Applications of ultrasound to analysis/quantitation of dairy lipids

Abstract

Food quality is a complex amalgam of objective, measurable features and not-so-easily measurable consumer perceptions (Vickers and Wasserman, 1979). Clearly, no one technique, or indeed any ensemble of techniques, presently available, can measure all important aspects of food quality. So, the question regarding any technique under consideration for the measurement of quality is: "What can it usefully measure?" From the general point of view, ultrasound has advantages regarding the measurement of dairy product quality in that it may be implemented inline, non-invasively and even using non-contacting techniques such as laser excitation and detection (Mulet et al., 1999). Ultrasound can also be safe, hygienic and economic in implementation, all characteristics desirable for any technique for the measurement of food quality (Povey, 1997a). Moreover, it can reveal aspects of the quality of dairy products which are not measurable by current techniques. An example is the extraordinary capability of ultrasound to detect crystal nucleation (Povey et al., 2001; Hindle et al., 2002). Ultrasound is one of the more dynamic areas of food quality measurement, as indicated by the rapid rate at which commercial ultrasound instrumentation is entering the market (Mulet et al., 1999; Povey, 2001; Povey and Higgs, 2001). Many reviews of the literature on the subject have also appeared recently (Povey, 1997a; Kress-Rogers, 2001). This dynamism is the result of two decades of developmental work (Povey and Wilkinson, 1980) in which ultrasound testing evolved from an empirical approach to a model-based approach, together with developments in electronics and computing which make model-based instrumentation commercially feasible, as opposed to a laboratory curiosity. The principles upon which ultrasound measurement are based are, therefore, very important to successful measurement of food quality (Povey, 1997b). An empirical approach is generally undesirable in the analysis of ultrasound data because of the great number of variables upon which the ultrasound velocity and attenuation may depend. It is highly desirable to have a physical model, which can be used to relate ultrasound measurements to interesting properties of food. Otherwise, initially encouraging results may be confounded in practical application in ways which cannot be understood, undermining conWdence in the technique. It is claimed that commercially- available ultrasound equipment can measure the following quality parameters of dairy products: levels of solids, solids non-fat (SNF), protein, water and fat; solid fat content (SFC), colloidal stability, gelation point, adulteration with oil, particle size, particle size distribution, oil composition, protein denaturation and fat oxidation. This incomplete list represents an impressive contribution towards the solution of food quality measurement although the present authors are slightly skeptical regarding some of these claims. In this review only those applications will be addressed which are regarded as robust. Foods, in general, present online physical measurements with quite a challenge, in particular the need to obtain 100% inspection arising from the highly variable nature of the raw materials. Ultrasound is one technique, which has the potential for 100% inspection using tomographic techniques (Beck et al., 1994), in which the entire sample is penetrated by the inspecting Weld. It is unlikely that ultrasound will be used on its own; rather, a range of techniques are likely to be brought to bear on process monitoring, fault detection and the achievement of a consistently high-quality product. Traditionally, this has focused on monitoring quality measurements, including sensory-based analysis, color analysis, measurements of texture and product consistency. More recently, online analytical measurements, such as NIR, Raman, NMR and ultrasound are used. These are combined with the routine collection of traditional process measurements such as temperature, pressure and Xow. Integration of all these data will be the next stage in the development of process measurement. In the following, the principles underpinning ultrasound measurement of food quality are discussed, online technologies are considered and applications examined, future trends identiWed together with sources of information.