What to do with bread once you notice mold on it is a common household dilemma. You want to be safe but not needlessly wasteful.

You may wonder whether the fuzzy spots of mold are safe to eat, can simply be scraped off, or whether the rest of the loaf is safe to eat if it doesn’t have visible mold.

This article explains what mold is, why it grows on bread, and whether it’s safe to eat moldy bread.

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Mold is a fungus in the same family as mushrooms. Fungi survive by breaking down and absorbing the nutrients of the material on which they grow, such as bread.

The fuzzy parts of mold you see on bread are colonies of spores — which is how the fungus reproduces. Spores can travel through the air inside the package and grow on other parts of the bread (1).

They’re what gives mold its color — white, yellow, green, gray, or black, depending on the type of fungus.

However, you can’t identify the type of mold by color alone, as the color of the spots may change under different growing conditions and can fluctuate during the lifecycle of the fungus (2).

Types of mold that grow on bread include Aspergillus, Penicillium, Fusarium, Mucor, and Rhizopus. What’s more, there are many different species of each of these types of fungus (3).

Summary Mold is a fungus, and its spores appear as fuzzy growths on bread. Many different types may contaminate bread.

Some mold is safe to consume, such as the types purposely used to make blue cheese. However, the fungi that can grow on bread give it an off-flavor and may be harmful to your health.

It’s impossible to know what kind of mold is growing on your bread just by looking at it, so it's best to assume it’s harmful and not eat it (1).

Additionally, avoid smelling moldy bread, as you may inhale spores from the fungus. If you have an allergy to mold, inhaling it could lead to breathing problems, including asthma (1).

Those with allergies to inhaled mold may also experience harmful reactions — including life-threatening anaphylaxis — if eating it in food. Still, this appears to be uncommon (4, 5, 6).

Lastly, people with weak immune systems — such as from poorly controlled diabetes — are vulnerable to infection from inhaling Rhizopus on bread. Though uncommon, this infection is potentially life-threatening (7, 8).

Summary Mold gives bread an off-flavor, may trigger allergic reactions and may cause harmful infections — particularly if you have a weak immune system. Therefore, you should never knowingly eat or smell it.

The Food Safety and Inspection Service of the United States Department of Agriculture (USDA) advises discarding the entire loaf of bread if it has developed mold (1).

Though you may only see a few spots of the fungus, its microscopic roots can spread quickly through porous bread. Therefore, don’t try to scrape off mold or salvage the rest of your loaf.

Some mold can produce harmful and invisible poisons called mycotoxins. These may spread through bread, particularly when mold growth is heavy (1).

High intake of mycotoxins may cause digestive upset or other illness. These toxins can also sicken animals, so don’t feed contaminated bread to your pets (9, 10, 11).

Furthermore, mycotoxins may negatively affect your intestinal health, possibly by altering the makeup of the microbes that inhabit your gut (12, 13).

Additionally, long-term, heavy exposure to some mycotoxins — including aflatoxin produced by certain species of Aspergillus — has been linked to increased cancer risk (14, 15, 16).

Summary The USDA advises discarding the entire loaf of bread if it has developed mold, as its roots can quickly spread in your bread. Additionally, some types of fungi produce harmful toxins.

Without preservatives, the shelf-life of bread stored at room temperature is generally three to four days (17).

Preservatives and other ingredients, as well as certain methods of handling and storing bread, may deter mold growth.

Ingredients That Inhibit Mold

Mass-produced bread from the supermarket typically contains chemical preservatives — including calcium propionate and sorbic acid — which deter the growth of mold (17, 18).

Still, a growing number of people prefer bread with cleaner ingredients, meaning bread made with no chemical preservatives (3).

An alternative is to use lactic acid bacteria, which produce acids that naturally deter mold growth. Currently, these are most commonly used in sourdough bread (3, 19, 20).

Vinegar and certain spices, such as cinnamon and cloves, may also deter mold growth. However, the spices may alter the flavor and aroma of bread, so their use for this purpose is limited (3).

Bread Handling and Storage Tips

Common mold spores generally can’t survive baking, but bread can easily pick up spores from the air after baking — for example, during slicing and packaging (18).

These spores can start to grow under the right conditions, such as in a warm and humid kitchen.

To deter mold growth on bread, you can (1, 21):

  • Keep it dry. If you see visible moisture inside the bread package, use a paper towel or a clean cloth to dry the package before sealing it. Moisture encourages mold growth.
  • Cover it. Keep bread covered, such as when serving it, to shield it from spores in the air. However, to avoid soggy bread and mold, don’t package fresh bread until it’s thoroughly cooled.
  • Freeze it. Though refrigeration slows mold growth, it also makes bread dry. Freezing bread stops the growth without altering the texture as much. Separate the slices with wax paper to make it easier to thaw only what you need.

Gluten-free bread is more vulnerable to mold growth, as it typically has a higher moisture content and limited use of chemical preservatives. For this reason, it’s often sold frozen (3).

Some bread is protected with special packaging instead of preservatives. For example, vacuum-sealing removes oxygen, which is needed for mold growth. Still, this bread is prone to contamination after you open the package (17).

Summary To inhibit mold growth, chemical preservatives are typically used in bread. Without them, bread generally starts to grow the fungi within three to four days. Freezing bread prevents the growth.

You shouldn’t eat mold on bread or from a loaf with visible spots. The mold roots can quickly spread through bread, though you can’t see them.

Eating moldy bread could make you sick, and inhaling spores may trigger breathing problems if you have a mold allergy.

Try freezing bread to prevent mold.