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Quercetin is a bioactive compound that has many beneficial effects on human health. Due to a lack of information about quercetin and quercetin derivatives in the Vietnam shallot, this work aimed to identify the quercetin family in shallot skin and flesh by liquid chromatography-mass spectrometry. Effect of extraction method (conventional solvent extraction and ultrasound-assisted extraction), including ethanol concentration (40–80%) and extraction time (15 to 60 mins) on the yield of quercetin were also investigated in this study. Quercetin, quercetin-glucoside and methylated-quercetin-hexose were commonly found in the extract of shallot flesh and skin. A very small amount of quercetin aglycone (0.7%) was found in flesh extract, while it was the most abundant in the skin sample (46.3%). The high content of quercetin-mono-glycoside was obtained in shallot extract (43.8% and 35.3% in the shallot flesh and skin, respectively). The flesh extract also contained high level of quercetin-di-glucoside (38.9%). About 14-16% of methylatedquercetin (-glucoside) were found in shallot bulb (both skin and flesh). It was found that ethanol concentration and extraction time directly affected on the quercetin extraction yield. The highest quercetin content was obtained in the sample which was extracted in 60% ethanol in combination with ultrasound-assisted extraction for 15 to 45 mins. The ultrasound-assisted extraction method improved quercetin yield by 13.38-15.64% and 49.46-56.88% for shallot skin and flesh compared to conventional solvent extraction. This study proved that ultrasound-assisted extraction could successfully be used for extraction of quercetin from shallot (both skin and flesh).

Some of the changes in physiological indicators during the growth and development of litchi fruit grown in Vietnam were studied. The length and diameter of litchi fruits reached their maximum value at 10 weeks after anthesis. The content of chlorophyll in litchi peel reached its highest value at 6 weeks and decreased rapidly. The content of carotenoids was low in fruit formation and then increased rapidly until fruit ripening. The vitamin C content and reducing sugar content increased continuously and reached a maximum at 10 weeks and then decreased slightly. The starch content and lipid content reached a maximum when the fruit was 8 weeks, the total organic acid content reached a maximum when the fruit was 9 weeks and then gradually decreased. The protein content decreased from fruit formation until fruit ripening. Based on these results, we found that the litchi fruit should be harvested at 10 weeks after anthesis to ensure the nutritional value of the fruit during storage.

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This research examined the ripening time of guava fruit to provide a scientific basis for better harvesting and preservation of these fruits. Biochemical research methods were used to analyse changes in physiological and biochemical parameters according to the growth and development of guava fruit. The fruit took 14 weeks after anthesis to reach its maximum size in terms of length and diameter. The chlorophyll content in guava peel peaked after 10 weeks, decreasing until week 15. The content of carotenoids, which was low at fruit formation, rose rapidly until fruit ripening, while the vitamin C and reducing sugar contents increased continuously and peaked at week 14. A gradual increase was seen in the starch and total organic acid contents from the beginning of fruit formation, with peaks at 10 weeks, followed by a moderate downward trend. The pectin content showed the same trend, as it declined gradually after peaking at 12 weeks. The first 4 weeks showed an increase in the tannin content, which decreased afterward. The study results show that guava fruit should be harvested after physiological maturity and before ripening completely (14 weeks) to ensure that the nutritional value of the fruit is maintained during storage. Highlights• Xa Li guava at 14 week after anthesis to reach its maximum size in terms of length and diameter. • The chlorophyll content in guava peel peaked after 10 weeks, decreasing until week 15. The content of carotenoids, which was low at fruit formation, rose rapidly until fruit ripening.• The vitamin C and reducing sugar contents peaked at week 14. A gradual increase was seen in the starch and total organic acid contents and peaks at 10 weeks, followed by a moderate downward trend. • The pectin content showed the same trend, as it declined gradually after peaking at 12 weeks. The first 4 weeks showed an increase in the tannin content, which decreased afterward.