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

Seedless wampee is one of the most characteristic fruits in the Yunan County, Guangdong Province, southern China. The whole fruit is also edible for deep processing, due to the nutrition with the health care function. The variety of seedless wampee is then characterized by the large size, juicy, sweet, and flavorful fruit, particularly with its dietary and pharmaceutical values. This study aims to explore the physicochemical features, main nutrients, flavors, and functional components using various techniques, including high-performance liquid chromatography, inductively coupled plasma-mass spectrometry, and headspace-gas chromatography-ion mobility spectrometry. The fruits were picked on June 25th (WP6) and July 25th (WP7), and then frozen. WP7 was stored at −18 ℃ for 6 months (WPS). Results showed that the WP6 and WP7 shared a ripeness of 7-8 degrees and 9-10 degrees, respectively. Two samples were finally harvested in the two periods. The results showed that: (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 with the WP6; (3) The total acid content of WP7 decreased by 18.32%, compared with the WP6; The citric acid and oxalic acid were the main organic acids of wampee fruit, where there was decrease from 13.308 and 6.851 mg/g in the WP6 to 9.132 and 4.570 mg/g in the WP7, respectively. The content of ascorbic acid increased significantly, with the higher level (0.581 mg/g) in WP7, compared with the citrus; (3) The content of both free amino acid (FAA) and total amino acid (TAA) increased from 301.5 and 801.6 mg/g in the WP6 to 380.7 and 1 203.8 mg/g in the WP7, respectively. In addition, the WP7 showed an increase in the content of flavor amino acids, such as alanine, serine, and aspartate. There was a significant effect on the taste and flavor of seedless wampee fruit; 4) The content of potassium element increased from 1533.35 mg/kg in the WP6 to 2608.47 mg/kg in the WP7. The content of calcium decreased from 184.63 mg/kg in the WP6 to 108.79 mg/kg in the WP7; 5) The phenolic and flavonoid compounds are also distributed in the peels of wampee fruits. The 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. The relative content of volatile ester compounds (e.g., butyl acetate, ethyl acetate, and methyl acetate) increased from 25.165% in the WP6 to 85.356% in the WP7. The ripe, seedless wampee fruit shared a strong fruity aroma but a weak, pungent aroma. There were fruit weight and a pulp ratio of 11.74 g and 78.47%, respectively, in WP7. The high degree of both sugar and acid content contributed to the tropical fruit's taste and fruitiness. There were the insignificantly (P>0.05) variations in the fruit weight and shape index of the WPS after storage at −18 ℃ for 6 months, compared with the WP7. Microstructure observations showed that there were broken cellular structures of pulp in the WPS. The total sugar content, total acid content, and volatile ester compounds of the WPS decreased by 5.78%, 42.31%, and 50.036%, respectively, leading to the deterioration of the taste, nutrition, and flavor for the WPS. The finding can provide for the quality evaluation and functional component of seedless wampee fruits.