Abstract:【Objective】To study the dynamic changes of sugar-acid components during the color transition of main cultivars and self-bred varieties including yellow passion fruit and purple passion fruit, and to lay a foundation for determining the appropriate harvest time, producing high-quality fruits and cultivating high-quality varieties of passion fruit. 【Methods】Using the main cultivar and self-bred varieties including Qinmi (QM), Zhuangxiang Mibao (ZXMB), Zhongbai No. 6 (ZB-6), Huangxiang No. 1(HX-1), Tainong (TN) and Mantiaxing (MTX) for the trial materials, the samples were collected at five different stages according to the degree of fruit color change and ripeness. The five stages are: T1 (after 12 weeks after flowering): green fruit skin, pale yellow flesh; T2 (after 13 weeks after flowering): yellowish fruit skin, yellow flesh; T3 (after 14 weeks after flowering): half-yellow fruit skin, yellow flesh; T4 (ripe period, after 15 weeks after flowering): yellow fruit skin, yellow flesh; T5 (after 16 weeks after flowering): slightly wrinkled fruit skin, yellow flesh. The soluble solid content of the fruit was measured using a sugar-acid meter (ATAGO PAL-BX/ACID 1), the acid content was determined using a ZDJ-4B automatic potentiometric titrator (Shanghai Lei-ci), and the solid-acid ratio was calculated. The organic acids and soluble sugar components and content were determined using high-performance liquid chromatography (HPLC). 【Results】As the fruits turn color and mature, the soluble solid content of the flesh of QM, ZXMB, ZB-6, HX-1, TN and MTX varieties of passion fruit presents a trend of increasing first and then decreasing, and it reaches its highest value at T3 (the fruit peel is half-yellow, the flesh is yellow). The titratable acid content of ZXMB, ZB-6, HX-1, TN and MTX varieties decreases gradually as the fruits turn color, while that of QM shows an up-and-down trend. Except for QM's solid-acid ratio increasing gradually as the fruits turn color, the solid-acid ratio s of ZXMB, ZB-6, HX-1, TN and MTX varieties all show an up-and-down trend. The sucrose content of passion fruit in the fruit turning color process all shows an up-and-down trend, but the highest value of sucrose content in the fruit turning color process varies among the varieties. The trend of fructose and glucose in different color transformation periods of the six varieties is consistent, among which QM, ZB-6, HX-1, and MTX show a trend of first increasing and then decreasing with fruit color transformation. Citric acid is the main organic acid component of passion fruit, and the trend of citric acid content change of the six passion fruit varieties all shows a gradual decrease as the fruits turn color. Malic acid is the secondly organic acid, while the content of succinic acid, lactic acid and tartaric acid present the lowest. There are significant differences in sugar and acid components among the six passion fruit varieties at maturity stage. The content of fructose and glucose in the fruits of ZB-6 is significantly higher than that of other varieties at maturity stage, while ZXMB has significantly higher soluble solid content than other varieties. The acid components in the mature fruits of the six passion fruit varieties are mainly citric acid, accounting for 68.27%-84.22% of the total. Among them, the citric acid, total acid and titratable acidity content of TN are significantly higher than those of other varieties. From the analysis of the solid-acid ratio of the mature fruits of the six passion fruit varieties, QM has a significantly higher solid-acid ratio than other varieties, and the solid-acid ratio of yellow passion fruit is higher than that of purple passion fruit. The solid-acid ratio from high to low is as follows: QM > ZXMB > HX-1 > ZB-6 > TN > MTX. 【Conclusion】The sugar and acid component ratios of different passion fruit varieties vary during different stages of fruit color change and ripeness. T2-T4 is the key period of fruit quality change for QM, ZXMB, ZB-6, HX-1, TN and MTX. With fruit maturity, there is a trend of reducing sugar, and the solid-acid ratio of yellow passion fruit is greater than that of purple passion fruit. When the fruit is fully ripe, the solid-acid ratio from high to low is as follows: QM > ZXMB > HX-1 > ZB-6 > TN > MTX. Both purple passion fruit and yellow passion fruit belong to the citric acid dominant type, and the fruit solid-acid ratio of passion fruit is mainly influenced by citric acid content, and it is inversely proportional. Through the above research, the optimal consumption period of six different passion fruit varieties is from the complete maturity of the fruit to the stage of slightly wrinkled skin. Among them, the fruit of MTX, HX-1, and ZXMB, has a fairly balanced sour and sweet taste at the stage of half-color change quite close to the state of full maturity, so this stage is also suitable for tasting and consumption.
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