Dynamic Light Scattering

Dynamic light scattering (DLS) is an alternative method for determining food microstructure that is based on scattering of light by moving particles. Details of this noninvasive and relatively rapid method are given elsewhere (63). In the food industry, DLS is used for determining particle sizes in the range of 1 to 300 /im in such diverse products as chocolate, wheat and soy flour, confectionery sugar, spices, and mayonnaise. More specific instruments have been developed to detect particles outside this range (64). The main application of this technique has involved very dilute systems (milk and oil in water emulsions), because the concentration of scatters had to be sufficiently low that an incident photon of light is scattered only once by the sample (63). Therefore, DLS can rarely be used directly on most foods, since particulate concentration is too high and native structure is difficult to maintain upon dilution. Nevertheless, DLS has been used to look at the changes in the average size of casein micelles with addition of calcium

Figure 4. «-Amylase action on a single starch granule obtained by contact mode AFM. A-D: 0, 105, 420, and 735 s, respectively. Source: Ref. 61, used with permission.

phosphate, increased temperature, and different pHs (63). These changes (reflecting processing protocols) could be related to the functional properties of milk. Additionally, DLS has been used to detect size distribution (65) and adsorbed monolayers (66) in emulsions.

A modification of DLS, diffusion wave spectroscopy (DWS), in which incident and back-scattered light are conducted through fiber optics, can be used to provide estimates of mean particle size in concentrated suspensions. For example, the particle-size changes in rennet (67) and acid-induced (68) gelation of milk were studied by DWS. However, to obtain accurate particle sizes with DWS, the instrument needs to be calibrated with separate samples with known size scatterers, which may not have the same optical characteristics as the food samples used, making the interpretation of results difficult (63).

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