Determination of optimal harvest period of early-maturing apples in South Basin of Shanxi Province

【Objective】The early-maturing apple cultivars Luli and David Gala are extensively cultivated at the south region of Shanxi due to earlier harvest and higher market value, compared with other cooler areas. The fruit qualities are determined by the harvest time. Traditionally, apple fruits are harvested according to skin coloring, seed color or days after full bloom (DAFB), which are influenced by climates in different years. Rapid fruit ripening of early-maturing apple cultivars under higher temperatures in the south region of Shanxi presents challenges in determining the optimal harvest timing, which directly affects postharvest fruit quality and storage potential. This study aimed to establish a starch-iodine staining criteria as a reliable indicator for harvest maturity, ensuring fruit quality and prolonging its shelf life.【Methods】The apple flesh of Luli (95 to 130 DAFB, 7 d harvest interval) and David Gala (91 to 126 DAFB, 7 d harvest interval) at different maturities was stained by iodine-potassium iodide solution to show starch degradation patterns, and correspondingly the fruit quality attributes of fruit size, single-fruit weight, firmness, skin color, soluble solids content (SSC), titratable acidity (TA), and sugaracid ratio were measured. Correlation analysis was conducted to evaluate relationships between the starch index and quality attributes. The fruits of Luli and David Gala at different maturities were stored at 1 ℃ for 60 days, and their indicators of firmness, SSC, and TA were monitored to assess storage quality.【Results】Starch-iodine staining results indicated that starch hydrolysis occurred in both Luli apples (95 to 130 DAFB) and David Gala apples (91 to 126 DAFB) during ripening. The starch index of Luli apples gradually increased from 1.16 (at 95 d DAFB) to 7.90 (at 130 DAFB), and David Gala apples gradually increased from 1.03 (at 91 DAFB) to 7.00 (at 126 DAFB). The hydrolysis of starch and the starch index could serve as core indicators of maturity. With fruit maturing, the starch hydrolysis in both Luli and David Gala started from the core and gradually spreaded to the pulp. Notably, vascular bundles and pericarp tissues were the last to undergo hydrolysis. At the later stage of maturity (staining grade 6), the starch in the cross-section of Luli apples was almost completely hydrolyzed, while some starch in the vascular bundles and pericarp tissues of David Gala apples had not been hydrolyzed. This indicated that the starch hydrolysis in Luli apples was faster and more complete during the maturity period (95 to 130 DAFB). Starch index of Luli and David Gala was significantly and positively correlated with single fruit weight and soluble solids content, and significantly and negatively correlated with fruit firmness and titratable acidity (P ＜ 0.01). With the prolongation of the harvest period, fruit firmness and titratable acidity decreased, and soluble solids content increased. Firmness can serve as an indicator of storage potential. The firmness of Luli decreased less than that of David Gala during storage, and its storability was better. Soluble solids content is a quality index reflecting the flavor and maturity of the fruit. The content of soluble solids in Luli apples gradually increased from 95 to 109 DAFB and remained stable from 116 to 130 DAFB, while the soluble solids content of David Gala apples gradually increased during the storage process at different harvest periods. Comprehensive analysis of fruit quality indicators at different maturity stages showed that Luli apple fruit and storage quality during 116 to 123 DAFB was the best, in which the starch index was 5.40-6.53, fruit size tended to be stable, skin changed from partial coloring to complete coloring, soluble solids content was 13.20%-14.33%, and fruit firmness was 9.42-10.49 kg · cm- 2 and decreased to 7.60 kg · cm- 2 after storage for 60 d. David Gala apple fruit and storage quality at 119 DAFB was the best, in which starch index was 5.93, fruit size tended to be stable, fruit skin was fully colored, soluble solids content was 13.33%, and fruit firmness was 11.02 kg· cm-2 and decrease to 7.13 kg· cm-2 after 60 d of storage. When harvested too early, for instance, Luli fruit was harvested at 95 to 102 d DAFB (starch index was 1.16-1.85) and David Gala was harvested 91 to 98 DAFB (starch index was 1.03-1.80), the fruit firmness was excessively high, SSC was insufficient, and the flavor cannot meet the standard. When harvested too late, for instance, Luli fruit was harvested at 130 DAFB (starch index was 7.90), and David Gala was harvested at 126 DAFB (starch index was 7.00), the fruit firmness decreased significantly after storage (＜ 7.00 kg · cm- 2 ) and was not resistant to storage, although the fresh flavor was the optimal (SSC ＞ 14%).【Conclusion】Starch staining can be used as a core evaluation index for the harvest maturity of Luli and David Gala apples. The starch staining pattern for appropriate harvest time of Luli and David Gala apples showed that vascular bundles of sepals and petals were still stained, and presented 10 discrete dark clusters in the core. The optimal starch index for Luli was 5.40-6.53, but for David Gala it was 5.93.