1. INTRODUCTION

Kiwifruit (Actinidia spp.) is an economically important fruit crop worldwide. The global annual production of kiwifruit is 4.41 million metric tons (Mt), with the Republic of Korea accounting for 7,991 Mt (FAOSTAT 2020). The green-fleshed ‘Hayward’ is the predominant kiwifruit cultivar worldwide (Fiorentino et al. 2009). The kiwifruit industry has diversified with the development of ‘Hort16A’ (Actinidia chinensis; marketed as Zespri^® Gold Kiwifruit), a yellow-fleshed kiwifruit cultivar (Kwack et al. 2017). The yellowfleshed kiwifruit cultivars are gaining popularity as they taste sweeter than the traditional green-fleshed cultivar (Kwack et al. 2017). ‘Sweet Gold’ is a new yellow- fleshed variety developed by the National Institute of Horticultural and Herbal Science, Rural Development Administration in 2010. It is characterized by a medium-sized soft core with juicy and soft yellow or greenish-yellow flesh. Additionally, it is high in firmness and sugar content (Kim et al. 2018).

Kiwifruit harvesting time can be determined using a variety of maturity indices, such as color, sugar content, soluble solids content (SSC), starch test, and days after full bloom (DAFB). Generally, domestic kiwifruit growers determine harvesting maturity by merely measuring the sugar content of sample fruits (Choi et al. 2019). Relying solely on sugar content as a maturity index results in mixed harvesting, which negatively impacts fruit quality and storability (Choi et al. 2019). However, these maturity indices rely on destructive methods. Developing a feasible, non-destructive maturity index that farmers can use onsite is crucial. Kiwifruits are harvested when mature and unripe, and ripen during storage and distribution. As ripening progresses, textural changes in the fruit from hard to soft and melting occur (Moggia et al. 2017). Fruit firmness at harvest has been shown to affect postharvest softening and the physicochemical characteristics of different fruits (Moggia et al. 2017;Ornelas-Paz et al. 2018). However, to the best of our knowledge, there have been no reports on the postharvest behavior of ‘Sweet Gold’ linked to firmness at harvest.

Kiwifruit is a climacteric fruit; however, major physiological changes associated with fruit ripening, such as starch conversion, color change, and flesh softening, occur in the absence of a marked increase in ethylene synthesis. Additionally, autocatalytic ethylene is produced late in the ripening process as the fruit begins to senesce (Richardson et al. 2011). However, kiwifruits are extremely sensitive to ethylene treatment. Moreover, ethylene has been suggested to mediate softening of ‘Hayward’ kiwifruit during storage (Tilahun et al. 2020). Ethylene production is greatly affected by variety, harvest maturity, and storage temperature. Thus, understanding ethylene production in kiwifruit is essential for determining fruit maturity and predicting the duration of shelf life.

This study analyzed the impact of fruit firmness levels (hard, medium, and soft) at harvest on the ripening progression of ‘Sweet Gold’ kiwifruit to develop a maturity chart.

2. MATERIALS AND METHODS

2.1. Plant material

The kiwifruit cultivar ‘Sweet Gold’ grown by farmers in Jeju, Korea, was used in the present study. Kiwifruits were harvested at 180 days after full bloom (DAFB) in November 2021 and categorized into three groups based on their firmness at harvest (hard: 30-40 N; medium: 18-28 N; soft: 6-12 N) (Fig. 2). A total of 200 fruits were used for each firmness level at harvest. Thereafter, fruits were transported to the Postharvest Technology Division at the National Institute of Horticultural and Herbal Science, Korea, and packaged in a cardboard box used for distribution and stored at room temperature (20°C). After transportation, kiwifruits were stored at room temperature, and their physicochemical properties were analyzed six times at 2-day intervals over 12 days.

2.2. Firmness

On each evaluation day, 15 fruits per treatment were randomly sampled to assess fruit firmness. The measurements were conducted using a texture analyzer (TA Plus; Lloyd Instruments Ltd., Fareham, Hampshire, UK) equipped with a 0.98 N load cell. The analysis was conducted at a rate of 2 mm s^-1 using a 5 mm-diameter flat probe, which penetrated the flesh to a depth of 10 mm. Firmness measurements were taken after removing the skin at the fruit’s equator (once per fruit). The results are expressed in Newtons (N).

2.3. SSC, Titratable Acidity (TA), and °Brix Acid Ratio (BAR)

The three kiwifruits used to measure firmness were sliced and wrapped with four layers of cheesecloth at room temperature. Juice was extracted using a FruX80 juicer (Goojung Engineering Co. Ltd., Seoul, Korea). SSC was measured using a digital refractometer (PAL- 1; Atago Co. Ltd., Tokyo, Japan) and expressed as a percentage. Titratable acidity (TA) was determined using 0.1 N NaOH in a titration of 5 mL juice and adjusted to pH 8.2. This was performed using an auto pH titrator (TitroLine easy; SI Analytics GmbH, Mainz, Germany) and expressed as a percentage. The Brix-toacid ratio (BAR) was calculated by dividing SSC by TA. The reported data are from three independent replicates per fruit category per evaluation day.

2.4. Fruit surface color

The flesh color of kiwifruit was measured using a colorimeter (CR-300 Minolta Chroma Meter; Konica Minolta Sensing Inc., Tokyo, Japan) on the cut surface at the equator of the fruit with five fruits measured per category. For fruit sampling, the kiwifruit was cut to a thickness of 1 cm centered on the equator and used to measure color difference. The measurements were conducted using d/0 geometry, characterized by dif- fuse illumination and an observation angle of 0°, as implemented in the Minolta CR-300. The fruit surface color was measured using a colorimeter, and the results were expressed as Hunter values: L (lightness, where 0=black and 100=white), a (red to green scale, with positive values indicating redness and negative values indicating greenness), and b (yellow to blue scale, with positive values indicating yellowness and negative values indicating blueness). The Hue value was calculated as tan^-1 (b/a), representing the angle of color in the red-green-blue color space (McGuire 1992).

2.5. Ethylene production

Ethylene production of kiwifruit was measured from 5 fruits that were sealed in 1.7 L airtight plastic containers (width 13.5×height 10.2×length 23.5 cm) for 2 h. A 1 mL gas sample was removed from the headspace of each container using a gas-tight syringe and injected into a gas chromatograph to measure ethylene production. Ethylene production was analyzed using gas chromatography (Bruker 450-GC; Bruker Corp., Billerica, MA, USA). The injection and column temperatures were maintained at 70°C. The thermal conductivity detector and flame ionization detector used for ethylene measurements were set to 250°C. The results were expressed as μL C₂H₄ kg^-1 h^-1. Ethylene production was determined from three independent replicates per fruit category per day at 2-day intervals for 12 days.

2.6. Statistical analyses

The experiment was conducted using a completely randomized design with 15 replicates for firmness measurements and 5 replicates for SSC, TA, color, and ethylene production. The data were subjected to analysis of variance using SPSS 20 (SPSS Inc., Chicago, IL, USA), and the significance of differences was assessed using Duncan’s multiple range test (p<0.05). The Pearson’s correlation test was used to determine the correlation of parameters.

3. RESULTS

3.1. Fruit quality at harvest

At harvest, ‘hard’ fruits had the greatest firmness (31.28-40.14 N) and TA (1.42-1.61) and the lowest SSC (8.3-8.9% Brix) and BAR (5.16-6.27). However, these parameters in ‘soft’ fruits were the complete opposite. In ‘medium’ fruits, values of these quality attributes were in between those of ‘hard’ and ‘soft’ fruits (Table 1). Similarly, the a values were highest in ‘soft’ fruits, followed by ‘medium,’ and then ‘hard’ fruits. However, the Hue values in ‘hard’ fruits were significantly higher than those of ‘medium’ and ‘soft’ fruits.

3.2. Changes in firmness

Firmness decreased over the storage period for all fruit categories (Table 2). ‘Hard’ fruits tended to loose firmness gradually (~3 N) until day 4; however, there was an almost 50% drop in firmness on day 6 compared to that at day 0 (during harvest). Thereafter, the firmness of ‘hard’ fruits reduced to ~6.5 N until day 10. Similarly, ‘medium’ fruits lost 46% firmness at 8 N on day 6. The firmness values of ‘medium’ fruits on days 4 and 8 were similar to those of ‘hard’ fruits on days 6 and 10, indicating a faster firmness loss in ‘medium’ fruits. In contrast, ‘soft’ fruits, whose harvest firmness was similar to that of ‘hard’ fruits on day 8, lost 40% of their firmness within the first 2 days of storage. The firmness of ‘soft’ fruits on day 4 was similar to that of ‘medium’ and ‘hard’ fruits on days 8 and 10, respectively.

3.3. Changes in SSC, TA, and BAR

The SSC gradually increased in all fruit categories during storage (Table 2). Regarding changes in firmness, SSC levels were in the order of lowest to highest in ‘hard’, ‘medium’, and ‘soft’ fruits throughout storage except on day 12. Medium’ fruits required 6 days to reach an SSC level comparable to that of ‘soft’ fruits at harvest, whereas ‘hard’ fruits needed 10 days. The decline in TA during storage was significant until day 6 among the fruit categories; however, there was no difference between ‘medium’ and ‘soft’ fruits thereafter. Within 4 d of storage, ‘soft’ fruits attained the maximum decline in TA (0.79%), as there was no significant difference in its reduction thereafter. However, ‘medium’ and ‘hard’ fruits required 8 and 12 d of storage, respectively, for this level of reduction. With the exception of the last day of storage, fruit categories had significantly different BAR values, with the order of highest to lowest in fruit firmness categories from ‘soft’ to ‘hard’. While ‘medium’ and ‘soft’ fruits recorded their greatest BAR on the last day of storage, ‘soft’ was achieved in half the storage time.

3.4. Changes in flesh surface color

The hue value was significantly higher in the ‘hard’ fruits than that in the other fruits at the harvest. The decrease in hue value in the ‘hard’ fruits was delayed compared to other fruits, and the values of the ‘hard’ fruits on day 8 and of the ‘soft’ fruits on day 2 were similar. The hue value of ‘soft’ fruits was the lowest during the storage period (Table 3). The a values of ‘soft’ fruits gradually increased until day 6 and were steadily maintained thereafter. The a values of ‘medium’ and ‘hard’ fruits were not significantly different from the values on the day of harvest, indicating delayed color transition in these fruits. Notably, a values of ‘soft’ fruits on the day of harvest were similar to those of ‘hard’ fruits on day 12 (Fig. 2).

3.5. Changes in ethylene production rate

‘Soft’ fruits showed the highest ethylene production rate throughout storage, while ‘hard’ fruits had the lowest. In ‘medium’ fruits, the ethylene production rate was in between that of ‘soft’ and ‘hard’ fruits (Fig. 1). Ethylene production began on day 8 after harvest for ‘hard’ fruits, on day 6 for ‘medium’ fruits, and immediately (day 0) for ‘soft’ fruits. In particular, ‘soft’ fruits began to generate ethylene from the first day of irradiation, making it difficult to maintain freshness and marketability.

4. DISCUSSION

Maturity at harvest is the most important factor that determines storage life and final fruit quality (Tilahun et al. 2020). Immature fruits are more susceptible to shriveling and mechanical damage and have inferior flavor quality when ripe (Meena et al. 2018). Overripe fruits are likely to become soft and mealy, with an insipid flavor soon after harvest. Fruits picked either too early or too late in their season are more susceptible to postharvest physiological disorders than fruits picked at proper maturity (Babu et al. 2017). Although sensory evaluation from the consumer’s perspective was not conducted in this study, the definition of the edible period of fruit was reported to be that the level of firmness and the riper the fruit, the higher the proportion of consumers who were satisfied with the eating experience (Harker et al. 2019).

Most commonly used maturity indices balance the need for optimal consumer eating quality and the flexibility required for marketing. Moreover, developing maturity indices using non-destructive methods is essential, as they are far more feasible to implement. In this study, we developed a maturity chart based on firmness at harvest time and predicted the optimal eating period and distribution limit (Fig. 2).

Harvesting fruits with appropriate firmness is critical for postharvest handling, including storage in bulk bins, passing across a grading line, or determining whether a particular line of fruit is suitable for export. Firmness at harvest determines fruit quality during storage and distribution. The relationship between firmness at harvest, post-harvest softening, and internal browning has been studied in blueberries (Moggia et al. 2017). Vilhena et al. (2022) reported that slight changes in fruit firmness at harvest can influence the storage potential of persimmon. To determine the relationship between kiwifruit firmness at harvest and postharvest ripening at room temperature, we evaluated the ripening and quality attributes of kiwifruits that differed in firmness from highest to lowest (hard, medium, and soft). ‘Soft’ fruits lost 40% firmness within 2 d after harvest with the greatest SSC and BAR and lowest TA. ‘Hard’ fruits were firmer than soft fruits throughout the storage period with the least SSC and BAR and the highest TA, indicating slower progression of ripening in these fruits. The quality attributes of ‘medium’ fruits fell between those of ‘hard’ and ‘soft’ fruits. Both hard and soft fruits demonstrated a major drop in firmness on day 6; however, ‘soft’ fruits showed faster softening than that of ‘hard’ fruits, as evidenced by the firmness values on days 4 and 8 in ‘soft’ fruits, which were similar to those of ‘hard’ fruits on days 6 and 10. Regarding ripening progression, ‘soft’ fruits had a higher respiration rate with an early climacteric rise of ethylene than those of ‘medium’ and ‘hard’ fruits.

Overall, ‘hard’ fruits had a longer shelf life than that of ‘medium’ fruits, and ‘soft’ harvested fruits were unsuitable for long storage. The maintenance of cell walls and membrane integrity was associated with delayed firmness loss and higher storage potential of persimmon fruits that had higher firmness at harvest (Vilhena et al. 2022). Variations in structural integrity among kiwifruits with different firmness levels at maturity might have influenced their overall quality and storability. Understanding ethylene production during kiwifruit ripening is crucial. Unlike typical climacteric fruits, major physiological changes occurred without a climacteric rise in ethylene (Mworia et al. 2010). However, kiwifruits are extremely sensitive to ethylene (Yin et al. 2010). Furthermore, fruits at different maturity levels responded differentially to ethylene. As ethylene is commonly used for commercial ripening, it is essential to determine when to administer the treatment. In this study, soft fruits exhibited a climacteric rise in ethylene on day 13, whereas ‘medium’ and ‘hard’ fruits exhibited it on days 19 and 21. These findings can guide the determination of the optimal ethylene treatment timing for ‘Sweet Gold’ kiwifruit.

5. CONCLUSIONS

In this study, the effect of firmness at harvest of ‘Sweet Gold’ kiwifruit on post-ripening characteristics was investigated. Firmness declined throughout the storage period across all fruit categories. Among them, kiwifruit in the ‘soft’ stage softened at a much faster rate than kiwifruit at other stages. The SSC consistently increased across all fruit categories during storage. Regarding changes in firmness, SSC levels were in the order of lowest to highest in ‘hard’, ‘medium’, and ‘soft’ fruits throughout storage with the exception of day 12. The decline in TA during storage was significant among the fruit categories until day 6. In particular, the acidity of ‘soft’ stage kiwifruit was very low from the day of harvest, indicating that it was consumed as a respiratory substrate during the ripening process. The a values of ‘soft’ fruits gradually increased until day 6 and remained steady thereafter. ‘Soft’ fruits showed the highest ethylene production rate throughout the storage duration. Thus, it was confirmed that the firmness of ‘Sweet Gold’ kiwifruit at harvest significantly affects its marketability, and its shelf life can be predicted when firmness is considered a key selection factor. Based on these findings, a maturity chart for kiwifruit was developed.