Changes on nutrients and functional components of seedless wampee fruits at different harvest periods and after frozen storage

Seedless wampee is a characteristic fruit of southern China, which is native to Yunan county in Guangdong province. This particular variety of wampee has characteristics of large fruit, seedless, juicy, tasted, and flavourful, and is recognized as the gem of wampee. To know comprehensively about the dietary and pharmaceutical values of seedless wampee fruits, detection and analysis techniques including high performance liquid chromatography, inductively coupled plasma-mass spectrometry, and headspace-gas chromatography-ion mobility spectrometry were used to analyse the differences and changes on physicochemical characteristics, main nutrients, flavours, and functional components between the fruits picked on June 25^th (WP6), the fruits picked on July 25^th (WP7), and frozen WP7 stored at -18 ºC for 6 months (WPS). Results showed that WP6 had a ripeness of 7–8-degree, and WP7 had a ripeness of 9-10 degree, and the two samples harvest at two key periods showed the following differences and changes: (1) the fruit weight and pulp ratio of WP7 were significantly (P<0.05) higher than those of WP6; (2) the total sugar content of WP7 increased by 37.92% compared to that of WP6; (3) the total acid content of WP7 decreased by 18.32% compared to that of WP6; the citric acid and oxalic acid, which were the main organic acids of wampee fruit, decreased from 13.308 mg/g and 6.851 mg/g in WP6 to 9.132 mg/g and 4.570 mg/g in WP7, respectively. However, the content of ascorbic acid increased significantly and reach a level (0.581 mg/g) in WP7 that was comparable to that of citrus; (3) the content of both free amino acid (FAA) and total amino acid (TAA) increased from 301.5 mg/g and 801.6 mg/g in WP6 to 380.7 mg/g and 1203.8 mg/g in WP7, respectively. In addition, compared to WP6, WP7 showed an increase in the content of flavour amino acid, such as alanine, serine and aspartate, which posed a significant effect on the taste and flavour of seedless wampee fruit; 4) the content of potassium element increased from 1533.35 mg/kg in WP6 to 2608.47 mg/kg in WP7, however, the content of calcium decreased from 184.63 mg/kg in WP6 to 108.79 mg/kg in WP7; 5) the phenolic compounds and flavonoid compounds were mainly distributed in the peels of wampee fruits, and flavonoid compounds showed a significantly (P<0.05) higher content of 229.43 mg/kg in WP6 than that in WP7; 6) the relative content of volatile acid compounds (e.g., propanoic acid and acetic acid) decreased from 48.127% in WP6 to 5.409% in WP7, however, the relative content of volatile ester compounds (e.g., butyl acetate, ethyl acetate, and methyl acetate) increased from 25.165% in WP6 to 85.356% in WP7, which made the ripe seedless wampee fruit had a strong fruity aroma but a weak pungent aroma. WP7 had a fruit weight and a pulp ratio of 11.74 g and 78.47%, respectively, and showed a high degree of both sugar content and acid content, which made this tropical fruit tasted and fruity. However, after being stored at -18℃ for 6 months, although the fruit weight and fruit shape index of WPS showed insignificantly (P>0.05) changes compared to those of WP7, microstructure observations showed that the cellular structures of pulp in WPS were broken. The total sugar content, total acid content, and volatile ester compounds of WPS decreased by 5.78%, 42.31%, and 50.036% respectively, leading to a deterioration on the taste, nutrition and flavour for WPS. This study indicated that seedless wampee is a nutritious fruit with health care function, and the whole fruit is edible and can be used for deep processing. The results of this study provided basis for quality evaluation and functional component research of seedless wampee fruits.