The holes in Swiss cheese are created by gas-releasing bacteria.

The formation of holes in cheeses like Emmental is a complex biochemical process driven by the bacterium Propionibacterium freudenreichii. During the fermentation stage, which typically lasts between 4 and 8 weeks in a warm cellar, these bacteria consume the lactic acid produced by other starter cultures. As they metabolize the acid, they release carbon dioxide gas, water, and propionic acid, which gives the cheese its distinct nutty flavor.Because the cheese has a firm, elastic structure and a thick rind, the carbon dioxide cannot easily escape. Instead, the gas collects at specific points within the curd, exerting pressure and expanding into spherical bubbles known as 'eyes.' In 2015, researchers at Agroscope, a Swiss government agricultural institute, discovered that tiny particles of hay dust often act as the 'seeds' for these holes. These microscopic hay specks create weak points in the curd structure where gas can accumulate more easily.Modern industrial milking methods have become so clean that they often remove these hay particles, leading to 'blind' cheese with fewer or no holes. To maintain the traditional appearance, some cheesemakers now add controlled amounts of hay dust back into the milk. The size of the holes is strictly regulated by international standards. For example, Grade A Swiss cheese must have eyes between 11/16 and 13/16 of an inch in diameter to be considered high quality.