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Flavonoids, secondary plant metabolites with many health-promoting properties, including antioxidant, are a valuable component of food products, especially functional foods. In the latter, plant extracts are commonly used, the properties of which are attributed to the characteristic main ingredients. However, in a mixture the antioxidant properties of the individual ingredients do not always show an additive effect. This paper presents and discusses the antioxidant properties of naturally occurring flavonoid aglycones and their binary mixtures. In the experiments, model systems were used that differed in the volume of the alcoholic antioxidant solution in the measuring system and its concentration in the range in which it occurs in nature. Antioxidant properties were determined by ABTS and DPPH methods. The presented data proved that the dominant resultant effect in the mixtures is antioxidant antagonism. The magnitude of the observed antagonism depends on the mutual relations of individual components, their concentrations and the method used to assess antioxidant properties. It was shown that the observed non-additive antioxidant effect of the mixture results from the formation of intramolecular hydrogen bonds between phenolic groups of the antioxidant molecule. The presented results may be useful in the context of proper design of functional foods.

In this study it was shown that the fungistatic and antioxidant activities of onion extracts are related to the type of liquid used as the extractant and the technique of its preparation. A change in the antioxidant properties of white and red onion extracts was demonstrated with the change of the temperature of the pressurized hot water extraction process, which can be easily related to the changes accompanying the process of thermal processing of vegetables and fruits during cooking. Owing to the experimental and mathematical approaches concerning both the main and characteristic components of onions, i.e., quercetin and phenols, respectively, with the biological activity of the extracts, it was possible to demonstrate the significant share of these compounds in the antifungal and antioxidant properties of the extracts. Considering that the research was carried out, inter alia, on onion husks, demonstrating a very high potential of biological properties of this waste material from agricultural production, the research results presented in the paper should encourage the popularization of the use of this so far underestimated raw material for the production of various functional materials.

In this study, to increase the content of quercetin in the isolates obtained from the white and red onion scales, in accordance with the zero waste philosophy, using water as an environmentally and healthy friendly extractant, a mathematical model was constructed, based on the Central Composite Design. The suitability of the model for the accurate and quick study of complex relationships was demonstrated by obtaining a statistically insignificant difference in the yield predicted by the model and that determined experimentally under optimal extraction conditions, i.e., 44.14 mg/g (33.57 mg/g) and 45.54 mg/g (33.31 mg/g) for white (red) onion scales, respectively. To present the obtained isolates applicability, their biological activity was assessed in terms of antioxidant activity and antifungal activity against various fungi, showing properties that are sufficient to encourage the use of this material, previously treated as worthless waste, for the production of various functional ingredients, including functional food additives. Moreover, they can be also used to produce safe and effective protectants meeting the GRAS criteria. [ABSTRACT FROM AUTHOR]

Reliable plant analysis is a challenging task due to the physical character and chemical complexity of plant matrices. First of all, it requires the application of a proper sample preparation procedure to fully isolate the analyzed substances from the plant matrix. The high-temperature liquid-solid extraction is commonly applied for this purpose. In the light of recently published results, however, the application of high-temperature extraction for polyphenolics analysis in plants is disputable as it causes their transformation leading to erroneous quantitative estimations of these compounds. Experiments performed on different plants show that the transformation/degradation of quercetin and its glycosides is not induced by sea sand disruption method (SSDM) and prove the method to be most appropriate for the estimation of quercetin and its derivatives in plants. What is more, the application of SSDM in plant analysis allows the researcher, to determine which quercetin derivatives are native plant components and what is their true concentration. In other word, the application of SSDM in plant analysis eliminates errors in the study of plant metabolism involving quercetin and its derivatives. [ABSTRACT FROM AUTHOR]

Pressurized liquid extraction (PLE) is considered as an effective and quick sample preparation method, especially in plant analysis. This quick extraction process favors the isolation of compounds for which long exposure time in high temperatures is undesirable. However, despite this advantage, the application of PLE for plant compounds extraction involves certain problems, such as instability of the compounds. The obtained data proved that even in short-lasting PLE, quercetin 4'-glucoside undergoing the hydrolytical degradation process produces the corresponding aglycon quercetin when water is a component of the applied extractant. The increase of water content in methanol-water extractant mixture heightens the transformation degree of quercetin 4'-glucoside into quercetin and increases its concentration in the PLE extracts. The degradation is accelerated by increased extraction temperature and time. In plant samples, the degradation of quercetin 4'-glucoside is much higher because of the plant matrix effects. The presented results explain why extraction of quercetin from plants by pure methanol is less efficient than by water containing alcohol solutions. More importantly, they prove that the application of water-alcohol extractant during sample preparation leads to the hydrolytical degradation of glycosides to aglycones, and, as a consequence, to false conclusions in plant analysis. [ABSTRACT FROM AUTHOR]

This study deals with the sea sand disruption method (SSDM) in sample preparation for chromatographic analysis of quercetin in different plants. Several SSDM variables were tested in relation to quercetin recovery: sample to sand mass ratio, type of dispersing liquid, volume of the solvent applied for analyte elution, and the time of grinding. The optimized procedure has been applied for the extraction of quercetin from the edible and inedible parts of different varieties of onion, apples, and tea. The SSDM results are comparable with those obtained by pressurized liquid extraction, regarded as one of the most effective sample preparation techniques. The presented results show that the hydrolysis of quercetin glycoside form in SSDM conditions does not occur or is negligible. Hence, the method is especially useful when the estimation of the real amount of aglycones in plants is required. [ABSTRACT FROM AUTHOR]