"Curdling" of soymilk in coffee: a study of the phase behaviour of soymilk coffee mixtures

Soymilk is often observed to “curdle” (become inhomogeneous) and then sediment in coffee, which makes it an undesirable product for consumers. This work investigates the phase behaviour (curdling) of an organic, unsweetened, long-life soymilk in instant coffee solutions for a range of coffee and soymilk concentrations, temperature and pH. The temperature vs soymilk concentration phase diagram exhibits a coexistence curve, separating a two-phase region at high temperature from a single-phase region at low temperature. The phase transformation is reversible and it is possible to recover a single phase mixture from the “curdled” mixture either by cooling the mixture or by increasing the soymilk concentration. Image analysis was used to investigate the time evolution of the phase separation and sedimentation processes. Fourier transformation of the images resulted in peaks corresponding to the growing length scale of the phase separation. The rate of growth of peak intensity over time shows a power law dependence on temperature characteristic of spinodal decomposition. However, the observed separation kinetics are not entirely straightforward to explain with common phase separation models.