ORIGINAL PAPER Sensory Thresholds of Selected Phenolic Constituents from Thyme and their Antioxidant Potential in Sunflower Oil Alain Bitar Æ Tarek Ghaddar Æ Amal Malek Æ Tharwat Haddad Æ Imad Toufeili Received: 17 August 2007 / Revised: 30 April 2008 / Accepted: 30 April 2008 / Published online: 18 May 2008 Ó AOCS 2008 Abstract The odor detection thresholds of carvacrol (5-isopropyl-2-methyl-phenol), thymol (2-isopropyl-5- methyl-phenol) and p-cymene 2,3-diol (2,3-dihydroxy-4- isopropyl-1-methyl-benzene) in sunflower oil, determined by the three-alternative, forced-choice procedure, were 30.97, 124 and 794.33 mg kg -1 , respectively. Sunflower oil containing 13, 70, or 335 mg kg -1 of carvacrol, thymol or p-cymene 2,3-diol, respectively, was judged to be similar (P \ 0.01) in taste and odor to its antioxidant-free counter- part. The rate constant of sunflower oil oxidation, measured from the increase in peroxide value during storage at 25 °C, was 9.2 9 10 -9 mol kg -1 s -1 while the rate constants were 9.3 9 10 -9 , 9.8 9 10 -9 , and 4.3 9 10 -9 mol kg -1 s -1 in the presence of 13 mg kg -1 carvacrol, 70 mg kg -1 thymol, and 335 mg kg -1 p-cymene 2,3-diol, respectively. At a level of 335 mg kg -1 , p-cymene 2,3-diol did not impart flavor taints and effected a 46.7% reduction in the rate of oxidation of sunflower oil. These findings indicate that the diphenolic p-cymene 2,3-diol could potentially replace synthetic antioxidants and is a valuable addition to the antioxidants used by the food industry in its quest to meet consumer demands for synthetic-additives-free and ‘natural’ foods. Keywords Carvacrol  Thymol  p-Cymene 2,3-diol  Sensory thresholds  Mono and diphenolic antioxidants  Sunflower oil  Thymus vulgaris  Natural antioxidants Introduction Bulk oils and fat- and oil-containing foods are susceptible to oxidation during processing and storage. Lipid oxidation negatively impacts the sensory quality of foods through production of unpleasant/offensive flavors, reduces the nutritional quality of foods, produces potentially toxic compounds, and causes significant economic losses to the food industry [1]. Synthetic phenolic antioxidants [e.g. butylated hydroxyanisole (BHA), butylated hydroxytolu- ene (BHT), tertiary butylhydroquinone (TBHQ) and propyl gallate (PG)] are extensively used to retard lipid oxidation in foods [2]. However, concerns about the safety of syn- thetic antioxidants [3, 4] and consumers’ demands for synthetic-additives-free foods triggered the search for nat- ural compounds with antioxidative properties [5]. Herbs and spices are rich sources of phenolic antioxidants and much effort has been spent in isolating and characterizing their active compounds [6]. However, the antioxidant potential of herbs and spices is largely limited by the characteristic flavors of their phenolic constituents, which as antioxidants must not impart flavors/taints to foods [7]. Carvacrol (5-isopropyl-2-methyl-phenol), thymol (2-isopropyl-5-methyl-phenol), and p-cymene 2,3-diol (2,3-dihydroxy-4-isopropyl-1-methyl-benzene) (Fig. 1) are phenolic compounds responsible for the distinct flavor of A. Bitar  A. Malek  I. Toufeili (&) Department of Nutrition and Food Science, Faculty of Agricultural and Food Sciences, American University of Beirut, Riad El Solh 1107 2020, Beirut, Lebanon e-mail: toufeili@aub.edu.lb T. Ghaddar Department of Chemistry, Faculty of Arts and Sciences, American University of Beirut, Riad El Solh 1107 2020, Beirut, Lebanon T. Haddad Department of Plant Sciences, Faculty of Agricultural and Food Sciences, American University of Beirut, Riad El Solh 1107 2020, Beirut, Lebanon 123 J Am Oil Chem Soc (2008) 85:641–646 DOI 10.1007/s11746-008-1244-8