TITLE: Determination of total polyphenol content and analysis of antioxidant activity of onion extracts 

 Determination of total polyphenol content and analysis of antioxidant activity of onion extracts

Summary

 

In recent years there has been great interest in the functional properties of fruits and vegetables, as well as their contribution to human health. Fruits and vegetables are source of many useful nutrients, among which polyphenols play an important role. Polyphenols are organic compounds that are naturally present in plant foods. They are antioxidants, i.e. they bind free radicals that damage cells and increase the risk of various diseases. Consumption of foods rich in polyphenols is of great importance, because it contributes to the prevention of various diseases. Researchers and food processors are increasingly interested in identifying and determining polyphenols in fresh and processed foods.

One of the richest sources of polyphenols in the human diet is the onion (Allium cepa L.). It is one of the oldest cultivated vegetables in the world, the second most produced vegetable after tomatoes. Flavonols such as quercetin and its derivatives are more common in onion varieties with yellow or brown husks, while anthocyanins that contribute to the purplish-red color are more common in varieties with red husks. According to the data from the literature, the presence of polyphenols is greater in the enveloping shells of onions. Therefore, research on the reduction of onion waste, which can be solved by using the enveloping onion peels in the diet, has been current lately.

Quantitative determination of polyphenols is difficult due to the complexity of their structure and diversity. First a suitable solvent for the extraction of polyphenols is selected, followed by a quantitative determination of total polyphenols, flavonoids and antioxidant activity. The most commonly used method for determining the content of total polyphenols and flavonoids, as well as for determining the antioxidant activity is UV-Vis spectroscopy. The determination of the content of total polyphenols and flavonoids is performed according to different standards. In the research of this project, gallic acid (3,5,7-trihydroxybenzoic acid) will be used as a standard for determination of total polyphenols, while the content of flavonoids will be determined in relation to quercetin (3,3',4',5,7-pentahydroxy flavonoids). The antioxidant activity is based on the use of the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). For the interpretation of the experimental data will use the parameter IC50 (IC - inhibition capacity), i.e. concentration of the substrate which reduces the activity of DPPH by 50 %.