Polyphenols are responsible for the astringency of many beverages and foods. This is thought to be caused by the interaction of polyphenols with basic salivary proline-rich proteins (PRPs). It is widely assumed that the molecular origin of astringency is the precipitation of PRPs following polyphenol binding and the consequent change to the mucous layer in the mouth. Here, we use a variety of biophysical techniques on a simple model system, the binding of beta-casein to epigallocatechin gallate (EGCG). We show that at low EGCG ratios, small soluble polydisperse particles are formed, which aggregate to form larger particles as EGCG is added. There is an initial compaction of the protein as it binds to the polyphenol, but the particle subsequently increases in size as EGCG is added because of the incorporation of EGCG and then to aggregation and precipitation. These results are shown to be compatible with what is known of astringency in foodstuffs.