Fire officials told ABC7 News they believe the substance is either hydrochloric acid or lye. There are concerns the chemicals would create a toxic vapor.
About eight employees were inside the plant at the time and all had to be evacuated.
"They create and process cheese and it looks like the actual cleaning materials that they use to clean the apparatus afterwards had some sort of catastrophic failure," said Menlo Park Fire Capt. John Wurdinger.
The fire department responded with three different hazmat teams. No one needed medical care.