mouldy bread in a plastic bag

Why Bread Turns Moldy – the Science of Bread Spoilage

Ever felt really hungry and wanted to make a quick sandwich? Ever found out that the only bread there’s left has bluish/grey hairy spots on it? Luckily it hasn’t happened to me that often, but when it does, it’s always a disappointment.

But why does bread at all get mouldy? And is there a way to prevent mouldy bread? Short answer, yes there is! And it’s not hard :-). This post will of course be the longer answer, digging into some more food microbiology.

Freshly baked bread & Start of spoilage

During baking several things happen in a bread that transform it in a great smelling, fluffy, brownie bread. We’ve discussed the details in a post dedicated on bread baking. But let’s summarize quickly:

  • The dough expands further due to the production of gas by yeast.
  • Water is evaporated and leaves the bread. The crust will become dry, which makes it crunchy, whereas the center will remain moister.
  • The gluten and starch ‘cook’ giving the bread its final structure. This prevents the bread from collasping once it comes out of the oven.
  • The high temperatures kill micro organisms, the yeast die, but also a lot of other micro organisms as well as moulds. The outside, which is hottest will be virtually sterile at the end.

However, as soon as the bread is out of the oven, some of these reactions start reversing themselves again. The decay of bread sets in almost as soon as the bread is taken out of the oven.

Once the bread comes out of the oven water will redistribute itself again, resulting in a softer crust. Also, the bread will start staling, it will become tougher and drier due to retrogradation of starch.

But also, as soon as the bread comes out of the oven and starts cooling down micro organisms will land on the bread and ‘contaminate’ the bread. The air will always contain moulds of some sort.

Freshly baked breads, fortunately not (yet) moldy. However, once they came out of the oven, micro organisms landed on the bread, since they sit all around us.

Why does bread mould?

Moulds can only grow under certain circumstances. The moulds need enough water, but not too much either, the temperature should be comfortable and there should be enough food. Bread always contains enough food due to all the carbohydrates.


Also, even though it might not seem obvious, bread contains quite a bit of moisture. Even though the outside is dry after baking, moisture from the inside will travel outwards. The crust won’t get wet, but it will get noticeably softer. If bread is stored in a plastic bag you can see the moisture very well. The inside of the bag may get a little damp (especially with warmer weather). It’s these conditions that yeast really appreciate. The slightly moist enironment is perfect for moulds to grow. Storing a bag in a paper bag on the other hand will not create such a nice environment. The paper will simply let the water through. Bread dries out more easily in a paper bag, but chances of it getting mouldy are also less!


Most moulds grow best between room temperature and 30-37 degrees Celsius. However, it does depend a bit on the type of mould. That said, storing you bread in a closed plastic bag on a hot summer day may result in moulds more easily than on a cold winter day.

In a freezer, well below zero degrees Celsius, growth of moulds is virtually zero. The growth will be stopped. Frozen bread won’t mould even after years (if stored at -18C).

In an oven moulds are killed, but as mentioned, these will probably come back again once the bread is out of the oven.

How to prevent mouldy bread

The two most simple solutions first:

  1. Freeze your bread. Freezing is really by far the best way to store bread. After thawing it will taste like freshly baked bread. Freezing doesn’t only prevent staling of bread, it wil also prevent mould growth.
  2. Eat you bread fast. Yes, simple as well, just eat your bread within 2-3 days and you’ll be fine (unless you decide to store it in a plastic bag on a very warm location).

However, the reason your bread had moulds is probably because you forgot about the bread or had a change of plans and had to postpone eating your bread. So we need a next level of preventive measures. These aren’t too complicated either, but take a little more trouble:

  1. Do not slice the bread yet. Slicing will speed up spoilage of the bread, not only will it dry out faster, it can also get mouldy more easily, the food is accesible more easily.
  2. Change the pack of the bread. That plastic bag is great for keeping bread moist, but storing bread in a warm place in a plastic bag is almost a guarantee for mould growth. So either pack it in a paper bag or bread box where moisture has less of a chance to build up. Or, the other option is to store the bread at cooler temperatures. Never store bread in the fridge (remember that’s horrible for the freshness of bread), but don’t store it at 30°C either.

Changing the recipe to prevent mould

If all those measures don’t help, you can also take another step. This is what a lot of bread manufacturers have done. They have started making their bread in such a way that it is less prone to mould growth.

When you bake your own bread, you will probably use only a few ingredients: flour (maybe several types), water, yeast (or a sourdough starter), salt and maybe some milk, butter or eggs, depending on the bread type.

Bread that you buy at a bakery or in a supermarket will most likely contain at least the ingredients mentioned above, but it might contain a lot more ingredients also. These additional ingredients are on the one hand added to improve the sensory properties of the bread, but others really serve to prevent mould growth. We won’t go into the details here, saving that for another post.

Sourdough breads

A freshly baked bread often cannot be kept for long. My homemade breads become dry and tough within 3-4 days and even 2 days after baking the quality has gone down. That said, though, my homemade baked breads don’t tend to mould. It has become stale well before it has time to mould.

I’ve also read that sourdouh breads don’t have a tendency to mould. Apparently the sourdough and acidity in the bread helps the preservation. That said though, I still have to find a proper reliable source (or do an experiment) to know this for sure.

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    • Hi Lewis,

      Thanks for coming by, glad to hear the post is useful for you.
      The pen name I’m using is Julie (aka the Science Chef). If you’d like more information, please consider visiting the About page and always feel free to send a message through the contact form :-)!

  1. Why there is mould growing on the bread when it was heated in the oven and left on the table for three days?

    • Hi Shia,

      Mold grows on bread when it’s moist enough for it to grow on. If it was left in a plastic bag that chance is quite large, especially if it’s warm. Breads in the supermarket with longer shelf life contain ingredients that prevent the growth of mold, but in most homemade bread you won’t add these ingredients, causing it to spoil. If you don’t pack the bread in plastic, but in a paper bag or cloth, mold won’t be able to grow (unless you’re in a very humid environment). The bread will dry out and grow stale, but you can overcome that by baking it again for a few minutes in a hot oven.

      Hope that answers your question! You can read more about staling of bread here.

  2. Nothing to do with modern trends for low sodium then – not enough salt in mass-produced bakery bread to ‘kill’ the rising agent ?

    • Hi Pete!

      Indeed, bread doesn’t contain enough salt to preserve eat. It would need a lot more for the salt to really extend the shelf life, bread simply contains too much moisture.

  3. Thanks for your sharing.when I baked bread I didn’t use the salt and bake it .I make it cool and keep in plastic bag only two day after that I saw a mould in the bread .can u please how can be make without bread mould .? Normal we can keep three or four day.

    • Hi Eheh,

      Your best bet here would be to not store it in a plastic bag. The plastic bag causes moisture to collect within and provides a great growing environment for microorganisms. Store it in a paper bag or wrap it in a towel. It might turn stale and a little drier, but it won’t spoil as easily. If you need an even longer shelf life, I would always suggest freezing it, you can store bread for months while frozen and it will almost be completely fresh when thawed.

  4. Great post . Although I do the wrong thing by keeping my loaf in plastic I’ve found that it seems to keep longer – up to 6 days – before the mould starts. I’ve started adding cider vinegar to the water and since then the storage time has improved by a couple of days and the taste has improved. I also used to have a problem with ‘Rope’ mould but fixed that by sterilizing the bench and all utensils.

    • Hi James,

      Interesting to hear that in your case the bread keeps for longer in a plastic bag! Could the impact of the environment be a reason you think? I can imagine that in more humid environments the bread benefits from being protected from the outside?

  5. Very informative ! Thank you I have been making/baking my own bread for about a year now…to start with I was anxious re “over kneading” but as time progressed I realised that when one jand kneads dough, that is, in all likelihood, not a likely ssue at all. My dough is now rising far more re the 100% wholewheat, although the grimd is quite coarse. I thank you for assisting me by explaining that it neefs longer to absorb the moisture better. Thank you again. Kind regards artisttrish (Instagram)

  6. I am still wondering why certain varieties of Finnish flat bread did not spoil in the old days.
    They made round loads with a hole in the middle. They would put the bread on a pole and hang it in the rafters. It would be used all winter. Apparently, it was dry enough but still, that is a long time for bread to keep from molding. .

    We have been baking my grandmother’s Finnish bread and freezing it. Maybe I will try keeping a loaf out and see how long it lasts.
    Her bread is baked longer at a lower temperature which might help. It does dry out to a hard bread in a short amount of time if left out.

    • Hi Dan,

      I’m afraid I’m not too familiar with this bread, but it seems like the way it was stored will have helped a lot. The outside of the bread will most likely dry out quite a bit, preventing growth of molds. Bread is especially prone to molding when packed in a plastic bag, where moisture can’t escape, in a humid environment. I would also expect that Finland during winter won’t be cooler in temperatures, but also have a drier air that helps slow down molding?

      Maybe the bread starts getting closer to crackers, where the water activity (amount of available water) is low enough to slow down growth of molds?

  7. Is there anything to add to my dough mixing to prevent my bread from mould,and my bread can be kept for six day or more in market

    • Hi James,

      Yes, there are some mold delaying ingredients you can add to bread such as sorbic acid (find more info here). Keep in mind that bread doesn’t jus turn mouldy, it can also turn stale over time. Sorbic acid delays the growth of microorganisms, but doesn’t do anything about staleness, that will require other ingredients.

      Hope that helps!

  8. Very informational, but I have a question. If a piece of bread and a wet napkin is left in an enclosed bag with no air inside, can it still grow mold and how/why? Thanks for reading.

    • Hi Lilly,

      If you truly take out all of the air, mold won’t grow. Molds need oxygen to grow. However, your bread won’t like it either, it will get squashed and pushed together. It’s why you won’t find bread being packaged in a vacuum pack. If you just close a bag, there will still be air in it and as such oxygen so mold can still grow until all oxygen has run out. There are plenty of breads that when wrapped in a plastic bag will form mold, even without the additional napkin. Adding a wet napkin will speed up the process since it will make the environment even more humid and attractive for molds to grow.

      Hope that helps!

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