In a typical food process, a hot mixture is made, which is then cooled rapidly. A main result of cooling is a change in physical state and/or molecular structure. Often, a succession of physical changes occurs as the product cools. Events occurring at the more rapid cooling rates can be difficult to characterize using common laboratory methods, and there is a pressing need for faster laboratory measurement techniques. The goal of this chapter is to show that X-ray fluxes available today at synchrotron radiation sources make it possible to characterize rapid process events by X-ray diffraction (XRD). Currently, synchrotron X-ray fluxes are up by 3 to 4 orders of magnitude over the best conventional laboratory sources. When used in conjunction with fast electronic detector systems, synchrotron radiation beams can be used to study events occurring on millisecond time scales. Owing to on-going technical developments, the prospect is to be able to characterize events on a microsecond time scale in the near future. Many processed foods start with the mixing of ingredients at higher temperature, often with the formation of an emulsion during mixing. Typically, the mix is then cooled rapidly. In the sequence of heating and coohng, chemical changes may occur, such as disulfide bond formation in dairy products, or the major change may be simply a temperature-induced change of physical state, such as starch gelation, protein aggregation or fat crystallization.
[...] The radiation process is analogous to the emission of radio waves from a radio tower, but with a twist: Because of a relativistic effect, the radiation from the high- energy particles cannot go off in all directions, as for a radio antenna. Instead, the radiation goes off in the near-forward direction. The result is a highly directional X-ray beam. The X-ray beam so generated has been compared to the light coming from the headlamp of a locomotive moving around a bend in the tracks. [...]
[...] In order to introduce the technique of XRD using synchrotron radiation, this chapter will begin with a discussion of the technologies for generating and detecting X-rays. Then some examples of structural kinetics determined using XRD will be presented, which are drawn from the more mature areas of biological and biomedical studies; these areas are chosen because the materials studied are akin to foods. Finally, one of the first applications of synchrotron radiation to food science - the kinetics of fat crystallization - will be summarized. [...]
[...] Although food processing rates fall far below the highest cooling rates possible biological specimens are frozen at up to 10,000°C/sec for electron microscopy our experience has been that unit operations in the Plant tend to outperform the laboratory equipment that we would choose to model process dynamics. For example, margarine is made by forming a hot water-in- oil emulsion. This emulsion is then cooled rapidly in order to cause the oil to crystallize, thereby stabilizing the emulsion. The cooling step from 110° to 40°F is in roughly 20 seconds, i.e., at an average cooling rate of about 200°F/min. [...]
[...] Using these twin technologies - high-intensity synchrotron X-ray beams and fast electronic detector systems allows one to do 'time-slice' exposures at a rate that was once inconceivable. After summarizing work in biological kinetics, two examples from the study of fat crystallization will be presented; those demonstrate second and even sub-second time resolution. Applications to Biological Kinetics There are only a few examples of the use of synchrotron XRD techniques to study food process kinetics. Therefore, the potential of the method will be illustrated first using examples drawn mainly from biological studies, where there is a substantial history of this kind of work. [...]
[...] Based on this fundamental understanding, it is clear that the way in which a food formulation is sheared, while nucleation and crystallization are under way, can be expected to have profound consequences for texture of the final product. SUMMARY The study of such a process using synchrotron radiation techniques can be expected to yield valuable insights. This chapter necessarily is speculative since the potential of synchrotron radiation in the food industry remains largely to be demonstrated. Nonetheless, the application to fat crystallization shows that this potential is real and not "academic". What actually happens in future may depend on the will of various food companies to pursue long-term, fundamental research, a pursuit to which [...]
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