Yogurt Fortification with Walnut Leaf Extract and Investigation of its Physicochemical and Sensory Properties

Walnut leaves contain high amounts of polyphenols and antioxidant compounds which exhibited potentially health benefits. In this research, the effect of incorporation walnut leaf extract into yogurt on its physicochemical and sensory properties was examined. For this purpose, ethanolic extracts of dried walnut leaves were added at three concentrations (1, 1.5 and 2 %) to the milk prepared for yogurt production immediately after adding starter culture. The produced yogurts were evaluated at definite time intervals during 28 days storage at 4 °C. According to the results the non-fat solid content of yoghurts enriched with extracts was higher than control while their protein and fat content was lower. The acidity of yoghurts was significantly enhanced in the presence of walnut leaf extract, while their pH and syneresis was decreased and, in this regard, the effect of storage time was significant. During the storage, the total phenolic contents and antioxidant activity of the yogurts were both reduced. Significant decrease (p <0.05) in the thiobarbituric acid (TBA) index of the enriched yoghurts, compared to the control sample, was indicative of the strong antioxidant properties of the extract. In terms of color properties, with increasing concentration of extract the value of L* and a* of yogurts decreased while their b* increased. The viability of starter cultures decreased significantly (p <0.05) with incorporation of extracts and during the storage period. At the end of storage, significant reduction in viscosity of yogurt fortified with extract was observed compared with the control sample. In addition, the sensory scores of different yogurts decreased with increasing concentrations of the extract and during the storage time. Generally, considering the highest score in terms of sensory evaluators, also reduction of syneresis and improvement of the physicochemical characteristics, the walnut leaf extract at concentration of 1.5% was introduced for enrichment of yogurt.