For Centuries, Nobody Knew Why Swiss Cheese Has Holes. Then, The Mystery Was Solved.

Not too long ago, Swiss cheese was on the brink. Its iconic hole-ridden body was filling in: the caverns that had for so many centuries dotted the alpine mainstay were shrinking, or worse – disappearing altogether. 

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A solution was needed – but the problem was deeper than it first seemed. Before scientists could figure out a way to save the cheesy Swiss holes – or, as cheese aficionados sometimes call them, “eyes” – they first needed to understand why they existed in the first place. 

As it turned out, that would be more difficult than expected.

Holey smokes

Swiss cheeses have been famously holey for centuries – and precisely why that is has been a mystery for just as long. 

“The quality of a prime Emmental cheese is determined not alone by its sweet, nutty flavor and pliant texture, but also by the character of its holes,” begins a seminal 1917 paper from USDA researcher William Mansfield Clark. But while “various attempts have been made to distinguish the fermentations responsible for various types of holes,” he wrote, “the biological origin of normal eyes is still in doubt.”

Scientists at the time had a theory, however. Perhaps, Clark suggested, the holes were made by certain bacteria present in the fermentation process. As the cheese ripened, these microbes would release carbon dioxide, forming bubble voids within the food that would be preserved as it hardened. 

Which bacteria, you ask? When in the process did the bubbles form, and why here and there rather than somewhere else? Well… don’t worry about that. After all, Clark didn’t: “Until the specific origin of the gas is more definitely known, and until these bacteria have been located in greatest abundance at points of eye formation, or their liberated enzymes have been shown to have their action confined to these localities, the evidence at hand is in favor of the view that the gas is produced more or less evenly throughout the whole body of the cheese,” he wrote, perhaps optimistically by modern standards.

Still, it was evidently a convincing hypothesis, since nobody seems to have seriously argued with it for quite some time. But then, something terrible happened. With the dawn of the 21st century, the holes started disappearing – and, if the bacteria hypothesis was right, that shouldn’t have been possible. 

Further confusing the matter was the fact that Swiss cheese made in the summer had fewer holes than that made in the winter. That shouldn’t make sense, should it? Especially in the modern age, where the cheesemaking process is more standardized than ever before; storage of the foodstuff is controlled to within a couple of degrees of temperature, and even the precise cultures of bacteria present can be tweaked to perfection.

Perhaps, then, the holes had nothing to do with the cheese curdling process at all. Maybe it happened earlier on – and the disappearing holes were to do with something else that was changing.

But here’s the question: what?

The hole picture

It took until 2015, but eventually, the mystery was solved. “It’s the disappearance of the traditional bucket,” said Regis Nyffeler, a spokesperson for the Swiss state center for agricultural research Agroscope, to The Guardian at the time.

Now, we know what you’re thinking: huh? But as it turns out, the holes in Swiss cheese aren’t random – they’re triggered by the presence of tiny, microscopically small particles of hay that find their way into the milk. It explains the seasonal variance – in fact, that’s how the 2015 research team came up with the idea, reasoning “that hay and grass feeding could affect the entry of microparticles into raw milk,” they wrote at the time.

Equally, it accounts for the disappearance of the holes in modern times. “Traditional milking in open-cup barns has been completely replaced by modern, closed milking systems in recent decades,” pointed out a statement from Agroscope after the results were published. 

“These technical improvements in milking technology have reduced the risk of unwanted microbiological contamination,” it explained, “and, at the same time, reduced the ingress of microscopic hay particles into the milk. This has also resulted in fewer ‘hole initiation sites’ in the cheese.”

It seemed like a slam-dunk argument – but would it hold up to scrutiny?

Plugging the hole

As convincing as their reasoning was, the team had yet to prove their hypothesis – so they set out to do just that, by grinding hay into power, and seeding it into some pristine milk. Then, over the 130-day cheese ripening process, the forming holes were recorded using CT scans.

Obviously, they were hoping to be vindicated – but the results of their experiments “were astonishing even for the researchers,” Agroscope stated at the time. “Depending on the dosage of hay particles, the number of holes in the cheese could be controlled almost at will.”

It was a victory for the scientists, and a relief for a dairy industry facing an existential crisis. “Thanks to this discovery, the mystery of ‘hole loss’ can now be scientifically explained,” Agroscope announced. “The fact that traditional cheese production requires a pinch of hay dust in addition to milk, rennet, and bacterial cultures is a good example of how the production and processing of raw milk into cheese are still closely linked today.”

But more than that, it was a solution for a little-known mystery that had puzzled food scientists for more than a century. The reason for the holes in Swiss cheese? Hay. Always has been. Replace traditional milking techniques with modern, industrial ones, and those hay particles can’t reach the milk. And no hay: no holes.

And at the very least, that’s good news for the cheese lovers among us. 

“While the holes have a more or less spherical form […], there are important distinctions superficially based upon their size,” Clark pointed out, all the way back in 1917. 

“Large ‘blow holes’ […] may mar the contour of a cheese [and] sometimes are associated with an ‘off flavor’,” he wrote. “Worst are ‘die Nissler,’ ‘the thousand eyes,’ the ‘pin holes,’ which frequently ruin a cheese by making it spongy.”

But, he added, “when uniform in size and distribution, they give ‘The King of Cheeses’ a distinction admired by the connoisseur.” 

Now, we know exactly how to make those holes – and as a result, we can make better cheeses than ever before. Cheers to that!