Introduction

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. Further changes can occur during aging. Chemical reactions may occur as well. This chapter will focus on the more rapid events, particularly changes in physical state, which happen early in the process.

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. A few other potential applications of synchrotron radiation are indicated at the end of the chapter.

1.1. The problem

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 cooling, 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. Whether the changes are physical or chemical, the problem for the food scientist or engineer is to control events in such a way as to produce the optimum product consistently.

The engineer or scientist who seeks to control a food process will do this best by an appropriate understanding of events; such understanding is the price we pay for reliable control. Problems encountered in developing this understanding are, first, formulating a model system that includes the essential elements of both the ingredients and the process, and second, recording data at sufficiently high rates. This chapter will concentrate on the second of these related problems.

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 accomplished in roughly 20 seconds, i.e., at an average cooling rate of about 200°F/min. In comparison, the Differential Scanning Calorimeter (DSC) that we use regularly (Perkin-Elmer DSC-7 with Intracooler II) cools at a maximum sustainable rate of <100°F/min over the same temperature range.

As another example, cream cheese is made in a multi-step process that involves two cycles of heating and cooling. First the mix is heated, in order to homogenize and pasteurize it, and then cooled to the culturing temperature. After culturing, the mix is heated once again, and then the curd is separated and cooled in stages, in order to make the final product. Both heating and cooling may be accomplished at average rates ranging from 200° to 400°F/min.

Given these moderately high cooling rates, the problem is to find a methodology capable of following the physical events of interest. The problem is not a trivial one. For example, the DSC is widely used to characterize exo- and endo-thermal process events. Although the maximum cooling rate depends on the temperature range under study, the maximum rate achievable by DSC can be less than that realized in some food processes.

In addition, shear is common in food processing, and shear certainly affects texture of the final product. In order to do DSC while shearing a sample, however, the heat generated by mechanical work must be accounted for. As far as the author is aware, no thermal analyzer has been made to accomplish this apparently difficult task.

1.2. Choice of method

DSC, spectroscopic methods (IR and UV absorption), scattering methods (light scattering, X-ray diffraction) and imaging methods (light and electron microscopies) all are capable of following food processing events. Among these methods, X-ray diffraction (XRD) has the advantages that it is a direct measure of structure over a broad size range (~lA up to 2 microns); that it is relatively non-destructive and therefore can be used to observe dynamic events; and that it averages over many copies of the structure of interest. Moreover, in principle the experiment can be done while the sample is being sheared. In comparison, light microscopy too is non-destructive, it provides a direct image of the material, and it also may be feasible on a sample undergoing shear; however, the size range is limited to 1/2 micron or greater. Electron microscopy provides a direct image as well, and the resolution is comparable to XRD; however, food samples must be physico-chemically stabilized ("fixed") before viewing. Hence dynamics must be inferred rather than observed directly. Finally, the microscopy methods require a great deal of effort to average over a large population, e.g., to determine droplet sizes. Light scattering methods allow dynamic experiments to be carried out, even as the sample undergoes shear; however, the resolution is limited, and there are restrictions on the allowable concentration of solids.

In passing, it may be useful to record different kinds of data simultaneously. XRD and calorimetric data, for example, complement one another nicely. The calorimetric device made by Mettler (M-84, designed for use with the light microscope) can be used for combined calorimetry and XRD.

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.

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