Eye development in cheese can be a sought-after attribute or an unfortunate defect, but in both cases, gas is the culprit. That’s why it is essential for cheesemakers to understand the basics of gas development and apply that knowledge, whether they are looking to make a tightly knit Cheddar or a Swiss with perfect eyes.

Understanding fermentation

Perhaps the most important piece of knowledge in relation to eye development is an understanding of fermentation by microorganisms. There are typically three categories of dairy microorganisms that are commonly involved in fermentations including: obligate heterofermentative, obligate homofermentative and facultative heterofermentative.

Without getting too deep into the science, the basic principle here is that each microorganism can be classified by how/if they ferment sugars (lactose and galactose), citrate or lactic acid to produce an end product like carbon dioxide. In Swiss cheese, the large eyes are the result of lactic acid fermentation by Propionibacterium while in Gouda the small eyes are due to citrate fermentation by added strains of Leuconsotoc and Lactococcus.

If carbon dioxide is produced from a fermentation process then this gas can collect in a weak spot within the cheese and as pressure builds, the gas pushes the curd apart, usually spherically, because gas bubbles form droplets to reduce surface tension. This is the ideal scenario for a Swiss or Gouda cheese, but if the gas forms in a cheese that is too hard (or too cold when cheese becomes much firmer) then these spherical holes can turn into splits and cracks, which is a visible defect for consumers.

Gas development in cheese

When gas is unwanted, we can learn a lot about the culprit organism by simply noticing the time at which the defect occurred. If, for example, gas develops within a week or so after cheese manufacture, it is very likely that the gas-forming microorganism was in the raw milk, or that there was a significant biofilm on processing or manufacturing equipment.

On the other hand, if gas occurs late in the ripening process, then residual galactose fermentation or amino acid decarboxylation may be to blame. These reactions are only likely to occur if certain contaminating bacteria are present and if the cheese is held at warm temperatures (>50 F) (which helps facilitate the growth of these unwanted microorganisms).  Temperature abuse at the retailer is a critical factor contributing to these defects, which are enhanced by intense lighting on cheese packages. Spoilage organisms can cause the curd to develop small holes, unclean flavors and a puffed up package, in addition to splits and cracks.

When gas development is desired, such as in Swiss or Gouda cheese, there are some precautions that should be taken in order to get the right number and size of eyes. For example, cheesemakers try to remove most of the entrapped air from the curd by pressing under water/whey to avoid producing too many holes, which can form around existing air bubbles.

Last year there were some media reports that picked up on a paper published by Swiss researchers and incorrectly suggested that the eyes in Swiss cheese were created by hay and not by bacterial fermentation. Unfortunately, these reports misunderstood what the Swiss researchers were actually saying. In reality, residual particles like entrapped air are essential nucleation sites, or weak spots, in the Swiss cheese curd where the carbon dioxide gas produced by fermentation processes can collect or aggregate together to form a bubble, which then eventually grows into an eye as more gas is generated. It is not surprising that if cheese curd has fewer of these nucleation sites then fewer eyes will occur in the cheese.

In any case, understanding and controlling eye development is an important topic for cheesemakers, both in cheeses where eyes are required, as well as where they are considered a defect. While the eyes themselves have no flavor, the fermentation reactions involved in their formation can create flavors ranging from sweet and nutty, to diacetyl to unclean. Therefore, it’s essential that cheesemakers understand this process and watch for signs of trouble.

By following these guidelines and bridging the gap between research and practical applications, we can all work together towards the end goal of a high-quality product.