Flavor and Stability of Pasteurized Milk with Elevated Levels of Conjugated Linoleic Acid and Vaccenic Acid* J. M. Lynch, 1 A. L. Lock, 2 D. A. Dwyer, 2 R. Noorbakhsh, 3 D. M. Barbano, 1 and D. E. Bauman 2 1 Department of Food Science, and 2 Department of Animal Science, Cornell University, Ithaca, NY 14853 3 Institute of Standards and Industrial Research, Mashad, Iran ABSTRACT The objectives of this study were to determine if flavor differences between 2% fat pasteurized milks with and without naturally enhanced vaccenic acid (VA) and cis- 9, trans-11 conjugated linoleic acids (CLA) levels could be detected over the commercial shelf life of the product and to determine if milk with elevated VA and cis-9, trans-11 CLA levels was more susceptible to develop- ment of light-induced oxidative flavor defects. Cows were fed a control diet or the same ration supplemented with 2% soybean oil and 1% fish oil (CLA diet). The milk, standardized to 2% fat, was pasteurized, homogenized, and stored in plastic containers at 4°C. Oxidation was induced by exposing half of the containers to light. Test- ing was conducted at 1, 7, and 14 d postpasteurization. Average cis-9, trans-11 CLA content of the milks from the control and CLA diet groups was 0.52 and 4.74 g/ 100 g of fatty acids, respectively (8-fold increase). Aver- age VA content of the milk from the control and CLA diet groups was 1.43 and 12.06 g/100 g of fatty acids, respectively (7.5-fold increase). Together, VA plus CLA represented almost 17% of the total milk fatty acids. There was no effect of light exposure on fatty acid com- position initially or over the 14-d storage period. Al- though VA, cis-9, trans-11 CLA, and degree of unsatura- tion were significantly elevated in the milk from the CLA diet group, untrained panelists were unable to detect flavor differences initially or over time in 15 of 16 triangle test evaluations. Similarly, sensory results indicated no difference in susceptibility to the develop- ment of oxidized off-flavors between the milk from the Received June 6, 2004. Accepted August 4, 2004. Corresponding author: Joanna M. Lynch; e-mail: JL72@ cornell.edu. *Use of names, names of ingredients, and identification of specific models of equipment is for scientific clarity and does not constitute any endorsement of product by authors, Cornell University, the Insti- tute of Standards and Industrial Research, or the Northeast Dairy Foods Research Center. control and CLA diet groups, even when oxidation was induced by light exposure. (Key words: conjugated linoleic acid, milk flavor, oxi- dation, vaccenic acid) Abbreviation key: CLA = conjugated linoleic acid, CN%CP = casein as a percentage of crude protein, FO = fish oil, VA = vaccenic acid. INTRODUCTION Dairy products are the major source of conjugated linoleic acids (CLA) in US diets (60 to 70 %; Ritzen- thaler et al., 2001) and the major CLA isomer in milk fat is cis-9, trans-11 CLA (75 to 90%; Bauman et al., 2003). Biomedical studies using animal models have identified many beneficial health effects of CLA. In par- ticular, the cis-9, trans-11 CLA isomer has been identi- fied as a potent anticarcinogen for many types of cancer (Belury, 2002; Banni et al., 2003) and it has been shown to prevent development and cause regression of athero- sclerotic lesions in animal models for coronary heart disease (Kritchevsky, 2003; Toomey et al., 2003). Conse- quently, surveys indicate a consumer interest in food products that are enriched in CLA (Ramaswamy et al., 2001a). A portion of the cis-9, trans-11 CLA in ruminant fat originates from incomplete rumen biohydrogenation of linoleic acid. However, the major source is from endoge- nous synthesis involving the enzyme  9 -desaturase, with the substrate being vaccenic acid (VA; trans-11 18:1) that is produced as an intermediate during rumen biohydrogenation (Bauman et al., 2003). Other species, including humans, have  9 -desaturase so they can also synthesize cis-9, trans-11 CLA endogenously using VA consumed in the diet. Because of the unique precursor- product relationship between VA and cis-9, trans-11 CLA in dairy cows, efforts to increase cis-9, trans-11 CLA result in a concomitant increase in the milk fat content of VA. Based on the typical relationship be- tween these fatty acids in milk fat and the extent of VA conversion to cis-9, trans-11 CLA in humans (Tur- peinen et al., 2002), Parodi (2003) has suggested that multiplying the amount of cis-9, trans-11 CLA by 1.4