Unlike their starter brothers, secondary cultures are not only bacterial but can be moulds or yeasts. They have a number of functions, depending on the cheese they are used in, which include rind forming, gas creation and veining of blue cheese.
As we saw in the history of cheese, many of these secondary cultures were derived from organisms already present in the milk, or were added during cheese making simply because of unsanitary conditions, be it unsterile utensils or a general unclean environment. Today, this sounds unpalatable and I’m a big advocate of keeping kitchens and equipment as clean as you can, so it’s ironic to consider that it was only as we discovered the importance of working in sanitary conditions with food that we realised the importance of secondary cultures. As conditions became cleaner, cheese became blander, and it was this change that revealed the necessity of ‘unwanted’ micro organisms in some of our finest cheeses.
Today, of course, our secondary or adjunct cultures are produced commercially in very pure form, which is why cleanliness in the kitchen is the way forward and days of adding mouldy bread to our milk to bring about blue veins in the cheese are behind us.
Yeasts do not directly impact the cheese but they do help other (secondary) bacteria thrive. There are many variants of yeast which are added to cheeses as diverse as Brie and Stilton, Limburger and Tallegio. The finished appearance of the cheese is dependent on the yeast strain being used, it’s crucial to get the correct one so that you don’t end up with a slimy finish if you were aiming for a velvety texture. A good cheese making kit supplier will be able to help with selection.At the surface of the cheese, they generally work by fermenting the lactose in the cheese, which reduces the surface acidity. In turn, the lower acidity encourages rind-forming bacteria or desirable moulds to thrive, i.e. the yeast creates an environment that’s ideal for our desired bacteria or mould. This brings an additional benefit because increased growth of desirable mould reduces the amount of undesirables on the surface of our cheese.Yeasts also produce aromatic compounds, so the desirable (or less than) smell of some cheese is, in part at least due to the addition of yeast as a secondary culture.
Moulds are used in the two situations described earlier: to create the mould on surface ripened cheeses like Brie, and the veins in cheeses such as Stilton and Roquefort. Indeed, when shopping for a secondary culture, you will come across P. roquerforti and P. camemberti as the two main varieties (P standing for Penicillium) producing the distinctive blue veining and white mould, respectively.In some respects, these are the ‘headline’ strains and there are many varieties available, fine tuned to the particular variety of cheese being created. They are selected for such properties as how long the mould filaments will grow on the cheese surface, the aroma compounds they encourage, the colour of blue/green vein produced and so on.Just like yeast, these cultures convert lactic acid in and on the cheese into other compounds, which reduces its acidity, and break down proteins (proteolysis), which increase its softness. A great example of the effect proteolysis is often seen in Camembert. Close to the mould encased edges, the cheese is soft and even runny, whereas at the centre, it is firmer and a lighter colour. This difference is due to the surface mould breaking down the protein near it but not being able to reach the middle of the cheese and so leaving the protein intact.
This is the final group of secondary culture. The propionic bacteria are responsible for producing the eyes in Swiss cheeses such as Emmental and Jarlsberg. As you can imagine, the key to a great strain is finding one that produces eyes in the quantity and size that makes the cheese recognisable and desirable.These bacteria (look for P. shermanii when buying) convert milk sugars into carbon dioxide during ripening and it is this build up of carbon dioxide pockets that creates the familiar eyes in the finished cheese. Just as we discovered earlier, these bacteria also impact the proteins in the cheese, so they also impact on the texture and flavour of the cheese we eat.Secondary cultures are a brilliant avenue to explore if you want to expand your cheese making repertoire. There is a lot of science goes into the selection of strains to be used, which you can read more about in Cheese: General Aspect, Vol 1, edited by P. F. Fox, if you are interested in more detail.