Evaluation of sugar and acid quality traits of different passionfruit varieties during coloration period

Abstract:【Objective】To study the dynamic changes in sugar and acid components during fruit coloration of the main passionfruit cultivars and some self-bred varieties including yellow varieties and purple varieties, in order to determine the appropriate harvest time, produce high-quality fruits and cultivate high- quality varieties of passionfruit.【Methods】The tested varieties included Qinmi (QM), Zhuangxiang Mibao (ZXMB), Zhongbai No. 6 (ZB-6), Huangxiang No. 1(HX-1), Tainong (TN) and Mantiaxing (MTX) for the trial materials, and the samples were collected at the five stages according to the degree of fruit color change and ripeness. The five stages were: T1 (12 weeks after flowering), with green fruit skin, pale yellow flesh; T2 (13 weeks after flowering): yellowish fruit skin and yellow flesh;T3 (after 14 weeks after flowering), with half-yellow fruit skin and yellow flesh; T4 (ripe period, after 15 weeks after flowering), with yellow fruit skin and yellow flesh; T5 (after 16 weeks after flowering), with slightly wrinkled fruit skin and yellow flesh. The soluble solid content was measured using a sugaracid meter (ATAGO PAL-BX/ACID 1); the acid content was determined using a ZDJ-4B automatic potentiometric titrator (Shanghai Lei-ci), and the solid to 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 matured and changed color, the soluble solid contents in the flesh of QM, ZXMB, ZB-6, HX-1, TN and MTX varieties displayed a trend of increasing first and then decreasing, and reached peak values at T3, when the fruit peel was half-yellow and the flesh yellow. The titratable acid content of ZXMB, ZB-6, HX-1, TN and MTX decreased gradually as the fruits changed color, while that of QM showed an up-and-down trend. Except for QM, solid to acid ratio increased gradually during fruit coloration. The solid to acid ratios of ZXMB, ZB- 6, HX- 1, TN and MTX varieties all showed an up-and-down trend. The sucrose content during coloration of the fruit in all varieties showed an up-and-down trend, but the highest value of sucrose content varied among the varieties. The change trends of fructose and glucose during coloration period were consistent in the six varieties, among which QM, ZB-6, HX-1, and MTX increased first and then decreased. Citric acid is the main organic acid component in passionfruit, and the change trend of citric acid of the six passionfruit varieties all showed a gradual decrease as the fruits changed color. Malic acid is the second major organic acid, while the contents of succinic acid, lactic acid and tartaric acid were very low. There were significant differences in sugar and acid components among the six passionfruit varieties at mature stage. The contents of fructose and glucose in the fruit of ZB-6 was significantly higher than that of the other varieties at mature stage, while ZXMB had a significantly higher soluble solid content than the other varieties. The major acid components in the mature fruits of the six passion fruit varieties was citric acid, which accounted for 68.27%-84.22% of the total. Citric acid, total acid and titratable acidity in TN were significantly higher than those in the other varieties. QM had a significantly higher solid to acid ratio than the other varieties, and the solid to acid ratio of yellow passionfruit was higher than that of the purple passionfruit varieties. The solid to acid ratio from high to low was: QM ＞ ZXMB ＞ HX-1 ＞ ZB-6 ＞ TN ＞ MTX.【Conclusion】Sugars, acids and sugar to acid ratio in different passionfruit varieties vary at different stages of fruit maturation. T2-T4 is the key period of fruit quality formation for QM, ZXMB, ZB-6, HX-1, TN and MTX. With fruit maturation, reducing sugars tend to reduce, and the solid to acid ratio of yellow passionfruit varieties is greater than that of purple passionfruit. When the fruit is fully ripe, the solid to acid ratio from high to low is as follows: QM ＞ ZXMB ＞ HX- 1 ＞ ZB- 6 ＞ TN ＞ MTX. Both purple and yellow varieties belong to the citric acid dominant type, and the fruit solid to acid ratio is mainly influenced by citric acid content. Through the above research, the optimal harvest period of the six passionfruit varieties is from the complete maturity to the stage of slightly wrinkled skin. Among them, the fruits of MTX, HX-1, and ZXMB, have 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 harvesting.