Lycopene degradation and isomerization in tomato dehydration

Lycopene is an important nutrient, since it appears to provide protection a gainst a broad range of epithelial cancers. Tomatoes and tomato products ar e the major source of lycopene, and are considered to be an important sourc e of carotenoids in the human diet. Biodegradation of lycopene not only aff ects the attractive color of the final products, but also their nutritive v alue. The main cause of lycopene degradation in tomato dehydration is isome rization and oxidation. The objectives of this study were to determine the retention of total lycopene and isomerization in different dehydration meth ods, and to optimize processing technology for the retention of lycopene bi ological potency in the tomato products. Experiments were carried out to co mpare the effect of osmotic treatment, vacuum-drying, air-drying and their combination on the retention of lycopene bioactivity. Firstly a skin treatm ent was applied to the tomatoes, following an osmotic treatment at 25 degre es C in 65 degrees Brix sucrose solution for 4 h, then vacuum-drying at 55 degrees C for 4-8 h, or air-drying at 95 degrees C for 6-10 h. In the fresh tomato samples, lycopene content is 75.5 mu g/100 g on dry weight basis. L ycopene occurs in nature primarily in the more stable all-trans form. h sig nificant increase in the cis-isomers with simultaneous decrease in the all- trans isomers can be observed in the dehydrated tomato samples in the diffe rent dehydration methods. The cis-isomers increased with temperature and pr ocessing time. In the osmotic treatment, the predominating mechanism is iso merization of lycopene. Since the total lycopene content remained essential ly constant, but the distribution of trans- and cis-isomers changed. In the air-drying processing, isomerization and oxidation (autoxidation) as two s trong factors affected simultaneously the decrease of total lycopene conten t, distribution of trans- and cis-isomers, and biological potency. A possib le explanation of this result is that sugar enters the tomato matrix and st rengthen the binding force on lycopene in the tomato matrix. Osmotic soluti on (sugar) remaining on the surface layer of the tomato prevents oxygen fro m penetrating and oxidizing lycopene. The osmotic treatment could reduce ly copene losses in comparison with other dehydration methods. (C) 1999 Canadi an Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.