Physicochemical quality of hardy kiwifruit (Actinidia arguta L. cv. Cheongsan) during ripening is influenced by harvest maturity

Abstract Hardy kiwifruit is popular among consumers, but it has a short shelf-life. In this study, the influence of harvest maturity on the physicochemical properties of ‘Cheongsan’ hardy kiwifruit was investigated during storage at 22 °C. Harvested fruit were divided based on the soluble solids content (SSC) into ‘harvest 1’ and ‘harvest 2’ with 6% and 8% SSC, respectively. As the fruit continued growing on the vine, the fresh and dry weights at harvest were 1.3 and 1.7 times higher in ‘harvest 2’ than in ‘harvest 1’, respectively. There was no significant difference in titratable acidity between the two groups. Harvest maturity significantly affected the content of total phenolic compounds; ‘harvest 2’ fruit had 2.1 times higher concentration than that of ‘harvest 1’ fruit. The respiration rate rapidly increased after harvest, and the SSC also increased throughout the ripening, regardless of fruit maturity. ‘Harvest 2’ fruit exhibited significant reduction of weight loss and retained firmness during ripening compared with those of ‘harvest 1’ fruit. The ‘harvest 1’ fruit were not of adequate quality and spoiled before ripening. These data suggest that ‘Cheongsan’ hardy kiwifruit should be harvested when the SSC is greater than 8% to retain premium quality fruit.


Introduction
Actinidia arguta hardy kiwifruit or baby kiwifruit, is a widely cultivated fruit species in East Asia, and consumer preference for the fruit has increased due to well-balanced sweet and sour taste and intense flavor (Fisk et al. 2006;Oh et al. 2017). Hardy kiwifruit have a smooth and edible skin, and the fruit size is smaller than green flesh kiwifruit (A. deliciosa) (Fisk et al. 2008). The fruit are considered healthy as they contain high amounts of vitamin C, approximately 150-200 mg per 100 g of fresh fruit (Krupa et al. 2011;Oh et al. 2017). The fruit is also rich in phenolic compounds, lutein, and minerals (such as, P, Ca, Fe, and Zn) . New cultivars of hardy kiwifruit have been developed in South Korea, such as 'Cheongsan', 'Saehan', 'Daesung', 'Chilbo', and 'Autumn Sense', and are mainly cultivated in Gangwon-do Oh et al. 2014;Lim et al. 2016).
One of the main disadvantages of hardy kiwifruit is their short shelf-life due to rapid softening and dehydration (Latocha et al. 2014). Therefore, the fruit is commonly harvested before being ripened on the vine because they are too soft to package when fully ripe (Fisk et al. 2006;Krupa et al. 2011). The fruit is ripened during cold storage, which is a common practice in green flesh kiwifruit (Strik and Hummer 2006). Furthermore, the optimal harvest time for green flesh kiwifruit has been reported as when the soluble solids content (SSC) of the fruit is reached to 6.5% (Fisk et al. 2006). However, there is limited information on the ideal harvest time for hardy kiwifruit. The ideal harvest date and postharvest storability of the hardy kiwifruit are important to produce fruit of intense flavor, taste, and texture for commercial markets.
The main objective of the study was to determine the effects of harvest maturity (based on the SSC) on the quality of stored 'Cheongsan' hardy kiwifruit by monitoring physicochemical parameters. The study will provide basic information on the postharvest quality of 'Cheongsan' hardy kiwifruit, which will be helpful for further studies on extending the shelf-life of the fruit.

Plant materials
Hardy kiwifruit cultivar 'Cheongsan' was grown in Wonju-si, Gangwon-do, Korea (127 89 0 E; 37 45 0 N). The fruit were harvested at two different maturity stages, based on the SSC, on August 29, 2018 (average SSC 6%, 'harvest 1') and September 10, 2018 (average SSC 8%, 'harvest 2'). The fruit selected were of uniform size, appearance, and free from defects, and they were packed in low-vent polypropylene containers ( container. The fruit was stored at 22 ± 1 C with relative humidity of around 85% for one week. The fruit was collected from three plastic containers on days 0, 1, 3, 5, and 7 of ripening for physiochemical analysis; 20 fruit were collected per plastic container. To analyses of the content of total phenolic compounds (TPC), the fruit was directly frozen in liquid nitrogen and stored at À80 C prior to analysis.

Physicochemical analysis
For respiration measurements, five fruit was placed into a 250-mL sealed jar with a septum in the lid for one replicate. After one hour, the headspace gas sample was collected through the septum and immediately injected into a headspace gas analyzer (CheckMate 3; Mocon dansensor Inc., Ringsted, Denmark). The respiration rate was calculated for five replicates and expressed in mg kg À1 h À1 . Fruit weight loss is presented as the percentage of weight at each sampling time relative to the initial weight immediately after packaging. Ten selected fruit was used for weight loss evaluation at each time point. Fruit firmness was determined using a texture analyzer (CR-3000EX-S; Sun Scientific Co., Ltd., Tokyo, Japan) with a 5-mmdiameter punch probe. Each fruit was subjected to a compression speed of 250 mm min À1 after contact and penetration to 10 mm, approximately at the center of the flat surface of the fruit. The results are presented as the mean maximum peak force from 36 replicates and expressed in Newton (N). Marketability was visually assessed by the number of fruit showing decay symptoms from fungal and mold developmental in 10 individual fruit. Marketability is presented as the percentage of the number of fruit with decay symptoms at each sampling time relative to the number of total fruit in the packaging. The SSC and titratable acidity (TA) were measured in nine replicates, each with a combined extract from four fruit. A refractometer (PR-101a; ATAGO Co., Ltd., Tokyo, Japan) was used to assess the SSC in the juice obtained from whole fruit squeezed, and results are expressed in percent. The TA was determined using a TA meter (GMK-835N; G-Won Hitech Co., Ltd., Seoul, Korea) and is expressed as percent anhydrous citric acid as anhydrous citric acid is the dominant organic acid in the genus Actinidia (Marsh et al. 2004).

Measurement of the content of total phenolic compounds
Ten fruit were collected from each container and macerated for 1 min, and then the tissues were homogenized using a mixer. A slurry sample (5 g) from the homogenate was mixed with 30 mL of 100% methanol and homogenized. The mixture was stirred for one hour in dark at 24 ± 1 C. The mixture was then centrifuged at 3,000 g for 15 min and the supernatant was collected. The TPC was determined using Folin-Ciocalteu reagent, with gallic acid as the standard (Slinkard and Singleton 1977); the experiment was performed in triplicate. The absorption of the solution was measured using a UV spectrophotometer (NanoPhotometer NP80; Implen Inc., Munich, Germany) at 735 nm, and the results are expressed as mg gallic acid equivalence (GAE) per kg of fresh weight (fw). The TPC was quantified using a standard curve, with gallic acid concentration between 50 and 200 mg L À1 .

Statistical analysis
All data are presented as the mean of replicates followed by standard error. The figures were produced using Sigma Plot 12.0 software (Systat Software Inc., San Jose, CA, USA). An independent t-test at p < .05 was conducted to analyze differences between the maturity stages ('harvest 1' vs. 'harvest 2') using SPSS statistical software version 18 (SPSS, Inc., Chicago, IL, USA). Statistical significance based on Tukey's multiple range test at p < .05, and analysis of variance (ANOVA) were performed during storage period using SPSS statistical software.

Quality of 'Cheongsan' hardy kiwifruit at harvest was affected by fruit maturity
The physicochemical and nutritional characteristics of 'Cheongsan' hardy kiwifruit at harvest are presented in Table 1. The fresh and dry weights of fruit were 1.4 and 1.7 times higher in 'harvest 2' fruit than in 'harvest 1' fruit, respectively, as the fruit were still growing before harvest. The fruit SSC was significantly higher in 'harvest 2' than in 'harvest 1'. However, there was no significant difference in the TA between the two groups. 'harvest 2' fruit also showed significantly higher firmness than 'harvest 1' fruit. 'harvest 2' fruit had significantly higher TPC than 'harvest 1'.

Respiration rate of 'cheongsan' hardy kiwifruit during ripening
Fruit stored at 22 C for seven days are shown in Figure 1. A high respiration rate, which is a critical factor associated with the ripening of climacteric fruit, was observed in both 'harvest 1' and 'harvest 2' fruit during ripening (Figure 2(A)). The respiration rate of 'harvest 1' fruit sharply increased after ripening for one day from 0.8 to 1.7 mg kg À1 h À1 , and then gradually decreased to its initial level during the seven-day ripening periods. 'harvest 2' fruit showed a similar respiration pattern to that of 'harvest 1' fruit; however, the initial level of respiration was 1.5 times higher in 'harvest 2' fruit than in 'harvest 1' fruit. The highest respiration rate was observed one and two days after ripening in 'harvest 1' and 'harvest 2' fruit, respectively, and was approximately 1.6-1.7 mg kg À1 h À1 .

Fruit firmness, weight loss and marketability of 'cheongsan' hardy kiwifruit during ripening
The primary factor responsible for reduced quality of stored hardy kiwifruit is rapid softening due to water loss and decay. Therefore, weight loss and firmness are basic indexes for determining fruit senescence (Ghiani et al. 2011;Atkinson et al. 2012;Wang et al. 2015). The rate of weight loss gradually increased during ripening to over 10% in 'harvest 1' fruit and 6% in 'harvest 2' fruit by the end of ripening (Figure 2(B)). 'harvest 1' fruit exhibited a rapid decline in fruit firmness, down to 4.0 N three days after ripening; however, 'harvest 2' fruit showed relatively high firmness until five days of ripening (Figure 2(C)). Fruit firmness decreased after ripening for seven days, and it reached 3.0 N and 6.6 N in 'harvest 1' and 'harvest 2' fruit, respectively. After ripening for seven days, the marketability of 'harvest 1' fruit decreased by 67% because the fruit was decayed by fungal and mold development (Figure 2(D)).

Soluble solids content and titratable acidity of 'cheongsan' hardy kiwifruit during ripening
The SSC and TA are important parameters determining the taste and consumer acceptability of vine-ripened fruit including hardy kiwifruit. There was a 190.2% and 195.2% increase in the SSC of 'harvest 1' and 'harvest 2' fruit, respectively, compared with their initial SSC (Figure 2(E)). Only a negligible change was observed in the SSC of 'harvest 1' fruit three days after ripeing, and the SSC was approximately 11.2-12.2%; whereas, the SSC of 'harvest 2' fruit consistently increased throughout the ripening period, and reached up to 16.2% seven days after ripening. The TA detected in the fruit of 'harvest 1' and 'harvest 2' was unstable during ripening (Figure 2(F)). In 'harvest 1' fruit, the TA decreased one day after ripening from 1.12% to 0.86%, and then slightly increased until 5 days in ripening. On the other hand, the TA in 'harvest 2' fruit increased from 1.04 at harvest to 1.55 at three days in ripening, but it was 45.8% decreased at the end of ripening compared to three days in ripening.

Discussion
We found that the respiration rate of 'Cheongsan' hardy kiwifruit peaked shortly after harvest. In a previous study, the A. aruguta cultivars showed a typical climacteric characteristic with a high respiration rates during storage (Lim et al. 2016). Wang et al. (2015) demonstrated that the respiration rate is highly correlated with the ethylene production rate during the ripening of climacteric fruit, including hardy kiwifruit, and thus postharvest treatments (such as, application of 1-methylcyclopromene [1-MCP]) are effective in delaying fruit ripening and softening by reducing the respiration and ethylene production rates (Lim et al. 2016).
The firmness and weight loss of 'Cheongsan' hardy kiwifruit decreased after harvest in our study, with greater decreases in 'harvest 1' fruit than in 'harvest 2' fruit. Similar to our findings, Fisk et al. (2008) reported that the firmness of 'Ananasnaya' hardy kiwifruit rapidly decreased in the first few weeks of ripening; however, Lim et al. (2016) found that the softening of 'Cheongsan' hardy kiwifruit was delayed during storage at 1 C when treated with 1-MCP. Studies have reported that the softening rate of fruit varies with species (White et al. 2005) and maturity stage (Krupa et al. 2011). Fisk et al. (2006 observed that 'Ananasnaya' hardy kiwifruit harvested at 8.7% SSC had significantly lower (p < .05) firmness than those harvested at 6.0% SSC, three weeks after ripening, unlike our results. The higher firmness in 'harvest 2' than in 'harvest 1' observed in the present study was not consistent with that reported previously. Latocha et al. (2014) reported that 'Bingo' fruit harvested at 6.5-8.0% SSC was firmer than fruit harvested at 8.0-9.5% SSC. Furthermore, the firmness of 'Saehan', 'Daesung', and 'Chilbo' hardy kiwifruits decreased with the increase of SSC, similar to fruit growing on the vine ). The temperature and relative humidity in July Figure 1. Later harvest improved hardy kiwifruit appearance after ripening. The appearances of 'Cheongsan' hardy kiwifruit harvested at two different maturities, at a soluble solids content of 6% in 'harvest 1' and 8% in 'harvest 2'. Table 1. Effects of harvest maturity on the physicochemical properties of hardy kiwifruit.
and August of 2018 was relatively hotter and more humid than usual according to the Korea Meteorological Administration (http://www.kma.go.kr). Moretti et al. (2010) noticed that the A. argute fruit qualities, such as firmness, SSC, TA, and fresh weight, could be strongly correlated with weather condition during the fruit ripening phase. Thus, it may be assumed that different vegetation season may influence the hardy kiwifruit firmness. The increase of SSC in 'Cheongsan' hardy kiwifruit during ripening observed in this study is consistent with that reported previously (Fisk et al. 2006;Krupa et al. 2011;Latocha et al. 2014). Latocha et al. (2014) reported that hardy kiwifruit SSC increased to approximately 175-200% and 150-175% relative to the initial harvest value in 'Ananasnaya' and 'Bingo' cultivars, respectively, eight weeks after cold storage. Furthermore, a relatively high SSC was observed after storage in fruit harvested at 8.0-9.5% SSC compared with fruit harvested at 6.5-8.0% SSC. According to MacRae et al. (1992), the increase in SSC during storage is mainly due to the degradation of starch and conversion to sucrose by glucolytic enzymes. Langenk€ amper et al. (1998) reported that the activity of these enzymes increased as fruit matured, regardless of storage conditions; however, the activity was different between cultivars (Choudhury et al. 2009). The TA of 'harvest 2' fruit increased from 1.04% to 1.55% citric acid for up to three days after ripening, and then decreased to 0.84% at the end of ripening. After seven , firmness (C), marketability (D), soluble solids content (E), and titratable acidity (F) of 'Cheongsan' hardy kiwifruit harvested at two different maturities, at the soluble solids content of 6% in 'harvest 1' and 8% in 'harvest 2'. Marketability was visually assessed by the number of fruit showing decay symptoms from fungal and mold developmental in 10 individual fruit. Marketability is presented as the percentage of the number of fruit with decay symptoms at each sampling time relative to the number of total fruit in the packaging. The data are expressed as mean ± error of replicates. (n ¼ 5 for respiration rate analysis, n ¼ 10 for weight loss and marketability analysis, n ¼ 36 for fimness analysis, and n ¼ 9 for soluble solids content and titratable acidity analysis). Ã , ÃÃ , or ÃÃÃ indicate significant difference between harvests determined by independent t-test at p < .05; p < .01; p < .001, respectively. days, 'harvest 1' and 'harvest 2' fruit lost $14.0% and 19.1% of their acidity, respectively; however, these decreases were not statistically significant. A similar level ($0.5-1.1%) of TA has been previously reported (Latocha et al. 2014). They also observed a consistent decrease in the TA during ripening, regardless of fruit maturity, unlike our results. Our results indicated that the harvest of hardy kiwifruit at 8% SSC could significantly reduce weight loss and improve firmness during ripening, thus effectively extending shelf-life.

Conclusions
This study showed the physicochemical changes in 'Cheongsan' hardy kiwifruit of different harvest maturities, determined based on the SSC, during ripening. 'harvest 2' fruit presented significantly reduced weight loss and high firmness during ripening compared with those of 'harvest 1' fruit. Additionally, 'harvest 1' fruit decayed seven after days in ripening, and they were not of adequate fruit quality and spoiled before ripening. These data suggest that 'Cheongsan' hardy kiwifruit should be harvested when the SSC is higher than 8% to retain high quality fruit after ripening.

Disclosure statement
No potential conflict of interest was reported by the authors.