"We are investigating the caterer regarding this incident," airline spokesman Prasad Rao told AFP in New Delhi.
The spokesman said the airline maintained high standards, including hygiene conditions, adding that this was a "very rare incident".
The loss-making airline has been hit by a string of issues, including over safety. In May, a flight to Bangalore was diverted to another city after the pilot returned from a toilet break and found the door to the cockpit jammed shut.
Air India also this year suspended a senior pilot caught drunk just before he was to fly a passenger aircraft.
Air India is the country's fourth-largest airline by market share, and has been hit hard by rising fuel prices and fierce competition from private carriers that have added to a legacy of labour problems and crushing debts.