Freezing Lactic Acid Hay Particulate Nitrogen Expansion 
Answer: Hay Particulate
It may not be the most pressing mystery in the history of the world, but a mystery it was. For years cheese makers and scientists alike were puzzled by the holes in Swiss cheese. Despite the numerous theories floated as to why Swiss cheese has holes in it, none of them satisfactorily explained exactly why the holes appeared in that particular type of cheese and not in other types of cheese.
Partial explanations hinged on the presence of specific bacteria that consumed the lactic acid in the cheese curd and produced carbon dioxide as a metabolic waste product. While that was the accepted theory for decades, it didn’t explain the size, variability, and amount of gas bubbles during cheese formation. Only recently did researchers finally isolate exactly what causes the formation of the bubbles: hay particulate.
For years the holes in Swiss cheese had been getting smaller, a development that generated significant curiosity among cheese makers. Where were the holes going? Why were they fewer and smaller in size? The answer to the mystery was simply better cleanliness. Dairy farms had gotten cleaner and cleaner over the years and that meant that particles of hay, ubiquitously present and abundant on farms for centuries, were not contaminating the milk used to create the cheese at the same rate they had been in the past.
Just like the tiny imperfections in the bottom of a champagne glass act as a point of nucleation for the formation of gas bubbles, the microscopic bits of hay in the cheese act as a nucleation point for the gas to gather. The Swiss cheese of yesteryear had larger and more abundant holes because there were more hay particles for the gas to attach to.
What’s particularly interesting about the whole affair is that historically, cheese makers considered the presence of the gas pockets (or “eyes” as they are known) as an imperfection in the cheese making process and they sought to eliminate them; they often resorted to pressing the blocks of cheese to force the air out. Now, however, the presence of the eyes is the most readily identifiable characteristic of Swiss cheese and when the cleaner production facilities led to a reduction in size and frequency of the eyes, the cheese makers sought a way to keep the signature look of traditional Swiss cheese.