Light-induced formation of free radicals in cream cheese Signe Westermann, Dagmar A. Brüggemann, Karsten Olsen, Leif H. Skibsted * Department of Food Science, Faculty of Life Sciences, University of Copenhagen, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark article info Article history: Received 21 November 2008 Received in revised form 12 March 2009 Accepted 16 March 2009 Keywords: Cream cheese Radical formation Light transmittance Aroma abstract Radicals were found, by electron spin resonance (ESR) spectroscopy, to accumulate in cream cheese (26% fat, 7% protein) and more significantly in low fat cream cheese (17% fat, 11% protein) upon light exposure. The decay of radicals following illumination (875 lux, with a strong UV-component for up to 80 min) followed first-order kinetics with a half-life at room temperature of around 0.5 h both for cream cheese and low fat cream cheese. The surprisingly long-lived radicals had a broad structureless ESR-spectrum (g-value of 2.006) which, for partly desiccated cream cheese, changed towards a nitrogen-centred ESR powder spectrum (g-value of 2.0014) typical for immobilised protein-based radicals. The protein oxida- tion product, dimethyl disulphide, and the lipid oxidation products, hexanal and 2-butanone, were detected by GC-analysis in higher concentrations in the outer 1 mm layer than in the second layer (of 1 mm thickness) of the product, in agreement with absorption of 99% of the UV-light intensity in the outer 1 mm layer. The low fat cream cheese had higher levels of both lipid and protein oxidation prod- ucts, in agreement with the higher steady state concentration of radicals and confirming the role of pro- teins in oxidative changes also for lipids in cheese. The low fat cream cheese was initially more oxidised than was the cream cheese, as seen from the peroxide values, and oxidation products in lipid droplets could be visualised in three dimensions by confocal laser scanning microscopy, using the fluorescent probe C 11 -Bodipy (581/591). Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Light-induced oxidation is important for quality and shelf-life of many foods, including dairy products. Riboflavin and chlorophyll degradation products act as photosensitisers and may initiate for- mation of radicals and other reactive species, resulting in forma- tion of lipid and protein oxidation products and off-flavours in the product. Light exposure of dairy products may accordingly de- crease the content of unsaturated fatty acids and vitamins, includ- ing riboflavin and a-tocopherol, in effect decreasing the nutritional value of the product (Mortensen, Bertelsen, Mortensen, & Stapel- feldt, 2004). The initial stage of oxidation involves formation of radicals, and detection and quantification of radicals may be used for prediction of further oxidative damage. Radicals can be de- tected by ESR spectroscopy, and ESR spectroscopy may accordingly be developed to become a valuable technique for prediction of oxi- dative stability of dairy products and other sensitive foods. ESR spectroscopy has previously been used to follow formation of rad- icals in whole milk powder during storage (Stapelfeldt, Nielsen, Jensen, & Skibsted, 1999; Thomsen, Lauridsen, Skibsted, & Risbo, 2005), and to determine lag-phase for oxidation in caprine milk and in dairy cream (Kondyli, Andersen, & Skibsted, 2005; Wester- mann, Møller, & Skibsted, 2008). Progression of oxidation in food is normally quantified by the peroxide value as a measurement of lipid hydroperoxides as the primary oxidation products or by the TBARS-value to measure aldehydes and ketones as secondary lipid oxidation products. Hexanal, dimethyl disulphide and 2-buta- none are among the most important oxidation products serving as indicators for damaging light exposure to dairy products (Anders- son & Lingnert, 1998; Juric, Bertelsen, Mortensen, & Petersen, 2003; Kim & Morr, 1996). Upon light exposure of cheese, fluores- cence (caused by riboflavin) declines as oxidation is initiated, but other compounds absorbing light at higher wavelengths may also contribute to oxidation by acting as photosensitisers. Among these are chlorophyll and porphyrins, both naturally occurring in dairy products, as shown by front face fluorescence spectroscopy of the product surface (Andersen, Wold, & Mortensen, 2006; Wold et al., 2005). Confocal laser scanning microscopy is a well known technique for investigation of structural properties of dairy products (Lopez, Camier, & Gassi, 2007). Fluorescent dyes have recently been intro- duced to detect oxidation in living cells upon light exposure and to determine antioxidant capacity in cell cultures, as well as in lipo- some systems. Among these, the fluorescent probe C 11 -Bodipy (581/591) seems to be very useful for the microscopic detection of oxidation in two or three dimensions in biological systems (Drummen, van Liebergen, Op den Kamp, & Post, 2002; Pap et al., 0308-8146/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2009.03.059 * Corresponding author. Tel.: +45 35 28 32 21; fax: +45 35 28 33 44. E-mail address: ls@life.ku.dk (L.H. Skibsted). Food Chemistry 116 (2009) 974–981 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem