Diffusing wave spectroscopy of gelling food systems: The importance of the photon transport mean free path (l*) parameter

The gelation of milk proteins by the addition of 1.5% (w/w) glucono-δ-lactone (GDL) as well as the effect of the addition of pectin to low pH oil-in-water emulsions stabilized with whey protein isolate were measured in situ using Diffusing Wave Spectroscopy (DWS). For both systems, large changes were observed in the 1/l* parameter before any marked increase in particle size, indicating a partial correlation between particles which increased as the pH dropped (for the milk case) and with each addition of pectin (for the emulsion case). Theoretical calculations of l* were made assuming no correlation between particles (S(q)=1) and compared to the experimentally-obtained 1/l* values. Without interparticle interactions the value of 1/l* remained essentially constant throughout. This indicates that the change observed experimentally is primarily due to a change in S(q) and that 1/l* is a good indication of system order. Finally, it is noted that the opacity of a gel is mainly determined by the initial properties of the dispersed particles.