Olive Leaf, Thyme and Green Walnut Husk Extracts as Functional Ingredients in Dairy Products: A Review
Abstract
Oxidative stress and the inflammation associated with it play key roles in the development of many chronic diseases. As alternatives to synthetic antioxidants, plant-derived materials with potential anti-inflammatory and antioxidant properties—such as olive leaves (Olea europaea), thyme (Thymus vulgaris) and green walnut husks (Juglans regia)—have attracted increasing scientific interest. These materials are rich in phenolic compounds, including oleuropein and hydroxytyrosol in olive leaves, thymol and rosmarinic acid in thyme, and ellagitannins and juglone in green walnut husks. Numerous studies have demonstrated the strong antioxidant activity of these extracts using analytical assays such as DPPH, FRAP and ABTS. However, such in vitro antioxidant capacity does not necessarily translate directly into biological efficacy in vivo, as polyphenol stability, release, metabolism and bioavailability may be strongly influenced by interactions with proteins, lipids and other components of dairy matrices. Their incorporation into dairy products has shown promising technological effects, including improved oxidative stability, reduced lipid oxidation, delayed microbial spoilage and potential shelf-life extension. Reported application levels vary substantially among studies and product types, including 0.4–5 % for olive leaf extract in yogurt systems, 0.4–0.8 % for thyme extract in yogurt, 0.1–1 mg/mL for thyme extract in milk systems, 0.5–3 % for olive leaf extract in cheeses, and 100–200 ppm for green walnut husk extract in cream-based products. However, these values should be interpreted cautiously because extract composition, matrix type and units of expression differ among studies. Nevertheless, evidence supporting direct health benefits in humans remains limited, and further in vivo and human intervention studies are needed to confirm their physiological relevance. This review summarizes the polyphenolic profiles of the olive leaf, thyme and green walnut husk extracts, their technological and sensory effects in dairy matrices, and advances in sustainable extraction methods. Overall, the available findings support their potential as natural, clean-label ingredients for innovative dairy products, primarily from a technological and functional formulation perspective.