I found this link which is basically what the prof said: http://www.bbc.co.uk/science/humanbody/mind/articles/persona... and an actual study: http://psycnet.apa.org/index.cfm?fa=search.displayRecord&UID...

Of course you should keep in mind potential confounding factors, but it's certainly interesting as an explanation. I'm not sure if there've been any studies where they took fMRIs while doing the lemon juice test, which would probably be a bit better.