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Content - Swiss scientists solve mystery of holes in cheese

  • June 17, 2015

Swiss scientists solve mystery of holes in cheese

Tokyo (SCCIJ) - Cheese is the most exported Swiss agricultural good. Traditional varieties like Appenzeller and Emmental have characteristic holes which are professionally called "eyes". But how do these eyes arise? Now after a century of research and much uncertainty, a scientific study of Agroscope, the Swiss centre of excellence for agricultural research, and Empa, the Swiss Laboratory for Materials Science and Technology, has solved this very old mystery once and for all.

Swiss cheeese with characteristic "eyes"

Microscopic plant particles

The study by scientists of Agroscope and Empa examined the potential of micro-particles of plant origin to influence eye formation in cheese. The basic idea of the scientists was that the eyes are caused by hay particles in combination with bacteria producing CO2. Thus, eight experimental Emmental cheeses were produced with one replicate from micro-filtrated milk with addition of a very small quantity of powdered hay to the milk and ripened for 130 days.  

Eye formation was quantified by means of X-ray computed tomography (between 30 and 130 days). The results demonstrated that micro-particles of plant origin act as eye nuclei that control the number and size of the eyes in cheese. Basically, the hay particles contain some air which is joined by the CO2 gas. If there are no hay particles, the CO2 leaves the cheese. The amount of hay particles needed to achieve the usual eyes is extremely small: 5 to 10 milligram in 1,000 kilogram of raw milk is sufficient.

Eye formation was quantified by means of X-ray computed tomography (click to enlarge).

Eye formation was quantified by means of X-ray computed tomography (click to enlarge).

Characteristic eye formation

According to the paper, the size, number, shape, and distribution of eyes in Swiss-type cheese, such as Emmental, is mainly the result of propionic acid fermentation during warm room storage, involving the conversion of lactate into propionate, acetate, and CO2. Some bacteria produce less CO2 than others. This explains why despite the presence of hay particles in the originating milk, cheese varieties such as Gruyère or Sbrinz do not contain any holes.  

Already at the beginning of last century, the unequal number and size of eyes was mainly associated with differences in the distribution and growth of bacteria in cheese. But experiments showed that the whole cheese body contained considerable amounts of CO2. Thus William Clark in 1917 postulated that eye formation in cheese could be similar to the formation of raindrops in a vapour-saturated atmosphere due to the presence of “dust particles”.  

Later it was found that certain treatments of cheese milk like cleaning filtration or centrifugation and other techniques usually lead to a drastic reduction of the number of eyes. The scientists reasoned that dust particles originating from hay could act as highly effective eye nuclei and induce the formation of eyes in cheese. 

The microscopic investigation of hay powder micro-particles revealed that capillary structures present in plant tissues from leaves or stems that act as eye nuclei in cheese milk and enable eye formation during cheese ripening. (click to enlarge)

The microscopic investigation of hay powder micro-particles revealed that capillary structures present in plant tissues from leaves or stems that act as eye nuclei in cheese milk and enable eye formation during cheese ripening. (click to enlarge)

Hay powder as eye nuclei

The microscopic investigation of hay powder micro-particles revealed that capillary structures present in plant tissues from leaves or stems that act as eye nuclei in cheese milk and enable eye formation during cheese ripening. Possible entrapment of air in such capillaries allows the diffusion of CO2 from the cheese body into the micro-particles. The subsequent growth of the cavities leads finally to the formation of visible eyes in cheese.  

The results of the present study clearly show that trace amounts of hay powder in micro-filtrated milk induce eye formation during cheese ripening. It is likely that trace amounts of hay dust entering in raw milk have always been the natural source of eye nuclei, triggering the start of eye formation in cheese. These findings highlight the competing interests of hygienic milk production and the need to contaminate milk with solid micro-particles to induce eye formation. But the discovery will make it possible to control the size and amount of eyes in Swiss cheese scientifically.    

 

Text: SCCIJ; Photos: Agroscope

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