Analysis of fruit quality differences between Zhongkelümi No. 1 and Zhongkelümi No. 2 at different harvest periods

【Objective】Kiwifruit（Actinidia chinensis var. chinensis）is a climacteric fruit, and its preharvest physiological maturity is closely related to the post-harvest storage characteristics, sensory quality, and nutritional quality of the fruit. However, it is difficult to accurately determine its quality based solely on its size, shape, and color at harvest. Moreover, the appropriate harvest time may vary among different kiwifruit varieties. In kiwifruit cultivation, both early and late harvests can have adverse effects. Therefore, timely harvest is crucial for ensuring kiwifruit quality. The Zhongkelümi No. 1 and Zhongkelümi No. 2 varieties for this study are green-fleshed kiwifruits developed by the Wuhan Botanical Garden of the Chinese Academy of Sciences. They have stable and consistent traits, with strong tree vigor, good productivity, and uniform fruit shape. This experiment aimed to determine the optimal harvest time for Zhongkelümi No. 1 and Zhongkelümi No. 2 kiwifruits and the quality differences between the two varieties, providing technical references for timely harvest in production.【Methods】The experiment utilized fruits from five harvest periods (Harvest Periods Ⅰ, Ⅱ, Ⅲ, Ⅳ, and Ⅴ, corresponding to 150, 158, 166, 174, and 182 days after full bloom, respectively) of Zhongkelümi 1 and Zhongkelümi 2. Measurements included fruit weight, longitudinal, transverse, and lateral diameters, fruit shape index, firmness, soluble solids content (SSC), soluble sugars, titratable acidity, ascorbic acid content, dry matter content, and incidence of soft rot. Sensory evaluations were conducted, and principal component analysis (PCA) was employed to select the harvest period with the highest score for volatile metabolite analysis, further exploring differences in aroma compounds between the two cultivars.【Results】The longitudinal and transverse diameters of Zhongkelümi No. 1 and Zhongkelümi No. 2 kiwifruits changed to varying degrees with the harvest period, but the fruit shape index remained consistently without significant differences, indicating that the fruit shape is relatively stable. The firmness of the fruits at harvest and softening showed an increasing trend initially and then gradually decreased. The firmness of Zhongkelümi No. 1 fruits at harvest showed no significant differences among the five harvest periods, and a significant decrease was only observed at Harvest Period Ⅴ after softening. The firmness of Zhongkelümi No. 2 fruits at harvest was significantly affected by the harvest period, with a range from 6.7 kg · cm-2 to 2.27 kg · cm-2 . The contents of the soluble solids, soluble sugar, ascorbic acid, and titratable acid in both varieties generally increased initially and then decreased with the harvest period. The content of the soluble sugar and ascorbic acid in Zhongkelümi No. 1 reached their peaks at Harvest Period Ⅲ, at 14.40% and 135.17 mg · 100 g- 1 , respectively. The contents of the soluble solids and soluble sugar in Zhongkelümi No. 2 reached their peaks at Harvest Period Ⅳ, at 16.10% and 13.79%, respectively. The ascorbic acid content of Zhongkelümi No. 1 was higher than that of Zhongkelümi No. 2 at Harvest Periods Ⅰ and Ⅱ, but was lower from Harvest Period Ⅲ onwards, with the difference gradually increasing. The sensory evaluation scores of Zhongkelümi No. 2 were generally higher than those of Zhongkelümi No. 1, and the scores of both varieties generally increased initially and then decreased. The highest score for Zhongkelümi No. 1 was 3.000 at Harvest Period Ⅲ, which was 36.36% higher than the lowest score at Harvest Period Ⅳ. The highest score for Zhongkelümi No. 2 was 4.130 at Harvest Period Ⅳ. The incidence of soft rot gradually increased with the delay of the harvest period. The incidence of Zhongkelümi No. 2 was generally higher, which might be related to the rapid decrease in firmness, making it more susceptible to mechanical damage and infection by pathogens, leading to rot, water loss, shrinkage, and softening. Eleven indicators including soluble sugar content, titratable acid content, and ascorbic acid content were selected for principal component analysis. Four principal components were extracted for Zhongke Lümi 1 and three for Zhongke Lümi 2. The comprehensive principal component value of Zhongke Lümi 1 was the highest at harvest period Ⅲ, reaching 1.69, followed by harvest period Ⅴ with a score of -0.05. The score was the lowest at harvest period Ⅰ, being -1.42, and the overall performance was the worst. For Zhongke Lümi 2, the comprehensive principal component value was the highest at harvest period Ⅳ, reaching 2.41, followed by harvest period Ⅲ with a score of -0.23. The score was the lowest at harvest period Ⅰ, being -1, and the overall performance was the worst. Based on the principal component analysis results, the fruits of Zhongke Lümi 1 at harvest period Ⅲ and Zhongke Lümi 2 at harvest period Ⅳ were selected for metabolite determination. A total of 1314 metabolites were detected, including 15 types of metabolites. The types of metabolites with a content over 5% were acids, alcohols, aldehydes, esters, heterocyclic compounds, hydrocarbons, ketones, and terpenoids. Among them, terpenoids, esters, and ketones had the highest proportion. There were 152 effective aroma substances (rOAV≥1) in Zhongke Lümi 1 and 136 in Zhongke Lümi 2. Bothcultivars shared abundant volatile compounds such as 2-methoxy-3,5-dimethylpyrazine, 2-nonenal, dodecanenitrile, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 3-octen-2-one, and 3-mercaptoacetate, contributing to nutty, grassy, fruity, and sweet aromas. The total relative odor activity value (rOAV) and metabolite content were higher in Zhongkelümi 1.【Conclusion】The optimal harvest period for Zhongke Lümi 1 is harvest period Ⅲ (166 days after flowering), and for Zhongke Lümi 2 is harvest period Ⅳ (174 days after flowering). Zhongke Lümi 2 has fewer effective aroma substances than Zhongke Lümi 1, but its soluble solids and soluble sugar content are generally higher than those of Zhongke Lümi 1, making it more favored by consumers. However, Zhongke Lümi 2 has a relatively high incidence of soft rot disease, so violent picking, transportation, and storage should be avoided.