The pungent aromas of cheeses have surprising effects on microbes, researchers in the US report.
Volatile organic compounds (VOCs) can be generated by fungi on the rinds of smelly cheeses such as camembert. Now, researchers report that fungal volatiles influence bacterial communities present in the cheese (Environm. Microbiol., doi:10.1111/1462-2920.15223).
‘These compounds affect the whole microbial community, but in particular Vibrio [bacteria], which seems to be able to metabolise and eat these compounds,’ says study leader Casey Cosetta at Tufts University in Boston.
The microbial biofilm that forms on cheese surfaces – cheese rind – contains yeast, fungi and bacteria that are intentionally or accidentally added. The microbes ooze enzymes that break down amino and fatty acids, generating a plethora of VOCs that often define the taste and quality of the cheese.
Vibrio bacteria are common on bloomy rind cheeses like Camembert, which are inoculated with fungi that then bloom on the rind, and on washed rind cheeses like Limburger that are rinsed with brine.
However, cheese is poor in nutrients for many bacteria. Energy is locked away in curd, protein and fat globules; easily metabolised sugars are especially rare in aged cheeses. The Tufts study shows that Vibrio shift from the typical citric acid cycle that breaks down glucose, to a shortcut involving two enzymes. This allows the organisms to chomp through smaller organic molecules such as the VOCs.
The distinctive taste of some cheeses may even hark back to a maritime origin. ‘Vibrio is thought to come from the sea salt that the cheesemakers add,’ says Cosetta, who believes these bacteria ‘are also contributing to the flavour, taste and smells of the finished cheeses’.
The study paired 16 different cheese bacteria with five common cheese rind fungi. The impact of the fungi ranged from strong stimulation to strong inhibition of the bacteria. Vibrio casei grew rapidly in response to all five fungi.
VOCs also changed the expression of many genes in bacteria, influencing how they used nutrients. The scientists say their results have implications for cheese producers around the world. ‘Volatiles from aged cheese could impact the fresh cheese in a cave when they are in close proximity,’ Cosetta explains. Often such cheeses are allowed to mature next to one another. Now it seems VOCs can influence microbial communities, which could impact consistency.
‘Spraying the surface of the cheese with VOCs such as acetate may enable Vibrio to dominate and thus you get a more controlled growth in the rind, producing a more consistent flavour profile,’ comments Kieran Kilcawley, a cheese scientist at Teagasc, the state agrifood research organisation in Ireland. ‘You would need to know how much to spray on without adversely impacting on the aroma of the cheese. Also, you might inadvertently change other biochemical processes. But it’s interesting.’