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Unformatted text preview: J. Dairy Sci. 88:13011310 American Dairy Science Association, 2005. Effect of Heating and Processing Methods of Milk and Dairy Products on Conjugated Linoleic Acid and Trans Fatty Acid Isomer Content S. M. Herzallah, 1 M. A. Humeid, 2 and K. M. Al-Ismail 2 1 Department of Nutrition and Food Technology, Faculty of Agriculture, University of Mutah, Jordan 2 Department of Nutrition and Food Technology, Faculty of Agriculture, University of Jordan, Jordan ABSTRACT The conventional heating methods of milk did not cause any significant increase in the trans isomer con- tent, with the exception of milk heated at 63 1.0 C for 30 min and milk microwaved for 5 min, which were significantly increased by 19 and 31%, respectively. The chemical changes of lipids were generally accelerated with the severity of the heat treatment and duration of storage. The conjugated linoleic acid content of cheese heated in a microwave oven for 5 min decreased by 21%, and microwave heating for 10 min caused a decrease of 53% compared with that of freshly boiled cheese. ( Key words: Nabulsi cheese, microwave, conventional heating, conjugated linoleic acid) Abbreviation key: CLA = conjugated linoleic acid. INTRODUCTION Milk lipids are considered one of the outstanding milk constituents with respect to presence of lipid classes, and variety and number of identified fatty acids, which was found to be more than 400 (Jensen et al., 1990, 1991). Milk lipids include anticarcinogenic compounds such as conjugated linoleic acid ( CLA ), sphingomyelin, and butyric acid (Ames et al., 1995; Parodi, 1999). Milk lipids may undergo chemical and physical changes dur- ing processing and storage such as auto-oxidation and formation of trans fatty acids (Semma, 2002). Trans fatty acids are naturally found in low concen- trations in dairy products because of the biohydrogena- tion process in the rumen but may also be formed during processing of dairy products at high temperatures such as in baking or frying. Trans fatty acids have been associated with biological and toxicological effects such as coronary heart disease, and disturbances of the me- tabolism of the essential fatty acids in the fetus, which could affect intrauterine human growth (Addis, 1990; Received August 28, 2004. Accepted December 20, 2004. Corresponding author: S. M. Herzallah; e-mail: saqermay@ yahoo.com. 1301 Addis and Warner, 1991; Kumar and Singhal, 1991; Willett and Ascherio, 1994; Boue et al., 2000). Milk and milk products usually undergo different changes during their preparation (boiling and micro- waving) or processing, which may include moderate or severe heat treatments that can lead to undesirable changes in lipids or proteins. Microwave ovens are widely used for cooking and reheating of foods in mil- lions of kitchens throughout the world. Food heating by microwave results from the conversion of microwave energy into heat by friction of vibrating water molecules due to rapid fluctuations in the electromagnetic field...
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