Assessment of phytochemical characteristics of walnut (Juglans regia) leaves: determination of nutritional value and quantitative content of phenolic compounds

Abstract

Objective. This study aimed to (1) assess the nutritional value of walnut leaves (Juglans regia), (2) investigate the content of major polyphenolic compounds in walnut leaves, and (3) assess the potential of walnut leaves as a phytogenic supplement to poultry diets. Methods. The nutritional composition of dried ground walnut leaves (crude protein, crude fat, crude fiber, calcium, phosphorus, vitamin E, b-carotene) was assessed according to generally accepted methods. The quantitative content of individual groups of polyphenols, such as total phenols, hydroxycinnamic acids, flavonoids, and juglon, was determined spectrophotometrically. The content of the sum of tannins was determined by the titrimetric method. Results. Walnut leaves contain a high content of b-carotene (295.0 µg/g) and natural antioxidant vitamin E (128.8 µg/g). The protein content was noted at the level of 12.5%, the fiber content - 12.86%. In a significant amount, calcium and phosphorus accumulated in walnut leaves - 2.04% and 0.23%, respectively. The content of the total hydroxycoric acids in terms of chlorogenic acid in walnut leaves was 24.3 mg/g, the total content of phenolic compounds in gallic acid equivalent was 14.4 mg/g, flavonoids in rutin equivalent was 20.2 mg/g, and juglone was 2.72 mg/g. The content of total tannins in walnut leaves was 124.5 mg/g. Conclusion. The high nutritional value of walnut leaves compared to alfalfa meal and the presence of a number of biologically active polyphenolic compounds in a significant amount give grounds to consider this phyto-raw material not only as a valuable feed component, but also as a source of natural antioxidants.