I've had a variety of singing teachers, all of whom assured me that eating chocolate prior to singing was bad for my voice. This is assumed fact amongst almost all other singers I have known. I've never come across the original research though, so is this claim based on scientific evidence or not?
According to Voice Care Knowledge Among Clinicians and People With Healthy Voices or Dysphonia Journal of Voice Volume 21,January 2007, Pages 80–91:
Chocolate as a solid caffeine may be detrimental to the voice
Four references (6, 18, 25 and 36) are cited for supporting this statement.
The effects of preventive vocal hygiene education on the vocal hygiene habits and perceptual vocal characteristics of training singers J Voice, 14 (2000), pp. 59–71
Clinical Voice Pathology: Theory and Management (3rd ed.)Singular, San Diego, CA (2000)
Vocal Arts Medicine: The Care and Prevention of Professional Voice Disorders Thieme Medical, New York, NY (1994)
Effect of caffeine on the vocal folds: a pilot study J Laryngol Otol, 113 (1999), pp. 341–345
This last reference says:
changes were seen to authenticate the fact that caffeine does produce alterations in voice quality