It is now possible to detect who is eating what by analyzing the sound of mastication. Researchers from the Wearable Group at ETHZ (Zürich) developed an automatic dietary monitoring system and demonstrated that sound from the user’s mouth can be used to detect that he/she is eating. As described in this paper:

The paper also shows how different kinds of food can be recognized by analyzing chewing sounds. The sounds are acquired with a microphone located inside the ear canal. This is an unobtrusive location widely accepted in other applications (hearing aids, headsets). To validate our method we present experimental results containing 3500 seconds of chewing data from four sub jects on four different food types typically found in a meal. Up to 99% accuracy is achieved on eating recognition and between 80% to 100% on food type classification.