I have searched "tasteless emetics", "flavorless emetics" and "undetectable emetics."
I am working on a play right now called Sunshine and Solomon. It takes place in New York in late May of 2009.
The basic premise of the play is that the main character, Robin Glass, sees someone she knows from somewhere. She doesn't know how she knows him, but she knows she is in love with him upon seeing him again. She leaves him a note telling him where to find her, and he follows the note.
What ensues is Robin being ridiculously forward, goofy and offensive on their first date. Instead of being repulsed, however, the man (Solomon Mercer) finds himself confused and intrigued by Robin and decides to stick around until he can at least figure her out.
Solomon burns his mouth on a scallion pancake and Robin decides to amend the situation by making out with him whilst having a mouthful of ice. She drools her Shirley Temple on him and he has to go and change his clothes. While he is in the bathroom, she puts something in his soy sauce. She believes in the "intimacy of puking" theory, the theory that once someone's seen you throw up, they've seen you at your most vulnerable and you are no longer simply acquaintances. She realizes how vulnerable she has made herself and wants him to do the same.
Long story short, is there any kind of substance that works as an emetic but would be relatively undetectable if mixed in with food or drink? It shouldn't be poisonous; I just need Solomon to hork, not die. An allergy wouldn't work here as she would have to know what he was allergic to, and this is the first time they've really spoken.