mouldy bread in a plastic bag

Why Does Bread Get 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 :-)!

  • 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.

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