It’s probably an even more familiar phenomenon than mouldy bread: mouldy cheese. And not just the cheeses that are meant to be mouldy, but mould growth on cheese that are not supposed to be covered in them. Dutch cheeses (like Gouda or Edam for instance) have a tendency to mould after a few weeks of storage outside of their original packs, although you prefer to eat them without the blue/grey moulds.
It occurred to me that when we buy grated cheese we can generally store it for a long time. Often longer than a block of cheese even. However, once the pack has been opened we have to watch out that we don’t keep it for too long, the grated cheese can go bad quickly at that point. A lot faster than a non-grated block of cheese. What is going on here? That’s what we’ll be exploring in this post.
Forgotten how cheese is made? Let’s start with a quick re-cap. Cheese is made from milk (as we’ve discussed extensively in a post on the science of cheese making). Most of the water is removed during this process (1 liter of milk will make about 200g of cheese) as well as the so called whey proteins.
There are several ways to induce the water to leave the milk and end up with cheese. It can be done by adding acids or enzymes for instance. Adding a lot of acid will even make for an acidic cheese since that acid will not be removed later on in the process.
What is left is a consistency with quite a lot of fat. In the Netherlands this fat is often included in the name of a cheese. Regular (not low fat) cheese generally contains over 48% of fat and tends to be called 48+ cheese. Similarly there is 30+ cheese as well as several others. Apart from fat there still is quite a bit of moisture in cheese, as well as proteins.
Cheese & moulds
In order for micro organisms to grow on food there has to be enough food (often sugars, but also proteins), enough water (but not too much) and the right air composition (most moulds would like oxygen). Also, it shouldn’t be too cold, nor too hot for the moulds.
A fridge in this case does slow down growth of moulds, however, moulds aren’t stopped by fridge temperatures. They can still grow, be it slower. It does greatly stop growth of most pathogenic micro organisms (those that make you sick).
Coming back to the food and air supply on cheese. Cheese has quite an abundance of food, there are a lot of proteins, but also some sugars in cheese. If cheese is stored in the fridge without a special pack, the air is also beneficial for mould growth.
The main factor that remains in the availability of water, by chemists expressed as ‘water activity’. The water activity will greatly determine whether mould growth will take place and how fast. Cheeses with more available water tend to have more mould growth and it tends to happens faster. A long aged cheese loses moisture during ripening, as a result it will be less prone to moulds. A younger, less ripened moist cheese on the other hand will have a higher chance of mould growth. This is however also greatly impacted by the amount of salt in the cheese. More salt will reduce water activity and thus mould growth.
Last but not least the pH (acidity) of cheese will play an important role. All micro organisms, including moulds, have an optimal pH to grow at. Moulds tend to be more resistant to extreme pH values than bacteria, but still, more acidic cheeses tend to slow down mould growth.
Is mould on cheese dangerous?
A lot of mould on cheese is not dangerous. These moulds do not produce toxins (which they do in fruits and vegetables for instance). And moulds themselves tend to be quite harmless. In the case of Gouda & Edam style cheeses, it is sufficient to cut the mouldy part off (I wouldn’t advice eating that part) and enjoy the rest of the cheese.
However, when it comes to grated cheese the story becomes a bit more complicated. You cannot really remove the mould from the cheese anymore and it will have spread far more easily. Therefore, mouldy cheese often just has to be thrown out.
Preventing mould growth – vacuum packaging
Mould growth can be eliminated in various ways. A common method to do so for blocks of cheese is the vacuum pack the cheese. The vacuum packs take away all the air. Since a lot of these moulds actually require oxygen to grow, this prevents mould growth to a great extent.
However, grated cheese is generally not packed vacuum. The reason being that the vacuum pack will cause all the cheese gratings to be squashed together in the pack. As a result, the cheese might clump together. This is not something a grated cheese manufacturer will want.
Grated cheese without moulds: MAP
The solution found for this problem is the use of modified atmosphere packaging (abbreviated MAP). We’ve discussed MAP before when it comes to packaging cut fruits and vegetables. Just like for cut fruits and vegetables, cut (or grated) cheese requires a bit more protection than the whole thing. A very similar concept is also used for parbaked breads. These breads have to be finished in the oven at home. They are not sold fresh, and thus have to be stored for up to several months after initial production. Therefore, these breads also require a bit of extra protection.
Modified atmosphere packaging does this by creating an environment in the package that is not suitable for mould growth. It does so by modifying the gas composition in the pack. Whereas the air around us contains 20% oxygen, little carbon dioxide and around 80% nitrogen gas, these packs will not. Instead, the oxygen content tends to be a lot lower (or zero) since a lot of moulds tend to require oxygen to grow. Also, the carbon dioxide levels tend to increase, since high carbon dioxide levels also slows down (or eliminates) mould growth.
Opening a grated cheese pack
When the package of grated cheese is opened though this protective atmosphere will disappear. Instead, the pack will fill itself with the regular air around us. This allows moulds in the pack to grow. Since it’s virtually impossible to make a sterile environment in these packs, especially after opening, this is bound to happen.
Unless, the cheese manufacturer has added mould growth inhibitors. There are certain components that can be added to inhibit growth of moulds.
Why doesn’t grated cheese clump?
Well, not using vacuum packaging is one reason. But you might recall that your own freshly grated cheese clumps a lot more easily than the store bought one. This is due to two main reasons. One, the grated cheese isn’t just packaged using MAP, manufacturers also tend to add some starch. This starch prevents the moist cheese from sticking by slightly drying out the outside. The second reason is the formulation of grated cheese. This isn’t always used either, but by choosing an appropriate formulation, stickiness can be reduced. An old cheese which has dried out slightly will be less prone to stickiness than a young moist one.
Only open the grated cheese pack when you actually intend on using it! And take care that you might be able to store it for quite a well when closed, once it’s been opened the shelf life will be limited.
Still excited to learn more about food science & cheese? Check out our posts on cheese making (either with acids or enzymes). Or check out which cheese is best to use on top of your quiche or pizza!
Yearning for more food packaging information? Have a look at our food packaging basics course. It’s our first course on food packaging, aiming to provide you with all the basics!
Please note, this article as any on this blog are for information purposes only. If deciding to eat cheese with moulds it remains your own responsibility and judgement at that moment in time.