Flavonoids are the most abundant functional compounds distributed in higher plants, and are used as important dietary components for human health protection. The development of natural flavonoids, such as functional food or medicinal food, has received extensive attention in recent years. The extraction, separation, and quantitation of flavonoids are the key techniques in the utilization of flavonoid resources. The traditional methods for flavonoid extraction and separation always used toxic solvents, which produce toxic residues and pollute the environment. Based on an analysis of the literature on flavonoid resources, the utilization, separation, quantitation, and green separation techniques of flavonoids were summarized. First, extraction by hot water or hot ethanol, assisted by pressurization and microwave-ultrasonication, then concentration and precipitation of flavonoids by cool water or cool ethanol or ethanol/water in specific ratios. This method could obtain over 85% purity in the first cycle and over 95% purity after three precipitation cycles in the separation of the most commonly used flavonoids, such as dihydromyricetin, rutin, and quercetin. In conclusion, flavonoids showed great prospects in human health protection and disease treatment. Chemical structure-based separation using the water–ethanol methods and assisted with microwave-ultrasonication, pressurization, and temperature regulation proved to be efficient and environmentally friendly, showing great potential for the flavonoid industry. These “green” processing techniques and mechanisms deserve further research.

Premna microphylla is a medicinal plant species distributed in Southeast Asia and China. P. microphylla leaves have been widely used for processing edible gels called Chai tofu, which have many medicinal values, such as clearing heat and detoxifying. However, the main functional components of P. microphylla leaves and Chai tofu are still unknown. In this study, selenium-enriched cultivation of P. microphylla was conducted, and the main compositions of pectins, flavonoids, total phenolics, carotenoids, chlorophylls, and proteins were separated and comparatively analyzed. The results are that kaempferol was the main composition of flavonoids, with the average contents of 5.19% DW (dried weight) in leaves and 3.83% DW in Chai tofu; the composition of the Chai tofu pectin included glucose, fructose, and mannose. Contents of phenolics, kaempferol, chlorophyll, and carotenoids were significantly increased by the selenium enrichment cultivation in a concentration-dependent manner (R² = 0.989, 0.994, 0.94, 0.948). Moreover, selenium enrichment produced selenized pectins with Se-O bonds. Selenium-enriched P. microphylla is an important plant source for functional foods. Related processing and extraction techniques deserve further research.