Characterisation of sugar content in yoghurt by means of microwave spectroscopy Xavier Bohigas a, * , Roger Amigo ´ b , Javier Tejada b a Dept. Fı ´sica i Enginyeria Nuclear, Universitat Polite `cnica de Catalunya, Diagonal, 647, 08028 Barcelona, Catalonia, Spain b Dept. Fı ´sica Fonamental, Universitat de Barcelona, Diagonal, 647, 08028 Barcelona, Catalonia, Spain Received 1 June 2007; accepted 3 October 2007 Abstract Measurements of the complex dielectric permittivity of sugar solutions in yoghurt were taken in the 1–20 GHz microwave frequency range. Quantitative relationships between parameters that define both the real and the imaginary part of the Debye relations and the weight fraction of sugar content in yoghurt were obtained. These data were used to model the dielectric behaviour of the sugar content in the microwave region. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Microwave spectroscopy; Yoghurt; Sugar; Permittivity; Ionic loss; Dielectric properties 1. Introduction In recent years there has been increasing interest in studying the dielectric properties of different foods using electromagnetic radiation in the microwave range. Micro- wave dielectric characterisation has been very effective in studying different kinds of foods, such as cheese (Herve, Tang, Luedecke, & Feng, 1998; Wang, Wig, Tang, & Hall- berg, 2003); treated chicken and scallop (Kent & Anderson, 1996); haddock (Martinsen, Grimmes, & Mirtaheri, 2000); untreated pork, poultry, fish and prawns (Kent, Kno ¨ ckel, Daschner, & Berger, 2001); macaroni (Wang et al., 2003); wine (Garcı ´a, Torres, Prieto, & De Blas, 2001); and butter (Shiinoki, Motouri, & Ito, 1998). It has also been observed that water activity is well correlated with dielectric proper- ties (Clerjon, Daudin, & Damez, 2003). We have previously reported on the dielectric properties of milk (UHT, skimmed, low fat, and homogenised whole milk) over the frequency range 1–20 GHz. Milk was chosen in this case because it is an important dietary staple that is easily han- dled and prepared for microwave exposure (Nunes, Bohi- gas, & Tejada, 2006). The aims of the present paper are to characterise sugar solutions in yoghurt by their complex permittivity spectra in the microwave frequency range and to analyse the pos- sibility of determining the sugar concentration present in the yoghurt from the model extracted from this characterisation. 2. Experimental 2.1. Samples Two types of yoghurt, natural and sweetened, were used in this study. Both were purchased at a local supermarket and were of the same brand. The composition listed on the label of the sweetened yoghurt indicated that the sugar content was 9.2% by weight. Samples of natural yoghurt with sugar solutions of up to 15% by weight were prepared. Additional tests were carried out on dilutions of sugar in water. All samples were beaten intensely before being measured to ensure they were homogeneous. Measurements were 0963-9969/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2007.10.005 * Corresponding author. Tel.: +34 93 401 5936; fax: +34 93 401 1839. E-mail address: xavier.bohigas@upc.edu (X. Bohigas). www.elsevier.com/locate/foodres Available online at www.sciencedirect.com Food Research International 41 (2008) 104–109