How Does Fat Content Affect Eye Formation in Cheese?

A quarter fat Appenzeller cheese usually has significantly more holes (‘eyes’) than a full fat Appenzeller. Is this really due to the fat content? Agroscope researchers looked into this issue.

Hole or ‘eye’ formation is a distinguishing feature of many cheese varieties from Switzerland, such as Appenzeller®, Tilsiter and Emmentaler PDO. The size, number and nature of the eyes are key parameters for assessing the quality of the cheese. Eye formation is the result of a complex process that is dependent on a wide range of parameters such as the filling of the curd into the mould, the pressing of the curd, the aggregation of the curd granules, CO2 formation through a starter culture or adjunct culture, the presence of eye starting points – the nuclei –, the texture and much more. Despite this, the importance of the composition of cheese has scarcely been studied to date. It is known that CO2 has good solubility in both water and in fat. This solubility is temperature-dependent: as the temperature rises, CO2 becomes less soluble in water and more soluble in fat. Knowing about the influencing factors allows cheese producers to steer eye formation in the desired direction.

Amount and solubility of CO2 in cheese determine eye formation

But it is not just the question of how much CO2 can dissolve in the main constituents of the cheese that is critical for eye formation. The amount of fermentable material converted into CO2 by the bacterial community in the cheese is also important. The investigations took account of the fact that adjusting the fat content always had an effect on composition, and hence on the availability of fermentable material.

Fat affects eye formation in semi-hard cheeses …

Appenzeller-type semi-hard cheeses were manufactured in various fat levels. A facultatively heterofermentative adjunct culture with Lacticaseibacillus paracasei ensured that CO2 was produced from citrate. With decreasing fat content, more CO2 was formed owing to the altered composition (more fermentable citrate thanks to the expansion of the water phase). At the same time, it was shown that in the cheeses with a lower fat content, less CO2 was dissolved than had been formed, owing to the lower solubility of the CO2 in the water phase as opposed to the fat phase. Consequently, more undissolved CO2 was present in the cheese and each effect intensified the other: eye volume increased markedly with decreasing fat content.

Eye formation in Appenzeller-type semi-hard cheeses of various fat levels, visualised on CT scan. (Photographs: Dominik Guggisberg, Agroscope)

… and in Emmetaler-type cheeses

Emmentaler-type hard cheeses were also manufactured in varying fat levels. In this case, strains of Propionibacterium freudenreichii were responsible for the CO2 formation from lactate. Compared to Appenzeller or Tilsiter, significantly more CO2 was formed in Emmentaler cheese, which explains the markedly larger eyes of the latter. The change in composition of the cheese resulting from the reduction in fat led to a dramatic increase in CO2 formation, and hence in eye formation. At the high fat level, CO2 formation and CO2 solubility remained in balance, with the result that few holes were formed

Eye formation in Emmentaler-type cheeses of varying fat levels, visualised on CT scan. (Photographs: Dominik Guggisberg, Agroscope)


  • Adjusting the fat content of cheese always affects eye formation because of the change in composition.
  • Reducing the fat content leads to an expansion of the water phase and hence to an increase in fermentable material. Consequently, both CO2 formation and eye formation increase.
  • An increase in fat content leads to increased CO2 solubility, and hence to a reduction in eye volume. These effects are increasingly observed at warmer temperatures. This is particularly apparent during the ripening of Emmentaler.
  • Knowing about these interactions enables production facilities to monitor the quality of eye formation.
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