Learn the science behind:
Ever opened a pack of grated cheese, left it in the fridge, and had it turn moldy, even well before its expiry date? If so, you’re not alone! By opening the pack you broke down an important barrier against mold growth: the absence of oxygen.
Grated cheese is in its original, closed, pack, can often be stored for weeks, if not months. However, once that pack has been opened, some of the protection that the packaging offered is gone. Even if you carefully closed the bag after its first use, it isn’t as effective as it was before.
That’s because the a grated cheese pack contains some invisible barriers. Don’t worry, it’s nothing frightening. All that’s changed, is the gas composition inside of that pack! We’ll have a look at how it protects the cheese!
Cheese & Molds
Cheese and mold are a common combination. Sometimes, it’s desirable way, such as in a camembert cheese. Other times, not so much.
Desirable molds are often (but not always) white in color, maybe a little fluffy. However, once you see small specks of blue/grey/green on your otherwise white or yellow cheese, you know something is off. These are molds that you don’t want on your cheese.
Cheese isn’t the only food that can spoil because of mold growth. A lot of foods can actually. Another very common example is (improperly packaged) bread.
For some cheeses, especially harder cheeses, you can simply carve off these moldy pieces and continue eating. However, if mold has infested your grated cheese, you will have to throw it out. The cheese can’t be saved, the mold is likely everywhere!
Why do molds like grated cheese?
Molds are living organisms and in order to survive they need:
- food (often some type of sugar and maybe some protein)
- water (but not much)
- a comfortable temperature
- oxygen (at least, most do)
Cheese, and especially grated cheese ticks off just about all these boxes, making it prone to spoilage. Let’s have a closer look
Molds are just one type of microorganism that can occur on your food. The other common ones are bacteria and yeasts. Learn more about microorganisms in your food in our Food Microbiology Basics course.
Cheese is a great energy source…
Cheese contains a high amount of protein, fats, as well as some sugars. These are excellent sources of foods for a lot of types of molds.
Grated cheese provides an even bigger supply of easily accessible food. By grating the cheese, the overall surface area of the cheese increases considerably. All surfaces are now available for microorganisms to be devoured.
… And contains a good amount of water
Molds need water to be able to grow. Whether a food contains enough water is expressed using a measure called ‘water activity’. The water activity of a product is a measure for the amount of available water. At a value of zero, no water is available. A value of 1 indicates pure water.
The water activity of cheeses varies. For hard cheese, which can be grated the water activity tends to be around 0,95 or lower. For cheese that have been ripened for longer this can sink down to 0,8, or even just below 0,6.
At higher water activity, more microorganisms will survive and thrive. Molds though are the hardier ones to survive. Most molds can still grow quite well at water activity values down to 0,8. Some can even still grow at values as low as 0,6. As such, most grated cheese still contain plenty of water for a mold to thrive.
Learn more about the relationship between growth of microorganisms and water activity.
The fridge slows things down
Most grated cheese is stored in the fridge. And whereas most molds grow more slowly in the fridge, they do still grow.
Freezing grated cheese (at -18°C/0°F) will stop the growth of molds. However, keep in mind that freezing doesn’t kill molds. Once thawed the molds may start growing again.
Oxygen keeps it going
Have you ever noticed molds growing within a solid block of cheese*? Probably not. This is because most molds need oxygen to grow. Within a solid piece of cheese, there isn’t any oxygen, so molds won’t grow.
However, a piece of cheese that’s not been wrapped airtight will be surrounded by oxygen. Remember that the air around us contains about 20% oxygen. This oxygen enables the molds to thrive!
*Yes, various types of blue cheeses to contain (blue) molds within the cheese. However, these cheese have been punctured with needles to enable these molds to thrive!
How to prevent mold growth on grated cheese
In other words, grated cheese is a very desirable place for molds to grow. Nevertheless, a lot of grated cheese packs can be stored for months on end, without any spoilage. So, how do manufacturers do this?
They use that last essential factor to fight the molds: oxygen. Or, better said, the absence of oxygen. By choosing a suitable packagining technique, they can eliminate the presence of oxygen around the cheese. This prevents growth of those undesirable molds.
They have a few options here.
In a vacuum pack, all air is pulled out of the pack, including the oxygen. As such, molds can no long grow. A big disadvantage of this method though is that by pulling the vacuum, all the separate pieces of grated cheese are pressed against each other. As a result, when opening the pack the pieces can stick to each other or may form one large clump.
This is why you do find this method commonly used for larger blocks of cheese, but not so much for grated cheese!
Modified atmosphere packaging
It’s why grated cheese manufacturers often this another alternative: modified atmosphere packaging. This technique doesn’t eliminate any and all gases from the packaging. Instead, it simply replaces the air with a gas of a different composition. One that doesn’t contain any oxygen.
These packs may for instance contain 100% nitrogen. The big advantage of this is that there’s still a gas in the pack that protects the individual grated cheese strips from being pressed together.
Modified atmosphere packaging is an invisible packaging technology that helps to extend the shelf life of wide range of food products! It is also commonly used for freshly cut fruits and vegetables, e.g. to prevent browning as well as par-baked breads.
Even though proper packaging of grated cheese can greatly extend its shelf life, there are still other options. Most of these will involve actually changing the cheese to improve the shelf life. First instance, manufacturers can add ingredients that slow down or stop the growth of molds. These are called antimicrobials. If these have been added, you will find those labelled on a pack of grated cheese.
So, better use that grated cheese quickly!
The use of modified atmosphere packaging, so changing the gas composition within the pack, is commonly used for grated cheese. However, this does mean that once you’ve opened the pack, you’ve broken this protective barrier. Once opened the composition within the pack will be the same as that of air again. This happens very quickly. So even closing the pack again quickly after use won’t be of any help!
When manufacturers determine the shelf life of their grated cheese, they do so for the closed pack. It’s why an opened pack of grated cheese might not make it to the best before date giving on the package!
The best solution? Buy a suitable size and use all the grated cheese within a couple of days after opening!
El-Nimr, A & Eissa, Hesham & El-Abd, M & Mehriz, A & Abbas, Hayam & Bayoumi, Hala. (2010). Water Activity, Color Characteristics and Sensory Properties of Egyptian Gouda Cheese during Ripening. Journal of American Science. 6. link
Dean Sommer, Why Some Cheeses are Sensitive to Food Safety Problems, June-11th, 2014, link