Dielectric study of milk for frequencies between 1 and 20 GHzA.C. Nunesa,*, X. Bohigasb, J. TejadacaDepartment of Physics, University of Rhode Island, Kingston, NY 02881, United States bDepartamento de F sica i Enginyeria Nuclear, Universitat Polite cnica de Catalunya, Diagonal, 647 08028 Barcelona, Catalonia, Spain cDepartamento de F sica Fonamental, Universitat de Barcelona, Diagonal, 647 08028 Barcelona, SpainReceived 4 June 2004; received in revised form 5 April 2005; accepted 25 April 2005 Available online 1 July 2005AbstractThe complex permittivity of milk at room temperature (1720 ?C) was studied over the frequency range of 120 GHz. UHT whole, low fat and skim milk were examined, fresh from the container, and over a period of two weeks while they were allowed to spoil at room temperature. In addition, the spectrum of whole milk at various dilutions was also measured. Both the real and imaginary parts of the permittivity were found to be smooth, slowly varying functions of frequency over this range. The Debye rela-tion, with an additional term for ionic conduction losses was fitted to the data, and six parameters were extracted. The variation of these parameters with fat content and dilution suggests that they may be useful to roughly determine the milk?s content in terms ofgroups of materials (ionic compounds, fats, and carbohydrates and proteins). Spectra also vary significantly with spoilage, but anal- ysis is complicated by concerns of physical (phase separation) in addition to chemical changes. ? 2005 Elsevier Ltd. All rights reserved.Keywords: Microwave; Milk; Spoil; Permittivity; Ionic loss; Dielectric properties; Composition1. IntroductionThe response of substances to electromagnetic radia- tion is complex, and dependent upon many parameters, including temperature, physical state, and the chemical composition of the material. The microwave range of the spectrum (frequencies between 0.5 and 300 GHz) is increasingly exploited in probing fundamental questionsin physics and chemistry. Microwaves also find more general, practical application for treating and character- izing foods. They are used in the pasteurization and cooking of foods, typically at frequencies of 0.915 and 2.450 GHz (Herve, Tang, Luedecke, Wang, Wig, Tang, Shiinoki, Motouri, Kent, Kno ckel, Daschner, fax: +1 401 874 2380. E-mail address: acnunesuri.edu (A.C. Nunes).www.elsevier.com/locate/jfoodengJournal of Food Engineering 76 (2006) 250255scientific interests, and our responsibility to the broader community that ultimately supports us, we have decided to set aside some instrument time for studies of ques- tions of immediate practical importance.As a first step in this direction, we report here an investigation of the dielectric properties of milk (UHT, skim, low fat, and homogenized whole milk) over the frequency range 120 GHz. Milk was chosen because it is an important dietary staple, and is easily handled and prepared for microwave exposure. This is not meantto be a definitive example of how microwaves will be used to characterize milk, but merely as an indication of the form such measurements might take, what the data may look like, and what components of the milk may be most easily detected. It should be noted that the instrumentation used inthis study was designed for scientific research. It is highly sophisticated, expensive, requires skilled person- nel to operate and maintain, and is not something many businesses would consider buying or using as part of their processing protocols. It is hoped, however, that studies of this sort will identify a small number of dis-crete frequencies most sensitive to those specific charac- teristics of a product that a company may wish to monitor. A much simpler instrument, operating only at those frequencies, would be cheap to build, would not require an operator with an advanced degree, and would permit rapid, on-line, monitoring of product quality.2. Materials and methods2.1. Sample preparationThree types of UHT milk, purchased from a local supermarket, were used in this study: skim, low fat, and homogenized whole milk. The compositions listed on the cartons are presented in Table 1. In addition to measuring the spectra of each of these fresh from the carton, other tests were performed. One series of measurements was made on successive dilutions of whole milk. Also, three samples were left at roomtemperature in parafilm covered beakers for nearly two weeks. Several spectra of these were measured over this period as the milk spoiled.2.2. Instrument and proceduresThe instrument used to measure the complex permit- tivity of these materials was a Hewlett Packard 8510C Network Analyzer coupled to an 83651B Synthesized Sweeper, and an 8517B S-Parameter test set. The probe used was an Agilent coaxial probe, model 85070C. All modules but the probe have 50 GHz capability. The probe limited the upper frequency to 20 GHz. The sys- tem was controlled by an Agilent program running u