The Snyder Test is used to determine if a person is susceptable to cavities.
It contains 2% glucose and a pH indicator.
The pH of the medium is acidic ( 4.8) and only some bacteria can grow with that pH. One of them being the Lactobacillus species. They fermentate the glucose, producing lactic acid and causing the pH to drop.
The more glucose present in saliva, more glucose Lactobacillus will fermentate.
So, the dietary item that causes more of this bacteria is sugars.