Quantifying the ethylene induced softening and low temperature breakdown of ‘Hayward’ kiwifruit in storage

Postharvest Biology and Technology 113 (2016) 87–94

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Quantifying the ethylene induced softening and low temperature breakdown of ‘Hayward’ kiwifruit in storage Abdul Jabbar, Andrew R. East* Centre for Postharvest and Refrigeration Research, Massey University, Palmerston North, New Zealand

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 July 2015 Received in revised form 30 September 2015 Accepted 4 November 2015 Available online xxx

Kiwifruit are considered highly sensitive to exogenous ethylene during refrigerated storage (0  C). This study aimed to quantifiably describe the effect of continuous application of exogenous ethylene (0.001, 0.01, 0.1 and 1 mL L 1) in the storage environment (0  C, 95% RH) on quality (softening and low temperature breakdown; LTB) of ‘Hayward’ kiwifruit when exposed either after harvest or after 10 weeks of storage. For both ethylene application times fruit exposed to 1 and 0.1 mL L 1 ethylene exhibited significant loss of firmness compared to control (0.001 mL L 1) after 2 weeks of application. Fruit exposed to 0.01 mL L 1 ethylene also softened rapidly compared to control fruit (0.001 mL L 1) when ethylene was applied at-harvest, but no substantial difference in softening was observed when applied after 10 weeks of storage. Most of the softening differentiation occurred in the first 4 weeks of exposure, after which the rates of softening returned to being relatively constant irrespective of the ethylene environment. Along with rapid softening, fruit exposed to 1 mL L 1 ethylene were higher in incidence of LTB, irrespective of exposure timing. This study demonstrates that ethylene concentrations as low as 0.01 mL L 1 can influence softening of ‘Hayward’ kiwifruit in a commercial cool storage environment. As the differentiation of treatments occurs solely in the initial period of ethylene exposure, more research is required to understand the impact of small exposure occasions, which are more likely to occur in real supply chain scenarios. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Actinidia deliciosa Climacteric Firmness Chilling injury Storability

1. Introduction Kiwifruit (‘Hayward’ Actinidia deliciosa (A. Chev.) C.F. Liang and A.R. Ferguson) produces autocatalytic ethylene during ripening and therefore is considered climacteric (Arpaia et al., 1994; Sfakiotakis et al., 1997; Antunes, 2007; Chiaramonti and Barboni, 2010). However, kiwifruit produces little ethylene (<0.01 nL kg 1 h 1) at harvest (Burdon and Lallu, 2011). Kiwifruit are usually stored at 0  C with relative humidity (RH) of 90–95% for up to 6 months (Arpaia et al., 1987; McDonald, 1990; Hewett et al., 1999; Burdon and Lallu, 2011). At low temperature (0  C) kiwifruit has a unique climacteric behaviour, as they do not produce substantial ethylene until softening dramatically (<10–15 N) later in storage (Hewett et al., 1999; Kim et al., 1999; Ritenour et al., 1999; Feng et al., 2003; Atkinson et al., 2011). Ensuring that no substantial ethylene accumulation occurs is essential for successful management of fruit quality (Atkinson et al., 2011). Kiwifruit are considered to be highly susceptible to small ethylene concentrations (i.e. 0.005–0.01 mL L 1) in storage

* Corresponding author. Fax: +64 6 350 5657. E-mail address: [email protected] (A.R. East). http://dx.doi.org/10.1016/j.postharvbio.2015.11.002 0925-5214/ ã 2015 Elsevier B.V. All rights reserved.

and may exhibit excessive softening, leading to fruit losses (Mitchell, 1990; Hewett et al., 1999; Kim, 1999; Antunes, 2007). A maximum threshold of 0.03 mL L 1 ethylene is used in industry to minimise ethylene effects on premature softening (Beever and Hopkirk, 1990; Jeffery and Banks, 1996). The vast majority of data demonstrating ripening responses of kiwifruit to ethylene exposure has been performed at ambient conditions (20  C) and still lack the quantification of effect of previously unmeasurable low ethylene concentrations (e.g. 0.001 mL L 1) in optimal storage environment (Sfakiotakis et al., 1997; Antunes et al., 2000; Antunes and Sfakiotakis, 2000, 2002; Sfakiotakis et al., 2001; Antunes, 2007; Albert et al., 2013). The exceptions to this is the work of Arpaia et al. (1986) who applied ethylene concentrations ranging from 0.05 to 5 mL L 1 during controlled atmosphere (2% O2 + 5% CO2) storage (at 0  C); Jeffery and Banks (1996) who applied ethylene at concentrations of 0.002–30 mL L 1 at 1  C; and Wills et al. (2001) who applied ethylene (<0.005–1 mL L 1) during storage at 0  C. However these studies are limited by the graduation in scale used to quantify the ethylene effect and the fact that ethylene exposure was initiated at the start of storage. Given that efforts are made to reduce ethylene concentrations in storage environments commercially, the only

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likely source of ethylene accumulation within a package is likely to occur from the fruit themselves when they reach a firmness of 20 N. At 0  C, ‘Hayward’ requires (10–12 weeks) to reach 20 N firmness and hence start autocatalytic ethylene production (Chiaramonti and Barboni, 2010). Quantification of the effect of exogenous ethylene on kiwifruit quality, when applied later in storage at the stage of autocatalytic ethylene production is not addressed in previous studies. In addition to premature softening, kiwifruit quality is impaired by a chilling injury referred to as low temperature breakdown (LTB). This physiological disorder is distinguished as a granular appearance of the outer pericarp, that develops into a water soaked appearance (Lallu, 1997; Burdon et al., 2007; Burdon and Lallu, 2011). LTB is the term used for the same injury symptoms referred to as storage breakdown disorder (SBD) in parts of the industry. Commercially, LTB results in discarded fruit, as the soft fruit are considered inedible (Bauchot et al., 1999). Storage temperature and cooling rate are known factors directly associated with the incidence of LTB in kiwifruit (Lallu, 1997; Yang et al., 2013; Zhao et al., 2015). A number of works have also associated the occurrence and induction of LTB with endogenous (Feng et al., 2003; Yin et al., 2009; Ma et al., 2014) and exogenous ethylene (Koutsoflini et al., 2013). This study aimed to quantitatively describe in industry relevant storage conditions and ethylene concentration ranges, the impact of ethylene on Hayward kiwifruit quality (firmness and LTB incidence). Ethylene concentrations were established both after harvest and after 10 weeks (70 d) of optimal temperature storage to better quantify the ethylene effect on fruit quality in a commercial supply chain scenario. As a result this research provides guidelines on the influence of realistic cool chain ethylene conditions on storability of ‘Hayward’ kiwifruit. 2. Material and methods 2.1. Fruit sampling ‘Hayward’ kiwifruit from three growers located in the Bay of Plenty, New Zealand were commercially harvested on May 27th, 2014. After initial commercial grading and packing, 60 (20  3 growers) modular bulk packs (of count size 36 fruit) were transported to Massey University, Palmerston North, New Zealand. Fruit were not cooled prior or during transport. Upon arrival, 61 mesh or onion bags containing 30 fruit each were prepared randomly from the 20 packs per grower. Mesh bags were randomly labelled to allocate storage condition (ethylene concentration) and time of removal from the experiment (storage time).

Once ethylene was applied a single pre-labelled mesh bag (30 fruit) was removed from each barrel at 2 week intervals. One (1) mesh bag (30 fruit) per grower was allocated for at-harvest quality assessments. 2.2.1. Storage conditions Four ethylene concentrations (0.001, 0.01, 0.1 and 1 mL L 1) were established in a flow through system to supply to the barrels. In the case where ethylene was applied after 70 days of storage, all barrels were initially supplied with air (0.001 mL L 1 C2H4). All the barrels containing fruit were stored at 0  C in the same cool room. Relative humidity of 95% was maintained in barrels by passing the gas mix through sealed 1000 mL jars filled with glycerol (21.1%) and water (78.9%). 2.2.2. Ethylene concentrations For the lowest ethylene concentration (0.001 mL L 1), only compressed air was used, with the measured ethylene being a result of residual contamination from the environment, despite efforts to scrub the supply (with KMnO4). Ethylene concentrations of 0.01, 0.1 and 1 mL L 1 were established by mixing air (99%) with 1% of 1, 10 and 100 mL L 1 ethylene b-standard (BOC gases, Auckland, New Zealand) concentrations respectively. A flow controlled mixer was used to mix air and standard ethylene gas. Mixed gases were later divided into channels by using a manifold to supply gas to each barrel representing each grower. A flow of 300 mL min 1 for each barrel was maintained by using control valves. This flow rate was designed to ensure no significant increase in ethylene concentration accumulated within the barrels as a result of ethylene production. Out flow from each barrel was attached to room ventilation to ensure removal of ethylene from the room environment. Purafil1 (KMnO4) was placed in the room to minimise ethylene accumulation during storage. Photoacoustic ethylene analysing equipment (ETD-300, Sensor Sense B.V., Nijmegen) was used to enable the project to be conducted at the accuracy necessary. Each ethylene concentration was checked and maintained every 7–10 days to ensure consistent gas concentration delivery throughout the experiment. The 1 mL L 1, concentration remained within 2.9% during the experiment (Fig. 1). Likewise, concentrations of 0.1 and 0.01 mL L 1 were also maintained consistently (8.7% and 17.2%, respectively). For the lowest concentration of 0.001 mL L 1, compressed air was used without added ethylene, with an average concentration of

2.2. Experiment setting Two different timings were used to apply ethylene. To replicate previous work, ethylene was applied at the initiation of storage. Alternatively, to evaluate the exogenous ethylene effect on softening more likely to be experienced during commercial storage, ethylene concentrations were established after 10 weeks of optimal storage. This timing of application of ethylene after 10 weeks (70 d) was assumed to be the stage when fruit started autocatalytic ethylene production, as was informed by Chiaramonti and Barboni (2010). Sixty (60) mesh bags for each grower were stored in 8 barrels (capacity 60 L) attached to a flow through gas delivery system. Of the 8 barrels, 4 contained 7 mesh bags each for the component of the experiment where ethylene conditions were established at the introduction of storage. The remaining 4 barrels contained 8 mesh bags each, that were used for when ethylene conditions were established after 70 d of storage.

Fig. 1. Different ethylene concentrations of 1, 0.1 and 0.01 mL L 1 were achieved throughout the experiment. All ethylene concentrations were assessed at mixers point before dividing into channels to maintain supply of gas to each barrel.

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0.003 mL L 1 achieved. The increase in variability as the concentration decreases is not unexpected given the logarithmic scale of the treatments applied. Overall, for each generated mix, ethylene concentrations were maintained, well separated and relatively stabilised.

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Low temperature breakdown was assessed by cutting each fruit into half and visually assessing the fruit flesh for grainy appearance, water soaking or chilling injury symptoms. Only incidence was recorded. 2.4. Data analysis

2.3. Fruit quality assessments Quality of 30 fruit was assessed at the beginning of the experiment and on 7 occasions (30 fruit at each time for each treatment/grower combination) during storage (at 2 weeks intervals), once the different ethylene concentrations were applied. On each measurement occasion fruit quality (firmness, and low temperature breakdown (LTB) incidence) were assessed, with dry matter (DM) and soluble solid contents (SSC) also assessed at the beginning of the experiment. Before each measurement from storage, fruit were placed overnight at room temperature (20  C), to equilibrate the fruit temperature. SSC (%) was assessed by using digital pocket refractometer (PAL1, Atago, Japan) calibrated to 0 by using distilled water. An approximately 15 mm slice was taken from both blossom and stem end with juice drops of equal amounts collected and mixed on the prism of a refractometer. Fruit firmness was assessed by using a QALink Penetrometer (Willowbank Electronics Ltd., Napier, New Zealand). A 7.9 mm Magness Taylor probe was attached to the penetrometer to puncture the fruit at two locations at a 90 angle at the equator after removing 1–2 mm of skin. Fruit was punctured to the depth of 8 mm at each location, at a speed of 20 mm s 1. Average peak force (N) of two measurements was recorded to represent the fruit firmness. For dry matter (DM) estimation, a 2–3 mm equatorial fruit slice of known initial (fresh) weight was dried in an oven (60–65  C) for 24 h. After drying, the slice was re-weighed and dry matter calculated as dry weight percentage of initial fresh weight.

Fruit quality at-harvest and during storage at 4 ethylene concentration environments were statistically analysed with a general linear model (at P = 0.05) using Minitab 16 (Minitab Inc., State College, PA, USA). Differences in quality were determined by Tukey’s Least Significant Differences (LSD0.05). 3. Results 3.1. At-harvest fruit quality Initial measurements showed that all three growers had similar DM (17.4%, P = 0.160) and firmness (59.5 N, P = 0.808). Meanwhile, at-harvest SSC was lower (P < 0.001, LSD0.05 = 0.8%) for grower C (9.7%) compared to grower A (11%) and B (11.2%). Overall, all three growers were at a commercial maturity stage at the time of harvest, having a SSC > 6.2% and DM > 14.5% (Burdon and Lallu, 2011). 3.2. Ethylene induced softening 3.2.1. Application of ethylene at storage Kiwifruit from each grower softened differently and hence each grower line is presented separately. Overall, application of ethylene concentration during storage (at 0  C) from storage initiation clearly resulted in differential softening of Hayward kiwifruit (Fig. 2). After 2 weeks, ethylene treatments of 0.01 and 0.001 mL L 1 were already differentiated in comparison to 1 and 0.1 mL L 1 and from each other (Table 1). Firmness differences between the control (0.001 mL L 1) and 0.01 mL L 1 ethylene application were consistent across

Fig. 2. Firmness of three ‘Hayward’ kiwifruit grower lines (A–C) when exposed to different ethylene concentrations after harvest during optimal storage (0  C). Each data point represents a mean of 30 fruit. Error bars represent LSD0.05. A black line represents the export threshold limit of 10 N.

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Table 1 Average firmness (N) of three ‘Hayward’ kiwifruit growers at 0  C, when continuously exposed to different ethylene concentrations (0.001–1 mL L Ethylene treatment (mL L

1

)

1

) after harvest.

Storage duration (weeks) 2

4

6

8

10

12

14

0.001 0.01 0.1 1

39.63a 35.54b 22.74c 20.04c

23.37a 19.99b 10.98c 9.50c

20.05a 14.72b 8.76c 7.08c

17.30a 12.71b 7.58c 5.87d

14.02a 10.89b 6.79c 4.57d

12.57a 10.24b 6.69c 3.98d

12.22a 9.14b 5.91c 3.39d

P-value (<0.001) LSD

*** 3.66

*** 2.13

*** 1.74

*** 1.49

*** 1.06

*** 0.76

*** 0.88

Means not sharing the same letter in a column are statistically significantly different as observed by Tukey’s test. ***Significant F test at P < 0.001(n =90).

growers after 2 weeks of storage, while 8 weeks of treatment application were required to separate 1 and 0.1 mL L 1 treatments (Table 1). For all growers, the vast majority of the differentiation in softening between treatments occurred in the first 4 weeks of ethylene application, after which the rate of softening of all the treatments seemed relatively constant (Fig. 2). Application of 1 mL L 1 ethylene in storage resulted in the average firmness reaching 10 N after just 4 weeks. By the end of 14 weeks storage kiwifruit at 1 mL L 1 ethylene has a firmness of 3.4 N while fruit of other treatments of 0.1, 0.01 mL L 1 and 0.001 mL L 1 ethylene were at 5.9, 9.1 N and 12.2 N respectively. 3.2.2. Application after 10 weeks of storage Before applying ethylene concentrations (0.001, 0.01, 0.1 and 1 mL L 1) after 10 weeks, fruit were placed in barrels attached with continuous air flow from the start of the storage. Regular monitoring of continuous flow to (inlet) and from (outlet) barrels revealed that the control treatment (no ethylene) for grower A did not maintain continuous flow and as a result fruit of this treatment was withdrawn from the experiment. While loss of this data limits the dataset, it does not prevent comparison of other ethylene

treatments for this grower (A) and all treatments for the other two growers. By day 70 (week 10 from harvest) of air storage, firmness of all three growers decreased substantially from an initial (average atharvest) value of 59 N to 12–17 N depending on grower. Grower B softened fastest with a recorded firmness of 11.7 N. Softening for Grower C was slightly slower (14.4 N), while Grower A softening was the slowest (17.2 N; Fig. 3A & C). Importantly, no differences in barrels that were to become treatments were observed at 10 weeks. Application of ethylene at 70 d of storage accelerated the softening of kiwifruit from all three growers (Fig. 3). After 2 weeks of subsequent ethylene application (12 weeks from harvest), firmness differences became established which were increased after an additional 2 weeks of storage (14 weeks, Table 2). Exposure to 1 mL L 1 ethylene caused the fastest softening followed by 0.1 and 0.01 mL L 1 for all three growers. Differences between 0.001 and 0.01 mL L 1 were only observed after 14 weeks of application of the treatments. Kiwifruit exposed to 1 and 0.1 mL L 1 of ethylene reached export threshold (10 N) approximately 2–3 week after treatment initiation. While fruit exposed to 0.01 mL L 1 ethylene

Fig. 3. Firmness of three ‘Hayward’ kiwifruit growers (A–C) exposed to different ethylene concentration after 10 weeks at 0  C. Each data point represents a mean of 30 fruit. Error bars represent LSD0.05. A black line represents the export threshold limit of 10 N. * Grower A is missing control data due to experimental problems. F0 indicates firmness at harvest.

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Table 2 Average firmness (N) of three ‘Hayward’ kiwifruit growers at 0  C when continuously exposed to different ethylene concentrations (0.001–1 mL L as control (0.001 mL L 1). Ethylene treatment (mL.L

1

)

1

) after 10 weeks of storage

Storage duration (weeks) 10

12

14

16

18

20

22

24

0.001* 0.01 0.1 1

13.87 14.20 14.73 14.25

12.18a 12.24ab 11.19bc 10.23c

12.46a 10.58a 8.19b 6.74c

11.19a 10.13a 7.53b 5.52c

9.48a 9.72a 6.78b 4.30c

9.41a 7.83a 5.62b 2.77c

7.96a 7.64a 5.26b 2.40c

8.14a 6.64b 4.67c 2.25d

P-value (<0.001) LSD

NS

*** 1.66

*** 0.99

*** 0.90

*** 0.89

*** 0.68

*** 0.73

*** 0.78

Means not sharing the same letter in a column are statistically significantly different as observed by Tukey’s test. *Firmness values for this treatment are means of two growers only (n = 60). ***Significant F test at P < 0.001 (n = 90).

reached the same firmness after 3–8 weeks depending on grower (Fig. 3). Following the differentiation in firmness between treatments after 4-6 weeks of ethylene application (after 10 weeks) storage, the rates of softening of all treatments became pseudoparallel to each other, consistently maintaining a similar difference in firmness. 3.3. Low temperature breakdown Low temperature breakdown (LTB) or chilling injury symptoms appear as kiwifruit soften during storage at 0  C. Differences also existed between growers for expression of LTB. Overall, fruit exposed to higher ethylene concentration (i.e. 1 mL L 1) were more prone to LTB development than fruit with lower levels or no ethylene in storage environment. When ethylene was applied from storage initiation, higher incidence of LTB was observed in fruit stored in 1 mL L 1 ethylene followed by fruit stored at 0.1 and 0.01 mL L 1 (Fig. 4). Meanwhile, fruit stored without additional ethylene (0.001 mL L 1) did not show incidence of LTB. When ethylene was applied after 10 weeks of air storage, higher incidence of LTB was also observed in fruit stored in high ethylene environments. LTB incidence occurred after 4 - 6 week of ethylene application for all treatments (Fig. 4). 4. Discussion This study confirms that exposure of ‘Hayward’ kiwifruit to exogenous ethylene (0.01–1 mL L 1) after harvest and after 10 weeks of optimal storage accelerates fruit softening. Notably, rate of softening not only depended on the ethylene concentration in the storage environment as previously reported by Beever and Hopkirk (1990) but also timing of exposure to ethylene (Tables 1 & 2). Kiwifruit exposed to 1 and 0.1 mL L 1 ethylene exhibited the fastest softening in both timings of application but were inseparable until 8 weeks after harvest while 0.01 mL L 1 and control (0.001 mL L 1) differentiated after 2 weeks of treatment (Table 1). In contrast, when fruit were first exposed to ethylene after 10 weeks storage, it took until 14 weeks later (week 24) for differences in softening to occur between 0.01 mL L 1 and control (0.001 mL.L 1) Previously Wills et al. (2001) also found ‘Hayward’ kiwifruit to be sensitive to exogenous ethylene levels of 0.01 mL L 1 at optimal (0  C) and ambient (20  C) conditions. However, timing of application of such low ethylene concentrations during storage substantially influences the softening response. Application of 0.01 mL L 1 at harvest affected the softening while when applied after 10 weeks it did not affect the firmness as compared to control (Tables 1 & 2). After 10 weeks of storage, when kiwifruit are at a firmness of 12–17 N, endogenous ethylene production should have initiated (Kim et al., 1999), meaning that exposure to ethylene concentrations of 0.01 mL L 1 may not significantly increase the

internal ethylene concentration that already exists due to endogenous production. As a result no stimulation of softening and LTB development is observed. Meanwhile exogenous ethylene at 0.01 mL L 1 during early optimal storage can significantly accelerate the softening rate of kiwifruit. Retamales and Campos (1997) also reported that exposure to ethylene as low as 0.01 mL.L 1 after harvest can accelerate the softening of kiwifruit during optimal storage compared to control fruit (without ethylene). Hence effects to reducing the ethylene concentration <0.01 mL L 1 commercially in the early period of storage could potentially result in extending the storage potential. Alternatively, it would seem that once fruit have softening to 12–17 N (after approximately 70 days) maintaining ethylene concentration to 0.01 could be considered best practice. In this study continuous ethylene exposure was established for 14 weeks with rapid differentiation for softening observed within the first 4 week of ethylene application irrespective of timing (Tables 1 & 2). Subsequent to this period, rates of softening of all treatments became pseudo-parallel demonstrating little effect of the continued ethylene exposure. Given this result, it raises a question whether ethylene application over only a 4 week period would have resulted in similar results? Literature suggests that exogenous ethylene accelerates softening and acts as a rate defining factor rather than a trigger for accelerated softening (Beever and Hopkirk, 1990). An alternative explanation is that exposure of kiwifruit to ethylene for only a brief period may trigger accelerated softening. However, both Jeffery and Banks (1996) and Ritenour et al. (1999) reported that short term exposure of even higher ethylene concentration at low temperature did not affect fruit softening. Antunes (2007) also reported that exogenous ethylene results in kiwifruit softening at 0  C but it does not trigger autocatalytic ethylene biosynthesis of fruit at this temperature. Hence, previous literature has established that continuous application of ethylene is required to cause acceleration in softening of kiwifruit at 0  C, but the mechanism of why this effect only applies for approximately 4 weeks is unknown. The potential reduction in commercial storage life as indicated by the results was assessed by presenting the firmness data (Tables 1 & 2) on an ethylene concentration scale (Fig. 5A & B). A firmness of >10 N signals a quality which enables export of kiwifruit from New Zealand (Jackson and Harker, 1997), whereas a firmness range of 10 to 6 N approximates an “eating window”, the firmness in which most consumers accept the firmness of the fruit (Woodward, 2006; Ilina et al., 2010). Within 4 weeks of 1 mL L 1 ethylene application at storage initiation, firmness could be considered edible (<10 N) and after a further 4 weeks (interpolated as 8 weeks) fruit became too soft to be considered edible (Fig. 5A). Alternatively, fruit exposed to 0.001 mL L 1 ethylene remained >10 N after 14 weeks of harvest. Likewise exposure to ethylene 1 mL L 1 after 10 weeks of storage reduced the storage potential by

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Fig. 4. Low temperature breakdown (LTB) in ‘Hayward’ kiwifruit during storage at 0  C when exposed to different ethylene concentrations after harvest (A–C) and after 10 weeks (D–E). Each data point represents the incidence of LTB (n = 30). * Grower A (D) is missing control data due to experimental problems.

more than 9 weeks, reaching < 6 N after 5 weeks (interpolated at 15 weeks) exposure as compared to treatment of 0.001 mL L 1 ethylene where fruit were still edible after at least a further 14 weeks (at 24 weeks; Fig. 5B). Even the lowest applied ethylene concentration of 0.01 mL L 1 at storage initiation was enough to cause a 29% reduction in storage potential (i.e. time to reach 10 N was 12 weeks compared to 17 weeks for 0.001 mL L 1). Likewise, Jeffery and Banks (1996) reported that exposure of ‘Hayward’ kiwifruit to 0.01 mL L 1 ethylene reduced the storage potential by 36% for time to reach 8 N. Pranamornkith et al. (2012) reported similar sensitivity to exogenous ethylene for ‘Hort16A’ kiwifruit. Ethylene exposure (0.001–1 mL L 1) during constant storage at  0 C, rapidly (within 4 weeks of application) differentiated

kiwifruit softening trends. Exposure to exogenous ethylene causes accelerated softening, but the magnitude of the acceleration is a function of ethylene concentration, duration of application and storage temperature (Beever and Hopkirk, 1990). Hence, it would be interesting and necessary to further understand these effects in interaction with variable temperature or short term ethylene exposure situations which occur in real supply chains (Bollen et al., 2015). Ethylene can also be a chilling injury inducer in fruit sensitive to low temperature (Sevillano et al., 2009). Koutsoflini et al. (2013) observed an increase in LTB incidence in kiwifruit exposed to 1 mL L 1 ethylene for 24 h before storage. Meanwhile Yang et al., (2013) found that a low temperature conditioning treatment

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standards (0.03 mL L 1) resulted in different firmness outcomes, especially when applied after harvest. While this work has identified the importance of knowledge of ethylene in the supply chain in being able to predict kiwifruit softening profiles and further understanding of responses under variable time, temperature, and ethylene exposure scenarios observed in real supply chains is required. Acknowledgments The authors appreciate Zespri International Ltd., New Zealand for providing project resources and for funding the Postdoctoral Fellowship of the first author. Sue Nicholson assisted in data collection. She also provided guidance during setting up of different ethylene concentrations. References

Fig. 5. Average firmness of three ‘Hayward’ kiwifruit as a function of continuous ethylene exposure after harvest (A) and after 10 weeks (B) during optimal storage (0  C, 95% RH). Red dotted lines mark the firmness range (10–6 N) for eating-ripe stage. Vertical line at 0.03 mL L 1 represents the industry standard threshold concentration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

reduced ethylene production and LTB incidence simultaneously. These findings are in agreement to this study where continuous exposure of 1 mL L 1 ethylene resulted in higher incidence of LTB in kiwifruit followed by fruit exposed to 0.01 mL L 1 (Fig. 4). This suggests that along with advancing the ripening of kiwifruit ethylene also increases the LTB incidence. Changes in cell wall metabolism leading to LTB occurs at the advanced stage of ripening and hence any factor accelerating the ripening process can enhance the susceptibility of kiwifruit to LTB during storage at 0  C (Koutsoflini et al., 2013). Yet, low temperature breakdown is usually the result of cell membrane damage and has been associated with reduced oxidative stress (Yang et al., 2013). Therefore, while it has been established that ethylene will result in accelerating LTB incidence, how this mechanistically occurs, or whether LTB is differentiable from advanced kiwifruit senescence remains undetermined. 5. Conclusion Storage of ‘Hayward’ kiwifruit in ethylene environments resulted in rapid decline of firmness and higher incidence of LTB. Different ethylene concentrations (0.01, 0.1 and 1 mL L 1) generated rapidly differential fruit firmness within 4 weeks. Concentration differences below current industry threshold

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