼800 mm). Trees were hand-thinned at 35 days after full bloom by reducing spur crop load to one to two fruit to optimize fruit quality at maturity (Castagnoli et al., 2011). All other production practices were performed according to commercial standards. Fruit were harvested when flesh pressures softened to 47.2 N. Defect-free fruit were randomly packed into wooden boxes (∼80 fruit per box) with standard perforated polyethylene liners and immediately placed in a RA room (−1.1 °C, > 90% RH). Boxes were divided into three groups and each group comprised three replicates. On the second day after harvest, one group of boxes was exposed to 0.15 μL L−1 1-MCP (SmartFresh®, AgroFresh, Spring House, PA, USA), another group of boxes was exposed to 0.3 μL L−1 1-MCP and the remaining boxes were non-treated. 1-MCP was applied in an airtight room (39.8 m3) at 0 °C. Air was circulated with a fan for 24 h according to the application procedures recommended by the commercial manufacturer. After treatment, all boxes of fruit were stored in a single RA room. Beginning one month after 1-MCP treatment, fruit were randomly selected from each treatment replicate, placed in a 20 °C room and analyzed after 24 h, or 5 d, depending on the response variable evaluated. The experimental period was seven months. Fig. 4. Sensory scores of crispness, juiciness, and flavor in ‘Gem’ pears as influenced by 0.15 and 0.3 μL L−1 1-MCP after 5 (A), 6 (B), and 7 (C) months RA storage at –1.1 °C plus 5 days at 20 °C.
2.2. Ethylene production and Rs rates Ethylene production and Rs rates were determined on five fruit per replicate following 24 h at 20 °C after removal from RA storage. Additionally, ethylene production and Rs rates were quantified after 5 d of 20 °C at the final, seven-month evaluation. Fruit were weighed, placed in a 3.8-L airtight jar at 20 °C, and sealed for 1 h prior to withdrawing a sample of headspace gas using a 1-mL gas-tight syringe. Ethylene production rate was measured by injecting the gas sample into a gas chromatograph (GC-8 A, Shimadzu, Tokyo, Japan) equipped with a flame ionization detector and a Porapack Q column (80/100 mesh, 3 mm diameter, 2 m long). The carrier gas was nitrogen at a flow rate of 0.83 mL s−1, the oven temperature was 90 °C, and the injector and
detector temperatures were 140 °C. An external standard of ethylene was used for calibration and data were expressed as ng kg-1 s-1. For quantification of Rs, the headspace gas CO2 concentration was measured using a CO2 analyzer (900161, Bridge Analyzers Inc., Alameda, CA, USA) and data were expressed as CO2 μg kg−1 s−1. 2.3. Fruit quality attributes and sensory score For determination of peel chlorophyll content and FF, twenty fruit were removed monthly from each treatment replicate and placed in 633
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Fig. 5. Internal browning (A and C) and scald (B and D) of ‘Gem’ pear as influenced by 0.15 and 0.3 μL L−1 1-MCP after 5, 6 and 7 months RA storage at –1.1 °C. Evaluations were performed after 24 h at 20 °C. Vertical bars represent standard error (SE). Means were separated among treatments by Fisher’s protected LSD. Different letters indicate significant differences at P < 0.05.
surface area displaying browning and/or cavities.
20 °C. Ten fruit were assessed after 24 h for chlorophyll content and FF and ten fruit were assessed after 5 d for FF only. Peel chlorophyll content was estimated at two locations on the fruit surface opposite each other near the equator using a DA meter (Sinteleia, Bolonga, Italy) and expressed as IAD value (Ziosi et al., 2008). Fruit firmness (N) was measured using a fruit texture analyzer (GS-14, Güss Manufacturing Ltd., Strand, South Africa) with an 8-mm probe and a penetration speed of 10 mm s−1 at similar locations on the fruit where chlorophyll content had been estimated after removal of a 2-mm thick disc of peel. After peel chlorophyll and FF determination, 100 g of flesh tissue was blended for 30 s in a juice extractor (Acme 6001, Acme Juicer Manufacturing Co, Sierra Madre, CA, USA) equipped with a uniform strip of milk filter (Chen and Spotts, 2005). The SSC (%) of juice was measured using a digital refractometer (PAL-1, ATAGO, Tokyo, Japan). TA (%) was determined by titrating 10 mL of the juice and 40 mL of deionized water to an end-point pH 8.1 using 0.1 N NaOH with an automated titration system (DL-15, Mettler-Toledo Inc., Columbus, OH, USA) and expressed as percentage of malic acid equivalents. Ten additional fruit per replicate were removed from RA storage and ripened for 5 d at 20 °C at 5, 6 and 7 months. A panel of five people was trained to recognize and score the sensory quality of both ripened and non-ripened pears. Sensory quality of crispness, juiciness, mealiness, and flavor was scored on a nine-point hedonic scale as previously described (Cliff et al., 1999), where 1 = extremely poor, 3 = poor, 5 = acceptable (limit of marketability), 7 = good, 9 = excellent.
2.5. Malondialdehyde (MDA), α-farnesene, and conjugated trienols (CTols) After 7 months RA and 5 d at 20 °C, skin tissue was removed (2-mm thickness) and immediately immersed in liquid nitrogen. MDA content was determined as described by Yuan et al. (2010). Briefly, tissue samples (1.5 g) were homogenized in 5 mL of 100 g L−1 trichloroacetic acid, then centrifuged at 10,000g for 15 min at 4 °C A 2 mL aliquot of the supernatant was added to 2 mL of 6.7 g L−1 thiobarbituric acid. Absorbance was measured at 450, 532, and 600 nm using a spectrophotometer (Ultrospec 3100 Pro, Biochrom Ltd, Cambridge, UK) and calculated according to the formula: 6.45 × (A532 – A600) – 0.56 × A450. Data were expressed on a fresh weight basis as mmol kg-1. αFarnesene and CTols were measured as described by Anet (1972). Excised discs of fruit skin (9 mm in diameter and 2 mm thickness) were removed from ten fruit per replication using a cork borer and immersed in 20 mL of hexane (HPLC-grade, Sigma-Aldrich Co., St. Louis, MO, USA) in a 50-mL centrifuge tube. Tubes were continuously shaken with a variable rotator (R4139-5 A, Tek-Pro Tek-Tator V, Miami, FL, USA) at 20 °C for 30 min. After extraction, absorbance at 232 nm (a-farnesene) and 281–290 nm (CTols) was recorded. Concentrations of a-farnesene and CTols were calculated using the molar extinction coefficients (ε) 27,740 at 232 nm and 25,000 at 281–290 nm for a-farnesene and CTols, respectively, and expressed on a fresh weight basis as mmol kg-1.
2.4. Physiological disorders 2.6. Statistical analysis After removal from RA storage, thirty fruit per replicate were placed in 20 °C for 24 h and assessed for surface scald, then cut longitudinally and transversely to assess internal browning. Scald incidence was expressed as the percentage of fruit affected with commercially unacceptable scald (> 0.6 mm2) as previously described (Wang, 2016). Internal browning was characterized by chocolate-colored tissue in the cortex of fruit, often associated with the development of cavities. Internal browning incidence was expressed as a percentage of the cut fruit
The experimental design was a CRD (completely randomized design). There were three replications per treatment for all parameters evaluated and for all evaluation dates. The data were subjected to analysis of variance (ANOVA) using StatSoft® Statistica (version 6, StatSoft, Tulsa, OK, USA). When appropriate, means were separated by Fisher’s protected least significant difference test (LSD) test at P < 0.05. 634
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Fig. 6. Effect of 0.15 μL L−1 1-MCP on scald development (A), fruit firmness (B), respiration rate (C), ethylene production rate (D), malondialdehyde content (E), αfarnesene (F) and conjugated trienols (G) of ‘Gem’ pears following 7 months storage at –1.1 °C plus 1 and 5 days at 20 °C. Vertical bars represent SE. Means were separated among treatments by Fisher’s protected LSD. Different letters indicate significant differences at P < 0.05.
the first four months of RA storage. Ethylene production rose slightly between 5 and 7 months to 1.01 and 0.56 ng kg−1 s−1 for 0.15 and 0.3 μL L−1 1-MCP, respectively, though these numeric differences were not significant. Both rates of 1-MCP were effective at significantly reducing Rs compared to non-treated fruit.
3. Results 3.1. Ethylene production and Rs rates At harvest, internal ethylene production was not detectable and fruit Rs rate was ∼ 5.08 CO2 mg kg−1 s−1 (Fig. 1A and B). After two months of RA, ethylene production and Rs rates of non-treated control fruit markedly increased reaching maximum levels of 5.25 ng kg−1 s−1 and 9.25 CO2 mg kg−1 s−1, respectively, after 7 months of RA storage (Fig. 1). Both rates of 1-MCP completely inhibited ethylene production
3.2. Fruit quality attributes and sensory score Over 7 months of RA storage, FF of control fruit gradually decreased from 47 to 38 N (Fig. 2A). 1-MCP maintained slightly higher FF, 635
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temperature conditioning to ripen to dessert quality and the fruit lost their ripening capacity after 5 months, characterized by mealy texture and overall poor eating quality (Einhorn and Wang, 2016). We observed similar results in the present study. Several years of informal sensory evaluations of ‘Gem’ indicated a strong preference for the fruit’s crisp and juicy characteristics (Turner and Einhorn, unpublished data) and support programs that maintain and/or improve ‘Gem’s’ non-ripened quality, and preferably lengthen storage life. 1-MCP extended the postharvest life of ‘Gem’ and completely inhibited ripening capacity by suppressing ethylene synthesis and Rs throughout the entire 7-month storage period resulting in the maintenance of FF, peel chlorophyll, and TA as similarly observed in ‘Bosc’ pear (Xie et al., 2017). For European pear, an increase in 1-MCP dose occupies more ethylene receptor-binding sites and results in greater ripening inhibition (Chiriboga et al., 2011). Doubling the concentration of 1-MCP from 0.15 to 0.3 μL L−1 had minimum effect on chlorophyll, ethylene generation and Rs rate, and no effect on FF, TA or the incidence of physiological disorders in the present study. Thus, 0.15 μL L−1 1-MCP appeared adequate to maintain quality and extend ‘Gem’ pear postharvest life. These results support preferential treatment of ‘Gem’ pears with 1-MCP for consumers that prefer crisp fruit. Under these conditions, it is clear that firmness would be maintained even if left unrefrigerated after purchase. Clearly, having a portion of one cultivar treated with 1-MCP to maintain crispness while another remained non-treated to allow ripening, side-by-side in the marketplace, would require supportive retail information, a significant marketing effort and physical separation of the two populations to avoid considerable confusion. Irrespective of the textural status of ‘Gem’, the retail chain has no tolerance for the presence of postharvest-related physiological disorders such as internal browning, storage scald, black-end, and cork spot. Among these disorders, internal browning occurs in the cortex as firm, brown, sharply defined areas of senesced tissue (Raese, 1989). For ‘Gem’, the occurrence of typical internal browning limited its postharvest life (Einhorn and Wang, 2016). In this study, internal browning was first observed at 5 months and increased in severity by 7 months. 1MCP delayed internal browning incidence and severity. In fact, all fruit treated with 1-MCP were within the commercially acceptable range for internal browning. Higher storage temperatures, ethylene, CO2 concentration, and metabolism were all closely associated with the development of internal browning in European pear (Franck et al., 2007; Giraldo et al., 2005; Ma and Chen, 2003; Saquet et al., 2003; Wang and Sugar, 2013). 1-MCP would appear an effective strategy for mitigating internal browning of ‘Gem’ pears, in a manner similar to that observed in ‘d’Anjou’ (Bai et al., 2006), ‘Starkrimson’ (Xie et al., 2015), and ‘Bartlett’ (Ekman et al., 2004). The onset of scald development of non-treated ‘Gem’ also appeared after 5 months but unlike internal browning, reached quite severe levels by 7 months. Symptoms appeared as brown to black discolored regions of the fruit surface, manifesting as lesions or wrinkled patches (Ingle, 2001). 1-MCP effectively inhibited scald development throughout the entire 7-month RA storage period even after 5 d at 20 °C. Similar results were reported for ‘Rocha’ (Isidoro and Almeida, 2006), ‘d’Anjou’ (Wang, 2016), and ‘Wujiuxiang’ (Zhou et al., 2017) pears, indicating that the combination of 1-MCP and cold storage inhibited ethylene biosynthesis and related receptor gene expression (Xie et al., 2014; Zhou et al., 2017). In the absence of 1-MCP, scald appears to be induced by chilling injury and may be associated with the production of reactive oxygen species (ROS) caused by α-farnesene (3,7,11-trimethyl,1,3,6,10dodecatetraene) metabolism (Lurie and Watkins, 2012). In pear, αfarnesene mostly accumulated at high levels in the wax layer and increased with ethylene production during cold storage and ripening (Yazdani et al., 2011). In the present study, ethylene production and αfarnesene content of non-treated fruit were significantly higher than 1MCP-treated fruit after 7 months of RA storage. The high concentration of α-farnesene may stimulate self-oxidation resulting in conjugated
irrespective of concentration (Fig. 2A). 1-MCP significantly enhanced peel chlorophyll content, as observed by higher IAD values, compared to non-treated fruit (Fig. 2B). IAD values were highest for 0.3 μL L−1 1MCP-treated fruit. SSC was unaffected by treatment and remained relatively constant throughout the 7-month RA period (Fig. 2C). TA of non-treated fruit consistently decreased between harvest (0.37%) and 7 months RA (0.25%) (Fig. 2D). 1-MCP significantly reduced TA loss over the entire storage period, though differences between 1-MCP concentrations were not detected (Fig. 2D). Non-treated fruit required two months of low temperature exposure to soften to FF levels characteristic of ripe fruit when held at 20 °C for five days (Fig. 3). Beyond five months of RA, flesh softening was impaired and increasingly higher FF occurred with each additional month of RA, indicating a loss in ripening capacity. In contrast, 1-MCP completely inhibited softening for the entire seven-month experimental period (Fig. 3). A sensory evaluation of ‘Gem’ pears between 5–7 months RA plus 5 d ripening at 20 °C indicated that control fruit were unacceptable for marketing after 5 months due to intermediate softening and mealy texture (Fig. 4A–C). However, ‘Gem’ pears exposed to 0.15 and 0.3 μL L−1 1-MCP maintained the crisp and juicy properties at harvest throughout the entire storage period, even after a 5 d ripening period, and ranked significantly higher than control fruit (Fig. 4A–C). 3.3. Physiological disorders The physiological disorder, internal browning was first observed in non-treated fruit at 5 months RA (Fig. 5A and C). Internal browning incidence increased over the subsequent two months to maximum levels of 26% (Fig. 5C). 1-MCP significantly delayed the onset of internal browning to 7 months, at which time only a slight percentage of internal browning was detected (Fig. 5C). There was no difference between 1-MCP concentrations on the inhibition of internal browning. Non-treated fruit began displaying brown and black patches on the skin surface following 5 months of RA storage, becoming more severe at 6 months and occupying 85% of the surface area after 7 months RA (Fig. 5B and D). In contrast, 1-MCP-treated fruit remained free of scald during the entire 7-month experimental period. α-Farnesene and CTols were significantly lower in 1-MCP-treated fruit compared to nontreated fruit after 7 months RA (Fig. 6F and G). MDA was initially higher in 1-MCP-treated fruit but was significantly reduced following 5 d at 20 °C (Fig. 6E). Notably, α-farnesene and CTols content did not increase after 5 d at 20 °C (Fig. 6F and G). Reduced content of these compounds was associated with 1-MCP inhibition of fruit softening, ethylene production and Rs (Fig. 6B–D). 4. Discussion The genetic, biochemical and physiological changes during ripening that profoundly alter the textural and sensory properties of European pears render them unique among climacteric fruits. The classical dessert pear, characterized by a soft, buttery and juicy texture is contingent on a preconditioning treatment by low temperature or ethylene gas (Chen and Mellenthin, 1981; Sugar and Basile, 2009, 2013; VillalobosAcuña and Mitcham, 2008). While cultivars differ with respect to the minimum duration of low temperature conditioning, conditioning periods can be reduced by delaying harvest and/or storing fruit at intermediate temperatures (i.e., between −1.1 and 10 °C) (Chen and Mellenthin, 1981; Ma et al., 2000; Sugar and Einhorn, 2011). Pear ripenability is a notoriously difficult task for consumers to accomplish, and is arguably the single-most important barrier to repeat purchases (Kevin Moffitt, personal communication) as indicated by consumers’ willingness to pay for properly conditioned and ripened pears (Gallardo et al., 2011). For ‘Gem’, we demonstrated that delaying harvest to FF values between 42–50 N promoted larger fruit size, higher SSC, juiciness and development of mild ‘pear’ flavor. However, when harvested within this pressure range, ‘Gem’ still required > 30 d of low 636
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Technol. 49, 187–200. Wang, Y., 2016. Storage temperature, controlled atmosphere, and 1-methylcyclopropene effects on α-farnesene, conjugated trienols, and peroxidation in relation with superficial scald, pithy brown core, and fruit quality of ‘d’Anjou’ pears during long-term storage. J. Am. Soc. Hort. Sci. 141, 177–185. Wang, Y., Sugar, D., 2013. Ripening behavior and quality of modified atmosphere packed ‘Doyenne du Comice’ pears during cold storage and simulated transit. Postharvest Biol. Technol. 81, 51–59. Wang, Y., Sugar, D., 2015. 1-MCP efficacy in extending storage life of ‘Bartlett’ pears is affected by harvest maturity, production elevation, and holding temperature during treatment delay. Postharvest Biol. Technol. 103, 1–8. Watkins, C.B., 2006. The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables. Biotechnol. Adv. 24, 389–409. Xie, X., Song, J., Wang, Y., Sugar, D., 2014. Ethylene synthesis, ripening capacity, and superficial scald inhibition in 1-MCP treated ‘d’Anjou’ pears are affected by storage temperature. Postharvest Biol. Technol. 97, 1–10. Xie, X., Einhorn, T.C., Wang, Y., 2015. Inhibition of ethylene biosynthesis and associated gene expression by aminoethoxyvinylglycine and 1-methylcyclopropene and their consequences on eating quality and internal browning of ‘Starkrimson’ pears. J. Am. Soc. Hort. Sci. 140, 587–596. Xie, X., Fang, C., Wang, Y., 2017. Inhibition of ethylene biosynthesis and perception by 1methylcyclopropene and its consequences on chlorophyll catabolism and storage quality of ‘Bosc’ pears. J. Am. Soc. Hort. Sci. 142, 92–100. Yazdani, N., Arzani, K., Mostofi, Y., Shekarchi, M., 2011. α-Farnesene and antioxidative enzyme systems in Asian pear (Pyrus serotina Rehd.) fruit. Postharvest Biol. Technol. 59, 227–231. Yuan, G., Sun, B., Yuan, J., Wang, Q., 2010. Effect of 1-methylcyclopropene on shelf life, visual quality, antioxidant enzymes and health-promoting compounds in broccoli florets. Food Chem. 118, 774–781. Zhou, S., Cheng, Y., Guan, J., 2017. The molecular basis of superficial scald development related to ethylene perception and α-farnesene metabolism in ‘Wujiuxiang’ pear. Sci. Hort. 216, 76–82. Ziosi, V., Noferini, M., Fiori, G., Tadiello, A., Trainotti, L., Casadoro, G., Costa, G., 2008. A new index based on vis spectroscopy to characterize the progression of ripening in peach fruit. Postharvest Biol. Technol. 49, 319–329.
trienols (CTols) that directly cause damage to cells in peel tissue (Gapper et al., 2006). Moreover, MDA levels are associated with membrane lipid peroxidation, indirect loss of membrane integrity during fruit deterioration in storage and chilling injury (Liu et al., 2013; Lurie and Watkins, 2012). In the present study, even though significant lower MDA was observed in non-treated ‘Gem’ after removal from cold storage at 7 months, MDA increased dramatically and was higher than 1-MCP-treated fruit after 5 d at 20 °C, implicating MDA levels in fruit skin tissue as a plausible contributor to cell damage. In contrast, 1-MCPtreated ‘Gem’ fruit had relatively low ethylene production, α-farnesene accumulation and CTols as previously shown for apple (Arquiza et al., 2005). Additional research to evaluate the effects of controlled atmospheres and antioxidants on extending postharvest life and controlling physiological disorders of ‘Gem’ pears would be valuable. 5. Conclusion A low dose of 1-MCP (0.15 μL L−1) effectively maintained fruit quality attributes (i.e., FF, IAD, and TA) and suppressed ethylene production, Rs and the incidence of internal browning and scald to prolong storage life of ‘Gem’ pears from 5 to 7 months in RA. The eating quality of pears treated with 1-MCP was superior to non-treated fruit between 5 and 7 months storage; thus, 1-MCP is a potential tool to maintain the unique crisp and juicy attributes of ‘Gem’. However, for consumers that prefer a ripened pear, 1-MCP ripening-inhibition would be undesirable. Acknowledgements We are grateful to the Columbia Gorge Fruit Growers Association and the NW Fresh Pear Research Committees for their generous financial support. References Anet, E.F.L.J., 1972. Superficial scald, a functional disorder of stored apples. IX. Effect of maturity and ventilation. J. Sci. Food Agric. 23, 763–769. Arquiza, J.M.R.A., Hay, A.G., Nock, J.F., Watkins, C.B., 2005. 1-Methylcyclopropene interactions with diphenylamine on diphenylamine degradation, α-farnesene and conjugated trienol concentrations, and polyphenol oxidase and peroxidase activities in apple fruit. J. Agric. Food Chem. 53, 7565–7570. Bai, J., Mattheis, J.P., Reed, N., 2006. Re-initiating softening ability of 1-methylcyclopropene-treated ‘Bartlett’ and ‘d’Anjou’ pears after regular air or controlled atmosphere storage. J. Hort. Sci. Biotechnol. 81, 959–964. Bell, R.L., Van der Zwet, T., Castagnoli, S., Einhorn, T.C., Turner, J.D., Spotts, R., Moulton, G.A., Reighard, G.L., Shane, W.W., 2014. ‘Gem’ pear. HortScience 49, 361–363. Castagnoli, S., Turner, J., Einhorn, T.C., Bell, R., 2011. Performance of US 71655-014, a fire blight resistant pear selection from the USA. Acta Hort. 909, 177–182. Chauvin, M.A., Ross, C.F., Pitts, M., Kupferman, E., Swanson, B., 2010. Relationship between instrumental and sensory determination of apple and pear texture. J. Food Qual. 33, 181–198. Chen, P.M., Mellenthin, W.M., 1981. Effects of harvest date on ripening capacity and postharvest life of ‘d’Anjou’ pears. J. Am. Soc. Hort. Sci. 106, 38–42. Chen, P.M., Spotts, R.A., 2005. Changes in ripening behaviors of 1-MCP-treated ‘d’Anjou’ pears after storage. Int. J. Fruit Sci. 5, 3–17. Chiriboga, M.A., Schotsmans, W.C., Larrigaudière, C., Dupille, E., Recasens, I., 2011. How to prevent ripening blockage in 1-MCP-treated ‘Conference’ pears. J. Sci. Food Agric. 91, 1781–1788. Cliff, M.A., Sanford, K., Johnston, E., 1999. Evaluation of hedonic scores and R-indices for visual, flavour and texture preferences of apple cultivars by British Columbian and Nova Scotian consumers. Can. J. Plant Sci. 79, 395–399. Daillant-Spinnler, B., Macfie, H.J.H., Beyts, P.K., Hedder-Ley, D., 1996. Relationships between perceived sensory properties and major preference directions of 12 varieties of apples from the southern hemisphere. Food Qual. Pref. 7, 113–126. DeEll, J.R., Ehsani-Moghaddam, B., 2011. Timing of postharvest 1-methylcyclopropene treatment affects Bartlett pear quality after storage. Can. J. Plant Sci. 91, 853–858. Einhorn, T.C., Wang, Y., 2016. Characterizing the effect of harvest maturity on ripening capacity, postharvest fruit quality, and storage life of ‘Gem’ pear. J. Am. Pom. Soc. 70, 26–35. Ekman, J.H., Clayton, M., Biasi, W.V., Mitcham, E.J., 2004. Interactions between 1-MCP
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Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti
Postharvest physiology, storage quality and physiological disorders of ‘Gem’ pear (Pyrus communis L.) treated with 1-methylcyclopropene Yu Donga,b, Yan Wanga, Todd C. Einhorna,c,
T
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a
Department of Horticulture, Oregon State University, Mid-Columbia Agricultural Research and Extension Center, 3005 Experiment Station Drive, Hood River, OR 97031, USA b College of Food and Bioengineering, Zhengzhou University of Light Industry, No. 166 Kexue Avenue, Zhengzhou, Henan 450002, PR China c Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Ripening Ethylene production Fruit respiration Crisp Texture Eating quality
‘Gem’ is a crisp, juicy European pear (Pyrus communis L.) that can be consumed immediately at harvest or directly from cold storage. Alternatively, ‘Gem’ pears can ripen (5 d of 20 °C) to a soft, buttery, juicy texture once fruit accumulate 30 to 60 d of low temperature chill. In either condition, ‘Gem’ has a relatively short postharvest storage life of 5 months in regular air (RA). The purpose of this work was to evaluate two concentrations of 1methylcyclopropene (1-MCP), 0.15 and 0.3 μL L−1, to maintain crisp, juicy textural properties and extend the postharvest storage life of ‘Gem’. Fruit were treated with 1-MCP, held in −1.1 °C RA and evaluated monthly (+1 d at 20 °C) for 7 months. Only minor differences were observed between 0.15 and 0.3 μL L−1 1-MCP for any of the response factors assessed. The respiration (Rs) and ethylene production rates of non-treated fruit increased ∼2- and 30-fold, respectively, between 2 and 7 months. Fruit firmness (FF), peel chlorophyll content (IAD), and titratable acidity (TA) all decreased linearly over the 7-month storage period. Treatment with 1-MCP completely inhibited internal ethylene production for the first four months. Ethylene production increased linearly between 5 and 7 months to a maximum value ∼15% of non-treated fruit. 1-MCP similarly suppressed Rs. FF, IAD, and TA were all significantly higher for 1-MCP-treated fruit than non-treated fruit. 1-MCP maintained the crisp and juicy textural properties of non-ripened fruit throughout the entire 7-month experiment by inhibiting ripening, despite a five-day 20 °C ripening treatment. In contrast, non-treated ‘Gem’ ripened after 2 months; however, the eating quality of non-treated fruit decreased after 5 months. Poor eating quality was associated with mealiness and insufficient softening after ripening. Internal browning and scald were first observed in non-treated fruit following five months of RA and reached levels of 26% and 85%, respectively, after seven months. The development of scald was closely associated with the accumulation of α-farnesene and conjugated trienols (CTols) in the fruit skin. 1-MCP significantly reduced the incidence of internal browning and completely inhibited the development of scald. Overall, 0.15 μL L−1 1-MCP maintained texture and fruit quality for 7 months RA and reduced the incidence of physiological disorders.
1. Introduction
rendering ‘Gem’ unique among European pear cultivars. ‘Gem’ can also ripen to a soft, buttery and juicy dessert quality but requires a minimum of 30 d low temperature conditioning to attain ripening competency (Einhorn and Wang, 2016). To optimize fruit quality, the recommended harvest maturity of ‘Gem’, as indicated by flesh pressure, is 42 to 47 N. While other European pear cultivars are harvested at markedly higher FF, scuffing or peel damage of ‘Gem’ was not observed during commercial postharvest operations when harvested at these pressures (Einhorn and Wang, 2016). One drawback associated with a delayed harvest, however, is a short postharvest storage life. Postharvest RA storage of ‘Gem’ is four to five months with additional time in storage
‘Gem’ pear is a new, fire-blight resistant European pear (Pyrus communis L.) with a smooth, russet-free finish and red blush (Bell et al., 2014). ‘Gem’ is crisp and juicy at harvest and can be consumed directly from the tree (i.e., without ripening). While crispness is a desirable sensory attribute of apple (Daillant-Spinnler et al., 1996) a small, but significant population of pear consumers prefer crisp pears (Jaeger et al., 2003). The textural sensory attributes crispness, hardness and fracturability were strongly correlated for several European pear cultivars, but juiciness and crispness were not (Chauvin et al., 2010),
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Corresponding author. Present address: Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA. E-mail address: [email protected] (T.C. Einhorn).
https://doi.org/10.1016/j.scienta.2018.06.073 Received 14 May 2018; Accepted 26 June 2018 0304-4238/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Ethylene production (A) and respiration rates (B) of ‘Gem’ pears influenced by 0.15 and 0.3 μL L−1 1-methylcyclopropene (1-MCP) following 7 months storage at –1.1 °C plus 1 day at 20 °C. Vertical bars represent standard error (SE). Means were separated among treatments by Fisher’s protected least significant difference test (LSD) (P < 0.05).
Fig. 2. Fruit firmness (A), peel chlorophyll content (B), soluble solids concentration. (SSC) (C), and titratable acidity (TA) (D) of ‘Gem’ pears as influenced by 0.15 and 0.3 μL L−1 1-MCP following 7 months storage at –1.1 °C plus 24 h at 20 °C. Vertical bars represent SE. Means were separated among treatments by Fisher’s protected LSD (P < 0.05).
and winter pears, ‘d’Anjou’ (Wang, 2016) and ‘Comice’ (Wang and Sugar, 2013) to 1-MCP has been previously examined. Generally, summer pears require two-fold 1-MCP concentration than winter pears (i.e, 0.3 μL L−1 versus 0.15 μL L−1 for summer and winter pears, respectively (Wang, 2016; Xie et al., 2015). 1-MCP increased storage life of pears, in part by reducing skin scald, internal browning and impact bruising but concomitantly inhibited fruit ripening capacity, which has limited its utilization (Bai et al., 2006; Chen and Spotts, 2005; Xie et al., 2014). ‘Gem’ pear, with its distinct crisp, juicy texture may prove an ideal candidate for 1-MCP given that the benefits of minimizing physiological disorders and extending the postharvest storage life would not come at the cost of softening inhibition. The objectives of this study, therefore, were to investigate the effect of 1-MCP on physiology, storage quality, eating quality and physiological disorder development of ‘Gem’ pears following 7 months of RA storage plus 5 days at 20 °C. Given that the physiology of ‘Gem’ cannot be easily characterized as either a summer or winter pear, two rates of 1-MCP (0.15 and 0.3 μL L−1) were evaluated.
resulting in a loss of ripening capacity (Einhorn and Wang, 2016) and the occurrence of storage disorders (Wang, unpublished data). Developing ‘Gem’-specific postharvest protocols to increase consistency of non-ripened fruit quality and storage life is an area in need of research attention. European pears are climacteric fruit and are susceptible to postharvest disorders during long-term RA storage due to high rates of internal ethylene production (Villalobos-Acuña and Mitcham, 2008). The use of 1-methylcyclopropene (1-MCP) to bind ethylene receptors has been successfully applied by commercial postharvest operations to inhibit ethylene action and maintain higher quality of climacteric fruits such as apple, pear and kiwifruit (Watkins, 2006). Several factors such as genotype, 1-MCP concentration, duration of exposure, and storage temperature can all modulate a fruit’s response to 1-MCP (DeEll and Ehsani-Moghaddam, 2011; Wang and Sugar, 2015; Xie et al., 2014). For pear, cultivars can be grouped broadly into two categories, summer and winter pears, based on differences in their development, storability, ripening and response to ethylene. The response of summer pears, ‘Bartlett’ (Wang and Sugar, 2015) and ‘Starkrimson’ (Xie et al., 2015) 632
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Fig. 3. Effect of 0.15 and 0.3 μL L−1 1-MCP and duration of RA storage after 5 days at 20 °C on ripening capacity (expressed as fruit firmness) of ‘Gem’ pears. Vertical bars represented SE. Means were separated among treatments by Fisher’s protected LSD (P < 0.05).
2. Materials and methods 2.1. Plant material and experimental design Twelve-year-old ‘Gem’ trees on Old Home × Farmingdale 97 rootstock were selected for uniformity at Oregon State University’s MidColumbia Agriculture Research and Extension Center in Hood River, Oregon (latitude 45.7 °N, longitude 121.5 °W, elevation 150 m, average annual rainfall ∼800 mm). Trees were hand-thinned at 35 days after full bloom by reducing spur crop load to one to two fruit to optimize fruit quality at maturity (Castagnoli et al., 2011). All other production practices were performed according to commercial standards. Fruit were harvested when flesh pressures softened to 47.2 N. Defect-free fruit were randomly packed into wooden boxes (∼80 fruit per box) with standard perforated polyethylene liners and immediately placed in a RA room (−1.1 °C, > 90% RH). Boxes were divided into three groups and each group comprised three replicates. On the second day after harvest, one group of boxes was exposed to 0.15 μL L−1 1-MCP (SmartFresh®, AgroFresh, Spring House, PA, USA), another group of boxes was exposed to 0.3 μL L−1 1-MCP and the remaining boxes were non-treated. 1-MCP was applied in an airtight room (39.8 m3) at 0 °C. Air was circulated with a fan for 24 h according to the application procedures recommended by the commercial manufacturer. After treatment, all boxes of fruit were stored in a single RA room. Beginning one month after 1-MCP treatment, fruit were randomly selected from each treatment replicate, placed in a 20 °C room and analyzed after 24 h, or 5 d, depending on the response variable evaluated. The experimental period was seven months. Fig. 4. Sensory scores of crispness, juiciness, and flavor in ‘Gem’ pears as influenced by 0.15 and 0.3 μL L−1 1-MCP after 5 (A), 6 (B), and 7 (C) months RA storage at –1.1 °C plus 5 days at 20 °C.
2.2. Ethylene production and Rs rates Ethylene production and Rs rates were determined on five fruit per replicate following 24 h at 20 °C after removal from RA storage. Additionally, ethylene production and Rs rates were quantified after 5 d of 20 °C at the final, seven-month evaluation. Fruit were weighed, placed in a 3.8-L airtight jar at 20 °C, and sealed for 1 h prior to withdrawing a sample of headspace gas using a 1-mL gas-tight syringe. Ethylene production rate was measured by injecting the gas sample into a gas chromatograph (GC-8 A, Shimadzu, Tokyo, Japan) equipped with a flame ionization detector and a Porapack Q column (80/100 mesh, 3 mm diameter, 2 m long). The carrier gas was nitrogen at a flow rate of 0.83 mL s−1, the oven temperature was 90 °C, and the injector and
detector temperatures were 140 °C. An external standard of ethylene was used for calibration and data were expressed as ng kg-1 s-1. For quantification of Rs, the headspace gas CO2 concentration was measured using a CO2 analyzer (900161, Bridge Analyzers Inc., Alameda, CA, USA) and data were expressed as CO2 μg kg−1 s−1. 2.3. Fruit quality attributes and sensory score For determination of peel chlorophyll content and FF, twenty fruit were removed monthly from each treatment replicate and placed in 633
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Fig. 5. Internal browning (A and C) and scald (B and D) of ‘Gem’ pear as influenced by 0.15 and 0.3 μL L−1 1-MCP after 5, 6 and 7 months RA storage at –1.1 °C. Evaluations were performed after 24 h at 20 °C. Vertical bars represent standard error (SE). Means were separated among treatments by Fisher’s protected LSD. Different letters indicate significant differences at P < 0.05.
surface area displaying browning and/or cavities.
20 °C. Ten fruit were assessed after 24 h for chlorophyll content and FF and ten fruit were assessed after 5 d for FF only. Peel chlorophyll content was estimated at two locations on the fruit surface opposite each other near the equator using a DA meter (Sinteleia, Bolonga, Italy) and expressed as IAD value (Ziosi et al., 2008). Fruit firmness (N) was measured using a fruit texture analyzer (GS-14, Güss Manufacturing Ltd., Strand, South Africa) with an 8-mm probe and a penetration speed of 10 mm s−1 at similar locations on the fruit where chlorophyll content had been estimated after removal of a 2-mm thick disc of peel. After peel chlorophyll and FF determination, 100 g of flesh tissue was blended for 30 s in a juice extractor (Acme 6001, Acme Juicer Manufacturing Co, Sierra Madre, CA, USA) equipped with a uniform strip of milk filter (Chen and Spotts, 2005). The SSC (%) of juice was measured using a digital refractometer (PAL-1, ATAGO, Tokyo, Japan). TA (%) was determined by titrating 10 mL of the juice and 40 mL of deionized water to an end-point pH 8.1 using 0.1 N NaOH with an automated titration system (DL-15, Mettler-Toledo Inc., Columbus, OH, USA) and expressed as percentage of malic acid equivalents. Ten additional fruit per replicate were removed from RA storage and ripened for 5 d at 20 °C at 5, 6 and 7 months. A panel of five people was trained to recognize and score the sensory quality of both ripened and non-ripened pears. Sensory quality of crispness, juiciness, mealiness, and flavor was scored on a nine-point hedonic scale as previously described (Cliff et al., 1999), where 1 = extremely poor, 3 = poor, 5 = acceptable (limit of marketability), 7 = good, 9 = excellent.
2.5. Malondialdehyde (MDA), α-farnesene, and conjugated trienols (CTols) After 7 months RA and 5 d at 20 °C, skin tissue was removed (2-mm thickness) and immediately immersed in liquid nitrogen. MDA content was determined as described by Yuan et al. (2010). Briefly, tissue samples (1.5 g) were homogenized in 5 mL of 100 g L−1 trichloroacetic acid, then centrifuged at 10,000g for 15 min at 4 °C A 2 mL aliquot of the supernatant was added to 2 mL of 6.7 g L−1 thiobarbituric acid. Absorbance was measured at 450, 532, and 600 nm using a spectrophotometer (Ultrospec 3100 Pro, Biochrom Ltd, Cambridge, UK) and calculated according to the formula: 6.45 × (A532 – A600) – 0.56 × A450. Data were expressed on a fresh weight basis as mmol kg-1. αFarnesene and CTols were measured as described by Anet (1972). Excised discs of fruit skin (9 mm in diameter and 2 mm thickness) were removed from ten fruit per replication using a cork borer and immersed in 20 mL of hexane (HPLC-grade, Sigma-Aldrich Co., St. Louis, MO, USA) in a 50-mL centrifuge tube. Tubes were continuously shaken with a variable rotator (R4139-5 A, Tek-Pro Tek-Tator V, Miami, FL, USA) at 20 °C for 30 min. After extraction, absorbance at 232 nm (a-farnesene) and 281–290 nm (CTols) was recorded. Concentrations of a-farnesene and CTols were calculated using the molar extinction coefficients (ε) 27,740 at 232 nm and 25,000 at 281–290 nm for a-farnesene and CTols, respectively, and expressed on a fresh weight basis as mmol kg-1.
2.4. Physiological disorders 2.6. Statistical analysis After removal from RA storage, thirty fruit per replicate were placed in 20 °C for 24 h and assessed for surface scald, then cut longitudinally and transversely to assess internal browning. Scald incidence was expressed as the percentage of fruit affected with commercially unacceptable scald (> 0.6 mm2) as previously described (Wang, 2016). Internal browning was characterized by chocolate-colored tissue in the cortex of fruit, often associated with the development of cavities. Internal browning incidence was expressed as a percentage of the cut fruit
The experimental design was a CRD (completely randomized design). There were three replications per treatment for all parameters evaluated and for all evaluation dates. The data were subjected to analysis of variance (ANOVA) using StatSoft® Statistica (version 6, StatSoft, Tulsa, OK, USA). When appropriate, means were separated by Fisher’s protected least significant difference test (LSD) test at P < 0.05. 634
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Fig. 6. Effect of 0.15 μL L−1 1-MCP on scald development (A), fruit firmness (B), respiration rate (C), ethylene production rate (D), malondialdehyde content (E), αfarnesene (F) and conjugated trienols (G) of ‘Gem’ pears following 7 months storage at –1.1 °C plus 1 and 5 days at 20 °C. Vertical bars represent SE. Means were separated among treatments by Fisher’s protected LSD. Different letters indicate significant differences at P < 0.05.
the first four months of RA storage. Ethylene production rose slightly between 5 and 7 months to 1.01 and 0.56 ng kg−1 s−1 for 0.15 and 0.3 μL L−1 1-MCP, respectively, though these numeric differences were not significant. Both rates of 1-MCP were effective at significantly reducing Rs compared to non-treated fruit.
3. Results 3.1. Ethylene production and Rs rates At harvest, internal ethylene production was not detectable and fruit Rs rate was ∼ 5.08 CO2 mg kg−1 s−1 (Fig. 1A and B). After two months of RA, ethylene production and Rs rates of non-treated control fruit markedly increased reaching maximum levels of 5.25 ng kg−1 s−1 and 9.25 CO2 mg kg−1 s−1, respectively, after 7 months of RA storage (Fig. 1). Both rates of 1-MCP completely inhibited ethylene production
3.2. Fruit quality attributes and sensory score Over 7 months of RA storage, FF of control fruit gradually decreased from 47 to 38 N (Fig. 2A). 1-MCP maintained slightly higher FF, 635
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temperature conditioning to ripen to dessert quality and the fruit lost their ripening capacity after 5 months, characterized by mealy texture and overall poor eating quality (Einhorn and Wang, 2016). We observed similar results in the present study. Several years of informal sensory evaluations of ‘Gem’ indicated a strong preference for the fruit’s crisp and juicy characteristics (Turner and Einhorn, unpublished data) and support programs that maintain and/or improve ‘Gem’s’ non-ripened quality, and preferably lengthen storage life. 1-MCP extended the postharvest life of ‘Gem’ and completely inhibited ripening capacity by suppressing ethylene synthesis and Rs throughout the entire 7-month storage period resulting in the maintenance of FF, peel chlorophyll, and TA as similarly observed in ‘Bosc’ pear (Xie et al., 2017). For European pear, an increase in 1-MCP dose occupies more ethylene receptor-binding sites and results in greater ripening inhibition (Chiriboga et al., 2011). Doubling the concentration of 1-MCP from 0.15 to 0.3 μL L−1 had minimum effect on chlorophyll, ethylene generation and Rs rate, and no effect on FF, TA or the incidence of physiological disorders in the present study. Thus, 0.15 μL L−1 1-MCP appeared adequate to maintain quality and extend ‘Gem’ pear postharvest life. These results support preferential treatment of ‘Gem’ pears with 1-MCP for consumers that prefer crisp fruit. Under these conditions, it is clear that firmness would be maintained even if left unrefrigerated after purchase. Clearly, having a portion of one cultivar treated with 1-MCP to maintain crispness while another remained non-treated to allow ripening, side-by-side in the marketplace, would require supportive retail information, a significant marketing effort and physical separation of the two populations to avoid considerable confusion. Irrespective of the textural status of ‘Gem’, the retail chain has no tolerance for the presence of postharvest-related physiological disorders such as internal browning, storage scald, black-end, and cork spot. Among these disorders, internal browning occurs in the cortex as firm, brown, sharply defined areas of senesced tissue (Raese, 1989). For ‘Gem’, the occurrence of typical internal browning limited its postharvest life (Einhorn and Wang, 2016). In this study, internal browning was first observed at 5 months and increased in severity by 7 months. 1MCP delayed internal browning incidence and severity. In fact, all fruit treated with 1-MCP were within the commercially acceptable range for internal browning. Higher storage temperatures, ethylene, CO2 concentration, and metabolism were all closely associated with the development of internal browning in European pear (Franck et al., 2007; Giraldo et al., 2005; Ma and Chen, 2003; Saquet et al., 2003; Wang and Sugar, 2013). 1-MCP would appear an effective strategy for mitigating internal browning of ‘Gem’ pears, in a manner similar to that observed in ‘d’Anjou’ (Bai et al., 2006), ‘Starkrimson’ (Xie et al., 2015), and ‘Bartlett’ (Ekman et al., 2004). The onset of scald development of non-treated ‘Gem’ also appeared after 5 months but unlike internal browning, reached quite severe levels by 7 months. Symptoms appeared as brown to black discolored regions of the fruit surface, manifesting as lesions or wrinkled patches (Ingle, 2001). 1-MCP effectively inhibited scald development throughout the entire 7-month RA storage period even after 5 d at 20 °C. Similar results were reported for ‘Rocha’ (Isidoro and Almeida, 2006), ‘d’Anjou’ (Wang, 2016), and ‘Wujiuxiang’ (Zhou et al., 2017) pears, indicating that the combination of 1-MCP and cold storage inhibited ethylene biosynthesis and related receptor gene expression (Xie et al., 2014; Zhou et al., 2017). In the absence of 1-MCP, scald appears to be induced by chilling injury and may be associated with the production of reactive oxygen species (ROS) caused by α-farnesene (3,7,11-trimethyl,1,3,6,10dodecatetraene) metabolism (Lurie and Watkins, 2012). In pear, αfarnesene mostly accumulated at high levels in the wax layer and increased with ethylene production during cold storage and ripening (Yazdani et al., 2011). In the present study, ethylene production and αfarnesene content of non-treated fruit were significantly higher than 1MCP-treated fruit after 7 months of RA storage. The high concentration of α-farnesene may stimulate self-oxidation resulting in conjugated
irrespective of concentration (Fig. 2A). 1-MCP significantly enhanced peel chlorophyll content, as observed by higher IAD values, compared to non-treated fruit (Fig. 2B). IAD values were highest for 0.3 μL L−1 1MCP-treated fruit. SSC was unaffected by treatment and remained relatively constant throughout the 7-month RA period (Fig. 2C). TA of non-treated fruit consistently decreased between harvest (0.37%) and 7 months RA (0.25%) (Fig. 2D). 1-MCP significantly reduced TA loss over the entire storage period, though differences between 1-MCP concentrations were not detected (Fig. 2D). Non-treated fruit required two months of low temperature exposure to soften to FF levels characteristic of ripe fruit when held at 20 °C for five days (Fig. 3). Beyond five months of RA, flesh softening was impaired and increasingly higher FF occurred with each additional month of RA, indicating a loss in ripening capacity. In contrast, 1-MCP completely inhibited softening for the entire seven-month experimental period (Fig. 3). A sensory evaluation of ‘Gem’ pears between 5–7 months RA plus 5 d ripening at 20 °C indicated that control fruit were unacceptable for marketing after 5 months due to intermediate softening and mealy texture (Fig. 4A–C). However, ‘Gem’ pears exposed to 0.15 and 0.3 μL L−1 1-MCP maintained the crisp and juicy properties at harvest throughout the entire storage period, even after a 5 d ripening period, and ranked significantly higher than control fruit (Fig. 4A–C). 3.3. Physiological disorders The physiological disorder, internal browning was first observed in non-treated fruit at 5 months RA (Fig. 5A and C). Internal browning incidence increased over the subsequent two months to maximum levels of 26% (Fig. 5C). 1-MCP significantly delayed the onset of internal browning to 7 months, at which time only a slight percentage of internal browning was detected (Fig. 5C). There was no difference between 1-MCP concentrations on the inhibition of internal browning. Non-treated fruit began displaying brown and black patches on the skin surface following 5 months of RA storage, becoming more severe at 6 months and occupying 85% of the surface area after 7 months RA (Fig. 5B and D). In contrast, 1-MCP-treated fruit remained free of scald during the entire 7-month experimental period. α-Farnesene and CTols were significantly lower in 1-MCP-treated fruit compared to nontreated fruit after 7 months RA (Fig. 6F and G). MDA was initially higher in 1-MCP-treated fruit but was significantly reduced following 5 d at 20 °C (Fig. 6E). Notably, α-farnesene and CTols content did not increase after 5 d at 20 °C (Fig. 6F and G). Reduced content of these compounds was associated with 1-MCP inhibition of fruit softening, ethylene production and Rs (Fig. 6B–D). 4. Discussion The genetic, biochemical and physiological changes during ripening that profoundly alter the textural and sensory properties of European pears render them unique among climacteric fruits. The classical dessert pear, characterized by a soft, buttery and juicy texture is contingent on a preconditioning treatment by low temperature or ethylene gas (Chen and Mellenthin, 1981; Sugar and Basile, 2009, 2013; VillalobosAcuña and Mitcham, 2008). While cultivars differ with respect to the minimum duration of low temperature conditioning, conditioning periods can be reduced by delaying harvest and/or storing fruit at intermediate temperatures (i.e., between −1.1 and 10 °C) (Chen and Mellenthin, 1981; Ma et al., 2000; Sugar and Einhorn, 2011). Pear ripenability is a notoriously difficult task for consumers to accomplish, and is arguably the single-most important barrier to repeat purchases (Kevin Moffitt, personal communication) as indicated by consumers’ willingness to pay for properly conditioned and ripened pears (Gallardo et al., 2011). For ‘Gem’, we demonstrated that delaying harvest to FF values between 42–50 N promoted larger fruit size, higher SSC, juiciness and development of mild ‘pear’ flavor. However, when harvested within this pressure range, ‘Gem’ still required > 30 d of low 636
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concentration, treatment interval and storage time for ‘Bartlett’ pears. Postharvest Biol. Technol. 31, 127–136. Franck, C., Lammertyn, J., Ho, Q.T., Verboven, P., Verlinden, B., Nicolaï, B.M., 2007. Browning disorders in pear fruit. Postharvest Biol. Technol. 43, 1–13. Gallardo, R.K., Kupferman, E., Colonna, A., 2011. Willingness-to-pay for optimal Anjou pear quality. HortScience 46, 452–456. Gapper, N.E., Bai, J., Whitaker, B.D., 2006. Inhibition of ethylene-induced α-farnesene synthase gene PcAFS1 expression in ‘d’Anjou’ pears with 1-MCP reduces synthesis and oxidation of α-farnesene and delays development of superficial scald. Postharvest Biol. Technol. 41, 225–233. Giraldo, E., Szczerbanik, M.J., Scottpez, K.J., Paton, J.E., Best, D.J., 2005. Effects of polyethylene bags, ethylene absorbent and 1-methylcyclopropene on the storage of Japanese pears. J. Hort. Sci. Biotechnol. 80, 162–166. Ingle, M., 2001. Physiology and biochemistry of superficial scald of apples and pears. Hort. Rev. 27, 227–267. Isidoro, N., Almeida, D.P., 2006. α-Farnesene, conjugated trienols, and superficial scald in ‘Rocha’ pear as affected by 1-methylcyclopropene and diphenylamine. Postharvest Biol. Technol. 42, 49–56. Jaeger, S.R., Lund, C.M., Harker, F.R., 2003. In search of the “Ideal” pear (Pyrus spp.): results of a multidisciplinary exploration. J. Food Sci. 68, 1108–1117. Liu, R., Lai, T., Xu, Y., Tian, S., 2013. Changes in physiology and quality of Laiyang pear in long time storage. Sci. Hortic. 150, 31–36. Lurie, S., Watkins, C.B., 2012. Superficial scald, its etiology and control. Postharvest Biol. Technol. 65, 44–60. Ma, S.S., Chen, P.M., 2003. Storage disorder and ripening behavior of ‘Doyenne du Comice’ pears in relation to storage conditions. Postharvest Biol. Technol. 28, 281–294. Ma, S.S., Chen, P.M., Mielke, E.A., 2000. Storage life and ripening behavior of ‘Cascade’ pears as influenced by harvest maturity and storage temperature. J. Am. Pom. Soc. 54, 138–147. Raese, J.T., 1989. Physiological disorders and maladies of pear fruit. Hort. Rev. 11, 357–411. Saquet, A.A., Streif, J., Bangerth, F., 2003. Energy metabolism and membrane lipid alterations in relation to brown heart development in ‘Conference’ pears during delayed controlled atmosphere storage. Postharvest Biol. Technol. 30, 123–132. Sugar, D., Basile, S.R., 2009. Low-temperature induction of ripening capacity in ‘Comice’ and ‘Bosc’ pears as influenced by fruit maturity. Postharvest Biol. Technol. 51, 278–280. Sugar, D., Basile, S.R., 2013. Integrated ethylene and temperature conditioning for induction of ripening capacity in ‘Anjou’ and ‘Comice’ pears. Postharvest Biol. Technol. 83, 9–16. Sugar, D., Einhorn, T.C., 2011. Conditioning temperature and harvest maturity influence induction of ripening capacity in ‘d’Anjou’ pear fruit. Postharvest Biol. Technol. 60, 121–124. Villalobos-Acuña, M., Mitcham, E.J., 2008. Ripening of European pears: the chilling dilemma. Postharvest Biol. Technol. 49, 187–200. Wang, Y., 2016. Storage temperature, controlled atmosphere, and 1-methylcyclopropene effects on α-farnesene, conjugated trienols, and peroxidation in relation with superficial scald, pithy brown core, and fruit quality of ‘d’Anjou’ pears during long-term storage. J. Am. Soc. Hort. Sci. 141, 177–185. Wang, Y., Sugar, D., 2013. Ripening behavior and quality of modified atmosphere packed ‘Doyenne du Comice’ pears during cold storage and simulated transit. Postharvest Biol. Technol. 81, 51–59. Wang, Y., Sugar, D., 2015. 1-MCP efficacy in extending storage life of ‘Bartlett’ pears is affected by harvest maturity, production elevation, and holding temperature during treatment delay. Postharvest Biol. Technol. 103, 1–8. Watkins, C.B., 2006. The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables. Biotechnol. Adv. 24, 389–409. Xie, X., Song, J., Wang, Y., Sugar, D., 2014. Ethylene synthesis, ripening capacity, and superficial scald inhibition in 1-MCP treated ‘d’Anjou’ pears are affected by storage temperature. Postharvest Biol. Technol. 97, 1–10. Xie, X., Einhorn, T.C., Wang, Y., 2015. Inhibition of ethylene biosynthesis and associated gene expression by aminoethoxyvinylglycine and 1-methylcyclopropene and their consequences on eating quality and internal browning of ‘Starkrimson’ pears. J. Am. Soc. Hort. Sci. 140, 587–596. Xie, X., Fang, C., Wang, Y., 2017. Inhibition of ethylene biosynthesis and perception by 1methylcyclopropene and its consequences on chlorophyll catabolism and storage quality of ‘Bosc’ pears. J. Am. Soc. Hort. Sci. 142, 92–100. Yazdani, N., Arzani, K., Mostofi, Y., Shekarchi, M., 2011. α-Farnesene and antioxidative enzyme systems in Asian pear (Pyrus serotina Rehd.) fruit. Postharvest Biol. Technol. 59, 227–231. Yuan, G., Sun, B., Yuan, J., Wang, Q., 2010. 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trienols (CTols) that directly cause damage to cells in peel tissue (Gapper et al., 2006). Moreover, MDA levels are associated with membrane lipid peroxidation, indirect loss of membrane integrity during fruit deterioration in storage and chilling injury (Liu et al., 2013; Lurie and Watkins, 2012). In the present study, even though significant lower MDA was observed in non-treated ‘Gem’ after removal from cold storage at 7 months, MDA increased dramatically and was higher than 1-MCP-treated fruit after 5 d at 20 °C, implicating MDA levels in fruit skin tissue as a plausible contributor to cell damage. In contrast, 1-MCPtreated ‘Gem’ fruit had relatively low ethylene production, α-farnesene accumulation and CTols as previously shown for apple (Arquiza et al., 2005). Additional research to evaluate the effects of controlled atmospheres and antioxidants on extending postharvest life and controlling physiological disorders of ‘Gem’ pears would be valuable. 5. Conclusion A low dose of 1-MCP (0.15 μL L−1) effectively maintained fruit quality attributes (i.e., FF, IAD, and TA) and suppressed ethylene production, Rs and the incidence of internal browning and scald to prolong storage life of ‘Gem’ pears from 5 to 7 months in RA. The eating quality of pears treated with 1-MCP was superior to non-treated fruit between 5 and 7 months storage; thus, 1-MCP is a potential tool to maintain the unique crisp and juicy attributes of ‘Gem’. However, for consumers that prefer a ripened pear, 1-MCP ripening-inhibition would be undesirable. Acknowledgements We are grateful to the Columbia Gorge Fruit Growers Association and the NW Fresh Pear Research Committees for their generous financial support. References Anet, E.F.L.J., 1972. Superficial scald, a functional disorder of stored apples. IX. Effect of maturity and ventilation. J. Sci. Food Agric. 23, 763–769. Arquiza, J.M.R.A., Hay, A.G., Nock, J.F., Watkins, C.B., 2005. 1-Methylcyclopropene interactions with diphenylamine on diphenylamine degradation, α-farnesene and conjugated trienol concentrations, and polyphenol oxidase and peroxidase activities in apple fruit. J. Agric. Food Chem. 53, 7565–7570. Bai, J., Mattheis, J.P., Reed, N., 2006. Re-initiating softening ability of 1-methylcyclopropene-treated ‘Bartlett’ and ‘d’Anjou’ pears after regular air or controlled atmosphere storage. J. Hort. Sci. Biotechnol. 81, 959–964. Bell, R.L., Van der Zwet, T., Castagnoli, S., Einhorn, T.C., Turner, J.D., Spotts, R., Moulton, G.A., Reighard, G.L., Shane, W.W., 2014. ‘Gem’ pear. HortScience 49, 361–363. Castagnoli, S., Turner, J., Einhorn, T.C., Bell, R., 2011. Performance of US 71655-014, a fire blight resistant pear selection from the USA. Acta Hort. 909, 177–182. Chauvin, M.A., Ross, C.F., Pitts, M., Kupferman, E., Swanson, B., 2010. Relationship between instrumental and sensory determination of apple and pear texture. J. Food Qual. 33, 181–198. Chen, P.M., Mellenthin, W.M., 1981. Effects of harvest date on ripening capacity and postharvest life of ‘d’Anjou’ pears. J. Am. Soc. Hort. Sci. 106, 38–42. Chen, P.M., Spotts, R.A., 2005. Changes in ripening behaviors of 1-MCP-treated ‘d’Anjou’ pears after storage. Int. J. Fruit Sci. 5, 3–17. Chiriboga, M.A., Schotsmans, W.C., Larrigaudière, C., Dupille, E., Recasens, I., 2011. How to prevent ripening blockage in 1-MCP-treated ‘Conference’ pears. J. Sci. Food Agric. 91, 1781–1788. Cliff, M.A., Sanford, K., Johnston, E., 1999. Evaluation of hedonic scores and R-indices for visual, flavour and texture preferences of apple cultivars by British Columbian and Nova Scotian consumers. Can. J. Plant Sci. 79, 395–399. Daillant-Spinnler, B., Macfie, H.J.H., Beyts, P.K., Hedder-Ley, D., 1996. Relationships between perceived sensory properties and major preference directions of 12 varieties of apples from the southern hemisphere. Food Qual. Pref. 7, 113–126. DeEll, J.R., Ehsani-Moghaddam, B., 2011. Timing of postharvest 1-methylcyclopropene treatment affects Bartlett pear quality after storage. Can. J. Plant Sci. 91, 853–858. Einhorn, T.C., Wang, Y., 2016. Characterizing the effect of harvest maturity on ripening capacity, postharvest fruit quality, and storage life of ‘Gem’ pear. J. Am. Pom. Soc. 70, 26–35. Ekman, J.H., Clayton, M., Biasi, W.V., Mitcham, E.J., 2004. Interactions between 1-MCP
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