The holes in Swiss are a by-product of some of the microbes added to milk to make the cheese. Specifically, there are three primary types of bacteria that are typically used to make Swiss cheese (these can vary slightly depending on the manufacturer): Streptococcus thermophilus, Lactobacillus helveticus and Propionibacterium shermanii.
The first two types of microbes produce significant amounts of lactic acid, which is, in turn, consumed by the latter type of microbe, Propionibacterium shermanii. It is this Propionibacterium shermanii that is responsible for the holes in Swiss cheese. Through the process of consuming the lactic acid, the shermanii produce acetate, propionic acid and carbon dioxide as by-products. The acetate and propionic acid give Swiss cheese much of its distinct flavour, while the carbon dioxide forms bubbles within the cheese block or wheel. These carbon-dioxide bubbles are left in as the cheese continues to ferment rather than being pressed out, giving Swiss cheese its distinctive holes. Historically, however, these holes were seen as an imperfection in the cheese, and most manufacturers would try to avoid them by pressing this type of cheese during the ageing process in order to force out the bubbles and keep the cheese solid throughout the block or wheel.
Get more cheese #funfacts on our Facebook page: Cheese from Europe Hong Kong